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Thermally Conductive Adhesives

There is a wide range of uses for thermally conductive adhesives within the electronics industry. The main applications include bonding SMDs (Surface Mount Devices) to PCBs (Printed Circuit Boards), bonding heatsinks for dissipating heat from circuit boards or other components, potting and encapsulating parts (including PCBs, transformers and coils). There are also applications for electric motors, batteries, lighting and LED heat transfer management.

Thermally conductive adhesives can be used in more general applications requiring good heat transfer between component parts e.g. for heating and cooling units (heat exchangers) and for tooling or mechanical parts.

Why are thermally conductive adhesives used?

Soldering is a quick and easy way of attaching components to PCBs. It is also very cheap and with lead-free solder now available, it has been made quite safe. However, some components parts are not suitable for soldering as they may not have “legs” which go through holes in the PCB, or they need some sort of electrical resistance to protect them and to prevent short circuits. Thermally conductive adhesives offer an ideal alternative for attaching SMDs when soldering isn’t practical. They can also be used to replace mechanical assembly – offering cost and process savings and helping to reduce component weight and prevent vibration loosening and rattling.

Components, PCB and transformer coils are often “potted” inside a plastic housing (or “pot”) with a potting adhesive to help dissipate heat away from the electronic part, to protect the parts against impact, vibration, environmental conditions and for security reasons.

Types of thermally conductive adhesive

Popular adhesive products for these applications include 1 and 2 part silicones, 2-part epoxies and polyurethanes (normally for potting). Occasionally 1-part epoxy adhesives are used, if components are not sensitive to the high temperature needed for curing the adhesive. Alternatively, special room temperature cure 1-part epoxies that do not require high temperatures for curing can be used, although these are very expensive to purchase and require special storage, shipping and handling at subzero temperatures.  Other recent developments include structural acrylic adhesives which combine rapid cure speeds with strength performance and high thermal transfer rate.”

Thermally conductive adhesives: Properties and benefits

On average, standard filled epoxy adhesives achieve thermal conductivity measurements of between 0.4 and 0.55 W/m.K , whereas an unfilled epoxy adhesive would achieve less (which is a pity as many potting applications require a low viscosity adhesive to fill all the gaps around the components). However, specially developed thermally conductive epoxies are available with thermal conductivity of between 1.5 and 3 W/m.K. It is possible to formulate epoxies with special fillers including ceramic, metallic or nano-fillers to give this level of thermal performance. In fact, epoxy adhesives can even be blended with silver powder to give both thermal and electrical conductivity!  Products can be formulated to be flame retardant and comply with standards such as UL94-V0 which is often required in the electronics industry.

As well as good thermal conductivity, there are other benefits commonly sought for electronics applications and for general bonding, these include;

  • High strength performance – good adhesion to a wide variety of substrate materials
  • Resistance to very low and very high temperatures – able to cope with differential expansion and contraction between dissimilar substrate materials (adhesive normally requires some degree of toughening)
  • Resistance to chemicals, water and humidity
  • Low-outgassing to minimise risk of damage to sensitive circuitry
  • Non-corrosive formulation
  • Resistance to thermal shock, impact and vibration
  • Able to withstand solder-reflow processes
  • Compliance with RoHS and REACH

Adhesive considerations

Before choosing a thermally conductive adhesive it is important to consider a number of factors– it is not just a case of going for the adhesive with the best conductivity;

  • What substrate materials are you bonding together? What is the size, dimensions and joint design?
  • What strength performance are you looking for?
  • How quickly do you need the adhesive to cure?
  • How will it fit into your production line? What cure mechanism is acceptable in your process?
  • Is there a gap?
  • What is the glue line thickness? (A thin glue line will aid with thermal transfer between components but if there is differential thermal expansion and contraction between dissimilar materials it could put a lot of stress on component parts – for this reason a  toughened adhesive and a controlled glue line thickness of 0.25µm would be recommended)
  • If you are potting, how big is the “pot”, how complex is the part that is being potted? We need to consider how well the adhesive will flow around the parts – also if a large volume of potting adhesive is required we have to consider a slower curing product to prevent it getting too hot during cure (exotherm).

Permabond chemists are able to formulate thermally conductive adhesives, modified epoxies and acrylic adhesives to meet your specific requirements.  If you have an application you would like to discuss with a technical advisor or would like to receive further information, please contact help.europe@permabond.com  or info.americas@permabond.com.