Structural design guideline for Cu pillar bump reliability in system in packages module

Molded underfill (MUF) process for system in package with WLCSP package with cu pillar bump would be the future application in SIP module assembly due to small form factor and low cost of WLCSP and good electrical, thermal performance, fine pitch and lower cost of cu pillar bump. However, higher stiffness of cu pillar could cause interface fracture between cu pillar and solder bump during SMT process and lower TCT reliability performance. In this work, finite element analyses were carried out to explore TCT reliability of a WLCSP with cu pillar bump in SIP module. Results indicate that a thin molding cap height, lower substrate flexural rigidity, large ratio of solder height over standoff, and a small ratio of cu pad diameter over under bump metallurgy (UBM) size are directions to follow in order to enhance cu pillar bump thermal cycling reliability.