Conformal coatings are widely used in high reliability and safety critical applications, where the impact of electronic failure can have very serious consequences including loss of life. To ensure a conformal coating meets the desired requirements, coated assemblies need to be exposed to a range of environments via appropriate test conditions to establish the performance range and limitations. Basic tests include electrical performance and accelerated humidity testing, whereas advanced testing can imitate severe conditions such as salt mist, temperature extremes or rapid environmental changes. Conformal coatings are designed to protect PCBs and ensure that they work efficiently in their end-use environment. However, there are instances where boards may not perform adequately or may even fail entirely despite being coated, which in turn produces an expensive, time-consuming nightmare that also impacts on your reputation. Phil Kinner, Electrolube’s Global Business & Technical Director for Conformal Coatings, explores why testing is so important and examines different variables that can cause board failure.
As electronics evolve to do more in an ever smaller footprint, the chance of failures increase more and more. As components get smaller and pitches become ever finer, many designs are getting closer and closer to design rule limits and sometimes even challenging the limits of manufacturability. It stands to reason, that the more designers push the boundaries, the chance of failure increases dramatically.
In addition to design related failure issues, the closer proximity of components due to the higher density inherently makes the hardware more vulnerable to corrosion, which is a complicated, diffusion controlled, electro-chemical process that takes place on an exposed metal surface, in the presence of water and ionic contaminants. Cleaning prior to conformal coating will go a long way to removing conditions for corrosion, enabling a more efficient coating process to increase the insulation resistance in these sensitive designs and mitigate against the effects of the operating environment on failure.
As PCBs are highly complex, there are so many variables that can potentially cause board failure. Some failure mechanisms occur slowly, which can help with detection, maintenance and even repair, however, with a sudden and unanticipated failure mechanism, complete PCB failure is often more likely. When a PCB fails despite being coated first, the chances are that there were problematic conditions already prevailing on the PCB prior to the conformal coating process, further still, the coating itself could have been inappropriate or the coating method deployed was not suitable for the application. Equally, there are many other threats to a PCB’s success, which include applying conformal coating to components and surfaces that have not been cleaned prior to coating, electro static discharge or even potentially poor construction of the unit itself.
The first and most critical procedure to employ is to carry out an inspection prior to coating. This is essential because it validates the overall quality of the PCB, ensuring it is fit for purpose and in keeping with customer specifications. Inspection at this stage is also paramount in detecting any conditions that could potentially cause board failure such as component failure, insufficient board thickness and loose connections that impact on connectivity. Conformal coatings do have their limitations and contaminants present on the surface prior to coating will be sealed in by the operation and may cause long term problems – such contaminants might include fingerprints, flux residues and moisture as well as other atmospheric pollutants. Boards should always be cleaned and dried before conformal coating to obtain optimum performance. Even when using so-called no-clean fluxes, cleaning boards before coating helps to increase performance and reliability.
Another renowned enemy of PCBs and components is heat. During operation, the materials within the PCB will undergo a wide range of temperature changes and each component has a specified range of heat that it can absorb, largely dependent on its size and shape. Higher power, greater density electronics produce more heat and excessive heat can cause significant mechanical stress that can impact on solder connections and burn out components. It is essential to manage heat transfer efficiently to increase the lifetime of the device and prevent failure. Overheating not only accelerates failure mechanisms but can also cause devices to become too hot to handle and, in some cases, present a fire risk.
In addition to the issues surrounding heat, the continuing drive towards miniaturisation means that we don’t have space for multiple boards, and there is an increase in the number of designs utilising mixed technologies where analog, digital, and RF circuits are closely combined with high voltage circuits, it becomes increasingly difficult to satisfy both clearance and creepage design requirements. Even very slight changes in the environment, whether it is an increase in dust, higher humidity, water-splash or exposure to potentially corrosive gaseous materials can be enough to take an otherwise safe, functional design outside of safe operating clearance and creepage, and cause performance failures.
Other contributing factors to board and component failure include poor solder joints, unconsumed or superfluous flux and tin whiskers. Cold solder joints, which occur when the solder fails to melt entirely during assembly, produce poor surface-mount connections that burn components and create power problems. Left over flux can also contribute to corrosion, due to its absorption of moisture, and produce short circuits and damage to components. Tin whiskers also cause short circuits. Conformal coatings can normally combat the formation of tin whiskers during operation, but are less effective when whiskers exist already within the assembly prior to coating. In general, the level of mitigation provided depends more on coating coverage than the properties of the coating, although there is some equivocal data that harder and tougher coatings provide more mitigation, but this must be balanced with an increased impact on the life of the solder joint. Overall, focussing on achieving 100% coverage of metal surfaces is likely to be a more efficient mitigation strategy.
Trace damage can occur from power surges, lightning strikes and overheating. Damage to the silver or copper conductive pathways can normally be seen with the naked eye, but this is not always the case. Trace damage causes considerable issues with conductivity, components and the reliability of the device. Fortunately, trace damage is normally detected and remedied during the initial inspection as it is visually very apparent.
Whilst this is not an exhaustive list of every single factor that contributes to PCB failure, it should help to provide a comprehensive overview of key elements to look out for. Lastly, the design itself could be responsible for PCB failure. To increase the lifetime of the board, it is imperative to ensure that the appropriate components and materials have been selected, the board layout is sufficient and the design is verified for its specific requirements. Design is also important in determining the appropriate coating application methodology and therefore the cycle times and the costs involved. Some simple things, like trying to keep connectors or other no-coat areas on the same edge of the assembly, can make a huge difference to the ease of coating an assembly, the cost of coating that assembly and, of course, the overall reliability of the assembly.
