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Locate the interface and its job

Draw the two surfaces that must exchange heat. Identify the gap, surface finish, flatness, electrical potential and any movement or service requirement. A thin interface between machined surfaces differs from a variable gap between a component and an enclosure lid.

Keep this material separate from the dielectric inside an IMS board. The supplier-role comparison explains why confusing those roles leads to poor comparisons. An interface may provide useful electrical properties, but those must be qualified for the actual insulation requirement.

Select the form around the gap

A grease or phase-change layer is often considered for a thin controlled bond line, while a compliant pad can accommodate a different gap and tolerance situation. The specific product's mechanical and processing data matter more than the category name.

Use the assembly's minimum and maximum gap, not only its nominal dimension. Check whether the material can establish contact without excessive force on components or the PCB. This page does not provide a universal compression percentage or mounting torque.

Distinguish conductivity and impedance

Conductivity describes bulk material behavior under stated conditions. Area-normalized thermal impedance can include the behavior of a particular installed layer and its contact boundaries. Its units and test conditions must be read carefully.

For an entered impedance of 0.3 K路cm虏/W over 400 mm虏, the uniform-interface resistance is 0.075 K/W. At 10 W, the reference rise is 0.75 K. This example assumes the actual area and test conditions are represented; it excludes partial contact and nonuniform pressure.

Avoid counting contacts twice

If a supplied impedance value includes interface contacts, adding separate contact resistances without a clear boundary can double count them. If a bulk conductivity is used instead, contact effects may still be absent. Draw the model boundaries before adding numbers.

The thermal principles guide helps organize series paths. Keep thickness, pressure and temperature with the selected value. A number measured at one condition does not establish the performance of a different installed gap.

Area-normalized interface impedance

Review assembly repeatability

Specify placement, coverage, surface preparation and any liners that must be removed. Define how the installed thickness or contact is controlled. A correct material placed over contamination, wrinkles or trapped air may not deliver the expected interface.

Consider whether the assembly can be reopened and whether the interface can be reused. Follow the product-specific process guidance rather than assuming every pad or compound survives disassembly in the same way. Keep replacement instructions with service documentation where relevant.

Review electrical and environmental needs

Electrical insulation, contamination, pump-out, aging and compatibility can influence the choice. A high conductivity number alone does not settle these questions. Obtain the grade data relevant to the product environment and the actual surfaces being joined.

If the interface is part of a safety barrier, use the approved insulation review. Do not infer continuous working-voltage suitability from an isolated short test value. For the wider barrier requirements, see high-voltage PCB design.

Test a representative interface

Use the actual mating surfaces, pressure, gap and heat-flow boundary when practical. Record the measured temperature locations and load. Compare before and after environmental exposure when that is part of the product's qualification requirement.

If a coupon method is used, explain how it relates to the assembly. A supplier datum can support selection without being direct evidence of the final product's temperature. Keep the difference visible in the engineering record.

Include the interface in the enquiry

Provide a section drawing, gap range, contact area, electrical requirement, material candidate and assembly scope. State who supplies and installs the interface. For thin-compound quantity planning, use heat-sink thermal paste.

For a metal-backed board review, continue to metal-core PCB. The calculator record is optional and stays local until you choose to attach it. It supports a well-defined interface question rather than replacing a material qualification.

Qualify the installed joint, not just the material name

Document both mating surfaces, the minimum and maximum gap, the contact area and how the assembly applies force. Identify whether the interface also has an electrical function. The selected product must suit that joint rather than only matching a conductivity target on a table.

Retain the supplier's test conditions with the property used in the model. Distinguish bulk conductivity, measured impedance and the effect of installed thickness. If contact effects are included in one datum, keep the model boundary consistent so they are neither omitted nor counted twice.

Agree a repeatable placement and mounting process. Consider disassembly and environmental exposure where relevant. Representative testing should use the actual surfaces and joint conditions or explain the relationship between a coupon and the product. A thermal paste volume or a pad thickness is an input to that process, not proof of its result.

Before release, place the chosen requirement, the proposed construction and the evidence in one review record. Separate measured values from supplier nominal data and from your own assumptions. If a value is unknown, identify who will obtain it and what decision depends on it. An empty field is more honest and more useful than an invented typical figure.

During an A/B comparison, change one relevant variable where possible. Preserve the original inputs and the reason for the change. If several things change together, describe the result as a comparison between complete configurations. Do not attribute the entire difference to one material or geometry feature without supporting evidence.

For a repeat order, confirm that the relevant drawing, material and process revisions still match the accepted configuration. Record alternatives explicitly. This small discipline helps purchasing avoid an apparently equivalent substitution that changes the engineering basis of the original decision.