On reliability of chips bonded on flex substrates using thermosonic flip-chip bonding process with nonconductive paste

The purpose of this study is to verify the reliability of chips bonded on flex substrates using thermosonic flip-chip bonding process with a non-conductive paste (NCP). High temperature storage (HTS) test, temperature cycling test (TCT), pressure cooker test (PCT) and high temperature/high humidity (HT/HH) test were conducted to investigate the reliability of chips bonded on flex substrates. The environmental parameters for reliability tests were complied with the JEDEC standards. After various reliability tests, the peeling test and microstructure observation on tested specimen were performed to evaluate the reliability. The bonding strength increased with increasing the storage durations of HTS test. The HTS test provided sufficient thermal energy to promote atomic interdiffusion between Au bumps and Cu electrodes. A metallurgical bonding between Au bump and the Cu electrode was formed, and the bonding strength is thus improved. The bonding strength of chips and flex substrates assembly without applying ultrasonic in bonding process was decreased with increasing the storage durations of PCT. The typical failure for PCT was the interfacial delamination between NCP and flex substrates. Approximately 80% specimen exhibited fully separated after PCT at 336 h, implying the NCP cannot withstand the PCT test and lost its adhesion strength. The mean bonding strength of chips and flex substrates assembly with an ultrasonic power of 14.46 W in bonding process slightly varied with increasing storage durations of PCT, and standard deviation of bonding strength increased dramatically in the range of storage durations from 196 h to 336 h. Although the adhesion strength of NCP decreased, a part of Au bumps well bonded on Cu electrodes as the storage duration increased to 336 h. Applying the adequate ultrasonic power to bonding process was not only to improve the bonding strength, but also the bonding strength could be maintained in high level after PCT. There was no significantl- y change in bonding strength for chips bonded on flex substrates after TCT test. It shows that specimen has great reliability for TCT test. The bonding strength increased with increasing storage durations of HT/HH test. Neither cracks nor defects at boding interface are observed. The reliability for chips bonded the flex substrate using thermosonic flip-chip bonding process with NCP meets the requirements stated in JEDEC specifications, exception of the adhesion strength of NCP for PCT should be improved.