Achieving optimum high-frequency printed-circuit-board (PCB) performance is not simply a matter of specifying the best possible PCB material, but can be significantly impacted by PCB fabrication practices. In addition to appropriate circuit materials and circuit design configurations to meet target performance goals, a number of PCB material-related issues can affect final performance, including the use of soldermask, the PCB copper plating thickness, the conductor trapezoidal effect, and plating finish; understanding the effects of these material issues can help when fabricating high-frequency circuits for the best possible electrical performance.
Many PCB applications use soldermask as a thin coating to prevent the adhesion of solder to different sections of a PCB, although it may be surprising to those not working with PCBs at higher frequencies to find that these circuits are formed without soldermask, or that it is used only sparingly for high-frequency PCBs. High-frequency PCBs are often multilayer constructions employing a combination of materials and circuit configurations. For high-frequency PCBs, it is not uncommon to have outer copper circuit layers consisting of microstrip circuits or grounded coplanar circuit constructions as shown in figure 1.
Initially Published in the IPC Proceedings