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Effective decapsulation of copper wire-bonded microelectronic devices for reliability assessment
Abstract
The wire bonding industry has made a major shift in wire materials from gold to copper, primarily due to cost concerns. Copper wire-bonds are now present in many commercial off-the-shelf (COTS) devices but minimally used in automotive, industrial, or military-grade applications due to lack of detailed understanding about reliability concerns. A thorough study of wire bond reliability includes performing bond shear and pull strength measurements before and after stress testing. This in turn requires a special decapsulation procedure for copper wire-bonded devices because, unlike gold, copper is chemically potent. Many techniques for copper wire-bonded device decapsulation exist and can be categorized into laser-, plasma-, and acid-based processes. This paper reviews some of these techniques and discusses the decapsulation mechanism, which involves decomposition of the epoxy resin. By understanding the decapsulation mechanism and available techniques, a unique decapsulation method was developed. The effectiveness of this method is presented along with scanning electron microscopy (SEM) images of the results, which indicate minimal etching of copper wire bonds. The critical parameters of this technique are also identified, a suitable range of input for each parameter is analyzed theoretically, and a design of experiment (DOE) is conducted to find optimal values for each parameter. Several SEM images are provided to show both the good and bad results from the DOE. An image method for measuring effectiveness of decapsulation is also presented.
... This is done by dissolving the package itself in concentrated 90% nitric acid at a temperature of 100°C. [61] At this temperature, the organic epoxy material is being oxidised by the nitric acid and dissolves completely. Any metals except for gold plating and gold bonding wires also dissolve (tin and silver from solder remains, copper wiring, etc.). ...
The invention of 14 C-based solid state diamond power-cells offers unique opportunities for demonstrating a circular economy in relation to fissile energy production; and closed-loop recycling of these 14 C Diamond Betavoltaic Batteries (C14-DBB). The extremely long half-life of the radioisotope 14 C on a human time scale, 5730 ± 40 years, means that the power output of a C14-DBB is constant from any practical power engineering perspective. The durability of a diamond crystal on geological time scales is quite long and is the result of strong covalent bonds between carbon atoms. 14 C as a beta emitter has a maximum decay energy of 156 keV and a weighted mean energy of 49.5 keV, insufficient to break the covalent bonds and therefore preserving integrity of the crystal over the entire "discharge" period/half-life of the C14-DBB. Whether as backup cells for real-time clocks (RTC), volatile memory, infrastructural health monitoring, medical implants, or any other use case, this class of betavoltaics will outlive any foreseeable engineering application they are used in. An examination of the socioeconomic , ecological, and regulatory interplay in relation to the use and life cycle of a 14 C-based diamond is meant to demonstrate the applicability of closed-loop circular economy as applied to the nuclear sector as a first of a kind demonstrator of how fissile energy production can operate in a closed-loop circular economy, not just as a means of producing energy with a significant waste stream, but a holistically integrated cycle. As a part of this cycle we demonstrate the utility of harvesting feedstock for radiovoltaics. In the next step we demonstrate the incentive to collect and recycle 14 C-based C14-DBB's thus completing a closed-loop economic cycle wherein the derivative waste stream from fissile energy production is reutilised and recycled. Though this approach is not a panacea solution for how to deal with the end state waste from fissile energy production, it is our hope that this first demonstration will evince new investigations along similar lines to make maximum use of every part of the process thus converting the entire value chain to a wholly integrated and cyclical 'value constellation' relating to fissile energy production. In this work, we specifically present a novel process flow to identify and recover C14-DBB's from Waste Electrical and Electronic (WEE) appliances repurposing the 14 C-diamond into new C14-DBBs. The process uses machine vision to detect C14-DBBs on Printed Circuit Boards (PCB) using inspection cameras, and fiducial marks from the SMD assembly process to calculate the exact location of the C14-DBB on the PCB. It is then recovered using selective hot air de-soldering. Electrical contacts and package are removed in a nitric acid bath to expose the C14-DBB and recover it in a controlled environment. The 14 C-diamond is packaged inside a layer of non-active 12 C-diamond, so no radioactivity is exposed at any point in the recycling process. It is inspected using surface analysis techniques before new ohmic contacts are applied and an electrical check is performed using a flying probe tester. The recovered diamond is finally repackaged as a repurposed C14-DBB device, and valuable metals are recovered from the nitric acid solution for re-used after electro-filtration and purification.
