Thermal Interface Materials Silicones

Heat sink RTV silicone

SILCOTHERM® Thermal Interface Materials

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Silicone polymers and elastomers have particular inherent physical properties including:

  • Wide operating temperature range -115 to 300ºC
  • Excellent electrical properties
  • Flexibility
  • Hardness range, soft gels to moderately hard rubbers
  • UV resistance
  • Good chemical resistance
  • Resistant to humidity and water
  • No or low toxicity
  • Easy to us

These natural properties can be further enhanced using fillers and chemical additives to provide additional features when needed, including flame retardancy, thermal conductivity, electrical conductivity and adhesion. Through the selection of polymers and fillers it is also possible to adjust viscosity and rheology and the final hardness and modulus of the cured rubber.  Control of the curing regime and speed can be achieved using the silicone chemistry to produce both heat and room temperature cure (RTV) systems.  Silicones can be supplied as 1 or 2-part systems.  In short silicone encapsulants are very versatile and provide design engineers with a wide product choice.

Types of Silicone Thermal Transfer Materials

Using silicone polymers we can then formulate a variety of materials that in addition to the ability to transfer heat, will also have some additional functionality which will be of benefit to the designer.  We can produce three basic types of silicone material:

  • Adhesive sealants
  • Encapsulation and potting compounds
  • Non-setting compounds
  • Gap Fillers

Adhesive Sealants

Silicone heat transfer adhesiveThe obvious benefits of having a thermally conductive adhesive enable you to permanently bond your component to some form of heat sink and eliminate the need for additional mechanical fixings.  It will also prevent the possibility of movement and air gaps forming which will reduce performance.  These products can also be used to form gaskets (see separate sheet on FIPG Gaskets) which will not only transfer heat but also form a seal against moisture and other environmental contaminants.

Adjustments can be made to control the physical properties such as; rheology, viscosity, hardness, colour etc, which can be selected to meet specific design requirements.  Using a flowable 1-part adhesive it is possible to apply a coating with thermal conductive properties.  This approach has successfully been used to coat the back of large scale mega screen LED displays thereby offering environmental protection and effectively removing heat from the diodes.

As the adhesive will be in contact with sensitive metals such as copper, it is essential that there are no harmful corrosive by-products such as acetic acid.  ACC have developed their AS1400, AS1700 and AS1800 ranges of neutral cure adhesives with the electronics market in mind and have included a number of thermally conductive products.

For further information regarding the benefits and certain curing chemistries please refer to the  Silicone adhesive page

Encapsulation and potting compoundsSilicone encapsulant

Using a thermally conductive encapsulant has become a very attractive option when trying to remove heat from a number of components within a single device.  Selection of a suitable flowable silicone will facilitate removal of all the air gaps in and around various components, thereby providing an effective path for the transmission of any unwanted heat.  In addition to the dissipation of heat, silicone encapsulants will also provide protection from harsh environments, vibration and thermal shock. Using the versatile silicone chemistry we can produce a variety of encapsulants with various physical properties.  Of particular interest is the new development of thermally conductive gels which reduce mechanical stress on delicate wire connections.

Typical applications include the manufacture of power supplies, under bonnet electronics and LED packaging.

The emergency vehicle light shown here, uses a thermally conductive silicone compound to seal and protect the electronics behind the array of HB LED's and also helps keep them cool, maintain performance and improve the working life.

Encapsulation materials can be supplied as either 1 or 2-Part systems using condensation and addition cure chemistries.
For a more detailed explanation of these chemistries please refer to the ACC Silicones encapsulation page

Non-setting compounds

Silicone thermal transfer compounds are non-setting in that they do not cure, have no adhesion and retain their physical properties, similar to that of grease.  The main reason to choose a compound rather than adhesive is the ability to easily rework the component.  Under normal circumstances the component would be held in place with some form of mechanical fixing and the compound applied to simply improve heat dissipation.  These silicone compounds are work stable and will with-stand reasonably high temperatures.

For a more infomation on these compounds please refer to the silicone grease/compound pages

Gap Fillers

These Thermally Conductive 1:1, 2-Part, Paste materials are designed to be used as flexible Gap Fillers in electronic assemblies. The cured material is not self-bonding but remains flexible allowing for a good interface between components or complete PCB’s and heat sinks or enclosures. They are easily applied and provide a very efficient path for heat dissipation between uneven surfaces, removing air gaps that would cause heat to build up in the component. They are supplied in bulk containers for automated dispencing or Semco® twin cartridges together with a static mixer which are widely used within the electronics industry.

Gap Fillers are used in applications with wide tolerances or differences in the gap between the component or PCB and the heat sink. In some areas the gap may be close to zero in other areas it may be several mm’s. If a hard interface material is used, the pressure exerted when trying to eliminate any air gaps could be very damaging to either the PCB or delicate components.

The advantage of using a liquid dispensable Gap Filler is the ability to apply the material to very specific areas and at varying thicknesses, thus offering commercial and technical advantages. One disadvantage of a liquid system is if a rework should it be necessary is more difficult to perform.

For a more information on these compounds please refer to the silicone Gap Filler pages

 

ACC Silicones Ltd, Amber House, Showground Rd, Bridgwater, TA6 6AJ
Tel: +44 (0) 1278 411400 Fax: +44 (0) 1278 411444