  In recent years, interfacial fracture becomes one of the most important
problems in the assessment of reliability of electronics packaging. Especially,
underfill resin is used with solder joints in flip chip packaging for
preventing the thermal fatigue fracture in solder joints. In general, the
interfacial strength has been evaluated on the basis of interfacial fracture
mechanics concept. However, as the size of devices decrease, it is difficult to
evaluate the interfacial strength quantitatively. Most of researches in the
interfacial fracture were conducted on the basis of the assumption of the
perfectly bonding condition though the interface has the micro-scale structure
and the bonding is often imperfect. In this study, the mechanical model of the
interfacial structure of resin in electronic components was proposed.
Bimaterial model with the imperfect bonding condition was examined by using a
finite element analysis (FEA). Stress field in the vicinity of interface
depends on the interfacial structure with the imperfect bonding. In the front
of interfacial crack tip, the behavior of process zone is affected by
interfacial structure. However, the instability of fracture for macroscopic
crack which means the fracture toughness is governed by the stress intensity
factor based on the fracture mechanics concept.
