Heatsinks and Thermal Interface Materials: Selection and Attachment

A heatsink is only as good as the interface between it and the component. The interface is a thin layer of material that exists to fill the microscopic gaps between two surfaces that are never as flat as they look, and its properties set the limit on what the assembly can dissipate.

Why an Interface Material Is Needed

Two nominally flat surfaces touch at a small fraction of their apparent area. The gaps between the contact points are filled with air, which conducts heat poorly, and the resulting resistance can dominate the whole thermal path.

The interface material displaces the air and provides a path across the gaps. Its performance depends on its thermal conductivity and on how thin a layer it forms, and the layer thickness matters more than the conductivity in many cases.

The product of thickness and conductivity is what counts, so a material with modest conductivity applied very thin can outperform a highly conductive material applied thickly.

Classes of Material

Thermal grease is a paste filled with a conductive powder. It forms the thinnest bond line and has the lowest resistance, and it does not cure, which means it can pump out over thermal cycles.

A gap filler is a soft, highly conformable material for uneven surfaces, used where the gap is large or where the surfaces are not parallel. Its resistance is higher because the layer is thicker, and it tolerates more variation.

A phase change material is solid at room temperature and softens at operating temperature, combining the handling of a pad with the thin bond line of a grease. A cured adhesive provides mechanical attachment as well as thermal conduction. Our thermal design notes describe how the interface term fits into the overall calculation.

Bond Line Thickness and Pressure

Every interface material needs pressure to reach its rated thickness. The pressure flattens the material and closes the gaps, and without it the layer stays thick and the resistance stays high.

The mounting hardware must supply that pressure evenly. A single screw in the middle of a heatsink gives pressure at the screw and little elsewhere, while a spring clip or several screws spread around the perimeter give a uniform load.

The pressure also must not damage the component. A ceramic package cracks under a point load, and a plastic package deforms. The mechanical design should distribute the force over the component body.

Attachment Methods

Mechanical attachment with screws or clips allows the heatsink to be removed and the interface material to be renewed, and it requires the hardware to be designed into the assembly.

Adhesive attachment uses a thermally conductive adhesive and requires no hardware. It is permanent, and the adhesive layer is thicker than a grease bond line, which costs thermal performance.

Direct soldering of the heatsink or the tab to a copper area gives the lowest resistance of all and requires the surface to be solderable, which is a constraint on the finish and on the layout.

Electrical Isolation

Where the heatsink is connected to a voltage, isolation is required between the component and the heatsink. The isolation is provided by a thin insulating layer, which adds thermal resistance.

The isolated version of a component avoids the separate insulating layer by providing it inside the package, which usually gives better thermal performance than an external pad.

Where isolation is provided externally, the assembly must maintain the creepage and clearance distance. Our metal core and ceramic notes describe the substrates used where isolation and thermal conduction are both required.

Mounting to the Board Rather Than to the Air

In a sealed enclosure with little airflow, a heatsink that radiates into still air achieves little. The thermal path is then from the component into the board copper and from the board to the enclosure wall.

This is often the better arrangement for a compact product, because it uses the enclosure as the heatsink and avoids a component that can be knocked off.

The design requirement is then a large copper area on the board and a controlled interface between that area and the chassis. Our copper area notes describe the sizing of the conductor and the thermal spreading.

Reliability of the Interface Over Time

Grease migrates under thermal cycling, and a joint that was thin at assembly can become thick and dry after years. The mechanism is accelerated where the temperature swings are large and where the pressure is uneven.

Pads and phase change materials resist migration better because they do not flow. Their initial resistance is higher and their long term resistance is more stable, which makes them the usual choice for a product with a long service life.

Where the assembly will be subject to vibration, a cured material is preferable, because a grease can act as a lubricant between the surfaces. Our quality notes describe the inspection of the assembled interface.

Verification

The verification is a temperature measurement on the assembled product at the worst case ambient and load. Measuring the case temperature and comparing it against the calculated value confirms whether the interface is performing as assumed.

Where the measured temperature is higher than predicted, the interface is the first candidate, followed by the airflow and the copper spreading.

A thermal image of the assembly shows where the heat is going and where it is not, and it often reveals a path that the calculation did not include. The measurement should be recorded with the assembly configuration so that a later change can be compared against it.

Additional Considerations for This Build

Practical attention to thermal interface material pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating thermal interface material explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, thermal interface material is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

FAQ

Is more interface material better? No. The material is a poor conductor compared with metal, so the objective is the thinnest layer that fills the gaps.

Can a heatsink be reused? A mechanically attached heatsink can be removed and the material renewed. Reusing the old material leaves a thick, contaminated layer.

What does gopcb provide for thermal interfaces? We provide interface material selection for the bond line and the reliability requirement, mounting hardware recommendations that spread the pressure, assembly records for the material and its thickness, and temperature measurements on the finished assembly. Where the interface is not the limiting term, we say so and focus the effort elsewhere.