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Advances in the semiconductor industry have focused on size reduction and enhanced package performance. This has resulted in significant interest in the use of flip-chips for commercial applications. The encapsulation or underfilling of flip chips is critical for the widespread success of this technology. The reliability of a flip chip assembly is affected by several factors, which are influenced by the material properties like the Coefficient of Thermal Expansion (CTE) and the elastic modulus. The mismatch in the CTE between the chip, solder joint, and the substrate has a strong influence on the reliability of the assembly. This mismatch causes crack initiation at the high stress points which eventually leads to delamination and electrical failure of the flip chip assembly. The CTE mismatch problem is much worse in the case of reflow or no-flow encapsulants, since these materials have CTE values that are much higher than regular encapsulants. The use of encapsulants reduces thermal stresses that result from the CTE mismatch at the die-solder-substrate interfaces. In addition to increased performance and reliability, enhanced characteristics for faster, in-line processing of these materials are also necessary for high throughput surface mount processes. The concept of reflow encapsulants allows for the underfill process to be a part of the surface mount process sequence, in-line, and not requiring a separate encapsulant dispense step. However, the use of these materials as underfills does lead to issues that affect the reliability. The concerns that affect the reliability evaluation include, among others, delamination of the encapsulant material from the passivation interface, encapsulant fillet cracking, solder extrusions and bridging, and solder mask misregistration issues. The delamination itself was of two types, either driven by corner fillet cracking, leading to a release of compressive forces at the corner, which allowed delamination from that corner, or in some cases, delamination from around the solder joints, which led to fatigue failure of the solder joints. Other factors affecting the reliability of these assemblies includes moisture ingress into boards prior to assembly, which led to voids in the encapsulant layer during the curing of encapsulant. Dispense patterns were also varied to achieve superior wetting of all the die sides. Factors like passivation type, die pitch, and layout were also evaluated to estimate their impact on reliability. As an estimate of the overall reliability or life in cycling achieved with these encapsulants, the assemblies lasted 1500 or more Liquid-to-Liquid Thermal Shock (LLTS) cycles and 6000 or greater Air-to-Air Thermal Cycles (AATC). The assemblies are susceptible to corner fillet cracks, which is similar to the trend seen with regular encapsulants. Thinner fillets seem to perform better in thermal cycling. Corners having no solder joints also show a delay in failure, indicating that delamination at the corners causes failure once the joints are reached. Reflow encapsulants materials appear to be a promising alternative to the conventional flip chip underfilling process for applications that do not require to perform in harsh thermal regimes.


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