When and how to clean selective coating equipment
Much time and planning are invested in the choice of the ideal conformal coating material and process to adequately protect your printed circuit boards. This often includes multiple qualification trials. There is also what can be long and detailed testing in areas such as electrical performance, flame resistance, and thermal or mechanical cycling.
By Matt Eveline, HumiSeal
Unfortunately, the qualification and testing process for conformal coatings is simply a snapshot of the process at the start. In order to maintain consistency, there remains an often-overlooked activity: regular cleaning and flushing of your selective conformal coating equipment.
In general, the following comments and guidelines from HumiSeal are designed for a discussion involving typical modern selective coating equipment as shown in the schematic below. However, nearly all of the principles are applicable to manual spraying operations as well.
Why clean and flush?
Before discussing the specific considerations involved in the clean and flush process, it is probably best to explain the reasoning behind it. Beyond the obvious answer that it is simply good practice, there are a few reasons to regularly clean and flush your selective coating system.
- The process improves the working life of key components such as:
- Nozzles and spray heads
- Feed and return lines
- The process minimises inconsistency due to clogs or buildups.
- Cleaning and flushing improve reliability and consistency of:
- Fow rates and material usage
- Application thickness
- Coating pattern width and edge definition
- Regular maintenance prevents cross contamination from incompatible chemistries.
(Note: When a silicone coating chemistry is used, never use the same fluid lines, valves, or pressure pot when converting to other chemistries, such as acrylics, urethanes, synthetic rubbers, and so forth.)
When to clean and flush?
There are a number of fairly obvious circumstances and times when it is advisable to thoroughly clean and flush your selective conformal coating equipment. They include the following:
- Startups or changeovers to different coating chemistries, such as acrylic to urethane and synthetic rubber to acrylic.
- At startups after shutdowns of 48 hours or more for acrylic, synthetic rubbers, and non-moisture sensitive materials.
- At the end-of-production day for moisture sensitive materials, including most silicones and many urethanes.
- Anytime inconsistent spray patterns or flow rates are observed.
How to clean and flush?
Here are the basic steps to cleaning and flushing your manual or selective coating equipment. (Note: Please consult your equipment manufacturer to discuss details before attempting).
- Remove excess coating from the reservoir or pressure pot.
- Remove the air cap (if applicable) of the valve and back off the micrometer (if applicable) to allow for more needle travel.
- Flush the coating from the coating reservoir, fluid lines and valve until there’s no coating coming out.
- Add a compatible solvent/thinner (MEK, xylene, commercial thinner) to the fluid reservoir.
(Important: Use thinners provided by your coating manufacturer, not stripping agents, which are used for removing cured coatings from PCBs.)
- Turn on the machine blacklight.
- Flush until the solvent/thinner stops fluorescing blue. Do this if the coating contains fluorescing agents; otherwise, flush until the coating appears to be eliminated.
- Add a small amount of additional thinner.
- Flush just until the moment when the fluid becomes spray. The fluid will go from a stream to a spray when the flushing fluid runs out. Then stop. If done correctly, the process will leave a very small amount of solvent/thinner in the fluid lines/valve.
Startup of process
- Flush the remaining solvent from the fluid reservoir/ fluid lines/ valves.
- Now your machine and valve are ready to be filled with fresh coating.