I had the same problem and addressed it on the smtnet.com site. I was using
a paste mesh that is suitable for screening, not dispensing (325/500 mesh).
I figured the only bad thing that would occur would be that the paste
wouldn't flow as easily through my nozzle. After purging the pump to remove
all air bubbles, the flow would stop and I would disassemble the pump and
find a hard, shiny chunk of solder at the end of the pump, before the
nozzle, impeding the flow, that appeared reflowed. My guess, coupled with
knowledgeable persons agreeing, was that the mesh indicates a larger ball
size and the "resistance" of the balls to flow through the nozzle was
causing great friction inside the pump, compounding the balls together and
possibly the constant friction was causing the backing-up solder to, in
essence, "flow". Check your paste mesh. BTW, I am using a Camalot 5000 with
an Archimedes-style pump. Hope some of this helps.

> -----Original Message-----
> From: [log in to unmask]
> [SMTP:[log in to unmask]]
> Sent: Friday, August 13, 1999 8:41 AM
> To:   [log in to unmask]
> Subject:      [TN] clogging dispensers
>
> Out of 1,700 netters, someone should have an idea about this:
>
> We need expertise advice immediately to solve a mystery with clogging
> high speed automatic dispensers. Three silver epoxy pastes give same
> clogging. Fresh pastes are bought. Same. Dispenser screws, tubes, pumps,
> all details are cleaned thoroughly. Same. New dispenser parts installed.
> Same. Whatever we do. Same. After running some minutes-hours: clogging
> in the needle. Some say that the room humidity must be very exact in
> order to make the dispenser epedize well. Others talk about local curing
> inside the paste. And some think sedimentation can be the plague.
> Experts from the paste manufacturer will be here in one or two days, but
> according to my experience, they seldom have that kind of knowhow,
> machine makers can have, this one not. Most oftenly, the knowhowers are
> people like you, dear nett(l)ers.
>
> I have my own theory, but can't prove it yet. Namely, that this
> "conglomerate" or mix of fluids and silver flaws (1-50um) can be
> PRESSURE DEPENDENT, i.e. at certain constant or momentary pressure,
> there will be a separation in the narrowest part, the needle, fluids
> will drop out and separate, leaving the coarser silver flaws without
> fluency, they will clog. Also, I have seen that the inside of the needle
> (inner dia 100um) is not polished, but has a rather grainy stucture,
> likwise the rotating helix that feeds paste. Both should be polished, I
> suppose. There is a science that use morphology and fluiding dynamics
> that possibly can give a clue, but I'm rather far from that art, maybe
> some of you have education of the sort.
>
> Familiar to someone?
>
> Remo@Alenia, if you read this, please contact me, I have lost your
> adress.
>
> Thanks in advance/
> Ingemar Hernefjord
> Ericsson Microwaves Systems
>
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