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Gap Fillers versus Thermal Pads

( 07/19/2018 ) Written by: Eric Wyman

In the quest to find the best thermal management material, battery manufacturers typically are drawn to one of two types of products: a cure-in-place, liquid-dispensable gap filler (gap filler) or a pre-cured thermal pad (thermal or gap pad). So, how do they differ?

Let’s start by understanding the variances in the materials and the methods they are applied. 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.    

To understand the differences, we’ve compared the thermal impedance of the metal-TIM-metal sandwiches measured on two LORD CoolTherm™ gap fillers against the thermal impedances obtained on commercial solid thermal pads having equivalent bulk thermal conductivities.

The result?  We’ve found that a thermal management material may have a high thermal impedance despite having a high thermal conductivity and thin bond line. This can be the result of the material’s poor physical contact with one or both substrates. The results show that thermal pads have significantly higher impedance values as compared to gap fillers.

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. We have found that gap fillers are better at accommodating macroscopic variations in thickness across the length of a bond line. It is not uncommon for bond lines to vary by several millimeters across the length of a battery pack due to tolerances in substrate planetary and battery module heights. 

Let’s review the contrasts. 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. As we mentioned before, heat dispensation is best when using the gap fillers because they can flow into the microscopic surface roughness. 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.     

So, when searching for the best thermal management material for your battery pack, consider a liquid-dispensed gap filler for better thermal performance coupled with ease of assembly, low applied forces, ability to span large, variable bond lines, and lower cost.  Contact us to learn more about LORD’s CoolTherm™ liquid-dispensed gap fillers and download our Gap Filler vs Thermal Pad White Paper.

 

ABOUT THE AUTHOR MORE BY THIS AUTHOR
Eric Wyman

Eric Wyman is Lead Global Application Engineer for the Electronic Materials Group at LORD Corporation with a focus on understanding the role of Thermal Management in the Electric Vehicle space.

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