Gap Filler Dispensing for PCB Thermal Paths: 6 Rules

A gap filler is a soft, heavily loaded polymer that is dispensed into the space between a board and a metal housing, frame or heat spreader. Unlike a thermal pad, it is applied as a liquid or a paste, so it can fill a space that varies across the assembly and it can conform to components of different heights.

Its job is to replace the air in that space. Air is a poor conductor of heat, and even a small void between a hot component and a metal surface raises the temperature of the component considerably. The filler is the thermal path, and its performance depends as much on how it is applied as on the material that is chosen.

What a Gap Filler Is For

The material is used where the clearance between the board and the metal part is too large for a thin interface material, or where the gap varies because several components of different heights have to be coupled at once. It is common in power electronics, in motor drives, in lighting products and in any assembly where a metal housing doubles as a heat sink.

Because it is dispensed, it can also be applied selectively, so that material is placed only where heat has to move. That saves weight and cost and avoids the mechanical problems that come with compressing a large soft interface across a whole board.

Choosing the Bond Line and the Gap

The bond line is the thickness of the filler after assembly, and it is the number that controls the thermal resistance. A thin bond line conducts better, but it demands tighter tolerances on the gap, while a thicker bond line tolerates variation at the cost of performance.

The tolerance stack decides which is possible. Board thickness, component height, housing flatness, standoff length and the compression of the material all add together, and the worst case combination has to leave enough filler to bridge the gap without leaving the surfaces in hard contact. Where the stack produces a range that is too wide for one material, the design usually needs a standoff that fixes the gap rather than a softer filler.

Dispensing Patterns and Volume

The material is normally applied as a bead, a dot pattern or a serpentine line, and the pattern has to deliver the volume needed to fill the gap after compression. A pattern that relies on the material to flow sideways will leave voids where the flow stops, so the pattern has to cover the area from the start.

The volume delivered by a dispensing system depends on the needle, the pressure, the time and the viscosity of the material, all of which change with temperature. A system that is set up in a cold room will deliver a different volume in a warm one, which is why the pot temperature and the material condition belong in the process record and why a periodic weight check on a sample is worth more than a setting on a screen.

Compression and Set

After dispensing, the parts are brought together and the filler is compressed to its final bond line. The compression has to be controlled by the mechanical design, usually by standoffs or by a hard stop, because a filler compressed by an uncontrolled force will squeeze out and leave a starved area.

Where the material is a curing type, the assembly has to be held until the cure has advanced enough that the bond line is fixed. Where it is a non curing type, the mechanical fixings hold the compression for the life of the product, and their torque and their creep behaviour become part of the thermal path.

Curing and Fixturing

Curing schedules for these materials are long compared with a solder joint, and the fixture that holds the assembly during the cure is a production asset rather than a convenience. A fixture that allows the parts to move during the first hour of the cure will produce a bond line that varies from unit to unit.

Heat accelerates the cure and also changes the viscosity, so the cure schedule should be developed with the actual assembly rather than with a coupon. Where the assembly contains materials with a lower temperature limit, the cure temperature has to be reduced and the time extended, and the resulting schedule belongs in the work instruction.

Inspection and Records

Inspection is difficult because the material is hidden once the parts are assembled. Where access allows, the squeeze out at the edge of the interface shows that material was present and that the parts closed fully. Where it does not, the process has to be controlled by the volume check on a sample and by the fixture rather than by looking at the finished unit.

The records should link the material batch, the pot life, the dispensing parameters, the compression and the cure to the units produced. A thermal problem found at test can then be traced to a dispensing parameter rather than to the components, which is the same discipline that supports the heatsink interface work and the void checks described for thermal pad assemblies. Where a liquid material is used instead of a pad, the patterns and volumes in the dispense pattern notes are a useful starting point, and the potting and coating alternatives are compared in the coating and potting review.

Material Handling and Pot Life

Gap filler is usually supplied in a cartridge or a pail and is dispensed from a heated pot or a syringe. The material has a pot life after it is brought to temperature, and the viscosity changes across that life, so the volume delivered at the start of a shift is not the volume delivered at the end. The pot temperature, the time since the material was brought to temperature and the batch number should all be recorded with the units produced.

Two part materials add a mixing ratio to the list. An off ratio mix cures soft, which reduces the compression the material can sustain and changes the bond line, and it may never reach the strength the design assumed. Where the material is filled with a thermally conductive powder, it also settles, so a cartridge that has been standing should be conditioned to the supplier instruction before use rather than shaken by hand. Material that has exceeded its pot life should be discarded, because the cost of the material is trivial compared with the cost of tracing a thermal failure in the field.

FAQ

How thick should the gap filler be? As thin as the mechanical tolerances allow, because thermal resistance rises with thickness. The practical limit comes from the tolerance stack, which has to guarantee that the gap is always filled without the parts touching.

Can a gap filler be reused after rework? No. Once the material has been compressed and cured it cannot be re-flowed into the same bond line. The old material must be removed and the joint re-dispensed when the assembly is reworked.

Why does the same assembly measure different temperatures after a repair? Usually because the bond line changed. Removing a board and refitting it changes the volume of material, the compression and the flatness of the interface, and the thermal path is not restored by simply adding more material.