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Reliability studies of surface mount solder joints - Effect of Cu-Sn intermetallic compounds
Abstract
Cu-Sn intermetallic compounds (IMCs), formed at the interface
between the solder and the copper substrate are found to play a dominant
role in determining the thermal fatigue life of surface mount solder
joints fabricated from a conventional infrared reflow process. In order
to predict the growth of this IMC layer during the operating life of the
solder joint and its effect on the thermal fatigue life, the formation
characteristics of the IMC's in 0805 and 1206 LCCC solder joints are
systematically studied in this investigation. Only the stable Cu<sub>6
</sub>Sn5% η-phase intermetallic compound was observed in
all as-solidified solder joints as confirmed by scanning electron
microscopy (SEM) and energy dispersive X-ray (EDX). The mean layer
thickness was found to increase almost linearly with reflow time up to
about 200 s. The thickness of the interfacial IMC layer increased with
increasing reflow temperature for 0805-type solder joints up to around
250°C and reached a saturated thickness of 2.5 μm beyond this
temperature. Additional intermetallic formation due to higher reflow
temperature or longer reflow time would appear as Cu-Sn whiskers in the
bulk solder of the joint. The copper land pad size and quality of
component lead metallization were also found to greatly affect the
formation of Cu-Sn IMC in surface mount solder joints, and hence its
reliability in terms of thermal fatigue life and mechanical properties
... Mechanical stresses, [6][7][8][9] local recrystallization regions, 10,11 intermetallic compounds, 12,13 and stress gradients [14][15][16][17] have been considered as MW driving forces. A recent electrostatic concept attributes MW growth to random electric fields generated by surfaces of imperfect metals. ...
... Using Eqs. (13) and (8) the characteristic barrier V 0 is estimated as, ...
... at distance h from the metal surface [recall that V 0 depends on h as specified in Eq. (13)]. Because for all practical cases one has to assume V V 0 , the distribution in Eq. (17) is not very different from uniform, ρ(V ) ≈ const ∼ 1/V 0 , which is typical of many models of disordered systems [note that V 0 is length dependent in Eq. (13)]. ...
The phenomenon of spontaneously growing metal whiskers (MW) raises significant reliability concerns due to its related arcing and shorting in electric equipment. The growth kinetics of MW remains poorly predictable. Here we present a theory describing the earlier observed intermittent growth of MW as caused by local energy barriers related to variations in the random electric fields generated by surface imperfections. We find the probabilistic distribution of MW stopping times, during which MW growth halts, which is important for reliability projections.
... Mechanical stresses, [6][7][8][9] local recrystallization regions, 10,11 intermetallic compounds, 12,13 and stress gradients [14][15][16][17] have been considered as MW driving forces. A recent electrostatic concept attributes MW growth to random electric fields generated by surfaces of imperfect metals. ...
... The following conclusions can be made based on the results obtained from Eqs. (13) and (17)- (19). ...
The phenomenon of spontaneously growing metal whiskers (MWs) raises significant reliability concerns due to their related arcing and shorting in electric equipment. The growth kinetics of MWs remains poorly predictable. Here, we present a theory describing the earlier observed intermittent growth of MWs as caused by local energy barriers related to variations in the random electric fields generated by surface imperfections. We find the probabilistic distribution of MW stopping times, during which MW growth halts, which is important for reliability projections.
... One hypothesis points at the mechanical stress 4-6,16-18 relaxing during whisker growth and thus providing the necessary driving force. Local recrystallization regions 3,19,20 and intermetallic compounds 17,21,22 have been referred to as possible stress sources. On the other hand, it was inferred that the stress gradient rather than stress itself is the whisker driving force. ...
... Once considered as whisker driving factors, the local intermetallic compounds 17,21,22 and their underlying dynamic recrystallization 3,19,20 can be linked to the electrostatic effects as well. Indeed, generally speaking, an intermetallic compound has a work function, different from that of its host by Dw $ 0:3 À 1 eV, typical of the difference between work functions of two chemically different metals. ...
We consider surface parameters responsible for variations in propensity for whisker formation and growth between (1) different metals and (2) different recipes of the same metal. The former is attributed to metal surface tension, while the latter is related to the surface charge density that is sensitive to structure imperfections, stresses, contaminations, etc. We propose a figure of merit combining these two parameters that describes a metal propensity for whiskers and the relative smallness of whisker concentration. We argue that many previously observed correlations between whiskers and stresses, stress gradients, intermetallic compounds, contaminations, etc., can be attributed to the effects of the above two parameters.
... 24 A succinct summary of whisker properties was given by G. Davy. 25 While the mechanism behind metal whiskers remains mysterious, one hypothesis points at the mechanical stress relaxing during whisker growth and thus providing the necessary driving force. [4][5][6]16,17,23 Local recrystallization regions 3,26,27 and intermetallic compounds 17,28,29 have been referred to as possible stress sources. It was inferred also that the stress gradients can be more im-portant than stresses itself. ...
We present TEM images of the interior of metal whiskers (MW) grown on electroplated Sn films. Along with earlier published information, our observations focus on a number of questions, such as why MWs' diameters are in the micron range (significantly exceeding the typical nano-sizes of nuclei in solids), why the diameters remain practically unchanged in the course of MW growth, what is the nature of MW diameter stochasticity, and what is the origin of the well-known striation structure of MW side surfaces. In an attempt to address such questions we performed an in-depth study of MW structure at the nanoscale by detaching a MW from its original film, reducing its size to a thin slice by cutting its sides by a focused ion beam, and performing TEM on that structure. Our observations revealed a rich nontrivial morphology suggesting that MW may consist of many side by side grown filaments. This structure appears to extend to the outside whisker surface and be the reason for the striation. In addition, we put forward a theory where nucleation of multiple thin metal needles results into micron-scale and larger MW diameters. This theory is developed in the average field approximation similar to the roughening transitions of metal surfaces. The theory also predicts MW nucleation barriers and other observed features.
... Furthermore, high electric current flows affect mass transport and enhance the growth of Cu 6 Sn 5 in the anode, the depletion of Cu, and the formation of voids in the cathode. Reliability issues caused by this phenomenon, known as electromigration (EM) [6][7][8][9][10][11][12] have become more important due to miniaturization and high-performance electronic devices, such as 3D integrated circuits with larger current densities and higher temperatures that accelerate mechanical failure [13,14]. ...
The miniaturization of solder bumps in flip-chip packages results in large variability in their microstructure making the prediction of joint lifetime challenging. This is of particular importance in Sn-rich solders due to the anisotropic diffusion behavior of Sn and the presence of fast diffusion pathways, such as grain boundaries and twins that vary from joint to joint. We developed a multi-physics phase-field model to predict the electromigration-enhanced evolution of intermetallic compounds in Cu/Sn system with different grain structures. We present simulations of solder joints with polycrystalline Sn with grains with different orientations to observe the effect of Sn anisotropy and grain boundaries on electromigration failure. The simulations predict Cu depletion in the cathode and the formation and accumulation of intermetallic compounds along Sn grain boundaries dependent on the Sn grain orientation. Additionally, Cu6Sn5 clusters are formed in the Sn matrix when high diffusion rates at the interfaces are considered.
... These aspects cannot be rapidly identified, hence the need for early consideration during the process design phase. Based on existing research, it has been found that the thickness of the IMC significantly affects the reliability of solder joints [21]. Moreover, using the heating factor concept proposed by Professor Wu Yiping [22], a robust correlation has been established between the solder joint temperature curve and the IMC thickness, defined as follows: ...
