The EMPF is working with both military and commercial OEMs to introduce modern commercial materials which are lighter and less expensive into military RF packaging. This effort is in keeping with the DoD goal of integrating commercial off-the-shelf (COTS) solutions wherever possible without sacrificing reliability and durability.

To date, the commercial sector has designed a limited number of high-end RF solutions suitable for DoD applications. The EMPF seeks to ensure the continued application of these commercial technologies across the complete spectrum of DoD needs.

Traditional military RF applications utilize heavy, costly ceramic materials for their RF devices. These materials are too heavy and expensive for most RF applications in the commercial sector, where lightweight, inexpensive plastics are predominant. Typical commercial uses include cell phones, wirelesss modems, RF ID tags for the retail industry, Bluetooth modules for electrical appliances, and automotive radars.

Hermeticity is the major barrier to the transition from ceramics to plastics in the defense industry. Hermeticity is the exclusion of water or corrosives. Military components which are exposed to environmental extremes demand this characteristic. This imposes a limit on which military applications can benefit from these newer, lighter, less expensive, yet less watertight, organic materials.

Traditional, ceramic RF materials
Alumina (Al203) is used extensively as the substrate for “hybrid” or integrated circuit chips, which are mounted to the Alumina substrate and interconnected with wire bonds and traces that have been screen-printed using thick film inks and then fired.

Hybrid circuits can be made on either a single-sided Alumina ceramic or a multilayer High Temperature Cofired Ceramic (HTCC) or Low Temperature Cofired Ceramic (LTCC) substrate material. These multilayer circuits provide additional routing trace length per square inch of circuit.

The greatest advantage of these ceramic materials is that they are very low RF loss. This means that minimal RF energy is lost as heat during signal transmission through the materials. The disadvantages of ceramics are the high cost and heavy weight. Besides adding to the weight of the end-application unit, the heavy weight and density of ceramic material means it cannot be formed into large sheet sizes like organic, plastic materials. This adds significantly to the initial cost and labor expense of ceramic fabrication and assembly.

Organic RF materials
Getek is an epoxy-glass laminate similar to FR-4 in its ease of fabrication. It has a highly consistent RF property known as dielectric constant (Dk). This uniformity of Dk is a highly sought after RF design characteristic.

R/T Duroid, marketed by the Rogers Corporation, is a flexible, fluopolymer, glass-fiber reinforced laminate with very low Dk and relatively low RF loss. Duroid is commonly used in high frequency RF applications. It is more expensive than FR-4 but less expensive than ceramic. Duroid is also a flexible material, affording wider design applications.

There are hundreds of commercial laminates available, each with a specific Dk and corresponding RF loss characteristics. These materials are made from woven or non-woven glass cloth with varied weave styles and glass fiber sizes.

Thermoplastic bonding materials are used to produce multilayer PTFE circuit boards or to bond PTFE boards to heatsinks or FR-4 boards. FoamClad is a material which is suitable for conventional PCB print and etch processing. It has a low Dk and low RF loss.

Although organic materials tend to have higher RF losses than ceramics, the advantages outweigh the disadvantages for many applications.

RF materials that have been considered for use in military systems:

1. LTCC (Low Temperature Cofired Ceramic)
• Lower RF loss
• Better thermal conductivity; better power dissipation
• Better for operation above 2 GHz
• Can integrate passive elements
• More brittle; limits to overall size
• More costly

2. Organic (commercial composites)
• Can integrate capacitive and resistive passives
• FR-4 is used in most cases up to 2.4 GHz
• Does not require change in current PCB manufacturing
• Limited commercial availability (but improving)

Also on the horizon of organic RF material options is Liquid Crystal Polymer (LCP). This material combines the positive characteristics of organic and ceramic materials, providing a lightweight, less costly solution with low RF loss. LCP is based on the molecular structure of Polyester. It is naturally hydrophobic, providing “near hermetic” or “functionally hermetic” enclosure for electronic components. LCP materials are also thermoplastic rather than thermoset, making them much more economical to process than epoxies.

Although some applications of LCP technology already exist within the DoD, the ability of the material to provide a functionally hermetic enclosure has not been fully exploited. A lower cost, lighter weight material supplying sufficient hermeticity could revolutionize many military RF applications. Currently, the EMPF and major defense contractors are looking into applications of LCP technology for phased array radar (T/R modules) and avionics for use within the DoD.

Summary
The EMPF will continue to serve the DoD’s interest by leveraging its partnerships and capabilities to provide functional solutions which integrate commercial materials with their less costly and lighter weight advantages, without sacrificing DoD-specific needs for assured ruggedness and reliability. For further information regarding RF packaging materials, please contact Fred Verdi at the EMPF
at (610) 362-1200, ext. 201.