Check Thermal Gap-Pad Compression Across the Mechanical Stackup

A thermal pad chosen only by conductivity and nominal thickness may leave an air gap at one tolerance corner and overload a component at another. The useful starting point is the assembled gap range. Connect that range to the selected material’s compression behavior and the structure carrying the resulting force.

Define the gap before selecting thickness

Build a stackup containing component height, soldered seating, board position, heat-spreader position, flatness and relevant tolerances. Identify minimum and maximum gap at each contact area. Several devices under one plate may have different gaps even when their package names look similar.

Henkel’s GAP PAD TGP HC3000 information identifies low-modulus behavior and thermal performance at low pressures as product characteristics. Obtain the actual grade’s thickness and compression data; another pad with the same conductivity need not apply the same load.

Calculate a geometric compression range

For an illustrative uncompressed thickness of 1.0 mm and an assembled gap of 0.8 mm, the nominal thickness reduction is 20%. If the gap ranges from 0.7 to 0.9 mm, the geometric reduction ranges from 30% to 10%. These values illustrate the stackup calculation, not an acceptable range for a particular material.

Include pad-thickness tolerance in the final evaluation. Use the manufacturer’s stress-versus-compression information and relevant conditions to assess force. Do not assume that doubling compression doubles pressure or that a softer pad always solves an unsupported-board problem.

Follow the mechanical load path

Identify where the spreader force reaches the component, solder joints and PCB supports. Review package loading limits and assembly requirements with the component and mechanical teams. A broad pad spanning several unequal-height parts can distribute force unevenly.

Check pad placement, liner removal and retention in the assembly process. Preserve the intended contact area without covering features that must remain clear. A successful thermal prototype using a hand-cut pad may not represent the tolerance and placement variation of the production die-cut part.

Verify thermal and mechanical performance together

Measure temperature under the required power and ambient conditions while inspecting the final assembly for unintended deformation or contact. Include relevant aging or reassembly evaluation when the product requirements demand it. A cool device during one test does not independently prove acceptable long-term mechanical loading.

Release the material grade, thickness, shape, placement and stackup limits together. If component height or heat-spreader geometry changes, recalculate the compression range before retaining the old pad specification. Keep the actual compressed thickness and contact coverage in the prototype record so that later thermal comparisons use an equivalent assembly.