Drop Performance of Land Grid Array Packages as a Function of Cu Content in Lead-free Solder Alloys

High strain-rate drop impact tests were performed for land grid array (LGA) packages with (electroless nickel immersion gold) ENIG metallization and lead-free solder interconnects. Sn-4Ag-0.5Cu (in wt%, SAC405), Sn-3.5Ag (in wt%), and Sn-0.7Cu (in wt%) were used for pretinning the LGA pads. SAC 405 alloy was used for the LGA-to-board mounting. Drop test results showed two distinct trends. Samples pretinned with Sn-4Ag-0.5Cu and Sn-3.5Ag showed poor drop performance while those pretinned with Sn-0.7Cu (in wt%) or without pretinning showed excellent drop performance. The dierence in behavior is related to the initial intermetallic morphology and structure. The needle shape Ni3Sn4-based intermetallics were observed for the first two cases. This led to poor adhesion between solder and NiP metallization. The samples with Sn-0.7Cu pretinning and no pretinning showed a continuous intermetallic morphology. In this case, sucient Cu improved the solder wetting and formed a continuous intermetallic morphology, leading to better drop performance. A sucient amount of Cu makes the better wetting, forming (Cu,Ni)