Study of intermetallic compound layer formation, growth and evaluation of shear strength of lead-free solder joints

Solder joints play a very important role in electronic products as the integrity of electronics packaging and assembly rests on the quality of these connections. The increasing demands for higher performance, lower cost, and miniaturisation in hand-held and consumer electronic products have led to the use of dense interconnections. This miniaturization trend means that solder joint reliability remains an important challenge with surface mount electronics assembly, especially those used in hostile environments, and applications such as automobile, aerospace and other safety critical operations. One of the most important factors which are known to affect solder joint reliability is the thickness of intermetallic compound (IMC) layer formed between the solder and the substrate. Although the formation of an IMC layer signifies good bonding between the solder and substrate, its main disadvantage is that it is also known to be the most brittle part of the solder joint. Thus as the miniaturisation trend continues, and solder joints become even smaller in size, the nature and impact of IMC layer thickness on solder joint reliability becomes even more of a concern with the introduction of new lead-free soldering. Other factors which are known to affect solder joint reliability include the bonding strength, the voiding percentage in joints, the size of the voids and their location within the joint. The work reported in this thesis on formation and growth of intermetallic compound layer, and evaluation of the shear strength of lead-free solder joints is divided into four main parts. The first part of the study is concerned with understanding of the effect of pad sizes on Inter-metallic compound layer formation and growth for lead-free solder joints. The second part concerns the study of the effect of temperature cycling and reflow profiles on intermetallic growth between Sn-Ag-Cu alloy and Cu substrate. The third part of the study concerns the investigation of the effect of reflow soldering profile optimization on solder volumes using design of experiment technique. The focus of the final part of the study is the investigation of the effect of Inter-metallic Compound thickness on shear strength of 1206 surface mount chip resistor. The results from the experimental work showed that the pad size has very little influence on the growth of the IMC. The result also shows that the growth of IMC depends on diffusion rate, temperature and time according to the power-law model; and that the IMC layer thickness is independent of pad size. The significance of this result is that with further reductions in joint size (with IMC layer thickness remaining the same), the ratio of the IMC layer thickness to solder joint size will increase and adversely impact the joint reliability. The work carried out on ageing temperatures and reflow profiles of Sn-Ag-Cu alloy and Cu substrate also showed the reaction-diffusion mechanism of intermetallic compound formation and growth in solder joints. The study also showed that the most significant factor in achieving lower IMC layer thickness and fine microstructures is the time to peak temperature of the reflow soldering process. The effect of IMC layer thickness on the shear strength of Sn-Ag-Cu solder joints was investigated. The relationship of shear strength, interfacial microstructures and fracture surfaces was considered. It is clear that formation of continuous Cu-Sn and SnNiCu layers are the reason for the weak interface strength. The results show that the shear strength of solder joints decreases with increasing ageing time. The results of this study have been disseminated through journal and conference publications and will be of interest to R&D personnel working in the area of high temperature electronics and in particular those working in the field of automotive electronics

A Study on Process, Strength and Microstructure Analysis of Low Temperature SnBi-Containing Solder Pastes Mixed with Lead-free Solder Balls

