Study of the Board Level Reliability Performance of a Large 0.3 mm Pitch Wafer Level Package

Abstract

Board level reliability investigations have been performed on 36 mm² wafer level packages (WLP) with a 0.3 mm ball pitch. Three different solder ball alloys were included in the temperature cycling, thermal shock and mechanical shock test. In addition to daisy chain test vehicles to address the solder joint reliability, 28 nm die packaged with WLP were included in the assessment to check the stress impact on extreme low K dielectric (ELK) in the die back-end-of-line (BEOL). The tests have included selected studies on the influence of printed circuit board (PCB) thickness and usage of a board level underfill. WLP with SAC-Q solder ball show a significant improvement versus the industry standard SAC405 solder balls. In temperature cycling on board (TCoB), WLP's with SAC-Q (SAC405 with 3% Bi) achieve more than 2000 cycles without fail in temperature range from -40°C to 85°C on a 0.8 mm thick PCB, whereby first fails with SAC405 balls were observed above 800 cycles. All WLP's with LF35 (SAC125Ni) solder alloy fail before 1000 cycles. The failure mode in TCoB changes from solder joint fatigue for SAC405 and LF35 to Cu redistribution layer (RDL) cracks for SAC-Q. The 28 nm functional die packaged with WLP show no ELK crack or any other fail after 1000 cycles for SAC-Q alloy balls. The TCoB with SAC405 WLP pass 2000 cycles without fail when an underfill was applied. Non-underfilled WLP with SAC405 balls assembled on a thickness reduced 0.4 mm PCB show also a significant improvement with a first fail above 1900 cycles. WLP with SAC-Q and SAC405 passed mechanical shock (24 drops at 10k g).