The process design of hot air reflow soldering is one of the key factors affecting the quality of PCBA (Printed Circuit Board Assembly) component products. In order to improve the product quality during the design process, this paper proposes a robust optimization-based finite element simulation analysis method including significant influencing factor screening, robustness evaluation, robust optimization, and reliability verification for the reflow soldering process. The simulation model of the reflow soldering process temperature field based on experiments is constructed and validated. Sensitivity analysis is used to select important influencing factors, such as the last five set temperature zones (T5 to T9) in the reflow oven and the thermal properties of materials such as PCBs (printed circuit boards), BGAs (ball grid arrays), and solder paste, as well as noise factors like the heating environment during the soldering process. Several surrogate models are used to construct the response surface, and the optimal fitting scheme is selected to effectively avoid poor fitting caused by inappropriate surrogate models. The 6σ robust optimization approach is introduced to evaluate and optimize the robustness of the process design parameter where the heating factor is chosen as the optimization target. The reliability analysis method is employed to validate the product quality. This paper establishes a comprehensive robustness analysis method for hot air reflow soldering, effectively reducing design costs and addressing the lack of robustness analysis in the current hot air reflow soldering process design.
... As the homologous temperature rises, the diffusion rate of atoms in solder increases, resulting in an increase in the thickness of the IMC in the solder joint [15,16]. Due to an increase in the thickness of the IMC, the solder joint may have reliability issues [17,18]. ...
Due to the inherent environmental and health toxicities associated with lead, the use of environmental friendly lead-free solder materials has become an unavoidable trend in the electronic packaging industry. Sn-58Bi alloy is gaining attention for its good material properties such as low melting point, reliability and high tensile strength. The presence of the bismuth-rich phase increases the brittleness of Sn-58Bi alloy. The purpose of this study is to suppress the brittleness of Sn-58Bi alloy by the addition of different wt% (0, 10, 20, 30) of Sn powder. The powder metallurgy method was used to prepare the samples. Scanning electron microscopy and energy-dispersive X-ray analysis were done to study the structural properties and a tensile test was done by a universal tensile machine to study the mechanical properties. The results reveal that the Sn particles partially dissolved in the Sn-58Bi solder matrix. The dissolution of Sn particles significantly improved the mechanical strength by 30%, suppressed the brittleness and improved the strain value by 1.3 times.
... To properly evaluate the reliability of the solder joints, these dependencies are necessary to know. Many previous studies proved the influence of the peak temperature and time above liquidus (TAL) on the growth of the intermetallic layers, and, thus, the solder joint properties for most of the nowadays used solder alloys, including Bi-Sn solders [6]- [8]. However, the studies rarely include the influence of the rest of the temperature profile, namely a pre-heat phase, when a solder flux is activated, although a major impact of the flux on the solder wetting and, therefore, solder joint creation, especially for lead-free solder alloys, is generally known [9]. ...
This work aimed to evaluate the change of properties of joints based on bismuth-tin eutectic alloy soldered under various reflow conditions during long-term accelerated aging tests. Four temperature profiles were used for the reflow soldering of testing boards. The profiles differed in the reflow phase and the pre-heat phase when the soldering flux is activated. Subsequently, the first half of the testing boards was aged for 1000 hours in a climatic chamber at 85 °C and relative humidity 85 %. The second half was closed up in a temperature-shock chamber for 1000 cycles with the following settings: a hot temperature of 125 °C, a cold temperature of-40 °C, and a cycle time of 30 minutes. The solder joint properties-electrical resistance, shear strength, and thickness of intermetallic layers-were measured four times during the aging to observe trends. It turned out from the measurement results that the most stable during temperature-humidity aging were the joints soldered with higher temperature in the pre-heat phase and lower temperature in the reflow phase. On the contrary, the joints soldered with a lower temperature in the pre-heat phase and a higher temperature in the reflow phase were more resistant against the thermal shocks. The shear strength was even higher than before the start of the test. Overall, both pre-heat and reflow settings play an important role in solder joint reliability.
... From a more general perspective, our work renders additional insights into the physics underlying whisker development. Mechanical stresses, [23][24][25][26][27] local recrystallization regions, [28,29] intermetallic compounds, [30,31] and stress gradients [32][33][34][35] have been considered as possible whisker driving forces. A more recent electrostatic concept [5,6] attributed whiskers to electric interactions that can underly the mechanical stress/stress gradient mechanisms or act through other agents. ...
We observed accelerated whisker development in thin tin films under non-destructive gamma-ray and x-ray irradiation sources. The effect is mediated by charges induced in glass substrates supporting films, and becomes significant reaching the characteristic range of radiation doses of 20-30 kGy. We were able to change the radiation induced whisker growth rate by electrically disconnecting some parts of our experimental setup, thus demonstrating the electrostatic nature of the whisker development. Subject to additional qualification trials, the observed acceleration factors suggest that ionizing radiation can be used for accelerated whisker propensity testing.
... The IMC layer serves as a mechanical bond between the substrate and the solder deposit. It can, however, have a detrimental effect not only on the solderability but also on the mechanical properties of the solder joints, as these layers are brittle compared to the solder (So and Chan, 1996). These concerns have prompted considerable attention in this area. ...
Purpose
– The purpose of this paper is to study the effect of intermetallic compound (IMC) layer thickness on the shear strength of surface-mount component 1206 chip resistor solder joints.
Design/methodology/approach
– To evaluate the shear strength and IMC thickness of the 1206 chip resistor solder joints, the test vehicles were conventionally reflowed for 480 seconds at a peak temperature of 240°C at different isothermal ageing times of 100, 200 and 300 hours. A cross-sectional study was conducted on the reflowed and aged 1206 chip resistor solder joints. The shear strength of the solder joints aged at 100, 200 and 300 hours was measured using a shear tester (Dage-4000PXY bond tester).
Findings
– It was found that the growth of IMC layer thickness increases as the ageing time increases at a constant temperature of 175°C, which resulted in a reduction of solder joint strength due to its brittle nature. It was also found that the shear strength of the reflowed 1206 chip resistor solder joint was higher than the aged joints. Moreover, it was revealed that the shear strength of the 1206 resistor solder joints aged at 100, 200 and 300 hours was influenced by the ageing reaction times. The results also indicate that an increase in ageing time and temperature does not have much influence on the formation and growth of Kirkendall voids.
Research limitations/implications
– A proper correlation between shear strength and fracture mode is required.
Practical implications
– The IMC thickness can be used to predict the shear strength of the component/printed circuit board pad solder joint.
Originality/value
– The shear strength of the 1206 chip resistor solder joint is a function of ageing time and temperature (°C). Therefore, it is vital to consider the shear strength of the surface-mount chip component in high-temperature electronics.
... Intermetallic compound (IMC), which is formed at the interface between the solder alloy and the soldered metal (Lee, 2014;Li et al., 2005), has a direct influence on the quality of the solder joint (So and Chan, 1996;Tu et al., 1997). Process of dissolution and diffusion of solder atoms to soldered metals (and vice versa) is happening during the soldering, and the layer of IMC is formed. ...
Purpose
The purpose of this paper is to increase the reliability of manufactured electronics and to reveal reliability significant factors. The experiments were focused especially on the influence of the reflow oven parameters presented by a heating factor.
Design/methodology/approach
The shear strength of the surface mount device (SMD) resistors and their joint resistance were analyzed. The resistors were assembled with two Sn/Ag/Cu-based and one Bi-based solder pastes, and the analysis was done for several values of the heating factor and before and after isothermal aging. The measurement of thickness of intermetallic compounds was conducted on the micro-sections of the solder joints.
Findings
The shear strength of solder joints based on the Sn/Ag/Cu-based solder alloy started to decline after the heating factor reached the value of 500 s · K, whereas the shear strength of the solder alloy based on the Bi alloy (in the measured range) always increased with an increase in the heating factor. Also, the Bi-based solder joints showed shear strength increase after isothermal aging in contrast to Sn/Ag/Cu-based solder joints, which showed shear strength decrease.
Originality/value
The interpretation of the results of such a comprehensive measurement leads to a better understanding of the mutual relation between reliability and other technological parameters such as solder alloy type, surface finish and parameters of the soldering process.