As the traditional eutectic SnPb solder alloy has been outlawed, the electronic industry has almost completely transitioned to the lead-free solder alloys. The conventional SAC305 solder alloy used in lead-free electronic assembly has a high melting and processing temperature with a typical peak reflow temperature of 245ᵒC which is almost 30ᵒC higher than traditional eutectic SnPb reflow profile. Some of the drawbacks of this high melting and processing temperatures are yield loss due to component warpage which has an impact on solder joint formation like bridging, open defects, head on pillow, and other drawbacks which include circuit board degradation, economic and environmental factors, and brittle failure defects in the circuit board like pad cratering. To overcome this, a detailed study has been carried out on low temperature lead-free solder paste that utilizes Bi bearing alloys. Three low temperature lead-free solder pastes, Sn-58Bi, Sn-57Bi-1Ag and Sn-40Bi-Cu-Ni with the melting temperatures of 138ᵒC (which is 45ᵒC below eutectic SnPb and 79ᵒC below SAC) were printed on Cu-OSP finish test boards. These pastes were then assembled with SAC305, Sn99CN and Sn100C solder spheres. The range of Bi concentrations for various mixtures used in this study was calculated to be in the range of 2 to 4 wt%. The mixtures were reflowed under two different low temperatures reflow profiles; (a) a traditional SnPb profile with a peak temperature 217ᵒC and (b) a low temperature SnBi profile with a peak temperature 177ᵒC (recommended by the paste manufacturer). After the assembly process, the mixed solder joints were shear tested to study the failure modes and shear strength at rate of 27.50mils/sec. Cross sectioning was performed to evaluate the possible microstructural changes at room temperature and after aging conditions that may have led to the changes in failure mode observed in shear testing. The isothermal aging condition used in the study is 125ᵒC for 200 hours, which mimics 21 years of field storage at 25ᵒC degrees using Arrhenius extrapolation for Cu6Sn5 intermetallic formation. Our study suggests that high temperature reflow profile (217ᵒC peak profile) had better mechanical strength than the low temperature reflow profile (177ᵒC peak profile). A metallurgical explanation for the improvement is presented in this paper. Thus, this paper describes that by generating a robust reflow assembly process for SnBi solder paste, the shear strength can be increased, cost of manufacturing can be reduced and high temperature assembly process (SAC) issues can be minimized which may improve product yield in production

MICROSTRUCTURAL CHANGES UNDER ISOTHERMAL AGING AND THEIR INFLUENCE ON THERMAL FATIGUE RELIABILITY FOR TIN-LEAD AND LEAD-FREE SOLDER JOINTS, INCLUDING MICROSTRUCTURAL CHANGES UNDER ISOTHERMAL AGING IN MIXED SOLDER JOINTS

Most electronics companies have transitioned to lead-free processes, both to comply with government legislation and to avoid issues related to mixing of tin-lead and lead-free metallurgies. However, exemptions from lead-free legislation have been granted for certain products, especially those intended for high-reliability applications. One major concern with these exempt products is that, during assembly or rework, lead-free components will have to be used due to the unavailability of tin-lead components. This will result in the mixing of tin-lead and lead-free metallurgies. The mixing of metallurgies can induce new reliability concerns. This study is focused on mixed solder joints formed by attaching lead-free components with tin-lead paste. Solder interconnect reliability is influenced by the environmental imposed load, solder material properties and the microstructure formed between the solder and the metal surfaces to which the solder is bonded. Several lead-free metallurgies are being used for component terminals, board pad plating and solder materials. These metallurgies react to form the microstructure of a solder joint. Microstructure of a solder joint continuously evolves and affects solder joint properties. A fundamental understanding on the microstructure is required to analyze the changes occurring in a solder joint with time and temperature and make predictions on solder joint reliability under thermal loading conditions. This dissertation determines key microstructural features present in SnPb, lead-free and mixed solder joints. Changes in the microstructural features were determined for SnPb, lead-free and mixed solder joints exposed to isothermal aging conditions. The effect of microstructural changes on reliability was determined by conducting thermal fatigue reliability tests for SnPb and lead-free solder joints. Whereas, for mixed solder joints, hypotheses has been determined based on microstructural analysis on their thermal fatigue performance compared to SnPb joints. This dissertation doesn't include the effect of microstructural changes on the reliability of mixed solder joints. This dissertation doesn't include the reliability tests for mixed solder joints. Two microstructural features namely, intermetallic compounds (IMC) and Pb phase were characterized for SnPb, lead-free and mixed solder joints. IMCs are formed at the solder to pad metallization interface and in the bulk solder. It was determined that reaction between Sn3.0Ag0.5Cu solder and Ni/Au component side metallization result in interfacial IMCs consisting of Ni3Sn4 IMC in the as-reflowed stage and IMCs such as (NiCu)3Sn4, (Cu,Ni)6Sn5 and (Au,Ni)Sn4 after thermal aging of 350 hours at 125ºC. With pad metallization of ImAg, ImSn and OSP, IMCs such as Cu6Sn5 are formed after reflow followed by formation of a new Cu3Sn IMC phase after thermal aging of 350 hours at 125ºC. Cu6Sn5 and Ag3Sn IMC were found distributed in bulk solder joints in the as-reflowed and aged (125ºC for 100, 350 and 1000 hrs) solder joint. This dissertation demonstrated that under thermal cycling, intergranular crack propagates between Sn grains in the bulk solder and Cu6Sn5 IMCs present at Sn grain boundaries in the bulk solder influence crack propagation. It was demonstrated that isothermal aging for 350 hrs at 125ºC causes coarsening of Cu6Sn5 IMC particles in the bulk solder which results in a 50% reduction in number of Cu6Sn5 IMC particles in the bulk solder, thus promoting the crack to propagate faster along the grain boundary. This dissertation determined that isothermal aging for 350 hrs at 125ºC would cause a 25% reduction in characteristic life for lead-free solder joints due to the changes associated with Cu6Sn5 IMC particles. In conventional SnPb solder joints Pb phase present in the bulk solder coarsens as a function of time and temperature and influences thermal fatigue reliability. Due to the presence of Pb in mixed solder joint, this dissertation determined the extent of coarsening in mixed solder joints compared with SnPb joints. It was determined that mixed solder joints are not prone to Pb phase coarsening under aging for 350 hrs at 125ºC as opposed to SnPb solder joints and therefore would have better thermal fatigue performance compared to SnPb joint under these conditions. This dissertation demonstrated that the presence of Pb in mixed solder results in a 30 to 40% lower IMC thickness compared to Pb-free and SnPb solder joints by being present at the interface as a diffusion barrier between Ni and Sn for IMC formation. Presence of Pb has been known to act as diffusion barrier for SnPb solder joints

