Analysis of microstructure and mechanical properties of bismuth-doped SAC305 lead-free solder alloy at high temperature

SAC305 lead-free solder alloy is widely used in the electronic industry. However, the problems associated with the growth formation of intermetallic compounds need further research, especially at high temperatures. This study investigates the doping of Bismuth into SAC305 in the various compositions of 1, 2, and 3 wt.%. The microstructure in terms of intermetallic compound particles and mechanical properties was examined after thermal aging at temperatures of 100 °C and 200 °C for 60 h. The microstructure examination was observed using scanning electron microscopy, and the chemical composition of each alloy was confirmed with an energy dispersive X-ray. Tensile tests were performed to find the mechanical properties such as yield strength and ultimate tensile strength. The intermetallic compound’s phase analysis was identified using X-ray diffraction, and differential scanning calorimetry was done to study the temperature curves for melting points. Results showed that the addition of Bismuth refined the microstructure by suppressing the growth of intermetallic compounds, which subsequently improved the mechanical properties. The thermal aging made the microstructure coarsen and degraded the mechanical properties. However, the most improved performance was observed with a Bismuth addition of 3 wt.% into SAC305. Furthermore, a decrease in the melting temperature was observed, especially at Bismuth compositions of 3 wt.%

Isothermal Aging of Low-Ag SAC with Al Addition

AbstractZinc sulfide (ZnS) thin films have been successfully deposited via spray pyrolysis using an aqueous solution of thiourea and zinc Amongst the candidates of lead-free solders, eutectic or near eutectic Sn-Ag-Cu or SAC has been found to be the most suitable candidate to replace the traditional tin-lead (Sn-Pb) solder. However, due to the higher cost of SAC (expensive Ag), research have now been directed to find a lower cost lead-free solder with comparable or better performance to the Sn-Pb or SAC, i.e low Ag SAC or Sn-Cu solders. The solder alloys developed in this work were SAC 0305 (0.3 wt% Ag, 0.5 wt% Cu and bal. Sn) and SAC0305 with 1 wt% Al and 2 wt% Al. The addition of Al is expected to refine the β-Sn grains and contribute to higher solder strength. Characterization of the solder alloys focused on the bulk solder microstructure, intermetallic compound (IMC) evaluation and wettability of solder alloys in reflowed and aged conditions. Reflow temperature was 270°C while aging was done isothermally in normal atmosphere for 100, 200 and 500hours at 100°C and 150°C. Microstructure of bulk solder and the IMC formed at interface between solder and Cu substrate were observed using SEM equipped with EDX. Wettability of solder was evaluated via wetting force and time, and the wetting angle between solder and the Cu substrate. A low Ag content resulted in fine Ag3Sn IMC as compared to much larger needle Ag3Sn observed in the commercial SAC 305 (Ag 3.0 wt%). With Al addition, SEM result showed finer β-Sn dendrites with fine Ag3Sn and Cu6Sn5 distributed within the eutectic colony of the bulk solder, and also thinner IMC layer in reflowed samples. Isothermally aged samples on the other hand, showed thicker IMC layer for Al added solder samples. This could be attributed to the finer β-Sn dendrites in the bulk solder providing more diffusion path and increased the thickness of IMC layer. The IMC formed at the interface between solder and the copper substrate was identified as Cu6Sn5 in reflowed samples, and both Cu6Sn5 and Cu3Sn when aged. Wettability of the solder alloys decreased with the addition of Al

A review: microstructure and properties of tin-silver-copper lead-free solder series for the applications of electronics

