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Microstructure, thermo-physical and mechanical properties of spray-deposited Si–30Al alloy for electronic packaging application
Abstract
In this study, Si–30Al alloy was synthesized by the spray atomization and deposition technique. The microstructure and properties of the alloy were studied using optical microscopy, scanning electron microscopy, coefficient of thermal expansion (CTE) and thermal conductivity (TC) measurements, and 3-point bending tests. It was found that the microstructure of the alloy after hot pressing is composed of a continuous network of globular primary Si and interpenetrating secondary Al-rich phase. The property measurements results indicate that the spray-deposited 70Si30Al alloy has advantageous physical and mechanical characteristics, including low coefficient of thermal expansion (6.8 × 10− 6/K), high thermal conductivity (118 W/mK), low density (2.42 g cm− 3), high ultimate flexural strength (180 MPa) and Brinell hardness (261).
... Scientists and engineers have toiled to fabricate metal matrix composites (MMCs) to match such a CTE range via adding reinforcement particles of low CTE into metal matrix [4][5][6]. For instance, 27%-50% (weight fraction) of Si were added into Al matrix to fabricate Al-Si MMCs to meet the CTE range of 17 9 10 -6 -11 9 10 -6°C−1 [7][8][9][10][11][12][13]. Unfortunately, these CTE-matched MMCs are hard to deform into desirable components since they contain high fraction of brittle reinforcement particles [9][10][11][12]. ...
... Scientists and engineers have toiled to fabricate metal matrix composites (MMCs) to match such a CTE range via adding reinforcement particles of low CTE into metal matrix [4][5][6]. For instance, 27%-50% (weight fraction) of Si were added into Al matrix to fabricate Al-Si MMCs to meet the CTE range of 17 9 10 -6 -11 9 10 -6°C−1 [7][8][9][10][11][12][13]. Unfortunately, these CTE-matched MMCs are hard to deform into desirable components since they contain high fraction of brittle reinforcement particles [9][10][11][12]. Thus, we are eager that a packaging material has both desirable CTE and good ductility. ...
... Hardness measurements were carried out on Zwick Roell Hardness Tester with a Vickers diamond pyramid indenter, by using a load of 500 g and dwell Si contents of 27%-50%, which meets the CTE requirements of microwave packaging, backing plates, and opto-electronic housings [7][8][9][10][11][12][13]. Figures 2, 3, and 4 illustrate the concentration segregations and phase decompositions of the 64-day annealed HEAs. ...
Application of an electronic packaging material is always limited by its established coefficient of thermal expansivity (CTE) and poor plastic working capacity. Here, we explored that a ductile high-entropy alloy (HEA) could achieve adjustable CTE values via decomposing at 500 °C. The carbon-doped equiatomic FeCoCrNiMn HEAs (0.8, 1.0, and 1.3 at% C) decomposed into B2, L10, M23C6 and σ phases during annealing. The adjustable CTE of the carbon-doped HEAs can be ascribed to the relatively low CTE values of the formed B2, L10, M23C6, and σ phases. The CTE value of a single face-centered cubic (FCC)-structured FeCoCrNiMn-1.3 at% C HEA is 16.7 × 10–6 °C⁻¹, yet it can be continuously adjusted to 11.3 × 10–6 °C⁻¹ when the FCC matrix is gradually decomposed into the B2, L10, M23C6, and σ phases. Furthermore, the decomposition rate and fractions of the B2, L10, M23C6, and σ phases can be controlled via changing carbon concentration and rolling reduction. More importantly, the CTE range of the 1.3C HEAs meets the requirements of electronic packaging. This work provides a way to design HEAs with desirable CTE for electronic industry via annealing at intermediate temperature.
... However, during the conventional casting process of such Al-high Si alloys, a coarse and highly brittle primary Si phase is formed, leading to crack formation in the casted parts and reducing their performance [3,4]. The Fabrication via rapid solidification technologies, like spray deposition or cold/hot pressing, can prevent the extensive coarsening of this Si phase and enable the production of alloys up to 90 wt.% silicon [5][6][7]. However, these production methods are limited to rather ...
... This could be attributed to the higher contiguity of the primary Si phases, as shown in Figure 4d. A comparison to a published hardness value of a spray deposited AlSi70 alloy, which was significantly lower at 270 HV5 [7], underlines the attractiveness of the additive manufacturing approach for the production of these alloys. ...
Hypereutectic Al-high Si alloys are of immense interest for applications in the automotive, space or electronic industries, due their low weight, low thermal expansion, and excellent mechanical and tribological properties. Additionally, their production by laser powder bed fusion (LPBF) technology provides high flexibility in geometrical design and alloy composition. Since, most of the alloy properties could be improved by increasing the Si content, there is much interest in discovering the maximum that could be realized in LBPF Al-high Si alloys, without the appearance of any material failure. For this reason, in this work the production of Al-high Si alloys with extremely high silicon content of up to 70 wt.% was fundamentally investigated with respect to microstructure and mechanical properties. Highly dense (99.3%) and crack-free AlSi50 samples (5 × 5 × 5 mm3), with excellent hardness (225 HV5) and compressive strength (742 MPa), were successfully produced. Further, for the first time, AlSi70 LBPF samples of high density (98.8%) without cracks were demonstrated, using moderate scanning velocities. Simultaneously, the hardness and the compressive strength in the AlSi70 alloys were significantly improved to 350 HV5 and 935 MPa, as a result of the formation of a continuous Si network in the microstructure of the alloy. With respect to the powder source, it was found that the application of powder blends resulted in similar alloy properties as if pre-alloyed powders were used, enabling higher flexibility in prospective application-oriented alloy development.
... The presence of large Si phases with sharp corners leads to not only low strength but also poor machinability. Therefore, various methods such as spray deposition [15], pressure infiltration [16], selective laser melting [17], and rapid solidification/powder metallurgy (RS/ PM) [18] have been applied for the refinement of Si phase in Al-Si alloys. However, their application in high power density electronic devices is limited due to the higher CTE of Al-Si alloys compared with that of Al-SiC p composites. ...
... In the Al-Si part, a continuous network of globular Si phases and a secondary Al-rich phase are exhibited. The presence of the Si phase network instead of Al-Si eutectics is attributed to the high cooling rate during rapid solidification [10,15]. Between these continuous Si phases, the Al matrix remains mostly connected, contributing to the high thermal conductivity of the composite. ...
A bilayer composite consisting of Al-50 wt% Si alloy and Al-70 vol% SiCp for electronic packaging was prepared by hot pressing. The results show that no obvious defects are observed, and strong interlayer bonding is obtained. With the Al–SiCp layer and the Al–Si layer as the load-bearing surface, the bending strength of the bilayer composite reaches 459 MPa and 241 MPa, respectively. The sample with the Al–SiCp layer as the load-bearing surface is subjected to the lowest tensile stress, while the sample with the Al–Si layer as the load-bearing surface is subjected to the highest tensile stress. The thermal conductivity and coefficient of thermal expansion (CTE) of the bilayer samples with different thickness ratios are between the values of the Al–SiCp composite and the Al–Si alloy. As the thickness percentage of the Al–SiCp layer increases, the thermal conductivity of the bilayer composite increases, while the CTE decreases gradually. Simple equations can be used to estimate the thermal conductivity and CTE of the bilayer composite.
