A Novel Lead-Free Low-Temperature Solder Paste for Wafer-Level Package Application

Abstract

An In-containing low-temperature solder paste (DFLT) has been developed and successfully used in mobile phone board-stacking application. It is now being tried for main-board application, targeting at using lower-temperature reflow profiles but delivering at least the comparable reliability to the mainstream SAC305. The customized drop-shock packages and the daisy-chained WLP256 assemblies were reflowed under a variety of reflow profiles, in which peak temperatures ranged from 200o C up to 240o C. SAC305 has been used as the control leg reflowed under a traditional 240o C peak temperature lead-free reflow profile. The joint morphology changed with the reflow profiles. Under the 200o C peak reflow, hybrid joints were formed in which the mixing zone, dominated by DFLT, was present at the PCB side while the SAC305 ball at the chip side maintained the original morphology. Increasing the reflow peak temperature led to the fully-merged SAC305 ball with DFLT and, finally the formation of the homogeneous joint. Based on the reflow profile, DFLT has shown the drop-shock performance (g-force>6000g) comparable to or substantially better than SAC305, in which 220o C peak reflow allows DFLT having the drop-shock performance more than 90% better than SAC305. Temperature cycling test (TCT) has been done with the profile of -40 to 125o C and 20 minutes dwelling time. Similar to drop-shock, depending on the reflow profiles DFLT has the TCT performance outperforming SAC305. Reflow under a peak temperature of 210o C results in best TCT performance among all profiles. Although the reflow profiles impacts the performance ranking, the failure modes remain the similar for all the assemblies in drop-shock and TCT, respectively.