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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: SMT boards: assembly and potting Q
References: <3E07D4B7.BA697DF5@earthlink.net> <email@example.com>
Date: Thu, 26 Dec 2002 03:17:15 GMT
NNTP-Posting-Date: Wed, 25 Dec 2002 19:17:15 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Fred Bartoli wrote:
> "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
> > soldering?
> > 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
> quantities ?
> 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.
1) I would worry about temperature uniformity and hot spots, unless the
air was stirred, or a metal plate was placed between the heater below
and the rack/PCB. The plate makes the heating more uniform and fets rid
of heat plumes from the heating element.
2) One does *not* need to laser-cut anything, much less steel for
screening. Hundreds of thousands of people have been making cards,
artwork, putting logos on panels, and making PCBs (*note* NO apostrophe,
NO ownership!) via the use of SILK screens. And if one needs to re-use a
silk screen, NaOH or KOH works well to remove even the nastiest of
resists and the silk screen is only slightly weakened. Many resists can
be removed with less harsh materials.
3) The term is "paste", not "past".
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