... Furthermore, the sharp ners of coarse lath-shaped silicon particles cause more complex stresses, resulting i discrepancy between the tested CTE and the predicted CTE. Due to the rapid solidific during the spray forming process, the silicon particles with fine microstructures a uniform size distributed in the matrix could be obtained [31][32][33]. During the densific process, the present bimetallic gradient alloy samples were reheated and held for a pe and some of the silicon phase grew into a slightly irregular shape, as shown in Figur As a result of such thermal effects, the solid solution containing supersaturated si underwent a series of changes; for example, silicon atoms diffused and precipitated The main reason for this difference is that the influencing factors considered in the two cases are different. ...
... Furthermore, the sharp corners of coarse lath-shaped silicon particles cause more complex stresses, resulting in the discrepancy between the tested CTE and the predicted CTE. Due to the rapid solidification during the spray forming process, the silicon particles with fine microstructures and a uniform size distributed in the matrix could be obtained [31][32][33]. During the densification process, the present bimetallic gradient alloy samples were reheated and held for a period, and some of the silicon phase grew into a slightly irregular shape, as shown in Figure 5d. ...
Bimetallic gradient alloys have attracted research attention recently due to their potential applications in the aerospace and automobile industries. In this study, Al-20Si/7075 bimetallic gradient alloys were successfully manufactured by co-spray forming and the roll process. We investigated the thermal expansion behavior of the gradient alloy. It was found that the coefficients of thermal expansion increased with silicon content and increased temperature, reaching the highest point at 573 K, after which they decreased on account of the relaxation of residual thermal stress and the silicon desolvation from the supersaturated aluminum phase. The measured thermal expansion coefficient can be roughly predicted through the traditional theoretical models. Our results revealed the thermal expansion behavior of Al-20Si/7075 bimetallic gradient alloys and would improve the development of new type aluminum–silicon alloy for electronic packaging.
... It is well known that the size, morphology and distribution of primary silicon in hypereutectic Al-Si alloy play an important role in determining their properties. For example, the main limitation for the extensive use of conventional ingot hypereutectic Al-Si alloys is the presence of coarse, irregular, and brittle primary Si that easily cracks, deteriorating their performance [3]. Therefore, the refinement of the primary silicon phase is essential in order to optimize the properties of these alloys and to widen their fields of application. ...
... The microstructure of the spray-deposited alloy consists of primary Si particles with an average size of 12.5 ± 0.1 lm and the Al matrix; no Al-Si eutectic is found in the alloy, as shown in Fig. 1(c) and (d). During the cooling step of the atomization process, a large number of fine semi-solid primary Si particles emerge in the droplets before depositing on the substrate [3]. The droplets impinge the substrate with a high speed and, as a result, they are remelted due to the latent heat of crystallization of the surrounding areas. ...
The evolution of microstructure and coefficient of thermal expansion (CTE) of the Al-50Si (wt.%) alloy manufactured by spray deposition followed by hot isostatic pressing (HIP) are systematically investigated. The results indicate that the microstructure of the deposited alloy is composed of primary Si with average size of 12.5 +/- 0.1 mu m and alpha-Al. The CTE of the deposited alloy is higher than the corresponding alloy produced by casting due to the high solid solubility of Al in Si. After HIP, the CTE is lower than the parent as-deposited alloy owing to the high solid solubility of Si in Al. The residual thermal stress results in a higher CTE during the second heating as a result of the CTE mismatch between the Al matrix and the primary Si particles. Furthermore, the measured CTE value is in good agreement with the Turner model after complete densification by HIP at 843 K.
... Aluminum has been widely applied as a matrix for composite materials due to its large reserves, good economy, low density (2.70 g cm −3 ), fine processability, and reasonable thermal conductivity (217 W m −1 K −1 ) [8][9][10]. For advanced electronic packaging material, the TC of aluminum needs to be further improved. ...
A novel DC-assisted fast hot-pressing (FHP) powder sintering technique was utilized to prepare Al/Diamond composites. Three series of orthogonal experiments were designed and conducted to explore the effects of sintering temperature, sintering pressure, and holding time on the thermal conductivity (TC) and sintering mechanism of an Al-50Diamond composite. Improper sintering temperatures dramatically degraded the TC, as relatively low temperatures (≤520 °C) led to the retention of a large number of pores, while higher temperatures (≥600 °C) caused unavoidable debonding cracks. Excessive pressure (≥100 MPa) induced lattice distortion and the accumulation of dislocations, whereas a prolonged holding time (≥20 min) would most likely cause the Al phase to aggregate into clusters due to surface tension. The optimal process parameters for the preparation of Al-50diamond composites by the FHP method were 560 °C-80 MPa-10 min, corresponding to a density and TC of 3.09 g cm−3 and 527.8 W m−1 K−1, respectively. Structural defects such as pores, dislocations, debonding cracks, and agglomerations within the composite strongly enhance the interfacial thermal resistance (ITR), thereby deteriorating TC performance. Considering the ITR of the binary solid-phase composite, the Hasselman–Johnson model can more accurately predict the TC of Al-50diamond composites for FHP technology under an optimal process with a 3.4% error rate (509.6 W m−1 K−1 to 527.8 W m−1 K−1). The theoretical thermal conductivity of the binary composites estimated by data modeling (Hasselman–Johnson Model, etc.) matches well with the actual thermal conductivity of the sintered samples using the FHP method.
... With the rapid evolution of modern electronic information technology, electronic systems and equipment are progressing toward large-scale integration, miniaturization, high efficiency, and high reliability [1][2][3][4]. Heat accumulation in electronic components has emerged as a significant challenge, with elevated operating temperatures resulting in shortened lifespans of electronic components and potential functional failures in severe cases [5][6][7]. It is worth noting that as the operating temperature of electronic devices increases, the failure rate of these devices increases exponentially [8]. ...
Spark plasma sintering is a process of rapid, low-temperature, and high-density sintering. Moreover, traditional sintering methods can solve the problems of large grain sizes and low densities. The sintering temperature plays a crucial role in influencing the physical properties of high-silicon–aluminum (Si-Al) composites. This work investigated the impact of temperature on the microstructure, interface, and physical properties of high-Si-Al composites by spark plasma sintering. The results demonstrate that when the powder was processed by ball milling at a sintering temperature of 565 °C, the material exhibited the densest microstructure with minimal pore formation. The average size of the silicon phase is the smallest. The material’s thermal conductivity is 134.6 W/m·K, the thermal expansion coefficient is 8.55 × 10⁻⁶ K⁻¹, the Brinell hardness is 219 HBW, the density is 2.415 g/cm³, and the density reaches 97.75%. An appropriate sintering temperature facilitates particle rearrangement and dissolution–precipitation processes, enhancing the material structure and performance.
