The answer is really you can't tell by testing. Even if you duplicated a
solder paste QA lab on your shop floor, you could test the paste and find it
OK, but still not know how much of its life had been used compared to new.
The best thing to do is to control the rate of issue of paste to the shop
floor and put in place some rules on re-use of paste to make an SOP for
operator. Then for your ISO guy demonstrate you follow the SOP.
Eg
Only issue what is used in a shift (commensurate with loading stencil)
Used paste goes in jars marked used paste. Never mix it with new paste.
Use only new paste to load stencil.
Old paste is only used to replenish stencil (if you are concerned to re-use
paste)
Discard used paste after x day
Also put in some best practice rules: Keep jars closed when not in use
And so on.
The precise rate of discard etc can be set arbitrarily or by reference to
your fault rates. [Would give your ISO guy something to do :)

Finally whilst no one condones unnecessary waste, don't forget the object is
to produce products not save paste. Solder costs are a tiny part of your
overall spend, you can't really save a huge amount of money on solder, but
you can easily incur costs with non spec materials. That is the real benefit
of the SOP.

Regards

Mike Fenner
Indium Corporation
Pb-free at Productronica
www.indium.com/productronica/


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Yannick Brisson
Sent: 10 November 2005 19:57
To: [log in to unmask]
Subject: [TN] Solder paste control

Hi,

        I had a question from our ISO guy asking me how I know that the
paste we use is in good condition?  The problem is that he saw our operator
put some old paste back in a empty jar to reuse it the next day. So his
question is how do you know the paste has been on the stencil below the
maximal hour wrote in the datasheet?  Do you know any measure than I can
take to certify that the paste in good? like viscosity or something else?
And how I do that?  Because I'm pretty sure that is next question will be
how I know the new paste we just receive is good too.

Thank You

Yannick


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