... Therefore, how to solve the electrical problems and improve the signal transmission quality of the gold bonding wire in circuit design, analysis and testing have become research hotspots. At present, research on the electrical properties in WB packaging is mainly focused on the following aspects: (1) the process and geometric parameters of bonding wire such as bonding process, bonding wire loop parameters, materials and so forth [4][5][6][7]; (2) the wiring form of bonding wire including wire spacing, signal pin distribution, interconnection etc., [8][9][10][11]; (3) the overall package design such as grounding copper cover, through silicon vias (TSV) technology, and the redistribution layer (RDL) of flip chip or the like [12][13][14][15]. Zhang analyzed the influence of bonding wire on parasitic parameters under different span and arch height. ...
In this paper, electrical characteristic analysis and corresponding experimental tests on gold bonding wire are presented. Firstly, according to EIA / JEDEC97 standard, this paper establishes the electromagnetic structure model of gold bonding wire. The parameters including platform lengthflat length ratio, diameter, span and bonding height are analyzed. In addition, the influence of three kinds of loops of bonding wire is discussed on the S parameters. The equivalent circuit model of bonding wire is proposed. The effect of bonding wire on signal transmission was analyzed by eye diagram as well. Secondly, gold bonding wire design and measurement experiments are implemented based on RF circuit theory analysis and test method. Meanwhile, the original measurement data is compared with the simulation model data and the error is analyzed. At last, the data of five frequency points are processed to eliminate the fixture error as much as possible based on port embedding theory. The measurement results using port extension method are compared with the original measurement data and electromagnetic field simulation data, which proves the correctness of the simulation results and design rules.
... The surge of introduction of Cu wire to replace Au wire due to high Au price [6] has leaded to new sets of bonding metallurgy and process issues. The effects are pre-mature failures and higher production yield losses [7][8][9][10][11]. Adversely, Cu wire brings along molding advantage of resistant to wire sweep due to its higher modulus. ...
Driven by today’s market demand, semiconductor is pushing towards the zero-defect direction. The improvement demanded in semiconductor manufacturing is becoming increasingly challenging. In this paper, common molding defects comprise of voids, incomplete fills, and piping holes are studied systematically, focusing on three key areas: 1) Potential mold flow weakness; 2) Molding temperature stability; as well as 3) Defined pressure effects. The in-depth understanding of mold flow in the LF design is achieved via mold flow numerical tool. The numerical model prediction is verified by short shots and end-of-line auto vision data. Advance Process Control (APC) is adopted to measure the stability of key molding parameters like temperature, transfer profile and pressure. The mechanism of transferring the compound in relation to pressure is also analyzed and its effect to molding quality is also assessed. A methodical approach is utilized to understand the process and equipment built-in capabilities from two different equipment manufacturers. The real time transfer profile monitoring is activated for diagnosis of the system issue which leads to the finding of design error of a critical component. The dual temperature controller on one of the systems is analyzed to stabilize temperature for improved compound flow-viscosity control. The process limitations are assessed and transfer profiles are optimized to modify the melt front. By shifting the molding defects to non-critical location, the formation of void at the 500um diameter bonded wire loop peak will be avoided. The verification of potential negative impacts resulted from changes to improve voids, incomplete fills and piping holes are also included in this study. Up-front analysis by adopting numerical tool as a means of understanding the existing design and identifying improvement approach are proven to be useful.
... Wire bonds in microelectronic devices experience failure by mechanisms such as excessive intermetallic compound (IMC) formation, bond fatigue, corrosion and electro-migration [4][5][6]. Some of these mechanisms occur not only during operation but also during their shelf life [7]. ...
