Based on above FA findings, no visible assembly defect but only serious burnout mark on dice surface which punch through dice. The failure mechanism is supposed reverse EOS, the higher reverse surge current or reverse bias voltage applied on the device during application in customer side which overwhelms die capability, the high electrical stress combined with resulting high thermal stress would damage the die starting from the current density concentration point and result in a fusion on the junction and localized burnout. Dig out FTA as below: