Studies on the effects of material design parameters on the board level reliabilities of wafer level chip size package (WLCSP)

Abstract

Board level reliabilities are important issues for the wafer level chip size package (WLCSP), because WLCSPs are directly assembled on the boards without any protective structure in general. Nowadays, satisfying the required reliabilities for the WLCSPs has become more difficult due to the reduced feature sizes of I/Os. Therefore, it is important to understanding the effects of WLCSP material design parameters for enhancing the reliability. Among many materials for WLCSP manufacturing, three important material design parameters of solder, under-bump metallurgy (UBM) and dielectric material which support the interconnections will be focused in this study. First, the effect of solder composition on thermal and thermo-mechanical reliability was investigated. For analyzing the effect of solder composition, WLCSPs having four different solder balls of $Sn_{3.0}Ag_{0.5}Cu$, $Sn_{1.0}Ag_{0.5}Cu$, $Sn_{0.7}Cu$ and $Sn_{1.2}Ag_{0.5}Cu_{0.05}Ni$ were prepared. And These WLCSPs were mounted on ENIG and OSP finished substrates. For analyzing the interfacial reaction of the WLCSPs, assembled samples were thermally aged at $150^\circ C$ up to 1000 hours. On Cu-OSP pads, $Cu_6Sn_5$ IMCs were observed at both interfaces of solder/UBM and solder/Cu-OSP pad in all solder compositions. And after thermal aging, $Cu_3Sn$ IMC layer with Kirkendall voids were observed between the $Cu_6Sn_5$ IMC and Cu layer. The amount of these Kirkendall voids increased in low Ag contained solders. Especially, in Sn0.7Cu solder, crack caused by the thermal oxidation of Cu UBM layer was observed along the Kirkendall voids. Because smaller $Cu_6Sn_5$ IMC grains were formed on the UBM In lower Ag contained solders, thicker $Cu_3Sn$ IMC layers and larger amount of Kirkendall voids were formed in lower Ag solders. In $Sn_{1.2}Ag_{0.5}Cu_{0.05}Ni$ solder, the formation of $Cu_3Sn$ was suppressed due to Ni substitution in IMC. On ENIG pads, after thermal aging, at the solder/UBM interface, thick $(Cu,Ni)_6...