... Thicker IMCs have shown to reduce shear strength and can cause easy fracture at the interface, due to its brittle nature, leading to an open circuit. Many researchers have developed an effect way of decapsulating copper wire boded microelectronic devices [3][16] [17] for analysis that have been adopted to study wire bond degradation due to mold compound material. ...
Copper wire bond is used in majority of microelectronic devices but has not been fully accepted by all industries due to its potential reliability issues. Good quality bond is believed to provide high reliability. Shear strength and Intermetallic compound (IMC)coverage are being used as indicatorsof good quality bond that will eventually lead to high reliability, however there is no clear relationship between the two. High shear strength is a result of large IMC coverage area, underthe ball bond and the reliability of the bond is related to growth in thicknessof IMC. This work involves studying the effect of shear strength on the reliability of ball bonds by performing temperature aging experiments on test devices, of QFN packages,consisting of different IMC coverage leading to a range of shear strength. The wire bonds are made by altering ultrasonic power, forceand time to obtaindifferent IMC coverage. All test packages are monitored for resistance change at specific intervals by performing four-wireresistance measurement. Resistance increase iscorrelated to the initial shear strength to find if good quality bonds led to high reliability.
Wire bonding remains one of the most widely adopted interconnection techniques in the field of electronic packaging. At present, the most effective way to ensure a long life and high reliability of wire bonds is to improve the bonding quality. In this study, both experiments and finite element analysis (FEA) were employed to develop a fundamental understanding of wire bond degradation. Sensors with two protective silicone gels were loaded with the same thermal shock at a temperature ranging from -40°C to 125°C, and the switching time was shorter than 10 s. The number of thermal shock cycles for the aluminum wire covered with transparent silicone reached 1200, but the maximum number of cycles for the other wire only reached 454. The experimental results indicated that the chosen transparent silicone performed better than did the black silicone originally selected, which was also verified by the simulation results. In addition, bond pull and shear tests were performed. The results revealed no degradation of either the Ag-Al or Ni-Al bonding joints under thermal loading. In summary, the root cause of failure was found to be improper protection silicone application, which, as often ignored in analysis, accelerated thermal fatigue of the aluminum wires. An explanation of the observed trend and a recommended aluminum wire bonding method were also provided.
In this study, the D-type latch devices with Ceramic Flat Package (CFP) and Shrink Small-Outline Package (SSOP) were tested and analyzed by experimental means. The effects of manufacturing technics on the pin structure, wire bond, chip layout design, die attach and temperature-affected electrical performance were subsequently investigated. Results show that manufacturing technics with CFP package possesses thicker special pin coatings, thicker different wire bond, stable and effective die attach compared with SSOP package. Meanwhile, the bare dies of two devices exhibit same layout structure design but different longitudinal dimensions. The temperature-affected electrical performances including voltage, current and propagation delay time reveal that the temperature sensitive parameters of CFP latch (variation about 1%) display 12 times better stability than that of SSOP latch (distinct variation about 12%) from −55 °C to 125 °C. Meanwhile, the SSOP latch displays 41.8% better switching speed and working frequency than CFP latch with similar other electrical parameters at room temperature.
Copper has been widely adopted as a viable alternative to gold for wire bonding, due to its lower cost, better electrical and thermal conductivity, lower tendency for wire sweep due to a higher modulus and slower intermetallic compound (IMC) formation with aluminum bond-pad. However, these IMCs can cause an increase in resistance and separation between bonding interface which causes an open circuit. Wire bond and component reliability must consider all factors in combination with the change of wire bond materials and processes. How can users of off-the-shelf components determine the effects of wire bond material changes on the reliability of the components in their systems? How will these changes affect high reliability applications such as automotive, military and harsh environments?
