Environmental Stress Testing of Power Transistors Encapsulated in Plastic Packages

Power HEXFET transistors encapsulated in TO-220-style and SOT223-style plastic packages were subjected to preconditioning per the JEDEC JESD22-A113 standard, which includes solder reflow simulation and flux application, and to environmental stress testing: highly accelerated biased temperature and humidity test (HAST) and multiple temperature cycling. Electrical measurements and C-SAM mode acoustic microscopy were performed after each of the stress tests. Thermo-mechanical and moisture characteristics of the molding compounds used have been analyzed to explain the origin and evolution of the observed delaminations. An anomalous increase in leakage currents was detected after preconditioning and was explained by flux penetration to the die surface through delaminated areas between the molding compound and the die/lead-frame assembly. Different modes of parametric degradation and failures of HEXFETs observed after HAST performed in two laboratories, where different types of fluxes were used for preconditioning, confirm that flux application might have a significant effect on the results of reliability testing. The role of delaminations in the reliability of power transistors, mechanisms of moisture-induced charge instability in HEXFETs, reproducibility of HAST, and the necessity to revise the JESD22-A113 standard regarding flux application are discussed.