On the relative contribution of temperature, moisture and vapor pressure to delamination in a plastic IC package during lead-free solder reflow

The elimination of lead in electronics assembly has presented challenges on the process. During the conventional eutectic tin-lead solder reflow process, the delamination of the interface between the leadframe pad and the encapsulant has been found to be a precursor to the popcorning of plastic IC packages. The melting point of lead-free solder is more than 30-40°C higher than that of eutectic tin-lead solder, which will require much higher peak reflow temperatures. Compared with the delamination during conventional lead-containing solder reflow process, the delamination problem would be expected to become more severe during a lead-free solder reflow process as the process temperature is higher and the thermo-mechanical stress caused by the CTE mismatch is larger than those during a eutectic solder reflow process. In this paper the entire thermal and moisture history of a plastic IC package is simulated from the start of level 1 moisture preconditioning to subsequent exposure to a lead-free solder reflow process lasting about 8 minutes. The transient development of the strain energy release rate due to thermal stress only G_t, hygrostress only G_h, vapor pressure only G_p and combined total strain energy release rate G_tot are computed and studied using the modified crack surface displacement extrapolation method (MCSDEM). Finite element models were constructed for a 160-leaded PQFP package. The initial crack length was varied from 0.1 mm to 3.5 mm to study the effect of crack size on the relative contributions of G_t, G_h and G_p.