Satoshi Hanaki

Study on Evaluation Method of Thermal Fatigue Life for BGA Solder Joints

The thermal fatigue strength of BGA solder joints is one of important properties for design. However, the traditional approach to estimate the fatigue life consumes much time and cost. As a method of evaluating the thermal fatigue lives more efficiently, the evaluation of S-N diagram by the combination of thermal fatigue tests and finite element analysis was described. To estimate the design margin, each fatigue data is transformed into normalized fatigue strength, because the distribution of normalized fatigue strength can be estimated by the small number of samples. As a result, it becomes possible to estimate the design margin with a given probability of failure and confidence level considering an effect of number of samples. Based on the obtained S-N curve with a design margin, distribution of fatigue lives for other types of BGA specimens can be also estimated without additional experiments. It is revealed that the proposed method is very useful for the reliability estimation of solder joints.

Fatigue Strength of Laser-Processed Hot Work Tool Steel and Its Reliability

A fatigue life of die-casting dies depends on heat checks, which are fine shallow cracks on a surface due to cyclic thermal stress. According to the recent study, the laser-processing is effective to repair cracks and extend the life of dies. Therefore, the purpose of this study is to evaluate the fatigue strength of laser-processed hot work tool steel to validate the reliability of repaired dies.In this study, the rotating bending fatigue test for the laser-processed hot work tool steel specimens has been carried out. Because test results have scatter, S-N curves are decided based on the evaluation of fatigue strength distribution. As a result, the fatigue strength of the laser-processed specimens decrease remarkably in compared with that of the non-processed ones. The notch sensitivity might have risen because the structure became brittle due to rapid cooling. However, it can be recovered by heat-treatment at 773K. This could be due to a high yield strength resulted in a secondary-hardening effect. The fractgraphic results also demonstrate the same tendency. The fracture surface of heat-treated specimen is similar to that of base metal, while that of laser-processed one shows different appearance.The laser-processing method can be carried out at relatively low cost and effective for the repair of heat check. Therefore, we can conclude that this method is available for the extension of the life span of dies.

On Reliability Evaluation of Diaphragm in Light Touch Switch.

Fatigue characteristics of a metal diaphragm in a light touch switch are evaluated with experimental approach considering a cyclic load acting on it. The approach consumes much time and costs for evaluation. In previous paper, we have described that the mechanical properties of stainless steel have anisotropy and the fatigue characteristic must be discussed with considering the large scatter of material properties induced by the anisotropy. Therefore, the purpose of this study is to establish a simple method to evaluate the reliability of diaphragms with the consideration of anisotropy. The proposed approach consists of three procedures. Firstly, residual stress after press forming is evaluated by FEM, and feeling curves when the pushing load by is applied on the diaphragm are also analyzed. And, the stress amplitude and the mean stress at every point on the diaphragm can be calculated. Secondly, the experimental database about S-N curve and Goodman diagram considering the effect of anisotropy have been prepared. Finally, the fatigue lives considering failure probability on the diaphragm can be estimated by comparison of Goodman diagram and numerical results. From the obtained results, it is revealed that the proposed approach is very useful for the reliability evaluation of the diaphragm.