Mechanical and board level reliability considerations of lidless flip chip BGA packaging

Lidless Flip Chip BGA packages offer significant thermal performance as well as cost benefits [1]. The lidless packages have to be tested after assembly and mounted with heat sink to improve the thermal performance. Such process can induce a lot of stress on the exposed die as well as the solder joints. This study has focused on evaluating the impact of various commercial heat sink attach processes on a lidless Flip chip BGA package. In this study, a commercially available heat sink was selected. The heat sink was mounted on the bare die using three different heat sink attach processes including a standard curing process, spring load screw down process and push pin process. Two different thermal interface materials and mounting forces were evaluated as well. Assembly evaluations were carried out to understand the various assembly challenges involved with the heat sink attach. Mechanical evaluation were carried out to understand the impact of heat sink attach on the bare die and solder joint. Finally, board level evaluations were conducted to understand the reliability of solder joint under thermo-mechanical as well as shock and vibration loading. The results of this study demonstrate the feasibility of a bare die flip chip package. Attaching a heat spreader on the bare die did not cause any damage on the die or solder joint. Further, the board level reliability of the lidless bare die package was comparable to the standard flip chip device.