Ask the Experts
April 6, 2018
Conformal Coating in Nitrogen Environment
We operate a conformal coat machine using nitrogen. We are thinking of changing it back to air for cost reasons, what are the pros and cons. We are under the impression that nitrogen is the better choice for purity reasons, is that correct?
Expert Panel Responses
Nitrogenuse for conformal coating system:
- Nitrogen (N2) is used inthis process for the following reasons:
A. N2 is low in moisture and if Compressed Air is used and thesystem is not maintained properly, moisture will be present and this will causeissues in the curing of the conformal coating on the parts, such as pooradhesion, moisture entrapment, etc.
B. Compressed air can contain oils if the filters are notmaintained, oil is a major problem with conformal coatings
C. Particle free as compared to compressed air
- Cost of N2 is higherthan compressed air, however there are issues, as stated above, that may bemore costly such as rework, field failures, etc.
A.Not knowing the supply mode that the M.E. is using, here aresome considerations that may lower his costs:
i. N2 generation membrane system
ii. Small liquid dewars that can be filled at your location and nottransported back to the supplier for fill
Global Segment Manager
Air Products & Chemicals, Inc.
Mr. Arslanian has been involved in electronics packaging processing and equipment since 1981 including flipchip, TAB, wirebonding and die attach. Current responsiblities include R&D, applications, marketing and customer interaction.
Conformalcoatings are basically a resin dissolved in a solvent. Most conformal coatingsutilize a flammable solvent. Once the coating is applied, the gas flow (air orN2) is needed to carry the solvent vapors from the machine/spray booth.
If airis the gas used, then the air flow must be sufficient to keep the solvent vaporconcentration below its LFL (Lower Flammability Limit). For example, the airflow for a spray booth that my employer uses is 7000 cfm. Using N2 as the sweepgas does not require as much flow since there's no oxygen in the gas flow. Hence,the LFL (also known as the LEL (Lower Explosive Limit) would not apply in a N2flow.
In addition to evaluating what air flow would be required to maintain.
Since air is 80% nitrogen, I don't understand how purity of the gas is involved.If the application was a wave solder unit, then N2 would prevent oxidizedsolder connections. But conformal coating???
Lee Wilmot has 20+ years doing EHS work in the PCB/PCBA industries, including environmental compliance, OSHA compliance, workers compensation, material content declarations, RoHS & REACH compliance. Active on IPC EHS committee and c-chaired committees on IPC-1331, J-STD-609A on labeling & marking, IPC-1758 on packaging and others.
Ipresume that you are using nitrogen to pressurize the pressure pot on thecoating system containing the coating and possibly power the air assist.Presumingthat this is a selective coating system.
Providingthat you filter and dry a compressed air supply you should not see anydifference.Theonly exception to this rule could be if you are using an oxygen cross linkingcoating, polyurethane with oxygen cross linking for example.
Therewould only be a negative effect here if the coating remained unused in the potfor a significant amount of time, weeks or months, this is not good generalpractice.Theonly other use of nitrogen in a coating system would be to form a blanket overa dip coating tank to reduce evaporation and solvent loss.
Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.
Without very specific knowledge of your process, including the coating materials involved, my comments must be rather general. The main benefit of nitrogen here is that it is very dry. By contrast, many factory compressed air sources contain a lot of moisture. Poor system design or maintenance can even result in water and/or oil in the lines.
If your specific process requires controlling the amount of moisture in the process, it may be as expensive to install and maintain drying and filtration systems as it is to run nitrogen for critical processes. On the other hand, if your compressed air system has effective dryers and filtration, then secondary filtration and/or drying may not be expensive, or even necessary.
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Besides purity & preventing solvent loss, the main reasons for choosing nitrogen as a blanket is its inertness and to keep the system dry. Many conformal coatings can exhibit "blushing" if moisture is absorbed into the solvents and then allowed to cure. In addition, RTV silicones require a minimum amount of moisture to initiate the curing process and so must necessarily remain moisture-free prior to application.
Another con of the air is that many operators will use "shop" air instead of dry compressed cylinder air. This has the potential to introduce contaminants into the coating through entrained oils etc that should be removed through the separators normally found in an air compression system. Unfortunately, this is not always the case, generally due to lack of frequent PM on the compressor, ie blowing down the separator system.
If you must switch to air due to economics, please consider using a dedicated cylinder of what is called "5.0" compressed air for your system. It is designed for use in chromatograph systems and is extremely dry and otherwise contaminant-free.
Pierce Pillon is the Laboratory Manager and lead formulations chemist at Techspray, a division of Illinois Tool Works (ITW) and a leading manufacturer of chemical products for the electronics industry.
In general we have not seen many conformal coat applications that benefit from nitrogen.
So, in general, it is probably without risk to stop using nitrogen.
But let's throw in the following caveats and diligence:
- Check with the vendor of the conformal coat material. They are really the bottom line on what type of atmosphere is best for their material. If they say ok to use air, that carries 1000 times more weight than what any other "expert" might be telling you.
- Try a few samples and test the results. Data rules! So before you turn off the main Nitrogen valve on the production line try a few sample batches and have your customer or your quality people confirm that this still provides a satisfactory result.
- Re-confirm the thermal profile of the curing oven. Some ovens have different thermal performance when running nitrogen vs. air. So it is a good idea to run a profile board just to make sure that you are getting the same temperature at board level with and without the nitrogen running.
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.
You may use air as you have a good filtering and drying system.
Senior Manufacturing Engineer
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.
Nitrogen is typically used to pressurize conformal coatingtanks when the material is moisture-sensitive and the factory air supply is notdry enough. The level of sensitivity to moisture varies by material and themoisture level found in house air will vary from factory to factory. Withmoisture-sensitive fluids, using moist air can cause buildup inside the fluidsupply lines. The build-up causes flow to decrease and introduces gelledmaterial particles that may enter the fluid stream and clog the applicator. Themore sensitive the material is to moisture, the more significant theconsequences.
Nordson ASYMTEK Conformal Coating Applications Team
Bottled nitrogen is typically very dry and thus offers aquick fix in these situations. You can add dryers to the air, however, that istypically not as effective as using the dry nitrogen. An alternative solutionis the use of "bladder-bags" - these are air-tight bags in which the fluidsupplier packages coating materials, eliminating direct contact withpotentially moist air. You would need to request this packaging from your fluidsupplier and be sure your conformal coating system will handle the bags.