I 've read all the posts now on this and the idea to use dispense cartridges is good.
However I find myself at slight odds with other netties so here is my more orthodox purist
viewpoint...

Start from the premise that the important thing in any continuing production operation is to
eliminate unknown and uncontrolled variables, to be in control in other words.

The best way to control paste is to buy it in containers that are no bigger than the amount of
material you will use in one work period.

That way you don't have anything to put back in the fridge .... All variations and worries about
how many times have I used this stuff, how long has it been out are thus eliminated in one go.

The problem for small users is that this is likely to be less than the amount needed to load the
printer. So you need some rules and reasons for rules:

Refrigerated paste should be allowed to come to ambient before use and  certainly even before
exposing material to air.
Reason: Cold paste has a different rheology to warm paste.
Cold paste will suffer from condensation on the stencil and in the jar.
A stencil printer is designed to expose the maximum area of paste to the air to help this
process and then to thoroughly mix it and then expose some more paste and so on.  (You think of
this process as printing.) Flux activators are probably more soluble in water and probably more
active as well. (Any paste formulators still reading: I know this is a massive generalisation
and yours is probably OK, it's the others I'm referring to).
So the effect of this on a production basis is that the rheology of the paste will change
immediately after the start of a new work period and each time you replenish your paste.
An uncontrolled variable.

The chemistry is changing too. The effect of the water is most likely to use up the activator
and maybe provoke a reaction in the paste that will send the viscosity up. plus the water is
diluting the
paste (viscosity down)and you get a little solvent loss as well [viscosity up], overall:
rheology tack etc now gone... sideways??
So, the first time you use paste with added humidity its probably OK, but if you keep on adding
humidity you are no longer using the paste you approved and  bought. Another uncontrolled
variable. [If it was better to add a little water then your paste supplier would already have
done it for you.] .

If you keep recycling the paste these effects will become significant from shift to shift not
just within shifts.

So the rules stand:
Don't use paste straight from the fridge
Don't open containers till paste has come to equilibrium. (This could be as much as 10 hours for
a 500gm jar.)
Don't return old paste to part used jars (Put it in an empty jar marked used 1x, used 2x ... for
recycle or whatever).
Buy small containers.

On a pragmatic basis you can decide how far you can go from these absolutes depending on the
quality of your staff, how much discretion you can allow. Bearing in mind the relative costs of
solder paste to boards components, to rectification costs and risks  and so on I would suggest
not much. Why give yourself the hassle?


Mike

----- Original Message -----
From: West, Jim <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, February 23, 2000 9:28 PM
Subject: [TN] Solder paste refrigerated after the container is opened.


Hi Techneters,

This questions is mostly geared towards the small manufacturers that does
not use that much paste, but all comments are welcomed.  Most manufactures
recommend you store your paste in a refrigerator and keep the temperature at
32-50°F.  We do this, but since we do not use that much paste, we will open
a jar and apply the required amount of paste on our stencil.  Our biggest
production amount only requires us to apply paste to our stencil maybe twice
or three times a day.  I was told by an outside source that once we open the
jar of paste we can not put it back in the refrigerator.  One reason they
said was we are exposing the paste to the air and once this happens you need
to use the paste ASAP.  That's a problem for us since we do not use that
much to begin with.  Currently, we will open the jar and take what we need
and then put the jar back into the refrigerator, it may be two days before
we need to use the paste again.  How do the small shops handle their paste?

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