Coatings fail for any number of reasons, some common and some not so common. I have more than 20 years’ experience and knowledge of the conformal coating business, and I’m still learning; but there are certain production related issues that cause problems time and time again, some of which could have been easily resolved. In my previous columns, I have examined a number of coating failure issues but this month I’m going to talk about some of the arch enemies of conformal coatings, the challenges posed by sensitive areas and higher profile parts, and I’ll also look at some of the best ways to avoid wicking to help you achieve a uniform and defect-free finish. Hopefully, it will provide useful background information to help you with your coating-related activities. For the full article, please visit the Electrolube website. 1) Why do taller/vertical pins on PCB’s represent more of a challenge to coat and can you suggest some key pointers for success? Tall and vertical surfaces present challenges to achieving coverage with liquid conformal coatings, predominantly due to gravity, and application method to a certain degree. Traditionally liquid conformal coatings rely on solvent evaporation to increase their viscosity and prevent the material flow away from sharp corners and vertical surfaces by the combination of gravity, or capillary forces. This process is relatively slow and some degree of coverage issues are always likely to be present. UV curable materials solidify very rapidly under UV radiation of the correct wavelength and intensity, however, there is nearly always some time between the application of the material and the exposure to radiation and during this time the material will still be subject to gravitational and capillary forces, again resulting in some degree of coverage issues…... 2) What steps should I take to protect particularly sensitive parts to be coated? This really depends on what the parts are sensitive to. If we are trying to protect parts from the effects of humidity, condensation or immersion in water then thickness and coverage are the most important factors to consider, along with the choice of conformal coating. Some coatings are inherently better at achieving coverage at higher thicknesses and have inherently greater barrier properties than others. Sometimes, parts are sensitive to actually being coated and either the component itself or the solder terminations can be damaged by conformal coating materials. This is especially true when the Glass Transition Temperature (Tg) of the material falls in the operating range of the board in question. As the coating material passes through its Tg, there is a delay between the decrease in elastic modulus and the increase in Coefficient of Thermal Expansion (CTE). This lag results in maximum stress being applied to the component or solder termination and can damage the component or solder. Over a sufficient number of cycles, solder can even be extruded by the coating, leading to shorts. 3) Is coating/board failure more common on higher profile parts of the PCB and why is this? This really depends on the design of the assembly and the conditions to which it is exposed in the field. As discussed in point 1, there are numerous challenges of actually coating taller devices effectively. Lack of coating thickness and coverage make taller components more susceptible to failures by tin whisker, arcing, corrosion, condensation or immersion. However, under conditions of high humidity, the board level terminations are often more susceptible to corrosion due to the presence of flux residues, which exacerbate the tendency for localised condensation due to their hygroscopic nature and subsequent corrosion due to their ionic nature. 4) Can you name 5 ‘enemies’ to a successful coating process? Lack of cleanliness or the presence of contamination on the board before coating can lead to open defects in the coating due to de-wetting or non-wetting. The contaminants can affect or compromise the ability of the coating to provide long-term protection by increasing the rate of moisture uptake, interaction with water and causing corrosion beneath the coating, compromising the adhesion of the coating or a combination of all of these factors. 5) What is wicking and how do I avoid this? Wicking is often used interchangeably with capillary flow, especially in a conformal coating context, to describe the ability of liquids to flow into narrow gaps without the assistance of any external forces. The narrow gaps are most often unsealed connectors or the standoff between component bodies and board substrate. In the case of unsealed connectors, switches etc, the main issue is the coating ‘wicking’ up and coating the connector mating surfaces, therefore reducing the contact or potentially insulating the mating surface, preventing either the form, fit or function of the connector or switch. I have only been able to give you a snippet of the information here, but understanding the major enemies of conformal coatings will go a long way in implementing a successful conformal coating process. Read The full version of this article Here as well as more content on the Electrolube Knowledgebase. Electrolube are a part of MacDermid Alpha Electronic Solutions