We at HumiSeal would like to dedicate a bit of time and space to one of the most often ignored factors related to successful conformal coating processes: Clean Air.
What Do You Mean “Clean, Dry Air”?
I think it is fair to say that we all have experienced, at least to some degree, air that is not as clean as we would like. In an industrial environment, we are talking about what is often described as “plant air” or the pressurized air supplied to industrial equipment by a compressor. Compressed air can be used to supply energy for brakes and motors, for the movement of fluids or spray operations, and even for basic cleaning and blowing operations.
Ideally, supplied compressed air will be as clean as possible and free from any contaminants. The most common and troublesome contaminants include:
- Dust and dirt
- Water and moisture
- Oils, silicones, or organics
Clean, dry air will contain as little as possible of these contaminants.
Where is Compressed Air Used in Conformal Coating?
In this case, we will get a little more specific about the importance of clean air in conformal coating processes.
Pressurized plant air from a compressor is used in at least two key areas during conformal coating-
- Pressure pot for fluid movement
- Air spray head/valve
Why Should I Care About Dust, Water, and Oils?
This section will take a deep dive into three of the most common contaminants.
Dust and Dirt
Let’s start with the simplest contaminant which is dirt. Plant air and compressed air will always contain a certain quantity of basic dirty stuff. Particle sizes run from large to microscopic. From a mechanical point of view, any solid particles in compressed air have the potential to agglomerate or stick to surfaces.
With conformal coating, this becomes a potential problem specifically at the spray valve unit where very tight tolerances and gaps can become clogged leading to poor spray patterns or no spray at all. In addition, the term dirt in fact includes any solids of many chemical variants including things such as silicones. These chemicals can build up on PCB surfaces causing poor adhesion or can even react in some instances producing bubbles or improperly cured coating.
Oils and Organics
A second undesirable contaminant present in plant-compressed air is oil. The source of oil in vapor form is generally the compressor itself which utilizes oil for lubrication and sealing purposes within the device. Small amounts of these oils vaporize and become part of the air stream supplied to the machinery. These oils are generally very low surface energy and can often cause poor wetting of conformal coatings when they are a part of the air supplied for the coating valve and heads.
Water and Moisture Vapor
Last, but certainly not least, we must discuss the contamination of plant air with water or moisture vapor. This can at times be considered the silent killer of your conformal coating operation if not properly eliminated.
The number one reason why moisture is undesirable is that many conformal chemistries utilize moisture or humidity for curing and crosslinking. Three particular chemistries are extremely sensitive to pre-curing with moisture-
- Polyurethanes
- RTV Silicones
- UV Curables with Secondary Moisture Cure
If conformal coatings of the above three chemistries are exposed to humidity either in the pressure pot or the coating head, premature curing will likely occur. This can lead to a rise in viscosity, poor flow rates, surface skinning, poor spray pattern, and clogging of lines of coating valves.
Chemistries such as acrylics and synthetic rubbers are often not affected significantly by reasonable humidity values.
The Best Equipment for Clean, Dry Air
Nearly all commercial compressors already have a fairly basic air cleaning and drying system attached to the unit. This is adequate for many basic needs, but experience has shown us that it is inadequate for conformal coating processes.
In order to clean up your plant air for conformal coating, you will need what is often described as a desiccant air drying system. More specifically, we often recommend a five-stage desiccant air-drying system as pictured below.
The drying/cleaning system is comprised of-
- Stage 1- Particulate Filter
- Stage 2- Oil and Organics Filter
- Stage 3- Desiccant Dryer for Water and Vapor
- Stage 4- Dust Filter for Desiccant and Dirt
- Stage 5- Regulator
The combination of filters and desiccant have the capability of resulting in plant air clean of dirt and particulates and free of humidity to typically a dew point of -30F to -40F.
Related Article: How to Clean and Flush Selective Coating Materials
Some Final Notes
There are some final logistical considerations to be considered for the optimal design of your cleaning and drying system.
First, it is important that the five-stage system be located as closely as possible to the actual conformal coating equipment. It is often possible to set up a system as close as five to ten feet away or even less. This helps to minimize the chances for re-contamination from longer air lines.
Second, regular observation and maintenance is necessary to keep the system effective. Suppliers of drying and cleaning systems will provide specific instructions, but suffice it to say that regular replacement of filters and desiccant is vital to keep your air clean and dry.
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 to discuss your application
