Terminal Board Materials — Choosing the Right Insulating Substrate

Terminal boards are among the most volume-intensive thermoset applications in electrical manufacturing — the right material choice balances punchability (for high-volume hole production), dielectric strength, moisture resistance, and cost against the voltage class and environmental conditions of the end application.

TL;DR — Key Takeaways

• Paper-phenolic (XXXP, XPC) is the traditional terminal board material — cold-punchable, low cost, adequate for dry indoor service ≤ 600V
• FR4 is replacing paper-phenolic in premium terminal boards where moisture exposure, higher voltage, or UL 94 V-0 is required
• GPO-3 is used for terminal boards in switchgear where arc resistance and V-0 are both needed at moderate cost
• G10 is used for precision machined terminal assemblies and HV terminal blocks (> 600V)
• UL 508A (industrial control panels) requires V-0 for exposed live-part insulation — XXXP phenolic (SE, not V-0) may not comply; check with the panel listing authority

What Terminal Boards Do

A terminal board (also called a terminal strip, connection board, or barrier strip board) serves as the insulating substrate into which terminal blocks, binding posts, or feedthrough terminals are mounted. Functions:

1. Electrical isolation between terminals and between terminals and metallic structures
2. Mechanical support for terminal hardware under vibration and insertion/extraction forces
3. Hole grid — the board must accept mounting holes punched or drilled to standard terminal pitch (typically 0.375″, 0.500″, 0.750″ or 25.4mm, 19.05mm pitch)
4. Code compliance — the board material must meet the flammability and creepage requirements of the equipment's listing standard

Grade Comparison for Terminal Board Applications

The Punchability Question

For high-volume terminal board production, cold-punching is the lowest-cost hole-making method. Standard progressive die tooling punches hundreds of terminal positions per stroke. Paper-phenolic grades support this:

Which Grades Can Be Cold-Punched?

• XXXP, XPC: Specifically formulated for cold punching — the phenolic resin is optimized for clean edge formation without cracking around hole perimeters. Standard punching recommended at room temperature.
• XX, XXX: Standard paper-phenolic — can be punched but with more edge cracking and die wear than XXXP
• LE (linen-phenolic): Can be punched in thin gauges; linen fiber increases edge quality vs. paper but still inferior to XXXP
• G10, FR4, GPO-3: Cannot be cold-punched at room temperature — glass fiber fractures rather than shears under punch loading. Must be drilled, routed, or water-jet cut.

Warm punching of G10 and FR4 (150–175°C substrate temperature): Allows punching of glass-epoxy materials, but requires heated tooling and is a specialized process. Only economical for very high volume production.

Voltage and Code Requirements

UL 508A — Industrial Control Panels

UL 508A requires UL 94 V-0 rated material for exposed insulating parts that are within 12.7mm of a live part in the listed panel. Terminal boards within the panel's live-circuit zone therefore require V-0 material.

Impact: Paper-phenolic (XXXP, SE rating) may not qualify for UL 508A-listed panels where the terminal board is in the live-circuit zone. FR4 (V-0) or GPO-3 (V-0) is required.

Workaround: Some panel builders use metal-covered terminal boards (the metal top plate serves as fire barrier) with phenolic substrate — the visible surface is metal, not phenolic. Verify this approach with the listing authority.

UL 891 — Switchboards

UL 891 for switchboards requires V-0 for all insulating parts that support or separate live conductors in equipment > 600A. FR4 or GPO-3 is required; XXXP phenolic does not comply.

IEC 60947-5 — Low-Voltage Switchgear and Controlgear

IEC 60947-5 covers terminal blocks and requires the terminal block body to achieve IEC classification "D" (CTI ≥ 400, Group II) or better for most industrial applications. The terminal board substrate (on which the block mounts) must provide equivalent insulation coordination.

Hole Patterns and Standard Pitches

Terminal boards are produced in standard hole pitches matching the terminal hardware being mounted. Common pitches:

Thickness Selection

Terminal board thickness determines:

• Thread strength in threaded holes (typically minimum 3× thread pitch = ~3/32″ for #6 screw)
• Mechanical stiffness between mounting points
• Available creepage path length through holes (IPC-2221 requires hole wall length contributes to creepage)

Common terminal board thicknesses:

• 1/16″ (0.062″): Minimum for thin panels; adequate for light-duty terminals ≤ 30A
• 3/32″ (0.093″): More common standard; adequate for most terminal blocks through 60A
• 1/8″ (0.125″): Standard for medium-duty panels; good thread engagement
• 3/16″ (0.187″): Heavy-duty; used for large terminal studs and bus-mounted terminals

Environmental Considerations

More related guides

Materials

• Phenolic-Paper Material Hub
• FR4 Material Hub
• G10 Material Hub

Applications

• Bus Bar Insulation Materials
• Arc Resistance vs Track Resistance