Akihiro Yaguchi

Impact strength evaluation of solder joints in BGA by dropping steel rod

The solder joints of ball grid array (BGA) packages in mounted in portable electronic devices like mobile phones suffer mechanical shock when the device is dropped during use. Accordingly, along with developing a quantitative method for evaluating the impact strength of BGA solder joints, improving the strength of these solder joints against such shocks are necessary. In the current work, aiming to establish this quantitative evaluation method, the authors developed an impact-testing procedure based on three-point bending produced by the impact of a drop steel rod on to a printed wiring board (PWB) mounting a BGA package. With the developed test, a PWB can be repeatedly subjected to impact bending deformation in a simple and stable manner. Since the test can reproduce the same mechanical impact as that suffered when a portable electronic device is dropped during use, it is well suited to strength evaluation during the development stage of BGA packages. In the strength evaluation, the maximum value of strain generated in the corner region of the package is used. A strong relationship between the maximum strain and solder-joint strength was found. Moreover, the impact strength of the solder joint depends on the rise time of strain in the PWB, namely, as rise time shortens, impact strength falls

Study of fracture mechanism and effect of particle volume fraction on fracture properties of silica-particulate-filled epoxy resins

Epoxy resins for encapsulating integrated circuit (IC) devices are filled with silica particles to reduce the thermal expansion coefficient and to improve thermal conductivity. Recently, the size of chips mounted in a package has increased rapidly with advances in large-scale integration technology. This trend creates a problem : increased mechanical stress in the package, which sometimes causes cracking in the encapsulation resin under temperature cycling. Hence, evaluation of the fracture properties of these materials has become an important issue in package design. This study reports the effects of the filler particle volume fraction on the static strength of smooth specimens and fracture toughness of silica-particulate -filled epoxy resins for IC encapsulation. The smooth specimen strength and fracture toughness were measured, respectively, by the three-point bending test and the double torsion test. It was found that the fracture toughness increased with increasing volume fraction. The static strength of epoxy resins with a small amount of silica particulate was smaller than that of unfilled resins. The crack propagation in resins containing 30% volume fraction of large size particles was unstable. At larger than 45% volume fraction of particles, the crack propagation was stable. These crack propagation properties have been related to the plastic zone size of matrix and particle spacing.