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Conformal coating material is applied to electronic circuitry to act as protection against moisture, dust, chemicals, and temperature extremes that if uncoated (non-protected) could result in a complete failure of the electronic system.
Most circuit board assembly houses coat assemblies with a layer of transparent conformal coating rather than potting.[1]
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.
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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.
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.[2] The coating also tends to be thicker and unless skilled operators used the coating, highly subjective in quality.
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.[3] 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.
This coating is a highly repeatable process and if the PCB is designed correctly can be the highest volume technique.[4] 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 double dipping the PCB, using several thin layers of atomised spraying to achieve good coverage without exceeding coating thickness recommendations or a combination of the two techniques.
This involves needle & 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.[5] 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[6] like all the other processes, such as potentially capillary effects around low profile connectors which "suck" up the coating accidentally.
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.[7] Choice of method is dependent on the complexity of the substrate to be conformally coated, the required coating performance, and the throughput requirements.
Different Methods of curing / drying are available depending on the conformal coating material.
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 max efficient curing without damage to the coating.[8]
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 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.[9]
Coating material when dry (after curing) should typically have a thickness of 30–130 μm (0.0012–0.0051 in) for situations where direct condensation of moisture occurs according to the IPC standards. This applies directly to acrylic and polyurethane coatings. For silicone 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.
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.
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[2] or when the coating material is too thin on the sharp edges of components due to poor application techniques. The former effect is a defect and can be eliminated by appropriate steps and the latter has been mentioned previously. These techniques effectively "pot" or "conform" to the components by completely covering them.[citation needed]
The selection of conformal coating material is a crucial factor that needs to be linked carefully, and simultaneously with the application method.[10] The wrong selection can not only effect the long term reliability of the circuit board but can cause massive difficulties in both processing and costs.
The most common[citation needed] standards for conformal coating are IPC A-610[11] and IPC-CC-830.[12] These standards give indications of good and bad coverage and indicate the various failures mechanisms that could occur such as dewetting[13] and orange peel.[14]
Conformal coating inspection is a critical factor in determining successful coating application and long term reliability to PCBs. Using the IPC standards this allows the coating operator to monitor the coating application performance. This can be done manually by the operator in an inspection booth who examines 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.
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 which meets your application needs. Below are five common strengths for each conformal coating chemistry.[15] Click here for a full list of pros and cons on the different conformal coating chemistries
The basics of conformal coating processing can be understood from a presentation available giving a summary of the areas covered above.[16]
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:
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:
