FR4 Specifications — IPC-4101, Thickness Classes, Glass Styles & Sizes
FR4 procurement is governed by two primary standards: IPC-4101 for PCB-grade copper-clad and unclad laminates, and NEMA LI 1 for general industrial laminates. Within those standards, the material is further differentiated by laminate thickness, glass fabric style, copper foil weight (for copper-clad), and resin content. Understanding these specifications prevents mismatches between ordered stock and fabrication requirements — particularly for controlled-impedance PCBs and precision machined electrical insulators.
At a glance:
- IPC-4101 /21 covers unclad structural sheet; /24 covers standard copper-clad; /41–/42 cover high-Tg grades
- Standard sheet sizes: 18" × 24", 24" × 36", 36" × 48", and 48" × 96" (varies by supplier)
- Nominal thickness range for structural stock: 0.031" to 2.0"+
- Glass styles: 1080 (thin, low weight), 2116 (mid-weight), 7628 (heavy, high-rigidity)
- Copper-clad laminate (CCL) uses 1/2 oz to 3 oz copper foil; unclad structural stock uses none
- Thickness tolerance: ±10% on nominal for NEMA LI 1 standard grades
Governing Standards
IPC-4101 — Printed Circuit Board Laminates
IPC-4101 ("Specification for Base Materials for Rigid and Multilayer Printed Boards") is the primary procurement standard for FR4 used in PCB manufacturing. It defines:
- Chemical and mechanical property minimums for each sub-class
- Acceptance test methods (referenced to IPC-TM-650)
- Dimensional tolerances for laminate thickness, length, and width
- Qualification requirements for laminate suppliers
Key IPC-4101 sub-classes for FR4:
| Sub-class | Type | Min Tg | Min Td | Halogen-free | Description |
|---|---|---|---|---|---|
| /21 | Unclad | 110°C | — | No | Standard Tg, unclad, structural use |
| /24 | CCL | 110°C | — | No | Standard Tg, copper-clad |
| /26 | CCL | 110°C | — | No | Standard Tg, enhanced electrical |
| /41 | Unclad | 150°C | 310°C | No | High-Tg, unclad |
| /42 | CCL | 150°C | 310°C | No | High-Tg, copper-clad |
| /99 | CCL | 150°C | 340°C | Yes | High-Tg, halogen-free |
NEMA LI 1 — Industrial Laminates
NEMA LI 1 defines the FR-4 grade for industrial laminated thermosetting products. It specifies minimum property values for flame retardancy, dielectric strength, moisture absorption, and mechanical properties in the structural (non-PCB) context. NEMA LI 1 FR-4 and IPC-4101 /21 are compatible in practice — distributors of structural FR4 sheet and plate typically supply material meeting both.
ASTM D229 — Test Methods for Sheet Laminates
ASTM D229 provides the test methods referenced by NEMA LI 1 for property measurement of rigid sheet laminates. Key tests:
- Dielectric strength (method D229-15, Test A for perpendicular, Test C for parallel)
- Flexural strength (Three-point bending per D229-15)
- Moisture absorption (D229-15, Method A: 24-hr immersion)
- Dimensional change (D229-15, Section 10)
Standard Sheet and Plate Sizes
Structural FR4 sheet and plate ships in standard panel sizes with slit or cut-to-size availability from distributors:
| Standard Panel Size | Common Use |
|---|---|
| 18" × 24" | Prototyping, small machined parts |
| 24" × 36" | Medium fabrication panels |
| 36" × 48" | Standard industrial sheet |
| 48" × 96" | Full-size industrial panel |
"Sheet" conventionally describes FR4 stock up to approximately 0.50" thick; "plate" refers to thicknesses of 0.50" and above. The distinction is informal — both are the same material in different thickness ranges. For transactional purposes, see the dedicated FR4 sheet and FR4 plate pages for stocked size tables.