... However, traditional packaging materials, including Al, Cu, Cu-W, C-Mo, and Kovar alloys, have difficulty satisfying the requirements of advanced electronic applications [9,10]. For instance, pure Al possesses favorable thermal conductivity as high as 217 W m −1 K −1 , but suffers a unacceptable coefficient of thermal expansion of 23.2 K −1 , which limits the utilization of pure Al in advanced electronic applications [11][12][13]. ...
In this paper, a novel power sintering technique, named fast hot-pressing sintering (FHP), which is able to achieve an ultrahigh heating rate similar to the spark plasma sintering (SPS) technique, but at a much lower cost, was applied to prepare a series of Al/Si composites with different Si volume ratios (12 vol.% to 70 vol.%) to meet the requirements of advanced packaging materials for electronic devices. In contrast to SPS, the FHP oven possesses a safe and budget-friendly current power supply, rather than a complex and expensive pulse power supply, for its heating power. The optimized sintering parameters (temperature, pressure and holding time) of FHP for preparing Al/Si composites were investigated and determined as 470 °C, 300 MPa and 5 min, respectively. In order to characterize the potential of Al/Si composites as packaging materials, thermal conductivities and coefficients of thermal expansion were studied. The thermal conductivity of the Al-40Si composite sintered by the FHP method is higher than that of the conventional SPS method (139 to 107 W m⁻¹ K⁻¹). With the increase in Si, the thermal conductivities and coefficients of thermal expansion on both decreases. Furthermore, the thermal conductivities obey the Agari model, whereas the coefficient of thermal expansion and Si volume ratios obey additivity. The numeric modeling would help develop required packaging materials based on the thermal performances of the substrate materials, like Si or GaAs semiconductor devices.
... The AleSi alloy's thermal expansion rate with an Si content of 50e80%wt. is similar to that of monocrystalline silicon, meaning that the alloy can be used for electronic packaging [3]. The techniques for manufacturing hypereutectic AleSi alloys include: (1) the conventional casting method; (2) rapid solidification e the powder metallurgical technique; and (3) the electrodeposition technique [4]. ...
In this study, a vertical twin-roll cast (TRC) thermal-fluid coupling model was established to determine a suitable roll-casting velocity. A slab of 0.9mm thick roll-cast Al-50 wt.% Si alloy with a two-phase zone width of 473°C was prepared at a velocity of 20m/min for the first time. In addition, the finite element simulation showed that the average heat transfer rate had a positive correlation with the roll-casting velocity. When the roll-casting velocity increased to 20m/min, the average heat transfer rate reached 1260K/s, at which point, the average size of the primary Si was merely 16.5μm. This was a decrease of approximately 90% when compared with the size of alloys cast with conventional methods. After carrying out semi-solid heating, an observation on the distribution of alloy elements identified a novel phenomenon of solute redistribution, i.e., inverse segregation. This inverse segregation process was highly significant in further controlling solute redistribution in the alloy. Additionally, it provided a new concept for improving roll-cast segregation.
... Al-high Si content alloys possess huge potential for broad application in automobile, aerospace and electronic packaging industries because of excellent physical and mechanical properties of light weight, low coefficient of thermal expansion (CTE), good wear and corrosion resistance [1][2][3][4]. Al-high Si content alloys fall in the range of hypereutectic Al-Si alloys, including primary β-Si and (α+β) eutectic phases upon solidification. Obviously, the amount of β-Si phase increases in the obtained structure when the Si content increases, which is favorable to improve the thermal and wear resistance of Al-Si alloys [5][6][7]. ...
The coarsening of primary silicon phases is one of the main challenges in the preparation and application of Al-high Si content alloys. In this work, AlSi40 alloys were manufactured by selective laser melting (SLM). The influence of ceramic nanoparticles on the morphology and size of primary Si phase(s) is investigated. With addition of ceramic nanoparticles, large snow-shaped primary Si change towards a polygonal shape in coarse regions, while blocky primary Si in fine regions shows insignificant change. The size of primary Si in the modified alloy decreases by ~27.1% and ~25.2% in the respective coarse and fine regions compared to that of the unmodified alloy. Due to the refinement and smoothening of primary Si in the modified alloy, the microhardness distribution becomes homogeneous and an average value of ~233 HV0.2 was obtained. The coefficient of friction (COF) value and the wear rate are ~0.44 and ~4.5×10-5 mm3N-1m-1, respectively. An ultimate compressive strength of ~0.76 GPa and a maximum compressive strain of ~7.99% are obtained, increasing by ~21.0% and ~18.0% compared to those of the unmodified alloy. The SLM-fabricated AlSi40 alloys upon heat treatment showed an improved plasticity and slightly decreased strength due to the coarsening of primary Si and grains, decreased solid solution of Si in the matrix as well as accommodation of residual stresses after heat treatment.
... By controlling the silicon content, a series of silicon-aluminum alloy materials called CE alloys (Control Expansion alloys) with a thermal expansion coefficient of (7~17) × l0 −6 /K were formed [16,17]. The current preparation methods mainly include rapid solidification [18], twin roll casting technique [19], electromagnetic directional solidification(EMDS) [20], powder metallurgy [21], spray deposition [22][23][24][25], semi-solid thixoforming [26] and selective laser melting (SLM) [27], etc. However, less research has been carried out on the joining of hypereutectic Al-Si alloys. ...
Hypereutectic Al-Si alloys are attractive materials in the fields of electronic packaging and aerospace. A Bi2O3-ZnO-B2O3 system lead-free brazing filler glass was employed to braze hypereutectic Al-50Si alloys in air. The hypereutectic Al-50Si alloys were pre-oxidized and the low-temperature glass powder was flake-shaped in the brazing process. The effects of brazing temperature and time on joints microstructure evolution, resulting mechanical strength, and air tightness were systematically investigated. The results indicated that the maximum shear strength of the joint was 34.49 MPa and leakage rate was 1.0 × 10−10 Pa m3/s at a temperature of 495 °C for 30 min. Crystalline phases, including Bi24B2O39 and Bi2O3, were generated in the glass joint. The formation of a diffusion transition layer with a thickness of 3 μm, including elements of Al, Si, Zn, Bi, Na, and B, was the key to form an effective joint. The elements of Al, Si, and Bi had a short diffusion distance while the elements of Zn, Na, and B diffused in a long way under brazing condition.
... However, the hyper-eutectic Al-Si alloys [17] have high liquidus and therefore are difficult to be fabricated via casting routes. To overcome the difficulty, many new technologies, such as spray deposition [18][19][20][21], the pressure/pressureless infiltration [9], the semi-solid squeeze [22,23] and powder metallurgy [2,10] were used in the fabrication of Al-Si alloys for electronics packaging usage. Hot extrusion is also regarded as a promising alternative to casting in hyper-eutectic Al-Si alloy processing [24,25]. ...
