Trace crack in molded thin substrate package, root causes and FEM modeling

Trace cracks in the substrate of a thin PBGA package were detected after temperature cycle testing during a product qualification. Characteristics of this package include a thin substrate core (0.15 mm) and relatively thin mold cap (0.7 mm). To understand the failure mechanism, the package was submitted for failure analysis (FA). Numerical simulations with the finite element method (FEM) were performed to investigate the impact of design parameters and material selection. Packages with various design changes were tested to seek and prove the findings. The investigation concluded that trace cracks were due to failure in the microstructure of the substrate core. Eliminating the epoxy rich region in the core was the key to improving the package reliability. This was accomplished by using a thinner multi-layered fiberglass cloth with a tighter knitting pattern. The FEM simulation also suggested that reliability improvement could be realized by using thicker core materials, thinner molding compound, and the use of a molding compound with a lower product value of elastic modulus and effective coefficient of thermal expansion. The improved package no longer had the trace cracking problem.