How to choose the correct PCB materials

The PCB Material Selection Process

The PCB materials you select will make-or-break your PCB. Choosing the incorrect materials sinks a PCB into a sea of performance and reliability issues.

But by taking a systematic approach to PCB material selection, you can avoid a slippery slope. Let’s take a look at what this involves. There are four key categories you need to consider in your selection process, and these are:

  1. Electrical properties
  2. Copper foil types
  3. Copper thickness
  4. Finished board thickness

1. Electrical Properties

Among the various electrical characteristics of PCB materials, electrical strength, dielectric constant, and moisture resistance are the most important.

Electrical strength/breakdown is the extent to which a material can resist electrical breakdown, and is it measured in Volts/mil or kV/mm. The breakdown value is the voltage at which an electrical insulating material begins to conduct electricity.

PCB material electrical strength

The dielectric constant is a measure of the effect an insulating material has on the surrounding conductive material’s capacitance. The dielectric constant and trace geometry define how a signal propagates along a trace.

Moisture resistance is the ability of a PCB dielectric material to resist absorbing moisture. Moisture impacts a PCBs’ performance and dependability since its presence changes the thermo-mechanical properties of printed circuit boards.

PCB material moisture resistance

2. Copper foil

Copper foil produces the signal routes on a PCB. Copper quality and its adhesion to the PCB’s dielectric layer are critical in the PCB’s performance and reliability. Electrodeposited (ED) or Rolled-annealed (RA) copper are the two types of copper foil generally used in PCB manufacturing.

The two foil types differ somewhat in their attributes and their performance as part of a PCB. For instance, PCBs fabricated using ED copper foil exhibit better operation in applications demanding mechanical stress, and RA copper is more suitable in applications where thermal shock might be a factor. So, ED copper foil offers mechanical robustness, and RA copper foil provides thermal robustness.

PCB Copper foil thickness

3. Copper thickness

You need to consider two key factors when deciding the suitable PCB copper thickness for your design.

The first is the thermal conductivity and the current capacity of the copper layer. The second is the mechanical strength resulting from the copper thickness, hole-size, and whether or not there are any support vias.

For high power DC currents using heavier copper foil is the preferred approach. But, thicker copper layers give rise to the overall board thickness, impacting the lamination and drilling processes during PCB production.

4. Finished board thickness

PCB thickness has an important impact on the performance of your board. So, in the last step of your PCB material selection process, you should calculate the finished PCB thickness. This is the distance between the outer copper layers.

PCB thickness impacts solder joint reliability and the board’s overall stiffness, possibly affecting its warpage or bending during the assembly process.
Adding more internal copper layers to the PCB structure improves the PCB thermal performance. Using 2, 4, or 6 layer PCBs is typical for power applications.

Finished board thickness

In Sum

The incorrect choice of copper foil and PCB substrates can smother a circuit assembly’s performance and reliability.

Selecting the optimal materials for your PCB requires you to research, analyze and evaluate the available materials’ characteristics and choose those that align with your desired board capabilities. The result is an optimized PCB primed to perform.