Conformal coating

Conformal coating material is a thin polymeric film which ‘conforms’ to the contours of a printed circuit board to protect the board's components. Typically applied at 25-250 μm it is applied to electronic circuitry to act as protection against moisture, dust, chemicals, and temperature extremes that, if uncoated (non-protected), could result in damage or failure of the electronics to function. When electronics must withstand harsh environments and added protection is necessary, most circuit board assembly houses coat assemblies with a layer of transparent conformal coating rather than potting.

Reasons for use

Because of the vast range of electronics in today's world, a printed circuit board is expected to perform in a huge range of environments. Circuit boards then are exposed to a vast array of different conditions such as moisture, salt, chemicals and temperature changes to name but a few, conformal coatings are designed to protect the electronic components from these varying conditions and prevent the board from corroding.

Applications

Precision analog circuitry may suffer degraded accuracy if insulating surfaces become contaminated with ionic substances such as fingerprint residues, which can become weakly conductive in the presence of moisture. (The classic symptom of micro-contamination on an analog circuit board is sudden changes in performance at high humidity, for example when a technician breathes on it). Furthermore, a suitably chosen material coating has proved to actually reduce the effects of mechanical stress and vibrations on the circuit and its ability to cope in extreme temperatures.

For example, in a chip-on-board assembly process, a silicon die is mounted on the board with an adhesive or a soldering process, then electrically connected by wire bonding, typically with .001-inch-diameter gold or aluminum wire. The chip and the wire are very delicate, so they're encapsulated in a version of conformal coating called "glob top." This prevents accidental contact from damaging the wires or the chip. Another use of conformal coating is to increase the voltage rating of a dense circuit assembly; an insulating coating can withstand a much stronger electric field than air, particularly at high altitude.

With the exception of parylene, most organic coatings are readily penetrated by water molecules. A coating preserves the performance of precision electronics primarily by preventing ionizable contaminants such as salts from reaching circuit nodes, and combining there with water to form a microscopically thin electrolyte film. For this reason, coating is far more effective if all surface contamination is removed first, using a highly repeatable industrial process such as vapor degreasing or semi-aqueous washing in a special machine. Extreme cleanliness also greatly improves adhesion. Pinholes would defeat the purpose of the coating, because a continuous contaminant film would be able to make contact with the circuit nodes and form undesired conductive paths between them.

Coating methods

The coating material can be applied by various methods, from brushing, spraying and dipping, or, due to the increasing complexities of the electronic boards being designed and with the 'process window' becoming smaller and smaller, by selectively coating via robot. Different methods of curing / drying are available depending on the conformal coating material. Nearly all modern conformal coatings contain a fluorescent dye to aid in coating coverage inspection.

Brush coating

This works by flow coating the material onto the board and is suitable for low volume application, finishing and repair. The finish tends to be inferior cosmetically and can be subject to many defects such as bubbles. The coating also tends to be thicker and unless skilled operators applied the coating, highly subjective in quality.

Spray application coating

Conformal Coating Spray booth

This coating can be completed with a spray aerosol or dedicated spray booth with spray gun and is suitable for low and medium volume processing. The quality of the surface finish can be superior to all other methods when a trained skilled operator completes the process, as long as the circuit board is clean and the coating has no adhesion issues. The coating application may be limited due to 3D effects but masking requirements are more "shield" than "barrier" since the penetration is less effective. However, the lack of penetration can be an issue where coating is desired to penetrate under devices.

Spray applications can be one of the most cost effective ways of applying conformal coating as it can be done on the bench top if necessary for small rework and repair jobs, or scaled up to be done in spray booths for medium scale production.

One of the key attributes of atomised spraying is giving excellent tip coverage to components. When conformal coatings are applied to a PCB they have a tendency to slump. The first layer of a coating can give a thin edge on the corner of components. This can be countered with a second coat through double dipping or over brushing but this is a repeat process and may not be acceptable. To counter this problem the technique of atomised spraying can be used.

Conformal coating dipping

Conformal Coating Dip System

This coating is a highly repeatable process and if the printed circuit board (PCB) is designed correctly, it can be the highest volume technique. Coating penetrates everywhere, including under devices, and therefore any masking must be perfect to prevent leakage. Therefore, many PCBs are completely unsuitable for dipping due to design.

