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Reply-To: "Fred Bartoli"
From: "Fred Bartoli"
Subject: Re: SMT boards: assembly and potting Q
Date: Wed, 25 Dec 2002 18:31:49 +0100
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Organization: Guest of ProXad - France
NNTP-Posting-Date: 25 Dec 2002 19:15:19 MET
"Robert Baer" a écrit dans le message news:
> I am hand assembling surface mount parts on a PCB.
> It is not easy to get chip resistors to align nicely, and SOT-23 parts
> are the worst.
> 1) What is the best way to keep alignment of these parts for hand
> 2) If hand soldering is not recommended, what is the least expensive way
> to assemble these parts at home (ie what do i get and where)?
> When completed, i would like to pot them, but need to keep ions and
> moisture, as these units are for voltages up to 2KV.
> I also am worried about mechanical stresses due to temperature cycling
> from (say) -55C to 150C.
> I think a conformal coating first may be best.
> 3) What is the best potting compound and method to use?
the site fab house that mention use of a toaster oven is www.pcbexpress.com
I recently came across and found the idea and the performances interesting
as I'm balancing about using BGAs for homebrewing.
They have "selected" 3 or 4 toaster ovens and as far as I understand the
important thing is that they have the power resistors on the bottom side of
the oven. This way the IR heat the PCB and then the solder past. This ensure
a good temperature uniformity across all the PCB's surface (even under the
component package) and a good temperature control.
To ensure an optimum soldering quality, i.e. follow as closely as possible
the recommended temperature profile, you could one or several thermocouple
on a dummy board and make some measures to optimize the process.
There's still the problem of solder past dispensing which is normally done
by printing through a laser cut steal stencil. That's of course out of
question for personnal use (cost).
Solder past dispensing can be done with a small syringe for the
resistors/caps/sot23... small packages, even maybe PLCCs. For the fine pitch
QFPs or BGAs the solder past quantity have to be well controled so I don't
think it can be done that way.
Maybe someone solve that problem or have experience with this for unitary
Oh, also don't use the oven you have in you kitchen but rather one you
dedicate to that use because of the presence of lead (and maybe other
nastiness) in the solder past.
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