SnPb Solder Research Articles

Influence of cooling conditions on the interfacial Cu6Sn5 intermetallic compound in Sn-37Pb/Cu solder joints during reflow

Despite a shift to Pb-free solders in the majority of electronic assemblies, Pb-containing solders are still used in a variety of applications. Recent research has revealed the complexities of the intermetallic Cu6Sn5 in terms of its crystal structure and stability when present as the reaction product between Pb-free solders and common substrates. In light of these findings, the effect of multiple reflow cycles with different cooling conditions on the microstructure of the interfacial Cu6Sn5 intermetallic compound (IMC) between Sn-Pb solders and Cu substrates was re- examined. For this purpose, the Sn-37Pb (in wt.%) solder was reflowed on a Cu substrate at a temperature of around 240 °C and cooled at a rate of around 30 °C/min under each of two conditions; (i) direct-cooling to room temperature, and (ii) interrupted-cooling with extended isothermal holding periods of 30 and 180 seconds at 140 °C. This second condition was chosen to encourage the polymorphic transformation that occurs in the Cu6Sn5 (at an equilibrium temperature of 186 °C) at a temperature which minimises the stresses involved. Smaller cracks and a thinner interfacial IMC layer were observed in the solder joints that were reflowed repeatedly with the interrupted-cooling conditions. However, despite the higher number of cracks in the samples undergoing conventional reflow, the shear strength was not significantly affected.

The effect of Aluminium addition on the microstructure of lead-free solder alloys: A short review

The ban on using lead in electronic packaging has been taken effect since the year 2006. The alternative to replacing lead (Pb) based solder has been given highly concerns by the electronic industry. Both Sn- Ag-Cu and Sn-Cu solder alloys were considered to be the most acceptable candidates to replace SnPb solder alloy. From the findings, it showed that β-Sn appears as island-liked shape and eutectic phases appear as a darker shade. The grain size is refined significantly after incorporated with a suitable amount of Al. When exorbitant amount of Al addition will cause Al-rich particles, which has high potential to acts as the heterogeneous nucleation site and result in a lower degree of undercooling. The most desired amount of Al addition is between 0.025 - 0.05 wt%. The primary intermetallic compounds found in Sn-based solder alloy are Cu6Sn5 and Ag3Sn. New intermetallic compouds were found after addition of Al, which are Cu-Al and Ag-Al intermetallic. The formation of new intermetallic compounds causes the formation of Cu6Sn5 and Ag3Sn to be suppressed.

Mechanical characteristics and fracture behavior of GaN/DBA die-attached during thermal aging: pressure-less hybrid Ag sinter joint and Pb–5Sn solder joint

Ag sinter joining provides superior mechanical and thermal/electrical properties and is considered to become a leading next-generation wide band-gap (WBG) die-attach material. However, the microstructural evolution and mechanical characteristics of Ag sinter joining when subjected to high temperature have never been directly compared to those same characteristics of solder materials. In this study, we have evaluated the high-temperature and long-term reliability of a GaN/DBA die-attached module by pressure-less Ag sinter joining and Pb–5Sn solder in a harsh thermal aging test. Both the Ag sinter joining and Pb–5Sn solder were subjected to a thermal aging test of up to 1000 h at 250 °C. Initial shear strength of the Ag sinter joint exceeded 42 MPa, and increased stably up to 1000 h without any defects such as interface oxidation, diffusion, or mechanical deformation. The increase in shear strength of the Ag sinter joints was the result of necking growth of the sintered Ag during thermal aging. On the other hand, the shear strength of the Pb–5Sn joints exhibited substantially decreased shear strength (by 60%) after aging 250 h. NixSnx intermetallic compounds (IMC) were also formed and serious interface degradation occurred during the aging process. These microstructure changes and mechanical characteristics have an important influence on mechanical reliability and, with that in mind, the tendency of fracture mechanism was investigated in detail by SEM–EDX. This study systematically examines the fracture mechanism on the microstructure of a DBA substrate and on high-temperature packaging during thermal aging tests for WBG semiconductor device applications.

