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Reliability of Underfill-Encapsulated Flip-Chip Packages


 Reducing the plastic strain in solder joints is necessary to prolong the life of the joints during the thermal cycling test for flip-chip packages. Without heat spreaders, it is known that underfills with suitable mechanical properties reduce the plastic strain in the solder joints for underfill encapsulated flip-chip packages. From the stress analyses of the heatspreader fixed packages, it is shown that although the heat spreader increases the strain in the joints, the underfills also have an effect on the reduction of the joint strain. Furthermore, the mechanical properties of the underfill that produces the least strain in the joints are the same as those for packages without heat spreaders.
 For the heat spreader fixed package, the delamination between the solder joint and the chip surface occurs. This is caused by the extension of the delamination in the interface between the underfill and the chip surface. From the stress analyses, it is made clear that this delamination is due to the stress in the underfill at the edge of the chip and this stress is significantly affected by the fillet shape of the underfill.
 Thermal cycling tests were conducted to relate the FEM analysis results with those of the tests. The effects of the heat spreader, the mechanical properties of the underfill, and the fillet shape of underfill on the mode of fracture and the life of the solder joint are investigated.