The current movement in the commercial RF electronics industry is toward plastic packages and assemblies that are lighter and cost less than traditional hermetic ceramic module packages. Since commercial applications account for 98% of the electronic components market share, market trends are not heavily influenced by the needs of Department of Defense (DoD) applications.

Specifically, the recent use of plastic materials to encapsulate the Laterally Diffused Metal Oxide Semiconductor (LDMOS) Monolithic Microwave Integrated Circuit (MMIC) radio frequency (RF) power amplifier chips for electro-optical power amplifiers is typical of the industry trend towards the wide-spread use of plastic packages to lower costs. Additionally, the movement to position RF hardware on commercial cellular towers close to the antennae to avoid costly and performance limiting amplification at a remote (ground level) site supports the trend from a performance standpoint. Both examples support the goal of many commercial competitors of all plastic electronics for RF and RF power applications. Once again, commercial applications are driving electronics technology trends.

It is interesting to note that these commercial applications of plastic packaging for RF electronics have DoD analogies. The LDMOS RF amplifier noted could easily have application on the digital battlefield, and the cell tower application is roughly analogous to Naval ship masthead located radars and communications gear.

The EMPF is helping to define where the DoD can take advantage of this commercial trend in RF packaging. The potential cost and weight benefits that the commercial world currently enjoys could then become available to the DoD.

One commercial method towards all-plastic RF electronic packaging is to utilize the concept of "Near Hermeticity." This concept allows the use of high performance organic materials for encapsulation to provide the required amount of hermeticity, as measured by the Highly Accelerated Stress Test (HAST) for the particular application. Hermeticity has a range of values as evidenced by the graph in Figure 1-1, which shows a progression from simple plastic packages up to a true hermetic DIP response, and then to HAST testing.

This continuum of hermeticity, from simple consumer item plastic packaging (inadequate for high reliability military applications) to the extremely high reliability of the hermetic DIP (Dual Inline Package), has been exploited by Dow Corning in their Chip Seal process (See Figure 1-1 for generalized Highly Accelerated Stress Testing (HAST) testing results). HAST is a measure of the degree of hermeticity exhibited by a test specimen). Chip Seal is a process of multi-level passivation applied at the wafer level to impart some level of hermeticity to each of the chips on the wafer. The results of HAST testing for some representative examples are shown in Figure 1-2. Commercial alternatives to the Dow

Corning wafer level hermetic coating are now available as less expensive, near-hermetic alternatives for RF packaging applications.
This technique, applying such a multi-stage hermetic coating to the RF die in the wafer form, represents significant cost and weight advantages over traditional ceramic packaging and is actively being pursued by some major defense contractors. However, this technique does require that all chips in a several-chip assembly be coated while they are still in wafer form, thus raising a significant logistical issue.

To further lower the cost of RF electronic modules, and also allow the application of a hermetic coating to a multi-chip assembly, the commercial industry, as well as the EMPF and some major defense contractors, are actively pursuing the introduction of all-organic, near-hermetic solutions. Unlike the wafer level chipseal type chip coating, these can be applied at the modular level to any and all kinds of chips used in the RF module, providing much greater flexibility to the module assembler.

A relatively new plastic material is now in the process of becoming a commercially available alternative to heavy, costly ceramics. It is called Liquid Crystal Polymer (LCP). LCP refers to the broad class of thermoplastic polymers that typically have long, rigid backbones with flexible ends. Polyester is the most common polymer backbone used in LCPs. When extruded, the rigid rods align in the flow direction and the polymer takes on some crystalline properties. Because of these crystalline properties and the excellent dielectric properties of the base (polyester-like resin), the properties of LCP are proving to be ideal for RF applications.

A comparison of the prosperities of the presently employed ceramic packages and the LCP plastic alternative that is becoming available is shown in Table 1-1.

LCP Plastic has a significant weight and cost advantage over traditional ceramic RF circuit substrates.

Navy aircraft require RF avionics that can withstand a wide range of operating temperatures and high levels of shock and vibration. Current rack-mounted circuit boards need heavy air or water cooling hardware so that thermally induced strains do not cause electronic components to fail at the surface mounted solder joints. Foster-Miller has made electronic substrates with LCP dielectric layers from 0.05 to 0.5 mm thickness to support and interconnect electronic chips in these critical RF applications.

Foster-Miller controlled the processing of the LCP substrates to achieve a coefficient of thermal expansion (CTE) matched to the printed circuits and electronic chips. Testing showed that these new interconnect substrates were more reliable and were lighter in weight than ceramic, fluoropolymers, or fiber reinforced composite dielectric substrates. Foster-Miller’s LCP circuit substrate technology was licensed to a major electronic laminate manufacturer and is being used in both flexible and rigid printed circuit boards for high-speed digital and RF circuit applications. There are already some applications of the LCP technology within the DoD. But the major implication of the use of LCP for RF electronic packaging is the ability of the LCP material to be used to make a "near hermetic" enclosure or package, thus saving the weight and cost of the traditional ceramic package while still displaying sufficient hermeticity for the DoD application.

Shown in Figure 1-3 are typical Foster-Miller LCP based RF circuit board and hermetic enclosure shapes, courtesy of Foster-Miller and Quantum Leap Packaging. The Foster-Miller RF commercial circuit substrate and package products shown in Figure 1-3 might be usable on military RF modules.

Major defense contractors are currently in active pursuit of applications of the LCP material in phased array radars (T/R modules) and avionics for use within the DoD. The incentive for this work is the promise of lower cost and lighter weight with sufficient hermeticity as compared to the traditional ceramic alternative.

In summary, the commercial industry has been actively engaged in replacing the heavy and costly hermetic ceramic packages used for RF packaging with lighter, less expensive, and adequate hermetic plastic packages for more than fifteen years, mostly for low frequency and digital applications. The EMPF is working to begin to introduce these relatively new commercial near hermetic technologies into higher frequency DoD RF packaging applications in order to gain similar benefits.

References
1) Byrne, Robert C., National semiconductor corporation, Santa Clara, CA and Camilletti, Robert C., Dow Corning Corporation Midland, MI “Reliability without Hermeticity (RWOH) for Integrated Circuits-Sealed Chips for Hermetic-Like Properties”.