Al-20Si alloy with high thermal conductivity and adequately low coefficient of thermal expansion is successfully produced by hot extrusion of pre-alloyed powder and used as a model material in investigating the influences of heat treatment on the thermal properties of the material. In the as-extruded samples, fine Si particles are distributed uniformly in the Al matrix. When the alloy is annealed at 250 °C or 400 °C for 2 h, neither the Al nor Si grains grow significantly, but the internal strain developed in extrusion is relieved. Therefore, thermal conductivity is improved to 148.0 W/(m·K) and the coefficient of thermal expansion is reduced to 17.3 × 10⁻⁶/K. With further increasing annealing temperature to 560 °C, the coarsening of Al and Si grains occurs. On one hand, it significantly reduces the grain boundaries and phase boundaries in the material, consequently further increasing the thermal conductivity to 174.8 W/(m·K). On the other hand, the coarsened structures lead to thermal stress during cooling, which offsets the constraining effect of Si on the thermal expansion of Al and results in an increase in the coefficient of thermal expansion.
... The advanced material of Si p /Al MMCs has characteristics such as a low coefficient of thermal expansion, low density, high thermal conductivity, good thermo-mechanical stability and is compatible with microelectronic assembly technology, etc. [11][12][13][14]. Due to the homogeneous distribution of Si particles in composites, components made from Si p /Al MMCs are nearly isotropic and readily formable [15,16]. Moreover, this material can be customized to meet specific requirements such as different coefficients of thermal expansion and thermal conductivity via changes in the Si content in the composites, making them especially attractive for aerospace engineering and the automotive industry, in addition to their electronic packaging applications [17,18]. ...
The vacuum brazing of dissimilar electronic packaging materials has been investigated. In this research, this applies silicon particle-reinforced aluminum matrix composites (Sip/Al MMCs) to Kovar alloys. Active melt-spun ribbons were employed as brazing filler metals under different joining temperatures and times. The results showed that the maximum joint shear strength of 96.62 MPa was achieved when the joint was made using Al-7.5Si-23.0Cu-2.0Ni-1.0Ti as the brazing filler metal at 580 °C for 30 min. X-ray diffraction (XRD) analysis of the joint indicated that the main phases were composed of Al, Si and intermetallics, including CuAl, TiFeSi, TiNiSi and Al3Ti. When the brazing temperature ranged from 570 °C to 590 °C, the leakage rate of joints remained at 10−8 Pa·m3/s or better. When the joint was made using Al-7.5Si-23.0Cu-2.0Ni-2.5Ti as the brazing filler metal at 580 °C for 30 min, the higher level of Ti content in the brazing filler metal resulted in the formation of a flake-like Ti(AlSi)3 intermetallic phase with an average size of 7 µm at the interface between the brazing seam and Sip/Al MMCs. The joint fracture was generally in the form of quasi-cleavage fracture, which primarily occurred at the interface between the filler metal and the Sip/Al MMCs. The micro-crack propagated not only Ti(AlSi)3, but also the Si particles in the substrate.
... According to the different Si contents, the alloy can be classified as hypoeutectic, eutectic and hypereutectic [3]. It is well known that the mechanical performance of Al-Si alloy is mainly influenced by the morphology and distribution of primary Si crystallites [4]. When the contents of Si exceed 20 wt%, the primary Si crystals in the conventionally cast Al-Si alloy show coarse star-like and plate-like habit. ...
The effect of different Cu-P admixtures and cooling rates on microstructure and wear behavior of hypereutectic Al-50Si alloys were studied systematically in this work. The growth patterns and the modified as well as over-modified mechanisms of primary Si crystals under various P contents are also illuminated in detail. It was found that the optimum content of the modifier in Al-50Si alloy is 3 wt%. At this modifier content, the mean size of Si crystallite is the smallest and the distribution is also the most homogeneous. Vickers hardness and wear test were employed to detect the mechanical performance of materials. The maximum value of hardness and the minimum value of average friction coefficient obtained under the modifier content of 3 wt% are 198 HV and 0.407, respectively.
... High-silicon aluminum alloy (high-Si Al alloy) is often used as the electronic packaging materials on the radar on ships because of its low density, suitable thermal expansion coefficient matching with semiconductor materials, good heat dissipating performance, and excellent mechanical property [1][2][3][4][5][6][7][8][9][10]. However, as high-Si Al alloy is often longterm exposed to corrosive atmosphere environment and seawater environment, it is easily corroded which can reduce the life of radar on ships. ...
High-silicon aluminum alloy (high-Si Al alloy) usually used as the electronic packaging materials on the radar on ships have a susceptibility to corrosion, reducing the life of radar on ships. How to improve the corrosion-resistance ability of high-Si Al alloy is still a challenge. In this paper, we introduced superhydrophobicity on high-Si Al alloy surface by electrochemical etching to improve the corrosion-resistance. The variation of surface wettability with the etching time was subsequently studied. A better superhydrophobicity with water contact angle of 163° and roll-off angle of 5° was obtained at 12 min etching time. This superhydrophobicity can effectively prevent the corrosive solution to corrode the high-Si Al alloy surface and increase the service life.
... Aluminum (Al) and its alloys are widely used in traffic, industrial production, electronics, communication, aeronautics, and military fields because of its superior properties such as low density, excellent specific mechanical properties, good resistance to stress corrosion, and high thermal and electrical conductivity [1][2][3][4]. Hypereutectic Al-silicon (Si) alloys and its composites have attracted much attention in electronic packaging applications due to their low density, high wear resistance, low thermal expansion coefficient and excellent thermal conductivity [5,6]. The requirement for packing materials is increasing, as electronic packaging has become an emerging technology featuring smaller size, higher integration, and complex geometries. ...
Powder injection molding is a promising low-cost technique for net shape processing of metal and ceramic components. This study aimed to investigate a new method for preparing aluminium (Al) – silicon (Si) alloy feedstock using the solvent hot mixing process. For this purpose, micron-sized Al-Si (20 wt. %) alloy powder was mixed with a binder consisting of 55 wt. % carnauba wax, 45 wt. % high-density polyethylene, and 3 wt. % stearic acid in a hot xylene bath. The scanning electron microscopy technique, thermogravimetric analysis, density measurement and torque measurements were used to verify the homogeneity of the feedstock. Moreover, the feedstock was chosen to perform the molding, debinding cycle and sintering. An Al-Si (20 wt. %) alloy part was successfully produced using this new method.
... Hypereutectic Al-high Si alloys (Si wt.% ! 50%) possess excellent physical properties that make them attractive for thermal management applications, including relatively high thermal conductivity (TC), low density, and low coefficient of thermal expansion (CTE) [7,8]. The physical properties of Al-high Si alloys are mainly dominated by the size and morphology of primary Si precipitates. ...
