In a past life, roughly co-existent with Tyrannosaurus rex, I did away
with jars altogether and thereby solved the problem. I used 1 kg Semco
cartridges of the stuff, in conjunction with an ordinary
electropneumatic dispenser (Glenmarc). The cartridge was fitted with a
100 cm plastic nozzle, which was cut diagonally so that the hole was
about 1.5 mm diameter. A slip-in gantry was fitted across the printer,
with a timer belt and motor, with limit switches, to move the cartridge
to and fro across the stencil. A simple footswitch pressure started the
cycle of movement and applying pneumatic pressure to the piston. This
spewed a thick thread of paste onto the stencil. To start off a clean
stencil, it required 4 or 5 presses and then 1 press every now and then,
as the paste got used up. This was particularly useful, because the user
had four different kinds of paste ("no-clean", RMA and two types of
water-soluble) and had to make frequent changes between batches. To slip
out a cartridge and put in another took about 30 seconds and the old one
was simply capped at both ends for re-use, even several days later (some
even went back into the fridge). By frequent recharging of small,
metered quantities of paste on the stencil, wastage was minimised and
the operators could easily judge when to hit that footswitch. There was
an added advantage (remember, this was when some pastes had a very short
open time): the quality of the printed paste was absolutely uniform.
I felt this was Columbus' egg and I mentioned it in a couple of
technical papers in the 1980s, but nobody seemed interested in copying
the idea.
To make sure that Glenmarc were still there, I checked at
http://www.glenmarc.com/products/1_dispensesys/1a.htm
The PD-2000 seems to be the modern equivalent of the dispenser I used
and the Semco cartridge holder is visible as the tall, black thing on a
stand at the back of the Optional Accessories photo. I see these guys
are now offering custom automation systems, so they may be able to
provide a turnkey system. The filled Semco cartridges came from
Multicore (I don't know whether they still do them under their Loctite
banner).
Hope this helps.
Brian
Steve Gregory wrote:
> Hi all!
>
> Hope everybody had a good thanksgiving holiday (our USA members anyway),
> but have a question about solderpaste life.
>
> We are a low-volume, high mix shop, and we may run 5-20 assemblies, then
> switch over to another assembly. The solderpaste between these
> switch-overs may change from a high activity OA to a low activity
> no-clean depending on the requirements called out on the assembly drawing.
>
> In the past, we have chosen the path, that once solderpaste is opened,
> it is either used, or scrapped. But looking closely at this policy,
> we're throwing a lot of paste away, and maybe we shouldn't.
>
> It's no small change, when I've looked at how much we throw away...just
> wondering now, and I understand the issues of using very old
> solderpaste, and mixing fresh with new, but does anybody have any
> guidelines of when they throw paste away, or put it back in the jar to
> reuse?
>
> I'm about to write some procedures concerning this. I don't think that
> once the jar, or container is opened, it needs to be consumed all at
> once, or scrapped. I do think solderpaste is a bit more robust than that
> (but you might have a hard time for the vendors to say
> that...hehehe....sorry Mike!). I do think that if good control is
> maintained, solderpaste can perform as advertised, whether it has been
> opened previously or not...
>
> Just curious if anybody else has encountered this issue...and adressed it.
>
> -Steve Gregory- ---------------------------------------------------
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