|Ask the Experts|
December 21, 2018
Pre-wash boards prior to conformal coating
Do we need to pre-wash boards prior to applying a conformal coating? My customer just required us to coat all boards, does it matter if we use an acrylic or urethane conformal coating?
|Expert Panel Responses|
Almost all material suppliers recommend pre-washing AND drying the boards before conformal coating. In some cases, plasma cleaning is called for. Many people do not pre-clean prior to coating because of the added cost and time, but they risk reducing the reliability of the board by not washing it. The risk increases as the expected life of the product increases or the environment factors (temperature, corrosives, humidity) increase. The IPC guidelines on conformal coating (IPC-HDBK-830) addresses cleanliness issue in section 7.3.3. Yes, it matters which coating you use. Acrylic offers excellent moisture production, but poor resistance to solvents. It is also re-workable. Urethane will provide better chemical resistance, but have other disadvantages. The way to approach the material selection is to start with your customer's requirements. Key questions that drive those requirements are: What is the operating temperature range? What moisture levels and / or chemicals will the board be exposed to? Do you need reworkabilty? What is the intended life of the product? Once you have answered these questions, you can take these answers and talk to you about the relative merits of the different coating products that will meet your needs. In some cases, you may find that your customer needs to meet a specific military, automotive, or other industry standard, which wraps these requirements together.
Director of Application Engineering
Ah, the typical conflict of no-clean versus conformal coating. As a general rule of thumb, I am against putting down any coating before washing. In other words, to paraphrase another expert, would you paint your house before washing it? Having said that, if there is some immutable conflict, typically cost, that prevents you from cleaning before conformal coating, you need to perform some qualification procedures to reduce the risk that the application of conformal coating will make things worse, not better. The center of these qualification procedures is ion chromatography and temperature/humidity/bias testing. Acrylic vs. urethane is somewhat dependent upon what you want. Acrylic tends to be cheaper, easier to rework. But, it tends to be less resistant to moisture and can crack some brittle components at low temperatures. Urethane tends to be more expensive, can still be reworked, but with slightly more difficulty, but tends to be a more effective moisture barrier and is more compliant.
CEO & Managing Partner
It sounds like you are using no-clean soldering materials (pastes, liquid fluxes and cored wires) for assembly and asking if it is required for you to remove these residues before applying a conformal coating. In many cases, conformal coatings can be applied on top of no-clean flux residues and cured without incident. Before making the decision regarding the cleaning step, it is important to determine if the flux materials are going to cause any adhesion problems with the conformal coatings. Material suppliers (for soldering products and/or the coatings) may be able to perform a laboratory test to determine the compatibility of a given flux residue with a given coating product. (ASTM Method 3359-92a can be referenced as the appropriate method for this test.) It is advised to let the boards to cool completely before applying the coatings; this will allow the flux residues to completely dry up and solidify. This helps with adhesion properties as the liquid coating will be applied to a solidified flux residue rather than a soft, fluid-like residue. Many modern solder pastes have been designed to have soft residues to enhance in-circuit testing properties. These soft residues behave more like a liquid than a solid and can create incompatibility between residues and various coatings. In most cases, no-clean flux residues are quite compatible with acrylic and urethane coatings. More incompatibility problems are likely with silicone-based coatings since there are ingredients found in some fluxes that inhibit the curing of such coatings. In any event, it is wise to perform the appropriate testing to verify compatibility between a flux residue and a conformal coating.
General Manager - Electronic Assembly Americas
Conformal coating requires a clean, dry board. Conformal coating adhesion is negatively effected by board contamination. In applications where conformal coating is the end process, at least one and frequently two cleaning processes are performed. The first cleaning process is to remove flux. Frequently, a second cleaning process is performed to remove residual contamination (hand oils, dust, etc) from any other post cleaning processes. One may eliminate the second cleaning process if the potential of post clean contamination is eliminated. As is true with any de-fluxing application, and particularly important in conformal coating applications, the use of DI water in the cleaning application is vital to good adhesion of the conformal coating.
Regarding pre-washing boards prior to conformal coating, this process is strongly recommended. First, many companies are using no-clean fluxes these days, and those residues will affect the ability of the coating to grip to the board. Secondly, there are many other types of contamination and residue which besides fluxes which need to be removed before coating. The most common examples are fingerprint oils and salts, plus fume residues from reworking, Kapton tape or other adhesive residues, solder balls, and even some inks or chip bonder which may have splattered unexpectedly. So cleaning before coating is absolutely essential. Regarding the best type of coating to use, my strong recommendation would be to avoid the urethanes. They are excellent coatings and offer excellent protection, but they are thick (and relatively heavy, if weight is a factor for your customer) and -- most importantly -- almost impossible to remove prior to rework. A good "second opinion" on this coating is available at: http://www.thermospray.com/ccfaq.html On that web site, you will also notice they strongly urge against epoxy conformal coatings, which are rarely seen in non-military applications. Also, there is a particularly nasty conformal coating called "parylene" which is a vapor deposited coating that is ultra-light, ultra-strong and impossible to rework. This product requires special equipment and handling, and the coating process is often out-sourced. See: http://www.nbtc.cornell.edu/facilities/downloads/Parylene%20Information%20Sheets.pdf As for selecting a coating, a major factor is also how easily it can be deposited and reworked on the PCB, as well as general housekeeping such as cleaning the coating machinery. Many companies make solvents that will quickly, safely and inexpensively remove acrylic coatings, so those are a good choice. But my personal favorites are the "solventless silicone" conformal coatings from Dow-Corning. They are light, very strong, very durable, and can be removed and reworked in seconds with the right cleaner. It's a 21st century answer. I hope this helps.
