What Are Printed Circuit Boards Made Of?

Printed circuit boards are engineered composites of several materials, each serving specific electrical and mechanical functions. Understanding these materials helps you make better design decisions.

Core Substrate Materials

FR4 (Flame Retardant 4)

FR4 is the most common PCB substrate, accounting for over 90% of all PCBs manufactured.

Composition: Woven fiberglass cloth impregnated with flame-retardant epoxy resin.

Properties:

• Dielectric constant: 4.2-4.5
• Glass transition temperature: 130-140°C
• Good mechanical strength
• Excellent electrical insulation
• Cost-effective

Limitations: Not suitable for high-frequency (>1 GHz) or high-temperature applications.

High-Frequency Materials

For RF and microwave applications, specialized materials are used:

Rogers (RO4003C, RO4350B): Lower dielectric constant, tighter tolerance, stable over temperature and frequency.

PTFE (Teflon): Excellent high-frequency performance but difficult to process.

Isola: Various high-speed materials for different performance needs.

High-Temperature Materials

Polyimide: Withstands temperatures up to 250°C, used in flex PCBs and high-temperature applications.

Ceramic substrates: Aluminum oxide (Al₂O₃) or aluminum nitride (AlN) for extreme temperature and thermal management.

Copper Layers

Copper provides the conductive pathways. It's measured in ounces per square foot.

Copper Weight	Thickness	Typical Use
0.5 oz	17.5 µm	HDI, fine pitch
1 oz	35 µm	Standard (most common)
2 oz	70 µm	Higher current
3 oz	105 µm	Power electronics
4+ oz	140+ µm	High power

Copper Types:

• Electrodeposited (ED): Standard, slightly rough surface
• Rolled Annealed (RA): Smoother, used for flex PCBs

Prepreg and Core

Core: Cured laminate with copper on both sides. The base building block.

Prepreg (Pre-impregnated): Uncured fiberglass/resin sheets used to bond layers together during lamination.

Multi-layer boards stack cores and prepreg, then cure under heat and pressure.

Solder Mask

The colored coating covering most of the PCB (typically green).

Function:

• Protects copper from oxidation
• Prevents solder bridges during assembly
• Provides electrical insulation
• Identifies PCB manufacturer/quality

Types:

• Liquid photoimageable (LPI) - most common
• Dry film - for complex topographies
• Spray coating - lower cost

Colors: Green (standard), blue, red, black, white, yellow, matte versions.

Silkscreen

The legend layer with component designators, logos, and markings.

Function:

• Component reference designators (R1, C2, U3)
• Polarity markings
• Company logos and part numbers
• Assembly instructions

Colors: White (on dark masks), black or yellow (on light masks).

Surface Finish

Protects exposed copper pads and enables soldering.

Finish	Shelf Life	Cost	Notes
HASL	12 months	$	Lead or lead-free, not flat
ENIG	12+ months	$$$	Flat, good for fine pitch
OSP	6 months	$	Organic, shortest shelf life
Immersion Silver	6 months	$$	Good solderability
Immersion Tin	6 months	$$	Flat, whisker concerns

Complete Layer Stack

A typical 4-layer board consists of:

1. Silkscreen (top)
2. Solder mask (top)
3. Copper layer 1 (signal)
4. Prepreg
5. Copper layer 2 (ground plane)
6. Core
7. Copper layer 3 (power plane)
8. Prepreg
9. Copper layer 4 (signal)
10. Solder mask (bottom)
11. Silkscreen (bottom)

Conclusion

FR4 substrate, copper foil, solder mask, and silkscreen form the basic PCB structure. Material selection depends on electrical requirements (frequency, current), thermal needs, mechanical constraints, and cost considerations. Most applications work well with standard FR4, but specialized applications may require high-frequency or high-temperature materials.