Gideon Analytical Laboratories, Inc. received on Vishay ES2D rectifier on a PCB. The initial condition and observation were that the diode had a conformal coating on a small circuit board in which when tested on a curve tracer had excessive leakage current across the junction and caused the PCB to fail in the application.
Rectifiers are primarily used for conducting in one direction and blocking in the other. The Vishay ES2D is an ultra-fast rectifier, which means the reverse recovery time should be less than 100 ns. Rectifiers are primarily used for power or signal conditioning in a variety of applications. This can range from high power output rectifier applications to low power switching rectifier requirements. They can be used for clamping networks on SMPS, dampening and modulating diodes on deflection circuits in CRTs, or freewheeling diodes on inductive loads etc.
The first steps in all failures are to verify the failure and perform any non-destructive analysis (radiography, CSAM, external optical inspection and SEM-EDS, XRF, XRD, etc.). The electrical testing confirms the failure and the radiography shows any internal damage as a result of the failure. The rectifier showed that it was indeed shorted but the radiography showed no damage. The conformal coating barely clung to the DO-214AA plastic package. The part was electrically tested again when it was removed from the PCB; it again failed. The part was decapsulated and tested between the sandwiched lead frames of copper where it passed the third testing. The die was removed and retested and again it passed.
This failure was due to ionic ingressing the package plastic seal. This can and does happen when vendors or board assemblers are not monitoring their process. The conformal coating should not be falling off in any process. There was nothing wrong with the part.
Gideon Analytical Laboratories, Inc. uses a variety of techniques to root out the cause of the failure. Some of these techniques are expensive and others are a matter of experimental know-how on deciphering a problem. We have over 40 years in the business and we keep out clients in business too.