Hello Lance et al,
I am not sure what type of spray system you are using, is it selective
coating atomised or non-atomised spray application ? Or is it hand
spraying?
If hand spraying, the way to avoid this would be to apply light coats,
building up the film thickness with 2 or 3 coats typically, applying
each additional one after the previous one is touch dry. This will give
the advantage of ensuring you get optimum coverage of coating across the
entire board. I would also recommend spraying the boards flat, using 4
"passes of coating" to make 1 coat......a pass can be described as
applying light overlapping stripes of coating, working your way down the
board, as though you are writing across and down the page, then turning
the board through 90degrees to add another pass and so on.
One of the key factors to give maximum performance of the coating is to
ensure you have full coverage across the board, if it is slightly uneven
in thickness particularly around components will not be critical, so
long as the UV inspection shows good coverage and the coating is of a
sensible thickness as we recommend (25-75microns DFT.) Aesthetics of the
final coating, to some customers is paramount, although of course
functionality and performance have to be primary considerations.
With coatings, the pull back, can be caused by reticulation or
de-wetting of coating from the surface due to surface contaminants
and/or capillary action of the wet coating. If it is capillary action
into narrow geometry voids eg. On leaded SMT IC's then thinner coatings
applied will minimise this because drying times will be reduced, which
in turn are influenced by the temperature and humidity conditions.
I hope this helps as a start.
If you would like to contact me off-line I am on
[log in to unmask]
Leigh Jansen (Mr)
Technical Support Manager
Concoat Limited - Engineering Electronics Reliability
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Douthit
Sent: 10 June 2004 06:26
To: [log in to unmask]
Subject: Re: [TN] Conformal Coating
Lance,
This a fairly common issue. The viscosity of the coating as well as the
temperature and humidity
are critical. The simple answer is to make multiple light spray passes
at different angles,
letting the coating dry past tackiness between passes. Be sure you do
not exceed a 3 mil thickness (cracking in extremely cold conditions
becomes an issue).
Contact the folks at Humiseal. They will be more than happy to assist
you with details.
The "pull back" is due to evaporation of the solvents/thinners.
David A. Douthit
Manager
LoCan LLC
Lance Mayes wrote:
>Hello again Techies;
>
>We are having issues with our conformal coating on some of our
components.
> It pulls back from some of the components leads along the edges.
>We are wondering if this will make a difference in the protection of
the
>board.
>We're using Humiseal Acrylic 1B31 and Humiseal Thinner 521 in a
mixture.
>The conformal machine sprays a nice coating and then the coating starts
to
>recede. Sometimes this happens even when we paint the acrylic
manually.
>Has anyone else ran into this problem, and what to do to overcome it?
>
>This is a good one for the end of the day. I'll be waiting.
>
>Thanks again
>
>
> P. Lance Mayes
>
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