Thermal Interface Material Application

A thermal interface material fills the microscopic gap between two surfaces so that heat can cross it, and its performance depends on the thickness of that layer as much as on its conductivity. A material with a high conductivity applied too thickly performs worse than a modest material applied thinly.

The application step therefore decides the result, and it is one of the few places where the process is more important than the specification.

Why the Interface Matters

Two surfaces that appear flat touch at a small fraction of their area, and the air in the gaps is a poor conductor. The interface material replaces the air with something better, which reduces the resistance of the joint.

The resistance has three parts: the material itself, the two contact resistances at the surfaces and the effect of the thickness. Reducing the thickness reduces the material term and usually the contact terms as well.

Bond Line Thickness

The bond line thickness is set by the mounting pressure, the material’s viscosity and the flatness of the surfaces. It is the parameter that is most often uncontrolled and the one with the largest effect.

The thickness should be specified as a range and verified on a sample by measuring the gap or by a cross section. A gap that is larger than intended usually means the mounting hardware is not applying the intended load.

Dispense Patterns and Volume

The material can be applied as a dot, a line or a screen printed pattern, and each distributes differently under pressure. The volume has to fill the gap and leave a small fillet without excess that spreads onto the board.

Too little material leaves voids and dry areas, while too much increases the bond line and can contaminate neighbouring parts. The volume should be calculated from the area and the target thickness.

Pump Out and Thermal Cycling

The material expands and contracts with temperature, and a large difference between the two surfaces can pump it out of the joint over many cycles. A low viscosity material is more prone to this.

Pump out is addressed by choosing a material with the right consistency and by reducing the temperature difference, which is usually a mechanical design change.

Surface Preparation

The surfaces should be clean and free of oxide and of release agent, since contamination adds a contact resistance that no material can overcome. Handling with bare hands leaves oil that matters at this scale.

Where a surface is rough, the material has to fill more of it, so the effective thickness rises. The surface finish of the heat sink and of the package should be part of the specification.

Materials and Their Uses

Grease is the best performer at a thin bond line and it never cures, which makes it unsuitable where the assembly is handled. Pads are clean to apply and thicker, so their performance is lower but repeatable.

A curing material combines a moderate performance with mechanical stability and is often used where the joint must survive vibration. The choice follows from the mechanical environment as much as from the thermal one.

Verification

The verification is a measurement of the case temperature under load, compared with a calculation or with a previous build. A cross section on a sample shows the bond line and any voids.

Where the result is worse than expected, the bond line is checked first, then the coverage and then the material. This order follows the likelihood of the cause.

Assembly Sequence

The interface material is applied after the electrical assembly and after any cleaning, because the material is difficult to remove from a joint. The sequence should be written into the process rather than improvised.

Where a coating is also used, the order relative to the interface material matters, since a coating that reaches the interface changes its behaviour. This is the same interaction described for adhesive and coating compatibility.

Records

The records should include the material, the lot, the dispensed volume and the measured bond line. Where a thermal problem appears later, those records show whether the interface changed.

They belong with the thermal design data described for heat sink assembly.

Process Control and Verification

On a design of this kind, bond line thickness is the item that decides how the rest of the board is arranged. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

Process Control and Verification

On a design of this kind, bond line thickness is the item that decides how the rest of the board is arranged. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Process Control and Verification

On a design of this kind, bond line thickness is the item that decides how the rest of the board is arranged. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Is a higher conductivity always better? Only if the bond line is controlled, because thickness dominates the resistance in most assemblies.

Can too much material cause a problem? It increases the bond line and the resistance, and the excess can spread onto adjacent parts.

Why does performance degrade over time? Usually through pump out or through drying of the material, both of which change the coverage.

How is the bond line checked? By a cross section on a sample or by measuring the gap after the hardware is tightened.