The issue of "thin tip coverage" where the material slumps around sharp edges can be a problem especially in a highly condensing atmosphere. This tip coverage effect can be eliminated by either double dipping the PCB or using several thin layers of atomised spraying to achieve good coverage without exceeding coating thickness recommendations. A combination of the two techniques may also be used.

Selective Coating By Machine

This method is the best choice for high volume applications as it is a fast and accurate way of applying the desired thickness coating, to precisely the areas of the board where it is required.

It works by using a needle and atomised spray applicator, non-atomised spray or ultrasonic valve technologies that can move above the circuit board and dispense / spray the coating material in selective areas. Flow rates and material viscosity are programmed into the computer system controlling the applicator so that the desired coating thickness is maintained. This method is highly effective at large volumes as long as the PCBs are designed for the method. However, there are limitations in the select coat process like all the other processes, such as potential capillary effects around low profile connectors which "suck" up the coating accidentally, it also requires a skilled operator trained to use the machinery.

The process quality of dip or dam-and-fill coating and non-atomised spray technology can be improved when necessary by applying and then releasing a vacuum while the assembly is submerged in the liquid resin. This forces the liquid resin into all crevices, eliminating uncoated surfaces in interior cavities.

The differences in application methods can be seen in a comparison presentation. Choice of method is dependent on the complexity of the substrate to be conformally coated, the required coating performance, and the throughput requirements.

Curing and Drying

Solvent & Water based conformal Coatings

For standard solvent based acrylics, air drying (film forming) is the normal process except where speed is essential. Then accelerated heat curing can be used, using batch or inline ovens / conveyors and using typical cure profiles which are designed for maximally efficient curing without damage to the coating.

Water based conformal coatings can be treated in the same manner but with more care with the application of the heat due to the slower drying times.

UV curing UV Conformal coatings

UV Inline Conveyor for curing conformal coatings

UV curing of conformal coatings is becoming increasingly important for high volume users in fields such as automotive and consumer electronics.

This increase in the popularity of UV curable conformal coatings is due to its rapid cure speed, level of processing ease, environmental friendliness and thermal cycling resistance, which have never before been achieved with UV conformal coating materials.

There are different types of UV lights (lamps) used in curing conformal coatings and they are Arc and Microwave lamps.

Thickness & Measurement

Coating material when dry (after curing) should typically have a thickness of 30–130 µm (0.0012–0.0051 in) when using acrylic resin, epoxy resin, or urethane resin. For silicone resin, the coating thickness recommended by the IPC standards is 50–210 µm (0.0020–0.0083 in).

There are several methods for measurement of conformal coating thickness and they fall into two categories. These categories are wet film & dry film conformal coating measurements.

Wet Film Conformal Coating Measurement

Wet film gauge for Conformal Coating Thickness Measurement

The wet film conformal coating thickness method ensures quality control while the coating is still wet.

Applying too much coating can be expensive. Also, wet film measurements are useful for conformal coatings where the dry film thickness can only be measured destructively or over application of conformal coating could be problematic.

The wet film gauges are applied to the wet conformal coating and the teeth indicate the thickness of the conformal coating. The dry film thickness can then be calculated from the measurement.

Dry Film Conformal Coating Thickness Measurement

Dry film Conformal Coating Thickness Measurement

An alternative method to wet film measurement is using a non contact technique using eddy currents. The system works by placing the test head on the surface of the conformal coating, the measurement is almost instantaneous and provides an immediate repeatable result for thickness measurement of conformal coating.

Test coupons are the ideal method for measuring the coating thickness, whether is it spraying or dipping, and can be kept as a physical record of the performance. Apply the coating to the test coupons at the same time as the circuit boards provides a permanent measurement and an accurate guide to the coating thickness.

Thicker coatings or better applied coatings may be required when liquid water is present due to potential microscopic pinhole formation in the coating or when the coating material is too thin on the sharp edges of components due to poor application techniques. This is considered a defect and can be eliminated with appropriate steps and training. These techniques effectively "pot" or "conform" to the components by completely covering them.

Conformal Coating Inspection

Conformal Coating Inspection Booth

Conformal Coating AOI

Traditionally conformal coating inspection has been carried out manually. A typical set up is an operator sitting in an inspection booth and examining each PCB individually under a high intensity long wave Ultraviolet lamp for workmanship, failures to meet the standards specified and defects.

