The stresses occurring in the solder joints during thermal cyclic loads have been assessed by finite element analysis and experimental tests in order to study the effect of hidden underfill imperfections on the reliability of flip chip modules. Imperfections have been found to exist quite frequently in real flip chip modules. They were categorized into voids, air inclusions, impurities, and delaminations. Stress analysis indicates that isolated voids and even larger air inclusions that are close to the neutral point of the module have no significant impact on the lifetime of the modules. Delamination from the solder surface does not seriously impair the reliability either. The substantial damaging effect of delamination from the chip surface can be mitigated by using a hard underfill. Even the serious impact of air inclusions next to the chip edge can be mitigated by choosing a hard material. Impurities like flux residues, however, are hazardous independent of the mechanical properties of the underfill. As seen in the experimental tests, they initiate underfill and UBM delaminations by combined physical and chemical effects.
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