... A large amount of work on the modification of the primary Si phase has been performed for Al-Si alloys with high Si content (Si wt% $ 50%) by spray deposition and casting. 4,[16][17][18][19] Hogg et al. 4 reported that the morphology of the primary Si phase in the Si-30Al alloy could be modified from a coarse flake shape to a continuous network shape using spray deposition technology. Our previous work found that rapid quenching through spray deposition could cause the primary Si phase in the Al-50Si alloy to be modified from a coarse platelet morphology to a fine irregularly blocky shape, 18 which further causes a decrease of the CTE in the Al-50Si alloy. ...
The Al–50Si alloy, as a kind of potential electronic packaging material, is manufactured by different methods, such as casting and spray deposition. The possible influences of the P refiner on the microstructure of the Al–50Si alloy are investigated at different cooling rates. The refinement mechanism of primary Si phase is discussed in view of the P refiner addition, and the variation of the cooling rates. The thermal conductivity (TC), as a key parameter for electronic materials, is measured. The coupled effects of the cooling rate and the addition of the P refiner during the solidification of the Al–50Si alloy on the TC are elucidated based on structural observations. Furthermore, the porosity in the Al–50Si alloy is treated as a second phase influencing the TC.
... Nevertheless, the knowledge of its mechanical properties and their variation with temperature is limited. Few studies [16][17] already present the values of Young's modulus and flexural strength but do not consider the temperature effect on mechanical properties. Moreover, the main feature of the mechanical behaviour of this material is its brittleness. ...
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The spray-deposited Si–Al CE9F alloy is a new material used for space applications involving temperature gradient. Thermal stresses arise and may affect the mechanical integrity of the components. It is therefore necessary to assess the critical temperature gradient to avoid failure. Hence this paper deals with the effect of temperature on the mechanical properties of the Si–Al CE9F alloy from − 50 °C to 130 °C: Young's modulus, coefficient of thermal expansion and Weibull's parameters of the material to account for its brittle fracture behaviour through the Weibull's model. The experiments indicate a linear dependence of the Young's modulus with temperature and show that the coefficient of thermal expansion and the Weibull's parameters are almost constant in the temperature range [− 50 °C, 130 °C]. From these results, an example of application schematizing a clamped plate under temperature gradient is studied to create abacus of probability of failure. These abacuses provide sizing guidelines and show the impact of the equivalent volume, highlighting the critical temperature gradients.
... However, the properties of traditional packaging materials can no longer satisfy the practical requirements [2][3][4]. Hypereutectic Al-Si alloys with high Si content (50-70 wt.%) are one of the ideal candidates for electronic packaging application as a result of the positive combination of properties, such as relatively low coefficient of thermal expansion (CTE), which closely matches that of GaAs or Si semiconductor materials, high thermal conductivity, low density and superior strength [5]. However, the main limitation of this type of material is the presence of the coarse, irregular, and brittle primary Si phase that can act as soft spots for premature crack initiation, deteriorating the overall mechanical properties of these materials [6,7]. ...
The effects of superheat temperature, content of modifier (P) and T6 heat treatment on the microstructure and mechanical properties of the Al-50Si alloy have been investigated systematically by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results indicate that the primary Si exhibits a plate-like morphology, with average size decreasing with increasing of the superheat temperature for the unmodified alloy. The morphology of primary Si changes to small blocky shape at an optimal P content of 0.5 wt.%, and the nucleation temperature increases for the alloy with 1.3 wt.% P because of the ease of formation of the AlP phase. The nucleation temperature is lower for 0.5 wt.% P due to lack of P atoms at relatively higher temperature. The ultimate tensile strength was enhanced by the addition of P followed by the T6 heat treatment, and the maximum ultimate tensile strength (∼ 160 MPa) was observed for the sample containing 0.5 wt.% P.
In this study, ZrMgMo3O12/Al-40Si (ZMMO/Al-40Si) composites with 15 wt.% ZrMgMo3O12 were fabricated by ball milling-vacuum hot pressing (VHP) processes. The effect of ball milling processes on the microstructure, compressive properties and coefficient of thermal expansion (CTE) of the composites were investigated. It is found that ball milling treatments of mixed ZMMO/Al-40Si powders refine the particles of ZMMO reinforcements and primary Si in ZMMO/Al-40Si composites and improve the distribution of ZMMO reinforcements in α-Al matrix, resulting in the increase of the compressive strength and the decrease of the CTE of ZMMO/Al-40Si composites. The highest compressive yield strength of 430.82 MPa and the lowest CTEs of 5.8 × 10⁻⁶/°C (RT - 400 °C) are obtained after ball milling for 8 hours at 250 rpm, increasing by 145.9% and decreasing by 61.3% respectively compared with the compressive yield strength and CTE of Al-40Si alloy. Among the reinforcements commonly used in aluminum matrix composites, ZMMO reinforcement has the highest reduction efficiency for CTE of ZMMO/Al-40Si composite. The application of high-energy ball milling to refine the microstructure is a promising method that can simultaneously increase the strength and reduce the CTE of ZMMO/Al-40Si composites.
Al−high Si alloys were designed by the addition of Cu or Mg alloying elements to improve the mechanical properties. It is found that the addition of 1 wt.% Cu or 1 wt.% Mg as strengthening elements significantly improves the tensile strength by 27.2% and 24.5%, respectively. This phenomenon is attributed to the formation of uniformly dispersed fine particles (Al2Cu and Mg2Si secondary phases) in the Al matrix during hot press sintering of the rapidly solidified (gas atomization) powder. The thermal conductivity of the Al−50Si alloys is reduced with the addition of Cu or Mg, by only 7.3% and 6.8%, respectively. Therefore, the strength of the Al−50Si alloys is enhanced while maintaining their excellent thermo-physical properties by adding 1% Cu(Mg).
A brittle reaction interface in molybdenum/Kovar joint acquired by electron beam welding-brazing, which consisted of three zones, was concretely analyzed with respect to microstructure evolution. Zone A was composed of α-Mo and σ (FeMo) intermetallic that was the origin of brittleness. The eutectoid structure in zone B was identified as α-Fe + μ (Fe3Mo2), indicating a decent ability of plastic deformation. The columnar phase of α-Fe extending into eutectoid structure showed a great tenacity considering quantities of gliding dislocations. Phase transformation in the reaction interface during solidification process was concretely analyzed, suggesting an amorphous area near α-Fe due to the blocking effect for eutectoid R (Fe5Mo3) on further eutectoid reaction between liquid metal and σ (FeMo), and, the subsequently excessive cooling rate. Thermodynamic calculation was conducted to explain the existence of μ (Fe3Mo2) with the absence of δ (MoNi) in zone B. The novel stacking faults and their overlapping were found within the body-centered cubic α-Mo due to the decrease in stacking fault energy of Mo with the addition of Fe atoms. Contrast change in diffraction fringes of twinning structure and new stacking fault caused by overlapping was explained.