Recent developments in conformal coating automated optical inspection (AOI) have begun to address these manual processed and issues. Automated Inspection Systems now exist which can be camera or scanner based so the technology can be matched to the project.

Conformal Coating Selection

The selection of conformal coating material is a crucial factor that needs to be considered carefully and in relation to the application method. The wrong selection can not only affect the long term reliability of the circuit board but can cause massive difficulties with both processing and costs.

The most common standards for conformal coating are IPC A-610 and IPC-CC-830. These standards list indications of good and bad coverage and describe various failure mechanisms such as dewetting and orange peel.

Conformal coating inspection is a critical factor in determining successful coating application and long term reliability of PCBs. Using the IPC standards allows the coating operator to monitor the coating application performance. This can be done manually by the operator in an inspection booth by examining the PCB under white and UVA light or it can be done automatically by a conformal coating inspection system.

Another type of coating called parylene is applied with a vacuum deposition process versus a spray or needle application. The parylene is applied at the molecular level by a vacuum deposition process at ambient temperature. Film coatings from 0.100 to 76 μm can be easily applied in a single operation. The advantage of parylene coatings is that they cover hidden surfaces and other areas where spray and needle application are not possible. Coating thickness is very uniform, even on irregular surfaces. The three main disadvantages are that (i) any desired contact points such as battery contacts or connectors must be carefully covered with an air-tight mask to prevent the parylene from coating the contacts, (ii) it is a batch process and does not lend itself to high volume processing, and (iii) the cost per PCB can be highly prohibitive due to the capital investment costs and the cost per batch.

Coating Chemistries

There are many chemistries of conformal coatings out on the market today. While the "Material Considerations" section below is very important to finding the correct conformal coating, it is also important to find a coating chemistry meeting the application needs. Below are five common strengths for each conformal coating chemistry.

Acrylic

• Ease of rework
• Simple drying process
• Good moisture resistance
• High Fluorescence level
• Ease of viscosity adjustment

Epoxy

• Useful to about 150C [302F]
• Harder durometer, abrasion resistance
• CTE closer to epoxy PCB substrate
• Higher Tg (Glass transition)
• Good dielectric properties

Polyurethane

• Good dielectric properties
• Good moisture resistance
• Solvent resistance
• Less reversion potential
• Abrasion resistance

Silicones

• Stable over wide temperature range (in general, -40C to 200C)[-40F to 392F]
• Flexible, provides dampening and impact protection
• Good moisture resistance
• High dielectric strength
• Low surface energy for better wetting

Fluorinated or non Fluorinated - Poly-Para-Xylylene (Parylene)

• Excellent uniformity regardless of part geometry
• Chemical inertness
• Minimal added mass and low outgassing
• Low environmental impact process
• Low dielectric constant

Amorphous Fluoropolymer

• Low dielectric constant
• High glass transition temperature
• Low surface energy
• Low water absorption
• Solvent resistance

The basics of conformal coating processing can be understood from a presentation available giving a summary of the areas covered above.

Material considerations

Selection of the correct choice of coating material (lacquer) is one of the process engineer's most critical decisions. Criteria for selection must be based on answering many questions, which will include:

• What is being protected against? (e.g., moisture, chemicals)
• What temperature range will the electrical device encounter?
• What are the physical, electrical, and chemical requirements for the coating material itself?
• Electrical, chemical, and mechanical compatibility with the parts and substances to be coated (for instance, does it need to match the coefficient of expansion of chip components?)

Answers will determine the suitability of a particular material, be it acrylic, polyurethane, silicone, epoxy, etc. Process, production and commercial issues will then enter the equation:

• How easily can the material be reworked once applied?
• How fast does the material dry (cure)?
• How fast can the material be applied and dried (throughput time)?
• What type of process and equipment is necessary to achieve the required coating quality (uniformity and repeatability)?
• Price of the material per litre.
• Quality of the material supplier (two acrylic material manufacturers will not make equal quality of material).

External links

• http://www.conformalcoating.co.uk/documents/BulletinNovember.pdf Conformal Coating application techniques
• http://www.ausbond.com/Product.asp?bigclassname=Conformal+Coatings
• http://www.electroniccoating.com/our-process-materials/materials/ List of different conformal coating chemistries
• Parylene Conformal Coating 101: Facts Sheet