Acrylics cure rapidly (reaching optimum physical properties in minutes), are fungus resistant and provide long pot life. Furthermore, during curing, acrylic coatings emit little or no heat (eliminating the risk of thermal damage to heat-sensitive components), and do not exhibit shrinkage (a phenomenon that places excessive mechanical stress upon components and joints).
Of the various types of conformal coatings on offer, acrylics offer the highest humidity resistance of all. They also have a relatively high dielectric characteristic (i.e. impedance), selective chemical resistance and good temperature resistance.
In production they are easy to apply and can be quickly removed during rework with specially formulated stripping solutions, or by soldering directly through them for spot repairs.
Polyurethane (commonly shortened to urethane) coatings offer excellent chemical resistance combined with good moisture, temperature and dielectric characteristics. Their high level of chemical resistance, however, can be a major drawback since their re-work can prove difficult and costly. To repair or replace a component, a special stripper compound must be used, but since its residues can introduce contaminants, the stripper compound may affect longer-term circuit performance and reliability, and inhibit adhesion of the new coating, if not properly cleaned.
Whilst polyurethanes can be soldered through during rework, there is usually a brownish residue that generally only affects the aesthetics, rather than the performance, of the coating. They are available as either single or two-component formulations. Single component polyurethanes however, whilst easy to apply, sometimes require 3-10 days at room temperature for final cure. Two component formulations, on the other hand, reach optimum properties at elevated temperatures within 1-3 hours, but have a pot life range of 30 minutes to 3 hours.
Epoxy coatings provide excellent Mar resistance (i.e. the ability to withstand rubbing) and chemical resistance, but poorer moisture and dielectric performance.
Cure time is average, but epoxies are virtually impossible to remove for rework, due to the other epoxy-based materials on a board (epoxy glass substrate and epoxy-based plastics in component housings), which could be vigorously attacked by the application of any epoxy dissolving stripping material. The only effective way to repair a board or replace a component is to burn through the epoxy coating with a knife or soldering iron.
When most epoxies are applied, a ‘buffer’ material must be used around fragile components to prevent their damage due to film shrinkage during polymerisation.
Curing of epoxy systems takes place in 1-3 hours at an elevated temperature or in 4-7 days at room temperature.
Short life creates a limitation on their effective use.
Epoxy-based coatings are usually supplied as two component compounds, but single part vinyl modified epoxy compounds are available for special applications.
Silicone coatings offer high temperature, moisture and dielectric resistance, but limited chemical resilience. A very low surface tension allows penetration into every part of an assembly.
Their primary use is in high temperature applications up to a maximum of 200ºC, making them desirable for assemblies with, for instance, large heat dissipating devices such as power resistors.
Like epoxies, silicone coatings can be difficult to rework, and they are relatively expensive compared to other coating material types. It should also be considered that high temperature protection would generally demand that the silicone coating be cured at or near to the maximum temperature it is designed to withstand.
Notwithstanding the above, it must be noted that Silicone coatings are not ideal as Conformal Coatings, as their behavior under humid conditions is roughly 10-20 times worse, as a rule than other types. Single component silicones usually require the presence of free hydroxyl radicals to cross link. That means that they will not cure in a perfectly dry atmosphere. In this sense, before curing, they are hygroscopic. After curing, the hygroscopicity is reduced, but they are porous, otherwise the trapped humidity would not have escaped. Two component products use other mechanisms of curing, so are less hygroscopic prior to curing. However, they are almost equally porous.
Any gap in the under-fill, especially if accompanied by flux residues or other hydrophilic contaminant, could even fill with water under humid conditions of use.
In addition, the Thermal Coefficient of Expansion (TCE) of silicones are enormous (e.g. 300-350 ppm/°C) compared with solder (c. 16 ppm/°C) Furthermore, silicones have a unique property that, however soft they may feel under the thumb, they are rock-hard when subjected to a shock. This combination could result in a squashed silicone at slightly elevated temperatures which would place a tension on a BGA ball- pad combination, which could increase by orders of magnitude should it be subjected to a small shock at the same time.
It should also be noted that silicone is classified as VOC’s under EU legislation.
When choosing coatings with environmental considerations, it is important to note the issue of Volatile Organic Compounds (VOC’s). Such products are photo-chemically reactive in the atmosphere and thus are “smog” formers.
In the USA, there is tax legislation that encourages users to reduce the consumption of these solvents however, in an attempt to differentiate certain solvents, there are a number that are less reactive than others and have thus been granted a “tax exempt” status.
In Europe however, a VOC is categorized as any solvent that has a vapor pressure >0.001mPa. This includes gin, whiskey, vodka, brandy etc., so it is totally incorrect that certain suppliers should label materials as being “VOC Free”. Low VOC is a more appropriate term.
The most major recent development in the field of conformal coatings is the advent of water-based variants (acrylic, urethane and epoxy types are all now available) that replace solvent as the carrier medium with water.
Their development has been driven by the fact that most traditional coatings are solvent-based, producing harmful solvent and VOC emissions through their industrial application.
This has created a large amount of environmental concern over the use of conformal coating materials in general, encouraging the development of ‘greener’ alternatives that use very little solvent. Like water based types, ‘greener’ solvent-based variants exist for virtually all the major category types listed below. The most notable are solvent-less silicone and UV curable types that are both available in “100% solids” format. However, the absence of a cheaper carrier material (users pay for the proportionately vastly more expensive but small solids content) can make their cost appear formidable.
It is expected, however, that most of the industry will eventually move completely away from all solvent based coatings to the newer water-based alternatives, particularly when the performance and application processes are so similar.
There is sometimes confusion about water reducible coatings and an water based “emulsion”: Water reducible are hydrophilic resins and thus have an affinity for water – they may be thinned with water. This is a truly water soluble polymer and will revert to liquid state when subjected to high levels of water i.e. very bad hydrolytic stability.
Emulsions are hydrophobic polymers and thus have no affinity for water. They have excellent hydrolytic stability.
The primary advantage of water-based coatings is that they can protect electronics assemblies at temperature extremes far beyond the capabilities of conventional coatings (up to 200ºC and beyond). Leading formulations share similar behavior and performance characteristics with older solvent-based products, and cure rapidly.
In addition to being extremely environment friendly, they are also non-flammable, as they do not use solvents.
They are therefore safer than most conventional coatings, reducing Health and Safety issues, demands for fume extraction and handling considerations of “hazardous” chemicals or substances.
Furthermore, unlike traditional solvent-based coatings, water-based types can be delivered ‘ready-to-use’ and as such they do not require any on-site handling and mixing, or packaging disposal.
On the down side, switching to a water-based coating from a conventional solvent based formulation, isn’t straightforward and should in no way be considered as ‘drop-in’ replacements for traditional materials. There are a number of processing issues that have to be addressed and assessed.
It is also important to note that the performance of existing water-based products can vary enormously from supplier to supplier, and some early water-based coating products have been known to fail minimum performance requirements.