PCB Design For High Frequency Applications

Choosing a PCB materials for high-frequency applications

Choosing a PCB materials for high-frequency printed-circuit board (PCB) is generally a compromise, 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 applications.

 What kind of effort is required to fabricate a desired circuit with a particular materials.

These two are generally a challenging factors among the customer and their needs.

Even though, these two factors may not mesh:

 One material may be well suited for a particular application but may pose some challenges in terms of the circuit fabrication.

 And other material may be well suited for different applications but may pose few challenges in terms of a circuit fabrication.

Selecting the PCB’s is not that easy, because Every PCB are made for different applications. There is no proof for selecting a Particular PCB material. 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..

Material Used Circuit Fabrication Electrical Performance

PTFE with micro glass fiber Difficult Excellent

Ceramic – filled PTFE Moderate Excellent

Ceramic–filled Hydrocarbon Difficult Good

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.

Choosing materials based on end-use applications:

There are several different concerns for choosing materials for high frequency applications.

Product Dissipation factor Thermal coefficient Volume Resistivity Surface Resistivity Moisture Absorption

PTFE ceramic 0.0022 -280 10^12 10^11 0.05

Woven Glass 0.0027 -212 10^7 10^7 0.05

Hydro Carbon 0.0037 +50 1.210^9 5.710^9 0.05

 There are many issues to consider when choosing a circuit material for high-frequency PCB applications. Some are related to fabrication issues for producing the most robust PCB possible, and some to achieving the best electrical performance possible for a given application.

 Because of various tradeoffs, the material for fabricating the most robust PCB may not be the same one for the highest electrical performance for an application.

 Multilayer hybrid PCBs represent one way to choose a blend of materials to combine robustness and good electrical performance.

Reference:

1. John Coonrod, "High Frequency PCBs Using Hybrid and Homogenous Constructions.

2. J.W. Reynolds, P.A. LaFrance, J.C. Rautio, and A.F. Horn, III, "Effect of conductor profile on the insertion loss, propagation constant, and dispersion in thin high frequency transmission lines."

These article explains about choosing a PCB materials for high-frequency printed-circuit board.

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