Selecting PCB materials for high-frequency applications
Choosing a circuit material for a high-frequency printed-circuit board (PCB) is generally a tradeoff, often between price and performance. But PCB materials are also selected by two key factors: how well they meet the needs of an end-use application and what kind of effort is required to fabricate a desired circuit with a particular material.These two factors may not mesh: one material may be well suited for a particular application but may pose challenges in terms of circuit fabrication, and vice versa. There is no foolproof, step-by-step procedure for selecting a PCB material. But by relying on some tangible guidelines designed to evaluate a material in terms of its suitability for circuit fabrication and for meeting the requirements of an application, the process of selecting a PCB for a particular application can be simplified. The approach will be demonstrated with some of the more popular high-frequency PCB materials, and where each stands in terms of fabrication qualities and suitability for end-applications.
Commercial high-frequency PCB materials can be categorized as one of seven generic material types, as shown in Table 1. High-performance FR-4 is included in Table 1 because it is often used in combination with other high-frequency materials for certain applications and requirements. However, in terms of electrical performance, FR-4 is not considered a true high-frequency circuit material.
Choosing materials based on circuit fabrication issues
A number of different mechanical processes are required as part of high-frequency PCB fabrication. In general, the most critical of these would be drilling, plated-through-hole (PTH) preparation, multilayer lamination, and assembly. The drilling process is typically concerned with creating clean holes, which will later be metalized to form viaholes for electrical connections from one conductive layer to another.
Some concerns with the drilling process include smear, burring, and fracturing of the material. Smearing can be lethal to PCB fabrication using a PTFE based material, since there is no way to remove the smear. Fracturing can be fatal for some of the nonwoven glass hydrocarbon materials; however, most of the woven glass hydrocarbon materials do not have this concern.
The PTH preparation process is relatively well defined and straightforward for most non-PTFE materials, although special processing is required when forming PTHs for PTFE-based materials. Ceramic-filled PTFE-based materials offer PTH preparation options which are more forgiving. However, non-ceramic-filled PTFE materials require a special process which can limit final circuit yields.
Fabricating multilayer PCBs presents many challenges. One is the fact that dissimilar materials are often being bonded together, and these dissimilar materials can have properties which complicate drilling and PTH preparation processes. Also, a mismatch between certain material properties, such as coefficient of thermal expansion (CTE), can lead to reliability problems when the circuit is thermally stressed during assembly. A goal of the material selection process is to find a good combination of circuit materials for a multilayer PCB which enable practical fabrication processing while also meeting end-use requirements.
Designers and fabricators have many choices of materials used to bond together the copper-clad laminates that ultimately form a multilayer PCB. As Table 2 shows, the materials differ in terms of dielectric constant, dissipation factor, and processing temperatures. In general, lower lamination temperatures are to be preferred. But if a PCB must undergo soldering or some other form of thermal exposure, it will be necessary to use a bonding material with high reflow (re-melt) temperature, one which is thermally robust and does not reflow at the elevated processing temperatures.