Novel numerical and experimental analysis of dynamic responses under board level drop test

Abstract

Board level solder joint reliability during drop test is a great concern to semiconductor and electronic product manufacturers. A new JEDEC standard for board level drop test of handheld electronic products was just released to specify the drop test procedure and conditions. However, there is no detailed information stated on monitoring and controlling of dynamic responses of printed circuit boards (PCBs) and solder joints which are closely related to stress and strain of solder joints that affects the solder joint reliability, nor there is any simulation technique which provides good correlation with experimental measurements of dynamic responses of PCB and the resulting solder joint impact life during the entire drop impact process. In this paper, comprehensive dynamic responses of PCB and solder joints, e.g., acceleration, strains, and resistance, are measured and analyzed in detail with a multi-channel real-time electrical monitoring system. It is found experimentally and numerically that the mechanical shock causes multiple PCB bending or vibration which induces the solder joint fatigue failure. According to the JEDEC board level drop test standard, the impact pulse is compulsorily monitored during drop test qualification of components. This impact pulse includes the effects of velocity before and after impact, and the effects of contact surface and material, as well as drop tester's characteristics such as friction of guiding rods, which are too complicated to be simulated. Therefore, a novel input acceleration (Input-G) method is developed to simulate the exact drop test process using ANSYS-LSDYNA software. This method is more accurate and much faster, and bypasses many technical difficulties in conventional dynamic model such as adjusting the parameters of contact surfaces, defining contact type, etc. The model established has excellent correlation with experimental measurement of peak strain, output acceleration, velocity, and vibration frequency as observed in actual drop test. More importantly, the impact pulse considered is the same for simulation and testing. The model can be applied to simulate the overall impact responses including PCB cyclic bending, which are very critical for understanding of board level drop test.