Types of PCBs
Throughout our PCB design course, you’ll see many photos of printed circuit boards on rigid laminate materials, most of them with the characteristic green solder mask and white silkscreen. The majority of design courses (and this course) will focus only on designing rigid-PCBs as they are the most common PCB produced every year. This is understandable; many electronic products use this type of circuit board and you can find the materials from any manufacturer.
Millions of rigid circuit boards are produced at high volume each year.
However, as was briefly mentioned in the first unit, there are other types of PCBs that are used in different electronic products. These other types of PCBs are built with a range of alternative materials that are better suited for different applications. Some examples include high temperature environments, or in systems that have very complex enclosures.
Oftentimes, determining the type of PCB you want to build will be the first step in starting a new design. The different types of PCBs are defined by the materials used to build their stackups, so some types of PCBs are more desirable in certain products than others. In this section, we’ll briefly review some of the common types of PCBs and review the materials used to build them.
Common PCB Types and Their Materials
Rigid PCBs
The primary type of circuit board you’ll find in most electronics is a rigid circuit board. The layer stack for these boards uses rigid laminates made from fiberglass impregnated with epoxy resin. The standard set of laminate materials used to build rigid PCBs is called “FR-4”. The term “FR-4” only applies to the PCB laminate, it does not apply to the copper, silkscreen, or solder mask.
This PCB was manufactured using a standard FR-4 grade laminate.
It’s common to hear some designers refer to an “FR-4 material” or use a similar expression, but it is important to note that the FR-4 designation does not refer to one specific material, company, or brand of laminate. Instead, FR-4 refers to a flammability rating assigned by the National Electrical Manufacturers Association (NEMA). Although NEMA defines specific requirements for this class of PCB laminates, manufacturers self-certify their compliance to this standard, and there is no independent body that audits laminate manufacturers. The laminate manufacturers can also freely choose various glass weights, epoxy materials, chemistries, and different filler types and content to use in their FR-4 laminates.
PCBs with PTFE Materials
While FR-4 laminates are most commonly used in the electronics industry, they are not the only material used to build PCB stackups. PCB laminates based on polytetrafluoroethylene (PTFE, better known as Teflon) are sometimes used in systems that operate at very high frequencies, a prime example of which is circuit boards for radar systems. Some PTFE-based laminates use a fiberglass weave as a structural support with PTFE as a filler, similar to FR-4 laminate materials.
The exposed white area in this circuit board is a PTFE laminate placed on the top layer.
These materials are used because electrical signals traveling on these laminates experience much lower losses compared to FR-4. However, these materials are expensive, so it is uncommon to see a multilayer PCB fabricated entirely from PTFE laminates. One common configuration is to place a sheet of PTFE laminate only on the top layer of a PCB stackup, and traces carrying high frequency signals will get routed on top of the PTFE layer so that the signal will experience very low loss.
Flex PCBs
Flexible PCBs, or flex PCBs, are fabricated using a flexible material called polyimide. Flexible PCBs can be shaped and molded into complex enclosures, and they can be assembled with through-hole or SMD components. A flexible PCB stackup uses rolled copper foil layers that are bonded to polyimide with an adhesive, as shown below. Multiple layers of alternative polyimide/adhesive/copper foil can also be used to build a multilayer flex PCB.
A special fabrication process is used to build flex PCBs because the materials involved are quite different from rigid laminates. During fabrication, flexible layer stacks are fragile, so they are handled very sparingly during fabrication to prevent damage to the layer stack. After fabrication and assembly, the PCB can be bent and molded as needed to fit into the device’s enclosure.
Rigid-flex PCBs
These circuit boards combine a flexible section with a rigid section. To build a PCB stackup for this circuit board, a stack of flexible layers is pressed between two rigid laminates. The pressing process forms a strong bond between the flexible and rigid layers, leaving a flexible section that protrudes from the side of the rigid layer stack. Just like in flex PCBs, polyimide is used to fabricate the flexible ribbon, which is then bonded to standard FR4 laminates to form a rigid-flex PCB stackup. Many rigid-flex PCBs will use the flexible region as a built-in connector between two or more rigid stacks, as shown in the image below.
Just like flex PCBs, the flexible region can have traces routed across the ribbon, and some components can be soldered onto flex ribbons. This flex region can then be bent to the desired shape once it is installed in its enclosure. Rigid-flex boards use a mix of the standard rigid PCB fabrication process and the flex PCB fabrication process. Although the fabrication process for these circuit boards is more complex than that used for individual flex PCBs, these boards can be used to eliminate the need for bulky cable assemblies and connectors that would normally be used to connect multiple rigid PCBs.
Metal-core PCBs
Some PCBs may have components that generate a lot of heat in a concentrated area, such as motor control boards or boards for industrial lighting. These products are sometimes manufactured using a metal-core PCB. This type of PCB includes a solid metal layer that very easily dissipates heat from components on the board, making it very useful for components that may reach very high temperatures.
In this type of PCB, the central core layer is replaced with a slab of metal, usually an aluminum alloy or copper. Dielectric materials are then bonded to the metal core layer to provide an electrically insulating barrier between any components on the surface layer and the internal metal core. Some of these boards will have additional plane or signal layers on the interior for additional routing space.
Example metal-core PCB stackup.
In the above board, the standard process for placing vias cannot be used because the metal core layer will create a short circuit. Therefore, a specialized drilling and plating process is used to route electrical connections between each side of the board. An alternate version of this board only has components, traces, and dielectric materials on one side of the metal core instead of on both sides, so this drilling/plating process is not needed.
Ceramic PCBs
Ceramic circuit boards can be designed as single-layer, double-layer, or multilayer boards using a specialized fabrication process. Typical materials used to fabricate the insulating layers are alumina, aluminum nitride, and beryllium oxide. Ceramic PCBs also offer very high thermal conductivity that is similar to that of metals. They also have lower thermal expansion coefficient compared to FR-4 laminates. For these reasons, ceramic PCBs may be used in environments where the board temperature is cycled between very high and very low values.
Through-Hole or Surface-Mount?
Some design companies will make a distinction between through-hole PCBs and surface-mount PCBs. This would suggest that designers have to choose between these two types of components and they do not have the freedom to use both types of components on the same PCB. This is incorrect; designers are free to use both types of components in their circuit boards as long as the board can be reliably fabricated.
This PCB uses both SMD components and through-hole components in the design.
Today’s ECAD software has plenty of built-in tools and rules that help ensure you comply with standard DFM requirements. PCB fabricators will also put a board through several checks to ensure DFM requirements are met when using both through-hole and SMD components. Therefore, you generally shouldn’t worry about using a mix of SMD components and through-hole components in a design.