The drop shock reliability of solder joints has become a major issue for the electronic industry partly because of the ever increasing popularity of portable electronics and partly due the transition to lead free solders. Most of the commonly recommended lead-free are high Sn alloys which have relatively higher strength and modulus. This plays a critical role in the reliability of Pb-free solder joints. Further, even though metallugically it is the Sn in the solder alloys that principally participates in the solder joint formation, details of the IMC layers formed with SnPb and Pb-free alloys are different. The markedly different process conditions for SnPb and Pb-free alloys also bear on solder joint quality.Brittle failure of solder joints in drop shock occurs at or in the interfacial IMC layer(s). This is due to the inherent brittle nature of the IMC, defects within or at IMC interfaces or transfer of stress to the interfaces as a result of the low ductility of the bulk solder.In developing improved performance alloys, Cookson Electronics has addressed both issues -improved ductility and modification and control of the intermetallic layer. A broad range of base alloy compositions together with selected micro-alloying additions to SnAgCu alloys have been evaluated with the objective of controlling bulk alloy mechanical properties and the diffusion processes operating in the formation and growth of the intermetallic interfacial layer(s).In the present article a detailed study of a range of microalloy additives is presented. The alloy additives generally act as diffusion modifiers slowing interdiffusion between substrates and solder thereby reducing IMC thickness or the propensity for void formation. Alternatively additions can be made that act as diffusion compensatorsD. It should be noted that the level of the micro-additions does not measurably modify the bulk mechanical properties of the base alloys. Our results show that dramatic improvements in the solder joint reliability, as demonstrated by high-speed ball pull and drop shock tests, can be achieved.
Based on the electronics industry's move to lead-free soldering, and increasing popularity of portable electronics, new concerns about solder joint reliability have emerged. Consequently, there is a resulting industry-wide effort to develop and understand new lead-free alloys for improved solder joint reliability. Until now, most of the work has focused on improving drop shock reliability which is a critical attribute for portable electronics. Significant work has been done to reduce the silver level in common SnAgCu Pb-free alloys, which lowers the bulk alloy modulus. Also, this work has focused on further modification of the characteristics of these "low-Ag" alloys using various micro-alloy additives. The net result of micro-additive addition is to either 1) alter the bulk alloy characteristics by changing the bulk microstructure and altering the formation and growth of intermetallics in the solder itself, or 2) control the interfacial intermetallic layer(s). Alloy microstructure can also be altered by thermal history of the solder. Therefore process conditions play an equally important role in determining the overall solder joint reliability. The alloy composition, both the Aglevel and presence of certain micro-additives, can have a profound effect on both drop-shock and temperature cycling reliability. At Cookson Electronics, we have an aggressive program to develop and study new alloys for BGA and CSP applications. We have investigated a wide range of low-Ag SnAgCu alloys with a broad selection of alloy additives. This paper examines the effect of micro-alloy additives on solder joint reliability and microstructure. Solder joint reliability as measured by drop-shock, high-speed ball pull, and temperature cycling tests is discussed in terms of the microstructural characteristics.
Recently. the industry has seen an increase in tlle number of Pb-free solder alloy choices beyond the common near eutectic Sn-Ag-Cu (SAC) alloys. The increasing munber of Pb-free alloys provides opportunities to address important issues. such as the poor drop/shock perfonnance. alloy cost. copper dissolution. and poor mechanical behavior in bend/tlex. Most recently. investigations into new solder paste alloys for mass retlow have begun. At the same time. tlle increase in choice of alloys presents challenges in managing the supply chain and introduces a variety of risks. Poor solder joint fonnation when BGAs with low Ag alloy balls are soldered at the low end of tlle conventional Pb-free process window is one example. TIle full impact of these new alloys on overall printed circuit assembly (PCA) reliability has yet to be detennined. TIns paper provides the results of an iNEMI study of tlle present state of industry knowledge on Sn-Ag-Cu alloy "alternatives," including an assessment of existing knowledge and critical gaps. Based on tlns assessment, focus areas have been identified for closing key gaps. Plans and progress in addressing tlle gaps are described. including efforts to update industry standards to account for tlle new alloys and to better manage supply chain complexity and risk.
Head-in-pillow (HIP) defect is a growing concern in the electronics industry. This defect is usually believed to be the result of several factors, individually or in combination. Some of the major contributing factors to the HIP defect are: surface quality of the BGA spheres, activity of the paste flux, improper placement / misalignment of the components, a non-optimal reflow profile, and warpage of the components.From the electronics components packaging industry's perspective, the contribution of the solder ball composition and its surface quality are two of the most important factors. To understand the role of each of these factors in producing the head-in-pillow defect and to find ways to mitigate the same, we designed an apparatus that simulates the reflow process and has an in-situ monitoring of the solder joint formation process. This apparatus facilitates the study of the effect of the thermal history of the components, in particular the oxidation of BGA spheres. A detailed comparative study of a number of lead-free solder spheres has been undertaken. The base alloy in these spheres is a low silver SnAgCu alloy. A number of minor alloy additions, focused on reducing the surface oxidation, have been tried. A comparative study of the fresh spheres and those oxidized at high temperature in air has been undertaken. Results show a big difference in wetting speed of the spheres with and without oxidation. Highly oxidized spheres have a thick oxide layer at the surface which, in certain cases, is impossible to breach. It is this "non-wetting" surface that is in-part responsible for the head-in-pillow defect.In this paper, quantitative measurements of the wetting time of spheres that come in contact with the solder paste at different times in the reflow cycle will be presented. Videos showing the in-situ soldering process will be shown along with the real time temperature and time measurements. Results show the effect of micro alloy additives on spheres' surface oxidation and their wetting behavior. Non-collapsing spheres resulting in the head-in pillow defect will be shown. In addition, effect of the solder paste activity will also be presented. Further, analysis of the study done under different reflow conditions will be presented showing the best and the worst reflow conditions for the formation of the head-inpillow defect.
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