Conformal Coatings - Challenges & 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 it’s 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 is eliminated, as well as reduced stress on the circuit are all key advantages. Boards can also see benefits of reduced mechanical vibration and analogue boards that can sometimes suffer performance changes with humidity - all of which 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 25-250um thickness depending on the type of compound in use and the method of application of the compound i.e. For acrylic, epoxy or urethanes - typically requires 30-130um, and silicone is typically 50-210um.
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” are 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.
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 its 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 with lower running costs (compared with more traditional lamp curing methods), whilst still providing significant bulb life with >17,000 mW/cm² initial intensity.
Key to controlling deposition
Custom dispnesing systems that support some of the spot curing hardware are ideal, where most 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.
PVA Delta 8 Conformal Coating/Dispensing Machine
One of PVA’s solutions, the Delta 8 systems is a good example with patented servo-controlled optional four-axis motion featuring valve tilt and rotate. This systems can be utilised inline integrated with other SMT solutions, or as a batch solutions.
This robotic system boasts repeatability of 25 microns and can be utilised with a range of PVA valves including the ultra fine spray valve which can achieve edge definition of ±1 mm with a 99% transfer efficiency. This precise functionality significantly reduces overspray and
Whether you require a solution for inline or batch processing, or 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.