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This paper presents finite element analyses done to understand delamination in flip chip assemblies. More specifically, the objectives of this paper were: 1) to investigate delamination at the encapsulant-chip interface along the thickness of the chip under thermal loading; and 2) to determine the tendency for interconnection failures resulting from such a delamination to occur. Under operating conditions, the mismatch in coefficients of thermal expansion (CTE) between the silicon die of the flip chip assembly and the organic substrate subjects the solder joints to enormous strains, which may lead to early failure of the solder connections. Although underfill encapsulation can lower the strains in the solder joints, it increases the potential for the occurrence of cracking at the chip-underfill-substrate interfaces during temperature cycling. Due to CTE mismatch, a strong interfacial shear stress concentration is produced near the free edge. When this stress exceeds the adhesion strength between the encapsulant and the silicon die, an interfacial crack will initiate. This may further propagate toward the encapsulated corner of the die, and then continue along the active surface of the die. Once the adhesion between the die and the encapsulant is lost, the solder joints are exposed directly to the strain resulting from the CTE mismatch, and are likely to crack under thermal cycling conditions. In this study, a crack was introduced
at the unencapsulated corner of the die, along the die-encapsulant interface. The crack tip driving force was then investigated for chips of varying sizes. Analyses have shown that for cracks whose lengths do not exceed 40% of the original uncracked length of the die-encapsulant interface, the energy release rates increase, making unstable crack growth possible. However, crack lengths beyond this critical value will cause the energy release rates to diminish rapidly. If the experimentally-determined critical energy release rate falls within or above the calculated energy release rate, the growth of the crack will stop before reaching the encapsulated corner of the die. In such a case, solder interconnection failures resulting from such delamination are not likely to occur.
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