A Plastic Encapsulated Microcircuit (PEM) uses organic packaging material, either transfer molded or coated, for environmental protection. This material is in direct contact with the active element or an inorganic barrier layer. 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. The vast majority of PEM usage has been in commercial, telecommunication, automotive and industrial applications. Militaryusage has been generally limited to high shock (munitions) and Nondevelopmental Items (NDI) or Commercial Off-The-Shelf (COTS) applications.
The major advantages that can be gained from their use are:
Greater availability (especially surface mount packaging)
Lighter weight
Lower cost (high volume procurement)
Concerns associated with their increased usage,especially military, include:
Uncertainty regarding their long term reliability in harsh environments
Lack of reliability/quality assurance procedures
Insufficient military environment reliability data (operating and storage)
Existing OEM procurement expertise
The concern over the lack of R&QA procedures is abating because of the following specification activity:
Automotive Electronics Council (Chrysler, Delco, Ford) CDF-AEL-Q1000, "Stress Test Qualification for Automotive Grade Integrated Circuits" (released 9 June 1994)
MIL-PRF-38535 "Integrated Circuits (Microcircuits) Manufacturing, General Specification for" which includes provisions for PEMs
JEDEC Standard 26 "Plastic Packages for Use in Rugged Applications" (being prepared)
Usage of early PEM's (1970's) was discouraged because of high failure rates. Table 1 summarizes predominate failure mechanisms and causes experienced in those devices.
Coefficient of thermal expansion (CTE) differences
Wire/metallization corrosion
Moisture/contamination
Voiding/poor adhesion
Processing/materials
Data/soft errors
Alpha particles (filler material)
However, major improvements have been made in the fabrication of PEM's. The following lists some of the processes/ materials/ testing that have been improved:
Recent data show that the failure rate of plastic packages has decreased from about 100 failures per million device hours in the 70's to those shown in Table 2.
Table 2: Average Early Life Failure Rates of PEMs
Application
Failure/106 Hours
Computer / Test Equipment
0.0007
Commercial Aircraft
0.04 - .07
Automotive
0.1 - 0.7
Status
Today the most popular molding compound is based on epoxy novolac resin. The basic composition contains, by weight, 15-30% epoxy resin and hardeners, 60-80% fillers, 1-7% pigment, mold release, coupling agent and stress absorbers, 1-5% flame retardant,and 1-2% catalyst. Reduction of chloride and other halides in the basic epoxy composition, stable flame retardants and ion scavengers have essentially eliminated aluminum wire and chip metallization corrosion problems. Singlebitloss and soft errors have been reduced through reduction of alpha emitting elements and by barrier coating of the integrated circuit (IC) die.
Delamination or "popcorning" associated with surface mount technology (SMT) using various soldering techniques is understood and can be controlled. Techniques used include baking the finished part and sealing it within an airtight plastic bag with a dessicant to reduce moisture levels. At the device level, delamination effects can be reduced by perforating leadframes, decreasing filler particle size, and stamping leadframes to eliminate burr formation sites that contribute to stress concentration. PEMs are used in harsh environments, such as automotive under-hood applications and commercial avionics systems. The mechanical ruggedness of plastic packaged devices makes them attractive in high shock and vibration applications that can damage ceramic packages.
To ensure PEM reliability, it is important to carefully review each vendor, his manufacturing process and reliability test results, and the customer base of each prospective plastic IC supplier prior to use. Additionally, PEM's are typically available and guaranteed by the vendor over the commercial temperature range of 070°C, to vendor electrical parameters. The industry has had success with the use of these devices at greater temperatureextremes. However,toensureperformance it is necessary for each OEM, to not only certify each vendor, but also to verify that each device will satisfy its intended application. For example, temperature can affect device parameter limits (e.g., speed) or reliability (e.g., excessive current density).
Some items that have been proven to enhance reliability that can be used in evaluating the integrity of a supplier of plastic parts include, but are not limited to:
reduced phosphorus levels in passivation
dual layer passivation in critical cases
perforated frames
benign (non-ionic) cleaning of frames after molding
use of copper frames
reduced stress trim and form
corrosion resistant mold compounds
nitride passivation
control/elimination of ionic contamination
comprehensive reliability program
Today there is general acknowledgement that there have been significant improvements in plastic encapsulated devices. Although their failure mechanisms have not been totally eliminated, they have been reduced by orders of magnitude.
When PEM attributes are discussed, it becomes obvious two camps exist, as illustrated in Figure1. However, both camps agree that the reliable application of PEMs requires the use of quality vendors and the characterization of PEMs for specific applications.
Figure 1. PEM Attributes (Click to Zoom)
The procedure described in Figure 2 can be used for the how, when and where determination for plastic packaged devices use.
Figure 2. Process Flow for PEM Use Determination (Click to Zoom)
The approaches used in the automotive specification CDF-AEL-Q1000 and MIL-PRF-38535 should be evaluated for use in procuring PEM's.
Bibliography
PEM Specifications:
MIL-PRF-38535 General Specification for Integrated Circuits (Microcircuits) Manufacturing
CDF-AEC-Q100 Stress Test Qualification for Automotive Grade Integrated Circuits
JEDEC - Standard 26 Plastic Encapsulated Packages for Use in Rugged Applications
Books:
Plastic Packaging of Microelectronic Devices, L.T. Manzione, AT&T.
Plastic Encapsulated Microelectronics: Materials, Processes, Quality, Reliability and Applications, M. Pecht, L. Nguyen and E. Hakim.
Reports:
"Plastic Microcircuit Packages: A Technology Review," (92 PEM (Plastic Encapsulated Microcircuits), RIAC Publication, M. Priore and J.P. Farrell.
"Reliability Considerations for Using Plastic - Encapsulated Microcircuits in Military Applications," Harris Semiconductor, W.L. Schultz and S. Gottesfeld.
Proceedings of 1993 and 1994 Advanced Microelectronics Qualification/Reliability Workshops.
"Reliability Data Supports PEMs in Many Military Applications," W. Schultz and S. Gottsfeld, Military and Aerospace Electronics, January 1995.
About the Authors
John P. Farrell is a senior engineer for IIIT Research Institute and serves as an advisor to the Reliability Analysis Center. Before joining IITRI, he was active in developing microelectronic reliability assurance procedures at the Air Force Rome Laboratory, formerly known as the Rome Air Development Center. At IITRI he has been active in technology transfer activities in both military and commercial microelectronics applications.
Mr. Farrell holds a bachelor's degree in Physics from Syracuse University. He holds the Defense Standardization Award for QML, VHSIC Pioneer Award, ISHM's Technical Achievement Award and the Outstanding Civilian Career Performance and Meritorious Civilian Service awards.
William K. Denson is a senior engineer for IIT Research Institute and is currently assigned to the Reliability Analysis Center. In this capacity, he has responsibility for RAC data operations, including data collection, analysis, and model development. He also performs circuit and system level reliability analysis, including Failure Modes and Effects Analysis, Fault Tree Analysis, and Worst Case Circuit Analysis.
Mr. Denson received a BS degree in physics from Buffalo State in 1980 and a MSEE from Syracuse University in 1989. He is a member of ASQC and is a certified reliability engineer.
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