... 24 A succinct summary of whisker properties was given by G. Davy. 25 While the mechanism behind metal whiskers remains mysterious, one hypothesis points at the mechanical stress relaxing during whisker growth and thus providing the necessary driving force. [4][5][6]16,17,23 Local recrystallization regions 3,26,27 and intermetallic compounds 17,28,29 have been referred to as possible stress sources. It was inferred also that the stress gradients can be more im-portant than stresses itself. ...
We present TEM images of the interior of metal whiskers (MW) grown on electroplated Sn films. Along with earlier published information, our observations focus on a number of questions, such as why MWs' diameters are in the micron range (significantly exceeding the typical nano-sizes of nuclei in solids), why the diameters remain practically unchanged in the course of MW growth, what is the nature of MW diameter stochasticity, and what is the origin of the well-known striation structure of MW side surfaces. In an attempt to address such questions we performed an in-depth study of MW structure at the nanoscale by detaching a MW from its original film, reducing its size to a thin slice by cutting its sides by a focused ion beam, and performing TEM on that structure. Our observations revealed a rich nontrivial morphology suggesting that MW may consist of many side by side grown filaments. This structure appears to extend to the outside whisker surface and be the reason for the striation. In addition, we put forward a theory where nucleation of multiple thin metal needles results into micron-scale and larger MW diameters. This theory is developed in the average field approximation similar to the roughening transitions of metal surfaces. The theory also predicts MW nucleation barriers and other observed features.
... A rather limited insight into the whiskers underlying physics so far revolved around the concept of mechanical stresses [8][9][10][19][20][21], their gradients [22][23][24][25], local recrystallization regions [7,26,27], and intermetallic compounds [20,28,29]. ...
We report on the growth of tin metal whiskers significantly accelerated under non-destructive gamma-ray irradiation. Sn thin film, evaporated on glass substrate, was subjected to a total of 60 h of irradiation. The irradiated samples demonstrated enhanced whisker development, in both densities and lengths, resulting in an acceleration factor of ∼50. We attribute the observed enhancement to gamma-ray induced electrostatic fields, affecting whisker kinetics. These fields are due to the substrate charging under gamma-rays. We propose that gamma-ray irradiation can be a much needed tool for accelerated testing of whisker propensity.
... [16][17][18] A rather limited insight into the whiskers underlying physics so far revolved around the concept of mechanical stresses, [8][9][10][19][20][21] their gradients, [22][23][24][25] local recrystallization regions, 7,26,27 and intermetallic compounds. 20,28,29 As a common drawback, the above approaches lack predictive power. They do not explain whisker parameters and multiple observations that are stochastic in nature, such as the intermittent whisker growth, 30 ordersof-magnitude variations in local concentrations, etc. 5 From a more conceptual standpoint, these approaches are FIG. ...
We report on growth of tin (Sn) metal whiskers that is significantly accelerated under gamma-ray irradiation. The studied Sn thin film, evaporated on glass substrate, was subjected to a total of ~60 hours of irradiation over the course of 30 days. The irradiated sample demonstrated the enhanced development, in both whisker densities and lengths, resulting in an acceleration factor of ~50. This makes gamma-ray irradiation a candidate tool for accelerated testing of whisker propensity. We attribute the observed enhancement to electrostatic fields created by charged defects in the glass substrate under ionizing radiation of gamma-rays.
... Interestingly, micrographs of hypereutectic alloys also showed a variety of intermetallics from irregular shaped particles to rod shaped structures. These sort of rod-like/flower shaped IMCs are very common in solder technology [27,28]. It is important to mention that mechanical stirring was employed during the addition of Mg into Zn melt and as the Mg content increased the time for stirring also increased. ...
Researchers have been working for some times to develop a Pb-free substitute for high-Pb containing solders in electronics. In this study the effect of Mg content on microstructure, melting behavior, thermal, electrical and mechanical properties of a new high-temperature Pb-free solder based on binary Zn–Mg alloy were investigated. The nominal compositions of Zn–xMg (x = 0.5–6.0) alloys were synthesized by conventional melting and casting route. The microstructures of the solders changed significantly on adding Mg content. Higher Mg containing alloys were found rich in intermetallic phases. The formation of the Mg2Zn11 and metastable MgZn2 intermetallic phases were identified and were also quantified by X-ray diffraction analysis. The presence of higher amount of intermetallic compounds in hypereutectic Zn–Mg systems deteriorated the tensile properties of the binary alloy. The results of electrical resistivity test indicated that the Mg could elevate the resistivity of newly developed Zn–Mg alloys. The melting behavior of the solder alloys studied by differential thermal analysis (DTA) analysis revealed that the melting temperature range of the solder alloys narrowed with Mg addition. Thermal mechanical analysis (TMA) analysis revealed that the co-efficient of thermal expansion (CTE) decreased on increasing Mg content.
... Intermetallic formation and kinetics at the solid-liquid interface during soldering processes have received great attention in the past for both Sn-Pb eutectic and Pb-free alloys [1,2,3,4]. In additions, there are many studied on the reliability of BGA components, and general effects of intermetallic compounds on the thermal fatigue life and mechanical properties of BGA solder balls [5,6,7,8]. However, no definitive relationship has been established about how intermetallic morphology and thickness affect mechanical strength of the solder joints in general due to complex stress-strain condition a BGA ball encounters in service and in testing. ...
Thermal exposures during soldering processes and during product testing, as well as in service, all together determine
intermetallic morphology and thickness in a solder interconnect for a given solder alloy and metallic surfaces to be
joint. The thermal history will also affect mechanical strength of the solder joint in general. In this study, a variety of
BGA components with balls made of Pb-free SAC305 alloy, Sn-Pb eutectic and high temperature 90Pb-10Sn alloys,
were subjected to different thermal history, including up to 10 reflow cycles, and aged at 125°C from 24 hrs up to 168
hrs. The intermetallic thickness and morphology after the thermal events were then examined under optical and
scanning electronic microscopes. Ball shearing tests were conducted to investigate effects of the thermal history and
intermetallic thickness and morphology on shearing strength of these solder balls. Results show that effects directly
from intermetallic layers may or may not be detectable; and shearing strength of solder balls are largely a reflection of
solder alloys and their microstructures. It is believed that a sound understanding of the effects of intermetallic
morphology and thickness on reliability of BGA solder balls can lead to more intelligent choice to soldering processes, as well as to rework/repair processes and to their operation limits.
... The growth of interface IMC in the isothermal aging process can be described by the following empirical formula [29]: ...
It is widely accepted that the structures and properties of solids are influenced greatly by its matrilineal liquid state. In the present paper, the correlation between the internal friction behavior of Zr55Al10Ni5Cu30 BMG samples and their quenching temperatures was investigated. The measured tan ϕ − T curves showed that the positions of internal friction peaks did not change with frequencies for all samples. But they were shifted to higher temperature with the quenching temperature elevated. In the supercooled liquid, the higher the melt temperature was, the lower the internal friction was. Based on the theory of Perez, the correlation parameter χ was deduced from tan ϕ − f curves in the vicinity of Tg. The change of the value of χ showed that the melt temperature had significantly influence on the mechanical relaxation of metallic glasses.
... The growth of interface IMC in the isothermal aging process can be described by the following empirical formula [29]: ...
The intermetallic compound (IMC) growth behavior of Sn–3.5Ag–3.5Bi/Cu joint was investigated with a change in the solder melt structure during soldering and 180 °C isothermal aging. The results show that when the solders undergo liqiud–liquid structure transition (LLST), the IMC of the joint is thinner and more evenly distributed during soldering. The interface IMC is also thinner, and the quantity of Ag3Sn as well as Cu6Sn5 in the solder is relatively lower. However, the IMCs are more bulky after long-time aging at 180 °C. When the solders do not undergo LLST, microcracks form in the solder. Kirkendall voids are more abundant and interconnected after long, high-temperature aging. This finding indicates better joint reliability after than before LLST. The growth rate constants of the interface IMC for the two kinds of joints are calculated to be 1.94 × 10−12 and 9.71 × 10−13. The correlation of IMC growth behavior and melt state is analyzed from the viewpoints of LLST and atom diffusion.
