PEMs_Introduction

PEMs are Plastic Encapsulated Microcircuits.

A microcircuit is an electronic device that is very small and implements several components (or a system) into a single integrated circuit.1 Microcircuits are usually fabricated by photolithography, which is a process used in micro fabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photo mask to a light-sensitive chemical photo resistor.2

A PEM uses organic packaging material, either transfer molded or coated, for environmental protection.3 One of the more popular materials in use is an epoxy novolac resin. This material is in direct contact with the active element or an inorganic barrier layer.3 This is in contrast to metal or ceramic packaging, which has a hermetically sealed cavity and no active element or organic barrier interface with the package material.3

PEMs and the Military/Space Industry

PEMs were originally never considered suitable for spaceflight applications because of their commercial connotation. With the decreased availability of military-grade hermetically sealed components, PEMs have now become a necessity.7

Cost and obsolescence issues are forcing the military/space hardware designers to move from military/space specific application products to the use of commercial grade PEMs.6 PEMs have been a consideration for military/space use for about thirty years.6

PEMs are being used in military/space systems, but there needs to be a critical realization that commercial components are often manufactured using uniform, highly automated lines that produce quite reliable parts for use in a typical commercial/consumer application.6

Reliability of PEMs

PEMs used in harsh military/space environment may cause intermittent and catastrophic failures with their unique reliability requirements.6 There are certain precautions and considerations that must be adhered to in order to select, screen, and test PEMs to acceptable levels of project risk. PEMs have been primarily (and initially) designed for use in benign environments where equipment is easily accessible for repair or replacement.4

NASA has even suggested PEMs should only be used when due to performance needs there are no alternatives in the military high-reliability market, and projects are willing to accept higher risk.4 Proper screening and reliability testing can be completed to mitigate risks to levels that may be more acceptable. The use of PEMs is permitted on NASA Goddard Space Flight Center (GSFC) space flight applications provided each use is thoroughly evaluated for thermal, mechanical, and radiation implications of the specific application, and if they are found to meet mission requirements.

PEMs shall be selected for their functional advantage and availability. Due to the additional reliability testing that must be incurred, cost savings for PEMs are not a reality.

Also, a PEM shall not be substituted for a form fit, functional equivalent, and high-reliability hermetic device in space flight applications. Shall statements are ‘non-negotiable’ and typically are directly mapped to mission requirements with implications to the contract statement of work (SOW).

Due to the rapid change in wafer-level designs typical of commercial parts and the unknown traceability between packaging lots and wafer lots, lot-specific testing is required for PEMs, unless specifically excepted by the Mission Assurance Requirements (MAR) lead for the project. Lot-specific qualification, screening, and radiation hardness assurance analysis and/or testing shall be consistent with the required reliability level as defined in the MAR. No PEMs are considered acceptable in high-reliability applications “as is” without additional testing and analysis to assure adequate reliability and radiation tolerance.