Shelf lives given by the manufacturer are based on the premise that the
material is
- in unopened original container
- container has been stored continuously in the conditions suggested from
date of receipt.

As paste ages you are running an increasing risk of introducing an unknown
variable to your line. Manufacturers set a use by date up to which this risk
is vanishingly small and certainly too small to worry about (conditional on
above). In the nature of things this is a bit arbitrary, six months and 1
day old paste is not going to be substantively different from 6 months
material, but there has to be a line somewhere.

You obviously do want to test pastes that are representative of the type you
finally decide on, and as you are using pastes up to three months old,
nitpickingly theoretically you should keep them all that long before testing
so as to be looking at a representative "worst case" scenario. That's
probably impractical.

Pragmatically I suggest you ask all your paste suppliers to deliver their
material at a certain time and then test it within say a month. A poor
result from a delayed test might raise doubts for you, and likely invites a
possibly aggrieved supplier to say that's not fair, our stuff was ... and so
on.


Hope this helps

Mike Fenner
Indium Corporation of Europe
T: + 44 1908 580 400
F: + 44 1908 580 411
M: + 44 7810 526 317
W: www.indium.com


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Mengers, William D.
Sent: Thursday, June 21, 2001 1:56 PM
To: [log in to unmask]
Subject: [TN]


Fellow Techies,

We are getting ready to do a somewhat "extensive" solder paste evaluation
for an SMT operation.  What to do seems fairly straight forward using
"standard" tests (wetting, solder ball, slump, etc.), but a concern I have
is age and relative age of the different pastes we are testing.  In
practice, we generally use paste within three months of the date
manufacture, so the paste we use is anywhere from one to three months old.
Manufacturers claim good performance for the paste until expiration of shelf
life, but I suspect there could be a difference in performance between fresh
paste and 6 month old paste.  Maybe not for all pastes, but probably for
some.  My question is, at what age should we test the paste, and does it
matter (significantly?) if some pastes are fresh and others are close to 6
months old, or am I sweating the small stuff by even being concerned about
this?

The pastes we will be testing are no-clean formulations, eg. Alpha LR737, UP
78N, Omnix 5000, Indium NC-SMQ92J, AIM NC 251, Heraeus SC3401HTP, Kester
Easy Profile 256, and Qualitek 691A.

Do we really need to make a science project out of this or should we just go
with something that is popular for the type of application we have and see
if it works on our hardware as long as it meets the standard (J-Std) we are
using?

Any ideas?  Thanks for any responses you give.

Bill Mengers
Process Engineer
Northrop Grumman Corp.
Baltimore, Md.

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