Al–50Si alloys for electronic packaging were prepared by gas atomization following hot press sintering, and the influences of adding minor Sc (0.3%) on microstructure and mechanical and thermo-physical properties were studied. The Si phase exhibits a semi-continuous network structure with an average size of 15–20 μm in the alloys with and without Sc addition. Transmission electron microscopy observation indicates that a fine spherical Sc-rich particle distributes at the interface between the Al matrix and Si phase in the Al–50Si-Sc alloy, which is further identified as AlSi2Sc2 (V-phase). The tensile strength, flexural strength, and hardness of the Al–50Si alloy are improved by 16.2%, 8.9%, and 14.7%, respectively, with the introduction of Sc. However, the coefficient of thermal expansion and thermal conductivity decreases slightly in the Al–50Si–Sc alloy as compared with the Al–50Si alloy. The increased strength is mainly attributed to the formation of fine spherical AlSi2Sc2 phase which strengthens the Al matrix.
The effect of sub-monolayer silicon on the oxidation of Al(111) and Al(100) surfaces was investigated using X-ray Photoelectron Spectroscopy (XPS) and density functional theory (DFT) calculations. On both surfaces the adatom site is preferred over substituting Si into the Al-lattice; on Al(100) the four fold hollow site is vastly favored whereas on Al(111) bridge and hollow sites are almost equal in energy.
Upon O2 exposure, Si is not oxidized but buried at the metal/oxide interface under the growing aluminum oxide. On Al(111), Si has a catalytic effect on both the initial oxidation by aiding in creating a higher local oxygen coverage in the early stages of oxidation and, in particular, at higher oxide coverages by facilitating lifting Al from the metal into the oxide. The final oxide, as measured from the Al2p intensity, is 25–30% thicker with Si than without. This observation is valid for both 0.1 monolayer (ML) and 0.3 ML Si coverage. On Al(100), on the other hand, at 0.16 ML Si coverage, the initial oxidation is faster than for the bare surface due to Si island edges being active in the oxide growth. At 0.5 ML Si coverage the oxidation is slower, as the islands coalesce and he amount of edges reduces. Upon oxide formation the effect of Si vanishes as it is overgrown by Al2O3, and the oxide thickness is only 6% higher than on bare Al(100), for both Si coverages studied.
Our findings indicate that, in addition to a vanishing oxygen adsorption energy and Mott potential, a detailed picture of atom exchange and transport at the metal/oxide interface has to be taken into account to explain the limiting oxide thickness.
In the present work, the sheets of Al-50Si alloys with different Mg content (0, 5, and 10 wt% Mg) are fabricated by vertical twin-roll casting (VTRC) technology at different roll-casting speeds. The size and morphology of primary Si are studied systematically. The central congregation mechanism of primary Si and the central fracture mechanism as well as the relationship between thickness and roll-casting speed are illuminated in detail. The thickness of the sheet increases with decreasing of roll-casting speed. The central congregation phenomenon of primary Si and the layer phenomenon caused by crack will appear when the roll-casting speed is slow. The size of primary Si and the hardness of alloy are mainly influenced by roll-casting speed and Mg content. The min size of primary Si and the max value of hardness obtained at the speed of 30 m min⁻¹ with 10% Mg content are 7.0 μm and 384, respectively.
Pre-treated Si powder (Sip) and 6061Al powder were used to fabricate high-fraction Sip/6061Al composites via pressureless sintering, and the effects of the Sip content and the sintering temperature on the microstructures and properties of the composites were studied. The results show that in the composites, there exist MgAl2O4 nanocrystalline particles, and the Si phase varied from a discontinuous particulate state to a semi-continuous skeleton state as the Si content increased from 30 to 50 wt-%. Densities, bending strengths, hardness, and thermal conductivities of the composites all increased initially and then decreased with the sintering temperature. The 680°C sintered 30 wt-% Sip/6061Al composites and the 700°C sintered 50 wt-% Sip/6061Al composites have the optimal mechanical and thermophysical properties.
A kind of novel packaging material, Al-27% alloy, was prepared by a method comprised of spray deposition and subsequent hot extrusion, then, the effects of heat treatments on the microstructures and mechanical properties of the alloy were studied. The microstructures and mechanical properties of hot extruded and heat treated Al-27%Si alloy were investigated with optical microscope, scanning electronic microscope and MTS-858 type fatigue testing machine. The experimental results show that the spray-deposited Al-27%Si alloy after hot extrusion has fine, homogeneous microstructure with no needle-like eutectic Si but small primary Si particles uniformly dispersed in the Al matrix, besides an increase of size to some degree, even the following heat treatments make little difference to it. Appropriate hot extrusion process can eliminate the porosities remarkably, so as to improve the relative density of the alloy to 99.5%. The microstructures of Al-27%Si experience further improvement after certain heat treatments, while the transformations of mechanical properties are opposite.
According to the application demand of novel integrated structural-functional materials for electronic packaging in electronic information industry, high-silicon aluminum alloys (50%-70%Si, mass fraction) were fabricated by a combination method of spray forming and hot compression. The microstructure, thermal-physical and mechanical properties were studied by optical microscopy, scanning electronic microscopy, hardness meter and Formastor-Digital equipment. The results show that the as-deposited microstructure of the spray-formed high-silicon aluminum alloy is fine and dispersive, and the primary silicon phases which are irregular distribute homogenously in the aluminum matrix. The appropriate hot compression process of the as-deposited preform can eliminate basically the porosities generated by spary-forming process to improve the relative density of the alloy. The spray-formed silicon aluminum alloys with hot compression process exhibit excellent thermal-physical properties and good mechanical performance, which is a new family of integrated structural-functional materials for electronic packaging.
High-energy ball milling and powder metallurgy process were applied to Al-Si powder to obtain Al-50Si composite. The microstructure and phases of the powder and alloy were investigated. And the density, hardness and TDC (thermal diffusivity coefficient) of the alloy were tested. The results show that Si particles were refined during high-energy ball milling process. The diameter of Si particles in the ball milled powder was between 1-15 μm. The diameter of Si particles in the consolidated alloy increased to 5-30 μm. The TDC of the alloy was 55 mm2·s-1 at room temperature.
A novel reactive technique has been employed to fabricate 2.0 mass%TiB2/70Si-Al composite. The growth behavior of primary Si phase in the 2.0 mass%TiB2/70Si-Al composite was investigated when the composite was heated in its semi-solid state. Both theoretical and experimental results have shown that the in-situ TiB2 particles can refine effectively the primary Si phase and restrain the Si phase growth. The coarsening rate constant of the 2.0 mass%TiB2/70Si-Al composite was calculated as 3.69 x 10(-18) m(3)/s, which is less than that of 70Si-Al alloy (7.6 x 10(-17) m(3)/s), suggesting that the in-situ TiB2 particles can effectively pin the grain boundaries and arrest the migration of liquid film when the composite was heated in its semi-solid state. [doi:10.2320/matertrans.M2011066]
The microstructures and thermal stabilities of high silicon aluminum alloys prepared by spray deposition (SD) and subsequent heat treatment were studied. The results indicate that the size of the primary silicon in the deposited ingots is much smaller than that prepared by normal casting. After the subsequent heat treatment, the pores in deposited ingots are disappeared and their density increase to about 100%. While the size and volume fraction of the primary silicon in solidified hypereutectic Al-Si alloy increase with the increasing of silicon content, the corresponding thermal expansion coefficient decreases.