An intermetallic layer (IML) between the solder alloy and the soldered surface affects the mechanical and electrical performance of the resulting joints. Numerous studies have explored the possibilities of influencing the IML to achieve more reliable interconnections. However, the type and composition of the used flux, crucial for the proper creation of solder joints, is rarely included as a possible influencing factor. In this article, a comprehensive study on the interfacial microstructure evolution of lead‐free SnAgCu solder joints, accounting for the flux type and the temperature of the preheating phase of reflow soldering, where the flux contained in the solder paste becomes active, is presented. In the results, it is shown that the IML of as‐reflowed and thermally aged solder joints depends significantly on the flux. The IML activation energy is 57% higher for rosin‐based low‐activity (ROL)0 flux compared to ROL1 flux. The ROL0 flux, containing fewer active components, also outperforms the ROL1 flux in both the mechanical and electrical properties of the joints. Furthermore, the temperature profiles also show slight differences in measured properties, with the fluxes responding differently to changes in preheating temperature. In the presented results, importance of the used flux on solder joint microstructure is demonstrated.
Effects of Mn-doped TiO 2 nanoparticles on the wettability, printability and thickness of intermetallic layer of SAC30 on a Cu substrate were studied in this paper. Concentration of Mn-doped TiO 2 nanoparticles was varied from 0.05 wt.% to 1[Formula: see text]wt.%. The wettability was evaluated in terms of contact angle of solder on the copper substrate and the printability was measured from the volume ratio of printed solder on the copper substrate to volume of stencil opening. The experimental results showed that 0.10[Formula: see text]wt.% Mn-doped TiO 2 led to the lowest contact angle while the printability was decreased with the increase of the added nanoparticles and the thinnest intermetallic layer was found at 0.50[Formula: see text]wt.% Mn-doped TiO 2 nanoparticles. Mn-doped TiO 2 concentration was then optimized using a desirability function to find optimal values for the three responses. It was found that 0.07[Formula: see text]wt.% was the optimal concentration for the three metrics — wettability, printability and thickness of the intermetallic layer.
This paper aims to introduce the development and implementation of uncertainty model, as well as, to discuss the quality assurance on metallic coating thickness measurement by using Scanning Electron Microscope (SEM) and delamination of failure integrated circuit (IC) chip by using Scanning Acoustic Microscope (SAM) under Laboratory Accreditation through the Hong Kong Laboratory Accreditation Scheme (HOKLAS), which is based on international standard ISO/IEC 17025 in Technology Support Centre of Hong Kong Science and Technology Parks Corporations.
The scaling of conventional solder-based flip chip bonding is facing its limitations due to thermal compression bonding overlay tolerance when using conventional bumping. To decrease the tolerance, planarization can be used to fabricate two flat surfaces for bonding. However, planarization of these soft and ductile surfaces is challenging by polishing. Here, we assess the creep-feed fly-cutting process, the so-called surface planer process for planarization of fine-pitch Sn bumps and polymer simultaneously. It is revealed that the polishing process causes a lot of scratches on the Sn and polymer surface; however, these surfaces are smooth for the case of the surface planer process. The planarized Sn and polymer surface has only a 50 nm step height which does not have any impact during thermal compression bonding. Using a planarized Sn and polymer surface, stacking of below 10 μ m pitch has been achieved.
Proeutectic zinc–aluminum‐based ternary Pb‐free solder for high‐temperature applications was developed by mechanically dispersing Ni particles into Zn–4 (wt%) Al alloy. The effect of Ni content on microstructure, melting behavior, thermal, electrical, and mechanical properties of the solder alloys was investigated. The amount of proeutectic Zn‐rich, β‐phase, and intermetallic compounds (IMCs) in the microstructures of the solders changed significantly on increasing Ni content. The development of ZnAl5Ni5 phase was observed in the microstructures of Zn–Al–Ni systems and their formation mechanism was also elucidated. Micrographs also showed a change in intermetallics from irregular‐shaped particles to rod‐shaped structures. All these structural changes improved the mechanical properties like tensile strength and hardness of the newly developed ternary alloys. The melting behavior of the solder alloys studied by differential thermal analysis (DTA) revealed that the formation of the Ni–Zn eutectic phase in the composite solders was increased with Ni addition. Thermal mechanical analysis (TMA) revealed that the coefficient of thermal expansion (CTE) decreased on increasing Ni content.
Finding a suitable solder alloy to completely replace Pb‐based high‐temperature solders in electronic packaging applications is a crying need for sustenance and further advancement of the present technology. Currently, Zn‐based alloys have emerged as a potential candidate in substituting the health hazardous Pb‐containing solders. This chapter focuses on the recent research in the development of Zn‐based high‐temperature solders comparing their microstructural, thermal, mechanical, and electrical properties and reliability issues. The major alloys discussed here include Zn–Al‐based, Zn–Sn‐based, binary Zn–Ni, Zn–Mg, Zn–Ag, and pure Zn alloys with trace element addition. Significant changes in structures and properties of the alloys due to addition of several alloying elements in different compositions have been critically analyzed and their suitability in desired applications has been assessed. Finally, the conclusion was drawn that further research and development of Zn‐based high‐temperature solders is essential for replacement of conventional Pb‐based high‐temperature solders.
This is a preliminary investigation on the mechanical properties of Pb-free Sn-1.0Ag-0.5Cu solder joints containing 0.02 wt.% to 0.1 wt.% Y under a range of thermal aging and reflow conditions. Despite the significantly thicker intermetallic compound (IMC) formed at the solder joint, the 0.1 wt.% Y-doped joint exhibited a higher fracture strength than its baseline Sn-1.0Ag-0.5Cu counterpart under most aging and reflow conditions. This may be associated with the formation of Y-Cu IMCs formed at the interface between the solder and the Cu substrate, because the Y-Cu IMCs have recently been referred to as relatively ‘ductile’ IMCs.
The growth kinetics of intermetallic compound layers formed between Sn-3.5Ag solder and Cu substrate were investigated as a consequence of solid-state isothermal aging. Isothermal aging was carried out in a temperature range between 70degreesC and 200degreesC for 0 to 60 days. A quantitative analysis of the intermetallic compound layer thickness as a function of time and temperature was performed. The diffusion couples showed a composite intermetallic layer comprised Of Cu6Sn5 and Cu3Sn. The growth of intermetallic compounds followed diffusion-controlled kinetics and the layer thickness reached only 9 mum after 60 day of aging at 150degreesC. The apparent activation energies were calculated for the growth of the total intermetallic compound (Cu6Sn5 + Cu3Sn); Cu6Sn5 and Cu3Sn intermetallic are 65.4, 55.4 and 75.7 kJ/mol, respectively.
The appearance of IMCs on the Sn/Cu interface plays an important role in the joint reliability. Their growth behavior during solidification of multiple reflows was investigated. Compressed air was employed to blow away the residual liquid solder immediately after heating preservation so that the morphology corresponding to heating preservation would remain unaltered; and then by compared they with samples experienced air cooling, the growth of interfacial Cu6Sn5 grains could be investigated separately. The morphology of Cu6Sn5 grains remained a scallop-type during heating preservations of multiple reflows and they always converted to prism-type after air cooling. The dissolution and precipitation of Cu in liquid Sn with the temperature fluctuations should be responsible to the transformation. Moreover, scalloped Cu6Sn5 grains appeared at heating preservation became larger with the increasing number of reflows due to the ripening reaction. Their growth in dimension induced a change in appearance of those prismatic Cu6Sn5 grains formed on them. Shorter but larger main facets formed on those prismatic Cu6Sn5 grains with the increasing number of reflows.
