Conformal coating is generally the last step in the PCB manufacturing process and the method of protection for all those valuable functional components, but it's not always that simple. Let's look at a fairly common scenario.
Everything was moving along fine through the design and testing process. You have begun production spraying and suddenly there are wisps of translucent webs floating in the air. Your taller components look like a scene from the movie Arachnophobia. This is the oft-dreaded cobwebbing effect; and while it may appear confounding and a little scary, it is often one of the simplest problems to eliminate when conformal coating.
In this brief article, we will discuss the likely causes of this odd situation and provide some simple solutions to get you back to applying your coating in a defect-free manner. Let's begin by exploring the phenomena in more detail.
Cobwebbing, as it is affectionately known, is a colloquial, informal term describing prematurely dried coatings or adhesives that effectively cure in the air during the spray process. The term is of course used since the floating or settling strands look a lot like cobwebs or spider webs. You may hear cobwebbing, spider webbing, or cotton candy to describe this application defect.
Cobwebbing is pretty much exclusively an effect that manifests itself during the SPRAYING of coatings (especially atomized spraying). It is not an issue usually seen with alternative methods of conformal coating. As such, you may instead opt for one of the following methods:
These may not work for your operation, so diagnosing the causes of cobwebbing is the first step to creating solutions.
Now, let’s take a deeper look to understand what causes cobwebbing in the conformal coating spray process. The cobwebbing effect, at its core, is one result of an adhesive or coating drying too quickly. Most traditional coatings consist of a polymer base (acrylic, urethane, synthetic rubber--the materials that make up the actual coating) combined with one or more solvents, sometimes referred to as thinners.
Solvents and thinners are almost always necessary as polymers on their own are too thick and flow well through machinery and coating equipment. Conformal coating viscosity is a tricky balance to manage. Solvents and thinners also help the coating to flow and cover the PCB surface evenly, which would not be possible with the polymer on its own.
Related Article: How to Measure Conformal Coating Viscosity: Tools and Devices
When a coating is applied via atomized spray, air is applied to the liquid coating in the application valve (usually at the nozzle) to break it up into smaller droplets. Cobwebbing occurs due to one or more of the following factors:
It is the manipulation and optimization of mainly these above four factors that will eliminate cobwebbing while still producing an acceptable spray pattern and flow of the coating.
Let's get to some answers! We have a basic understanding of what causes cobwebbing, and we can take a look at the steps you can take to eliminate these problems. Unlike some trickier application issues, improving or eliminating conformal coating cobwebbing involves some steps that are genuinely quick and reliable.
It may be that you can eliminate the problem using just one of the following actions, but it is often the case that you may end up using multiple or even all these fixes.
Probably the easiest and often the first thing to try. Since it does not involve changing mixing and such, just turning a dial and lowering the air assist pressure is often the best thing to try first. When valve atomization pressure is too high it can effectively “boil” the solvent in the coating mix due to its low vapor pressure.
The thinners and solvents used in most conformal coatings have low vapor pressures by design. This reduces the drying and cure time of the applied liquid coating.
During the application setup process, users should start with an atomization pressure set to the lowest possible setting. Next, increase until the coating comes out in an even and consistent width. If you observe cobwebbing, then the atomization pressure is too high and should be lowered.
If you have tried lowering the atomization pressure to the absolute minimum and you still see cobwebbing, we then move on to adjustments to the valve height. When the application valve is too high, the thinner / solvent in the atomized droplet will evaporate before reaching the substrate.
Due to the wide variation in electronic assembly layouts, it’s not realistic to expect or plan for a single, set valve height for the entirety of the run. For most applications, the user will need to program the valve in such a way that the valve height changes to meet the topography of the underlying substrate, raising and lowering to the ideal distance for each component. The user should fine-tune the combination of atomization pressure and valve height to ensure cobwebbing is avoided.
If at first and second you do not succeed, you still have one final option. Increase the amount of thinner mixed with the coating, or use a slower evaporating solvent altogether. (A combination of both works too!)
At the heart of it all, the coating is drying too fast is the root of all our problems. That can be slowed down either through the use of more solvent or a slower evaporating solvent. Both approaches will result in a slowing of the drying process and a reduction or elimination of cobwebbing.
We often use 1:1 or 2:1 (coating to thinner) as a starting point for our mix ratios; however, certain conformal coatings and thinners can require the user to adjust this ratio (either up or down) to best suit their unique application process.
HumiSeal manufactures many standard and specialty thinning solvents designed for the optimization of specific characteristics--the speed of drying being an especially relevant one for our purposes here. In general, a slower evaporating solvent will result in a lowered risk of cobwebbing defects. Additionally, this is a great way to remedy problems with conformal coating bubbling.
The only limiting factor in these cases can be things such as environmental regulations a line speeds. For example, there are growing regulatory concerns with aromatic solvents and thinners that may require operational changes.
Related Article: Toulune-Free Solvents for Conformal Coating Applications
Just as a quick summary, cobwebbing is one of the most common processing issues that PCB manufacturers face when conformal coating. It is caused in general by premature drying of the coating as it is atomized and sprayed. It can be solved using a number of approaches designed to slow down the drying process. While challenging, methods to eliminate cobwebbing are fairly straightforward and have been tested and proven to last.
As a supplier of all major forms and chemistries of conformal coatings, Chase Corporation and HumiSeal® can help you with an unbiased approach to evaluating your application and process. We’ll show you how to maximize efficiency, minimize cost, and improve product reliability. Our outstanding manufacturing and technical support groups can provide your organization with reliable global supply, unmatched quality, and superior technical support.
Please contact us today for a customized solution to your electronics coatings needs.