Purpose The research on lead-free solder alloys has increased in past decades due to awareness of the environmental impact of lead contents in soldering alloys. This has led to the introduction and development of different grades of lead-free solder alloys in the global market. Tin-silver-copper is a lead-free alloy which has been acknowledged by different consortia as a good alternative to conventional tin-lead alloy. The purpose of this paper is to provide comprehensive knowledge about the tin-silver-copper series. Design/methodology/approach The approach of this study reviews the microstructure and some other properties of tin-silver-copper series after the addition of indium, titanium, iron, zinc, zirconium, bismuth, nickel, antimony, gallium, aluminium, cerium, lanthanum, yttrium, erbium, praseodymium, neodymium, ytterbium, nanoparticles of nickel, cobalt, silicon carbide, aluminium oxide, zinc oxide, titanium dioxide, cerium oxide, zirconium oxide and titanium diboride, as well as carbon nanotubes, nickel-coated carbon nanotubes, single-walled carbon nanotubes and graphene-nano-sheets. Findings The current paper presents a comprehensive review of the tin-silver-copper solder series with possible solutions for improving their microstructure, melting point, mechanical properties and wettability through the addition of different elements/nanoparticles and other materials. Originality/value This paper summarises the useful findings of the tin-silver-copper series comprehensively. This information will assist in future work for the design and development of novel lead-free solder alloys

A review: microstructure and properties of tin-silver-copper lead-free solder series for the applications of electronics

Purpose: The research on lead-free solder alloys has increased in past decades due to awareness of the environmental impact of lead contents in soldering alloys. This has led to the introduction and development of different grades of lead-free solder alloys in the global market. Tin-silver-copper is a lead-free alloy which has been acknowledged by different consortia as a good alternative to conventional tin-lead alloy. The purpose of this paper is to provide comprehensive knowledge about the tin-silver-copper series. Design/methodology/approach: The approach of this study reviews the microstructure and some other properties of tin-silver-copper series after the addition of indium, titanium, iron, zinc, zirconium, bismuth, nickel, antimony, gallium, aluminium, cerium, lanthanum, yttrium, erbium, praseodymium, neodymium, ytterbium, nanoparticles of nickel, cobalt, silicon carbide, aluminium oxide, zinc oxide, titanium dioxide, cerium oxide, zirconium oxide and titanium diboride, as well as carbon nanotubes, nickel-coated carbon nanotubes, single-walled carbon nanotubes and graphene-nano-sheets. Findings: The current paper presents a comprehensive review of the tin-silver-copper solder series with possible solutions for improving their microstructure, melting point, mechanical properties and wettability through the addition of different elements/nanoparticles and other materials. Originality/value: This paper summarises the useful findings of the tin-silver-copper series comprehensively. This information will assist in future work for the design and development of novel lead-free solder alloys. © 2019, Emerald Publishing Limited

An investigation into nano-particulates reinforced SAC305-based composite solders under electro- and thermo-migration conditions

