Nov 28, 2018|General
A PCB is the heart of an electronic device. Without one the device would not function as it was intended. That is why it is crucial that the PCB be made with the utmost quality, down to the selection of materials and equipment used.
What are PCBs made of?
Table of Contents
Many materials come together to develop a printed circuit board. Each material has a purpose that contributes to the functionality of the board. Depending on what the intended purpose of the board is, that will factor into the type of materials you use.
Each board will be different, but they will mostly share the following elements:
- The core layer – contains glass fibre reinforced epoxy resin
- A conductive layer – contains traces and pads to make up the circuit (usually with copper, gold, silver)
- Solder mask layer – thin polymer ink
- Silkscreen overlay – special ink that shows the component references
- A tin solder – used to attach components to through holes or surface mount pads
The silkscreen is usually the final layer of the board, which allows the PCB manufacturer to write labels on the important areas of the board. It is a special ink that shows the symbols and component references for the component locations during the assembly process. The silkscreen typically is white, though you may see other colors like red, yellow, gray and black used.
RoHS Compliant PCB
RoHS Compliant PCBs are those that follow the Restriction of Hazardous Substances from the European Union. The ban is on the use of lead and other heavy metals in consumer products. Every part of the board must be free of lead, mercury, cadmium, and other heavy metals.
The laminates are created by curing under high temperatures and pressure layers of cloth with a thermoset resin. This process forms the uniform thickness that is essential to the PCB. If your board has a high layer count, then the laminate must be made up of woven glass for dimensional stability.
Key Substrate Parameters
The substrate layer is the foundation of the PCB. It is made up of fire retardant materials of fibreglass. It is typically known as FR4, with the FR standing for “fire retardant.”
Three key parameters characterize the substrate:
- Chiefly thermomechanical (glass transition temperature, thermal expansion, tensile strength, shear strength).
- Electrical (dielectric constant, dielectric breakdown voltage, leakage current, loss tangent, tracking resistance).
- Others (moisture absorption).
Some of the other common materials used for the substrate include FR2 phenolic paper. This is most common with single-sided boards, as the electrical properties are less than the FR4 material.
There is also aluminum or an insulated metal substrate. Typically, this is a single layer (although can be a double layer) thin circuit board.
You may encounter flexible substrates that have standalone copper-clad foil. It can also be laminated to a thin stiffener.
The copper thickness can be the weight of the copper in the area. Heavier copper layers are for higher currents to help dissipate heat.
The Solder Mask
The solder mask layer protects the copper from corrosion, which can damage your PCB. The solder mask can be either tin-lead, nickel, gold, or all three of them. Quite often, the solder mask is a green color, which gives the PCB a distinctive look.
FR4 materials are the most common type used for building a PCB. However, in some instances, non-FR materials are better suited. One material you can use is polyimide.
Polyimide (also seen as polyamide) is a material you can find in nature or you can create it synthetically. Examples of natural polyimide are materials like silk and wool.
Many PCB manufacturers use polyimide because it is flexible, durable, and strong. In addition, polyimide is heat-resistant, which is critical for building a PCB. If you’re building a flexible board, it’s best to use polyimide over an FR material.
Another material you can use is ceramic. Many prefer using ceramic because it has a high level of thermal conductivity along with low expansion coefficient for the substrates. If you are building a high-power circuit board, using ceramic is the best option.
Laminate materials are preferred when dealing with microwave frequencies with a board. The high-frequency material, the most common being polytetrafluoroethylene (PTFE) and teflon, has excellent dielectric properties. With high-frequency requirements, you need very precise controls on the dielectric constant and thickness.