Dosing of thermally conductive materials.

How much thermally conductive material do you need? And what is the pattern for applying the material to the substrate?

The importance of the layer thickness of the thermally conductive material

Under high magnification, you can clearly see that the surfaces of heat-generating devices and heat sinks never match 100%, because their surfaces are not perfectly flat. The physical contact areas are small and there are also many gaps between the heat sink and the heat source where air is trapped - these air spaces act as insulation, preventing efficient heat transfer. To facilitate heat transfer and avoid this problem, filled thermally conductive materials are used between the surfaces, which provide better thermal conductivity than air.

The bond line describes the area where the thermally conductive material contacts two surfaces in between. As a rule, the basic feature of the joint is its thickness. In general, a thinner bond layer reduces the distance the heat has to travel to be dissipated from the heat source. Therefore, to minimize thermal resistance, thin bond lines are preferred.

Filling materials

Thermal conductivity is expressed in W / mK. Unfilled polymers have a thermal conductivity of about 0.1 W / mK, while the filler materials provide thermal conductivity in the range of 1-1,000 W / mK. Inorganic particle fillers include aluminum, oxides (e.g., magnesium oxide), aluminum nitride, boron nitride, and diamond powder. Metal fillers, especially silver, are also used to increase thermal conductivity. The thermal conductivity of filled polymers is typically in the range 1-10 W / mK.

Significant procedural issues

How Much Thermally Conductive Material Do You Need?

In short, enough material should be used to cover the component - or the bond area - with a bonding layer that provides adequate adhesion and performance when the material is compressed. The amount of material to be dispensed is the following calculation:

Material volume = final bond line thickness x bond area

However, additional external factors must be taken into account, such as viscosity differences, substrate material composition and mechanical tolerances, which may affect the final thickness of the bond layer. To compensate for these factors, calculate the volume of material dispensed using the upper tolerance limit for your application. By using the thickest bond layer acceptable in the calculation, a sufficient amount of material is ensured, regardless of whether the part is at the lower or upper end of the mechanical tolerance range.

When calculating the volume, it is necessary to take into account:

• Dimensions of the heat generating element

• Coverage of the bond area required

• Application tolerance upper limit

• Desired final bond thickness after compression of the material

Tip: If the mechanical tolerance results in a bond layer of the lowest allowable thickness in the range, test the bond thoroughly to find out if it may fail and if this affects the overall performance. If necessary, adjust the thickness of the bond layer.

Selecting the dispensing pattern

The geometry of the dispensing pattern has a significant influence on the efficiency of the process. A dosing formula should be selected that balances:

• Dosing rate

• The force required to achieve the desired thickness of the bond layer

• Material consumption

• Capacities and consumption of dosing equipment

For example, if you are dispensing onto a part that has a low tolerance to mechanical stress, a more complex spiral pattern can be selected that takes longer to dispense but will provide better quality.

Source: The Essential Guide to Large-Volume TIM Dispensing © Nordson  & https://tek.info.pl
Cover photo: © Scanditron