The high operating frequencies of semiconductor chips generate heat fluxes it is important to be evacuated in order to avoid the destruction of the chip. A standard module in the field of power electronics is composed of a silicon chip, an electrical insulator (substrate) and a heat sink (drain) for the evacuation of heat. This heat induces thermomechanical stresses due to differential expansion of materials.Two new concepts proposed can overcome these problems and increase the overall reliability of electronic systems. The first is the design and development of a drain composite adaptive thermal properties (thermal expansion coefficient and thermal conductivity). In the second, a new assembly method is presented. It allows, by means of a metal film Sn or Au, intermetallic compounds to create stable over time.
Electronic systems are often stored for long periods prior to deployment in the intended environment. Aging has been previously shown to effect the reliability and constitu-tive behavior of second-level leadfree interconnects. Deployed systems may be subjected to cyclic thermo-mechanical loads subsequent to deployment. Prognostication of accrued damage and assessment of residual life is extremely critical for ultrahigh reliability systems in which the cost of failure is too high. The presented methodology uses leading indicators of failure based on microstructural evolution of damage to identify impending failure in electronic systems subjected to sequential stresses of thermal aging and thermal cycling. The methodology has been demonstrated on area-array ball-grid array test assemblies with Sn3Ag0.5Cu interconnects subjected to thermal aging at 125 °C and thermal cycling from −55 to 125 °C for various lengths of time and cycles. Damage equivalency methodologies have been developed to map damage accrued in thermal aging to the reduction in thermo-mechanical cyclic life based on damage proxies. Assemblies have been prognosticated to assess the error with interrogation of system state and assessment of residual life. Prognostic metrics including α − λ metric, sample standard deviation, mean square error, mean absolute percentage error, average bias, relative accuracy (RA), and cumulative RA have been used to compare the performance of the damage proxies.
In order to use a flip chip method for bonding the Si chip directly to an organic substrate, compatible under bump metallization (UBM) must be available. Conventional schemes with a copper-based solderable layer are not well compatible with the high-tin solders (such as eutectic Pb-Sn) used with organic substrates. This is due to the rapid reaction between Sn and Cu which depletes the UBM of copper. Ni-based schemes exhibit slower reaction with the solder and have been identified by the semiconductor industry as preferable replacements to Cu-based UBM's. However, Ni-containing metallurgies are often associated with high stresses, which results in poor practical adhesion between the silicon chip and the metallization, leading to interfacial failure during fabrication or service. In this research, several nickel-containing UBM schemes are studied experimentally. Stress measurements are made for each metallization before patterning of UBM pads. An optimal Ni concentration for the UBM is suggested based on the results from this study.
The effect of ageing and intermetallic compound formation on the surface mount solder joints and its shear strength behavior under extreme mechanical and thermal conditions have been discussed in this paper. The specimens used are solder paste (Sn3.8Ag0.7Cu), bench marker II printed circuit boards (PCB), resistors 1206 and the fabrication of solder joints makes use of conventional surface mount technology (SMT). Reflow process was carried out at a peak temperature of 250 °C and the test samples were exposed to isothermal ageing at a constant temperature of 150 °C for a period of 600 h. Shear test was conducted on the PCB’s. The shear strength of the solder joints rapidly increased during isothermal ageing to a certain time period and then started decreasing. Field emission scanning electron microscopy (FESEM) micrograph of the solder joint and energy dispersive X-ray (EDX) was performed on the solder sample to verify the formation of intermetallic compounds.
While the process fabrication and the electronic applications develop quickly, the die size and the package type also confront the improvement from the customers’ request to achieve the better signal performance in IC products. In cost consideration and marketing competition, most of commercial IC products still adopt the wire-bond technology, except some high performance products with solder/gold balls adhesion process. For consumer ICs, the gold wire is the major material to connect the IC chip and the lead frame through the bondability technology. In the recent era, the bond pad size and pitch is always shrinking. Therefore, the bond performance strongly depends on wire bond machine to provide lighters, thinner, and more reliable IC. After pad size shrinkage, the quality of pad is more impressed for wire bond reliability, especially in fine-pitch assembly process. It’s a challenge in reliability requirement. In this study, the root cause and promising solution of bond pad contamination were investigated. One is to improve the bonding yield in wire-bond process; the other is to promote the bonding reliability after the whole assembly process.
Electronic systems are often stored for long periods prior to deployment in the intended environment. Aging has been previously shown to effect the reliability and constitutive behavior of second-level leadfree interconnects. Deployed systems may be subjected to cyclic thermo-mechanical loads subsequent to deployment. Prognostication of accrued damage and assessment of residual life is extremely critical for ultrahigh reliability systems in which the cost of failure is too high. The presented methodology uses leading indicators of failure based on microstructural evolution of damage to identify impending failure in electronic systems subjected to sequential stresses of thermal aging and thermal cycling. The methodology has been demonstrated on area-array ball-grid array test assemblies with Sn3Ag0.5Cu interconnects subjected to thermal aging at 125 °C and thermal cycling from -55 to 125 °C for various lengths of time and cycles. Damage equivalency methodologies have been developed to map damage accrued in thermal aging to the reduction in thermo-mechanical cyclic life based on damage proxies. Assemblies have been prognosticated to assess the error with interrogation of system state and assessment of residual life. Prognostic metrics including α - λ metric, sample standard deviation, mean square error, mean absolute percentage error, average bias, relative accuracy (RA), and cumulative RA have been used to compare the performance of the damage proxies.
Thickening behavior of interfacial η (Cu6Sn5) phase and ɛ (Cu3Sn) phase intermetallic layers was investigated in liquid tin/solid copper reaction couples over reaction times from 30 sec to over 4,000 min and temperatures from 250°C to 325°C. A scanning electron microscope (SEM) was used to quantify the interfacial microstructure at each processing condition. The η developed with a scalloped morphology, while the ɛ always grew as a somewhat undulated planar layer in phase with the η. The thickness of each phase was quantitatively evaluated from SEM micrographs using imaging software. Thickening kinetics of the ɛ and η compounds were modeled using time- and temperature-dependent empirical power-law equations. From the model, values for the kinetic exponent, rate constant, and activation energy were established for each intermetallic layer. Measured values for the kinetic exponents and activation energies suggest that thickening of the η is controlled by a grain-boundary diffusion mechanism, and growth of the ɛ occurs by solid-state diffusion, probably grain-boundary diffusion.
Interdiffusion and intermetallics formation between metallization conductor Pd–Ag and solder 62Sn-36Pb-2Ag have been studied. Silver and palladium dot mapping images of solder joints demonstrate that the Pd–Ag metallization layer gradually disappears after 11 days of ageing. It is observed that silver-rich areas exist in the bulk of the 62Sn-36Pb-2Ag solder after ageing but palladium-rich areas are not evident. The diffusion coefficient of the silver in the solder joints for this material system was measured. The activation energy and pre-exponential factor for the silver diffusion were found to be about 0.475 eV and 0.56×10-10 m2 s−1, respectively for the configuration of surface mount thick film solder joints studied. X-ray diffraction results reveal the formation of the intermetallic compounds Ag5Sn, Ag3Sn, Pd3Sn2, Pd2Sn, PdSn2, PdSn4, PdSn, PbPd3, and Pb3Pd5.
Solid-state interactions taking place at the interface region of bi-layer thin film of Sn(500 Å)/Cu(500 Å) is studied by depth dependent X-ray photoelectron spectroscopy (XPS). Composition variation across the cross-section is measured. The distribution of copper is found to be uniform over a distance from the Cu–Sn interface and decreases sharply near the surface region. On further annealing at higher temperature (200 °C) copper redistributes uniformly over larger distance to a lower concentration. XPS peak shape parameter is determined to characterize the nature of the interface. It is shown that for freshly prepared and also for annealed samples no sharp interface could be found. Microstructural characterization is done by X-ray diffraction (XRD) and SEM techniques. From a comparative study with thicker films (Sn(1000 Å)/Cu(1000 Å)), it has been concluded that the interface of thinner films are made of nano-sized crystallites. It has also been shown that the grains in thicker samples grow faster than the thinner samples. Formation of inter-metallic compounds and their transformations are found to be same in both films.
