Board Level Reliability Cyclic Bend Test Study On WCSP Packages
Authors: Andy Zhang, Siva Gurrum, and Vikas Gupta Company: Texas Instruments, Inc. Date Published: 10/14/2012
Abstract: Wafer Chip Scale Packages (WCSPs) have been used widely in handheld wireless devices. Board level cyclic bend test has been adopted by the industry to characterize the use life of handheld devices under use condition of repeated key strokes. In this study, board level cyclic four-point bend test based on Jedec standard was performed on two large 0.4 mm pitch WCSP packages with LF35 (Sn-1.2%Ag-0.5%Cu-Ni) solder balls. The daisy chain packages and bend test boards were designed such that the corner BGA balls and remaining inner balls are monitored independently during the test. Different maximum board deflections (2, 3, 4 mm) were employed with a fixed bending frequency (1 Hz). A power law relationship between the number of cycles to failure and maximum board deflection was established. Failure analysis of the bend test assemblies indicated that the failure mode is solder joint fatigue failure and is the same for any of deflection levels studied. Finite element analysis (FEA) was conducted to establish a correlation between simulated damage and number of cycles to failure. Results of this study suggested that cyclic four-point bend test can be accelerated using larger board deflection up to 4 mm.