Conformal coatings have long been used to protect and prolong the life of a PCB’s components. But as with all things, technology and techniques evolve, and recently new coatings and methods of applying and checking have been developed to enhance the possibilities in this area.
Let’s start by defining exactly what conformal coatings are before we look at the latest trends and the reasons they are needed.
‘Conformal coating material is a thin polymeric film which conforms to the contours of a printed circuit board to protect its components. It is typically applied to thicknesses between 25 and 250 μm (micrometres) to offer protection against moisture, dust, chemicals and temperature extremes.’
The challenges facing conformal coating
Several factors are at play here which necessitate new ways of doing things.
- Increased concern with many conformal coatings about the concentration of volatile organic compounds they use and their adverse impact on both human health and the environment
- The need to reduce the formation of tin-whiskers and prevent current bleed between closely positioned components has become critical for the aerospace industry
- The continued miniaturisation of electronics and their circuitry presents challenges for precise coating
- Conformal coatings are often not effective in offering protection in applications that involve significant exposure to liquid water (as distinct from water vapour), especially when it contains ionic material
- Conventional liquid conformal coatings do not cover well over sharp spikes or sharp edges on components – with inadequate quality control checks, the coating will be much thinner or, sometimes, not even present in such areas
- In particular the defence sector needs to ensure electrostatic charge, electromagnetic interference and radio frequency interference are reduced, to avoid equipment failure and locational detection
The latest advances in conformal coating
Our expert team is fully trained in the manual application of coatings to both components and boards. However, increasingly, for high volumes we use automated equipment to apply coatings to selected board locations, increasing efficiency and reducing the opportunity for human error.
The benefit of automatic application is that it negates the need for masking and unmasking, application is vastly more accurate, consistent and repeatable, and once the initial setup costs have been amortised, costs are a small percentage compared to manual- 3mins automatic compared to 90mins manual (inc. masking and unmasking) is typical.
Flash UV cure
UV curing is increasingly used for conformal coatings, especially for high volume board production, and where rapid turn and process efficiency are key metrics. The rapid cure and lower temperature UV curing process removes delay without damage to sensitive electronic components.
It also enables in-line testing, further assembly, minimized handling introducing imperfections, increased throughput and lengthened product life – all while requiring minimal energy and space requirements.
This increase in the popularity of UV curable conformal coatings is due to its rapid cure speed, ease of processing, environmental friendliness (no VOCs) and thermal cycling resistance.
The one drawback is known as the ‘shadow effect’. Areas under components that are shielded from the UV radiation and can be slow to cure. This can be resolved by including a secondary curing process, which can involve a chemical, thermal or moisture cure, which takes place whilst undergoing other processing, transport or storage stages.
Two-stage cure with outer layer
Epoxy resins are usually available as two‐part compounds and create a very hard coating.
As a result, this conformal coating provides very good humidity resistance and isn’t generally permeable, as most conformal coatings are. It also has high abrasion and chemical resistance. It is typically used in potting compounds, which in contrast to conformal coatings, completely fill and enclosure and all the electronics within in a solid mass.
High solids coatings
A high solids coating is formulated to have higher concentrations (at least 65%) of solid components (binders, pigments and additives). It can be solvent- or water-based but due to the higher percentage of solids will have a much lower proportion of volatile organic compounds.
High solids became widely used in the aerospace and other industries as a way to comply with legislative regulations on emissions. A benefit of a high-solids material is that what is deposited, remains in place, with minimal shrinkage- thereby increasing consistency.
The miniaturisation of electronics has led to dense circuitry and more delicate construction. This in turn leads to a lowering of the operating voltage and the increased likelihood of electrostatic charge causing damage.
New insulating coatings allow for closer conductor spacing. They also increase the voltage rating of a dense circuit assembly as they can withstand a much stronger electric field than air. This is especially true at high altitude, an important consideration for the aviation and aerospace industries.
When electrostatic charge is discharged through an integrated circuit it can create sufficiently large a current flow and energy dissipation to cause damage to the circuit.
Similarly, stray energy emitted from various other sources can interfere with the circuit, lowering its performance or, in the worst case, causing a malfunction.
As devices get smaller, and as more devices are used around us, both of these scenarios are an increased risk.
Get your coats
With the varied uses of electronic devices, we can help you decide early in the day which conformal coating will be right for your product and its intended use.
Whether you need a product that:
- Will be used in highly charged atmospheres (such as for the aerospace, defence, oil and transport sectors)
- Will be used in corrosive or challenging environments (underwater, exposed to heat or wind)
- Needs to conduct electricity to move it around
- Needs to ground the electricity or provide a barrier so it stays in one place
- Improves sustainability and makes your products more environmentally friendly
As they say, there’s a coat for every season. Read about our specialist services in manufacturing to learn more.