Thickness Range and Tolerances
Standard PCB laminate thicknesses (nominal, pre-clad):
| Nominal Thickness | Common Use |
|---|---|
| 0.031" (0.8 mm) | Thin 2-layer PCBs |
| 0.047" (1.2 mm) | Mobile device PCBs |
| 0.062" (1.6 mm) | Standard PCB thickness (by far most common) |
| 0.093" (2.4 mm) | Power boards, thicker mechanicals |
| 0.125" (3.2 mm) | Bus bar insulators, thicker structural |
| 0.187" (4.8 mm) | Heavier structural insulation |
| 0.250" (6.4 mm) | Panel insulators, switchgear spacers |
| 0.500"–2.0" | Plate applications: bus bar supports, fixtures |
Glass Fabric Styles
The glass fabric reinforcement in FR4 is woven from E-glass yarns. The weave pattern and yarn weight determine laminate properties — stiffness, dielectric uniformity, and drillability. Three glass styles dominate PCB and structural FR4 production:
1080 Glass Style
- Yarn count: 60×47 per inch (warp × fill)
- Thickness per ply: ~0.0027" (0.069 mm)
- Resin content: high (~65%)
- Characteristics: thin, low weight, uniform dielectric; used for thin PCB cores and high-speed signal layers where consistent Dk is critical
- Applications: HDI PCBs, thin rigid-flex cores, controlled-impedance signal layers
2116 Glass Style
- Yarn count: 60×58 per inch
- Thickness per ply: ~0.0044" (0.112 mm)
- Resin content: moderate (~52%)
- Characteristics: balanced weight, good dimensional stability, good via quality; the most common glass style for mid-thickness prepreg layers in multilayer PCBs
- Applications: Standard multilayer PCB prepreg, 4–8 layer boards
7628 Glass Style
- Yarn count: 44×32 per inch
- Thickness per ply: ~0.0072" (0.183 mm)
- Resin content: lower (~43%)
- Characteristics: heavy, high glass content, maximum stiffness, coarser weave; used for thick outer cores and structural laminates where rigidity matters more than Dk uniformity
- Applications: Thick PCB cores (0.062"+ cores), structural sheet for electrical insulation, bus bar spacers
Copper-Clad Laminate (CCL) Specifications
For PCB-grade FR4 with copper foil bonded to one or both surfaces:
Copper Foil Weights
Copper foil weight is expressed in ounces per square foot. Common weights and their nominal thicknesses:
| Copper Weight | Nominal Thickness | Common Use |
|---|---|---|
| 1/4 oz | 0.0003" (8.5 μm) | Fine-line HDI |
| 1/2 oz | 0.0007" (17.5 μm) | Standard low-current signal layers |
| 1 oz | 0.00135" (35 μm) | Standard PCB (most common) |
| 2 oz | 0.00270" (70 μm) | Power distribution layers |
| 3 oz | 0.00405" (105 μm) | High-current power boards |
Peel Strength (Copper Adhesion)
IPC-4101 requires minimum peel strength values between copper foil and FR4 substrate. Typical values:
Unclad Structural FR4 Specifications
For FR4 used as structural electrical insulation (bus bar spacers, arc barriers, fixture plates — not PCB substrate), unclad stock is procured to NEMA LI 1 Grade FR-4 or IPC-4101 /21.
Key acceptance criteria per NEMA LI 1 FR-4:
Rod and Tube Specifications
FR4 rod is produced by mandrel winding glass-fabric prepreg and curing under pressure. Tube is similarly mandrel-wound but leaves a bore. These are not governed by IPC-4101 (which covers sheet laminate); they are typically produced to NEMA LI 1 or manufacturer-specific standards.
Standard size ranges:
| Form | Diameter / OD Range | Length |
|---|---|---|
| Rod | 0.25" to 6.0" OD | 36" standard |
| Tube | 0.50" to 12.0" OD | 36"–48" standard |
| Tube wall thickness | 0.062"–0.500" | Varies by OD |
For stocked sizes and lead times, see FR4 rod and FR4 tube.
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