Low Dk in Microwave Laminates

How is the dielectric constant kept so low in microwave laminates?

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Great question. When it was realized that fiberglass cloth had an irregular Dk as a result of the non-uniform nature of fibers and cloth, efforts began to find something more uniform. Substrates like Alumina had been used quite successfully in microwave hybrid circuits, and being ceramic, they were very uniform. So companies like Rogers came up with a couple breakthroughs. First, instead of epoxy resin, they came out with laminates based on Teflon (PTFE). These were much lower in Dk, even with cloth, but were hard to process requiring things like caustic soda to achieve reliable attach. Then they got the brilliant idea of basically just grinding up the fiberglass cloth and using it as filler. This accomplished two things. It improved the uniformity greatly so the Dk would be constant across the panel, and by mixing the ground up glass particles with other materials they could lower the Dk overall. This was way back in the very early 80’s and those same products are going strong today. Check out Rogers website for microwave laminate offerings and useful application notes and guides. Tell them Allan sent you. And, of course, Sierra stocks the majority of these products so you’ll never have to worry about lead time.