This month, I examine some of the key differences between conformal coatings, encapsulation resins, and potting compounds to help designers make more informed decisions, and ultimately help increase the performance, reliability and lifetime of your electronic circuitry. I also look at issues that some of you may have with mixing resin packs and air bubbles, what can go wrong and why.
No one wants a product recall, or worse still, a product failure. Therefore, protecting circuitry for its intended purpose and end-use environment is imperative to extend product lifetime and maintain your brand’s reputation.
Resins are an excellent way of protecting electronic circuitry, however, there are some excellent conformal coating alternatives on the market as well. For instance, we have created a two-part 2K coatings range, which provides more resin-like protection, with the application ease of a coating. This has been a great success with applications in aerospace and automotive, where protection in harsh environments is absolutely critical, whilst minimising the increase in product weight. I will be exploring coatings vs. resins in a little more detail, using my usual five-point Q&A format.
1. What are the main differences between resins and coatings?
The first difference between resins and coatings regards the various methods of application. For instance, there are aerosols, conformal coating spray equipment, manual spray guns, and brushing for coatings, compared to mixing and dispensing equipment, and resin packs for resins. Then there is the thickness of the application to consider:500 microns for resins, as well as approval ratings (coatings are generally approved to UL746, while for resins it is very much dependent upon the application).
Due to the coating thickness, coatings occupy less volume and give a much smaller increase in overall weight compared to resins. There are both coatings and resins based on epoxy, polyurethane, and silicone chemistries, but there are also acrylate, acrylic, and parylene coatings which do not have a direct resin equivalent. Most resins are 100% solids systems, so that few or zero VOCs are released during curing with minimal shrinkage, while many coatings are solvent-based, although there are two-component (2K), UV-curable acrylate systems, and solvent-less silicone coatings that are also 100% solid.
In particular, 2K coatings are designed to be applied more thickly, without cracking during thermal shock testing, and enable a greater degree of component lead coverage to be achieved, resulting in improved performance during thermal shock, powered salt-spray testing, MFG testing and condensation testing (traditionally gruelling test regimes, commonly used during automotive qualification campaigns). The 2K series is also VOC-free, solvent-free, and fast-curing, whether by thermal or the revolutionary UV/chemical dual-cure mechanism.
2. What are the main considerations for choosing a resin instead of a coating?
The main considerations for choosing between a resin and a conformal coating are normally down to the specifics of the application. If the housing is designed to be the primary protection against the environment, then a conformal coating is generally a more appropriate choice. However, if the housing is not the primary environmental protection or if the unit involved is to be subject to long-term immersion in various chemicals, then a resin is generally a preferred, more appropriate choice. Also, if there are many tall and heavy components on a PCB that is expected to encounter significant vibrational loads during use, then an encapsulation resin might be a more appropriate choice. Further consideration should also be given if the unit is to be used in a situation where it is not easily accessible or if a long continuous service life is required, in which case a resin might be recommended to provide the extra protection and durability needed.
3. What is the best way to mix a resin pack and what can go wrong?
Remove the resin pack from the outer packaging. In the case of polyurethane and silicone resins, don’t remove it from the foil pack until ready to use. Lay the pack out onto a flat surface and remove the centre clip. Use the clip to push the resin from one half of the pack to the other, then pick the pack up in both hands and mix in a circular motion for a couple of minutes. Place the pack back on the flat surface and use the clip to push the resin from the corners into the centre. Pick the pack up in both hands and continue mixing for a further minute. Repeat, pushing the resin from the corners into the centre of the pack, and then continue to mix for another minute to ensure that the material is fully mixed and is a uniform colour. Use the clip to push the mixed resin to the side of the pack with the angled seam. Roll the pack film up so that the pack fits into the hand. Cut the corner of the pack off and then tilt the pack to dispense the resin, applying slight pressure as required to maintain the flow.
If the material from the corners of the pack is not pushed into the centre of the pack then unmixed material can be dispensed. If the resin is not mixed long enough, then the resin might not cure, or have a patchy cure. In the case of filled resin systems, some sedimentation might have taken place over time, so it might take a little more mixing to ensure that the fillers are correctly distributed throughout the resin. With the optically clear resins, when first mixed, the resin may appear hazy. This is perfectly normal, and the haziness will disappear as the material cures.
4. What happens when air bubbles get trapped in encapsulation resins and how can this affect performance?
There are numerous effects that air bubbles can have on the performance of the cured resin. Depending upon the number and distribution of the bubbles, the actual thickness of the polymer layer applied will be decreased, hence the level of protection will also be reduced, particularly against chemical attack. If the air bubbles are next to components, wiring or tracks, and bridge conductors, then the insulation between those points is compromised and design rules around spacing and clearance may be broken. Ultimately, this can result in premature failures by creepage and clearance breakdowns. Voids also act as a weak point under thermal and physical shock, which can lead to the resin cracking in service.
5. What are the differences between encapsulation resins and potting compounds?
There is little difference in terms of the resins’s chemistry used, but there is a difference in how the resin is applied and the performance expected from it. An encapsulation resin will totally cover the PCB and the components and can act as the protective support structure, while a potting compound is used to fill a housing or enclosure containing the PCB and components.
So, an encapsulation resin will adhere to the PCB and the components, and its outer faces will act as the primary barrier to protect the unit. The encapsulation process may or may not be performed with the assistance of a mould. Without a mould, the flow characteristics of the material will be required to control the deposition of material, and a dam and fill process using two different resin chemistries may be used to ensure correct material deposition.
A potting compound must adhere not just to the PCB and components, but to all materials within the device, including the housing. In this case, the differences in CTE (coefficient of thermal expansion) between all the materials used are often a critical factor, as all the materials within the housing will be subjected to different rates of expansion and contraction due to the differences in the materials’ CTE values. This can put the resin, components, circuit board, and even the housing under extreme stress, and lead to failure over time.
If in doubt, it is always advisable to discuss with your suppliers which material is best suited to your application. The technical support teams of reputable suppliers have a wealth of experience to call upon and, should it become necessary, they have the expertise to modify chemical formulations to meet your application needs. I hope the points covered this month have been informative. Please do look out for my next column, where I’ll be covering more issues on getting the most out of conformal coatings.
This column originally appeared in the March 2021 issue of Design007 Magazine.