Evaluation of Fatigue Crack Initiation and Propagation in Thin Solder Joints Using a Lap-Joint Shear Specimen With High Stiffness Fixtures

Fatigue damage in solder joints is one of the most significant factors in the failure of electronic components. Accordingly, many research studies on the fatigue lifetime evaluation of solder joints have been undertaken to improve the reliability of the components. The authors have devised a lap-joint specimen with high stiffness fixtures in order to carry out shear fatigue testing on thin solder joints, which have thickness of a few hundred μm and are manufactured via a reflow process similar to that used in actual printed circuit boards (PCBs). In this work, using the developed lap-joint specimen, the fatigue properties, including crack initiation and propagation of Sn-3.0Ag-0.5Cu solder joints were evaluated under low cycle shear loading conditions with creep deformation. The lap-joint specimen was fabricated by the reflow soldering of two copper adherend, and was assembled with high stiffness loading fixtures. The dimensions of the solder joint are 4 mm (length) × 2 mm (width), with a thickness ranging from 100 to 400 μm. In the shear fatigue test, under the assumption of thermal loading conditions of actual PCBs, the inelastic strain amplitude and total strain rate were set to from 0.5 to 1.2 % and 1×10−4 s−1, respectively. In addition, the fatigue crack initiation lifetime is defined as the number of cycles N20% at which the load amplitude has decreased by 20 % from the initial value. As the first study result, the experimental relations between the fatigue crack initiation lifetime and the inelastic strain range were obtained. Next, in order to apply the experimental data to the evaluation of fatigue crack initiation in actual solder joints via finite element analyses, the lifetime data were related to the calculated inelastic strain at the interface corners of the solder joint of the specimen, where fatigue cracks initiate due to strain concentration. Finally, assuming that the reduction of the load amplitude corresponds linearly to the fatigue crack length, the experimental relations between the fatigue crack propagation rate and J-integral range were also obtained. The experimental data are regarded to be valid, given a comparison to other crack propagation curves for solder obtained by tensile cyclic loading of a flat specimen with a center crack. Consequently, the developed lap-joint specimen with high rigidity is effective for acquiring the material properties regarding fatigue crack initiation and propagation in actual thin solder joints.Copyright © 2015 by ASME