By Phil Kinner, Electrolube
Over the last few months, I’ve covered quite a bit of ground regarding the important considerations for conformal coating selection and performance, and the suitability of conformal coatings for LEDs and protecting circuitry from the harshest environments. I hope that my regular guideline pieces have provided plenty of food for thought as well as giving you a basic understanding of coatings, their benefits and limitations. In this piece, I’m going to look at the different angles that design engineers and purchasing professionals come from and explore how these can sometimes conflict when selecting conformal coatings.
Differing motivation and conflict is an area we are often confronted with at Electrolube. However, if experienced production personnel are reviewing the designer’s suggestions at an early enough stage, they could potentially prevent the perpetuation of some common problems. Thoughtful design will always pay huge dividends down the line and the designers among you will have friends for life among your production colleagues if you make their jobs just that little bit easier!
In an ideal world, the design engineer would work closely with either production engineers or with the industrial engineers who oversee the coating process in the factory. Identifying potential production problems at the design stage will always be preferable and far easier than trying to fix problems or concerns following finalization of the engineering drawings. Here are a number of factors to consider when designing out productions issues with coatings:
Simplify board layout
By the ‘simple’ act of placing connectors and components that must not be coated along one edge of the assembly, the conformal coating application process will be simplified. This might allow dip coating to be explored as a potential alternative methodology, speeding up application times and reducing costs. Also, avoid large arrays of discrete components, which can pose a huge coating challenge due to the high levels of capillary forces present. The net result is often areas of no coverage/protection on the board as well as areas of excessive thickness prone to stress-cracking, de-lamination and other coating defects. Similarly, tall components present challenges of their own by the creation of shadowed or hard to reach areas. Splashing is another associated problem. The trick is to avoid placing tall components next to ‘must-coat’ components in order to avoid this eventuality.
Be mindful of the processes that can impact on the coating
The designer should ideally be aware of what kinds of manufacturing practices may occur following the application and cure of the coating, as other materials such as thermal greases/putties and rework/repair chemicals, can all have an impact on the integrity and overall performance of a coating. Also, when selecting adhesives for assemblies, care should always be taken to ensure that they are compatible with the selected coating materials and processes. Adhesives that are not compatible can have a detrimental effect on the overall performance of the coating.
The importance of pre-coat cleaning
It is worth bearing in mind that choosing the most appropriate conformal coating is merely the first stage in the protective process. The condition of the assembly prior to coating, is an often over-looked part of the protective process. The designer should always consider the process steps leading up to conformal coating and a key element of this is cleaning. The overall cleanliness of the substrate or the potential presence of residues on the substrate (such as no-clean flux residues) can have a critical impact on coating performance. If the substrate is not adequately clean, the residues present may interfere with the curing mechanism, lead to poor adhesion of the coating to the substrate and trap conductive/ionic materials under the coating.
Without meticulous attention to preparation or pre-coat cleaning regimes, corrosive residues bridging the PCB’s conducting tracks can cause failures over time. Whilst the coating may delay failure for many years, at some point failure will inevitably happen. Contaminants present on the surface before conformal coating will be sealed in by the operation and may cause long term problems – such contaminants might include fingerprints, flux residues and moisture and other atmospheric pollutants. Boards should always be cleaned and dried before conformal coating to obtain optimum long term performance. Even when using so-called no-clean fluxes, cleaning boards before coating will usually improve long term performance and reliability.
Create a buffer to make coverage easier for production
Conformal coatings are usually liquid when applied and will flow with a combination of gravity and the capillary forces present. Conformal coating materials will simply go where they are placed – with some naturally occurring wetting/spreading to a greater or lesser-extent, which is a function of the material’s surface tension, thixotropic index and drying time. Most materials will tend to slump away from sharp edges of the component, leads and solder joints due to gravity, and this behaviour can be made worse by longer dry-times and also by baking, if the initial viscosity drop is greater than the increase due to solvent-evaporation.
Understanding and controlling this behaviour, and its effect on the conformal coating coverage, will be key to the performance of coated assemblies operating in harsh environments. So, whether the process specifies masking or a reliance upon the process of selective conformal coating, a production team will be greatly relieved if you leave a buffer (or don’t care area) of at least 3mm clear between the areas that must be coated and those that must not. This small buffer will make the production process easier and prevent future issues in production.
So, having started the conversation about the importance of making sound early-stage design decisions, it is important to understand “what effects what” on the surface of the board to ensure successful conformal coating is achieved. Implementing these lessons will prevent potential production disasters not only with conformal coatings, but also in other areas of production as well. In a nutshell, choose the right material for the protection required, apply it appropriately and cure it well. Check for interactions with other process chemistries, and thoroughly clean and dry the assembly prior to coating, and ultimately you will establish a solid and reliable electronic assembly process.