Regeneration of Sn-Pb solder from waste printed circuit boards: A hydrometallurgical approach to treating waste with waste

A green approach was introduced to regenerate Sn-Pb solder from waste printed circuit boards (PCBs). For this purpose, waste Al-based heat sinks were used as cementing agent to precipitate Sn and Pb from pregnant leach solution (PLS) obtained from the dissolution of waste PCBs in HCl. 97 % and 94.9 % of Sn and Pb were recovered, respectively, under optimum conditions at Al powder size of 300 μm, Al dosage of 1.516 g/l and reaction time of 15.41 min. Thermodynamic analysis was performed to predict the effect of temperature on the main reactions relevant to the cementation process. The structure of the Sn-Pb cement changed as function of temperature, leading to enhancement of the cementation rate via improving cathodic area. Kinetic modeling indicates that product layer diffusion is the rate limiting step for Sn and Pb cementation. However, the reaction mechanism shifted to chemical reaction control at high temperatures. The results of solder characterization indicated that the melting point of solder was 184.76 °C. The electrical resistivity and conductivity of the recovered pure alloy were measured to be 34.73 μΩ-cm and 2.88 × 106 Sm−1, respectively. The characterization revealed that the regenerated product is adequate as an alternative solder alloy.

Enhancement of the Unified Constitutive Model for Viscoplastic Solders in Wide Strain Rate and Temperature Ranges

Eutectic 63Sn-37Pb solder is a typical viscoplastic material widely used in aerospace applications. In order to benchmark the mechanical reliability of electronic packaging structures in harsh environments, this study focused on the constitutive behavior of the solder. A unified creep-plasticity constitutive model is proposed with an enriched flow rule to satisfactorily describe the plastic and hardening stages. The strain rate range of 10-4 and 10-2 s-1 and the temperature range of -55 and 125°C are covered by a series of comprehensive uniaxial tensile experiments. Moreover, the experimental results are analyzed to elucidate the underlying mechanism of the calibrated parameters. It is found that the drag strength increases with temperature, while its slope is reduced with the strain rate. The short range of back stress is closely associated with temperature to influence the hardening slope of the stress–strain relationships. The value of hardening exponent is sensitive to strain rate and thus determined to be 0.52 and 2.72 for the strain rate greater and less than 2·10-4 s-1, respectively. The strain rate and temperature ranges in this study are capable of covering the temperature cycling tests for reliability qualification of electronic devices. Thus, the proposed enhancement of the unified creep-plasticity constitutive model facilitates the finite element simulations of packaging structures under temperature cycling conditions. This framework of the proposed constitutive model is readily extended to other types of viscoplastic solders within wide strain rate and temperature ranges.

Review of Power Electronics Components at Cryogenic Temperatures.

In order to apply power electronics systems to applications such as superconducting systems under cryogenic temperatures, it is necessary to investigate the characteristics of different parts in the power electronics systems. This paper reviews the influence of cryogenic temperature on power semiconductor devices including Si and wide bandgap switches, integrated circuits, passive components, interconnection and dielectric materials, and some typical cryogenic converter systems. Also, the basic theories and principles are given to explain the trends for different aspects of cryogenically cooled converters. Based on the review, Si active power devices, bulk CMOS based integrated circuits, nanocrystalline and amorphous magnetic cores, NP0 ceramic and film capacitors, thin/metal film and wirewound resistors are the components suitable for cryogenic operation. Pb-rich PbSn solder or In solder, classic PCB material, most insulation papers and epoxy encapsulant are good interconnection and dielectric parts for cryogenic temperatures.

