Dave, Mike and Dick,
You are all honorable and well respected men in your field (some of you even when you come indoors). Unfortunately, it is beyond the scope of an Auditor to reconcile that as an artifact of compliance. Perhaps my pros and cons on this issue was too simplistic: used paste; makes waste. Let's look at this empirically: A - B = W.
A is the amount of paste issued, dispensed, removed from environmental envelope, etc. and on hand at a point of use.
B is the amount of paste used in the printing and processing
W is the amount of paste left over, not used, open to environmental exposure (ie- waste), needing some dispositioning
The goal is to have 0 waste. Therefore, to have A - B = 0, then the effort s/b be to minimize the differential between A and B.
The cost to feed the Elephants is peanuts, if they're riding the Subway to work.
Dewey
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner
Sent: Wednesday, March 13, 2013 9:46 AM
To: [log in to unmask]
Subject: Re: [TN] Cooling of Solder Paste Overnight
Translating 60F = 15C 85F =29.4C
I think you will find that most paste suppliers set an upper limit of about 30C for max room temp storage. Paste life is going to come down fairly rapidly above that and at 40 is going from months to days. Above 40 hours.
So storing in a temp limited environment makes sense.
Don't forget if you are storing at a lower temp than your usage temp you will then have to wait a long time for it to come back to operating temperature - and not open containers before they have done so.
So better to store at your day time ambient, which is likely to be higher than 15C.
Limiting the rate of issue from long term to line store is also a good way of keeping paste quality consistent.
If you do have to use previously opened paste: Use new paste to preload stencil and then the previous day's opened to top up. That will minimise any variations that may have become present.
Keep in mind when setting procedures that though no one wants unnecessary waste, money spent on paste is a very small part of your buy (typically <0.X%)and therefore many times less than cost of paste that doesn't work.
Your main driver therefore is quality of work.
Regards
Mike Fenner
Bonding Services & Products
M: +44 [0] 7810 526 317
T: +44 [0] 1865 522 663
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Richard D. Krug
Sent: Wednesday, March 13, 2013 12:40 PM
To: [log in to unmask]
Subject: [TN] Cooling of Solder Paste Overnight
It has been proposed that we take left over solder paste and store at ~60 F overnight to extend life and integrity of paste. This would be paste in its original tube or jar, not recovered from a stencil. We are located in Florida. During the summer, when the AC is reduced overnight, temperature in the building gets to ~85 F.
Has anyone done this? What are the pro's and con's?
Dick Krug, CSSBB, CSMTPE
Senior Process Engineer
Sparton Complex Systems
30167 Power Line Road
Brooksville, FL 34602-8299
p (352) 540-4012 (Internal Ext. 2012)
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