MOC101 Photocoupler

Gideon Analytical Laboratories received three failed Vishay MOC8101 isolating photocouplers for failure analysis. These photocouplers are components that transfer electrical signals between two isolated circuits by using light. Like the MOC8102, MOC8103, MOC8104, and MOC8105, the MOC8101 consists of a consisting of gallium arsenide infrared emitting diode optically coupled to a silicon planar phototransistor detector in a plastic plug-in DIP-6 package ( The MOC8101 were reported to have intermittent function, it was the goal of Gideon Analytical Laboratories to diagnose the cause of these failures.

The photocouplers were tested electrically for functionality. These tests revealed leakage current when the input was turned off. A non-conducting fluorescence dye with a surface tension less than water was used to determine that moisture ingression was causing leakage current to rise on the output. Pictured below at the top left is a curve trace of the junction deficiency in one of the samples. The photocoupler was then ground down under the phototransistor; a picture below at bottom left is of the package under a black light in which the dye is clearly visible.

The diode had something affecting the junction. This diode was analyzed by SEM-EDS. A micrograph of the corrosion of the contaminated diode is pictured below, at the top right. EDS analysis of the diode revealed the presence of chlorides; silver is a getter for chlorides. It was located on the die sides and lead frame. This demonstrates that chlorides entered by a liquid rather than migrating in under voltage. The silver would react with chloride. For comparison, the micrograph below (bottom right) is of the diode that had no corrosion and had no chlorides present.

Gideon Analytical Laboratories came to some interesting conclusions. Dye penetration demonstrated that all failures were susceptible to moisture ingress via the lead frame under normal pressure. Moisture will cause leakage within the device, which bypasses the die. This leakage path diminished the output intensity within the die giving an intermittent performance. These devices should be treated as non-wettable.

Gideon Analytical Laboratories has been an extremely beneficial resource to a variety of electronics companies for 35 years. Failure analysis provided by Gideon Analytical Laboratories has allowed numerous companies to know exactly how and why their devices are failing. Using this information, companies can better prepare and plan for the future business.


MOC8101 Optocoupler


New photo-diode


Dye ingress along the lead frame.


Dye penetration to the die.


GaAs die showing surface corrosion.


Chloride penetration to the die surface