Although physics-of-failure based models can represent the failure mechanisms individually, the complex nature of the mixed failure mechanism has inhibited development of an analytical equation to represent it. In addition, geometrical and material characteristics of the entire package along with bonding process parameters play a crucial role in magnitude of stress experienced by these bonds, portraying difficulty in developing one such equation for use to all copper wire bonded devices. This study has aimed to develop a data-based predictive method based on the principal component analysis technique to provide an estimate of the wire bond time to failure for plastic encapsulate components. Extensive thermal cycling experiments have been performed on multiple commercial devices to obtain training data for the model. This method of wire bond fatigue and failure prediction is an adaptive technique which can achieve greater accuracy as more data is added. This method aims to provide a life-time estimate of copper wire bonded component with inputs such as initial geometrical, material properties, and wire bond related attributes for a specific package.
Silver is a leading competitor to gold and copper in fine pitch wire bonding used in the interconnection of microelectronic devices. Primary material for wire bonding has been gold, which gave way to copper in order for original equipment manufacturers to realize cost benefits. However, copper wire bonding has exhibited several reliability issues, especially in industrial and high temperature applications. Corrosion is the major problem, which was mitigated by coating the wire with palladium, which increased overall cost of production. Other concerns include harder free air ball (FAB) leading to under pad metallization cracking, smaller process window, excessive aluminum splash especially in fine pitch bonding, and lower throughput and yield arising from the hardness and stiffness of copper. Due to the above concerns, automotive, military and aerospace industries are still reluctant to fully adopt copper wire bonding. Light emitting diodes (LEDs) are also not manufactured with copper wires due to its low reflectance. Some of these industries are still using gold wire bonds in most of their packages, but are continually looking for an alternative. Silver wire bonds have good electrical and thermal conductivity, are less prone to corrosion than copper, have low melting points and comparable hardness to gold. Also, cost of silver has been shown to be similar to that of palladium coated copper wire, hence making it a good alternative. Silver wire bonding, a relatively new area of research, has attracted a lot of research focused on wire dopant material, bonding process, quality and reliability. This paper is aimed to serve as a comprehensive review of research done in this area, by summarizing the literature on silver wire bonding, establishing benefits and drawbacks over other wire bond materials and indicating reliability concerns along with failure modes and mechanisms.
Abstract:
The cost of gold has increased drastically in the last few years forcing IC manufacturers to switch to copper wire bonding. Copper is used in many plastic-encapsulated devices, but manufacturers are still using the same qualification tests for copper wire-bonded devices as for gold wire-bonded devices, even though damage accumulates differently in these two interconnections, and the failure mechanisms and models are significantly different. To study the damage to the wire, bond shear and pull tests are performed. However, due to the absence of a decapsulation procedure to reveal the ball bond and the wedge bond, these tests are usually performed on non-molded compounds, which does not account for the effect of the molding compound in the failure. To study critical failure mechanisms such as corrosion, wire axial fatigue, and wire bond fatigue, a novel method for wire bond decapsulation was developed, that uses a sequential process, resulting in minimum degradationor, in some cases, no degradation of wire bonds. Previous studies presented decapsulation methods that either revealed the ball bond or the wedge bond separately, but these methods cannot be used to perform wire pull tests. Exposing both the ball bond and the wedge bond is not easy, as the wire tends to break at the loop due to the long exposure time. This paper, describes - a successful decapsulation technique and presents bond shear results for devices with copper wire bonds, thereby permitting studies of encapsulated devices.
Environmental stress screening (ESS) is a process to eliminate defects caused by materials and manufacturing variations in electronic products by a 100% screening, with the goal to eventually improve manufacturing processes that cause the defects. MIL-HDBK 344A- Environmental Stress Screening (ESS) of Electronic Equipment describes a quantitative approach for planning, monitoring, and controlling an ESS process. This handbook was last updated in 1993 but is still widely used today within and beyond the military and aerospace industries. While the processes and overall philosophy of ESS are well presented in the handbook, the mathematical foundations and the data it uses have become obsolete over the years. These concerns are explained with a thorough analysis of the initial defect estimation and screening strength estimation steps described in the handbook. With the rapid evolution in electronics technology, the handbook’s estimation methods are meaningless if not counterproductive for the planning of an ESS program for today’s electronics systems. Based on the critical concerns presented in this article, we urge the U.S. Department of Defense to cancel and stop use of MIL-HDBK 344A.
