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Conformal Coating Material Challenges in Electronics Processing
May 22, 2013 |Estimated reading time: 7 minutes
Conformal coating materials are ultimately applied to improve long-term reliability of a product and are a value-added process, but do add additional costs. This is traded against improvements in product reliability. However, it is often found that in the era of no-clean, lead-free globalization of the market and process engineers having little time to fully understand the process completely, conformal coating materials and the processes are not optimised, leading to more problems than initially expected. These problems can be divided into different areas, but the main issue is the conformal coating material itself.
Conformal coating materials themselves generally do not cause problems to a PCB if applied correctly and there are no compatibility issues. In fact, most conformal coatings do a great job of protecting a PCB if rules are followed. The difficulties come when the coating is expected to work beyond its boundaries or we fail to understand application techniques and their limitations.
This brief paper highlights conformal coating material issues and their effects on long-term reliability. Each of these topics will be covered in forthcoming technical articles in much greater detail. The idea of this article is to highlight the main challenges encountered during the process of conformal coating and why they matter. The assumption here is that the conformal coating is being applied to increase the assembly’s inherent resistance to corrosion or humid/condensing conditions.
Areas to be considered include:
- Conformal coating cracking, deamination, or loss of adhesion;
- Cleanliness of the PCB, the processes before coating, and components;
- Compatibility of conformal coating with no-clean processes;
- Customer specifications;
- National and international standards;
- Workmanship;
- Material selection;
- Support/the “mystery” failure phenomenon; and
- Coating application equipment.
Cracking, Delamination, or Loss of Adhesion
For the onset of corrosion on a metal surface, an electrolyte (water) and an ionic substance are required. Conformal coatings prevent corrosion by acting as a barrier to both ionic species and humidity to a certain extent. When the coating film is non-continuous or has been delaminated there is the possibility for a chemical soup to mix in this cavity and begin the corrosion process.
Compatibility with a No-clean processes
In addition to the bare board cleanliness issues described above, the no-clean process itself adds a significant number of potential sources of corrosion, as well as residues that interfere with the ability of the conformal coating to adhere to the various substrates in question. With solvent-based conformal coatings, the solvent often acted as a "quasi" cleaning process, with the solvent being able to cut through the contaminant, to enable the coating to make a chemical bond to the substrate.
However, the purpose of the rosin/resin binder is to encapsulate the potentially corrosive flux activators, so this approach needs to be well understood, researched, and documented. With the advent of lead-free, the coatings have become less adept at cutting through the binders that have been baked on by the higher reflow temperatures, and as the shift continues towards lower VOC processes, with less effective "quasi" cleaning, this issue will become more and more important, thus you must fully understand the compatibility of all of process chemistries.
PCB Cleanliness, Processes Before Coating, and Components
As described above, ionic contamination is required for the onset of corrosion. If these species are sealed between the coating and the substrate, the delamination process and corrosion mechanism will usually be accelerated. An often overlooked source of contamination is the bare PCB, which has been through at least 16 separate chemical treatments. If any of these processes have been insufficiently rinsed, then there exists an opportunity for failure. Since most conformal coatings are applied over a no-clean process, if you are starting dirty, you won’t make your assembly any cleaner through your process, thus it will be difficult for you to make reliable assemblies.
Customer Specifications
End customers demand longer and longer service lives in ever more extreme operating environments. Many of the leading conformal coating materials are more than 50 years old. Whilst it is tempting to use the old favourite on the next project, it is important to prove whether that is the best approach through testing, rather than relying on the adage, "We’ve used this material for the last 50 years and never had a problem." Guess what? The new design you’re working on, or the new customer specification, may prove to be "a bridge too far." Finding out about a problem in the field is obviously a disaster that should have been eliminated during design or, preferably, prior to that in material capability testing before product inception.
National and International Standards
These standards give you, the user, an idea of how the material has been tested and therefore deemed appropriate for use. The US MIL-I-46058C, for example, is perhaps the oldest and most widely used specification. It requires that any material listed is re-qualified on a biannual schedule. Users should always ensure they are looking at the most recent qualified products list (QPL) and that the coating they are using is listed. Many products have disappeared from the QPL over time, and thus you see the dichotomy on many drawings use product X, MIL spec acrylic. Well, product X is no longer a MIL-spec acrylic, so do you use product X or a Mil-spec acrylic? This is probably the customer’s decision.
The reverse is also true. On many occasions I've seen users continue to work with a product after the Mil-spec qualification has gone because they were not notified. Since it was the main product they used, they consolidated on it for every other Mil-spec job they were asked to do. There is no obligation for the manufacturer of the conformal coating to notify the user in the event they choose to let the qualification elapse. Have you checked the latest QPL recently?
Workmanship
Bubbles trapped in coatings, areas where the coating has de-wetted, or insufficient coating thickness and coating coverage over sharp edges are all issues. It should be clear why these defects provide the potential for moisture and ionic species to penetrate the coating and initiate corrosion. These defects can, and should be eliminated through process optimisation as far as possible, to minimize the risk of failure.
Taken to the other extreme, some users believe, "If some is good, more is better." Despite being extremely wasteful, excessive coating thicknesses is not a great idea. This action will generally exacerbate any tendency to crack, delaminate, or lose adhesion, as well as generate bubbles during curing/drying, as the solvent tries to outgas through a semi-cured coating film, which generates yet another path for the initiation of corrosion.
Material Selection
These days, an overload of information available. Claims are made about product performance which can make it difficult to select a product mix for further evaluation and internal testing, reducing the chances you introduce the most cost-effective process to achieve the performance level you require.
Support/The “Mystery” Failure Phenomenon
An often overheard complaint from users of conformal coatings is that the manufacturers can be difficult to reach with regards to process support, problem diagnosis, and troubleshooting. This difficulty to reach level can take on new levels when the problem is multilayered, clearly complex, time-consuming, and likely to be comprised of fixing multiple process stages. The use of either internal experts or independent experts is on the rise for exactly this reason.
Coating Application Equipment
The challenges for the manufacturers of coating application equipment are numerous and varied. Increasingly, as assemblies become smaller and more densely populated, the distinction between areas to be coated, and areas not to be coated are becoming blurred, with very tight separations. This can usually be managed; however, the success is largely down to the conformal coating material’s characteristics such as viscosity, flow, and dry time. It will usually involve some kind of dispensing application process which is much slower than coating and can therefore lengthen cycle time.
In addition, to increasingly dense assemblies, and ever faster cycle time/productivity demands, the adoption of 100% solids materials continues at a quick pace. These materials bring many desirable properties and improved performance to the end-user, but the application process is usually more complex. Balancing cycle time requirements against the need for thin, uniform films, applied in a repeatable, selective fashion, can be a challenge. These materials are usually best applied by a low-volume, low-spray (LVSP) atomised spray valve in combination with a dispensing valve for fine detail work.
Conclusions
A PCB can fail for many different reasons. Conformal coating is there to increase the reliability of the product. However, without understanding the relationship of the coating material within the process, it is very easy to find trouble lurking not far around the corner. When considering your conformal coating material, it really is wise to consider all issues that may have an impact to ensure implementation goes as smoothly as possible. Dr. Lee Hitchens has worked in the conformal coating and reliability sector of the electronics industry for over 15 years and has considerable experience in all areas of reliability and conformal coating processing. He now divides his time between the roles of technical director of SCH Technologies in the UK, Diamond SCH in Shenzhen China, and Nexus3c, a Conformal Coating Centre, providing unique global services of independent consultancy, troubleshooting, and training in conformal coating and long-term reliability issues.