In this study, copper powders were added into a molten Sn-Ag solder to form in-situ Cu6Sn5 disperoids. The composite solder formed more uniform and thinner intermetallic layer at the solder/Cu contact pad interface. However, the disperoids coarsened very significantly during the reflow processing and their gravitational sedimentation was observed.
The interfacial reactions between Sn–0.4Co–0.7Cu eutectic alloy and immersion Au/electroless Ni(P)/Cu substrate were investigated after reflow soldering at 260°C for 2min. Common Sn–4.0Ag–0.5Cu and eutectic Sn–0.7Cu solders were used as reference. Two types of intermetallic compounds (IMC) were found in the solder matrix of the Sn–0.4Co–0.7Cu alloy, namely coarser CoSn2 and finer Cu6Sn5 particles, while only one ternary (Cu, Ni)6Sn5 interfacial compound was detected between the solder alloy and the electroless nickel and immersion gold (ENIG) coated substrate. The same trend was also observed for the Sn–Ag–Cu and Sn–Cu solder joints. Compared with the CoSn2 particles found in the Sn–Co–Cu solder and the Ag3Sn particles found in the Sn–Ag–Cu solder, the Cu6Sn5 particles found in both solder systems exhibited finer structure and more uniform distribution. It was noted that the thickness of the interfacial IMCs for the Sn–Co–Cu, Sn–Ag–Cu and Sn–Cu alloys was 3.5μm, 4.3μm and 4.1μm, respectively, as a result of longer reflow time above the alloy's melting temperature since the Sn–Ag–Cu solder alloy has the lowest melting point.
The growth and morphology of the intermetallic compounds formed at the solder joint between the Sn–9Zn–2.5Ag lead-free solder and the Cu substrate have been investigated in this study. The planar Cu6Sn5 formed after soldering transforms to a scalloped Cu5Zn8 after aging at 180°C for 250h. However, the Cu5Zn8 decomposes and the scalloped Cu6Sn5 reforms at the interface close to the solder alloy after aging for 400h. The η′-Cu6Sn5 transforms to the η-Cu6Sn5 as aged at 180°C. The interdiffusion coefficient of Cu and Sn in the Cu6Sn5 is determined as 2.58×10−13cm2s−1, and that of Cu and Zn in the Cu5Zn8 layers is determined as 3.36×10−14cm2s−1 at 180°C, from the estimated thickness of the intermetallic compound (IMC). On the other hand, they are determined as 3.43×10−13 and 2.36×10−14cm2s−1, respectively, from the microbeam EDS analysis.
The growth kinetics of intermetallic compound layers formed between Sn-3.5Ag solder and Cu substrate were investigated as
a consequence of solid-state isothermal aging. Isothermal aging was carried out in a temperature range between 70°C and 200°C
for 0 to 60 days. A quantitative analysis of the intermetallic compound layer thickness as a function of time and temperature
was performed. The diffusion couples showed a composite intermetallic layer comprised of Cu6Sn5 and Cu3Sn. The growth of intermetallic compounds followed diffusion-controlled kinetics and the layer thickness reached only 9 μm
after 60 day of aging at 150°C. The apparent activation energies were calculated for the growth of the total intermetallic
compound (Cu6Sn5+Cu3Sn); Cu6Sn5 and Cu3Sn intermetallic are 65.4, 55.4 and 75.7 kJ/mol, respectively.
The Ni-based under-bump metallurgies (UBMs) are of interest because they have a slower reaction rate with Sn-rich solders
compared to Cu-based UBMs. In this study, several UBM schemes using Ni as the diffusion barrier are investigated. Joints of
Sn-58Bi/Au/electroless nickel (EN)/Cu/Al2O3 and Sn-58Bi/Au/electroplated nickel/Cu/Al2O3 were aged at 110C and 130C for 1–25 days to study the interfacial reaction and microstructural evolution. The Sn-Bi solder
reacts with the Ni-based multimetallization and forms the ternary Sn-Ni-Bi intermetallic compound (IMC) during aging at 110C.
Compositions of ternary IMC were (78–80)at.%Sn-(12–16)at.%Ni-(5–8)at.%Bi in joints of Sn-58Bi/Au/Ni-5.5wt.%P/Cu, Sn-58Bi/Au/Ni-12wt.%P/Cu,
and Sn-58Bi/Au/Ni/Cu. Elevated aging at 130C accelerates the IMC growth rate and results in the formation of (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5 adjacent to the ternary Sn-Ni-Bi IMC for the Sn-58Bi/Au/Ni-12wt.%P/Cu and Sn-58Bi/Au/Ni/Cu joints, respectively. The Cu content
in the (Cu,Ni)6Sn5 IMC is six times that in (Ni,Cu)3Sn4. Electroplated Ni fails to prevent Cu diffusion toward the Ni/solder interface as compared to EN-based joints. Cracks are
observed in the Sn-58Bi/Au/Ni-5.5wt.%P/Cu/Al2O3 joint aged at 130C for 25 days. It is more favorable to employ Ni-12wt.%P for the Sn-58Bi/Au/EN/Cu joint. Electroless nickel,
with the higher P content of 12 wt.%, is a more effective diffusion barrier during aging. In addition, P enrichment occurs
near the interface of the EN/solder, and the degree of P enrichment is enhanced with aging time. The Au(Sn,Bi)4, with pyramidal and cubic shape, is observed in the Sn-58Bi/Au/Ni/Cu/Al2O3 joint.
The nucleation kinetics of the η-phase (Cu6Sn5) intermetallic compound were investigated by hot dipping copper coupons in molten tin for 1 and 2 sec, at temperatures varying
from 240C to 300C. In the scanning electron microscope (SEM), the Cu6Sn5 phase appears as small, rounded bumps of varying sizes, jutting out from the surface of the copper. The experimentally determined
nucleation curves show the typical inverse C type behavior with a maximum nucleation rate occurring at an intermediate temperature.
The role of surface finish on nucleation rate was also studied. Experimentally determined “effective nucleation rate” per
unit area is presented and compared with theoretical predictions over the temperature range investigated. Relevant nucleation
models and nucleation energetic parameters are also discussed.
PBGA has the advantage of being compatible with existing surface-mount process. The eutectic solder balls collapse during reflow accommodates the lack of co-planarity, the surface tension between solder ball and associated pad creates a strong self-centering property to compensate for any placement offsets. However, some problems emerged in practice due to its special features. The major challenges for manufacturer are the inspection of solder joint and to perform touch-up. In most cases, the defect of solder joint cannot be found until in-circuit or functional testing is performed. At this stage, it becomes difficult to determine whether the defect is due to a solder joint or the component itself. Hence controlling the open defect is very important in the PBGA assembling process. It was found that three kinds of defective modes are responsible for the open defect; insufficient heating in the solder melting phase, poor thermal stability of PCB and PBGA and insufficient amount of printing solder paste. Based on the defect mechanism analysis, by optimizing the soldering process, very high assembly yield was achieved.
