Know the limitations of the materials you use to coat electronic assemblies
According to Murphy’s Law, if there is chance of anything going wrong, it will in fact go wrong. Although, it sounds very cynical, it’s true nonetheless. We must be prepared for all the things that can go wrong in life, more importantly in electronics.
Let’s take the case of conformal coatings for the protection of electronic assemblies. In this article, we are going to talk about different conformal coatings, their methods of application and the potential risks associated with each one of them.
When it comes to conformal coating, the method of application is equally important as much is the choice of material and supplier. For instance, if a product is transferred from one circuit manufacturer to another after dip coated at one place but selectively coated at another using the same specification of the material, the problem arises in the second step when a material made for dip coating is used in selective coating resulting in poor quality yield due to bubble entrapment and excessively quick drying.
Methods of application
If you are using acrylic conformal coating and are trying to achieve a 130 micron dry film thickness from single selective coating proocess, you are rooting for disaster as what you’ll get as a result will be full of bubbles, film shrinkage, coating de-lamination at various places and additional stress on components.
Typically, the required dry film thickness must be between 30 to 150 microns. Greater thickness is achieved by multiple coating layers. Trying to achieve it in a single coating will only result in poorer protection. If you can’t afford multiple coating, then the best option is to aim for a uniform thickness of 30–50 microns and focus on achieving a perfect coverage at each application. Paying attention to the viscosity of the coating is of utmost impost importance. If a conformal coating is applied at a lower or higher viscosity than that is specified, it may result in getting “sucked” beneath the components due to capillary action and lead to non-uniform fluorescence (if fluorescent conformal coating is used to allow backlight inspection of the PCB after coating)
If the coating applied is less than 20 microns, sometimes it becomes difficult to inspect the fluorescence. As a result, inexperienced people tend to apply more coating material which increases the component reliability issue due to coefficient of thermal expansion (CTE) mismatches in Z-direction. The better solution here is to apply a thin ‘primer’ layer as a base coat which will improve the ability of the coating to reduce bubble entrapments and cover sharp, vertical edges of components before applying the main coating layer.
While it is very tempting to uses shortcuts to speed up the production or reduce costs, you have to inevitably pay the cost. Knowing the special properties and limitations of the material that you are using to coat electronic assemblies will enable you to make maximum use of your resources and time.