Meaning
Surface roughness elements consisting of tiny peaks and valleys on solid materials affect the actual contact area and thermal transfer at an interface. In battery module cooling, microscopic asperity represents the primary obstacle to achieving low thermal resistance between the metal casing of the cell and the cold plate. Squeezing these peaks flat requires a compliant interface material to fill the gaps and displace trapped air.
This local mechanical interaction governs the thermal path at the microscopic level.
Contact Area
True contact occurs only at the highest points where these peaks meet. When two dry metallic surfaces are pressed together, the microscopic asperity prevents complete contact, leaving up to ninety percent of the interface as insulating air gaps. Applying compression forces can deform these asperities to increase the thermal transfer area.
Interface Optimization
Using a soft polymeric pad or liquid gel fills these microscopic voids to eliminate the thermal resistance of the air.
Measurement Standard
Sourcing specifications use surface finish metrics to define the maximum allowed peak-to-valley height on machined cold plates. Plates with high roughness require thicker and more expensive gel to establish a solid thermal connection. Defining this limit ensures that the manufacturing process does not produce parts that require excessive gap filler.
This control keeps assembly costs down.
