It can be challenging for failure analysts to know if device damage caused electrical breakdown or did electrical breakdown cause the damage. Here’s an example where a failed Power MOSFET device shorted source-to-drain. The image below shows the device geometry and pin descriptions. The device package is a small-outline-no-leads (SON) with 26-mil pad pitch.
The solder joints were suspected to be a potential failure cause by our client, so microsection analysis was chosen as the appropriate analysis technique (versus chemical decapsulation of the device and SEM analysis of the device die). The figure below is a BSE SEM image of the microsection of the device as-mounted on the PCBA.
This is one of the source solder joints, which has a void and shows an excess solder condition at the toe. Neither of these issues likely contributed to the failure.
The thermal pad also shows solder voids, but they are not so severe as to significantly increase the thermal impedance of the device.
The source-to-drain short was found at the front edge of the source lead frame.
The die attachment had failed on the left end of the die as shown below.
The molding compound separated from the die surface on the right end of the die (see below). This damage raises a question about whether thermal stresses might have caused the separations (e.g. popcorn damage) causing the die to to overheat and break down electrically? Or, did the device break down causing the thermal damage due to the heat generated by the short?
The failure current at breakdown can be estimated from the radius of the alloyed region (e.g. 1-mil radius =~ 1 ampere) [1] . The short site (melt pipe) diameter was ~ 265 microns (or 10.3 mils), suggesting a failure current of ~ 5.2 amps, which is within the acceptable operating range at 25C (i.e. 13 amps max continuous operating current) per the device specification.
The safe operating conditions diagram for this device is shown below. The brown line at 5.2 amps is the current estimated from the radius of the melt pipe. The diagram suggests that the device likely failed due either to a voltage spike above 30 volts or an intermediate voltage exceeding the pulse width indicated by the colored-dashed-lines.
It seems likely that thermal damage was caused by the failure event rather than being the cause of the failure. The rapid thermal expansion in the region of the melt pipe likely failed the interfaces that were found to have separations.
In summary, this device most likely failed due to an over-voltage condition on the source signal.
REFERENCES
[1] Microelectronic Failure Analysis – Desk Reference 3rd Edition, ASM Press, ISBN 0 -87170-479-X, p.327.