Optical Coupler Failure Analysis

Gideon Analytical Laboratories received several Avago HCPL-2202 Optocouplers for failure analysis. These optocouplers have a totem pole output stages and optical receiver input stages with built-in Schmitt triggers to provide logic-compatible waveforms. They feature low input current (1.6mA to 1.8mA), Mbd typical signal rate, and guaranteed performance from -40 to 85 degrees Celsius. Some of their applications include isolation of high-speed logic systems, microprocessor system interfaces, and ground loop elimination. Our goal was to determine why the Avago HCPL-2202 Octocouplers had been failing.

Gideon Analytical Laboratories received four Fairchild 74OL6000 field failed Optocouplers along with three good (virgin green dot) devices in which a comparison could be done. The LSTTL input compatibility is provided by an input integrated circuit, with industry standard logic levels. This input amplifier IC switches a temperature compensated current source driving a high speed 850 nm AlGaAs LED emitter. This integration scheme eliminates CTR degradation over time and temperature. The emitter is optically coupled to an integrated photodetector/high-gain, high-speed output amplifier IC. The TTL compatible output has a totem-pole with a fan-out of 10. The CMOS compatible output has an open collector Schottky clamped transistor that interfaces to any CMOS logic between 4.5 and 15 volts. More information on this part can be seen at http://www.alldatasheet.com/datasheet-pdf/pdf/179405/FAIRCHILD/74OL6000.html. Three of the optocouplers were tested in a circuit (the input voltage on pin 2 could be varied from 0 volts to 2.3 volts) and found the output remained low. One intermittent opto functioned in the test circuit, as did the green dotted optos. There was no electrical difference between this “quasi-failed devices” than a good green dotted opto. Next, the optos were tested for leakage current from pin to pin to establish which side of the opto was causing the problem.

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 (http://www.vishay.com/docs/83660/moc8101.pdf) The MOC8101 were reported to have intermittent function, it was the goal of Gideon Analytical Laboratories to diagnose the cause of these failures.

Gideon Analytical Laboratories 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.

Gideon Analytical Laboratories received several Fairchild QTC H11N1s, a high-speed logic optocoupler, in 6-pin plastic DIPs. The devices were reported as some good, some intermittent and some not working in the application.

The main function of an opto-isolator is to block high voltages and voltage transients, so that a surge in one part of the system will not disrupt or destroy another part of the circuit. Opto-couplers allow a signal to be sent in digital (some analog) between circuits. This isolation used to take place using isolation transformers via inductive coupling between input and out sides separated by a dielectric. Four of the date codes were 9607E and one 9942K. All the optocouplers were tested under conditions that corresponded to the application; these failed, however, when the current was increased, the output received the signal. Several of the failed devices had reverse leakage current on the diode (input) as measured on a curve tracer.

Gideon Analytical Laboratories received two failed photocouplers for failure analysis. These photocouplers feature a high isolation voltage, high-speed switching, and high collector to emitter voltage. They are used in many types of applications, including power supply, telephones, and fax machines. The optocoupler was electrically tested; there was a short on both input diodes. The goal was to determine the cause of this failure.

After curve trace tests were performed, both failed devices were immersed in a low surface tension dye penetrant This is a modified Mil-Std 1014.2 with alterations reflecting current processing. After several other tests, these were confirmed as hard shorts. Any dye penetration constitutes a failure and demonstrates that moisture could have ingress the package of these failures.