Abstract

During package qualification, a 5-die-stacked chip scale package was being marginally triggered on high stand-by current collectively known as Power ICCS failure. Affected lots are subjected to 3x reflow at 240°C. Post reflow failures include blown_up, high standby current in Vcc pin (ISBLO), and high standby current in Vccq pin (ISBLOQ). Backside chip-outs are observed on Die 1 and Die 3 of the three failures. Electrical validation showed that only Die 3 is failing. Corner crack on Die 3 is common to the blown_up and ISBLO failing units while crack on Die 3 backside is observed to propagate toward the active area on ISBLOQ failing units. Fracture analysis results show that the crack of the three failures all originated from die backside chip-out. Thermo-mechanical model of the package shows that, by design, Die 3 generates the highest stress concentration. Results show that if chip-outs are present on the area of the die with the highest stress concentration and the unit is subjected to reflow temperature of 240°C, die crack will propagate from the chip-out. This paper presents the unique failure mechanism observed on a 5-die-stacked chip scale package and the corrective actions applied to solve the issue.

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