Abstract
This paper focuses on the formation of solder voids in the large-area die-attach (DA) in power modules. The finite volume simulation method was used to study the motion behaviors of bubbles in molten liquid solder. The experiments were carried out by changing the reflow atmosphere. The voids distribution in DA was characterized by X-Ray detection. Results suggest that vacuuming the molten solder can significantly reduce the void rate of the DA. The bubbles only need a short stroke to escape from the molten solder under the influence of negative pressure. From a mechanical point of view, the collapse of the solder during the phase change is one of the main forces for driving the bubble to coalesce and move horizontally, but it may also lead to an increase in void ratio due to the reduction of BLT (Bond Layer Thickness). Solders with larger wall contact angles tend to have a larger void rate due to the increase in viscous resistance. The nitrogen atmosphere has a positive effect on reducing the void rate by reducing the wall contact angle of liquid solder. Besides, the wall adhesion plays a leading role in the movement of the bubble, and the influence of the buoyancy is small.
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