Start with constraints that can reject a candidate
List the assembly process before comparing attractive datasheet numbers. Record soldering exposure, joining methods, cleaning, and handling. Next describe operating temperature, humidity or fluid exposure, and motion. Separate requirements that are mandatory from goals that may trade against thickness, routing area, or cost. The material selection worksheet converts these inputs into questions for a material review.
Avoid scoring every property equally. For a moving interconnect, a small loss improvement may be less valuable than a construction that can be tested in the available motion envelope. For a short static tail, a difficult material change may add little system benefit. Use the product’s actual limiting condition to decide which comparison deserves engineering time.
Material-system review by function
| Function | Specify | Check together |
|---|---|---|
| Base dielectric | Grade and nominal thickness | Copper type and available laminate construction |
| Conductor | Foil grade and finished copper assumptions | Etched geometry and bending duty |
| Protection | Coverlay or flexible mask grade | Openings, cure or bonding process |
| Bonding | Adhesive grade and finished bondline | Temperature exposure and substrate surfaces |
| Shielding | Film, attachment and ground interface | Local thickness and channel behavior |
Compare grades and constructions
Polyimide, PET, and LCP names are entry points, not interchangeable material specifications. Compare the exact laminate, its copper, and its processing instructions. A supplier’s film measurements do not automatically describe copper-clad material or a finished circuit. Keep frequency, test method, conditioning, thickness, and typical-versus-guaranteed status alongside each value.
The component list should identify separately purchased materials as well as the base laminate. For example, Qnity’s LF coverlay combines polyimide film with acrylic adhesive. That construction illustrates why an insulation thickness needs more detail than a film name. Review coverlay selection and bonding-layer definition when closing the complete stack.
Make material substitutions manageable
Set a substitution policy while the drawing is still being developed. Identify the properties that affect your geometry, assembly process, or qualification. Ask the proposed fabricator to return exact candidate constructions and deviations instead of accepting an unrestricted ‘equivalent’ note. A comparison table should expose what is unknown, not hide missing evidence behind a single preferred score.
Route candidates to a prototype decision: what would cause you to reject the material after fabrication, assembly, or environmental testing? Define this before ordering. Preserve the selected supplier sheet revision and lot traceability expectations with the released package so a later supply change can be assessed against the same baseline.
Before you release the design
- Write the assembly and operating conditions separately.
- Identify each material by function and proposed grade.
- Compare properties only with their test conditions.
- Mark critical substitution-sensitive features.
- Request a reviewed construction before final layout release.
Watch points
- A broad comparison table cannot establish material equivalence.
- Minimum datasheet loss is not necessarily minimum complete-channel loss.
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Practical questions
Should a drawing lock every material brand?
Lock what the product qualification requires. Where alternatives are useful, define a controlled review process and the evidence needed to approve them.
Where should an unknown property go?
Keep it visible in the material comparison and RFQ. Do not substitute a generic internet value for the missing grade-specific information.
Technical references
References support the material or design context. Check the exact grade, part number, document revision and project conditions before release.Updated 27 September 2026 · Technical content policy · Engineering guidance supports project review; the agreed drawing defines the build.