ISPET: Interface Sintering Process Enhanced Technology

The research presented in this thesis was carried out in VISHAY Semiconductor Italiana S.P.A. at Borgaro Torinese - Italy. The framework of this thesis is the study of new materials for power electronics application, analysing their thermal, mechanical and electrical properties. Emerging application of high power systems requires new methods for power electronics integration and packaging. Stringent requirements in size and weight, reliability, durability, ambient and operation temperatures are pushing to go beyond the limits in industrial applications. As a consequence, our studies are focused on power modules, incorporating new materials and technology processes (sintering) for dies or chips (silicon), substrates and interconnection materials (wire bonding). This thesis work starts introducing the power semiconductor devices used in power electronics and their integration on Power Integrated Circuits (low and medium power density) and Power Modules (medium, high and very high power density). This chapter will explain technology evolution, power semiconductor device utilization mode and some applications. Chapter 2 will be focused on power modules packages. They have an important role for providing cooling, electrical connection and correct insulation, between the internal semiconductor devices and the external circuit. Isolated and non isolated packages are analysed and compared. Chapter 3 will make a point on the methods of thermal characterization and reliability tests, that were implemented to evaluate the impact of the introduction of new materials and processes into the device. In chapter 4, first experimental results, related to the sintering process will be discussed. In this chapter the attention will be focused on the Chip to substrate Joint of the device, analysing methods to mechanically fix die to substrate. The sintering process will be treated, analysing the process and the results will be thermally and mechanically characterized. The chapter 5 will present the experimental part oriented to the combinations of materials to produce a better heavy wire bonding, supported by a Design of Experiments (DOE). The behaviour of didifferent wires will be compared through thermal characterization methods and reliability test

