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Distinguish nonwoven aramid from woven glass

The official 55NT sheet reports low in-plane CTE in the 6–9 ppm/°C range and identifies an epoxy system with Tg 170°C. These typical material statements explain why the product is considered for low-expansion package interfaces. They do not establish the expansion of every finished copper-patterned board.

Copper distribution, layer symmetry, direction and temperature range can affect the composite response. Treat the published range as a source-qualified input to further analysis, not a universal coefficient for every stackup.

Evidence and construction decision

Start with the package and its distance from the neutral point

A ceramic package, organic package and connector can place different mechanical demands on the board. Record the package dimensions, attachment geometry, temperature excursion and mechanical support. The relevant free mismatch generally depends on the length scale being compared, not just the difference between two CTE values.

Do not use the full board outline as the length in a package-edge comparison unless that is the intended physical boundary. Define the datum and direction so another engineer can reproduce the calculation.

Use the mismatch tool to check units and direction

The worksheet calculates differential free expansion from two entered coefficients, a common length and a signed temperature change. The generic example uses 50 mm, 15 and 5 ppm/°C, and 100°C change, producing 50 µm differential movement. It is not a built-in 55NT versus package prediction.

The sign indicates which entered material expands more under the chosen direction of temperature change. The model excludes joint compliance, board bending, solder creep and fatigue. It cannot convert a smaller free mismatch into a guaranteed service life.

Compare woven and nonwoven aramid deliberately

45NK uses woven aramid, while 85NT combines nonwoven aramid with polyimide. These are different construction choices. Reinforcement architecture and resin system should remain separate fields in the comparison.

If the main requirement is high-temperature processing of a glass-reinforced polyimide, review 35N on its own terms. A low in-plane expansion value does not automatically provide the desired through-thickness behavior or manufacturing route.

Include dimensional processing in the build review

Specify registration-critical features, board support, flatness and the dimensional evidence required after fabrication and assembly. Organic reinforcement does not eliminate drilling, moisture or lamination questions. Use the exact material processing guidance and a qualified construction.

The cost comparison should include the agreed dimensional and assembly evidence as well as the board allowance. The local budget tool has no 55NT rate database. A material premium is meaningful only relative to the requirement it helps address.

Free differential thermal expansion

Optional cost comparison from your own allowances

Use a quotation or a clearly labeled internal assumption for each input. Keep the quantity, construction and inspection scope the same when comparing alternatives. This uncalibrated planning model adds entered values; it contains no current market rates and does not issue a ³Ô¹Ïtv quotation. You can contact the team without completing it.

Request a package-interface review with measurable limits

Send the 55NT callout, stackup, copper pattern, package data, attachment details, temperature range and mechanical constraints. Identify the acceptance criterion and any relevant prior failure or measurement. Include quantity and the policy on alternatives.

The desired reply is a proposed construction and a plan for evaluating the interface. Keep modeled free mismatch separate from observed package or solder-joint behavior. A downloadable calculation can support the discussion, but it does not qualify the assembly or establish a fatigue-life improvement.

Review the package, board and attachment together

The finished board combines a dielectric, copper patterns, holes, surface coatings and a manufacturing history. Its behavior cannot be inferred from a family name alone. State the supplied grade, reinforcement, thickness, copper and processing conditions that matter to the intended application.

Keep handling and assembly requirements with the fabrication drawing. A thin panel, punched outline or unusual support condition can change the production route even when the electrical circuit is simple. Ask the fabricator to review the actual features and panel format rather than assuming a dimensional example represents a production capability.

For cost comparison, hold the drawing, quantity, test scope and acceptance requirements constant. Record one-time tooling separately from recurring work, and distinguish raw material utilization from accepted-board yield. Any calculator on this page uses supplied assumptions; an actual quotation requires the files and an agreed scope.

A source-qualified table should be read alongside the exclusions and footnotes, not separated from them. If the supplier limits an application, identify whether that limitation affects the current design before requesting a price. A familiar family classification does not override an explicit restriction, and a positive calculator result cannot remove it.

For a repeat build, compare the newly proposed supply with the previously approved material record. Exact grade, manufacturing site where relevant, foil treatment, reinforcement and dimensional basis can matter. Define which changes require notification and which need technical approval. This avoids turning a purchasing substitution into an unnoticed design change.

Receiving records should support the agreed identity and traceability requirement. A certificate for a raw material, a finished-board inspection report and an assembly test result cover different boundaries. Request the records that actually answer the project questions rather than collecting unrelated certificates.

When a source contains conflicting values, keep the discrepancy visible until a controlled revision or written supplier response resolves it. Do not create a new table by combining the most favorable entries from several sources. The released drawing, material list and quotation should all describe the same accepted construction.

Compare in-plane expansion along the same direction and temperature interval. A package body, ceramic carrier, copper pattern and reinforced board need not move equally. The free differential-expansion result identifies a dimensional mismatch; it does not calculate solder-joint strain because joint geometry, distance from the neutral point, constraints and temperature cycling are excluded.

Record woven or nonwoven reinforcement explicitly. The fiber architecture can affect drilling, dimensional behavior and moisture response independently of the resin family. Do not choose a replacement solely because its nominal expansion is close to a package value. The finished construction and attachment must be reviewed together, including any permitted assembly repair.