In this study, the diffusion behavior and microstructural evolution of Cu-Sn intermetallics at eutectic Sn-Pb solder/copper substrate interface of PBGA solder joints was studied. The PBGA solder joints were formed by different profiles, which was devised to have the same "heating factor"-the integral of the measured temperature above the liquidus (183°C) with respect to dwell time in the reflow profile, but to have different conveyor speeds. Using the theory of heat transmission, it is shown that the solder joint cooling rate during solidification increases with increasing conveyor speed. As a result, the "crystallization degree" of the solder joint microstructure decreased with the increasing of cooling rate. The thickness of IMC layer increased with extension of aging time. The growth of IMC is a diffusion-controlled process, i.e., tin diffuses into copper, and the diffusion coefficient in the "disordered region" (Db) is much bigger than that in the "crystallization region" (Dl), so the IMC growth rate of solder joint with faster cooling rate was larger. On the other hand, although Db>Dl at all temperatures, the difference increases as temperature decrease, consequently, the difference of IMC thickness growth among different cooling rate solder joints varied according to the aging temperature.
Low-resistance copper-tin (Cu-Sn) microbumps, with sizes varying from 5 lm 9 5 lm to 20 lm 9 20 lm and formed by electroplating–evaporation bumping (EEB) technology for three-dimensional integration of large-scale integrated chips, have been evaluated for their microstructure and electrical resistance. It was inferred from x-ray diffraction data that the formation of low-resistance Cu 3 Sn intermetallic compound (IMC) is facilitated at higher bonding temperature. Electron probe microanalysis mapping showed that, even before bonding, Cu-Sn IMCs were formed at the interface between Cu and Sn, whereas they were sandwiched between the Cu of the upper and lower microbumps after bonding. Electron backscatter diffraction analysis revealed that the crystal orientation of Sn grains was sharply localized in the (100) orientation for physical vapor deposited (PVD) sample, while electroplated Sn film exhibited a mixed crystal orientation in all (100), (110), and (001) axes. A resistance value of $35 mX per bump was obtained for Cu-Sn microbumps with area of 400 lm 2 , which is several times lower than the resistance value reported for Cu-Sn microbumps fabricated by a pure electroplating method. The low resistance value obtained for EEB-formed Cu-Sn microbumps after bonding is explained by (i) the reduced surface roughness for evaporated Sn, (ii) the high degree of crystal grain orientation resulting from layer-by-layer growth in the PVD Sn, despite their smaller grain size, and (iii) the absence of impurity segregation at grain boundaries.
The crystal growth, morphology and structure of the intermetallic compounds (IMCs) at the Sn–9Zn–xAg/Cu interface have been investigated. During isothermal aging, the local strain and crystallite size of the IMCs vary with aging time due to the growth and decomposition of the IMCs. The Kirkendall voids form at the Sn–9Zn/Cu interface after aging at 180°C for 250h but are not observed at the Sn–9Zn–xAg/Cu interface at the same aging parameters. Ag is repelled from the monoclinic η′-Cu6Sn5 which transforms to the hexagonal η-Cu6Sn5 and reacts with Sn to promote the growth of the Ag3Sn. The diffusion coefficients of Sn in Cu6Sn5 are determined 5.76×10−10 and 2.82×10−10cm2/s at 250°C when the Ag contents in the solders are 0.5 and 1.5wt%, respectively.
Purpose
To investigate effects of the thermal history on intermetallic thickness and morphology and on the resulting shear strength of the ball attachment for a variety of BGA components.
Design/methodology/approach
In this study, a variety of BGA components with balls made of Pb‐free Sn‐Ag‐Cu (SAC) 305, Sn‐Pb eutectic and high‐temperature 90Pb‐10Sn alloys, were subjected to different thermal histories, including up to ten reflow cycles, and aged at 125°C from 24 to 336 h. The intermetallic thickness and morphology after these thermal events were then examined under optical and scanning electronic microscopes. Ball shearing tests were conducted to investigate effects of the thermal history and intermetallic thickness and morphology on shearing strength of these solder balls.
Findings
The results show that effects directly from intermetallic layers may or may not be detectable; and the shear strength of solder balls is largely dependent on the solder alloy and its microstructure. Shear strength increases are observed after multiple reflow cycles and ageing at elevated temperature for the two Pb‐bearing alloys, while the SAC305 lead‐free alloy shows slight reductions in both strength and ductility after thermal exposure.
Practical implications
Presented results can be used for estimation of reliability for electronic assemblies subjected to multiple rework and repair operations, which expose sensitive components, such as BGAs, to elevated temperatures.
Originality/value
It is believed that a sound understanding of the effects of intermetallic morphology and thickness on reliability of BGA solder balls can lead to more intelligent choice of soldering processes, as well as to rework/repair process optimisation and to establishing their operational limits.
A novel method of mechanical reliability analysis on vibration fatigue failure of muBGA solder joints, based on the heating factor Qeta, is introduced. Firstly, a two-parameter weibull distribution is used to model the collected data of vibration fatigue lifetime for different Qeta. After that, two explicit functions are deduced in a unified mathematic expression form, which give an intuitionistic description of the MTTF and reliability of solder joints against induced variable Qeta, thus revealing definitely the effect of Qeta on the mechanical fatigue lifetime of solder joints suffering from cyclic vibration loading. Numerical analysis and calculation are performed. The results show that the solder joints reflowed at Qeta near 500 have higher reliable, and those reflowed farther away this optimal process parameter have less reliability.
Soldering in electronics is in many respects different from soldering in other branches of industry. Although the physical principles of all soldering (and brazing) processes are the same, the features specific to their use in electronics are so numerous that it is possible to speak of soldering in electronics as a subject in its own right. This book deals with the technology of soldering in electronics, attempting to give not only a basic knowledge and comprehension of soldering phenomena, but also to deal with the systematic application of this knowledge to the practical manufacturing of electronic products. Considered are wetting of surfaces, thermal aspects, solder alloys, fluxes, solderable materials soldering joints, reflow soldering and printed boards.
This paper reports the use of X-ray radiography and computer image processing for the study of porosities in surface mount solder joints fabricated from different types of no-clean RMA and RA solder pastes. In order to understand the mechanism of pore formation in the solder joints, experiments are conducted on the processes of the pore formation during reflow soldering. The effects of different infrared (IR) reflow temperature profiles are also studied. From this work, it was found that the composition and structure of solder pastes had a significant effect on the pore formation, and the lower metal content and/or higher heating rate did not necessarily cause a greater percentage of pore formation in the solder joints. X-ray radiography, in conjunction with Optimas image processing, is shown to be very useful in quantitatively testing pores in surface mount solder joints and allows real-time radiographic image processing.
The growth rates for stable Ni-Sn intermetallic compounds (IMC) are much lower than those for Cu-Sn IMC. Therefore, Ni appears to be a good choice for a diffusion barrier between Cu and Sn. However, growth of a metastable plate-like IMC is a potential cause for long-term solderability degradation. This IMC has an approximate composition NiSn3, which does not correspond to any of the stable Ni-Sn IMC's on the equilibrium phase diagram. A long-term, low-temperature aging study confirmed the undesirable effects of NiSn3 growth upon solderability. Consequently, the growth rates for NiSn3 were studied as a function of aging temperature, lead content, and plating type, and were found to be affected by all of these variables. Lead was determined to reduce the NiSn3 growth in matte Sn-Pb. The growth rate reaches a maximum between 100° and 140°C (212° and 284°F) and then decreases.
This study characterizes the interfacial reactions that occur when Cu is soldered with 95 Pb-5Sn solder. A continuous layer
of Cu3Sn ε phase forms during the soldering process. Previous studies suggest that the intermetallic layer spalls off during soldering.
However, the present work shows that the intermetallic layer is intact after soldering and that any spalling observed is due
to improper polishing. A new polishing technique was developed to preserve the intermetallic layer. The Cu3Sn has a fine columnar grain structure that is very brittle. Both intergranular and transgranular fracture modes are observed.
The size of the intermetallic layer is dependent upon the length of time the solder is molten. The rate of formation of e
phase was measured and used to determine an activation energy for diffusion of Sn in 95Pb-5Sn of 13 kcal/mol.