Development of high-strength and superior thermal shock-resistant GaN/DBA die attach structure with Ag sinter joining by thick Ni metallization

With the advent of wide band-gap (WBG) semiconductors like silicon carbide (SiC) and gallium nitride (GaN), the operation temperature of WBG power modules are expected to be over 250 °C. High temperature die-attach materials that can withstand these harsh conditions are emerging as one of the main issues of the reliability of power electronic applications. In this study, high-strength and superior thermal shock-resistant GaN/DBA die attach structures with Ag sinter joining by thick Ni metallization (7 μm) were developed and compared to conventional Pb5Sn soldering die attach. A thermal shock test was conducted at a temperature range of −50/250 °C, holding 30 min at each extreme temperature up to 500 cycles. Consequently, the shear strength of the Ag sinter joining structure was sustained above the initial strength after 500 cycles, which showed durable sinter necks and non-deformed interfaces due to the thick Ni metallization. On the other hand, the Pb5Sn solder joined structure showed a 50% reduction in shear strength after 500 cycles compared to the initial state. Some disadvantages of these solder joints occurred such as an intermetallic compound and interface degradation. Thus, with the new design and evaluation reported in this study, Ag sinter joining can be expected as an environment-friendly replacement die material for the long-term reliability of WBG semiconductor module packaging.

(Invited) Ag Sinter Joining Technology for Different Metal Interface (Au, Ag, Ni, Cu, Al) in Wide Band Gap Power Modules

Wide-bandgap (WBG) semiconductor materials are expected to be widely utilized in next-generation power devices as modules to replace Si-based devices owing to their higher voltage-blocking capability, higher temperature operation, and switching frequencies, as well as their reduced power loss [1][2]. In WBG devices, die-attached materials that bond WBG dies to substrates are crucial for maintaining high performances in environments reaching 250 °C [3]. Conventional die bonding materials, such as Pb-5Sn solder [4], Au-Sn solder [5], and Bi-based solder [6], are not suitable for use in next-generation packaging because cracks can form at the intermetallic solder interface in high-temperature environments . One of the promising alternative approaches is Ag sinter joining technology, which allows for a sintering temperature below 300 °C at low pressure. As the melting point of Ag is higher than 900 °C, Ag sinter joining can be used in high-temperature environments exceeding 250°C .The quality of die-attach by Ag sinter joining depends on the bond-line cohesion from Ag sintering and interfacial adhesion from atomic inter-diffusion. Therefore, the bonding quality depends on the surface finish such as its chemistry and microstructure, on both device and substrate . Usually, electro-plated Ag, electroless-plated Ni(P)/Ag, electro- and electroless-plated Ni(P)/Au is used as surface finish layer for sinter Ag joining. On the other hand, the electro- and electroless-plated Ni(P) technology was well developed and used widely, which can achieve a sufficient resin adhesion resulting from the chemical interaction between the base metal Ni and the resins. However, this additional metallization layer increase costs due to the additional raw materials and processing steps. Ag paste-based, die-attachments on a bare the direct bonded copper (DBC) substrate or direct bonded aluminum (DBA) seems an ideal option due to reduced cost and a simplified process .Here we report a robust joint by Ag sinter joining technology for different metal interface (Au, Ag, Ni, Cu, Al) in wide band gap power modules. Micron-scale Ag flakes (AgC239, Fukuda Metal Foil, and Powder Co. Ltd, Japan) were used as the Ag fillers. The thermal behaviors and morphology of the Ag paste sintered at different temperatures were firstly studied to understand the sintering behavior of the prepared Ag paste. Sinter Ag joint structure for different metal interface was implemented by a sintering process under temperatures of 250 °C in air without pressure. In addition, each sinter Ag joint structure was investigated to obtain a comprehensive understanding for different metal interface bonding. A possible mechanism was proposed based on the SEM and TEM observation of the cross-sectional part of each sinter Ag joint structure.Fig. 1 shows the sinter Ag joint structure for Ag metallization metal interface, and the die shear strength of sinter Ag joint structure for different metal interface. Sintered Ag shows a microporous network structure and bonding well with the Ag metallization layer at the both SiC chip and substrate side. The die shear strength of each sinter Ag joint structure is larger than 25 MPa which comparable with the value of traditional Sn–Pb solders (19~24MPa. This study helps to understand the Ag sinter joining for different metal metallization interface, enlarger the Ag sinter joining for wide band-gap power modules in high temperature applications. ACKNOWLEDGMENTS This work was supported by the JST Advanced Low Carbon Technology Research and Development Program (ALCA) project “Development of a high frequency GaN power module package technology” (Grant No. JPMJAL1610). The author is thankful to the Network Joint Research Centre for Materials and Devices and Dynamic Alliance for Open Innovation Bridging Human, Environment and Material.

