Reliability Assurance of Plastic Encapsulated Microcircuits

(PEMs) by Temperature Humidity Bias (THB) Tests for High Rel Applications

Reliability of plastic encapsulated microcircuits (PEMs) has been continuously improved since they were introduced more than three decades ago. It is well known that plastics are permeable to moisture, and moisture ingress can cause corrosion of aluminum metallization, which leads to chip failures. The industry standard for testing life of PEMs in humidity conditions is at 85° C temperature, 85% humidity for 1000 hours. In recent years, improvements on chip passivation and molding compounds have greatly improved life span of PEMs, and have made it possible for PEMs to be used for Hi-Rel applications.

Sensitron has been an industry leader to provide improved plastic packaging for power semiconductors and power modules, i.e., modular assemblies of groups of power chips associated in some cases with hybrid driver circuits to provide a lower cost and higher level of functionality. An essential part of these low cost power modules is a low cost and reliable plastic packaging design. To use traditional hermetic packaging for such devices is inconsistent with low cost goals. Yet Sensitron has successfully designed such modules to operate reliably in harsh industrial and hi-reliability environments.

Temperature humidity bias (THB) tests are vital to long term reliability of PEMs. Extensive research and experimental data have demonstrated that today’s "clean" epoxies with extremely low concentrations of chlorine, sodium and potassium ions have prolonged median life to over 5000 hours under 85° C/85%RH conditions. Testing for 1000 hours tells little about long life properties of epoxy, and can only measure the "freak" level. Epoxies with median life from 10,000 to over 20,000 hours in 85° C/85%RH are available at Sensitron. These time periods put 85° C/85%RH testing out of the question for control or acceptance purpose. Sensitron is using accelerated THB tests to assure long term reliability of plastic power modules. For example, at 140° C/100%RH, the 20,000-hour equivalent is only 400 hours. When Hi-Rel conditions are a real objective, then 38° C/98%RH might be an extreme condition. For a maximum 50-FIT failure rate (or a MTBF of 20 million hours) for 10 years in this environment, a median life can be determined at 85° C/85%RH to be 24,000 hours. This could be established by a requirement of a 10% failure rate in 465 hours at 135° C/95%RH, or 60 hours at 150° C/100%RH with 5% failures.

From the given failure rate, Sensitron can determine the conditions for accelerated THB tests to ensure that reliability of the plastic power modules can meet the requirement. Such practice guarantees the reliability level of PEMs to be comparable to the level that previously only could have been achieved by hermetic devices.