The correlation investigation between cyclic bending and thermal cycle testing in CSP package on board

In this paper, a new application of 4-point cyclic bending test for the solder joint reliability evaluation is discussed. In light of the long cycle time of thermal cycle testing, the industry has been searching for a more cost-effective and time-effective way to achieve the evaluation task. The 4-point cyclic bending test is considered a good candidate in this study. The thermal cycle test and 4-point cyclic bending test both apply a repetitive stress to the tested objects to accelerate the appearance of fatigue. The effects of stress on solder joints and printed wiring board reflect the fatigue properties of the solder joints. The study in the past has shown that the damage mechanism of these two testing methods is similar. For these reasons, this paper discusses the correlation between the mentioned two testing methods. The data collection and comparison of their failure modes are used to investigate the correlation. The components used for both types of testing are 0.5 mm-pitch lead-free TFBGA. Under the optimized bending and thermal cycle testing conditions, the evaluation reveals that both testing methods share a similar failure mode, that is, ductile fracture. The microstructure observation through Scanning Electron Microscope (SEM) and Focused Ion Beam (FIB) plays a major role in the metallurgical analysis. The strain range of the thermal cyclic test samples is 800-1200 ? strain while the strain range of the 4-point bending test samples is 1200-2000 ? strain. After the thermal cycle test and the 4-point bending test in various conditions are completed, the fatigue life is estimated for the SnAgCu solder joints in accordance with the proposed Coffin-Manson equation. The curves are used to investigate the co-relation between strain amplitude and fatigue life for different given testing conditions. The behavior of the components on the boards in various conditions can be regarded as the foundation to simulate the lower strain rate, strain deformation and fatigue life in the cyclic fatigue test (i.e. thermal cycle test or 4-point cyclic bend test) so as to optimize the experimental design intended for solder joint reliability evaluation. Eventually, the objective of find a new effective testing method can be achieved.