Gap fillers are often applied by meter-mixing a two-part system, dispensing on one of the two substrates, and pressing the two substrates together to reach a specified thickness. The material is then allowed to form a solid, but compliant interface. Thermal pads, on the other hand, are pre-cut to a desired shape, applied to one substrate, compressed down to set thickness, and fixed in place.
The applied compressive load forces the solid, yet compliant, pad to make intimate contact with the rough surfaces. Also, gap fillers, unlike solid thermal pads, flow into the small valleys, and create more intimate contact with the surface. This allows a more efficient transfer of heat between the upper and lower substrates.
When comparing the key attributes of the two types, the relative cost for using thermal pads is high due to the costly scrap that results. Air entrapment is more frequent with thermal pads since they can’t reach those tiny spaces that result from the surface roughness. Gap fillers are the answer for design flexibility since the hardness and working time can be adjusted using the mix ratio of the gap filler’s two parts. And lastly, when it comes to applying the product, the large form factor thermal pads can be difficult to apply without trapping air and automation is difficult. On the other hand, gap fillers are well-suited for high volume production.