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Reliability of Flip-Chip Ball Grid Array (FC-BGA) package greatly depends on the mechanical properties of underfill material. In the package with Ultra Low-k (ULK) die and high-lead solder bumps, low glass transition temperature (Tg) and low elastic modulus (E) are effective for ULK delaminations. However, same properties are not enough for the package with lead-free solder bumps. The lead-free solder bumps are brittle and easily cracked without the support of underfill by thermal stress. To acquire the package reliability in the thermal cycle test, we noticed the viscoelastic property. Although developed underfill material having low storage and high loss modulus were tested in thermal cycle, neither delamination of ULK nor crack of solder bump occurred until 1500 cycles. It was presumed that low storage modulus was effective for low thermal stress and high loss modulus induced relaxation of thermal stress at low temperature and during thermal cycle test. These results were verified by the simulation technique based on viscoelastic finite element method (FEM) analysis with shift factors and relaxation modulus of underfill material. Low storage modulus and high loss modulus were presumably due to the content of low modulus material (elastomer) in underfill material. These results indicate that it is successfully to utilize the fabricated underfill material as a highly functional package having ULK die and lead-free bumps.