Reliability of HTS and HH/HT tests performed in chips and flex substrates assembled by a thermosonic flip-chip bonding process

This study assesses the reliability of the high-temperature storage (HTS) test and high humidity/high temperature (HH/HT) test for an assembly of chips thermosonically bonded onto flex substrates. Environmental parameters used in the HTS and HH/HT tests were consistent with joint electron device engineering council (JEDEC) specifications. The die-shear test was applied to examine changes in die-shear forces for specimens subjected to HTS and HH/HT test. The microstructure of test specimens was analyzed to evaluate reliability and to identify possible failure mechanisms. Die-shear force decreased slightly as HTS test duration increased. When the duration of the HTS test was increased, the percentage of gold bumps that peeled off of the surface of copper pads on the chip side increased, and crack existed at the bonding interface between gold bumps and bond pads of silicon chips. This crack was formed due to thermal stress generated during the HTS test, and degraded the die-shear force of the assembly of chips and flex substrates. Cracks and blisters that occurred at the bonding interface between gold bumps and bond pads of silicon chips after HH/HT tests of varying durations deteriorated the die-shear forces. Blisters that formed on the bond pad surfaces resulted from moisture penetrating the deposited layers of bond pads before being vaporized. The reliability of HTS and HH/HT test must be improved to prevent crack forming at the bonding interface between the gold bumps and bond pads and to prevent moisture from penetrating the deposited layers of bond pads.