With this entry I’d like to tackle the issue of thermal conductivity vs. thermal impedence in adhesive bondlines. Although some adhesive manufacturers may claim to have bulk thermal conductivity values higher than 30 W/m°K, device manufacturers need to appreciate that the bulk value is only an indicator of potential for heat transfer through the adhesive bondline. The material with the lowest thermal impedance should actually be the goal, as it is with this that the best heat transfer can actually be obtained.
The material with the lowest thermal impedance should actually be the goal
The factors that affect thermal impedance include: adhesion at the interface, surface wetting, thin adhesive bondlines and void-free bondlines. If the interfacial adhesion is weak or impacted by filler alignment, then the thermal resistance across the interface will be so great that the bulk resistivity becomes meaningless. If the bondline can be kept thin, for example by using a solventless adhesive, then heat transfer will be very efficient regardless of the difference in the bulk thermal conductivity.
What is the thermal conductivity of air?
Consider for a moment that air has a thermal conductivity of 0.0261 W/m°K (@25°C). Even a moderately-filled silica epoxy has a thermal conductivity of 0.3 W/m°K which is already more than 10 times more conductive than air. By the same token, consider a highly solvent-filled adhesive. Should the least amount of air be present, whether through a void or delamination, then the bulk thermal conductivity is immediately no longer the same.
With this in mind, I believe that the highest solvent-free, organic-filled pastes we will see will never be higher than 10 W/m°K, but that these pastes are still good candidates for applications requiring higher thermal conductivity — even those applications requesting values greater than 10 W/m°K. I remain open to the possibility that higher thermal-conductivity products may appear on the market, but I would look first to the test method used to determine the thermal conductivity and then to the void-free bondlines that these adhesives can achieve before being convinced that they make superior candidates for high thermally conductive applications.
CAPLINQ is a specialty plastics supplier offering a range of conductive materials including our broad range of electrically conductive plastics and antistatic tapes and films. If you have any questions, please don’t hesitate to contact us to find out how we can help you with other questions related to bulk thermal conductivity or thermal impedence.