ISOTHERMAL MECHANICAL AND THERMO-MECHANICAL DURABILITY CHARACTERIZATION OF SELECTED PB-FREE SOLDERS

Due to the hazards of Pb in the environment and its effect on humans and marketing competition from Japanese electronics manufacturers, the conversion to Pb-free solders in the electronics industry appears imminent. As major mechanical, thermal, and electrical interconnects between the component and the PWB, solder joints are crucial for the reliability of the most electronic packages. There is an urgent need for constitutive properties, mechanical durability and thermo-mechanical durability of Pb-free solders. A partitioned constitutive model consisting of elastic, plastic, primary creep and secondary creep models is obtained for the Sn3.9Ag0.6Cu solder and the baseline Sn37Pb solder from comprehensive monotonic and creep tests conducted on Thermo-Mechanical-Microscale (TMM) setup. The comparison between two solders shows that Sn3.9Ag0.6Cu has much better creep resistance than Sn37Pb at the low and medium stresses. The isothermal mechanical durability of three NEMI recommended Pb-free solders, Sn3.9Ag0.6Cu, Sn3.5Ag, Sn0.7Cu, is tested on the TMM setup under low creep and high creep test conditions. The damage propagation rate is also analyzed from the test data. The generic Energy-Partitioning (E-P) durability model is obtained for three Pb-free solders by using the incremental analytic model developed for TMM tests. The scatter of the test results from the prediction by these E-P durability model constants is small. The thermo-mechanical durability of the Pb-free Sn3.8Ag0.7Cu solder is investigated by a systematic approach combining comprehensive thermal cycling tests and finite element modeling. The effects of mixed solder systems, device types, and underfill are addressed in the tests. Thermal cycling results show that Sn3.8Ag0.7Cu marginally outperforms SnPb for four different components under the studied test condition. The extensive detailed three-dimensional viscoplastic FE stress and damage analysis is conducted for five different thermal cycling tests of both Sn3.8Ag0.7Cu and Sn37Pb solders. Power law thermo-mechanical durability models of both Sn3.8Ag0.7Cu and Sn3Pb are obtained from thermal cycling test data and stress and damage analysis. The energy-partitioning durability models of two solders are also obtained. It is found that the slopes of the plastic and creep curves in the E-P damage model of Pb-free solders for thermal cycling are steeper than those for mechanical cycling and those of Sn37Pb solders

Effect of Palladium Thickness and Extended Isothermal Aging on the Reliability of Solder Interconnects Formed on ENEPIG Surface Finish

Surface finishes for copper on printed wiring boards play an important role in the reliability of electrical interconnects. Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG), developed in the mid-1990s to alleviate the "black-pad" problem created by Electroless Nickel/Immersion Gold (ENIG) surface finish, has gained interest for critical system applications. This thesis investigates the effect of palladium layer thickness and extended isothermal aging on the reliability of both tin-lead and tin silver copper solder interconnects under temperature cycling, vibration cycling, and drop loading conditions. Chip array ball grid array (CABGA) packages soldered onto ENEPIG-finished PCBs are subjected to the three previously listed conditions. Reliability and failure analyses are conducted to determine the overall effect of palladium layer thickness and isothermal aging on the reliability of these solder interconnects

MICROSTRUCTURAL CHARACTERIZATION AND THERMAL CYCLING RELIABILITY OF SOLDERS UNDER ISOTHERMAL AGING AND ELECTRICAL CURRENT

Solder joints on printed circuit boards provide electrical and mechanical connections between electronic devices and metallized patterns on boards. These solder joints are often the cause of failure in electronic packages. Solders age under storage and operational life conditions, which can include temperature, mechanical loads, and electrical current. Aging occurring at a constant temperature is called isothermal aging. Isothermal aging leads to coarsening of the bulk microstructure and increased interfacial intermetallic compounds at the solder-pad interface. The coarsening of the solder bulk degrades the creep properties of solders, whereas the voiding and brittleness of interfacial intermetallic compounds leads to mechanical weakness of the solder joint. Industry guidelines on solder interconnect reliability test methods recommend preconditioning the solder assemblies by isothermal aging before conducting reliability tests. The guidelines assume that isothermal aging simulates a "reasonable use period," but do not relate the isothermal aging levels with specific use conditions. Studies on the effect of isothermal aging on the thermal cycling reliability of tin-lead and tin-silver-copper solders are limited in scope, and results have been contradictory. The effect of electrical current on solder joints has been has mostly focused on current densities above 104A/cm2 with high ambient temperature (≥100oC), where electromigration, thermomigration, and Joule heating are the dominant failure mechanisms. The effect of current density below 104A/cm2 on temperature cycling fatigue of solders has not been established. This research provides the relation between isothermal aging and the thermal cycling reliability of select Sn-based solders. The Sn-based solders with 3%, 1%, and 0% silver content that have replaced tin-lead are studied and compared against tin-lead solder. The activation energy and growth exponents of the Arrhenius model for the intermetallic growth in the solders are provided. An aging metric to quantify the aging of solder joints, in terms of phase size in the solder bulk and interfacial intermetallic compound thickness at the solder-pad interface, is established. Based on the findings of thermal cycling tests on aged solder assemblies, recommendations are made for isothermal aging of solders before thermal cycling tests. Additionally, the effect of active electrical current at 103 A/cm2 on thermal cycling reliability is reported

