How to Increase Flowability & Thermal Conductivity of Your Potting Material

Posted by Kris Hanson on Dec 20, 2018 9:38:39 AM
Kris Hanson

Thermosink-35-3Thermally conductive silicone products are widely used in the electronics industry to radiate heat away from locales for component survivability while maintaining optimum operating temperatures. For potting and encapsulation applications, circuit board engineers typically specify flowable products that will quickly surround the components after dispensing and become solid at room temperature within an hour or less.

Higher performance thermal pottants above 3.0W/mK are ideal for these applications but are prone to the following potential issues:

  • High thermal conductivity usually means high filler content, which inhibits flowability causing trapped air and poor thermal conduction
  • Filler settling can also be difficult to remix with inconsistent cured properties.

Since air is a poor conductor of thermal energy, it acts as a barrier and inhibits heat transfer into adjacent heat sinks, metal housings, and cooling plates. Silicones are inherently low in surface energy, around 25 dynes/cm, which aids in “wetting out” edges and displacing air between circuit board components.

Another important attribute is thermal conductivity and, in this case, the higher the better. However, higher thermal conductivity usually equates to higher viscosity due to increased thermal filler levels, which can inhibit the flow after dispensing as well as create high dispense head pressures and slow outputs. Since thermal fillers are fairly dense and heavy compared to unfilled silicone systems, they are likely to settle out during storage or transport. Many of these historical products can form a hard pack at the bottom, making them difficult to remix prior to use and usually increase in viscosity.

ThermoSink 35-3 provides all of these attributes.

Why You Should Consider ThermoSink 35-3 

  • The product is formulated for both excellent flow AND resistance to settling.
  • It flows easily into and around electrical devices and into deep cavities.
  • ThermoSink 35-3 is silicone based with low surface energy to “wet out” against electrical components.


The dispensing video demonstrates the low viscosity and degree of flowability with this material, even with >3.4W/mK thermal conductivity.

Related article: How to Select the Right Thermal Interface Material

The chart below shows the overall drop in both viscosity and thixotropy when compared to an earlier formulation used as a control. These combined improvements not only displace trapped air among circuitry elements but also greatly increase manufacturing throughput with automated meter-mix dispensing operations.

Overall drop in both viscosity and thixotropy  when compared to an earlier formilation used ads a control.


Brookfield viscometer testing Part B with spindle #6 -1Brookfield viscometer testing Part B with spindle #6 -2

Above Image: Brookfield Viscometer testing Part B with spindle #6

ThermoSink 35-3 is quickly redispersed into a low-viscosity liquid and ready for the vacuum degassing step prior to use to remove air during the mixing step.

ThermoSink 35-3 A vac chamber side1ThermoSink 35-3 A vac chamber side2   ThermoSink 35-3 A vac chamber side3

Above Images - ThermoSink 35-3 A vac chamber side/ under full vacuum / vac chamber open and completed

Dispensing equipment with transfer hoses and right-angle fittings is less likely to become clogged due to the anti-settling nature of ThermoSink 35-3, especially with programmable agitation and recirculation options.

For smaller volume applications, dual-component cartridges are ideal for experimentation with varying fill levels or the preparation of assemblies for thermal cycling tests. Video2 shows a 400CC cartridge preparation. This way, the bulk of the material can be transferred to smaller containers instead of handling large pails.

The video below is a summary of the entire process to prepare ThermoSink for dispensing.   

For more information on ThermoSink 35-3, please complete the contact us form and we will be in touch shortly.

Topics: Resin Designs, Thermosink, Thermal Conductivity, Potting Material

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