2nd level reliability drop test robustness for Wafer Level Packages

Abstract

2nd level reliability performance during drop impact is critical for Wafer Level Packages (WLP). Accompanying the popularization of portable and mobile phone products, high reliability under board level drop test is a great concern to semiconductor manufacturers. A 0.4mm pitch Cu under bump metallization (UBM) type has been developed for mobile computing application. In this paper presents the impact on solder joint reliability with various approaches to achieve higher Drop Test (DT) robustness. Polymer Core solder ball, solder ball Sn1.2AgCu (additive Ni + α), polymer flux with SAC107 solder ball, solder ball Sn1.2Ag0.5Cu (doped), copper core solder ball, and additional 6µm passivation layer (polyimide) have been investigated. The test vehicles were 49 pins and 0.4mm ball pitch with Cu UBM. Ball shear test was carried out to measure the solder joint performance after reflow process and units were performed cross sectioned for IMC formation analysis. Board level drop test was performed as per JESD22-B111 test method. The drop test results showed polymer core solder ball gives the best performance which is more than 1000 drops, followed by Sn1.2Ag0.5Cu (doped) solder ball, polymer flux, additional 6um polyimide, Sn1.2AgCu (additive Ni + α) solder ball & copper core solder ball. It indicated the stress relaxation within IMC & strength improvement to achieve higher drop test performance for polymer core solder ball during drop test. On the other hand, copper core solder ball has the worst drop performance (66 drops) as it has rigid material (Cu) inside the ball and less solder amount that can absorb the drop impact and stress.