IMPACT OF DUST ON THE RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES

Dust is a ubiquitous component of the environments in which we live and work. It can deposit on printed circuit assembly to act as a source of ionic contamination. Two common consequences of dust contaminations in the printed circuit boards are loss of impedance (i.e., loss of surface insulation resistance) and electrochemical migration between traces and component leads. Both failure mechanisms involve the contamination forming a current leakage path on a printed circuit board. Based on studies on ionic contaminations, researchers have argued that the impact of dust in these two failure mechanisms is dependent on its pH, its hygroscopic compositions, and the critical relative humidity of the salts in it. However, due to the lack of experimental results and the complexity of dust compositions, the argument is not substantiated. Very few papers concerning the impact of different natural dusts on these two failure mechanisms can be found in the literature. In practice, mixtures of Arizona dust and salts are used as a substitute for dust in experiments. In this research, natural dusts were collected from four locations: natural outdoor and indoor dust samples from Massachusetts, U.S., natural outdoor dust from Tianjin, China, and the ISO standard test dust (Arizona test dust). Loss of impedance in dust contaminated printed circuit boards was investigated under controlled temperature (20ºC to 60ºC) and relative humidity (50% to 95%) ranges. The impact of dust on electrochemical migration and corrosion was evaluated under temperature-humidity-bias tests (50ºC, 90% RH, and 10 VDC). In addition to the conventional DC measurement where only resistive data can be obtained, electrochemical impedance spectroscopy were adopted to obtain nonlinear equivalent circuit models of the electrochemical process, which helps to understand the underlying physics-of-failure. The variation of impedance with relative humidity exhibited a transition range. Below the range, the impedance was constant, and above it, the impedance degraded by orders of magnitude. The value of the transition range decreased with an increase of dust deposition density. The equivalent circuit modeling showed that the dominant resistive path gradually shifted from the bulk to the interfacial with the increase of temperature from 20 ºC to 60 ºC. There were big variations among different dusts, which were quantified using the degradation factor introduced in the research, the critical transition range, and time-to-failure. This result demonstrated that a single salt or a mixture of compounds can not be representative of all dusts. It also indicated that using the ISO standard test dust in place of natural dust samples for reliability evaluation could lead to inaccurate results. Dust should be collected from the field in order to evaluate its impact. It is showed in this thesis that some critical characteristics of dust can be used to classify different dusts for the failure mechanisms of interest. Moisture sorption capability of dust can be used to classify different dusts regarding the loss of impedance failure. The dust with the highest moisture sorption capability had the highest degradation factor. Ion species/concentration or conductivity of dust aqueous solution can be used to classify dust regarding the electrochemical migration related failures. Dust with the highest ion concentration and conductivity had the lowest time-to-failure. The underlying principals behind those critical characteristics were described and discussed based on the physics-of-failure

