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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.