A novel method for copper wire bond decapsulation was developed, which involves conducting a sequential process to ensure minimal corrosion or, in some cases, no corrosion/degradation of the copper wire. It requires a pre-treatment which involves mechanical milling or acid etching; the main decapsulation process, utilizing a mixture of nitric and sulfuric acid under electrically biased conditions; and a post decapsulation procedure of encapsulant debris removal using a solvent. Previous studies use decapsulation methods that reveal either the ball bond or the wedge bond, separately, and thus cannot be used to perform wire pull tests. Exposing both the ball and wedge bond is not trivial, as the wire tends to break at the loop due to the long exposure time. The three-step process, when properly executed, ensures that the entire wire along with both ball and wedge bonds are exposed with minimal damage. Five parameters were found to affect the decapsulation process. These are as follows: acid ratio, ...
Decapsulation of plastic packaged components is frequently necessary for failure analysis or product development. Conventional techniques utilize an acid etch process, which is not compatible with copper and other acid-sensitive materials. There is limited ability to alter etch geometry or selectively decapsulate individual circuit elements. Laser ablation using a raster scanned Nd:YAG laser was used for selective decapsulation of temperature sensitive passive components in a PDIP packaged DC-DC converter. Copper welds, nylon insulated ferrite, and polyesterimide insulated magnet wire were decapsulated without damage, allowing failure analysis of a miniature transformer and its associated welded leads.
The continuous increase in application temperature of power electronic packages/devices demands new packaging technologies capable of working reliably at high temperatures. Among different packaging technologies, the need for a new kind of interconnection is more serious due to the expanding ban on application of currently used lead containing solders in electronics. In this paper performance of a potential interconnect technology (TLPS) with low processing and high application temperatures is investigated under power cycling loading condition. A specific test setup compatible with prepared power packages for this paper was designed and assembled. This test setup cycles and continuously monitors temperature of power packages fabricated from a commercially available power diode, TLPS joints, and three types of substrates under constant current condition until failure. The failure criterion was defined as either extreme increase in maximum temperature of power device or electrical failure of the semiconductor device. The failed samples were destructively analyzed to identify failure modes and mechanisms. Optical microscopy, Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS) were implemented to perform a comprehensive failure analysis. The results show that Cu-Sn TLPS joints' stiff mechanical properties result in fracture of the semiconductor device. The prevalent failure mode was diode failure (short-circuit) and fracture of semiconductor device under thermos-mechanical loading was identified as failure mechanism. Finally, performances of different types of substrates in reliability of prepared packages were investigated and compared.
IC packages have been greatly improved over the past several years. With the adoption of Cu wires and new green EMC (Epoxy Molding Compound), the suppression of lead, the use of Cu pillars and the increased number of dies, the verification of the quality of the assembly and failure analysis becomes critical. Starting twelve years ago, LASER ablation was introduced as a means to facilitate the pre-decapsulation of packages aiming at a completion by wet chemistry (acids) or dry chemistry (plasma). The decapsulation process with acid at medium temperature (75°C) does not permit to keep the Cu wires intact. Our studies and work in the past several years has consisted in lowering the temperature of acid use in order to minimize the effect of acid attack on the Al pads and Cu wires. Currently the thinnest wires used are 0.6 mil in diameter (approximately 15µm). In this article we will demonstrate that decapsulations at sub-ambient temperatures are now possible and give expected results. Moreover, openings at near ambient temperature reduce the component deformation and also the deformation of its constituents compared to decapsulations at medium or high temperature.
Decapsulation of plastic packaged components is frequently necessary for failure analysis or product development. Conventional techniques utilize an acid etch process, which is not compatible with copper and other acid-sensitive materials. There is limited ability to alter etch geometry or selectively decapsulate individual circuit elements. Laser ablation using a raster scanned Nd:YAG laser was used for selective decapsulation of temperature sensitive passive components in a PDIP packaged DC-DC converter. Copper welds, nylon insulated ferrite, and polyesterimide insulated magnet wire were decapsulated without damage, allowing failure analysis of a miniature transformer and its associated welded leads.
