Mis-identified Failures in FETs

Several recent failure analyses have found that what appeared to be typical source to drain over current damage, was actually caused by intermittently open circuited gate bonds. In power switching applications, such as inverters and switching power supplies, the timing of the transistor’s turn ons and turn offs can be critical. For example in an inverter if the transistor between the positive supply and phase A does not turn off before the transistor between the negative supply and phase A turns on there will be a short circuit between the positive and negative supplies resulting in a high current condition and the failure of both transistors. The original cause of the failure can be masked after the catastrophic failure of the die. The gate does not necessarily remain open circuited. It can reestablish continuity due to the short circuiting of the gate on the die, which causes arcing at the open bond. It is easy to overlook this mechanism in the normal FA process, since there is obvious and sometimes spectacular damage to the die and usually pressure to make a rapid determination of the root cause of failure. The gate wires are more likely to become open circuited because there is usually only 1 wire and it is sometimes a much smaller wire. The wires can become open circuited for a variety of reasons: Mechanical damage to the leads. Inadequate bonds at either end of the wire. Excessive intermetallic formation. Several case histories of open gate wires as well as other open bonds, how they were discovered, and possible screening methods will be discussed.