In addition to Graham's comments on Ed's comments on someone elses'
comments, I would like to comment.
If you are using dip coating, the board really needs to be designed with
dip coating in mind or masking becomes a nightmare. Connectors in the
middle of boards, unclad areas, uncoated RF sections running all over all
make aerospace boards incompatible or at least very difficult for dip
coating.
Doug Pauls
Graham Naisbitt
<Graham.Naisbitt@
GEN3SYSTEMS.COM> To
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12/01/2006 05:19 Re: [TN] conformal coatings as
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Graham Naisbitt
<Graham.Naisbitt@
GEN3SYSTEMS.COM>
I cannot let Ed's message pass without comment in response.
First up, conformal coatings are to be at a thickness of 50 microns
±25 (0.002 ±0.001 inch (MILS if you must)) on a flat unencumbered
surface. Of course it will be thicker around the component and
thinner on the leads.
Dip coating by hand will NEVER be able to achieve such a thin film.
Faster = thicker. Plus, you will have the problem of runs and the
infamous wedge effect that are specifically regarded as "non-
acceptable" under IPC-A610.
Having manufactured coating equipment for over 25 years, and run a
coating service, I have never encountered such a phenomenon as Ed
described unless the wrong solvent was employed.
Immersion should be slow enough to allow air that may be trapped
under the components to be displaced by the coating. Of course the
surface tension of the coating will have a controlling influence on
the distance it is able to penetrate especially in low stand-off
areas. The withdrawal speed should be ideally slower than the cascade
effect of the coating material and is generally in the range of 100mm
to 150mm (4 to 6 inches) per minute. Try doing that by hand! By this
means you can minimise the wedge effect and, at least with air drying
coatings, minimise runs and drips as the coating is drying as it is
withdrawn.
Dip coating provides the benefit of applying the coating to both
sides simultaneously.
Of course masking is the major drawback and can only be avoided if
the components that should not be coated can be placed on 1 side of
the assembly.
Ed is right to point everyone in the direction of the IPC-HDBK830, a
lot of effort was put into that document from a significant number of
aerospace engineers and coating specialists.
Graham Naisbitt
BTW Dewey, Planet Zog!
[log in to unmask]
On 24 Nov 2006, at 16:45, Edward Mines wrote:
> Mr. Naisbitt is correct that machine dipping PC boards in diluted
> conformal coating will get the coating under every component.
>
> The performance of the machines that he sells compared to dipping
> boards in by hand surprised me and I had a lot of experience in
> chemical engineering pilot plants prior to working with conformal
> coatings. The slow speed of the machines (dipping inches per
> minute) causes many small bubbles to form that are not formed when
> dipping by hand. Slow withdrawal keeps these tiny bubbles on the
> surface of the liquid rather than the surface of the board.
>
> As mentioned in an earlier message, solvent borne coatings (types
> AR, UR & even some silicones) loose considerable volume as the
> solvent evaporates so the cavity beneath BGAs will have a
> significant gas space.
>
> I have seen several instances where these "air" spaces blew out
> between wires when the board was baked (the solvent comes out
> quickly and the gas is expands). If the board is not baked solvent
> will slowly diffuse through the cured coating until there is no
> more solvent.
>
> Oxygen or moisture is necessary to cure one part polyurethane
> conformal coatings and most silicone conformal coatings. Diffusion
> of air through cured conformal coatings is slow so it's likely that
> it will take a long time for these coatings to completely cure, if
> ever. Some heat curing silicone coatings rely on the release of a
> reaction inhibitor when heated; these may not cure completely or
> even cure at all in thick cross section (the distance between the
> center of a BGA and fresh air is much greater than the maximum
> conformal coating thickness called out by IPC-CC-830).
>
> These issues should be addressed in both IPC-CC-830 and the IPC
> conformal coating handbook.
>
> Ed Mines
>
>
> ---------------------------------
>
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