A fracture mechanics analysis of the popcorn cracking in the plastic IC packages

Popcorn cracking phenomenon in surface mounted packages is treated by assuming an inherent edge crack at the die pad/EMC interface of a TQFP and subsequent interface delamination under thermal and/or vapor pressure loadings. Using the finite element methods and the methods of interface fracture mechanics, path independent energy release rate is calculated and compared to the interface toughness which is assumed to be a function of the phase angle. Results indicate that the edge crack propagates toward the center leading to the delamination of the entire die pad/EMC interface most notably for the vapor pressure loading, then mixed loading when thermal and vapor pressure loadings are applied simultaneously. For the thermal loading, only the cooling process is likely to lead to the entire delamination where both the energy release rate and interface toughness decrease with the crack length. For the vapor pressure loading, the energy release rate increases parabolically with the crack length but proportionally with the vapor pressure while the interface toughness remains almost constant. In the case of the mixed loading, the energy release rate increases as in the vapor pressure loading, but the interface toughness decreases with the crack length; Stress states near the crack tip were closer to mode II for thermal loading but to mode I for vapor pressure loading, and changed from mode II to mode I with the crack length for the mixed loading.