The Kinetics Study of Dissolving SnPb Solder by Hydrometallurgy

Abstract A large amount of electrical and electronic equipment is being discarded every year, which produces a large number of waste printed circuit boards (WPCBs). The WPCB is a heterogeneous mixt...

Effect of polyvinyl chloride fire smoke on the long-term corrosion kinetics and surface microstructure of tin–lead and lead-free solders

With the rapid increase of electric power use, wire and cable fire occurs more and more frequently. While the fire smoke could corrode the solders on the printed circuit boards, which seriously affects the function of electronic equipment. In this research, the long-term corrosion behavior of Sn–37Pb, Sn–0.7Cu, and Sn–3.0Ag solders under 140 g m−3 polyvinyl chloride fire smoke atmosphere were investigated. The mass loss of all solders increased sharply on the first day and then varied exponentially in the subsequent time period. Particularly, the mass loss of Sn–37Pb solder gradually stabilized after 10 days, while that of lead-free solders was after 12 days. Meanwhile, the corrosion morphology indicated that the corrosion products of both Sn–37Pb and Sn–0.7Cu solders grew to be larger and denser. While those of Sn–3.0Ag solder grew larger firstly and then became smaller and denser because the grain growth dominated firstly and then the nucleation dominated with corrosion time. The corrosion mechanism was analyzed by using EDS, XRD, and XPS. The result showed that the carbon particulates from the fire smoke was detected which might promote the corrosion process and all solders had a common corrosion product, Sn21Cl16(OH)14O6. Besides, Sn–37Pb solder also contained other corrosion product, PbCl2. The present work could provide guidance to the risk assessment for electronic equipment rescue after a fire.

Fabrication of Novel Geopolymer Reinforced Tin Copper Solder in Suppressing Intermetallic Layer Growth

Currently, lead-free composite solder technology system is the new approach that have been considered as a promising replacement for toxic SnPb solder alloy. Sn-0.7Cu was used as matrix solder alloy while fly ash geopolymer and kaolin geopolymer with different compositions (0.5 wt.%, 1.0 wt.%, and 1.5 wt.%) was added as a reinforcement to increase the properties of solder alloy. The solder composite was prepared by using powder metallurgy method which consist of mixing, compaction, and sintering process via microwave oven. Microwave sintering approach results in more uniform heating, lower pore coarsening with cost-effective and energy efficiency. The chemical composition and morphology of geopolymer reinforcement was analysed by using X-ray Flourescene and Scanning Electron Microscope. The influences of the geopolymer particulates in the monolithic matrix solder on the interfacial intermetallic compound thickness and wettability were investigated by using optical microscope, while the specific value was measured by using J-image software. The melting point temperature were investigated by using differential scanning calorimeter (DSC). The influences of adding types of geopolymer particulates into Sn-0.7Cu lead free solder were investigated in terms of interfacial intermetallic compounds (IMCs) and thermal properties. It is noted that several addition of fly ash geopolymer and kaolin geopolymer particles can remarkably supressed the thickness of intermetallic layer between composite solder and the substrate and thus gives higher melting temperature but somehow certain composition resulted low in mechanical properties. Overall, the addition of 0.5 wt.% of kaolin geopolymer reinforcements into Sn-0.7Cu lead-free improved all the listed properties of the solder materials.

