| Abstract: | Plastic encapsulation is now a highly reliable method of packaging microelectronics and can be significantly more reliable than military specified “hermetic components” in field deployments of electronics in humid tropical climates. MIL883 hermeticity is an inadequate safeguard against penetration by high levels of moisture in the atmosphere. In this new work, tropical climates are analysed and shown to be significantly more severe than those safeguarded by current MIL883 standards. Field failure returns have revealed that MIL 883 hermetic CerDIP (ceramic dual-in-line) packaged ICs installed in digital switching systems failed catastrophically with a failure rate of 1755 FITs. The cause of failure was severe ingress of moisture, resulting in dewpoints up to 30°C. Alternative indigenous developments of modular digital switches for widespread rural use in India have incorporated plastic encapsulated components selected according to the criteria developed from earlier extensive and successful reliability work by British Telecommunications (BT). Such criteria include the use of HAST (highly accelerated stress technique, invented at BT Labs). The BT Labs pioneering work demonstrated that commercial transfer-moulded epoxy packaged devices from certain sources were more reliable than their hermetic counterparts.Evidence from other published pioneering work has confirmed that plastic packaging now endures as a high reliability method of packaging microelectronics. Such pioneering work is also reviewed in this paper. In the USA, the IEEE Gel Task Force followed the earlier BT Laboratories initiative and concluded that silicone gels are indeed suitable for high reliability applications. AT&T undertook comprehensive materials analyses to show that silicone gels possess remarkably useful properties to safeguard high reliability integrated circuits. CALCE, University of Maryland, conducted studies of simple logic circuits and showed that the achieved reliability in temperature climatic conditions is three times greater than that predicted for military parts. BT Labs continued its work and extended its applicability to optoelectronics. Analyses of MCM applications for satellites by the author also show that plastics are a preferred option. The evidence is that plastic encapsulation is now a very reliable and cost-effective option. |
