As complex electronic products move up the speed curve, the question arises as to which materials should be used for high-speed boards. The broad perception is that the general class of materials designated as FR-4 may not perform adequately for such designs. What should be used instead?

At least two problems complicate the answer: knowing which material properties are important and obtaining accurate information on each material from laminate suppliers.

In terms of material properties, it's perceived that a lower dielectric constant is most important. Next in importance is a lower loss tangent, or dielectric loss. As it turns out, however, making a low dielectric constant an important criterion can force a pc board to be excessively thick to attain the desired impedance of each signal path.

The property that does make a difference is the loss tangent. Here, the problem becomes complex.

There are dozens of materials that have very low loss tangents. Most of them have been designed for microwave or RF operations, where material cost is not a major factor. Second, such circuit boards have very few layers and don't require lamination of multiple layers to create the final board.

Moving to a lower-loss material always increases board cost, however, so it is important to make sure a low-loss material is really needed. This can only be accomplished by taking the candidate materials, simulating the worst-case circuit and then observing how much each material degrades the signals. Simulation should be performed only on materials that can successfully be manufactured in volume with the number of required layers.

In many cases, when the simulation results are in, it will be found that the standard FR-4 materials already in use are adequate.

When that is not the case, these questions will have to be asked: Is the alternative material multiple-sourced? Is it capable of being laminated into high-layer-count pc boards? Does it work well in the board fabrication process? Is it reliable in its intended application? Is it cost-effective? When all those questions are answered, it is likely that many "high-speed" materials will prove to be unsatisfactory.

Lee Ritchey is the founder and president of Speeding Edge (www.speedingedge.com), a consulting firm specializing in the high-speed pc-board and system design disciplines.