Wettability, Interfacial Behavior and Joint Properties of Sn-15Bi Solder

Sn-15Bi solid solution solder provides a potential replacement for Sn-40Pb solder due to lower cost and similar melting point. Sn-15Bi solder was compared with Sn-40Pb solder to assess the microstructure, wettability, bulk tensile properties, interfacial microstructures in solder joints with a Cu substrate, the interfacial evolution in joints during isothermal aging, and the shear strength on ball solder joints with the effect of aging conditions. The microstructure of Sn-15Bi bulk solder was composed of the primary β-Sn phase with Bi in solid solution and with precipitated Bi phases along the grain boundaries. The wettability of solder alloys was evaluated with wetting balance testing, and the results showed that Sn-15Bi solder had a lower wetting force and longer wetting time than Sn-40Pb solder. The poorer wettability of Sn-15Bi solder was not influenced by different types of soldering flux, but improved with increasing temperature. Tensile tests on bulk solder alloys indicated that Sn-15Bi solder had a higher tensile strength but lower elongation than Sn-40Pb solder. Also, the tensile strength of Sn-Bi solder decreased with decreasing strain rate and with increasing temperature, while the elongation of Sn-Bi solder was independent of the temperature and strain rate. In as-soldered joints, Sn-15Bi solder produced a thicker Cu6Sn5 layer than Sn-40Pb solder. Following isothermal aging, the thickness of the interfacial Cu-Sn intermetallic compound (IMC) increased with the aging time, and the growth rate of the IMC layer in Sn-15Bi solder joints was slightly higher than that in Sn-40Pb solder joints during aging. Ball shear test on solder joints illustrated that Sn-15Bi solder joint had a higher shear strength than Sn-40Pb solder joint, and that with increased aging time, the solder became stronger due to precipitation of Bi, leading to a change in the fracture mode from ductile failure in the solder to brittle fracture at the IMC interface.

Effects of electroless nickel plating method for low temperature joining ZnS ceramics

ZnS has been widely used as a material for windows in optical systems. This is because it has a good transmittance in both mid-infrared and far-infrared bands, a stable chemical performance, a moderate linear thermal expansion coefficient and an adequate mechanical strength. ZnS windows need the so-called splice technology implementation to satisfy the integrity of large size applications or special surface structures. Traditional ZnS light windows are mechanically assembled with metal frames. However, the strength of the large-size ZnS formed by mechanical assembly is low. Consequently, an adequate joining technology is needed to overcome the low splice strength and improve the mechanical strength of the assembly. In this paper, we proposed a pre-metallization method by using an electroless plating of Ni on ZnS surface. Subsequently, a low temperature joining by using a Sn63Pb37 solder was completed. By a study of the process and the corresponding mechanism of the electroless nickel plating, a Ni–P layer with a controllable P content and thickness has been formed, and a smooth surface and a good combination with ZnS has been obtained. The microstructure of the Sn63Pb37 and Ni–P/ZnS was analyzed, and the phase composition and element distribution within the joint were determined. The influences of process parameters on the microstructures of the joints have been explored, including the brazing temperature, the time of thermal energy preservation and the thickness of the coating. Finally, the shear strengths of joints under different imposed parameters were optimized leading to the peak soldering temperature of 250 °C and the dwell at the peak of 1 min. The fractures were along Ni–P/ZnS ceramic interface, therefore, a successfully low temperature joining process on ZnS ceramics using Ni–P plating and Sn63Pb37 solder had been performed.

(Invited) Ag Sinter Joining Technology for Different Metal Interface (Au, Ag, Ni, Cu, Al) in Wide Band Gap Power Modules

Ag sinter joining as a promising alternative die attach material have received more and more attention for next generation wide band gap (WBG) power modules. Here we report a robust joint by Ag sinter joining technology for different metal interface (Au, Ag, Ni, Cu, Al) in WBG power modules. Micron-scale Ag flakes were used as the Ag fillers. Sinter Ag joint for different metal interface was implemented by a sintering process under temperatures of 250 °C in air without pressure. The die shear strength of all the joint structures was larger than 25 MPa and the sinter Ag on the Ag metallized substrate shows the maximum value about 40 MPa. The die shear strength was larger than the value of traditional Sn–Pb solders (19~24MPa). Bonding mechanism for Ag sinter joining for different metal interface also was analyzed by SEM, EDS, and TEM observation. This study will add to the understanding of Ag sinter joining and expand its application in various power electric device interconnects.

