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How to Select a Conformal Coating Material
January 16, 2013 |Estimated reading time: 10 minutes
Choosing a conformal coating material is a complicated and involved process usually balancing a number of trade-offs and compromises. There is no such thing as a universally applicable conformal coating. The aim of this article is to give you an idea of the questions to ask yourself and your suppliers to shortlist suitable materials for the end application.
Factors to Consider
When selecting a conformal coating you must consider several factors which, if you are lucky, will all add up to at least one material choice. If not, compromises are required.
Factors to be considered are:
- Temperature operating range;
- International, customer, or national standards;
- Environmental considerations;
- Chemical resistance;
- Requirement to rework the conformal coating;
- Application process considerations;
- Curing the conformal coating;
- National, state, and regional legislation and insurance;
- Supplier performance; and
- Price.
Each of these factors is examined briefly to allow insight into the complexity of selecting a conformal coating material.
Temperature Operating Range
Conformal coated assemblies will often be exposed to a wide range of operating temperatures during their service life. It is important to understand the maximum and minimum temperatures to which the unit will be exposed. This needs to a holistic and realistic appraisal.
For example, if your boards are operating in an unheated part of an aircraft, they could easily see -65ºC when airborne. When on the runway, they could easily see +100ºC. You must consider the heating effects of the circuit--certain local hot-spots on the assembly could reach local maximums of 125ºC. Then your operating range becomes -65 to +125ºC.
If your operating range goes below -65ºC or above 150ºC, your choice is simple because silicone conformal coatings are the only materials that will survive extended use at this range. If your operating range is within this window, keep reading.
International, Customer, or National Standards
The next question to ask is whether your customer or end application requires the conformal coating to be qualified or approved to international, national, or customer-specific standards.
Examples of these would be UL746E (which includes UL94) North American Safety Standard, MIL-I-46058C, or BMW GS95011-5. The qualified product lists (QPL) will significantly reduce the number of coatings available to you. Specifications such as the IPC-CC-830 standard does not have a qualified product list. However, any product that appears on the MIL-I-46058C QPL, is automatically said to meet the requirements of IPC-CC-830. There are a lot of materials that state that they "meet the requirements of IPC-CC-830." Any material making this claim that does not appear on the MIL QPL must have a publicly available test report to backup these assertions. You will need to see this report to keep a material in your shortlist.
Environmental Considerations
A conformal-coated PCB can be exposed to many different types of deleterious environments. For any unit that will be placed in a partially-sealed protective housing, the conformal coating would be considered secondary protection only and therefore likely to be subjected only to condensation and humidity as the temperature within the housing changes. Acrylic materials are widely used in such applications, although silicone materials generally offer the best protection in condensing environments. The following describes the most commonly encountered environmental conditions which may be met:
High Humidity
When high humidity is the issue, silicone materials are an order of magnitude more porous to water vapour than other coating chemistries. Acrylics and urethane chemistries all have good resistance to high humidity environments. Two-component (2K) materials and materials that chemically cross-link to a significant degree would all be expected to provide a better barrier to humidity.
Salt-Spray Environments
This is a very common requirement in many customer specifications, but not within any of the common general specifications. The key to success with salt spray environments are even and uniform, void-free coverage of the assembly. Chemically, salt-water is benign to most coatings. However, the tests and the environments will test workmanship and find defects in the applied coating. Silicone materials and urethane chemistries are often used in these applications, although acrylics can pass most test regimes.
Corrosive Gases
These environments are perhaps the toughest of all, since the corrosive gases will expose any defects or voids in the applied coating. In addition, there is a real differentiation between coating chemistries in protective capability. Acrylics and single-component alkyd, or urethane chemistries are all extremely susceptible to corrosive gases. 2K urethane and epoxy chemistry mostly show good resistance, but some single-component UV curable materials and single component silicone chemistries provide very good resistance too. UV-curable materials are significantly cross-linked so this is not surprising. Silicone materials seem to work by reacting to form chemical bonds with the metal surfaces which largely inert the metal substrate from corrosion.
Immersion Environments
When the assembly is likely to be immersed in water, there is only one real choice, and that is to use parylene, a vapour deposited conformal coating that provides exceptional coverage and insulation. Liquid applied silicone coatings can provide decent protection if applied at a generous thickness. However, the key is quality workmanship and process quality control since immersion will find any weak areas of coverage.
For immersion-in or splash by solvents, the results are very dependent upon the particular coating chemistry and solvent of interest. In general, 2K materials would be the best choice, although some UV curable materials provide outstanding chemical resistance.
Requirement to Rework the Conformal Coating
It is possible to rework most types of conformal coating if necessary, although some conformal coatings can be very difficult to rework. If your assembly has a long service life, will be placed in a housing and be subjected to multiple upgrades and repairs over its life (common in military and aerospace applications) then acrylics are without a doubt the easiest type of coating chemistry to remove and rework, due to their very limited chemical resistance. 1K urethane and alkyd materials can be removed by chemical methods. 2K chemistries, silicones, parylene’s and UV-curable materials are best removed locally by media blasting with an ESD safe media blaster. Provided the coating is less hard than the solder mask, then a good degree of selectivity can be achieved with this method.