Electronics are used in a wide range of applications including computing, communication, biomedical, automotive, military and aerospace. They must operate in varying temperature and humidity environments including indoor controlled conditions and outdoor climate changes. Moisture, ionic contamination, heat, radiation and mechanical stresses are all highly detrimental to electronic devices and can lead to device failures. Therefore, it is essential that the electronic devices be packaged for protection from their intended environments, as well as to provide handling, assembly, electrical and thermal considerations. Currently, more than 99% of microelectronic devices are plastic encapsulated..
This critical volume provides an in-depth presentation of copper wire bonding technologies, processes and equipment, along with the economic benefits and risks. Due to the increasing cost of materials used to make electronic components, the electronics industry has been rapidly moving from high cost gold to significantly lower cost copper as a wire bonding material. However, copper wire bonding has several process and reliability concerns due to its material properties. Copper Wire Bonding book lays out the challenges involved in replacing gold with copper as a wire bond material, and includes the bonding process changes-bond force, electric flame off, current and ultrasonic energy optimization, and bonding tools and equipment changes for first and second bond formation. In addition, the bond-pad metallurgies and the use of bare and palladium-coated copper wires on aluminum are presented, and gold, nickel and palladium surface finishes are discussed. The book also discusses best practices and recommendations on the bond process, bond-pad metallurgies, and appropriate reliability tests for copper wire-bonded electronic components. In summary, this book: Introduces copper wire bonding technologies Presents copper wire bonding processes Discusses copper wire bonding metallurgies Covers recent advancements in copper wire bonding including the bonding process, equipment changes, bond-pad materials and surface finishes Covers the reliability tests and concerns Covers the current implementation of copper wire bonding in the electronics industry Features 120 figures and tables Copper Wire Bonding is an essential reference for industry professionals seeking detailed information on all facets of copper wire bonding technology. © Springer Science+Business Media New York 2014. All rights are reserved.
Low-temperature transient liquid phase sintering (LT-TLPS) can be used to form high-temperature joints between metallic interfaces at low process temperatures. In this paper, we will describe the processing and shear strength, along with the shock fatigue resistance of sintered joints made by this process. Joints made from different ratios of Ni and Cu high melting temperature constituents paired with Sn-based low melting temperature constituents have been evaluated. For the shear studies, the softening behavior of test samples joined by Ni-Sn3.5Ag and (Ni,Cu)-Sn3.5Ag sinter pastes have been assessed using a fixture designed for high temperature shear testing up to 600°C'. The reliability of sinter paste joints in drop-shock environments will be discussed. It is shown that joints formed from these sinter pastes possess improved drop-shock reliability compared to Sn3.5Ag solder joints and melting temperatures considerably higher than those of conventional high temperature solders (e.g. Pb5.0Sn2.5Ag).
Three kinds of epoxy resins were decomposed by nitric acid in order to establish a recycling method of epoxy resin. Specimens were immersed in 80°C nitric acid (4M and 6M) and decomposed products were recovered. The molecular weight distributions and chemical structures of the products were analyzed by using Size Eliminated Chromatograph and FT-IR, respectively. For the case of bisphenol A type epoxy resin cured with amine, a derivative of monomer was formed due to the decomposition at C-N bonds. The derivative of monomer was dissolved in nitric acid and was extracted with ethyl acetate. 2, 4, 6-trinitrophenol, which was produced by the breakage of main chain and subsequent nitration, was also recovered. Nitric acid attacked the C-N bonds in bisphenol F type epoxy resin cured with amine more selectively because of the existence of methylene group in the main chain. The yield of recovered decomposed product which contained derivative of the monomer of bisphenol F type epoxy resin was more than the case of bisphenol A. With regard to tetraglycidyl diamino diphenyl methane (TGDDM) resin cured with diamino diphenyl methane (DDM), the chemical structure of cured resin is symmetric and repetitive. Hence, decomposed product of TGDDM/DDM was recovered as a nearly single compound of 2, 4, 6-trinitroaniline.
