Key takeaways
- Flatness, waviness and roughness are three different calls, and each affects the interface differently.
- Contact pressure has to sit over the device, not at the screws, so the bracket and the base must work together.
- A thicker pad is easier to assemble and usually worse thermally than a thin pad under proper pressure.
- Thermal cycling can pump grease out of the interface or fatigue a pad, so the mounting needs a spring element.
Contact is a system, not a surface
A heat sink does not touch a device over its whole nominal area. It touches at the high points, and the rest of the gap is filled with air, grease or a pad. The real contact area depends on the flatness of both surfaces, the roughness, the pressure and the stiffness of the interface material. A sink that measures flat on a surface plate can still make poor contact once it is bolted to a warped package.
The mounting hardware is part of the thermal path. A screw at each corner pulls the sink down at the corners and can bow the centre upwards. A central screw pulls the centre down and lifts the edges. A spring clip applies a known force at a defined point, which is easier to control than a torque value that depends on thread friction. Below the device, the solder joint and the board carry heat into the copper, and a board that flexes under the clip load changes the contact.
The design should therefore treat the sink, the device, the interface and the fastener as one stack. A change to any of them changes the contact resistance, and the change is not always in the expected direction.
Flatness, waviness and roughness
Flatness is the deviation of a surface from a plane. Waviness is the longer wavelength variation across the surface, and roughness is the short wavelength texture. A surface can be flat overall and still have a wave that holds the centre of the device away from the sink. A surface can be smooth and still be bowed.
For a thin thermal pad, a flatness call of about 0.05 mm across the mounting area is a common starting point, with a roughness of Ra 1.6 micrometres or better. For grease, the surfaces need to be closer, because the grease layer is thin and the contact resistance depends on the metal to metal high points. For a thick gap pad, the pad conforms and the flatness call can be looser, at the cost of bulk resistance.
The measurement method matters as much as the number. A surface plate and a dial indicator measure the face in a way that may not match the installed condition. A coordinate measuring machine can map the surface, but it samples points and may miss a local high spot. State the method, the area and the datum, or two inspectors will disagree about a face that is within tolerance.
Pressure and fastener layout
Pressure closes the gap and squeezes out the air. Too little pressure leaves a thick interface and a high resistance. Too much pressure can crack a ceramic device, deform a plastic package or bow the base. The window depends on the device and the interface, and it is usually narrow enough to need a controlled spring rather than a torque guess.
Fastener layout decides where the pressure lands. Four corner screws on a stiff base give good pressure at the corners and less in the middle. A central screw with a stiff bridge gives the opposite. A spring clip that bears directly over the device is often the most repeatable solution for a small package, because the force path is short and the clip deflection sets the load.
The base stiffness matters. A thin base deflects under the screw load and follows the package, which can be good for contact and bad for the device. A thick base spreads the load and stays flatter, but it needs more force to conform. The mounting design should be checked with the base and the device together, not with a rigid test block.
Torque is not pressure
The same torque gives different clamp loads on a lubricated screw, a dry screw and a thread-forming screw. If the thermal design depends on pressure, specify a spring rate or a measured load rather than a torque alone.
Interface materials and what they need
The interface material fills the gap and conducts heat across it. The choice is a compromise between thickness, conductivity, pressure and assembly. A thin material has a lower bulk resistance but needs flatter surfaces and more pressure. A thick material conforms to a rough surface but adds resistance and can pump or tear.
| Material | Typical thickness | Pressure | Strength | Watch |
|---|---|---|---|---|
| Thermal grease | 0.05 to 0.1 mm | Moderate to high | Very low bond line resistance | Pump-out and migration under cycling |
| Gap pad | 0.5 to 2 mm | Low | Conforms to rough or stepped surfaces | Bulk resistance and compression set |
| Phase change | 0.025 to 0.05 mm | Moderate | Thin at operating temperature, dry to handle | Needs a defined clamp force and a warm up |
| Graphite sheet | 0.1 to 0.5 mm | Moderate to high | Spreads heat in plane as well as through | Electrical conduction and edge damage |
Thickness values are illustrative. The supplier's datasheet and the assembly pressure define the actual bond line.
