PCB Material Selection: How to Choose PCB Material

PCB material selection is most critical part of its fabrication process. That’s because different material types perform differently, and the wrong choice can mean a failed circuit board. In that regard, proper analysis of the application requirements is made before the manufacturing process can begin.

This article, with emphasis on PCB material properties, will help you understand what would best fit your project. To begin with, here’s the definition and identity of the different material types.

What is PCB Material?

PCB material means the materials, usually in layers, that manufacturers use to fabricate printed circuit boards. These can vary from the PCB substrate or dielectric and trace to the masking layer and silkscreen. PCB raw materials can also be rigid or flexible, and sometimes also both rigid and flex materials combined. Here’s a deeper description:

PCB Material Composition

PCB material composition here is used to mean the different types of layers that make a typical circuit board. These are — from top to bottom — the silkscreen, prepreg, solder mask, trace, and the substrate.

PCB Substrate Material

This is the PCB base material or core, the part that supports all other components. In addition to acting as structural support, this part also provides the insulation that the board needs.

PCB substrate is typically resin-treated fiberglass or FR4, with FR standing for flame retardant. PCB substrate material can also be metal like aluminum or iron, as well as Teflon or other plastic when the part must flex.

PCB Trace Material

PCB trace material means the foil that’s laminated to the core to form a conductive layer that will connect the different components. Typically made of thin copper tracks, the traces can be said to be the PCB signal and power pathways. In some applications, other materials are used for the traces. These include gold and silver.

PCB Solder Mask Material

Just above the trace surface is the solder rnask. This is the layer that serves to hold in molten solder, preventing it from spreading to unwanted parts. Most often, this PCB material is a green color, and what’s come to be associated with circuit boards. Other colors that may be used are red and blue.

PCB Prepreg Material

The PCB prepreg material is a dielectric, an insulating material that’s placed between the layers of a PCB. This can either be between cores, or between the core and trace layers. It’s normally made from glass fibers that have been woven together and then impregnated with a partly cured polymer.

Made in different levels of thickness, the materials used in PCB prepreg essentially insulate the different layers from one another. With the partly cured resin, prepreg also serves to bind the layers together.

PCB Silkscreen Material

This is the uppermost layer and is usually applied during the last stages of PCB fabrication steps. The silkscreen is typically printed using epoxy ink. It primarily indicates the identity of the board’s components and test points.

The silkscreen may also be used to display other important information such as the manufacturer’s name and other details. PCB silkscreen material is normally white, although other colors such as black, grey, yellow and even red are also common.

PCB Material Type

Based on the operating conditions and other application needs, a circuit board’s material may be one of these options: rigid, flexible, or rigid flex. These types of PCB materials, together with their required properties and other features, are explained below.

Rigid PCB Material

Rigid PCB is typically made from a rigid material called FR4. As a cheaper option, this type of PCB material can also be a composite such as CEM. Today, rigid PCB material is available as metal such as aluminum and iron.

FR4 materials are classified into FR4 1, FR4 2, and FR4 3. CEM is, on the other hand, available as CEM 1 up to 5. These different material grades offer different performance characteristics, and the application would guide on the best type of the material to use.

Flex PCB Material

Flex PCB material is mostly polyimide due to its many beneficial properties. Other commonly used materials are PTFE — or what’s also commonly called Teflon — and PEEK. These materials, in addition to flexibility, offer the benefits of being resistant to chemicals and thermal stress.

Rigid Flex PCB Material

Rigid flex PCB material can be said to combine the properties of both a rigid and non-n rigid or flexible materials. This is crucial when the application demands so, such as when the PCB must remain structurally rigid on side and flexible on another.

PCB material types, layer by layer
PCB material types, layer by layer
Resource: https://www.mdpi.com

PCB Material Selection Guide

Because usage conditions and other requirements can be different, proper PCB material selection is the most critical part of its production process. Based on your project needs, your manufacturer may prefer some material more than others, and even advise so, usually by comparing their properties and cost.

PCB Material Properties

These are characteristics that define a specific circuit board material, and that predict how the board will perform in an application. The main PCB board material properties range from mechanical and thermal to electrical and chemical.