With the rapid development in electronic packaging due to product miniaturisation, the size of solder joints is decreasing considerably, thus the failure of solder interconnects induced by electro-migration (EM) and thermo-migration (TM) became a reliability concern. The incorporation of foreign reinforcement can effectively improve properties of the solder alloys. However, this presents an imperative need for a further investigation to elaborate the underlying fundamentals associated with the reliability of reinforced solders. In this study, the Sn-Ag-Cu (SAC) based solder alloy powders as matrix were incorporated with Fullerene (FNS), TiC and Ni-coated graphene (NG) reinforcements to form composite solders through powder metallurgical method. These composite solders were then characterised in terms of their microstructure, physical property, solderability, followed by a systematic investigation of their performance under isothermal ageing, current stressing and large thermal gradient, respectively. The results showed that three types of reinforcements were successfully incorporated into the solder matrix; with all reinforcements added being embedded in the solder matrix or around the intermetallic compounds (IMC). The average loss of FNS and TiC particles in the solders was approximately 80% after the initial reflow, while this was only 40% for NG particles. It has been observed that β-Sn and Ag3Sn in the SAC solder alloys can be refined by adding appropriate amount of FNS and TiC, which is beneficial to the wettability with a reduced coefficient of thermal expansion (CTE) with the minimal influence on the melting point and electrical resistivity of solder alloys. For the SAC alloys without reinforcements, obvious extrusion of interfacial IMC at the anode was present after 360 hours of current (1.5×104 A/cm2) stressing, while the changes of surface profiles of all reinforced solders were unnoticeable. Under the current stressing regimes, a continuous increase of interfacial IMCs at the anode of the original SAC alloys was observed, but decreased at the cathode with stressing time. For the composite solders, both anode and cathode showed a continuous growth of interfacial IMCs; the growth rates of IMCs at the anode were greater than that at cathode. In addition, NG and TiC were found to be most effective to retard the growth of Cu3Sn IMC under current stressing. A gradient in hardness across the stressed SAC joints was present, where it was harder at anode. However, no such obvious gradient was found in SAC/FNS and SAC/NG solder joints. FNS and NG were proven to be beneficial to prolong the service life of solder joints up to approximately 7.6% and 10.4% improvements, respectively. Thermal stressing made the interfacial IMC in the original SAC joints to grow at the cold end considerably; causing serious damage at the hot end after 600 hours under temperature gradient of 1240K/cm stressing; a large number of IMCs, cracks and voids appeared in the SAC solder joints. However, a uniform increase of IMCs at both sides in the composite solders was observed without apparent damages at the interfaces under the same thermal stressing conditions, indicating an effective reduction of the elemental migration in the reinforced solders. Although there were also some voids and IMCs formed in the composite solder joints after a long-term thermal stressing, the integrity of the composite solder joints was enhanced compared with the SAC alloys. During thermal stressing, the dissolution rate of Cu atom into the SAC solder joints was estimated to be 3.1×10-6 g/h, while the values for SAC/FNS, SAC/NG and SAC/TiC were only 1.22×10-6 g/h, 1.09×10-6 g/h and 1.67×10-6 g/h, respectively

The durability of solder joints under thermo-mechanical loading; application to Sn-37Pb and Sn-3.8Ag-0.7Cu lead-free replacement alloy

Solder joints in electronic packages provide mechanical, electrical and thermal connections. Hence, their reliability is also a major concern to the electronic packaging industry. Ball Grid Arrays (BGAs) are a very common type of surface mount technology for electronic packaging. This work primarily addresses the thermo-mechanical durability of BGAs and is applied to the exemplar alloys; traditional leaded solder and a popular lead-free solder. Isothermal mechanical fatigue tests were carried out on 4-ball test specimens of the lead-free (Sn-3.8Ag-0.7Cu) and leaded (Sn-37Pb) solder under load control at room temperature, 35°C and 75°C. As well as this, a set of combined thermal and mechanical cycling tests were carried out, again under load control with the thermal cycles either at a different frequency from the mechanical cycles (not-in-phase) or at the same frequency (both in phase and out-of-phase). The microstructural evaluation of both alloys was investigated by carrying out a series of simulated ageing tests, coupled with detailed metallurgical analysis and hardness testing. The results were treated to produce stress-life, cyclic behaviour and creep curves for each of the test conditions. Careful calibration allowed the effects of substrate and grips to be accounted for and so a set of strain-life curves to be produced. These results were compared with other results from the literature taking into account the observations on microstructure made in the ageing tests. It is generally concluded that the TMF performance is better for the Sn-Ag-Cu alloy than for the Sn-Pb alloy, when expressed as stress-life curves. There is also a significant effect on temperature and phase for each of the alloys, the Sn-Ag-Cu being less susceptible to these effects. When expressed as strain life, the effects of temperature, phase and alloy type are much diminished. Many of these conclusions coincided with only parts of the literature and reasons for the remaining differences are advanced

Reliability Assessment Of Self-Alignment Assemblies Of Chip Component After Reflow Soldering Process

