High Performance, Economical Substrates: Myth Or Reality?

By Nick Santhanam
Revised by Jim Francey
Taconic

In the past decade, the wireless industry has undergone a metamorphosis. The industry has turned from a predominantly military-driven market to a cost-conscious, consumer-driven commercial market. At the same time, wireless applications are moving up in the frequency spectrum. A typical example would be the PCS system. The PCS systems are currently designed for the 1.9 GHz spectrum, but the newer designs for wireless communications are moving up to 5.8 GHz.

As the commercial applications move up in the frequency spectrum, there is a renewed interest in high-performance, high-frequency, low-cost substrates. In the various military applications, PTFE substrates were always the materials of choice. With a very low loss tangent (0.0032 at 10 GHz), excellent resistance to processing chemicals, negligible water absorption, and a high temperature resistance (PTFE, popularly known as Teflon®, has a melting point of 621F [327C]), PTFE substrates were able to meet all of the technical requirements of RF/wireless design.

When commercial applications demanded high-performance substrates, there were certain disadvantages with conventional PTFE substrates. First, PTFE substrates were expensive; typically the price of PTFE substrates is roughly 10x the price of the epoxy glass laminates (popularly called FR-4). Second, due to the inert nature of PTFE, the processing of the PTFE substrates entailed one extra step: pre-treating the hole wall by a plasma process or by a chemical process (such as a sodium napthanate etch) to achieve excellent adhesion of the copper to the hole wall. Third, conventional PTFE substrates were "soft", meaning that they needed special care during the processing step. Finally, the CTE of conventional PTFE substrates was high (typically, 180-250 ppm/C, depending on the dielectric constant of the substrates).

To fill the requirement in the commercial arena, several proprietary and generic products were attempted. These products did offer some advantages over the conventional FR-4 substrates, but all of these products were "me too" products, and did not match or exceed the superior electrical performance of PTFE, nor did they offer the cost advantages of FR-4.

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