ENGINEERING & SOURCING GUIDES
FR-4 Grade Selection for PCB Buyers
Choose an FR-4 grade by matching the board’s assembly exposure, operating environment, electrical requirements and approved construction to a named material’s current data. Then confirm that the fabricator can build that construction and document any substitutions. A quotation saying only “FR-4” or “high Tg” leaves too much unresolved for a design whose performance depends on the laminate.
This guide is a buyer’s selection workflow for rigid boards. It does not prescribe a universal temperature limit or rank suppliers. Start with the requirements below, then use the multilayer stack-up guide to connect the selected material to an actual board construction.
Build the requirement list before comparing grades
A useful material comparison begins with what the board must experience. Ask the design and assembly owners for the conditions they know, and mark estimates as provisional. “Industrial use” is too broad: a board in a dry control cabinet and one in a humid enclosure may have different qualification questions even if both use the same layer count.
| Design or process condition | Information to collect | Question for the material review |
|---|---|---|
| Assembly heat exposure | Intended reflow profile, number of passes and expected rework. | What evidence supports this exposure for the proposed material and construction? |
| Operating environment | Board temperatures, hot spots, humidity, exposure duration and cycling. | Which material data and board-level qualification address these conditions? |
| Electrical performance | Relevant frequency range, impedance targets and permitted channel loss. | Which construction-specific dielectric data will the design model use? |
| Mechanical construction | Layer count, dielectric thicknesses, copper and critical holes. | Can the proposed cores and prepregs produce the approved stack-up? |
| Recognition or customer requirements | Required material identity, documentation and applicable approval scope. | Does the evidence cover the exact material and intended board construction? |
| Supply and change control | Quantity, required order date, approved alternatives and repeat-build expectations. | What is available, and which changes require written approval? |
Keep the comparison tied to one released board revision. If a design requirement changes while quotations are being collected, send the revised requirement to every candidate supplier. Otherwise, the offers may describe different products even when their headings look identical.
Use a named product and retain the test method
Material names help locate the correct documentation, but a name alone is still incomplete. Record the manufacturer, grade, data-sheet revision and construction information. Separate a manufacturer’s typical published values from the acceptance limits you require for the order. A typical value should not silently become a guaranteed minimum.
A practical example of why methods matter appears in Isola’s FR408HR documentation: it reports Tg as 190°C by DSC and describes 230°C by DMA. These are different reported methods for the same resin system. Copying the larger number into a comparison without its method would obscure the meaning of the data.
For another documented example, the 370HR technical data lists Tg by DSC separately from decomposition temperature, expansion, moisture absorption and dielectric properties. Use those categories to organize questions; do not treat one favorable property as proof that all application requirements are met.
These examples explain how to read manufacturer information. They do not establish stock availability, a supply relationship or a recommendation that either grade is suitable for your board. The fabricator still needs to confirm the proposed construction, and the design owner needs to approve it.
Keep assembly exposure and operating conditions separate
Write two separate entries in the requirement list: what happens during assembly, and what happens during service. A short manufacturing exposure and a long operating life are different questions. Do not use a Tg figure as the board’s continuous operating-temperature rating or assume that a material-level test qualifies the complete assembled product.
For assembly, ask which profile and process history the supporting evidence covers. Include anticipated rework instead of evaluating only the first soldering pass. For service, ask what board-temperature measurements or estimates, environmental conditions and qualification plan the design owner has established. Where the evidence is missing, retain an open requirement instead of substituting an arbitrary temperature margin.
Make the response specific enough to review. “Lead-free compatible” may identify a useful candidate, but it does not explain whether the proposed board construction has been evaluated for your sequence. Record the supporting document, its conditions and who must resolve any gap before release.
Connect dielectric data to the actual stack-up
A headline dielectric constant is a starting point for finding documentation. For design work, identify the grade, construction, resin content, frequency and data source used. Isola’s FR408HR construction tables, revision G of January 5, 2026, list separate core and prepreg entries across multiple frequencies. The table structure shows why one number should not stand in for every construction.
Ask the fabricator to identify the proposed dielectric layers and the values used in its impedance proposal. Ask the electrical designer to confirm whether those values and assumptions suit the intended model. Keep finished geometry and any verification coupon requirements in the same approval package. A material substitution should trigger a review of the affected model rather than an automatic copy of the old trace dimensions.
At this stage, the deliverable is an agreed set of inputs and a verification approach. It is not a promise that a specified material name alone will produce a particular impedance or channel performance.
