How to make the most of heat conducting tapes

In most cases, heat conducting tape is chosen on the basis of its conductivity, meaning that tape with the best heat transmission properties is the most commonly used. Amongst the characteristic parameters of a heat conducting tape, the value that indicates its heat transmission capacity is defined by the thermal conductivity (expressed in Wm^-1K^-1) or by its opposite (thermal impedance). The thermal conductivity of a tape is determined by the material of which it is made and its thickness. Very thin tape containing metal materials has higher thermal conductivity (lower thermal impedance). However, the main characteristic of tape is still its ability to stick two surfaces firmly together. The fact that the interface also has to allow the passage of heat from the hotter body to the colder body is still firmly tied to its adhesive properties. If you wish to apply a heatsink to the surface of a chip to cool it down, it is best to set some selection criteria so that the adhesive properties do not let you down during use. It is good practice to choose tape on the basis of the following information: the material used to make the heatsink and the surface of the chip, the weight of the heatsink and the flatness of the surfaces to be joined. Once you have identified the tapes compatible with these characteristics, you should then select the tape that offers the best thermal conductivity. Let’s take the example of Southbridge cooling on the motherboard. Most of these chips are encapsulated in a black plastic package, which considerably increases the surface area, allowing for the application of medium-size copper heatsinks. Although a large surface like this facilitates the installation of the heatsink, it actually complicates adhesion on the other hand, as it is often not perfectly flat. In fact, the surface of the polymeric housing is slightly concave, meaning that the edges are prominent while the centre (where the Southbridge chip is positioned) is slightly set back. In this case, the choice of heat conducting tape must be subordinate to the fact that the adhesion surface is not flat and also the fact that the copper heatsink is of a certain weight.

The mark test demonstrates that the surface of the Southbridge is concave: the heat conducting paste at the centre has not adhered to the base of the heatsink through contact

In order to resolve the problem, we used Thermattach T411 tape, which is of an adequate thickness and possesses superior adhesive power. In this particular case, if we had chosen a heat conducting tape with greater thermal conductivity, such as Thermattach T412, the benefits would have been cancelled out by the material risk of the heatsink becoming detached during use.

To secure the heatsink to the Southbridge correctly, just cut a small square of T411 tape and position it on the centre of the chip, so that the copper heatsink is secured firmly by simply applying light pressure