PLASTIC-STRAIN OF ALUMINUM INTERCONNECTIONS DURING PULSED OPERATION OF IGBT MULTICHIP MODULES

Catastrophic burn-out occurring during power-cycling of insulated gate bipolar transistor (IGBT) multichip modules has been observed to aris e as a secondary failure mechanism caused by the lifting of the emitte r aluminium bonding wires. In fact, the successive lift-off of the alu minium wires turns in a current crowding through few IGBT cells with c onsequent triggering of the internal parasitic thyristor-structure. Ba sed on failure analysis data, this paper presents an exaustive descrip tion of the symptoms and a simple qualitative model for the time-depen dent lift-off of aluminium bond wires in IGBT modules. This power-cycl ing induced failure mechanism (occurring in the field and during accel erated tests) is described in terms of plastic deformation of the alum inium interconnections (bond wires and chip-metallization) during puls ed operation. Some practical conclusions are finally drawn for power c ycle testing and for optimal thermal design.