Photocoupler Vishay IL420 Failure Analysis

Gideon Analytical Labs received one failed photocoupler, a Vishay IL 420, for failure analysis along with two virgins (unused) optocouplers for comparison.

The basic function of a Photo Coupler is to have electronic isolation, including potential isolation (as in the case of the transformer) in an electrical circuit. Devices of the Photo Coupler family include Photo Transistor, Photo Darlington Transistor, Photo Triac and OPIC. These devices have the characteristics of excellent isolation, high CTR, compact package, high-speed operation, low decay and unit control function not being influenced by field effects.

There was a hard short from pin 4 and 6 to pin 5. The input diode tested normal and optical intensity had not diminished over time. This was compared to a known good device as well and the diodes were indistinguishable from one another.

Dye penetration reveals the devices are susceptible to moisture ingress. Both phototransistors and the input diodes have the dye surrounding the chip. The die was taken out of the package. The silicone was removed and the die was re-tested to determine functionality. Both the transistor and input diode functioned normally. The ionic contamination was found using SEM-EDS which entered during the processing of the PCB, found to migrate with voltage and was invariant in its effect once it shorted. Thus, the optocoupler was determined to be moisture sensitive and non-hermetic and often contacted contamination.

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