Key takeaways
- The interface material fills the gap between two surfaces; its performance depends on the gap, the pressure and the contact area.
- A thin paste gives the lowest resistance when the surfaces are flat and the clamp load is controlled.
- A pad is easier to assemble and rework, but it needs enough compression and a gap it was designed for.
- Graphite spreads heat well in plane but is not a substitute for a gap filler in every direction.
What a thermal interface material does
Two metal surfaces that look flat are rough at the microscopic scale. They touch at a small fraction of the nominal area, and the rest is air. Air is a poor conductor, so the interface becomes a bottleneck even when both metals are good conductors.
A thermal interface material displaces the air and fills the gaps. Its job is to give a continuous, thin, low resistance path between the surfaces. It is not there to conduct heat along the part, and it is not a structural adhesive unless it is specified as one.
The interface resistance has two parts: the resistance through the material and the contact resistance at each face. A material with a high bulk conductivity can still perform poorly if it does not wet the surfaces or if the bond line is thick.
Bond line thickness and gap
Bond line thickness is the thickness of the material after assembly. It is set by the surface flatness, the gap between the parts, the compressibility of the material and the clamp load. A thin bond line is good only if the material can cover the gap without being squeezed dry.
For a flat interface with a paste, the bond line may be tens of micrometres. For a gap pad, it may be several tenths of a millimetre. The material and the mechanical design have to agree on the same number.
A common error is to choose a pad to fill a gap that changes across the part. If the gap is 0.3 mm at one edge and 0.6 mm at the other, a 0.5 mm pad is under compressed on one side and over compressed on the other. The result is a non uniform bond line and a hot spot.
The interface material should be selected after the flatness and the clamp load are known, not before. Otherwise the material is being asked to solve a mechanical problem.
Thermal pads
A pad is a preformed sheet, usually filled with a ceramic or a metal oxide. It is easy to place, does not migrate and can be reworked. It is a good choice for a manual assembly, a field service part or a gap that is too large for a paste.
Pads come in different softness levels. A soft pad conforms at low pressure, which suits a delicate package. A harder pad needs more force but can give a more stable bond line. The datasheet should give the compression curve, not just a single thermal conductivity value.
The thermal conductivity of a pad is usually lower than a paste, but the pad can be thicker and still be practical. The total resistance, not the conductivity alone, is the number to compare.
A pad can also act as a mechanical damper and a dielectric barrier. Those may be useful, but they should be stated as requirements rather than assumed.
Paste and phase change materials
A paste is applied as a thin layer and flows under pressure. It gives a very thin bond line and a low resistance when the surfaces are flat. It needs a controlled dispense volume, a clamp load and a cure or a burn in step, depending on the chemistry.
A phase change material is solid at room temperature and softens at operating temperature. It is easier to handle than a paste and flows at the working temperature to fill the gaps. It is a common choice for a CPU or a power module with a defined clamp.
Paste can pump out under thermal cycling. The material moves away from the high pressure area and the bond line becomes uneven. A phase change material or a pad can be more stable in a cycling application, but the choice depends on the movement and the clamp.
Dispense volume is a process control, not a suggestion. Too little paste leaves voids. Too much paste gives a thick bond line and can contaminate nearby parts.
Graphite and other spreaders
Graphite sheet has a high in plane conductivity and a lower through plane conductivity. It is useful for spreading heat sideways from a small source, not for filling a gap between two rough surfaces.
A graphite interface is often a composite with a soft layer on each side. The graphite carries the heat laterally and the soft layers make contact. The orientation matters: a sheet installed the wrong way can perform worse than a simple pad.
Graphite is electrically conductive. That is an advantage for grounding and a risk for isolation. If the part needs electrical isolation, the graphite must be paired with a dielectric layer or replaced.
Other options include metal foil, indium and solder. They have their own place, but they change the assembly process and the rework route. Do not treat them as drop in replacements for a pad.
| Material | Typical bond line | Assembly | Best use | Watch out for |
|---|---|---|---|---|
| Thermal pad | 0.2 to 1.0 mm | Place and clamp | Large gaps, serviceable parts | Compression and gap variation |
| Paste | Under 0.1 mm | Dispense and clamp | Flat, high flux interfaces | Pump out and dispense volume |
| Phase change | Under 0.1 mm | Place and clamp | Cycling power modules | First cycle flow and clamp |
| Graphite sheet | 0.05 to 0.2 mm | Place and clamp | Spreading from a small source | Direction and electrical conductivity |
| Metal foil or indium | Very thin | Clamp, sometimes solder | Very high flux, tight gap | Oxidation and rework |
Typical ranges only. The datasheet, the surface finish and the clamp load decide the real performance.
Selecting by gap and power
Start with the gap. Measure it on the real parts, not on the CAD. If the gap is small and consistent, a paste or a phase change material can give the lowest resistance. If the gap is larger or varies, a pad is more forgiving.
Then look at the heat flux. A high flux needs a thin bond line and a material with a high through plane conductivity. A lower flux can tolerate a thicker pad and a simpler assembly.
Then look at the mechanical load. A delicate package cannot take a high clamp force. A soft pad or a low modulus material may be the only option. A stiff material may need a stronger lid and a stiffer heat sink.
Finally, look at the service model. A part that will be opened in the field needs a material that can be replaced cleanly. A paste that has cured may not come off without a solvent and a scraper.
Assembly, pump out and rework
Clean the surfaces before assembly. Oil, dust and oxide reduce wetting and add resistance. A pad should not be touched with bare hands, because skin oil changes the surface.
Apply the clamp load evenly. A four screw pattern tightened in one pass can tilt the lid and squeeze the material to one side. Tighten in stages, diagonally, and use a torque value that matches the material.
Thermal cycling can move the material. A paste may pump out, a pad may take a set, and a phase change material may flow. The design should leave enough material to cover the movement over the life of the part.
Rework is part of the cost. A paste needs cleaning and re-dispensing. A pad can be replaced if the surfaces are clean. A cured adhesive material may need a destructive removal. Choose the material with the service route in mind.
What to specify and test
Give the material type, the thickness, the thermal conductivity or the thermal resistance, the compression range, the clamp load and the operating temperature. State the surface finish and flatness of the mating faces. Add the electrical isolation requirement if there is one.
Test the assembly, not only the material. A thermal test on the real stack shows the bond line and the contact resistance that the datasheet cannot. A cycling test shows whether the material stays in place.
Measure the gap before choosing the material
A pad chosen from the CAD gap can be wrong on the first article. Measure the real parts, then pick the thickness and the compression.
Frequently asked questions
Is thermal paste better than a thermal pad?
Paste usually gives a thinner bond line and lower resistance on a flat, well clamped interface. A pad is easier to assemble and better for a larger or variable gap. The right choice depends on the gap, the flatness and the service model.
How thick should a thermal pad be?
It should be slightly thicker than the measured gap so that it compresses by the amount the datasheet recommends. Too thin leaves voids. Too thick adds resistance and may not compress enough.
Does graphite work as a thermal interface material?
Graphite spreads heat well in plane, so it is useful for spreading from a small source. Its through plane conductivity is lower, and it is electrically conductive. Use it with a soft contact layer and check the direction.
Why does thermal paste pump out?
Thermal cycling expands and contracts the materials at different rates, and the paste moves away from the high pressure area. A phase change material, a pad or a change to the clamp can reduce the movement.
Sources and further reading
- ISO 22007-2, Plastics. Determination of thermal conductivity and thermal diffusivity
- ISO 1101, Geometrical product specifications (GPS). Geometrical tolerancing
- Qumek thermal interface and assembly notes
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