Does underfilling enhance BGA reliability?

The effect of underfill on various thermomechanical reliability issues in SBGA (super ball grid array) packages is studied in this paper. Nonlinear finite element models with underfill and no-underfill have been developed, taking into consideration the process-induced residual stresses. In this study, the solder is modeled as time and temperature-dependent, while other materials are modeled as temperature and direction-dependent, as appropriate. The stress/strain variations in solder joints due to thermal cycling are analyzed. The effect of underfill is studied with respect to magnitude and location of time-independent plastic strain, time-dependent creep strain and total inelastic strain in solder balls. The possibility of delamination at the interposer-underfill interface and at the board-underfill interface is studied with the help of qualitative interfacial stress analysis. Results on SBGA packages indicate that the underfill does not always enhance BGA reliability, and that the properties of the underfill have a significant role in the overall reliability of the BGA solder balls