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Effect of underfill staging time on fillet depression

Underfill is a material typically used in flip-chip technology to provide a mechanical bonding between the die and the substrate. During the recent development of a new FC-BGA package, one of the main challenge was the selection of a single dispense underfill with a self-filleting capability. In the course of the evaluation a new fillet defect was encountered. The defect resulted in a yield loss of approximately 30% at the post epoxy visual inspection. However, upon closer inspection, what appears initially as a crack was later found out through FA cross sectioned results to be a deep groove running parallel to the edge of the die, a short distance away from the die edge. The defect was consequently referred to as Fillet depression. There were also micro depressions that were visible only under a microscope at 40x magnification. Closer inspection showed these micro depressions have resin rich tips. From the commonality study conducted the additional staging time for the underfill was determined to be the dominant factor that cause fillet depression. This paper includes a detail description of the new defect and a hypothesis for the effect of staging time on the filleting quality.