Printed circuit boards (PCBs) in the US are mostly a flat laminated composite. PCB manufacturers use non-conductive substrate materials along with layers of copper circuitry on the external surfaces and buried in internal layers.

Printed circuit boards may be simple, with one or two layers of copper on the substrate, or complex, such as in high density applications, with more than fifty layers. Manufacturers use a flat composite surface, as this is ideal for mounting components that are ultimately attached to the copper layer by soldering. The copper traces interconnect the components electrically.

Basic Components and Materials of a Standard PCB

A standard PCB usually has six basic components:

• Prepregs
• Laminates
• Copper Foil
• Soldermask
• Silk Screen
• Surface Finish

Prepreg is made from a thin fabric of glass and coated with resin. Manufacturers cure the resin in special treaters. The glass fabric forms the basic mechanical support for holding the resin in place. The resin can be of various materials, like FR-4 epoxy, Teflon, Polyimide, and others. Initially, the resin is in a liquid form when coated onto the glass fabric. The combination moves through the treater, and the epoxy begins to dry as it travels through the oven section. Once it exits the treater, the epoxy is dry to touch.

When exposed to higher temperatures, usually above 300 °F, the resin in the prepreg begins to soften and melt. The resin soon reaches its thermosetting point, where it rehardens to become rigid again and very strong. Manufacturers use prepreg sheets to build the second component of a PCB—the laminate.

When laminated together with heat and pressure, sheets of prepreg and copper foil form the copper clad laminates for PCBs. Once the resin hardens, these PCB laminates with copper foil on both sides establish the basic structure of which PCB manufacturers make printed circuit boards.

To produce the circuitry on the laminate surface, the PCB fabricator must image and etch away the unwanted copper from the copper foil. These copper circuits create the tracks or conductors on the internal and external layers of the board. Once the fabricator images and etches each internal layer to constitute the circuits, they laminate them together using the prepreg.

Solder Mask is another epoxy coating on the board to cover the copper tracks on its outer layers. The coating protects the copper tracks on the external layers from oxidizing and corroding over time. As the internal circuitry remains buried within layers of prepreg, they do not need additional protection. Copper pads, where components will be soldered, are not covered by the soldermask.

Silk screen or nomenclature, forms the lettering on the PCB surface showing component numbers, their location, and their orientation. They can also show additional mounting information or handling precautions.

Besides the common green and white colors for the solder mask and silkscreen, manufacturers often use other colors also, in consultation with their customers.

Surface finish protects the copper pads left exposed by the soldermask. Fabricators use various types of surface finishes such as silver, tin/lead solder, lead-free solder, gold, nickel and others. Apart from protecting the exposed copper pads, surface finishes also provide a surface suitable for good solderability.

Selecting Materials for Printed Circuit Boards

Printed circuit board manufacturers offer a variety of materials for making the substrates and other components of PCBs. PCB designers select materials depending on the application requirements. Different material choices provide the PCB with varying qualities, facilitating the optimum performance for specific applications.

Designers make a choice of materials depending on:

• Electrical performance — high voltage, high speed, high frequency
• Mechanical performance — rigidity, flexibility, flame retardant
• Thermal performance — ability to survive high temperatures, good thermal conductivity, low coefficient of thermal expansion
• Regulatory/Governmental requirements — WEEE, RoHS, UL

For instance, a common popular consideration is complying with UL or Underwriters Laboratories requirements for flame retardation characteristics. Many electronic devices come with a UL rating to show that in the event of the device catching fire, the circuit board will retard the flames and will self-extinguish. This is a critical consideration for several types of consumer and other electronic equipment.

Resins and glass cloth fabrics that make up the laminates, must offer distinct insulating capabilities. Designers use different dielectric materials offering distinctive electrical and thermal characteristics for determining which laminate will best suit a PCB design for a specific application.

When making a material selection for their design, designers must consider various performance features. Most important among these are:

• Dielectric Constant and Loss Factor — key electrical factors essential for high-speed performance and signal integrity.
• High Glass Transition Temperature — capability to withstand high temperatures during assembly and field operation.
• Flame Retardation — for UL qualification
• Thermal Conductivity and Coefficient of Thermal Expansion — capability to perform in harsh service environments
• Mechanical Strength and Dimensional Stability — mechanical attributes like shear and tensile strength when the PCB is in service, changes in dimension when manufacturing, undergoing thermal cycles during assembly, or when exposed to humidity.

Some Popular Materials for PCBs

FR-4 Prepreg and Laminates

FR-4 is the most popular PCB substrate material and is fire-retardant. The material has good thermal, electrical, and mechanical properties, along with a favorable strength-to-weight ratio as necessary for electronic applications. These are also one of the lowest cost PCB materials available.

Among printed circuit board manufacturers, FR-4 is a popular material for laminates and prepreg made from glass cloth and epoxy resins, for PCBs with low layer counts up to 14 layers. To improve its performance manufacturers often blend the epoxy resin with additives. This significantly improves its electrical, mechanical, and thermal properties, along with its ability to retard flames. Manufacturers use FR-4 laminates and prepregs as they are versatile, adaptable, and offer predictable yields.

Polyimide Prepreg and Laminates

For higher temperature performance compared to FR-4 materials, manufacturers use Polyimide laminates. Costing more than FR-4 materials, Polyimide materials offer better electrical performance, improved capacity to survive in higher temperature and harsh environments. With lower expansion characteristics, Polyimide materials are more stable during thermal cycling. Polyimide is more suitable for higher layer count PCBs.

Teflon Prepreg and Laminates

Bonding materials and laminates made of Teflon offer superior electrical properties, making them suitable for high-speed applications. Although more expensive than Polyimide materials are, Teflon offers designers better signal integrity in their designs. Printed circuit board manufacturers can enhance the characteristics of Teflon with special fillers and additives. Working with Teflon for PCBs requires specialized equipment and processing techniques.

Conclusion

Careful selection of materials for laminates and prepreg is necessary to ensure the Printed Circuit Board has the necessary electrical, dielectric, mechanical, and thermal properties for the end application.