Conformal coatings have to be tested against the particular flux that is on the board. Typically a low residue wave flux can be coated with either type acrylic or urethane. Solder paste residues can be trickier to coat. It depends on coating methods, type of curing and flux type. Typically to conformal coat solder paste residues you need to use a hard residue paste in order to insure proper curing of the coatings and help eliminate pin holes. How ever all should be tested for compatibility.
Deck Street Consultants
Good question - and like any good engineer I will answer it simply... It depends! The coating results you obtain will depend on the nature of residues on your board and will be largely independent of coating type. The choice of acrylic or urethane will depend on the end application of your board (what it will be exposed to) and whether it needs to be repaired. If the board is in a box, and only exposed to condensation then acrylics are the best choice, having the best inherent moisture resistance. If the board is directly mounted and may be exposed to more aggressive solvents (brake fluids, fuels etc.) then a urethane would be the better choice. Acrylics are extremely easy to rework, even in the field. Urethane coated boards need to be repaired in a workshop to achieve optimum results. The debate on cleaning is long, involved and probably beyond the scope of your question. My short answer is to use the old adage, "if in doubt, it's better to be safe than sorry". However, I have attempted to construct a reasonably detailed response below that outlines the issues involved and should give you enough information to make an informed decision... Why clean? Every bare board, component and circuit assembly undergoes numerous handling and production processes that each leave a variety of chemical residues. These can be ionic or non ionic and may become reactive when the board is live in a humid or more aggressive environment. This can cause dendritic growth, electromigration, corrosion or purely coating de-laminaton (followed by dendrites, corrosion or electro-migration) or plain breakdown of the coating. The types of residue on circuit assemblies which can cause coating issues and reliability issues in unit the field include flux, (correctly or incorrectly processed,) metallic salts, mould release agents on components, oily and salty fingerprints, hand contamination such as hand creams, food residues, fungus, mould or bacteria, halides and certain hygroscopic glycols. Any of these residues can be the cause of many aesthetic and functional conformal coating defects such as: Wet state : Poor coverage, de-wetting, pinholes Dry state : Craters, fish eyes, blisters, loss of adhesion, cure inhibition. Conformally coated un-cleaned assemblies can have the potential to cover up, but not make benign, chemical species which subsequently cause circuit failure or reduced efficiency. If circuits are cleaned prior to conformal coating all uncertainties regarding reliability caused by residues is eliminated, this is also true of uncoated circuits. However, the coating offers the additional benefit of keeping the surface clean and free from potential harmful contamination. Residues can also reduce the efficiency and high performance of the meticulously engineered solder pastes or fluxes, which in turn can lead to poor soldering or joint reliability. The majority of manufacturers of complex assemblies for advanced and high reliability applications (Military, Avionics, Aerospace market sectors) are well aware of the hazards of circuit contamination and cleaning is thus an integral part of their production process. Whether they conformally coat or put the assembly in a hermetically sealed housing, maximum long term reliability particularly in aggressive environments is achieved. By cleaning, these manufacturers ensure optimized production yields, throughput and quality, and also guarantee reliability and longevity. However, many of our customers successfully coat over no-clean processes – the most important things to remember are: Your incoming boards must be of very high cleanliness and each batch should at least be tested for ionic cleanliness. Your process must be as clean as possible. This means your operators must wear gloves whenever they handle boards. Your process must be under absolute control. No-clean only works if you apply just enough flux to achieve good soldering... Anything more than that will leave residues that can lead to corrosion and other coating related defects. I often use 2 analogies when discussing this issue with customers. Imagine you are painting your walls at home with DIY emulsion paints. If you try to paint directly over the top of hand-prints and other grease and grime, you aren't surprised when the coating fails to wet, or adhere. This is kind of what you are doing when coating over a no-clean process. In addition, car manufacturers go to great lengths with surface preparation prior to painting, using acid-etching baths prior to painting, to ensure the metal is free from oxide and scrupulously clean so that the paint gives maximum adhesion and corrosion protection. If they drove it "naked" through a muddy field prior to painting, again, one wouldn't be surprised to see coating defects or long-term issues. Finally, with the transition to lead-free, we notice that solder resists are being formulated with lower, and lower surface energies, to help prevent solder bridges. For the coating manufacturer, this is hard enough to adhere to, without the additional presence of unknown contaminants. A good cleaning process will remove any doubt from the process, the problematic silicone mould-release compounds used to manufacture ICs will be removed, bare board cleanliness becomes less of an issue, and process control is not so critical. Cleaning covers a multitude of sins. If you can guarantee the cleanliness of bare boards and of your process, and that the "chemical soup", produced from the residues of the 12 or more chemical processes your board goes through before coating is benign, then you are in a good position to coat over the no-clean process. If you have any doubt, then cleaning is the best option. Please feel free to contact me offline (firstname.lastname@example.org) if you would like to discuss your particular process and requirements at greater length. Best Regards,
Global Business Director conformal coatings division
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