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March 2002

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From:
"d. terstegge" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 28 Mar 2002 09:33:29 +0100
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Hi Edward,

An answer to your question about using adhesives: yes, when there are heavy components going through "inverted reflow" we apply a few gluedots to the corners of these parts. We use the same Hereaus glue that we use for wave soldering of smd's, it is applied after soldering and cleaning of the first side. 
Instead of using an additional curing step we let the adhesive harden in the preheat-zones of the reflow oven, so that it is cured before the solder melts.
It works, but I still see it as a makeshift for badly designed boards, and it should not be standard practice.

Daan Terstegge
SMT Centre
Thales Communications
Unclassified mail
Personal Website: http://www.smtinfo.net


>>> "Edward S. Wheeler" <[log in to unmask]> 03/26 9:14 pm >>>
Hello Tech-Net users! I am new to the group here, and would like your
assistance on a couple of matters concerning a double sided reflow process
we are undertaking. We are new to this process, so we would appreciate any
expertise on the matter.

We are using an OA type solder paste, and like to wash our boards within 2
hours, 4 being the maximum in house limit. To perform our double sided
reflow process in mass production, we may want to run all day on one side
and do the other side the next. My question is... does any of the residual
flux material aid the second reflow process, or can we wash it off before
the second reflow and get as good of results? I am concerned that the
solder joint may have an outer layer of oxidation or graininess due to the
lack of flux once it reflows again. Is there any flux left over that is
active, or is it simply the remainder of the carrier?

What luck are people having with BGAs populated on the bottom side? Is
there more of a concern with warp on the second pass?

Is there a rule of thumb for the pad surface area / part weight ratio to
maintain enough surface tension to correctly hold the part?

Does anybody use adhesives to stake heavy parts on the first pass?

Does anybody have a solid rule of thumb for pin-in-paste apertures? On our
first try we used an 8 mil step up and had the aperture the same size as
outer edge of the PTH. Would an oversize print work better?

Thanks in advance for any help.

Ed

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