To make some money during school, I used to push a wheelbarrow around building sites. While this didn’t prepare for my eventual career, it gave me an opportunity to see how buildings are constructed and gain an appreciation for using the best materials. If you skimp on material quality while constructing a building, you risk collapse due to overloading, inclement weather or premature structural failure. Similarly, the operation and lifetime of your PCB depend on your choice of circuit board material.
For buildings, there is a well-defined order of construction, typically beginning with a concrete foundation followed by the steel or wood framing, floors and roofing. Likewise, board fabrication consists of the substrate, laminate, solder mask and silkscreen. The silkscreen is primarily used for identification markings that are invaluable for PCB assembly. Solder mask is a protective layer that guards against external contaminants and provides the required isolation between surface elements like pads, copper traces and drill holes. The structural integrity or foundation of your board is defined by the substrate and laminate. Selecting the best circuit board material for these foundational elements, which can be chosen based on your board type, determines your board’s manufacturability, functionality and life cycle.
In order to make the best circuit board material selection for your board type, it is necessary to examine the materials available and categorize board types.
Circuit Board Material
The circuit board foundation, sometimes referred to as the base, comprises the substrate (or core) and the laminate. The substrate is the non-conductive dielectric material, typically chosen based upon the dielectric constant, dk. Laminates when used in conjunction with substrates provide the copper foil or surface material. However, laminates may themselves be used as core material in some board constructions.
Substrates are typically dielectric composite structures that are comprised of epoxy resin and paper or glass weave (sometimes unwoven), which may be supplemented by ceramics to increase the dielectric constant. Substrates are made to meet certain property requirements, like the glass transition temperature (Tg), which is the point where heat causes the material to deform or soften, and may be customized. However, there are a wide variety of standard substrates from which the majority of PCBs are fabricated. These include FR-1 thru FR-6, CEM-1 thru CEM-5, G-10 and G-11, Aluminum or insulated metal substrate (IMS), Polytetrafluoroethylene (PTFE), RF-35, Polyimide, Alumina and flexible substrates Pyralux and Kapton. FR-4 is by far the most commonly implemented of these substrates.
Laminates are manufactured under pressure and consist of cloth or paper layers and thermoset resin. Just as with substrates, laminates can be made to meet custom requirements or properties. Properties of interest for laminates include tensile and shear strength, the coefficient of thermal expansion, CTE, and Tg. Dielectrics commonly used for laminates include FR-1, FR-4, polytetrafluoroethylene (Teflon), CEM-1 and CEM-3. Also common are pre-preg materials FR-2 thru FR-6, CEM-1 thru CEM-5 and G-10.
Together the substrate and laminate define the basic electrical, mechanical and thermal circuit board properties that, when used in conjunction with an accurate board type, enable you to make the best selection of PCB material for your design.
Circuit Board Types
Circuit boards may be classified in a number of ways:
- component location - single-sided, double-sided, embedded
- stackup - single layer, multilayer
- design - module-based, custom, special
- bendability - rigid, flex, rigid-flex
- strength - electrically strong, mechanically strong
- electrical functionality - high frequency, high power, high density, microwave
The first three criteria above are poor candidates on which to base your selection as they are not based on circuit board material properties. While bendability and strength are good physical criteria, they do not provide guidance on your board’s ability to function as desired. Electrical functionality is based on PCB function, which makes it a good criterion for design-based circuit board material selection. According to function, PCBs may be classified as the following board types:
- High Frequency (High Speed) - These boards can accommodate frequencies in the 500MHz - 2GHz range.
- High Power (High Current) - These boards must be able to carry high currents, which usually requires wider traces and thicker copper, withstand and effectively dissipate high temperatures.
- High Density (High Density Interconnect) - Typically complex, these boards have thin traces, utilize laser microvias and thin high-performance materials.
- Microwave - Microwave boards usually refer to PCBs where signal speeds from 1GHz to hundreds of GHz are present; however, the microwave spectrum actually extends from 300MHz to 300GHz.
The board types above can be used to select the circuit board material best suited for your design.
5 Common Mistakes for High-Speed Board Builds
Selecting the Best Circuit Board Material for Your Board Type
As stated above, there are a number of physical characteristics or properties that can be used to identify circuit board materials. These characteristics are important and may be used for in-depth substrate and laminate analysis. Additionally, they may be used as metrics for comparison, which is done below. In the following table, common substrates and laminates used to form the foundation of PCBs are compared according to dielectric constant (dk) and ability to withstand heat (Tg):
The above table does not list all possible combinations of circuit board materials. However, it does provide circuit board materials that can be used for standard and special PCB board types and can, therefore, help you make the best selection for your design. As previously mentioned, there are additional circuit board properties that you should include in an overall evaluation of a PCB material. This evaluation should be done in conjunction with your CM to ensure that your design requirements align with your CM’s capabilities.
|Tempo‘s Custom PCB Manufacturing Service
The manufacturability, functionality and life cycle of your PCB depend on its construction. It is, therefore, essential that you make the best circuit board material selection. At Tempo Automation, the industry leader in fast turnkey PCB manufacturing, we offer rigid, flex and customized circuit board material options.
And to help you get started on the best path, we furnish information for your DFM and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on circuit board material options or how to select the best material for your design, contact us.