Application Process Considerations
With enough engineering expertise and budget, any material can be made to work in any type of application method, with the exception of parylene, which can only be applied by vapour deposition in a dedicated application system. However, there are certain combinations that lead to extreme process difficulties and/or cost implications and are worth considering:
Solvent-based Materials
All single component solvent-based materials, with the exception of some moisture cure urethane chemistries can be successfully applied by any of the common techniques. 2K solvent based materials work best with spray or brush applications and can be applied through robotic applications with the right system specification. Unless you are willing to throw away a dip tank of material when the pot-life is exceeded then dipping is far from ideal.
UV-curable Materials
UV cure conformal coatings are ideally designed for high volume, high throughput environments and are best applied by selective coating equipment with an immediate in-line, conveyorised cure operation. However, due to their outstanding protective properties, they find application in low volume applications too. In this case, manual spray coating is acceptable, as long as the coating thickness and cure process are carefully controlled to ensure the thickness is within specification and to avoid over-cure. Also, when curing UV materials and masking is involved, consideration must be given to this.
Water-based Materials
Water-based materials work best in a manual application process, be it dip, spray, or brush. They are difficult to apply with repeatability and reproducibility with selective spray equipment and can be impossible to cure in an in-line process due to the excessive thickness of material applied in these methods.
Silicone-based Materials
RTV silicones are not really suitable for dip coating applications without inert gas systems in place, but heat-cure silicone materials are ideal. With this exception, silicone chemistry can be successfully applied by any methodology.
Selective Robotic Coating
Nearly all robotic manufacturers can apply conformal coatings successfully. The key is to understand the valve technologies and how that relates to the material, the quality of the spray process and the speed of processing. For instance, when considering switching from a solvent based coating using a non-atomised valve to an environmentally friendly coating such as a UV cure or silicone cure product then it should be realised that a change of valve technology is probably needed and there will be speed limitations, and therefore throughput penalties.
Dipping
Generally, all coatings can be dipped. However, a wise rule of thumb is to avoid dip coating methods for any material containing a moisture-cure mechanism unless you wish to add in expensive process controls such as inert gas systems. If you are constrained by existing capital equipment then your choices may be more limited, but if new equipment can be purchased, then there are far more choices.
Curing the Conformal Coating
When talking about cure times, several definitions exist:
1. Drying time to handle - The time at room temperature at which the coating is dry to the touch and can be handled or moved onto another operation.
2. Time to full cure - The time until any physical drying process has been completed and any cross-linking reaction has taken place. This can usually be accelerated by elevated temperatures or other methods of catalysis.
3. Time to optimum properties - Intuition would tell you would think this would be the same as time to full cure. However, even when the coating is fully cured, other properties such as adhesion can develop over time
From a production flow point of view, it is desirable to minimize the time-to-handle and time-to-full cure as much as possible, taking into account the factors previously discussed. From a lean production point of view, minimising the time to optimum properties enables you to ship product you know is at its optimum, or enables these properties to develop during transport time to the end customer, without having to store a significant number of days of inventory.
National, State, and Regional Legislation and Insurance
Most countries, states, and even regions have their own legislation relating to the emission of volatile organic compounds (VOCs), with differing definitions and interpretations. When installing a new process you need to be mindful of these, and if possible, use a compliant material. For existing processes, there are options including carbon filtration, solvent collection, or oxidising (afterburner) units that can be installed at significant expense, although this can be more effective than re-qualifying new materials with multiple end customers. Sometimes, process and/or performance considerations will require this type of solution, even for new qualifications.
Shipping flammable, hazardous materials is expensive and requires a specialised freight firm in most countries. Using a safer material can simplify logistics and reduce cost. Using flammable, hazardous materials is inherently more dangerous with a greater risk of serious injury, long-term exposure effects or catastrophic incidents, and this is reflected in insurance premiums, which is another opportunity to save cost. Sometimes, incidents within plants, such as fires will necessitate a change in material, regardless of process, lack of business case or any other driver, other than compliance.
Supplier Performance
Choosing a conformal coating material is a long and exhaustive process, with many considerations and often conflicting choices and trade-offs to be made. Most suppliers would walk you through a similar framework to this to decide on the most suitable conformal coating for consideration on your shortlist.
This consultation provides a valuable insight into your potential material supplier. If they are unwilling or unable to help you through the selection process, then they are unlikely to be much help to you with any ongoing issues that need to be resolved. If you get great support and back-up prior to qualification, then this is an indicator that your supplier knows your business well enough to become a value-added supplier during production.
The sampling process will help you understand the geographic reach of your potential supplier, local availability, etc. If your sample has to ship halfway across the world, then your supply chain is likely to be unsuitable or difficult in production.
Finally, a key factor is quality control of the manufacturing process. Consider the costs for withdrawing a product that has been coated in a conformal coating that has been produced incorrectly. Traceability and quality control of the process is critical to avoid a disaster at a later point.
Price
Many materials are selected on price. Although this is a valid point, consideration of all the other factors is arguably as important if not more. Ultimately, the cost of the coating is fractions of the true of manufacture and the wrong material specified at the design stage can cost many times the saving originally considered.
Conclusions
Selecting a conformal coating is a crucial stage of the design process. However, it should be considered in the context of all the factors outlined above. If not, then it should not be a surprise to find problems later down the process stream that could have been avoided from the start. If in doubt, check with the supplier and independent experts.