Conformal coatings are materials that will adhere and conform to the surface of electronic assemblies in order to create a barrier against multiple conditions. Although conformal coatings are typically utilized to provide protection against external stresses, these materials can also be formulated to help dissipate heat produced internally by coated components.These thermally conductive conformal coatings (TCCC) are particularly useful when the substrate is prone to heat energy emissions, as it allows the excess heat energy generated to be regulated and removed - preventing overheating, whilst still retaining a protected surface.
Typical applications that utilize TCCC's include: Circuit boards, motors, power resistors, power semiconductors, chip bonding, opto-electronics, micro-electronics, and drill bits.
In this blog post we'll work to answer the following questions regarding TCCC's:
- What types of materials can TCCC's be made from?
- What are the typical properties of TCCC's?
- How are TCCC's applied?
- What is the optimum TCCC thickness?
Although traditional conformal coatings can be made from a long list of materials including acrylics, polyurethanes, epoxies, silicones, and paralenes, thermally conductive conformal coatings are normally limited to either epoxy or silicone base chemistries.
This is mainly a result of the high temperature capabilities of thermosetting epoxy and silicone materials. Some epoxies are able to withstand temperatures in excess of 150C, and some silicones are able to withstand temperatures in excess of 300C.
However, simply choosing a high temperature polymer is not enough to create a thermally conductive coating. Pre-cured resins are mixed with thermally conductive filler materials in order to induce thermally conductive properties in the end polymer product. Typical filler materials include metal oxides such as alumina, which provide substantial increases to thermal conductivity without reducing the electrical resistance of the coating (which might result in electrical shortages). These same filler materials are typically used in thermally conductive adhesive and potting compound formulations as well.
Typical TCCC Properties
Typically TCCC's are able to achieve thermal conductivity in the range of 0.5-5.0 W/mK. Thermal conductivity is entirely dependent on chosen filler materials and filler concentrations. Specialized materials utilizing unique filler blends are able to achieve even higher levels of thermal conductivity.
Beyond thermal conductivity, epoxy and silicone TCCC's have slightly different pre-cured and cured properties.
Epoxies are available either as one-part, heat curable systems, or two-part, room temperature curable systems. Both methods have their advantages and disadvantages with respect to production efficiency. One-part systems require large ovens and long heating times in order to achieve complete polymerization. Two-part systems, on the other hand, require mixing equipment, which generally result in more waste. Both methods, however, generally result in similar end properties/ benefits.
Epoxies can be applied by varying methods. Generally they are applied by brush spray or dipping. For larger components, spray or machine dipping is recommended. For smaller components, however, hand brushing and dipping is recommended instead. Hand brushing is also recommended for products requiring a lot of masking. For more delicate components, a buffer should be used.
As with anything, the properties as a whole are more important than just one stand alone property. In addition to thermal conduction, TCCC epoxies also exhibit:
- Resistance to humidity
- Resistance to abrasion
- Resistance to chemicals and organic solvents (inert)
- Electrically insulating
- Resistance to thermal stress
- Resistance to mechanical stress
- Ability to bond to heat sensitive components
- Environmentally friendly
However, one disadvantage of epoxy coatings are that they are extremely difficult to remove/ rework by chemical methods, because any stripper will destroy the coating/ dissolve the epoxy. The only way to effectively repair your product underneath is to burn through the coating with a soldering iron or a knife. However, burning through an epoxy can have it's own complications and has to be done carefully in order to avoid damaging the components by heat or physical damage. If you wish to use chemical means, the only viable method is to carefully apply methylene chloride and an acid activator with a cotton swab.
The primary advantages of using a silicone rather than an epoxy are high temperature performance, and reduced coating rigidity.
Silicone coatings can be applied screen printing, syringe dispensing, dipping and spraying. Additionally silicone TCCC's typically exhibit the following properties:
- High compression set
- High tensile strength
- High tear/shear strength
- High di-electric strength at high voltages
- Fire resistant
- Resistant to Ozone
- UV resistant
- Shock resistant
- Easy to apply and repair
As previously mentioned, unlike rigid epoxies, silicones are flexible due to high bond angles and bond lengths where the oxygen atoms are attached to the hydrocarbon units. High bond angles allow for greater rotation around the bonds, resulting in more flexibility in the final polymerized material. Silicones also have a strong bond energy, meaning they are less likely to be broken and are able to withstand higher temperatures.