Concern for the resistance to damaging moisture in electronics has been the primary driver for the continued use of truly hermetic glass-to-metal-sealed packages. These types of electronic packages, however, are expensive to build, large in size, and heavy in weight. Consequently, the commercial industry has responded with “Reliability Without Hermeticity” (RWOH) programs. These programs have shown the long-term reliability of electronic packages built from plastic (non-hermetic) materials, rather than glass and metal, for commercial computer and communications uses. As a result, very few applications in the commercial world require the bulk, weight, and cost of the hermetic solution.

The military world, on the other hand, needs most of its electronics to be sufficiently moisture resistant, a requirement that warrants full hermetic enclosures for its components. However, within the last several years, military electronics makers and users have made some effort to capture the benefits of RWOH – specifically, the lower costs, reduced sizes, and lighter weights being enjoyed by the commercial electronics industry.

The military’s exploration of non-hermetic packaging in applications where the truly hermetic is not absolutely necessary is encouraging. The Dow Corning ChipSealTM process is just one example of this exploration. The EMPF’s ManTech effort, in partnership with Raytheon, to discover MMIC chip coatings that could provide near-hermetic performance for phased array radar T/R modules is another example of this trend.

The organic (plastic) material that is most favored currently for potential application in these packages is the Liquid Crystal class of polymers, known as LCP (Liquid Crystal Polymer). These materials are high-melting-point thermoplastic materials. Because they are thermoplastic, they can be injection molded, with none of the waste associated with the usual epoxy (thermoset) electronic packaging plastic materials. LCP also exhibits natural hydrophobic properties that limit moisture absorption (see Table 4-1). Another natural trait of LCP is radiation hardening, the ability to withstand significant doses of radiation without degradation of physical properties. For purposes of chip packaging and high reliability of electronic systems, the LCP materials exhibit low CTE (Coefficient of Thermal Expansion) values.

Companies such as Foster-Miller are using these desirable electronic packaging traits to offer LCP-based packages to the commercial and military electronic markets. Figure 4-1 shows an example of these LCP molded packages and laminated packaging materials from Foster-Miller.

Another feature of LCP that is being used by Linden Photonics (a NAVSEA Small Business Innovative Research (SBIR) grant recipient company) is the ease with which LCP can be ultrasonically welded to itself. By cladding their optical fiber in a sheath of LCP material, and by using an injection molded LCP package for the electronics needed for an electro-optical system, Linden has been able to demonstrate the ultrasonic weld of the optical cable sheath to the electrical package. This system demonstrates near-hermetic performance and has the opto-electronic capability to potentially fill the military need for near hermetic opto-electronic current and voltage sensor systems for Navy high power applications such as DD(X) or CVN 21. EMPF is examining the potential of this novel use of LCP packaging technology.

Large defense OEMs have also begun to evaluate the electrical packaging utility of the LCP materials. This trend, combined with the EMPF partnership to define the near hermeticiy requirements of military applications, is expected to allow the use of lighter, more affordable, near-hermetic electronics applications for use in the new Navy ships.