What Is Heat Transfer Paste? Types and Selection Criteria
Heat transfer paste (thermal paste) is an interface material that improves heat flow by filling the microscopic air gaps between a heating electronic component and its heatsink.
Heat transfer paste (thermal paste) is an interface material that improves heat flow by filling the microscopic air gaps between a heating electronic component and its heatsink. It is used when mounting power transistors, thyristors, triacs, bridge diodes, solid state relays (SSR) and LED modules on heatsinks, and in driver and power supply maintenance.
Even when two metal surfaces look flat, they touch only at small points; the air between them conducts heat very poorly. The paste fills these gaps. Because the paste itself conducts heat worse than metal, the aim is not a thick layer but filled gaps.
Types of heat transfer paste
- Silicone-based: A thermally conductive ceramic or metal oxide filler in silicone oil. It generally does not conduct electricity and is the most common type in power electronics and panel work.
- Silicone-free: Chosen where silicone oil must not spread onto surfaces, for example near relay and switch contacts, or on surfaces that will later be painted or coated.
- Metal-filled and liquid metal: High thermal conductivity; some conduct electricity and can cause a short circuit if they spill over.
- Thermal pad: A soft sheet of set thickness, used when the gap between the surfaces is too large to fill with paste.
All the products in the catalog are silicone-based.
Selection criteria
- Thermal conductivity (W/m·K): Take the value from the manufacturer's datasheet. Different test methods give different results, so compare only values measured the same way.
- Electrical insulation: If the paste could spread onto neighbouring leads or conductive tracks, choose a paste that does not conduct electricity.
- Operating temperature: The component's highest case temperature must stay within the paste's operating range.
- Silicone restriction: Near contacts, optics or surfaces to be painted, a silicone-free paste is needed.
- Package size: Small packages suit service and maintenance kits; in workshops and production lines doing series assembly, large packages are used with less waste.
Common application mistakes
- Applying a thick layer: Too much paste hinders heat flow and squeezes out at the edges. A thin, gap-free layer over the surface is enough.
- Applying new paste without removing the old: Dried paste and dust raise the surface. Wipe the old paste off with isopropyl alcohol and a lint-free cloth.
- Treating paste as electrical insulation: If the component's metal tab is live, it must be separated from the heatsink by an insulating pad. A mica pad gets a thin coat of paste on both faces; silicone insulating pads are generally used without paste.
- Tightening each screw fully one at a time: The component sits crooked and the paste collects on one edge. Tighten the screws in turn and evenly.
- Confusing it with solder paste: Solder paste is for soldering and electrical connections; heat transfer paste does not solder, it only conducts heat.
Heat transfer pastes in the catalog
The Heat Transfer Pastes category has 5 active products; all are Hawcon silicone heat transfer paste and differ only in package size.
Heatsinks for mounting solid state relays are in the SSR Heat Sinks category.
Frequently asked questions
How much heat transfer paste should be applied?
Enough to cover the contact surface in a thin, gap-free layer. If paste clearly squeezes out at the edges when the component is tightened down, too much was applied.
Does heat transfer paste conduct electricity?
Silicone-based pastes generally do not; some metal-filled and liquid metal pastes do. Check the product datasheet for the exact value.
When should heat transfer paste be renewed?
Every time the component is removed from the heatsink, and whenever the paste looks dried out or cracked during maintenance, clean off the old paste and apply new.