Interesting stuff.
Modern solder pastes are much less delicate than their predecessors from
which we have formative memories on what can or can't be done, hence the
longer times which people are finding and being surprised at.
Paste manufacturers do indeed do stress tests on pastes in accordance with
IPC etc, but they are somewhat artificial, they provide a useful comparison
rather than absolute values. Actual usable life will vary according to
criticality of application and actual temperatures/humidities on shop floor.
These will be different at different sites, probably different from a nice
lab. (as required in the statutory tests).
So at risk of singing to choir, I would just caution:
There is a difference between what is possible in a controlled trial and
what could happen on the shop floor.
In a controlled trial you generally just look at one thing, in use the paste
will have many different stresses simultaneously.
You need to write SOPs which are simple and easy to follow with minimal
discretion.
So do not return paste to original jar is easy to follow.
Do not return paste to original jar except if it is a test paste [or some
other condition] is not so clear.
You also need to clearly define when your count starts, when you first open
the jar is easy to understand. So the clock counts for all opened jars no
matter what happens.
Allow for Sod's Law (Murphy's Law). How confident are you can track which
jars have been opened for how long in say a 20 day period? During the
working year that will take in a number of long weekends. The longer the
paste is out the more likely Sod's/Murphy's has to bite you.
No one likes unnecessary waste, but keep in mind in all this that solder is
likely to be <0.1% of your costs. You can't save a lot, but you could risk a
lot.
Regards
Mike
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, September 27, 2011 9:32 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste Shelf Life Extension
You know, I never saw any degradation, even after 20 days. We just stopped
after 20 days because we were really looking to validate being able to leave
the cartridges out over a 3 or 4-day weekend. The viscosity did creep up
over the 20 days, but did not exceed the viscosity range on the TDS. Did not
even come close.
-----Original Message-----
From: Guy Ramsey [mailto:[log in to unmask]]
Sent: Tuesday, September 27, 2011 2:11 PM
To: 'TechNet E-Mail Forum'; Stadem, Richard D.
Subject: RE: [TN] Solder Paste Shelf Life Extension
Very interesting. At what point in time did you see measurable changes in
performance?
What was the first property to fail? Print, hot slump, or wetting?
We did some similar testing and found the results to be type dependent. Type
3 performing better than Type 4, with different properties failing first.
Guy
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, September 27, 2011 3:00 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste Shelf Life Extension
I can tell you that some major military and avionics manufacturers and CEMs
use a ten-day limit on paste removed from refrigeration, as long as it is in
sealed tubes, and the tubes are kept re-sealed immediately after dispensing
to the printer, and the tubes are not re-refrigerated after initial removal
from the fridge.
This was done based on qualification results of the individual paste
candidates, which was then added to the original solder paste qualification
results. Paste was tested for viscosity, wetting, slumping, etc., etc.,
every day for a period of 20 days, without seeing any degradation of paste
or increase in DPMO (in fact, it improved!). Then the agreed-upon pot life
was agreed to be 1/2 the qualified time, to err on the side of caution, and
that was 10 days. This has been working extremely well for at least 4 years
now.
I have done that for three different pastes, from two paste manufacturers.
All were water soluble, standard Sn 63.
I would NOT allow that to be done with jars of pastes, only paste received
in sealed Semco cartridges. Once you open a jar of paste, you should never
return leftover paste to the jar. For jars, a 3-day pot life might very well
be as long as I dared to use it. The amount of exposure and the inability to
seal a jar very well are the reasons I do not use paste in jar form if I can
possibly help it. Too many bad experiences with operators leaving jars open,
mixing one alloy with another, mixing different flux types, partially
closing the cover without putting the internal air purger back in, etc.,
etc. None of those things is very likely to happen if the paste is in a
sealed Semco cartridge.
In order to cause no harm, I am not advocating this for anyone else unless
you perform the qualification for your paste and your printing processes.
But it is a very good thing to undertake as you will learn a lot about how
your paste and your handling processes hold up under worst-case scenarios.
You should do this in July or August under high humidity conditions (in
North America anyway) if possible.
dean
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Amol Kane
Sent: Tuesday, September 27, 2011 11:51 AM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste Shelf Life Extension
Leland,
After going thru the technet archives, talking to our paste manufacturers,
and reading their SOPs for paste handling, we concluded that 7 days at RT
was the usable limit (as long as the paste tube/tub is sealed). We have been
using that time limit with no paste issues.
How did you arrive at the 3 days? (experimentation, past experience etc?).
what I am getting is that there may be no need to re-certify after 3 days
Regards,
Amol Kane | Process Engineer
Catalyst Manufacturing Services, Inc.
941 Route 38, Owego NY 13827
Phone: (607) 687-7669 Extn 349 | Website: www.catalystems.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
Sent: Tuesday, September 27, 2011 11:42 AM
To: [log in to unmask]
Subject: [TN] Solder Paste Shelf Life Extension
Folks,
Quick question here...
We have a rule here to scrap solder paste after it's been out of the fridge
for 3 days. If we recertify and extend the expiration date by first
verifying the proper viscosity, is it permissible to return the paste to the
jar that was used for the viscosity testing?
Thanks for your help,
Leland
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