Plastic Encapsulated Microcircuits (PEMs)
Traditional failure prediction methodologies have indicated the failure rate of Plastic Encapsulated Microcircuits (PEMs) to be high compared to comparable hermetically packaged microcircuits. Data collected by RIAC and others have identified that the quality of these non-hermetic devices has improved and their failure rates may be similar to their hermetic counterparts when properly selected and used.
In some cases, the failure rate of a non-hermetic device can be lower than that of a similar hermetic device. A potential reason for the better failure rate is the possibility that processes used in PEM manufacturing may be more robust than those for hermetic parts. This may be due, in part, to the quantity of PEM vs. hermetic devices manufactured. There are caveats when selecting PEMs to assure reliability. The Original Equipment Manufacturer (OEM) using PEMs must have an effective parts control program, assure that components are used within specification and in an operating scenario. When using 217Plus™ to calculate failure rates of any system, it is recommended that 217Plus™ users define the processes that are being used in the design, development, manufacturing, and maintenance, etc. of their systems. These processes define what steps are (or are not) being taken to assure reliability in a design, while some questions specifically address the use of PEMs. Additionally, the values for Junction-to-Case and Junction-to-Ambient thermal resistance used in a 217Plus™ analysis should be modified to reflect the actual values for each PEM device analyzed.
The ideal scenario for PEMs is to operate at a 100% duty cycle. When a PEM is operating, moisture, which is an accelerant of corrosion mechanisms, has minimal opportunities to reach the die area of the PEM device since it is driven off by self heating, which reduces contamination-related effects for PEMs.
RIAC data, which was used to develop the 217Plus™ microcircuit model, shows that PEMs can be as reliable, or in some cases more reliable, than their hermetic counterparts. Other studies confirm this finding. In a study by ELDEC Corporation/CALCE Electronic Packaging Research Center, University of Maryland, Condra, et al, performed a unique comparison of PEMs and hermetic microcircuits. Devices were manufactured using the same microcircuit die in both plastic encapsulated and hermetic packages. This was then evaluated under conditions of temperature cycling and temperature/humidity/bias (THB) [L. Condra, et al, "Comparison of Plastic and Hermetic Microcircuits Under Temperature Cycling and Temperature Humidity Bias", IEEE Transactions on Components, Hybrids and Manufacturing Technology, Volume 15, Number 5, October 1992]. A semi-custom monolithic microcircuit, in an 18-lead Dual Inline Package (DIP), was used as the test vehicle. Two hundred non-hermetic and two hundred hermetic devices were divided into groups and tested under conditions of temperature cycling (-55°C to + 85°C) and powered-on, powered-off THB (85°C, 85% relative humidity, 14-volt bias applied to the microcircuit, 30 minutes on, 30 minutes off). Variations included testing loose parts and testing parts that had been soldered to printed circuit boards. Throughout testing, only one valid failure was experienced. This failure occurred in a ceramic package. It was concluded that the overall performance of the PEMs was virtually identical to that of the ceramic devices with regard to temperature cycling and THB environments.
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