50vol. %Si/Al composite was prepared by the separation of liquid and solid in semi-solid. The microstructures of composite were obtained using OM, SEM and EMPA. The primary Si particles distribute uniformly on the Al matrix which surrounds the Si particles and makes-up a continuous network. The thermal expansion coefficient and thermal conductivity of composites experienced different thermal process were examined. It shows that the thermal process history has a significant effect on the microstructure and properties. The residual stress and size of Si particles varied during thermal processing which were responsible for the thermal expansion coefficient alternation. The thermal process of high temperature insostatic pressing reduces the porosity in composite and improves thermal conductivity obviously
Microstructure evolution of high energy milled Al-50wt%Si alloy during heat treatment at different temperature was studied. Scanning electron microscope (SEM) and X-ray diffraction (XRD) results show that the size of the alloy powders decreased with increasing milling time. The observable coarsening of Si particles was not seen below 730°C in the high energy milled alloy, whereas, for the alloy prepared by mixed Al and Si powders, the grain growth occurred at 660°C. The activation energy for the grain growth of Si particles in the high energy milled alloy was determined as about 244 kJ/mol by the differential scanning calorimetry (DSC) data analysis. The size of Si particles in the hot pressed Al-50wt%Si alloy prepared by high energy milled powders was 5-30 m at 700°C, which was significantly reduced compared to that of the original Si powders. Thermal diffusivity of the hot pressed Al-50wt%Si alloy was 55 mm2/s at room temperature which was obtained by laser method.
2wt.%TiB2/Si-30Al composite was prepared by in-situ reaction and spray forming first and then by hot isostatic pressing (HIP). The microstructure and thermo-physical properties of the composite were investigated by means of scanning electron microscopy, and thermal expansion analyzer respectively. The results show that the microstructure of the TiB2/Si-30Al composite consists of a continuous network of primary Si (~35μm), interpenetrating secondary Al phase, and fine TiB2 particles (1~2μm). The TiB2 particles were uniformly distributed in the Si-30Al alloy matrix. After HIP, the pores in the TiB2/Si-30Al composite were almost eliminated, and the relative density of the composite was up to 98.9%. The 2wt.%TiB2/Si-30Al composite after the HIP exhibits good thermo-physical properties, including lower coefficient of thermal expansion (CTE) (6.6´10⁻⁶×K⁻¹) and higher thermal conductivity (84 W×m⁻¹×K⁻¹).
In this paper, nanoindentation tests with CSM technique were conducted on Sn–3.0Ag–0.5Cu(SAC) lead-free solder under different strain rates at room temperature. Results show that the nanomechanical properties of SAC solder depend on the strain rate remarkably. The contact stiffness increases almost linearly with indentation depth. Under different strain rates, contact stiffness and elastic modulus basically remain unchanged, but the hardness increases with increasing strain rate. For the same holding time, creep deformation is more evident under high strain rate. The “bulge” phenomenon for SAC solder weakens as unloading rate increases, residual indentation depth after unloading is larger under high strain rate.
In the present work, 50 vol% Sip/Al–20Si composite was prepared by hot-pressed sintering technology. Si particles were uniformly distributed in the Sip/Al–20Si composite, and only the presence of Si and Al phases were detected by XRD analysis. Dislocations, twins, and stacking faults were found in the Si particles. Several Si phases were found to be precipitated between Al matrix and Si particles. Si/Al interface was clean, smooth, and free from interfacial product. HRTEM indicated that the Si/Al interface was well bonded. The average CTE and thermal conductivity (TC) of Sip/Al–20Si composite were 11.7 × 10−6/°C and 118 W/(m K), respectively. Sip/Al–20Si composite also demonstrated high mechanical properties (bending strength of 386 MPa). Thus, the comprehensive performance (low density and CTE, high TC, and mechanical properties) makes the Sip/Al–20Si composite very attractive for application in electron packaging.
The rapid solidified process and hot press method were performed to produce three hypereutectic 55%Si–Al, 70%Si–Al and 90%Si–Al alloys for heat dissipation materials. The results show that the atomization is an effective rapid solidified method to produce the Si–Al alloy and the size of atomized Si–Al alloy powder is less than 50 μm. The rapid solidified Si–Al alloy powder were hot pressed at 550 °C with the pressure of 700 MPa to obtain the relative densities of 99.4%, 99.2% and 94.4% for 55%Si–Al, 70%Si–Al and 90%Si–Al alloys, respectively. The typical physical properties, such as the thermal conductivity, coefficient of thermal expansion (CTE) and electrical conductivity of rapid solidified Si–Al alloys are acceptable as a heat dissipation material for many semiconductor devices. The 55%Si–Al alloy changes greatly (CTE) with the increase of temperature but obtains a good thermal conductivity. The CTE of 90%Si–Al alloy matches with the silicon very well but its thermal conductivity value is less than 100 W/(m·K). Therefore, the 70%Si–Al alloy possesses the best comprehensive properties of CTE and thermal conductivity for using as the heat sink materials.
A novel reactive technique has been employed in fabrication of 2.0wt.%TiB2/70Si–Al composite for electronic packaging applications. The microstructure and properties of composite were studied using scanning electron microscopy, energy dispersive X-ray spectrometer, coefficient of thermal expansion and thermal conductivity measurements, and 3-point bending tests. The results indicate that the in situ TiB2 particles can effectively refine the primary Si phase. The property measurements results indicate that the 2.0wt.%TiB2/70Si–Al composite has advantageous physical and mechanical properties, including low density, low coefficient of thermal expansion, high thermal conductivity, high Flexural strength and Brinell hardness.
An in situ reaction and spray forming technique were employed in the synthesis of 2% TiB2/Si-30Al composite. The formation mechanism of TiB2 particulates was explained based on thermodynamic theory. The modification of the primary Si in the Si-30Al alloy was interpreted in the light of the knowledge of atomic diffusion. The experimental results show that adding 2% TiB2 to the Si-30Al alloy can effectively refine the primary Si. Moreover, the coarsening and growth of primary Si phase in its semi-solid state was retarded effectively due to the existence of the TiB2 particulates.
A family of lightweight, controlled coefficient of expansion silicon-aluminum alloys has been developed for electronic packaging applications. These alloys are produced by the Osprey spray deposition process, which achieves homogeneous microstructures and isotropic properties. The properties of high-silicon aluminum-silicon alloys required for electronic packaging are discussed, along with the spray-forming process.
