Advances in the drop-impact reliability of solder joints for mobile applications

This manuscript presents the research studies in the drop-impact reliability of solder joints in the PCB assemblies intended for mobile applications. The works cover stress–strain characterisation of solders, evaluation of test methods at component and board levels, and investigation of the fatigue characteristics of solder joints. The stress–strain characteristics of four solder alloys were generated for the low and medium strain rate regimes up to the strain rate of 300 s � 1 . In the study on test methods, the board level high speed cyclic bend test and the component level ball impact shear test were correlated with the board level drop-shock test using 23 groups of diverse solder joints. The high speed cyclic bend test was found to be able to replicate the failure mode and the performance ranking of the solder joints in the board level drop-shock test, while the ball impact shear test was found to have poor correlation with the board level drop-shock test. The S–N characteristics of solder joints between the PCB fibre strains of 1 � 10 � 3 to 3 � 10 � 3 , as a function of bending frequency (from 30 to 150 Hz) and test temperatures (�10 C and 25 C), were generated. The propagation of cracks in the solder joints were monitored through electrical resistance measurement, and the rates of propagation were correlated with the observed crack paths in the solder joints. The effects of load history on crack propagation were investigated using two-step load tests, and the use of Miner’s rule was found to be non-conservative. Design rules were formulated based on analytical solutions, and a robust solder joint design was proposed and validated.