Ask the Experts
April 28, 2008 - Updated
April 28, 2008 - Originally Posted

Carbon Dioxide generated and trapped by conformal coating

We have two complete circuit card assemblies that are conformal coated using a urethane (PC18M) and they display "material, or clumping/bubbles" in the coating after processing. We sent the batch number and a sample to our supplier who could not verify the failure. They stated: "One possible cause for this type of defect is moisture. Moisture can cause generalized bubbling over the surface of the coating. PC18M is a urethane coating which can react with moisture and release carbon dioxide." My question is, would carbon dioxide "trapped" in a circuit card assembly have any long term affects on an aerospace assembly?

D. K.

Expert Panel Responses

Not 100% certain of the details on this particular urethane material but we have seen issues when conformal coating is cured using Convection Heating. In particular, convection can "skin over" the surface of the coating in the early stages of the curing process. When the bubbles try to escape they either get stuck under the skin or break through and create small crater-like pock marks on the surface. However, when using an Infra-Red heat source, the heating is basically the opposite. The material is heated from the inside out and the bubbles are allowed to escape first and the outer surface is skinned over toward the end of the process. The result is a super smooth surface. You may want to try a sample or two in an IR based oven to see if this has an impact. Oh, and to answer your question, we cannot comment on the long term effects of the trapped bubbles but maybe they can be made to go away.

Marc Peo
Heller Industries Inc.
Mr. Peo has been with Heller Industries for over 20 years and has been President for the past 8 years. Marc has authored several industry articles on Soldering, Flux collection, nitrogen use and Lead Free conversion.

I don't pretend to be an expert on this; however there are several reasons for cause for alarm in my view i.e. Carbonic acid forms in the presence of Carbon Dioxide and moisture. Carbonic acid is a weak acid that could cause the potential for failure of the PCB. Further conformal coatings are usually applied to act as a barrier to prevent moisture from the PCB, whereas it seems that moisture may already be present and encapsulated in the PCB, reacting with the conformal coating, and causing delaminating of the coating. I would insure that the materials are compatible and that the boards are baked or processed to remove moisture before the coating is applied.

Jess Baker
RPS Automation
Jess Baker has been in manufacturing management positions for over 47 years. He is the President and founder of Repco Inc., and Robotic Process Systems. He is currently President of RPS Automation a manufacturer of Selective Soldering, and Solderability Test Equipment.

While I am no expert on aerospace assemblies and coatings. I would expect that these carbon dioxide bubbles would react to the changes in atmospheric pressures and eventually rupture, this would cause the PCB to be exposed where ever these fractures occurred, causing to potential for board and assembly failures.

Edward Zamborsky
Regional Sales Manager
OK International Inc.
Ed Zamborsky is a Regional Sales & Technical Support Manager for Thermaltronics, located in New York. His position requires frequent customer visits throughout North America and the Caribbean and his position encompasses not only sales but the role of trainer and master applications engineer for all of Thermaltronics products. His expertise includes such specialties as hand soldering, convection and conduction reflow techniques, array rework, fluid dispensing equipment, and fume extraction. Ed has authored many articles and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead-Free Hand Soldering, High Thermal Demand Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.
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