Thermal gradient phenomena in SnAg3.5 solder joints under high current density operating were both acquired from whole device simulation and observed from experiments. A series of structural solder bump models were created and simulated by finite element software under different current density. The thermal gradient and temperature increasing were investigated under current stressing of 103 A/cm2 to 104 A/cm2 at 70°C by infrared microscopy. The magnitude of the thermal gradient is increased by increasing the applied current. The measured temperature increase due to Joule heating is as high as 55.9°C, and the thermal gradient reached 333°C/cm when stressed by 104 A/cm2, yet only 8.3°C temperature increased and the thermal gradient reached 83°C/cm when stressed by 103 A/cm2. The temperature increasing in this model is 53.1°C and thermal gradient in the solder is 168°C/cm under 104 A/cm2 current stressing, yet only 10.4°C temperature increased and thermal gradient decreased to 35°C/cm under 103 A/cm2. After verification, only less than 0.5% error is achieved between simulation results and experiment results. It proves this built model can be used to project thermal characteristic of different module designs.

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