PCB Malfunction Post Coating
There are a number of variables that could be responsible for a malfunctioning printed circuit board following the coating process. Generally, the results could indicate poor product selection and/or application, or some underlying problem arising from insufficient surface preparation or some chemical activity going on beneath the coating that is entirely unrelated to the coating chemistry. Poorly performing coatings risk loss of insulation at the PCB surfaces when water condenses in combination with ionic impurities to form conductive pathways between PCB tracks. Without doubt, condensation can severely test the insulation resistance of a coating. There are many coating products that resist these sorts of conditions, so this type of problem can be avoided by making an appropriate material selection at the outset.
If the coating has not cured properly, it will not be able to develop its protective properties to the full. In this incidence, the application process is to blame. Correct application is a pre-requisite to coating success and by achieving this, a whole raft of problems can also be solved in one hit. For instance, poor coverage, insufficient thickness and sharp edge coverage can be difficult to achieve with many coatings and it can be hard to ensure sufficient thickness in these areas to maintain protection. A combination of material selection and application technique/workmanship will remedy these sorts of issues. The IPC specification allows a dry film thickness of between 30 and 130 microns, the greater thickness being achieved by the application of multiple coating layers. Trying to achieve a 130 micron dry film thickness from a single selective-coating process with a solvent-based acrylic material, for instance, is a recipe for a disaster, likely to result in excessive bubble formation, film shrinkage, coating de-lamination and additional stress on components. The result is poorer protection, rather than an improved overall level of circuit protection. Aiming for a uniform 30-50 microns and focusing on achieving perfect coverage at each application is a much better approach to improving the protection of electronic circuits.
Achieving the correct coating thickness is important; bear in mind that if the coating is too thick it can lead to entrapment of solvents in areas where the coating does not fully cure. Similarly, it can cause the coating to crack as it cures, or even cause cracking of the coated components themselves, or as the result of changes in temperature, or due to mechanical shock and vibration. The number one determinant of success for coating reliability lies in the application. Often a poor material applied well can be just as good or sometimes better than a material with great properties that is applied badly. Coating is about getting sufficient coverage of the sharp edges and metal surfaces without applying the material too thickly elsewhere. Of course, some materials ‘apply better’ than others and make this process as easy and foolproof as possible; but in the end, the performance of liquid applied coatings will always be determined by how well they were applied.
Large arrays of discrete components also represent a massive coating challenge due to the high levels of capillary forces present and the result is often quite disastrous, with areas of no coverage/protection on the board and conversely areas of excessive thickness prone to stress-cracking, de-lamination and other coating defects. Ultimately this will lead to premature failure of the assemblies and should be avoided if at all possible!
There may also be an unexpected interaction with some other process material used to prepare/build the PCB. Flux residues are particularly illustrative of this type of problem. In a ‘no-clean’ process, for example, these can inhibit the cure of some types of coating or lead to a loss of insulation of the system, greater than either material in isolation. Unless there has been meticulous attention to preparation or pre-coat cleaning regimes, corrosive residues bridging the PCB’s conducting tracks can, over time, cause failures. And while the coating may delay failure for many years, at some point failure will inevitably happen.
The greatest test of conformal coating’s performance is posed during power-up under wet conditions, whether this is due to condensation, immersion or salt-spray. Water with soluble impurities is electrically conductive and, finding any weak spots in a coating, will eventually leading to short-circuits at the PCB surface. In order to provide protection in these circumstances, it is essential to achieve 100% defect-free coverage of the PCB’s metal surfaces, and this poses a real challenge for both the material itself and the application process. Fortunately, a new class of two-part conformal coating materials dubbed ‘2K’ enable a much greater thickness and perfect application coverage to be achieved, resulting in a far higher level of protection. The superior performance advantages of Electrolube’s 2K coating materials, which combine the tough, resistant properties of an encapsulation resin with the easy application of a coating, have been positively demonstrated in three of the harshest tests that these materials can be subjected to, including powered condensation testing and powered immersion testing in salt-water.
Conclusion
PCBs are the life force of all electronic devices that we rely on every day, from smart phones, tablets, PCs and laptops to street lighting, TVs, refrigerators, microwaves and cars. When a PCB fails it can be enormously disruptive and in cases such as aerospace applications, it can be critical. Therefore, the materials selected to protect electronic assemblies can literally make or break a printed circuit board, particularly if it is required to endure substantial physical shock and thermal cycles.
Pre and post coating inspections and tests are essential to ensure that the PCB will perform reliably and increase its lifetime, particularly if the PCB is destined to operate in a harsh environment. Select the appropriate material for the protection required, apply and cure it well. Check for interactions with other process chemistries and thoroughly clean the assembly prior to coating. If possible, spend time simplifying the coating process at the design stage. By the simple action of placing as many connectors and components that must not be coated, as possible along one edge of the assembly, the conformal coating application process will be simplified. PCB failures can unfortunately occur following the application of a conformal coating but the positive news is that failure can be prevented with systematic pre-coat inspections, correct material selection/application method and further rigorous post-coating tests.
