Hello Again!  The last time we discussed bubbles in conformal coating, I wrote how to resolve process related root causes.  There are other simple ways to reduce the amount of bubbles:

• by varying the types of solvents used in a product
• by using a product that contains a surface tension modifier, hence increasing the rate at which bubbles burst.

If you recall, most of the bubbles are basically solvent or air entrapped in the coating.  As you might expect, the air and solvent try to find their way up through the liquid to escape.  The problem is that the surface dries first, creating a thin skin that prevents those bubbles from escaping.  Therefore, the solution is to eliminate the bubbles in the coating prior to the skin formation.   In other words, use a solvent set and other surface tension reducing chemistries to accelerate elimination of bubbles.

Figure 1: Thermally induced cracking in conformal coating

Does your conformal coating require long term thermal shock testing?   Do you struggle to overcome cracking and adhesion loss with your current conformal coating after 1000 thermal shock cycles?   With the drive for increasing functionality for end devices, and increased life cycles being placed on all electronic assemblies, the need to have materials endure more stringent testing becomes increasingly vital.  Necessary robustness to these stringent conditions force suppliers like HumiSeal to innovate new material solutions to address these requirements.

What is so special about synthetic rubber conformal coatings?   Why do many high reliability applications require synthetic rubber?  Would synthetic rubber be a better fit for your application?  Synthetic rubber copolymer provides higher operating temperature and exhibits superior mechanical properties compared to traditional materials. The products have been developed to withstand the demanding conditions in the most severe environments required in today’s applications.  Commonly used conformal coatings are acrylic, polyurethane and silicone.   Less known are synthetic rubber conformal coatings, however these have been used for many years and HumiSeal continues to develop new and improved varieties of this chemistry.

Have you ever experienced uncontrolled conformal coating migration?   Have you had to strip your recently coated boards because conformal coating wicked into non-desired keep out zones?   Wicking is a phenomenon caused by a combination of low surface tension coating, which is usually characterized by low viscosity, coupled with strong capillary forces generated by low standoff gaps possessed by the SMT process.   Wicking into connectors; onto grounding hole; or under sensitive components will create defects that will have to be reworked, as it may cause reliability concerns with the finished product, which we will describe in detail.   There are ways to either reduce and/or eliminate wicking issues through decisions made during preparation, application, and conformal coating selection.    

Does your conformal coating not wet properly? Do you struggle with craters, fish-eyes, or film separation? The culprit could be that the surface energy of your PCB is lower than the surface tension of the conformal coating.   The above defects are discussed frequently with customers on our technical support line, and any time we receive a technical enquiry regarding poor wetting or loss of adhesion, we ask “what is the surface energy of your substrate?” In this blog, we will explain surface energy, surface tension, and how to quickly troubleshoot defects resulting from low surface energy.

Have you ever wondered if you can use the same material to pot or conformaly coat your electronics?  HumiSeal has launched a new fast curing, two-part silicone (Humiseal 2C51) that can be easily applied as a conformal coating or as a potting material.  This option allows users to standardize on one product for multiple applications.

Humiseal 2C51 is a high solids, low viscosity, fast curing product.  Humiseal 2C51 comes as a two-part kit.   Part A and part B are mixed with a 1:1 ratio to either encapsulate or conformally coat your electronics.  Part A and B are yellow and blue for easy verification of complete mix which is slightly turquoise.   When cured, 2C51 is trasparent even in high thicknesses.  Humiseal 2C51 is primarily used in high temperature applications where component stress relief is required.

During your selection of conformal coating you ran in to a specification that you have seen before but not fully understood, MIL-I-46058C.   Conformal coating choices are vast, but with so many available, how does a user pick the correct one? Are there minimum standards that define what a conformal coating is supposed to do? Thankfully, yes there are. This article will focus on one such standard: MIL-I-46058C.

The official title for the specification is MIL-I-46058C, Insulating Compound (For Coating Printed Circuit Assemblies). The standard serves as a material standard, used to evaluate and document that a particular coating meets a list of specific performance attributes (more on those later). The MIL-I-46058C was developed to define a uniform set of test methods and performance requirements for conformal coatings and gives users confidence that the material they select will perform.

Your conformal coating film is supposed to be clear and transparent, but it’s cloudy-white and dull…it’s BLUSHING.  This condition is an indication that your coating film is trying to tell you something, and it isn’t that it’s embarrassed.  This blog will help you better understand why your conformal coating film is blushing from acquired moisture contamination and help you eliminate this defect.

Coatings appear cloudy because they have inadvertently acquired moisture, either due to their hygroscopic nature or by artificial means (a cooling mechanism created by solvent evaporation), which then reacts negatively with the coating resin -  exhibiting itself as a milky, colloidal-type substance.  Many different types of conformal coating chemistries can be sensitive to blushing under certain circumstances.  These circumstances are usually related to specific ambient conditions, application, cure, condition of the assembly/substrate/associated components, storage environment and/or equipment under use.

"Have you ever experienced air entrapment in your conformal coating?  We would like to offer some assistance to help you to get rid of those exasperating bubbles in the conformal coating process.

Contrary to what most people think bubbles are not material related, bubbles are mostly caused by process related issues.  Yes, it is true that the conformal coating contents would influence in having more or less bubbles.  Solvents will definitively generate bubbles if the process is not properly set but that is not all!  You have to think about  several other factors that could negatively affect the result of your coating application in regards of bubbles.

So, we will talk about not one, but three clever techniques to reduce and/or get rid of the bubbles.

UV curing of conformal coatings can appear to be a complicated topic, however, a few simple steps will help to correctly set up your production process.   The main difference between UV curable coatings the traditional cure mechanisms is the utilization of UV energy to effect the change from a liquid to a solid. After this rapid UV curing step the coating is tack-free and the PCB can continue its production journey negating any requirement for long curing, post-bakes etc.