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| Date: | Mon, 30 Apr 2007 08:00:52 -0500 |
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Hi Joel! I can pass on this experience with the use of excess conformal
coating. I have witnessed design teams mandate the use of conformal
coating in excess of 5 mils due to humidity qualification testing
failures. The conformal coating becomes a "potting material" rather than a
use environment coating. Most conformal coating materials are not designed
to functional as potting materials and the typical result is a loss of
solder joint integrity due to a conformal coating CTE impact. A phase that
Doug Pauls uses to emphasis this misapplication is "the bigger the glob,
the better the job" - bigger is not always better.
Dave Hillman
Rockwell Collins
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Joel Alexander <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
04/30/2007 07:49 AM
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Re: [TN] bubbles in conformal coating
Please humor my question regarding coating thickness.
Besides adding weight to the end product, what is the significance of
listing a maximum thickness to conformal coating? Why would 0.010 be
less reliable then the maximum thickness of 0.005?
Joel Alexander
Quality Assurance Manager
TT APSCO, INC.
Edward Mines <[log in to unmask]>
04/29/2007 12:54 PM
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Re: [TN] bubbles in conformal coating
Joel-
The thickness you are using is far greater rthan the maximum coating
thickness called out by MIL-I-46058 & IPC-CC-830. Some customers would
reject the coating for that reason alone.
Contact Dymax.
Ed
[log in to unmask] wrote:
We are using Dynamax Multi-Cure 984LVUF and it is about 10 mils thick on
the flat section. I know this sounds thick but the machine drop dispenses
so the coating will not run into keep our areas.
Joel Alexander
Quality Assurance Manager
TT APSCO, INC.
Edward Mines <[log in to unmask]>
04/27/2007 10:17 AM
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Re: [TN] bubbles in conformal coating
Joel-
You're right. 830 (the son of MIL -I-46058) is used by manufacturers to
qualify the material. As a former employee of a manufacturer I know this
document very well.
I am less familiar with IPC-A-610.
46058 used to be on the internet. The versions of IPC-CC-830 that I am
familiar with (there may be a newer one) followed the same format as
46058.
Near the very beginning in MIL-I-......it states something like "the
coating must be free from etc." Our interpretation was that this applied
to the users too.That MIL SPEC is vague on many other things like
thickness. Our interpretation was that the thickness the SPEC called out
was on flat area of the board.
I think foam is mentioned in the statement "the coating must be free
from....." I know of many instances where coaters did things that weren't
specifically prohibited that would adversely effect coating performance.
Which coating are you using? Is the thickness on flat sections within the
1-3 mil range?
Some HumiSeal polyurethane coatings do not cure properly if they are too
thick. To the best of my knowledge no document says you have to follow the
manufacturers instructions but if the don't the coating may not provide
the protection for which the coating was designed.
Ed
[log in to unmask] wrote:
IPC-CC-830 is a standard used by the manufacturer of the conformal coating
to qualify their material. Can these also be applied to the application by
the end user. IPC-A-610 states that bubbles are acceptable as long as they
do not bridge adjacent conductors.
Also is there a specific reference that states anything about the
appearance of foam?
Joel Alexander
Quality Assurance Manager
TT APSCO, INC.
Edward Mines <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
04/25/2007 10:26 AM
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[TN] bubbles in conformal coating
MIL-I-46058 & now IPC-CC-830 say conformal coatings should be "bubble
free". Some have interpreted this as meaning that a few small bubbles away
from components are acceptable.
Foam is not acceptable.
Some polyurethane conformal coatings generate gas as they cure. They
should not be coated on very humid days.
Bubbles are also apt to form in conformal coatings when the coating is too
thick. I've seen this happen in between closely spaced tall components
even when the coating on most of the board is within customary limits and
has no bubbles. The solution was to change the application technique.
Get the coating and equipment manufacturers involved. They want your
future business.
Ed Mines
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