The trend in the electronics packaging industry is towards smaller, more powerful devices. With these small, high power components, however, there are also higher heat fluxes. As a result, engineers must find ways to minimize the thermal resistance from the electronic device junction to ambient air.
This thermal resistance can be expressed as Rja, where:
Rja= Rjc+ Rcs+ Rsa
- Rja - Thermal resistance from the device junction to ambient air or water
- Rjc - Thermal resistance from the device junction to the package case, determined by the electronic device manufacturer (designer has no direct influence)
- Rcs - Thermal resistance from the package case to the heat sink or cold plate, determined by the size and quality of the contact areas between the electronic device and the heat sink or cold plate, the materials used, and contact pressure
- Rsa - Thermal resistance from the heat sink or cold plate to ambient air or water, determined by the heat sink or cold plate design (material and geometry)
Therefore, one way to reduce Rja is to reduce Rcs, the contact resistance between the electronic device case and ambient-cooled, finned heat sinks or liquid-cooled cold plates. There are several factors that impact Rcs, including surface flatness, surface roughness, contact force or clamping pressure, surface cleanliness, and interface materials.