Thermal management materials are needed to dissipate heat associated with high performance microelectronic circuits. Challenges arise both with the material selection and the component fabrication. Powder metallurgy techniques provide a means of fabricating high quality composite systems with tailored thermal properties. Mathematical analysis has identical several candidate P/M materials, many already familiar to the P/M community, including W-Cu an SiC-Al. In the near term, W-Cu is a viable heat sink and packaging material while SiC-Al will be primary composite for avionic applications. However, major opportunities are apparent in systems such as AlN-Cu and SiC-Cu, but these will require novel fabrication routes. This presentation outlines the design of thermal management materials, identifies the possible properly combinations, and isolates processing options for some target applications.
High Si content in Al-Si alloys usually leads to the formation of coarse, brittle Si phase under slow solidification conditions. In the present study, an Al-17Si-4.5Cu-0.6Mg (referred to hereafter as AS17) was synthesized using spray deposition to modify the Si phase. In the spray deposition process, the master alloy of AS17 was atomized using N2 gas, and was deposited on a collecting substrate directly into a three-dimensional material. The microstructure and mechanical behavior of the spray-deposited AS17 were studied using optical microscopy (OM) scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction, and tensile tests. The present results indicate that in the spray-deposited AS17, the eutectic Si phase was modified from a ``flakelike'' morphology, characteristic of ingot metallurgy (IM) materials, into a ``particulate'' morphology. The formation of the coarse primary Si blocks was suppressed. Moreover, the size and morphology of Si particulates were found to have significant influences on the deformation behavior. During plastic deformation, extensive fracture of Si occurred. The percentage of fractured Si increased with the increasing amount of plastic deformation and the size of Si particulates. Finally, the room-temperature mechanical properties of the spray-deposited AS17 were compared with its IM counterpart A390 (an IM alloy with identical composition as AS17). The strength and ductility of AS17 were improved over those of A390. In the T6 condition, the yield strength and tensile elongation of AS17 were 503 MPa and 3.0 pct, respectively, whereas those of A390 were 374 MPa and 1.3 pet, respectively.
This paper presents a study of the microstructure of a spray formed Si–30 wt.%Al alloy used in electronic packaging applications. The microstructure consisted of ∼5 μm equiaxed primary Si grains and a coarse grained Al-rich phase with occasional regions of ∼10 μm equiaxed Al-rich grains interpenetrating the Si network with no evidence of a lamellar Al–Si eutectic. This unusual microstructure arose because of the particular solidification conditions during and immediately after the spray forming process and the large alloy freezing range.
A variety of new advanced composite materials are now available that provide great advantages over conventional materials
for electronic packaging thermal control, including extremely high thermal conductivities (more than twice that of copper);
low tailorable coefficients of thermal expansion; weight savings up to 80 percent; extremely high strength and stiffness;
low-cost, net-shape fabrication processes; and cost reductions as high as 65 percent. In addition, composites are in a state
of continual development that will provide even greater benefits. This article provides an overview of advanced composites
used in thermal management, including properties, applications, and future trends. The focus is on materials having thermal
conductivities at least as high as those of aluminum alloys. Future trends and the potential for composites in other aspects
of the electronics industry, such as high-speed assembly machine materials of construction, are also examined.
The application of the Osprey process to the fabrication of newly developed Al-20Si-X alloys is at present a subject of considerable interest. This paper reports the results of a study on the as-spray deposited structural characteristics of an Al-20Si-5Fe alloy preform and their development during subsequent hot extrusion and high-temperature exposure, by means of X-ray diffraction, differential scanning calorimetry and electron microscopy. It is shown that in the as-spray-deposited preform of the alloy an unusual increase in the lattice parameter of the aluminium matrix was detected, which persisted throughout the processing. No evidence of supersaturation in the preform aluminium matrix could be found, which is considered to be associated with the characteristics of solidification and subsequent decomposition of the hypereutectic alloy during spray deposition, this allows the extensive formation of second phases and thus a substantial decrease in the solute enrichment of the solution. A metastable intermetallic phase, identified as -Al4FeSi2, together with silicon phase, was present as the predominant dispersed phase in the preform, and its transformation into the equilibrium phase, -Al5FeSi, through a peritectic reaction under the equilibrium conditions, must have been effectively suppressed during the Osprey process. The -phase initially with a platelet shape was fragmented into short rods during the extrusion subsequent to spray deposition, while a part of this phase was transformed into the equilibrium -phase under the combined influence of heat and deformation. The refinement of the -phase, on the other hand, was found to decrease its metastability and thus to promote its decomposition during subsequent annealing at 400 C. The coexistence of the high volume fractions of the intermetallic and silicon phases in the extruded material greatly modified its restoration kinetics, resulting in a partially recrystallized microstructure, after prolonged soaking at 400 C for 100 H. Also shown is a peculiar microstructure of the as-spray-deposited material with numerous spherical colonies of 10–20 m, characterized by the finer silicon particles and -phase platelets in their interior and occasionally decorated with micropores at their peripheries. These colonies are considered to originate from the remains of very fine droplets and particles in the larger droplets, which are presolidified in flight and then partially remelted at the deposition surface. The colonies were mixed up with the rest of the microstructure and the micropores closed by applying an extrusion operation.
Aluminum matrix composites reinforced by a high volume fraction of ceramic particles provide a novel solution to electronic packaging technology, because of their high thermal conductivity, compatible and tailorable coefficient of thermal expansion (CTE) with chips or substrates, low weight, enhanced specific stiffness, and low cost. In this paper, SiC-particle-reinforced aluminum matrix composites are fabricated by the cost-effective squeeze-casting technology, and their microstructure characteristics, thermo-physical, and mechanical properties are investigated. The reinforcement volume fraction is as high as 70% and composites with linear CTE of 6.9–9.710–6 C–1 and thermal conductivity of 120–170 W m–1 C–1 are produced. The composites can be electric-discharge machined, ground, and electric-spark drilled. An electroless nickel layer is plated on the composite by the conventional procedures. Finally, their potential applications in electronic packaging and thermal management are illustrated via prototype examples.
As part of a four-year BRITE/EURAM collaborative project supported by the European Commission, a new family of low-expansion alloys based on aluminium-silicon has been developed; and the Hirst Division of GEC-Marconi Materials Technology is establishing procedures for fabricating packages for microwave unusually low density of these materials. They are also readily machinable and can be electroplated with relative ease, in sharp contrast with silicon carbide containing metal matrix composites and copper-tungsten. These characteristics make the new materials particularly suitable for short runs of homogeneous packages and carriers to particular designs, facilitated by the ability to insert hermetic feedthroughs and lid-seal. It has also been demonstrated that high integrity heterogeneous packages with kovar side-walls can be made, utilizing Hirst's unique diffusion soldering process for bonding the kovar and aluminium-silicon components together.