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May 27, 2008

Changing Conformal Coating Material

We are considering changing over our pcb conformal coating process from a silicone based material to a urethane based material.

Are there any concerns regarding the current equipment other than obviously replacing the material delivery lines and cleaning/replacing the spray nozzles?



M.J.

Experts Comments

There are several considerations when making a switch like this.

First, sounds like you are taking the correct precautions for preventing cross contamination. Depending on your process, you may have overspray on the conveyors. Make sure to clean all parts that come in contact witht the board.

Next, you have to consider the material rheology and your coating method. There are many methods of coating: air atomized spray, airless atomized spray, ultrasonic atomization, film coating, jetting, and needle dispensing.

If the material that you are switching to has a similar viscosity, you may be in luck. If the material viscosity is very different, you will probably have to adjust your settings or use a differnt size nozzle.

Worst case, your current method may not be suitable to the new material and you will haveto switch methods.If that is the case, you should first go back to the equipment manufacturer and provide thefluid part number and manufacturer to see ifthey have recommendations. In somecases, the supplier of the fluid will have hardware recommendationsfor your specificequipment.

If the material you are changing to contains solvents or solvents that are different than the material you were using, you should check to make sure your equipment meets NFPA codes for fire / explosion prevention and make sure that you comply with EPA, OSHA, state, and local requirements for ventilation and emissions.

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Alan Lewis
Director of Application Engineering
Asymtek
Mr. Lewis worked for The Aerospace Corporation for 6 years before joining Asymtek in 1993. He holds multiple patents in dispensing technology for electronics assembly and packaging. He has a Master's Degree in Mechanical Engineering from University of Missouri-Rolla.
NOTE: Mr. Lewis is no longer working at Asymtek.

Changing from a silicone to a urethane conformal coating should not cause too many problems if you are careful not to contaminate the polyurethane with any remaining silicone. The main differences between the products are related to their chemistries and cured film properties.

The main thing you need to check is the viscosities of the two materials to make sure they are similar. If this is the case then spraying should be fine, providing you have the correct set up. Silicone contamination can be difficult to remove and so it is important to thoroughly clean any non-replaceable parts with a solvent that is compatible with the silicone.

Following this you should flush the spray equipment with the solvent used to dilute the urethane material before filling with the new coating. Silicone contamination can cause issues with wetting and overall coating appearance therefore it is best to run a couple of spray tests first on scrap material prior to carrying out any production work.

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Jade Bridges
European Technical Support Specialist
Electrolube Ltd
Jade Bridges is the European Technical Support Specialist for Electrolube. She is responsible for technical support within Europe, offering assistance to customers with product selection, implementation and after sales support across the range of Electrolube products. Her expertise is carried over from her position as R&D Manager for Electrolube, where she was responsible for the new product development and technical support across an array of chemical products for the electronics industry, including conformal coatings, encapsulation resins, thermal management products, contact lubrication and electronic cleaning solutions.

The following steps should be performed when changing from a silicone conformal coating to another material type such as a urethane:

  • Replace all the liquid delivery tubing and fittings.
  • If you are using a pressure reservoir designed for use with moisture proof bags for the silicone material, a new style reservoir will be required for theurethane material. Theurethane will not be packaged in bags, it will come in metal containers and will need to be mixed with the appropriate thinner for the spray coating process.
  • Thoroughly clean the coating heads and replace all the internal o-rings. The coating heads may also need to be reconfigured for theurethane material.
    For example, the nozzle tip of the spray valve will most likely need to be changed to one with a smaller orifice to accommodate theurethane sinceits viscosity will be lower than the silicone material.
  • Thoroughly clean the inside of the coating system including the conveyor to ensure that all the silicone material has been removed.

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Stuart Erickson
President
Ultrasonic Systems, Inc.
Mr. Erickson is co-founder of Ultrasonic Systems, Inc. a technology leader in ultrasonic spray technology. Hi is holder of several patents related to ultrasonic spray technology. His roles include product development, technology application, sales and marketing.
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