The materials used to make a PCB are not arbitrary. Every layer in the stack, from the substrate and copper foil through to the solder mask and surface finish, is chosen to meet specific electrical, mechanical, or thermal requirements. This page explains what each layer is, what it does, and why material selection matters for performance and manufacturability.

What is a PCB made of?

A printed circuit board is a precisely engineered stack of materials, each chosen for specific electrical, mechanical, or thermal properties. The combination of those materials determines how the board performs in service, how it behaves during manufacture, and how reliably it will function over its working life.

Substrate

The substrate is the base material of the PCB. It provides the structural foundation of the board and acts as the electrical insulator between conductive layers.

FR4 is the most widely used substrate material. It is a woven glass fibre cloth impregnated with an epoxy resin system, offering a good balance of mechanical strength, electrical insulation, and thermal stability for the majority of standard applications.

For more demanding applications, alternative substrate materials are used. High-frequency and RF designs often require low-loss laminates such as Rogers or PTFE-based materials, which have tightly controlled dielectric constants and low dissipation factors. High-temperature applications may call for polyimide-based materials, which maintain their properties at elevated temperatures where FR4 would degrade.

The choice of substrate has a direct impact on signal integrity, thermal performance, and manufacturability. Getting it right at the design stage is critical.

Copper

Copper forms the conductive layers of a PCB. It is laminated onto the substrate as a thin foil, then selectively etched away to leave the tracks, pads, and other features that carry electrical signals and power across the board.

Copper weight is specified in ounces per square foot, with 1oz (approximately 35µm) being the most common for standard signal layers. Heavier copper weights, such as 2oz or 3oz, are used for power layers or applications requiring higher current carrying capacity. Exception PCB supports copper weights from 9µm through to 210µm depending on the layer type and build specification.

Prepreg and core materials

In multilayer PCBs, the individual circuit layers are bonded together using prepreg, a partially cured glass-reinforced resin sheet that flows and bonds under the heat and pressure of the lamination process. Core materials are fully cured laminates with copper on both sides, forming the inner layers of the stack.

The combination of core and prepreg thicknesses determines the overall board thickness and the spacing between layers, both of which affect electrical performance and impedance control.

Solder mask

The solder mask is a polymer coating applied over the outer copper layers of the board. It serves two purposes: protecting the copper from oxidation and contamination, and preventing solder from bridging between pads during assembly.

Green is the most common solder mask colour, but black, blue, red, white, and other colours are available. The choice is generally cosmetic unless specific optical inspection requirements apply. Solder mask is applied as a liquid photoimageable coating and exposed to UV light through a patterned film or direct imaging system to define where it is and is not present.

Surface finish

The exposed copper pads on a PCB, the areas left clear of solder mask where components will be soldered, require a surface finish to protect the copper and ensure good solderability.

Common surface finishes include ENIG (Electroless Nickel Immersion Gold), ENEPIG, electrolytic hard gold, OSP (Organic Solderability Preservative), immersion tin, and immersion silver. Each has different characteristics in terms of shelf life, solderability, flatness, and suitability for specific assembly processes or contact applications.

Exception PCB offers a comprehensive range of surface finishes in-house and via subcontract, including dual finish capability for boards requiring different finishes on different areas.

Silkscreen

The silkscreen is a printed layer of ink applied on top of the solder mask. It carries component reference designators, polarity markers, logos, and other information used during PCB assembly, inspection, and servicing. It does not perform an electrical function but is an important part of the finished board for assembly and traceability purposes.

Exception PCB works with a wide range of substrate materials, copper weights, and surface finishes to support complex and high-reliability PCB designs. To discuss your material requirements, get in touch with our team.