Thermal cycling and pump-out
Every power cycle expands and contracts the stack. The device, the interface and the sink move by different amounts because their coefficients of thermal expansion differ. Grease can be squeezed out of the joint over many cycles, leaving a dry region in the centre. A pad can take a compression set and lose contact pressure. A phase change material can flow away from the edges if the joint is overfilled.
The mounting should therefore keep a spring element in the load path. A disc spring, a wavy washer or a flexible clip maintains force as the stack expands and the pad compresses. A rigid screw joint loses preload when the materials creep, and the loss may not be visible until the unit runs hot after a year in service.
The test for this is a power cycle or a temperature cycle with a thermal resistance check at intervals. A single cold measurement does not show pump-out. If the design cannot be cycled, a conservative interface and a spring loaded mount are cheaper than a field return.
Measuring flatness and contact
Measure the sink base before assembly, then measure the assembled stack. The first tells you whether the part is capable. The second tells you whether the mounting is bending it. A base that is flat on the bench and bowed in the assembly is a bracket problem, not a machining problem.
Pressure sensitive film can show the contact pattern between the device and the sink. The film is compressed where the pressure is high and stays light where it is low. It is a useful development tool, and it can be a production check for a critical assembly. The film has a pressure range, so the grade has to match the expected load.
A thermal test is the final check. Measure the sink base temperature under the device and the device case temperature, and compare them with the calculated interface resistance. A large difference points to the interface or the contact area. A small difference with a hot sink points to the sink or the airflow.
Drawing calls that prevent an argument
Call out the flatness of the mounting face over the contact area, not over the whole base. Give the roughness and the machining method if the surface texture matters. Show the fastener positions, the clip location and the direction of the spring force. State the interface material, the thickness range and the assembly pressure or torque.
Add the surface treatment and the mask areas. An anodised base is insulating at the contact face unless it is masked, and a painted base is worse. If the sink is also an earth path, the contact face needs a conductive finish and a defined bond resistance.
Finally, state the test. A flatness measurement on the finished part and a thermal check on the first assembly are different records, and both are useful. The drawing should say which one is required and what the acceptance limit is, so the supplier and the buyer are measuring the same thing.
Frequently asked questions
What flatness does a heat sink mounting face need?
A common starting point is about 0.05 mm across the contact area for a thin pad, with a roughness of Ra 1.6 micrometres or better. A thick gap pad can tolerate more. The requirement depends on the interface, the device and the pressure, so it should be set from the thermal test rather than copied from a template.
Is a thicker thermal pad better?
A thicker pad conforms to a rough or stepped surface, which helps assembly. It also adds bulk resistance, so it is usually worse thermally than a thin pad under proper pressure. Use the thinnest pad that covers the gap and the surface variation.
Why does thermal grease pump out?
The stack expands and contracts with each power cycle, and the grease is displaced from the joint. Over many cycles the centre can become starved. A spring loaded mount and a grease with better stability reduce the effect, and a phase change material avoids the problem in some designs.
How do I check contact pressure?
Use pressure sensitive film between the device and the sink to see the contact pattern, or measure the clamp force with a load cell. A torque value alone is not a pressure measurement, because friction and thread condition change the clamp load.
Sources and further reading
- ISO 1101:2017, Geometrical product specifications. Geometrical tolerancing. Tolerances of form, orientation, location and run-out
- ISO 4287:1997, Geometrical product specifications. Surface texture. Profile method. Terms, definitions and surface texture parameters
- JESD51-2A, Integrated circuits thermal test method environmental conditions. Natural convection (still air)
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Have a sink that is flat but still runs hot?
Send the stack drawing and the measured temperatures. The engineering desk will check the contact area, the clamp path and the interface together.
Send the stack