Solder joint reliability performance study and shear characterization of low-Ag SAC lead-free solders for handheld application

Abstract

As handheld electronic products move toward smaller and thinner form factor, the demand of high reliability under stringent field usage also increasing. Drop test, based on the JEDEC JESD22-B111A, accompanied with temperature cycling (TC) test, based on JEDEC JESD22-A104F, are two of the most critical board-level reliability tests for handheld makers. In terms of solder joint reliability (SJR), total 7 candidates of LF35-liked lead-free solder alloys are investigated for its drop performance, and TC reliability evaluation. Among 7 solders, leg 4 and 5 exhibit both decent drop and TC behavior, which are likely owing to the silver (Ag) precipitation hardening and bismuth (Bi) solid solution hardening effect respectively. In addition, the shearing mechanical characterization of 7 solder alloys are being studied. Shear strength of leg 4 and 5 shows higher strength value than others. In terms of failure analysis under drop test, three main fracture modes are observed, including via cracking at PCB side, solder/intermetallic compound (IMC) cracking at package side, and Cu trace broken at package side. Furthermore, in terms of failure modes of TC test, solder/IMC cracking at package side are the main fracture modes. In summary, this study provides an overall reliability performance mapping for 7 solders, and list out a table comprising of shear strength, void% analysis, TC reliability, and drop reliability performance for each LF35 solders which lays out potential future solder alloy to meet higher handheld phone drop reliability margin.