Head-in-pillow defect - role of the solder ball alloy

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 re flow 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.