Conformal coatings: challenges and considerations 101

Conformal coatings can be a blessing and a curse, but there is no question that they are a valuable process for protecting PCBs from moisture and contamination. It has become an integral part of the PCB production process for some manufacturers, but it doesn’t come without its challenges — we explore them in this article, along with some of the solutions.

The main objective for applying a conformal coating for your printed circuit board (PCB) is to ensure it maintains the functional integrity, which is imperative for critical applications. Temperature conduction issues and corrosion attributed to gaseous or liquid penetration are all minimised, and often completely eradicated, if a conformal coating is applied to the board.

Key features of conformal coatings

In addition to the factors mentioned above, a board benefiting from a conformal coating can see up to an 80% reduction in component density, compared with ‘normal’ boards. This is because boards that don’t utilise the conformal coating method are designed to ensure the components are placed within a set distance, ensuring current bleed is minimised. When coating is applied, the ingress of moisture between tracks and/or components is reduced or eliminated; therefore, designers can put components closer together without fear of changes in resistance or capacitance between devices.

The requirement for complex mounting enclosures being eliminated, as well as reduced stress on the circuit, are key advantages. Boards can also see benefits of reduced mechanical vibration, and while analog boards can sometimes suffer performance changes with humidity, these don’t occur if the board is coated.

It is the job of the coating medium to prevent ionisable contaminants reaching circuit nodes — which, combined with moisture, will form electrolytes. To aid the process, degreasing PCBs via aqueous cleaning (prior to the coating process) will promote enhanced adhesion of the coating compound.

Types of conformal coatings

There are four main classes of conformal coating compounds: Acrylic, Urethane, Silicone and Varnish. Conformal coatings are not sealants and as such allow the escape of moisture while maintaining protection from contaminates. Most coating compounds contain a fluorescent dye which aids with inspection of coverage.

Coverage typically requires a coating of 25–250 µm thickness, depending on the type of compound in use and the method of application of the compound. For acrylic, epoxy or urethanes this typically requires 30–130 µm, and silicone is typically 50–210 µm.

Compound application methods

Spray, dipping or brushing: all of these methods are usually applied to boards in low-volume production, touch-up or repair, and tends to be thick, wasteful and contains bubbles. What is more, vulnerable components such as connectors, piezo devices and sockets may be compromised.

Manual spray application: this method requires each individual PCB to be masked by hand, ensuring the ‘keep-out area’ is adhered to, albeit with minimal protection for vulnerable components. Application is by aerosol or by hand spray gun, with the removal of the remaining mask (post application) a time-consuming process.

Selective conformal coating: this method is best suited to high-volume applications, with high accuracy and repeatability. The need for masking is eliminated with critical components still avoided. Spray atomisation promotes good tip coverage and the deposition thickness can be easily controlled.

Curing methods

The curing method will depend on the type of coating applied, but typically air drying is required for most solvent-based acrylics. Curing can be accelerated by applying heat utilising a conventional forced convection reflow system (FCR).

The FCR system operates by set profiles which are configured specifically for each application. Water-based coatings also respond to heat curing, but more care is required due to longer curing times. Ultraviolet (UV) is another method that can be utilised for curing. This method is preferable for applications such as automotive manufacturing because it’s fast and effective. Traditionally, the UV was applied utilising bulky arc lamps, but today microwave electrodeless lamps and LED type lamps are much more suited to the space saving and efficiency required in manufacturing environments.

Parylene coating solution

However, not all coatings require further drying or curing; some coatings, such as a Parylene Coating, can be applied at nearly room temperature and require no initiator (solvents). Parylene Coating is applied using a high-purity powder by chemical vapour disposition (CVD).

Similar to liquid coatings, a Parylene Coating will protect the PCB from moisture and corrosion, and it usually achieves this with a much thinner coating. As the CVD process can be applied at room temperature, it means Parylene can be utilised for a range of substrates including silicone and paper. However, this process requires significant investment, so it will usually depend on the critical nature of the specific application.

UV LED spot curing

Some boards less critical in nature can benefit from utilising a ultraviolet (UV) LED spot curing method, which is ideal for boards such as mobile phone modules. LED spot curing has evolved into a viable method which has low environmental impact and lower running costs (compared with more traditional lamp curing methods), whilst still providing significant bulb life with >17,000 mW/cm² initial intensity.

One example is the BlueWave 200, which can be configured with the PVA Delta solutions. The BlueWave 200 has a notable patented intensity adjustment feature which allows users to both validate an appropriate intensity range and it is then maintained throughout the production process, ensuring accuracy and repeatability.

Key to controlling deposition

Custom dispensing systems that support some of the spot curing hardware are ideal for those who can’t justify the investment required for applying a Parylene Coating and simply require traditional coatings such as acrylic, but applied using a combination of speed and accuracy (+/- 25 microns). Historically, this has been hard to achieve, but with the PVA’s custom dispensing systems, not only do they provide up to four-axis robotic platforms for dispensing a wide variety of adhesives, sealants and conformal coatings, but their wide range of valves means a PCB can be coated with both speed and accuracy.

Utilising the spray valve to achieve speed and large-area coverage, and the needle valve to penetrate into the areas where tolerances are tight, bordering between the coating zone and areas where no spray is wanted or desired (typically connectors), users can achieve a balance between speed and precision.

PVA Delta 8 Conformal Coating/Dispensing Machine

One of PVA’s solutions, the Delta 8 system is a good example with patented servo-controlled optional four-axis motion featuring valve tilt and rotate. This system can be utilised inline, integrated with other SMT solutions, or as a batch solution.

This robotic system boasts repeatability of 25 microns and can be utilised with a range of PVA valves, including the ultrafine spray valve which can achieve edge definition of ±1 mm with a 99% transfer efficiency. This precise functionality significantly reduces overspray and allows for tight adherence to keep-out zones.

Optimisation

Whether you require a solution for inline or batch processing, your business is operating in defence, aerospace or medical devices and working on critical PCBs, or your focus is on speed, there is an ideal solution for you — but it requires a holistic approach and this has to start from the board design up.

There is always going to be balance between design, budget, process, materials, speed and accuracy, but looking at all of the options from the time of R&D idea inception with a collaborative approach could iron out any potential challenges early on.

If you require any assistance or advice on different conformal coatings solutions for your SMT production, contact us here.

Image credit: ©stock.adobe.com/au/Gordon Naldrett

Originally published here.