Board level drop test is one of the key qualification tests to ensure the solder joint reliability. It becomes critical due to leadfree solder. In this paper, we study the effect of underfills on drop test performance of a fine-pitch Ball Grid Array package (BGA), experimentally and numerically. Failure mode is also compared. There are good correlations between testing and modeling on the effect of underfill on failure modes and failure mechanisms of solder joints. Moreover, the results of testing and modeling show that important parameters affecting drop test performance are position of the package on the board, modulus of the underfill, and, circumstantially, the interfacial fracture toughness of the underfill. An unfilled underfill, as opposed to a silica-filled underfill, can provide satisfactory, or even superior, drop test performance if its interfacial fracture toughness is sufficiently high.The benefit of an unfilled underfill is better processability.

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