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March 2013

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Subject:
From:
Phil Kinner <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Phil Kinner <[log in to unmask]>
Date:
Tue, 26 Mar 2013 17:59:16 +0100
Content-Type:
text/plain
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text/plain (134 lines)
I've heard lots on my time Lee. The only reasons that resonated with me were:

1. To overcome moisture uptake of the boards between cleaning and coating if significant delay between 2 processes. High levels of moisture can create havoc with certain solvent-based materials developing their adhesion, or lead to blushing, where coating becomes cloudy, hazy or milky in appearance. Can lead to bubbling with moisture curing mechanisms occurring at a rate greater than the permeation of by products, eg CO2 thru the film.

2. To help prevent excessive capillary flow or dripping on inversion with low viscosity solvent-based materials, theory being drive solvent-off quicker, build viscosity faster. This works well with primer or 'dusting' layers, less well with traditional cc process.

I don't see this 2nd reason cited much any more.  Sure I missed some reasons though...

Best Regards,

Phil
+44 7703 795422

Sent from my iPhone

On 26 Mar 2013, at 17:39, lee Hitchens <[log in to unmask]> wrote:

> Can I also ask a question. What is the purpose of baking a PCB before
> coating, assuming that the cleaning process drying section is adequate? I
> have never really got a good answer to this?
> 
> Best Regards
> 
> Lee Hitchens
> Technical Director
> 
> SCH Technologies is a Division of SCH Services Limited
> Mob: +44 (0) 7862 25 33 21 www.conformalcoating.co.uk
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> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Fox, Ian
> Sent: 26 March 2013 16:17
> To: [log in to unmask]
> Subject: Re: [TN] Conformal Coat and Board Edges
> 
> No we don't. Since the purpose of the coating is not to prevent moisture
> ingress into the board (we all know that happens), but really to prevent
> issues with condensation, and since there likely will be no exposed tracks
> on the edge, there's no point. We typically have a ground/thermal strip
> around the edge of the card anyway so the masking may sometimes cover, or
> partially cover the edge. I couldn't tell you how many millions of on wing
> hours our assemblies have accumulated over the past 30+ years, but we've
> never had an issue in service as a result of uncoated edges, nor in the
> umpteen unit qualifications we've carried out. In fact if you carry out
> weight gain measurements with high (85%+) humidity exposure the laminate is
> typically saturated after about 4 days and we always run the 10 day tropical
> exposure test
> 
> Regards
> 
> Ian Fox
> Lead Materials and process Engineer
> 
> Aero Engine Controls
> York Road, Hall Green, Birmingham B28 8LN | Tel: +44 (0)121 607 3763 | Fax:
> +44 (0)121 607 3975
> Email: [log in to unmask] | Web: www.aeroenginecontrols.com
> 
> Aero Engine Controls is part of the Rolls-Royce Group
> 
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
> Sent: 26 March 2013 15:43
> To: [log in to unmask]
> Subject: [TN] Conformal Coat and Board Edges
> 
> OK, the conformal coat question of the day:
> 
> Do you conformally coat the edges of a printed circuit assembly?  Why or why
> not?
> 
> OK, that is two questions, or perhaps a half fractional question.....
> 
> Doug Pauls
> 
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