One method of reducing costs in the packaging sector is to switch from gold bond wires to copper. Thicker copper wires (over 2 mils) can be safely decapsulated using a ratio mixture of fuming acids. Some surface etching of the copper will occur, but the wire will remain electrically viable. Microwave Plasma can provide a safer alternative for decapsulating packages with copper bondwires and exposed copper metallization. In this paper, experimental deprocessing of copper bond wire and copper metallization using laser ablation and downstream microwave plasma has found that 1 mil stressed wires can be safely exposed and examined, showing slip plane fractures in the corner wires. Topside copper metallization remains intact, even the thin protective nickel plating. Sensitive copper metal structures on top of the passivation (such as antennas) will remain electrically viable following decapsulation with plasma, but are often lost and defective following acid decapsulation.
By utilizing a NdYAG lamp pumped marking laser, along with unique mixes of specific acids, reproducible decapsulation of copper bonded devices without damage to the bond wires, packaging material, or to the silicon die circuitry itself can be achieved. With the copper bond wires, die, or substrate exposed, typical failure analysis methodology can then be applied to drive root cause failure analysis or device characterization.
Decapsulation is the process of removing mold compound from the die surface of a plastic encapsulated device. Typically, the mold compound is removed only in the area above the die and bond wires for failure site isolation and analysis. Hot fuming sulfuric and nitric acids are the most commonly used decapsulating agent. Jet etching system employing these acids or mixture of these acids have become the dominant method for decapsulation. However, due to the increase of the gold price, the IC industry has already begun to switch from gold wire bonding to copper wire bonding. However, the use of copper wire bonding also introduces a problem in failure analysis. The traditional encapuslation techniques of acid results in corrosion. Therefore, many improvements for wet chemical etching and dry etching methods are found such as laser and plasma methods. In this paper, these methods are summerised and further analysis of these methods are discussed, such as comparisons, advantages and disadvantages of these methods. It is a guide for choosing the decapsulation methods and preform accurate IC package analysis.
Copper (Cu) wirebond technology in IC packaging has become popular these days due to its better characteristic and cost compare to Gold (Au). Nonetheless, failure analysis remains to be very challenging as Cu is easily dissolved when it is reacted with fuming nitric acid used during standard decapsulation process. Hence, by utilizing enhanced manual decapsulation technique with mixture of fuming nitric acid and concentrated sulfuric acid at low temperature, successful Cu wire package decapsulation happen to be reproducible mainly for die level failure analysis purposes.
A dry etching de-capsulation method using pre-laser ablation, microwave plasma etching and diluted methane sulfonic acid cleaning to expose the Cu wire is introduced. Comparison study of wire bond strength shows that dry plasma etching is superior over wet chemical etching method. The experiment leads further to determine the characterization result of wire ball bonding strength for different bond pads and wire materials of samples produced by the two types of de-capsulation method.
Decapsulation of plastic integrated circuit (IC) packages with copper wire bonding is achieved by using an atmospheric pressure microwave induced plasma. A thermal model is built to estimate the bulk IC package temperature under different plasma etching conditions. Temperature measurements of the plasma effluent and IC package are made to validate the model. Due to the low heat transfer rate from gas to solid, the plasma effluent of 700°C raises the bulk temperature of an IC package to 150°C only. This brings a great advantage in processing because a high temperature on a focused area where the plasma etching takes place results in a high etching rate, while a low IC package bulk temperature ensures minimum thermally induced damage to the internal components. Recipes for three etching steps are developed. An IC package with 38 um copper wire bonds and a 2 mm * 3.5 mm die is decapsulated in 20 minutes. Copper bond wires, aluminum bond pads, and structures on the die are undamaged after decapsulation.
