A Comparative Study on Drop Test Performance of Fine Pitch BGA Assemblies Using Pb-free and Tin-lead Solders

This paper described a new board-level free fall drop (BFFD) and its application in the evaluation of solder joints in fine pitch ball grid array (BGA) or chip-scale packaging (CSP) assemblies. This drop test method simulates the product-level free fall or phone level drop test (PFFD), which was widely used in the industry. A comparison study between solder alloys using Sn-Ag-Cu (SAC) and SnPb were performed experimentally using BFFD. The test vehicle was built from different alloys (SAC vs. SnPb) with various surface finishes and structures. The packages include 0.4mm pitch ultraCSP (chip scale package), uSMD (micro surface mount device) and micro leadless package and stacked die packages. In order to understand the differences between the two alloys (SAC and SnPb), an FEA (finite element analysis) was employed to calculate the stress and strain in the free fall drop for various packages. From the experimental work, it shows there is a clear difference between those two alloys but a mixed trend across all packages. In general, SAC is not as good as SnPb for larger packages. When combining with the strain-stress analysis, it was found from this study that the strain plays an important role.