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Common substrates


Often encountered materials:
  • FR-2 (Flame Retardant 2), phenolic paper or phenolic cotton paper, paper impregnated with a phenol formaldehyde resin. Cheap, common in low-end consumer electronics with single-sided boards. Electrical properties inferior to FR-4. Poor arc resistance. Generally rated to 105 °C. Resin composition varies by supplier.
  • FR-4 (Flame Retardant 4), a woven fiberglass cloth impregnated with an epoxy resin. Low water absorption (up to about 0.15%), good insulation properties, good arc resistance. Well-proven, properties well understood by manufacturers. Very common, workhorse of the industry. Several grades with somewhat different properties are available. Typically rated to 130 °C. Thin FR-4, about 0.1 mm, can be used for bendable circuitboards. Many different grades exist, with varying parameters; versions are with higher Tg, higher tracking resistance, etc.
  • Aluminium, or metal core board or insulated metal substrate (IMS), clad with thermally conductive thin dielectric - used for parts requiring significant cooling - power switches, LEDs. Consists of usually single, sometimes double layer thin circuitboard based on e.g. FR-4, laminated on an aluminium sheetmetal, commonly 0.8, 1, 1.5, 2 or 3 mm thick. The thicker laminates sometimes come also with thicker copper metalization.
  • Flexible substrates - can be a standalone copper-clad foil or can be laminated to a thin stiffener, e.g. 50-130 µm
  • Kapton, a polyimide foil. Used for flexible printed circuits, in this form common in small form-factor consumer electronics or for flexible interconnects. Resistant to high temperatures.
  • Pyralux, a polyimide-fluoropolymer composite foil. Copper layer can delaminate during soldering.
  • FR-1 (Flame Retardant 1), like FR-2, typically specified to 105 °C, some grades rated to 130 °C. Room-temperature punchable. Similar to cardboard. Poor moisture resistance. Low arc resistance.
  • FR-3 (Flame Retardant 3), cotton paper impregnated with epoxy. Typically rated to 105 °C.
  • FR-5 (Flame Retardant 5), woven fiberglass and epoxy, high strength at higher temperatures, typically specified to 170 °C.
  • FR-6 (Flame Retardant 6), matte glass and polyester
  • G-10, woven glass and epoxy - high insulation resistance, low moisture absorption, very high bond strength. Typically rated to 130 °C.
  • G-11, woven glass and epoxy - high resistance to solvents, high flexural strength retention at high temperatures. Typically rated to 170 °C.
  • CEM-1, cotton paper and epoxy
  • CEM-2, cotton paper and epoxy
  • CEM-3, non-woven glass and epoxy
  • CEM-4, woven glass and epoxy
  • CEM-5, woven glass and polyester
  • PTFE, pure - expensive, low dielectric loss, for high frequency applications, very low moisture absorption (0.01%), mechanically soft. Difficult to laminate, rarely used in multilayer applications.
  • PTFE, ceramic filled - expensive, low dielectric loss, for high frequency applications. Varying ceramics/PTFE ratio allows adjusting dielectric constant and thermal expansion.
  • RF-35, fiberglass-reinforced ceramics-filled PTFE. Relatively less expensive, good mechanical properties, good high-frequency properties.
  • Alumina, a ceramic. Hard, brittle, very expensive, very high performance, good thermal conductivity.
  • Polyimide, a high-temperature polymer. Expensive, high-performance. Higher water absorption (0.4%). Can be used from cryogenic temperatures to over 260 °C.
Copper thickness
Safety certification (US)
Less-often encountered materials:
Copper thickness of PCBs can be specified as units of length (in micrometers or mils) but is often specified as weight of copper per area (in ounce per square foot) which is easier to measure. One ounce per square foot is 1.344 mils or 34 micrometers thickness.
The printed circuit board industry defines heavy copper as layers exceeding three ounces of copper, or approximately 0.0042 inches (4.2 mils, 105 μm) thick. PCB designers and fabricators often use heavy copper when design and manufacturing circuit boards in order to increase current-carrying capacity as well as resistance to thermal strains. Heavy copper-plated vias transfer heat to external heat sinks. IPC 2152 is a standard for determining current-carrying capacity of printed circuit board traces.
On the common FR-4 substrates, 1 oz copper (35 µm) is the usual, most common thickness; 2 oz (70 µm) and 0.5 oz (18 µm) thickness is often an option. Less common are 12 and 105 µm, 9 µm is sometimes available on some substrates. Flexible substrates typically have thinner metalization; 18 and 35 µm seem to be common, with 9 and 70 µm sometimes available. Aluminium or metal-core boards for high power devices commonly use thicker copper; 35 µm is usual but also 140 and 400 µm can be encountered.
Safety Standard UL 796 covers component safety requirements for printed wiring boards for use as components in devices or appliances. Testing analyzes characteristics such as flammability, maximum operating temperature, electrical tracking, heat deflection, and direct support of live electrical parts.

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