Thermo-mechanical reliability studies of lead-free solder interconnects

N/ASolder interconnections, also known as solder joints, are the weakest link in electronics packaging. Reliability of these miniature joints is of utmost interest - especially in safety-critical applications in the automotive, medical, aerospace, power grid and oil and drilling sectors. Studies have shown that these joints' critical thermal and mechanical loading culminate in accelerated creep, fatigue, and a combination of these joints' induced failures. The ball grid array (BGA) components being an integral part of many electronic modules functioning in mission-critical systems. This study investigates the response of solder joints in BGA to crucial reliability influencing parameters derived from creep, visco-plastic and fatigue damage of the joints. These are the plastic strain, shear strain, plastic shear strain, creep energy density, strain energy density, deformation, equivalent (Von-Mises) stress etc. The parameters' obtained magnitudes are inputted into established life prediction models – Coffin-Manson, Engelmaier, Solomon (Low cycle fatigue) and Syed (Accumulated creep energy density) – to determine several BGA assemblies' fatigue lives. The joints are subjected to thermal, mechanical and random vibration loadings. The finite element analysis (FEA) is employed in a commercial software package to model and simulate the responses of the solder joints of the representative assemblies' finite element models. As the magnitude and rate of degradation of solder joints in the BGA significantly depend on the composition of the solder alloys used to assembly the BGA on the printed circuit board, this research studies the response of various mainstream lead-free Sn-Ag-Cu (SAC) solders (SAC305, SAC387, SAC396 and SAC405) and benchmarked those with lead-based eutectic solder (Sn63Pb37). In the creep response study, the effects of thermal ageing and temperature cycling on these solder alloys' behaviours are explored. The results show superior creep properties for SAC405 and SAC396 lead-free solder alloys. The lead-free SAC405 solder joint is the most effective solder under thermal cycling condition, and the SAC396 solder joint is the most effective solder under isothermal ageing operation. The finding shows that SAC405 and SAC396 solders accumulated the minimum magnitudes of stress, strain rate, deformation rate and strain energy density than any other solder considered in this study. The hysteresis loops show that lead-free SAC405 has the lowest dissipated energy per cycle. Thus the highest fatigue life, followed by eutectic lead-based Sn63Pb37 solder. The solder with the highest dissipated energy per cycle was lead-free SAC305, SAC387 and SAC396 solder alloys. In the thermal fatigue life prediction research, four different lead-free (SAC305, SAC387, SAC396 and SAC405) and one eutectic lead-based (Sn63Pb37) solder alloys are defined against their thermal fatigue lives (TFLs) to predict their mean-time-to-failure for preventive maintenance advice. Five finite elements (FE) models of the assemblies of the BGAs with the different solder alloy compositions and properties are created with SolidWorks. The models are subjected to standard IEC 60749-25 temperature cycling in ANSYS 19.0 mechanical package environment. SAC405 joints have the highest predicted TFL of circa 13.2 years, while SAC387 joints have the least life of circa 1.4 years. The predicted lives are inversely proportional to the magnitude of the areas of stress-strain hysteresis loops of the solder joints. The prediction models are significantly consistent in predicted magnitudes across the solder joints irrespective of the damage parameters used. Several failure modes drive solder joints and damage mechanics from the research and understand an essential variation in the models' predicted values. This investigation presents a method of managing preventive maintenance time of BGA electronic components in mission-critical systems. It recommends developing a novel life prediction model based on a combination of the damage parameters for enhanced prediction. The FEA random vibration simulation test results showed that different solder alloys have a comparable performance during random vibration testing. The fatigue life result shows that SAC405 and SAC396 have the highest fatigue lives before being prone to failure. As a result of the FEA simulation outcomes with the application of Coffin-Manson's empirical formula, the author can predict the fatigue life of solder joint alloys to a higher degree of accuracy of average ~93% in an actual service environment such as the one experienced under-the-hood of an automobile and aerospace. Therefore, it is concluded that the combination of FEA simulation and empirical formulas employed in this study could be used in the computation and prediction of the fatigue life of solder joint alloys when subjected to random vibration. Based on the thermal and mechanical responses of lead-free SAC405 and SAC396 solder alloys, they are recommended as a suitable replacement of lead-based eutectic Sn63Pb37 solder alloy for improved device thermo-mechanical operations when subjected to random vibration (non-deterministic vibration). The FEA simulation studies' outcomes are validated using experimental and analytical-based reviews in published and peer-reviewed literature.N/