Prior to conformal coating, how long can we keep assemblies in an open condition before they will be impacted by moisture absorption? Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis.
He has vast experience in SMT equipment, materials and processes.
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics.
He is a pioneer in the science of reflow.
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, Pick and Place here, to answer your technical and philosophical questions about surface mount processes, materials, equipment and what not.
Today we have a question for you, Mr. Conformal Coating. "Prior to conformal coating, how long can we keep a circuit board assembly in an open condition before they will be impacted by moisture absorption?"
So I assume that they're saying, "My board is completed, soldering and everything else. I want to conformal coat it. Do I have to be concerned about the circuit board or any of the materials absorbing atmospheric moisture over time?"
Based on my understanding, the most critical thing prior to applying conformal coating is to ensure the board is dry and that's one of the reasons we look very carefully at the drying section on cleaners.
As far as talking about absorbed moisture, I am not aware of any conditions or prerequisites in terms of a requirement to pre-bake boards prior to application of conformal coating assuming the surface is dry.
There may be a misconception here that the conformal coating prevents moisture absorption and that you've got to apply it while the board is dry so you get that coating on or that moisture will ultimately get into the board. With most conformal coating, that is a misconception.
Very few, if any, of the conformal coating processes will give you a long term, hermetic seal on a board. Yes, they will slow down moisture absorption, but you take a conformally coated board, put it out in an atmosphere and leave it there for weeks, months or a year and it is slowly going to absorb moisture as it diffuses through the coating.
I'm not that familiar with parylene and some of the more exotic coatings, but certainly for acrylics and silicone's, they do not prevent moisture absorption. The moisture will diffuse through the coating over time and eventually set off the electrolytic cell and get you dendrites and other things.
So if that is the question that's being asked, I think the answer is, don't worry about it, because you're ultimately going to absorb moisture over the long term anyway, even before or after coating.
Our conformal coating buddies will weigh in on this and label us a bunch of bozo's which a number of people do anyway.
And beyond that, no matter who made the joints - whether it's Jim Hall or Phil Zarrow, whatever you do ...
Don't solder like my brother.
Don't solder like my brother.
And Bob Diamond has you nailed, Dick. Well and accurately stated Bob. It is not a hermetic seal ! And moisture does, indeed, ultimately ingress as Jim stated. Conformal coating slows it down. And having boards come out of the cleaner dry is a best practice- whether you're going to conform ally coat ( with additional baking) or not.
Walt Bishop, MIT
Using the words "hermetic seal" relative to conformal coating is not correct. Hermetic is very specifically defined as a metal-to-metal or glass-to-metal seal with Helium leak rates down in the 1E-8 cc/sec range. Materials such as conformal coatings and epoxy coatings do NOT provide a hermetic seal. There will always be moisture migration through seals of this nature. The question is at what rate? That depends on many variables.
Bob Diamond, L3 Technologies, Narda-MITEQ Division
Barry Ritchie has it nailed. Surface moisture is the enemy of conformal coating, but absorbed moisture is part of that. In order to form a good adhesive bond to the PWB, it must not only be free of contamination, it must also be very dry. If the boards are left sitting around in a somewhat humid environment such as >40% RH for any length of time (lets say more than 6 hours), the adhesive bond of the conformal coating can be greatly reduced and it will be easier for weak spots to form, allowing moisture into the PWB. On the other hand, a good cleaning process, followed by at least a 4 hour bake at 105 deg. C., followed by coating within 2 or 3 hours after that, usually provides a good durable bond of the coating to the CCA. If that is achieved, the boards WILL remain hermetically sealed for a long period of time, dependent on the coating type and other factors. The drying section of a wash machine will NOT provide a sufficient moisture removal process, unless perhaps the conveyor is approximately 400 yards long.
Richard Stadem, General Dynamics, USA
Boards should be typically baked out prior to conformal coating to drive off any ambient moisture that has been absorbed into the substrate and/or components during storage. The comments about moisture slowly absorbing through the conformal coating over time is inaccurate. Providing there is adequate adhesion and the coating has been applied to a clean and dry surface to begin with, moisture should not ingress to the interface of the coating and the PCB, thus prohibiting the mixing with residual ionic species and causing a corrosion cell. The coating layer itself can and will absorb some moisture but that moisture should not get past to the board substrate interface providing the conformal coating has adequate adhesion. This topic is well covered in the IPC-HDBK-830.
Barry Ritchie, Dow Corning, USA
Other factors to consider when determining the amount of time between processing and coating are the ambient conditions that vary from region to region. My location in Melbourne, Florida is much different than our facilities in Cedar Rapids, Iowa, especially during this time of year. In the winter months, Florida has a relative humidity of +/- 40%, while Iowa is generally < 20%. In the Summer, when we have RH > 60%, we can experience more potential problems, especially when related to drying times and moisture entrapment under components. Parylene is a whole other ballgame, but is mostly un-reworkable, so we only use it in absolutely critical applications.
Michael Marzec, Rockwell Collins, Inc, USA
Assemblies may acquire moisture on the surface, but that's not absorption. If moisture absorption is going to occur post-assembly, it is most likely to occur along the PCB edges -- not the surface. The most common PCB substrate is fiberglass reinforced; the fiberglass bundles terminate at the PCB edges and act as a conduit for moisture. So, unless you have conformal coating sealing the PCB edges, it won't matter when you conformal coat for purposes of mitigating moisture absorption. But if you really want to get a handle on moisture and it's effects on PCBs and PCAs, check-out the IPC-1601.
Bob Lazzara, Circuit Connect, Inc., USA
To answer that question you have to know first how long moisture gets absorbed by a dry board at RT condition. To determine this you weigh a relatively dry board. A "dry" board can be achieved (or close to it) by placing it in the stand alone oven at 70 - 80 C for about 1 to 1.5 hour. Weigh the board on an accurate digital scale immediately. Then leave the board at RT condition but weighing it every hour; the weight would go up due to moisture absorption. Do this until the weight becomes stable to 0.10 to 0.05gm variations. My experiment run into about 6 to 8 hours. This is the time when moisture stabilizes the board. Your criteria should be 3/4 of this many hours that you should do conformal coating. Any more than this, your board would have acclimated with the moisture in your area.
Tomas Geronimo, DAS Product & CC Process Engineer, Sanmina-SCI