PCB Mechanical Properties

PCB mechanical properties refer to its behavior when exposed to physical stress or force. These properties include density, peel strength, time to delamination, and flexural strength.

  • Densitymeasured in g/cubic centimeter or pound/cubic inch, this is the density of the dielectric. PCB material density signifies its weight for a given size.
  • Peel Strength – this is the bonding characteristics between the PCB dielectric material and trace layer as observed when the board is exposed to thermal and chemical extremes.
  • Time to Delamination – this property measures the time that PCB layers would take to separate from one another, specifically when subjected to high heat levels and thermal shock or moisture.
  • Flexural Strength – flexural strength is the ability, by the PCB material, to stand up to flexural stress or forces before it can break.

PCB Thermal Properties

During PCB material selection, thermal properties play an important role in determining the right board for extremely hot conditions. The most critical of these properties are:

  • Decomposition Temperature – this is used to mean the temperature at which the PCB substrate would chemical decompose and not return to its original state, normally by losing not less than 5 percent of its mass.
  • Glass Transition Temperatureindicated as Tg, this is the temperature that would soften a PCB material and make it deformable but able to return to earlier state when cooled.
  • Thermal Conductivity – the PCB thermal conductivity is its ability to conduct and, thus, transfer or dissipate heat.
  • Coefficient of Thermal Expansion – this property measures the rate at which the circuit board material expands with rise in temperature.

PCB Electrical Properties

These properties concern the board’s electrical functions. For an insulating layer, the dielectric constant must reach a specified value, while the trace must adequately conduct current. These and other electrical properties of PCB material are explained below.

  • Dielectric Constant – the PCB material dielectric constant will determine how fast signals will travel through, with higher values for slower signals and vice versa.
  • Dissipation Factor – also called dielectric loss tangent, the PCB dissipation factor is a measure of its susceptibility to power loss. High end PCB loss tangent is normally about 0.0001 while most other boards offer around 0.02 to 0.3.
  • Electrical Strength – this denotes the ability of the PCB board material to stand up to, or resist, electrical breakdown. This property mostly ranges from 800 to about 2500 Volts/mil.

PCB Chemical Properties

PCB chemical properties pertain to its ability to withstand the effects of various environmental factors such as fire, moisture, and specified chemicals that might cause damage. These properties include the following.

  • Flammability Specifications – these are requirements related to fire hazards or, in other words, how flame-retardant to not ignite or spread a fire.
  • Moisture Absorption – most PCBs have absorption values that range from 0.01 % to about 0.20% which denotes its ability to withstand wet environments.
  • Methylene Chloride Resist – this measures the ability of the PCB dielectric to resist absorbing methylene chloride. For most materials, this ranges from 0.02% to 0.20%

Flex PCB Material

Flex printed circuit board material properties are slightly different from those of other boards, mostly owing to its unique requirements. As an example, this board is designed to bend as required, whether permanently or repeatedly. So it’s usually made from flexible but strong materials like Teflon or polyamide.

In addition to the bending ability or flexibility, PCB material specifications for the flexible types include those listed below. (Note that these requirements are similar to those of rigid boards, only that values may vary based on application).

  • Density
  • Flexural strength
  • Peel strength
  • Time to delamination
Testing PCB material properties for heat and moisture resistance
Testing PCB material properties for heat and moisture resistance
Resource: https://www.youtube.com/watch?v=40md2r0CuFs

PCB Material Cost Comparison

During PCB material selection, cost is as important a consideration as any other. Depending on the application requirements and budget, one may go for costly, high-end materials, or opt for low-cost materials.

Expensive but high quality printed circuit board material types are necessary when reliability and durability are key factors, such as when fabricating aerospace and medical or military PCBs.

On the other hand, common consumer electronics can do well with low cost materials, since the applications requirements are usually generally less demanding.

Conclusion

PCB material selection, in addition to being the first step of making circuit boards, is also the most important. It determines the performance of the final product, which includes durability and reliability, among others. An experienced manufacturer is able to ensure that your PCBs are made from the best materials based on your application, while also factoring in the costs to produce it.

Share:

Table of Contents