Sierra Circuit Certification

The importance of materials for performance

PCB material selection is the first step in the design process. Selecting the right material is very important as it can impact the overall performance of the circuit board. When dealing with high-speed designs, the material chosen significantly impacts the quality of the signals traversing through the traces. Hence, depending upon the operating signal frequency, the materials are categorized as normal speed, medium speed, high speed, and very high speed. The type of materials you choose defines the quality of your end product.

Now, if you’re wondering what materials you should go for, which material characteristics you need to consider to avoid signal loss then, our PCB Material Design Guide will surely help you out.

An overview of printed circuit board materials

Material selection is important for all PCB designs. The goal is always to select the right material for manufacturability, at the same time, meets your temperature and your electrical requirements. When dealing with high speed designs the material chosen significantly impacts the quality of the signals traversing through the traces. The type of materials you choose defines the quality of your end product.

A printed circuit board is manufactured using the following 3 items:

Prepreg is a sheet of woven-glass reinforcement impregnated with a resin that is not fully cured (B stage material). This is a tacky material and allows the bonding of different laminates or foils. Prepregs are available in a variety of glass weaves and epoxy compositions.

Copper foil
Copper foil on the outermost layer is the conductive medium through which electric current flows. Copper traces on a PCB are made by selectively etching copper foil. Traces create the electrical connections on the PCB. In case of multilayer PCBs, the electrical connections between the layers are made through the holes that are drilled on the PCBs.

Copper clad laminates (core)
Cores are basically made up of prepregs and copper foils that have been laminated and cured. A variety of materials with different thicknesses, epoxy properties, and types of glass weave are available. The designer must define the desired combination that is well suited for his/her application.

Copper loss or skin effect loss

Copper loss is essentially associated with the current that flows through the conductors. Electrons may not always flow through the centers of the conductors. If copper trace is finished with nickel, most of the current might flow through that nickel layer. The skin-effect loss gets larger as frequencies go up. This can be compensated by increasing the width of the traces which in turn creates larger surface area. Wider trace will always have lower skin effect loss. Copper foil-dielectric toothy interface profile increases the effective length and thus increases the copper loss.


Copper foil selection

The physical parameters of an ED copper foil can be controlled with the addition of organic additives into the plating bath. Certain additives – known as levelers – can decrease the roughness of the surface. During the electrodeposition process, the current density and electric field strength tend to be higher on the peaks of the substrate which therefore tend to grow faster. This yields a rough surface. However, some organic molecules are adsorbed preferentially on the peaks of a surface and the deposition is therefore favoured on the valleys, yielding a product with a much smoother profile.

Key considerations for choosing PCB materials

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    Our best practices

    As a PCB manufacturer, we can recommend you to follow these few guidelines: match the coefficient of thermal expansion, opt for tight substrate weave, avoid FR4 for high-frequency applications, use lower moisture absorption materials, and always use CAF-resistant materials.

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    Quality for HDI materials

    The material should be dimensionally stable; this also applies to the non-HDI PCBs as well. All materials shrink and stretch to some extent during manufacturing processes, and patterning must be scaled to compensate, which is not an issue provided the material movement is predictable.

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