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The underfill process was designed to improve the reliability of flip-chip packages, specifically the fatigue resistance of C4 interconnect bumps during thermal cycling. The mismatch in the coefficient of thermal expansion (CTE) between the chip and the substrate induced stress on interconnect bumps. It was later adopted for direct fl ipchip attach to circuit boards to improve not only resistance to fatigue, but also reliability during bending and mechanical shock. BGA packages were designed with larger interconnection bumps (interconnection balls), so fatigue during thermal cycling was not an issue. When BGA packages entered use in portable electronics, most notably in mobile phones, it was found that many BGA packages were not reliable during mechanical shock, such as dropping a phone.
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