Reliability of surface mount components and interconnect are significant issues in electronic manufacturing. Although the reliability of devices has been broadly studied,here we are focusing on the reliability of the solder joint after the self-alignment phenomena during reflow soldering.In this study,the quality of the self-alignment assemblies was analyzed relate to the joint shear strength according to the JIS Z3 198-7 standard and the inspection according to IPC-A-610E standard.The results from reliability study indicate that the shear strength of the misalignment component of solder joints indeed depends on the degree of chip component misalignment.For shift mode configuration in the range of 0-300µm,the resulted chip assembly inspection after the reflow process was in line with the IPC-A-610E standard

Eutectic growth and microstructure selection in electronic solder alloys

This thesis develops the understanding of eutectic growth and microstructure selection in electronic solder alloys, including the Sn-Ni, Sn-Cu and Sn-Ag binary systems, and alloys from the ternary Sn-Ag-Cu system. These systems are relevant to Pb-free soldering and the development of optimised Pb-free solder compositions. Past research has reported the presence of both stable Sn-Ni3Sn4 eutectic and metastable Sn-NiSn4 eutectic in Sn-Ni alloys and Sn-3.5wt%Ag/Ni solder joints. The present work follows up on these initial findings with a detailed investigation into competitive growth between these stable and metastable eutectics. As no information was available for the metastable Sn-NiSn4 eutectic phase diagram, the eutectic points of the two eutectics and the Ni3Sn4 liquidus line were measured in this thesis. Controlled Bridgman solidification experiments were then used to explore eutectic growth mechanisms of the stable and metastable eutectics and investigate the origins of metastable eutectic formation in this system. The dynamics of eutectic growth and the transition between the two eutectics were investigated by synchrotron radiography, and the crystallography of eutectic growth was measured by EBSD. The competition between Sn dendrite growth and metastable Sn-NiSn4 eutectic growth was then measured and compiled into a microstructure selection map in the range of C0 = 0.05 wt%Ni – 0.26 wt%Ni and V = 0.5 µm/s – 1000 µm/s. Sn-Cu, Sn-Ag and Sn-Ag-Cu (SAC) alloys are commonly used as soldering materials and past researchers have focussed on microstructure selection maps in these systems. However, there has been limited research on the eutectic crystallography and growth mechanisms. In the present work, laboratory and synchrotron Bridgman solidification is used to observe the eutectic growth front and its response to changes in pulling rate, and to analyse the eutectic crystallography and growth mechanisms in Sn-Ag, Sn-Cu and Sn-Ag-Cu alloys. First, the binary Sn-Ag3Sn and Sn-Cu6Sn5 eutectics are investigated. The growth of the ternary Sn-Ag3Sn-Cu6Sn5 eutectic is then compared with the two binary eutectics. Next, research explores the more complex case of Sn-1Ag-0.9Cu (wt%) which lies on the Sn-Cu6Sn5 univariant eutectic groove where there is competition between Sn dendrite growth, univariant SnCu6Sn5 growth, and invariant ternary Sn-Ag3Sn-Cu6Sn5 growth. Orientation relationships between Sn and the intermetallic compounds (IMCs) and the IMC growth directions are determined in all systems. The experimental observations of the growth microstructure as a function of velocity and composition in the Sn-Ni, Sn-Ag and Sn-Cu systems were combined with literature data to construct experimental eutectic coupled zones for the three binary systems and for the Sn-Pb system. The transition between fully eutectic growth and Sn dendrite growth ahead of a eutectic front was considered using the criterion that the microstructure whose tips can grow at highest temperature is selected (i.e. wins). Since eutectic growth often did not occur at the extremum for these nonfaceted-faceted eutectics, the eutectic growth temperature versus growth velocity was measured for the Sn-NiSn4 and Sn-Ag3Sn eutectics in this thesis, and literature data for this relationship in the Sn-Pb system was used. To calculate the fully eutectic / Sn dendrite transition, a combination of the measured eutectic growth temperature response function (TE* vs. V) and Lipton, Glicksman and Kurz (LGK) calculated dendrite tip growth temperature response function (Td* vs. V) were used to calculate eutectic coupled zones. With this approach, reasonable agreement between the calculated and measured coupled zones was obtained.Open Acces

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/