Derivation of novel creep-integrated fatigue equations

This manuscript presents a systematic derivation of novel creep-integrated fatigue equations that possess the following unique characteristics: (i) creep damage and creep-fatigue interaction damage are embedded into the fatigue equation and requires no deliberate computation; (ii) the embedded creep damage is formulated from the constitutive equation of creep; and (iii) the integrated equation is capable of modelling the full range of creep fatigue from pure creep to pure fatigue. These characteristics ensure that these creep-integrated fatigue equations are fundamentally sound, easy to use, and applicable over a wide range of creep fatigue conditions. The creep-integrated fatigue equations based on three constitutive equations of steady-state creep: Dorn equation, Larson-Miller equation, and Manson-Haferd equation are derived and successfully validated using two metals of vastly different creep and fatigue characteristics – the SS316 stainless steel and the Sn37Pb solder.

Fabrication of Novel Printable Electrically Conductive Adhesives (ECAs) with Excellent Conductivity and Stability Enhanced by the Addition of Polyaniline Nanoparticles.

Electrically conductive adhesives (ECAs) are one of the low temperature bonding materials. It can be used to replace toxic Sn-Pb solder. The key issue for the application of ECAs is how to improve their electrical properties. In the present study, we develop an effective method to promote the electrical properties of ECAs by addition of polyaniline (PANI) nanoparticles. PANIs were synthesized via a facile one-step chemical oxidative polymerization method. After adding 0.5 wt% PANI nanoparticles, the conductivity of ECAs increased dramatically by an order of magnitude. The bulk resistivity of 8.8 × 10−5 Ω·cm is achieved for 65 wt% silver fillers with 0.5 wt% PANIs loaded ECAs. Besides, this improvement has no negative effect on the shear strength and the aging life of ECAs. Moreover, the use of PANIs not only lowers the percolation threshold of ECAs, but also reduces the cost and improves the bonding reliability. Finally, PANIs enhanced ECAs patterns were successfully printed by a stencil printing method, which proved their potential applications in replacing conventional solder pastes and printing functional circuits.

Interconnect Fatigue Failure Parameter Isolation for Power Device Reliability Prediction in Alternative Accelerated Mechanical Cycling Test

In this work, a rapid and low-cost accelerated reliability test methodology which was designed to simulate mechanical stresses induced in flip–chip bonded devices during the thermal cycling reliability test under isothermal conditions, is introduced and demonstrated using power device analogous test chips. By stressing these devices in a controlled environment, mechanical stresses become decoupled from the design and temperature, such that useful lifetimes can be predictable. Mechanical shear stress was cyclically applied directly to device relevant, flip–chip solder interconnects while monitoring for failure. Herein, finite element analysis (FEA) is used to extract various damage metrics of different solder materials, including PbSn37/63, SAC305, and nanosilver, in both thermal operation and the introduced alternative mechanical testing conditions. Plastic work density and strain are calculated in the critical solder interconnects as factors that indicate the amount of the damage accumulation per cycle during the mechanical cycling, thermal cycling, and power cycling tests. The number of cycles to failure for each test was calculated using the fatigue life model developed by Darveaux for eutectic PbSn solder, while for SAC305 Syed's method was used, and for nanosilver, the Knoerr et al. equations are applied. The effects of environmental temperature and shearing force frequency were studied for the mechanical cycling reliability test, where a modified Norris–Landzberg equation for mechanical cycling tests was explored using the simulation results. Finally, comparing the mechanical cycling with the equivalent thermal cycling and power cycling demonstrated a significant reduction in required test duration to achieve a reliability estimation.