Interdiffusion and intermetallic compound formation in Cu-Sn thin film couples have been investigated by X-rays using a Seemann-Bohlin diffractometer. The films were prepared by consecutive evaporation at room temperature on fused quartz substrates and subsequently annealed between - 2 and 100°C. The η′-phase (Cu6Sn5), which is ordered, was found to grow at all temperatures. In contrast the ordered ε-phase (Cu3Sn) was found only in those specimens that had been annealed above 60°C. The formation of the η′-phase is interpreted in terms of interstitial diffusion of Cu into Sn at low temperatures. In specimens maintained at room temperature the unreactecl Sn and Cu layers were inferred to be in compression and tension, respectively. Tin whiskers were observed to grow spontaneously at room temperature from the Sn surface of the Cu-Sn bimetallic films, but not from Sn films without the Cu underlaver. The driving force for whisker growth is attributed to interdiffusion and reaction that occur in the bimetallic films.
A multiphase diffusion model was constructed and used to analyze the growth of the ε- and η-phase intermetallic layers at a plane Cu-Sn interface in a semi-infinite diffusion couple. Experimental measurements
of intermetallic layer growth were used to compute the interdiffusivities in theε andη phases and the positions of the interfaces as a function of time. The results suggest that interdiffusion in the ε phase(≈D
ε) is well fit by an Arrhenius expression with D0 = 5.48 × 10−9 m2/s andQ = 61.9 kJ/mole, while that in the η phase (≈Dη) has D0
= 1.84 × 10−9 m2/s andQ = 53.9 kJ/mole. These values are in reasonable numerical agreement with previous results. The higher interdiffusivity in theη phase has the consequence that theη phase predominates in the intermetallic bilayer. However, the lower activation energy for interdiffusion in theη phase has the result that theε phase fills an increasing fraction of the intermetallic layer at higher temperature: at 20 °C, the predicted ε-phase thickness
is ≈10 pct of that ofη, while at 200 °C, its thickness is 66 pct of that ofη. In the absence of a strong Kirkendall effect, the original Cu-Sn interface is located within theη-phase layer after diffusion. It lies near the midpoint of theη-phase layer at higher temperature (220 °C) and, hence, appears to shift toward the Sn side of the couple. The results are
compared to experimental observations on intermetallic growth at solder-Cu interfaces.
Hollow, Cu6Sn5 intermetallic rods form within molten 60Sn-40Pb solder when it reacts with Cu. It is suggested that these rods form at the
Cu surface by a screw dislocation mechanism and break off into the bulk solder. Hollow hexagonal intermetallics result when
the core of the rod dissolves away and fills with molten solder. The mechanical properties of bulk 60Sn-40Pb solder with and
without the Cu6Sn5 intermetallic rods were tested in tension, at -196° C, 20° C, and 125° C. The intermetallics had no effect on strength, but
decreased elongation at the lower temperatures. The intermetallics had a large effect on the fracture characteristics. At
-196° C failures initiate by interfacial separation between the intermetallic and solder matrix. At 20° C failures initiate
at cleaved intermetallic rods. At 125° C the intermetallic rods appear to have little effect on the mechanical properties.
This article reports a comparative study of the formation and growth of intermetallic phases at the interface of Cu wetted
with a thick solder joint or a thin, pretinned solder layer. The η phase (Cu6Sn5) forms when Cu is wet with eutectic solder at temperatures below 400 °C. The intermetallic layer is essentially unaffected
by aging at 70 °C for as long as 13 weeks. On aging a eutectic joint at 170 °C, the η-phase intermetallic layer thickens and
ε phase (Cu3Sn) nucleates at the Cu/intermetallic interface and grows to a thickness comparable to that of the η phase, while a Pb-rich
boundary layer forms in the solder. The aging behavior of a thin, pretinned eutectic layer is qualitatively different. At
170 °C, the Sn in the eutectic is rapidly consumed to form η-phase intermetallic, which converts to ε phase. The residual
Pb withdraws into isolated islands, and the solderability of the surface deteriorates. When the pretinned layer is Pb-rich
(95Pb-5Sn), the Sn in the layer is also rapidly converted into η phase, in the form of dendrites penetrating from the intermetallic
at the Cu interface and discrete precipitates in the bulk. How ever, the development of the intermetallic largely ceases when
the Sn is consumed; ε phase does not form, and the residual Pb remains as an essentially continuous layer, preserving the
solderability of the sample. These observations are interpreted in light of the Cu-Sn and Pb-Sn phase diagrams, the temperature
of initial wetting, and the relative diffusivities of Cu and Sn in the solder and intermetallic phases.
Solders of nominal 95Pb-5Sn and 60Sn-40Pb were used to join Cu plates. The effect of ternary additions of In, Ag, Sb, and
Bi to the near-eutectic solder were also investigated. Bulk solder and interfacial joint microstructures were characterized
for each solder alloy. The solder joints were strained to failure in tension; joint strength and failure mode were determined.
95Pb-5Sn/Cu and 60Sn-40Pb/Cu specimens were tested both as-processed and after reflow. 95Pb-5Sn/Cu as-processed and reflow
specimens failed in tension in a ductile mode. Voids initiated at β-Sn precipitates in the as-processed specimens and at the
Cu3Sn intermetallic in the reflow specimens. 60Sn-40Pb/Cu failed transgranularly through the Cu6Sn5 intermetallic in both the as-processed and reflow conditions. The joint tensile strength of the reflow specimens was approximately
half that of the as-processed specimens for both the high-Pb and near-eutectic alloys. The Cu6Sn{5} intermetallic dominated the tensile failure mode of the near-eutectic solder/Cu joints. The fracture path of the near-eutectic
alloys with ternary additions depended on the presence of Cu6Sn5 rods in the solder within the Cu plates. Specimens with ternary additions of In and Ag contained only interfacial intermetallics
and exhibited interfacial failure at the Cu6Sn5. Joints manufactured with ternary additions of Sb and Bi contained rods of Cu6Sn5 within the solder. Tensile failure of the Sb and Bi specimens occurred through the solder at the Cu6Sn5 rods.
Kinetics of formation of Cu6Su5 between Cu and Sn thin films at room temperature and of Cu3Sn between Cu6Sn5 and Cu at temperatures from 115° to 150°'C were measured by Rutherford backscattering spectroscopy and glancing-incidence X-ray diffraction. The growth of Cu6Sn5; showed a linear rate and the reduction of Cu6Sn5 due to the growth of Cu3Sn showed a parabolic rate with an activation energy of 0.99 eV. A flash of W film was deposited between Cu and Sn as diffusion markers during their room temperature reaction. Marker displacement measured by the backscattering technique showed that Cu is the dominant diffusing species in forming Cu6,Sn5. By annealing a Cu3Sn/Cu sample at 640°C for 20 min and by cooling (not a rapid quenching) it to room temperature, we found that the high temperature phase Cu4Sn can be made metastable at room temperature.
analysis of low-temperature [17] H. H. Manko, Solders and Soldering: Materials, Design, Production, and Analysis for Reliable Bonding
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Mei. A. J. Sunwoo, and J. W. Morris, Jr., " analysis of low-temperature
[17] H. H. Manko, Solders and Soldering: Materials, Design, Production,
and Analysis for Reliable Bonding. New York: McGraw-Hill, 1964,
ch. 4, pp. 135-139.
The morphology of Cu&ndash,Sn intermetallics in Cu/Pb&ndash,Sn solder joints
- L F Felton
- K Rajan
- P J Ficalora
- P Singh
Handbook of Fine Pitch Suvface Mount Technology
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J. H. Lau, Handbook of Fine Pitch Suvface Mount Technology. New
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Isothermal aging characteristics of external lead solder connections
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- R C Reinert
SI T. F. Marinis and R. C. Reinert, "Isothermal aging characteristics of
external lead solder connections," in Proc. 35th ECTC, 1985, pp. 73-80.
Mechanical behavior of solder joint interfacial intermetallics
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