Check recognition evidence and manufacturing availability
If the order requires recognized materials or a recognized board construction, identify the exact scope with the responsible compliance owner. UL Solutions describes PCB and material recognition and its searchable listings. A logo on a generic sales page is not enough information to match a specific order to the required evidence.
Ask for the relevant material designation, board-manufacturer identification and supporting listing or documentation where applicable. Verify what it covers instead of assuming that one material listing establishes every certification needed for the finished product. UL’s Follow-Up Services guidance also explains the importance of matching materials and components to the applicable documented construction. No certification ownership is claimed by this website.
Availability is a separate commercial check. A manufacturer’s catalog offering does not confirm that a fabricator has the required core thickness, prepreg construction or copper option for your quantity and date. Ask the quotation to identify the offered construction and state any alternative explicitly. Keep requested delivery dates separate from dates the supplier has confirmed.
Approve alternatives with a short comparison record
A proposed replacement deserves a written comparison even when its Tg is equal to or higher than the original. The purpose is to show which design assumptions stay valid, which need review and which facts are still unknown.
| Record | Required comparison | Release decision |
|---|---|---|
| Material identity | Original and proposed manufacturer, grade and document revisions. | Reject an unspecified “equivalent” until its identity is known. |
| Thermal and environmental evidence | Relevant properties, methods and qualification conditions. | Resolve differences against the application’s requirements. |
| Electrical inputs | Dielectric data source, frequency and affected stack-up layers. | Obtain design approval for revised modeling assumptions. |
| Construction and documentation | Core/prepreg options, copper, thicknesses and required recognition evidence. | Approve the revised stack-up and required records together. |
| Change authorization | Approver, board revision, affected quantity and permitted future use. | State whether approval is for one lot or repeat production. |
Use the quality evidence guide to define the records needed at delivery. The manufacturing process guide explains why unresolved construction questions should be closed before lamination.
Illustrative example: a higher Tg does not close the review
Example only, not a customer case: a six-layer controller has a controlled-impedance link and is intended for a humid enclosure. The assembly plan includes two reflow passes and possible rework. A supplier proposes an alternative laminate with a higher reported Tg, but does not identify the test method or the available prepreg construction.
The buyer records the offer as a candidate, not an approved equivalent. First, the supplier provides the exact grade and current data. Next, the assembly owner reviews the exposure evidence, while the designer checks the proposed dielectric construction and electrical inputs. The compliance owner reviews any applicable recognition requirement. Open items remain visible on the comparison record.
If a required construction or supporting qualification cannot be confirmed, the team can retain the original material, evaluate another candidate or change the design through its normal approval process. The useful outcome is a documented choice with known conditions, rather than a selection based on a single larger number.
What to put in the material section of an RFQ
- Board number, released revision, quantity and proposed stack-up.
- Named manufacturer and grade, or a clear property-based requirement awaiting selection.
- Core and prepreg construction, thickness and copper requirements that affect the design.
- Relevant property values with methods, units, conditions and document revisions.
- Assembly exposure and operating conditions needed for the material review.
- Required qualification or recognition evidence, where applicable.
- Substitution rules, approval responsibility and traceability documents.
Attach this information to the PCB quotation file package. Mark unknown values as open questions. This lets a supplier explain its assumptions instead of silently choosing defaults.
Prepare a material-specific inquiry
Share the stack-up, exact grade or required properties, thickness, copper, quantity and substitution restrictions. Include the assembly and operating conditions that need review, so the quotation can identify its proposed material and unresolved assumptions.
Frequently asked questions
Is “FR-4, high Tg” a complete purchasing specification?
No. Identify the material or the required properties, the applicable methods and conditions, the proposed construction and the rules for substitutions. Leave unresolved requirements visible for review.
Can a higher Tg make a material an automatic equivalent?
No. Review the properties, test methods, construction, electrical inputs and documentation relevant to the board. Record who approved the alternative and which builds the approval covers.
Can one published Dk value represent every stack-up?
Do not assume so. Use the material manufacturer’s relevant construction data and confirm the values, frequency and geometry used in the board’s design and manufacturing proposal.
What if the specified grade is unavailable?
Ask the supplier to identify a proposed alternative and its documentation. Keep it unapproved until the affected design, process and documentation requirements have been reviewed and the substitution is authorized.
Primary sources checked October 7, 2026. Manufacturer products are cited as documentation examples, not as availability or suitability guarantees. The procurement workflow and hypothetical example are editorial guidance; final material approval belongs to the responsible project team.