Bonding Strengths and Thermal Degradation of Photovoltaic Module Ribbon Solder Joints

In this study, solar ribbon solder joints were investigated to ensure the reliability of photovoltaic (PV) modules. Ribbon joints comprising two different solder compositions (wt. %: 60Sn40Pb, 62Sn36Pb2Ag) were used to perform thermal aging tests at three different temperatures (150 °C, 120 °C, and 90 °C) during a 1000-h period to analyze the resultant thermal degradation; shear tests were also performed to measure bond strengths. Initial bond strengths were 271.3 MPa and 241.3 MPa for 62Sn36Pb2Ag and 60Sn40Pb solder joints, respectively. Bonding strength decreased as a result of thermal aging, but was maintained at a value of around 130 MPa regardless of solder composition. Fracture surfaces from the shear test were observed to analyze for the constant bond strength phenomenon. It was verified that the shear fracture surface changed from the solder/sintered Ag interface and the sintered silver (Ag)/silicon (Si) wafer interface during thermal aging. Thus, the bond strengths and bonding characteristics of PV ribbon solder joints decreased under a thermal load, which could be attributed to a weakening of the bonding characteristics for sintered Ag silicon interfaces as opposed to a degradation of solder metallurgy.

Investigation of the interfacial reactions and growth behavior of interfacial intermetallic compound between Sn37Pb solder and Au/Ni/Kovar substrate

The interfacial reactions and microstructural evolution of Sn37Pb/Kovar with Au/Ni platings solder joints were investigated during isothermal aging at 120 °C, 150 °C and 170 °C for different duration, respectively. The experimental results indicated that a thin interfacial intermetallic compounds (IMCs) layer formed at the Sn37Pb/Kovar interface after reflowing process. The accurate compositions of thin IMCs were double-layers of (Ni, Au)3Sn4 and AuSn4, which were confirmed by utilizing TEM-EDS analysis. However, during the aging process, the compositions of interfacial IMCs of solder joint obviously changed, which transformed from AuSn4/(Ni, Au)3Sn4 to (Au, Ni)Sn4/Ni3Sn4 and the AuSn4 IMC formed in the bulk solder gradually changed to the (Au, Ni)Sn4 IMC. Furthermore, the total IMC thickness increased dramatically with increasing isothermal aging time, as well as aging temperature. The growth process of IMC layer is mainly dominated by the volume diffusion mechanism during the solid-state aging. The square of growth rate constant (K) equals to 2.559 × 10−17, 1.307 × 10−16 and 7.781 × 10−16 m2s−1 for the aging temperature of 120 °C, 150 °C and 170 °C, respectively. The activation energy (Q) value of the Sn37Pb/Kovar couple was calculated to be approximate 95.96 kJ·mol−1.

Influence of thermal shock cycles on Sn-37Pb solder bumps

PurposeWith continuous concerning on the toxic of element Pb, Pb-free solder was gradually used to replace traditional Sn-Pb solder. However, during the transition period from Sn-Pb to Pb-free solder, mixing of Sn-Pb and Pb-free is inevitable occurred in certain products, and in China where Sn-Pb solder was still used extensively in certain areas especially. Correspondingly, understanding reliability of Sn-Pb solder joints was very important, and further studies were needed.Design/methodology/approachThermal shock test between −55°C and 125 °C was conducted on Sn-37Pb solder bumps in the BGA package to investigate the microstructure evolution and the growth mechanism of interfacial intermetallic compound (IMC) layer. The effects of thermal shock on the mechanical property and fracture behavior of Sn-37Pb solder bumps were discussed.FindingsPb-rich phase was coarsened and voids were increased at first; Pb-rich phase was refined and voids were decreased secondly with the increase of thermal shock cycles; the shear strength of solder bumps was slightly decreased after thermal shock, but was back up to 73.67MPa at 2,000 cycles; interfacial IMCs of solder bumps was from typical scallop-type into smooth, the composition of IMCs was from Cu6Sn5 into Cu6Sn5 and Cu3Sn after thermal shock with 1,500 and 2,000 cycles; 20.0 per cent of solder bumps at 1,500 cycles and 9.5 per cent of solder bumps at 2,000 cycles were failure respectively.Originality/valueCompared with the board level test method, the impact shear test for the single solder bump is more convenient and economical and is actively pursued by the industries. The shear strength of solder bumps was slightly decreased after thermal shock, but was back up to 73.67 MPa at 2,000 cycles; 20.0 per cent of solder bumps at 1,500 cycles and 9.5 per cent of solder bumps at 2,000 cycles were failure.