Wire bonding technology has been extensively used to interconnect IC chips and substrates. Gold (Au) and aluminium (Al) has been used for wire bonding interconnect for decades. Recently, copper (Cu) wire bonding is used in high temperature applications and general cost down approaches. Despite its many benefits, copper wire has not yet been widely used like gold wire, as copper wire bonding also introduces many new challenges. One challenge is that copper wire bonding process needs more ultrasonic energy and a higher bonding force, which can damage the Si substrate, form die cratering and induce cracking and peeling of the bonding pad. Compared with copper, Ag is similar in conductivity, but softer in terms of mechanical properties. The lower Young's modulus and Yield stress of Ag could help to reduce the bond force and ultrasonic power during wire bonding process which leads to less damage on interconnect metallization under bond pad. In this paper, transient mechanical responses of the interconnect metallization beneath bump pad are investigated. Under the assumption of elastic-plastic behavior of the bonding pad and elastic behavior of the oxide, parametric studies are carried out to examine the effect of wire bonding material, interconnect metallization structure and thickness of bump pad on the stress distribution and elastic deformation of interconnect metallization layers.
This paper reports a second reliability concern related to the introduction of the low-CTE green mold compounds in advanced IC packaging. The increased mismatch between the CTE’s of copper wire and mold compound causes high mechanical stresses in the copper wire bonds during temperature cycling tests. The repeated plastic deformation in each temperature cycle results in fatigue cracking of the copper wire bond.
This paper gives an explanation for this new failure using thermo-mechanical finite element modelling. It shows that only under the combination of copper wire and low CTE overmold, the stresses become high enough in the wire in order to get plastic deformation, which finally leads to low cycle fatigue cracking.
The interfacial shear (IS) force of copper ball onto aluminium-based bond pad in microelectronics packaging depends on the formation and growth of Cu–Al intermetallic. This paper reports the study on the behaviour of the IS force of the Cu–Al bonds that were subjected to pressure cooker test up to 576 h. Initially, the IS force increases with test readout point until 192 h, due to Cu–Al intermetallic growth that has strengthened the bonding. However, IS force decreases significantly from 157.4 gf at 192 h to 97.6 gf at final test readout point, 576 h. The number of shear-induced cratering shows similar reduction trend after 288 h. Result of scanning electron microscopy (SEM) on bonding morphology shows the evidence of crack at the aluminium bond periphery or outer Cu–Al interface and the cracking trend continues at higher time point. Surface analysis of ball-peeled bond pad using X-ray photoelectron spectroscopy (XPS) indicated that the cracks were due to stress corrosion cracking at aluminium that has been stimulated by copper. The concentration of CuO at the surface bonded area was found to be increased at higher readout point and reached 100% at 576 h. These results indicated that the Cu–Al bond had been weakened by stress corrosion cracking at outer bond interface and reduced the IS force.
Epoxy molded IC packages with copper wire bonds are decapsulated using mixtures of concentrated sulfuric acid (20%) and fuming nitric acid in an automatic decapping unit and, observed with minimal corrosion of copper wires (0.8-6 mil sizes) and bond interfaces. To attain maximum cross-linking of the molded epoxies, the post mold cured packages (175 degC for 4 h) were further, aged at high temperature of 150 degC for 1000 h. These packages are decapsulated using mixtures of higher ratio of concentrated sulfuric acid (40%) along with fuming nitric acid. The shear strength of copper wire bonds with 1 mil (25 mum) diameter of the decapsulated unit is higher than 5.5 gf/mil<sup>2</sup>. The present study shows copper stitch bonds to Au, Cu, Pd, and Sn alloy plated surfaces are less affected on decapping, with a few grams of breaking load on stitch pull test, while stitch bonds on silver plated surfaces reveal lifting of wire bonds on decapping
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Pit area measurement. SEM image (left) and gray scale thresholding on
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Fig. 18. Pit area measurement. SEM image (left) and gray scale thresholding on Image J (right).
Copper wire bond shear strength dependence on process and bond pad parameters
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Laser decapsulation of Electronics Packages
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Low temperature plasma decapsulation of copper wire-bonded and exposed copper metallization devices
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Estimating bond-wire current carrying capacity
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