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March 2013

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From:
Mike Fenner <[log in to unmask]>
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Date:
Wed, 13 Mar 2013 18:31:41 +0000
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Hi
HO! just indoors from walking the dog in an actual field :) 
A=B is certainly an ideal to be desired. 
Problem is things like container size, amount required to load or prime
machine and usage rate can intervene to make a high perceived cost.
At low usage rates and a 500 gm stencil load, this could give you W= >0.9A
.
Almost everything to do with the practice of soldering is empirical, and how
far you can afford to stray from A=B, taking into account writing a doable
SOP is an empirical judgement which will differ from one place to the next
according to circumstances.
Let me put what I said in my last paragraph another way, perhaps what I said
was not clear enough.
If your build buy is about a million bucks a year, you probably spend about
$1000 on paste. If you could get your paste for nothing you would still be
spending about a million bucks.  How much effort do you want to put into
saving paste, taking into account the possible cost of rework, scrap etc
which could easily exceed in a day what you are hoping to save in a year?


Regards 
 
Mike 


-----Original Message-----
From: Whittaker, Dewey (EHCOE) [mailto:[log in to unmask]] 
Sent: Wednesday, March 13, 2013 5:39 PM
To: [log in to unmask]; [log in to unmask]
Subject: RE: [TN] Cooling of Solder Paste Overnight

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)
[log in to unmask]

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