Open failure mechanisms of FCBGA package under temperature cycling stress

Large scale and high density packaging of ASICs are usually achieved by FCBGA forms. The structure and materials are more complicated in FCBGA, which would cause reliability concerns in situations where thermo-mechanical stressing is dominant. Accelerated temperature cycling reliability test was performed on 90-nm/8-level copper based FCBGA packaging devices, and open failures dominated by thermo-mechanical stress when exposed to temperature cycling were observed. Combined with physical failure analysis and finite element analysis method, the failure mechanisms were explored. Results indicated stress concentration occurred around the low-k dielectric layer under UBM and stacked via in the organic substrate, forming delamination or crack of low-k dielectric and broken contact of stacked via, resulting in open failures. The thermo-mechanical stressing comes from mismatch of CTE at different interfaces.