Although alternatives to good old FR-4 exist, it remains the pc-board material of choice for most applications. It's inexpensive and well understood in terms of manufacturing. In high-end applications, it's getting close to its limits with regard to performance. But the cost of materials that are optimized for microwave frequencies can be prohibitive. So, it's not surprising that designers would like to wring the last drop of performance from FR-4.
FR-4 imposes considerable losses at signal speeds of 1 Gbit/s and higher. At those speeds, designers can't rely on the standard rules of thumb that they're accustomed to. According to Rob Hinz, principal engineer at SiQual, provider of interconnect engineering consulting services, there are tricks that designers can use to help sidestep some of the manufacturing issues when working with FR-4 in high-speed applications.
For example, in cases where dielectric losses are severely impacting signal integrity, consider implementing microstrip layout for surface layers. "This allows you to get some of the field up in the air and off the board," says Hinz. But be aware that manufacturing variations can make use of microstrip construction on outer layers a bear in terms of predictable characteristic impedance.
A key point is that above about 1 GHz, dielectric loss in FR-4 starts to dominate over metallic loss. At 5 GHz or so, it's all about dielectric loss, which is proportional to frequency. If you double frequency, dielectric loss doubles. Metallic loss, however, is proportional to the square root of frequency. So although metallic loss starts out higher, it doesn't rise as fast with frequency.
Keep the material characteristics of FR-4 in mind when attempting to control impedances. Two types of material make up an FR-4 board—the core material and what's known as pre-preg. Core material is fully cured and thus fairly stable in terms of dimensions. Pre-preg, which is the material between core layers, is only partially cured and prone to expansion and contraction with temperature.
"You want to have your impedance controlled across core material rather than pre-preg," Hinz warns. "Otherwise, your impedance will change across the length of the line and you'll get mismatches and reflections. When you build up microstrip, you want a core piece on the outside. When you're building up stripline, once you've got a reference plane on either side of the conductor, then you want the core material thinner than the pre-preg. So you can play those sorts of games with the FR-4 to extend the usefulness."
SiQual offers an excellent white paper on gigabit pc-board design challenges. It can be found at www.siqual.com.