Influence of the Chip Temperature on the Moisture Induced Failure Rate of Plastic Encapsulated Devices

Moisture-induced failures have frequently been cited as a significant disadvantage of plastic-encapsulated devices compared with more expensive ceramic packaged components. An investigation has been carried out to determine why in systems predominantly in operation, the moisture-induced failure rate is many times lower than expected. The results show that self-heating of the component or of the total system in the rack is an active protection against moisture, because the relative humidity (RH), which decreases as the temperature increases, is the dominant parameter in moisture-induced failures, rather than water partial pressure. Since the relative humidity depends exponentially on the temperature at a constant water partial pressure, an increase of 10°C compared to ambient, increases the lifetime of a plastic component in accelerated tests, and consequently under actual operating conditions, by about one order of magnitude. This temperature effect is therefore the key to employing plasticencapsulated components in long-term systems. A very Careful analys!s of the operating conditions, however, is necessary in the case of intermittent operation. The protection by chip excess temperature is not present during the switch-off time of the system and the moisture can easily diffuse through the plastic material to the chip surface. An investigation was carried out which examined the water uptake and release as a function of ambient conditions and chip excess temperature of the components. Based on these results a computer program was developed and various intermittent operating conditions were simulated. Worst-case conditions are small time ratios (on-time divided by sum of on- and off-time), almost independent of the chip excess temperature. Information is given on how to simulate intermittent operation during accelerated life tests.