FR-1, FR-2, FR-3, FR-4 and FR-5: Material Differences and Selection

Compare the reinforcement and resin first

The paper-family descriptions are consistent with Lamitex’s NEMA FR-1, FR-2 and FR-3 material guidance. That source discusses industrial insulating laminates; it should not be interpreted as proof that every named grade is available as a PCB-qualified copper-clad sheet. In particular, FR-3 is not a woven-glass epoxy grade.

FR-1 and FR-2 should not be distinguished by an invented universal Tg boundary or a simple “good versus better” label. Their precise grade requirements and offered products matter. For a board already qualified on a particular paper laminate, inspect the existing drawing, supplier designation and assembly history before proposing a replacement.

For a multilayer rigid board, glass-epoxy systems provide a familiar construction route using cores and compatible prepregs. The resin system, glass construction and processing details still matter. The FR-4 PCB construction overview addresses that manufacturing route; this page concentrates on the differences between the FR families.

Do not confuse an FR grade with a UL 94 rating

“FR” identifies flame-retardant material grades in this context. It does not mean fireproof, and the numbers 1 through 5 are not a flame-resistance ranking. A material’s flammability classification must be read from the applicable product documentation and recognition record, including its tested thickness and other conditions.

UL’s explanation of plastics flammability testing distinguishes different test methods and classifications. For example, VTM classifications for thin materials are not interchangeable with the corresponding V classifications. It is therefore misleading to assign every FR-1 material “fair” flame resistance and every FR-4 material “best” without an actual test basis.

A supplier’s laminate recognition also does not automatically certify a finished electronic product. Ask which material and board recognition applies to the proposed construction and what documentation the product approval process requires. If the design needs a particular class at a particular thickness, state those requirements explicitly rather than assuming they follow from the family name.

Separate Tg, decomposition, expansion and long-term use

Temperature comparisons become useful only when the property and test method are identified. Glass transition temperature, Tg, describes a transition in the resin’s behavior. Decomposition temperature, Td, describes degradation under a defined test criterion. Neither is a universal maximum continuous operating temperature for an assembled PCB.

Expansion through the board thickness affects the copper interconnect structure during thermal excursions. A laminate can have an attractive Tg value yet require a closer review of its expansion behavior, moisture condition and repeated soldering exposure. The board construction and plated holes must survive the manufacturing and service conditions together.

A useful counterexample to “FR-5 means every high-Tg board” is Isola 370HR. The manufacturer describes it as an FR-4 system and lists a typical DSC Tg of 180°C and a typical Td of 340°C using a 5% weight-loss criterion. These are properties of that named product under stated methods, not values to assign to all FR-4 laminates or acceptance limits for an unspecified board.

The Gund glass-epoxy comparison discusses NEMA and IEC material classifications and the need for comparable test evidence. Use the exact classification required by the design or purchasing specification. Do not rename a high-Tg FR-4 laminate “FR-5” based only on one temperature number.

When considering a dedicated FR-5 PCB requirement, first establish why that grade is specified and whether the offered copper-clad system meets it. A substitution review should address all relevant properties and available constructions, not merely match or exceed Tg.

Compare dielectric data at the frequency and construction that matter

A single generic dielectric constant cannot accurately describe every FR-4 product. Resin content, glass construction, frequency and test method influence published data and the model used for a finished board. For controlled impedance, ask for the proposed stackup and the data the fabricator will use for that construction. A catalog value and a design value are not automatically interchangeable.

Loss tangent is also a condition-dependent property. FR-5 should not be assumed to provide lower high-frequency loss than every FR-4 material. If the project has an insertion-loss requirement, compare suitable named materials and copper constructions at relevant frequencies. Define the channel requirement before choosing a material family from a list.

For insulation performance, review the required voltage, spacing, environment and contamination conditions. Moisture resistance, tracking behavior and board cleanliness answer different questions from Dk and Df. A lower Dk does not by itself make a board safer at high voltage, and a flame-retardant designation does not establish creepage or clearance.

Keep the electrical review proportionate to the application. A simple low-frequency control board and a fast serial channel may both use glass epoxy, but they need different evidence. Over-specifying a laminate without a defined failure mechanism can increase sourcing constraints without resolving the real design risk.

Apply the comparison to three purchasing situations

A simple single-sided legacy control board

Begin with the material already called out in the approved drawing. Confirm that the intended supplier offers the specified copper-clad grade, thickness and copper weight. Check the soldering process, operating environment and mechanical features. If punched features are part of the design, verify the relevant processing conditions rather than assuming any inexpensive paper laminate will behave identically.

A move from paper phenolic to glass epoxy may change fabrication methods, tooling assumptions and cost. It can be appropriate, but it is an engineering change that requires review. Do not infer the material reliably from board color, surface finish or a photograph of an old assembly.

A multilayer controller with plated through-holes

Specify a compatible core/prepreg system, stackup, final thickness and copper requirements. Review the assembly thermal history, expected rework and service cycling alongside plated-hole construction. If an impedance requirement exists, include its tolerance and measurement scope. The multilayer FR-4 guide is the more appropriate next step than comparing generic family numbers again.

Ask for a proposed named laminate and any permitted alternatives before fabrication. “Equivalent FR-4” should lead to a documented comparison and approval, especially where a change affects electrical modeling, recognition or an already qualified process.

A design calling for higher-temperature operation

Identify the actual stress first: repeated reflow, local component heating, high ambient temperature or long-term thermal cycling. Those are different requirements. Review the laminate, copper interconnections, solder joints and component ratings as a system. A board material with a higher Tg cannot correct an overheated component or an inadequate thermal path by itself.

If the existing requirement explicitly calls for FR-5, preserve that requirement until the design owner approves a change. If the requirement merely says “high temperature,” replace the ambiguity with measurable conditions and suitable evidence. Avoid adding a high-temperature grade name as a substitute for defining the product’s operating limits.

Turn the family choice into an orderable specification

A useful fabrication note identifies the material manufacturer and grade where required, the applicable material specification, finished thickness and tolerance, copper requirements, layer stackup and any controlled-impedance conditions. It also identifies permitted substitutions and who must approve them. Distinguish a mandatory requirement from a suggested starting point so the quotation can answer the actual design need.

Ask the supplier to flag unavailable constructions before work begins. A datasheet showing that a material exists does not establish present stock, lead time, available prepregs or a manufacturer’s experience with your exact stackup. Those are project-specific sourcing and process questions. No universal FR-1-to-FR-5 price multiplier can replace a quote for the specified board.

For a PCB manufacturing review, provide the fabrication files, layer count, stackup, quantity and assembly conditions. Include the existing material callout and any required test or recognition documents. RAYPCB can review the requested construction and sourcing scope against the project; this family comparison does not certify a proposed substitute.

Review a PCB material requirement

Common questions