The EMPF participated in a project to improve the wafer applied seals that protect plastic encapsulated modules. The project’s goal was to demonstrate and develop a protective coating that has an economical application process, is widely accepted by the semiconductor industry, and provides excellent environmental protection that meets the standards of many military applications.

Commercial off-the-shelf integrated circuits are usually embedded in a plastic encapsulation that serves two purposes. First, environmental protection is provided for the integrated circuit (IC). Second, mechanical support is supplied for the IC’s electrical connections to the outside world. However, plastic encapsulation does not provide hermetic protection.

Hermetic seals are airtight and impervious to external influences. Military applications typically require hermetic enclosures to eliminate device failures caused by moisture and ionic contamination.

Since plastic is not hermetic, it is susceptible to the intrusion of water vapor. As a result, any impurities within the encapsulation or on the IC can participate in moisture-assisted reactions with the printed circuit board and the IC interconnecting metals. Figure 1-1 shows aluminum lines and bond pads that have corroded because of the interaction of moisture and ionic contamination.

Irregular performance and eventual device failure may result from the corrosion of the interconnected wires and bonding pads on the IC. Individual components may also be adversely affected by moisture within the package. Figure 1-2 displays how moisture corrodes component leads.

While hermetic seals improve the functionality and increase the lifetime of ICs, such enclosures also increase the package size, weight, and cost. Furthermore, manufacturers are closing their manufacturing facilities for military specification parts because orders from the military account for only two percent of the total semiconductor market. This phenomenon is causing the availability of currently used military-grade hermetic parts to diminish rapidly. In addition, because of the small market, commercial semiconductor manufacturers are reluctant to meet military requests for increased reliability. Therefore, for plastic encapsulated modules (PEMs) to be a viable replacement for military-grade hermetic de-vices, an inexpensive and effective protection scheme must be employed to eliminate common failure mechanisms in PEMs. Concurrently, this scheme must encourage semiconductor manufacturers to adopt the protection method by adding value to commercial applications.

As a result of this project, materials and processes were selected that enabled advanced development and cost reduction of PEMs. Reliability and cost data were collected by processing and testing wafers coated with the protective material. The improved processes were demonstrated on a semiconductor package manufacturing line. Additionally, a business case was developed that encouraged semiconductor manufacturers to incorporate the process and materials for most or all of their products.

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The project also resulted in further collaborations between the EMPF, Dow Corning, and Raytheon RF Components. The purpose of this alliance was to significantly reduce the cost and weight of radar transmit and receive modules by eliminating hermetic enclosures. To accomplish the project goal, Dow Corning developed ChipSealTM processing to provide essentially hermetic protection for ICs. The near hermeticity was verified by results from highly accelerated stress testing (HAST). The graph within Figure 1-3 shows test data from the program, indicating a high failure rate for standard PEMs. In contrast, the ChipSealTM coated PEMs exhibit failure rates approaching those of hermetic controls.

The ChipSealTM coating was then further optimized to reduce cost. Raytheon evaluated and developed a production process for a wafer level coating to achieve near hermetic environmental protection for GaAs MMIC (monolithic microwave IC) chips.

During the next project phase, the EMPF will continue to work with industry partners to develop polymer-based, near hermetic solutions.