By Phil Kinner, Technical Director For Electrolube’s Conformal Coatings Division
In recent months, Electrolube’s Technical Director of the Conformal Coatings Division, Phil Kinner, has taken a closer look at coating issues – particularly those that circuit designers need to address at the earliest stages of a project. In a series of regular posts, Phil provides key design pointers to help electronics manufacturers avoid some common pitfalls when applying conformal coatings and lists five essential coating ‘Facts’, highlighting certain production related issues that are frequent sources of problems – problems, he asserts, that are so much easier to resolve at the design stage, rather than at the prototype or – more importantly – at the production stage.
Thoughtful design will 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! So, having started the conversation about the importance of making sound early-stage design decisions, for my third column, let’s look at some of the issues that the production department is likely to face when applying conformal coatings.
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 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.
Adhesion results with conformal coatings can vary from supplier to supplier and this can create problems when applying solder resists. A quick and very effective solution to this is by specifying a surface energy of >40 dynes/cm on incoming bare boards and ensuring that each batch is religiously tested and rejected if they do not meet this minimum value.
Conformal coatings are usually liquid when applied and will flow with a combination of gravity and the capillary forces present. Whether you are masking or relying specifically on selective conformal coating, leaving a buffer of at least 3mm clear between the area to be coated and uncoated areas will make the production process easier.
That old adage: ‘if some is good, more is better’ doesn’t necessarily hold true with conformal coatings, which are designed to be applied at the thickness specified on the datasheet. Exceeding the recommended thickness is unlikely to provide better protection, but may introduce a range of production issues ranging from dramatically increased cycle time to solvent-entrapment, stress-shrinkage, de-lamination and cracking, to name just a few!
If you need to apply a thick coating, two thin coating applications are better than a single thick one. And if you need more thickness than specified, use a coating that is designed to be applied thickly or consider a resin product.
Conformal coatings should not be used as under-fill materials, as they generally contain no filler and have relatively high Z-axis thermal coefficients of expansion. Make this mistake and you could see the lifetime of ball grid array (BGA) and quad flat no-lead (QFN) terminations considerably reduced when exposed to thermal cycling conditions. If you need to under-fill a device, use one of the many under-fill formulations especially designed for this purpose.
Ok, enough of the small talk; let’s move on to my top tips for design engineers:
1. Conformal coatings are not waterproof, although having said that, we have just developed a hydrophobic two part coating system, which is water-resistant, but as a rule of thumb coatings are NOT waterproof. The housing around the assembly can often act as a water-trap due to condensation and poor drainage. Boards that are conformal coated and subjected to long-term immersion in condensed water probably won’t operate as you might expect! Airflow and drainage are important considerations during the design of the housing.
2. Speaking of airflow, coatings can be abraded by particles drawn from the environment by cooling fans. Once the coating is abraded, unprotected parts of the circuit will be vulnerable to high humidity and condensation, which will lead ultimately to failure. If these operating conditions are expected, it is well worth considering the use of a breathable membrane or particulate filter – or choosing a more abrasion resistant coating.
3. The interaction between solder paste, solder mask, fluxes and coatings is not easy to predict and should always be tested for each design. The geometry and thermal profile of assemblies can significantly affect these interactions; just because it worked on the last assembly is no guarantee it will work on the next.
4. Returning to the subject of housings, many boards are qualified without their housings as presumed worst case exposure. However, mechanical fasteners and fixtures in the housing can significantly affect the behaviour of the board during thermal shock or cycling by causing additional stresses on certain areas and changing the expansion and contraction dynamics of the system. And remember: condensation in the housing can lead to worse environmental conditions than testing outside of the housing.
5. Getting liquid coatings to cover sharp edges of components can be difficult. Poorly coated edges, leads etc, can be worse than not coating, due to the formation of ‘micro environmental hotspots’ where the effect of contamination, corrosion and so on will be maximised. Try to avoid the use of high, sharp cut ceramic components in large arrays, since this will increase the difficulty of achieving good coverage through a combination of capillary flow, and the tendency of the coatings to pool. This is particularly true for ceramic capacitors, which are extremely susceptible to corrosion due to the chemicals used in their manufacture.
It is no easy task to choose the correct conformal coating for your product, let alone have confidence that, in applying it, you will have achieved the ultimate goal of protecting your electronics. Conformal coatings are available in many generic types; each has its strengths and weaknesses. Choose the right coating for the intended use and operational environment, rather than one that is used by your subcontractor or qualified on another product line for a different end-use environment. And be sure to test your design to ensure that it has sufficient robustness for the intended application.
Look out for Phil’s next post!
For further information, please visit www.electrolube.com.