So ist a bit like bud light then??
Happy Friday Mike,
John
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Mike Fenner
Sent: Friday, November 11, 2005 7:49 AM
To: [log in to unmask]
Subject: Re: [TN] Solder paste control
A jar of paste from us is exactly the same as the last jar. It will have all
the properties you desire and originally specified for your production.
Not so for old paste. As paste ages, its print, tack and reflow properties
decline/change and quality of joints produced will decrease.
Thus the used paste represents a variable. [How much variance you can
tolerate will depend on your specific circumstances, inspection criteria
etc. ] The name of the game is to eliminate all uncontrolled variables, or
at least control variables to acceptable limits. Hence my suggestions. The
actual amounts you set yourself, (this is a variant of the It depends
answer) Also all the time the solder paste is out of the jar it is at risk.
You need to control/limit the risk.
As it is Friday permit me to ramble a bit:
Think of it another way. Suppose we are not talking about solder but high
quality paint and you are going to use it on a precious object.
Now instead of carefully keeping the lid on to stop it skinning etc we want
you to tip the paint on a flat surface and spread it out to get the max
surface area. Next wait a moment and then push it all back and then spread
it out again. [This will ensure the most evaporation and the biggest area
for absorbing contamination. Re-spreading will ensure any dried out paint or
contaminated paint is mixed well in to mess up the rest] Do this several
times a minute for several hours. Now apply the paint to your work. Wold you
expect a perfect result? If not why not? You do with solder paste! The
chemistries are not a million miles apart.
Regards
Mike Fenner
Indium Corporation
We are at Productronica
click for details
www.indium.com/productronica/
-----Original Message-----
From: Ivanoe Pedruzzi [mailto:[log in to unmask]]
Sent: 11 November 2005 11:19
To: [log in to unmask]; TechNet E-Mail Forum
Subject: Re: [TN] Solder paste control
Mike,
why do you advice not to mix used paste with fresh one? What someone can
expect to happen, from a reflow and soldering point of view?
(please consider that we are talking about used paste I woudn't think to
have more than 20% in volume to mix on a stencil and also I wouldn't go for
mixing it in only one time, so it gets diluted over a working shift)
THX, Ivan
Il giorno 11/nov/05, alle ore 12:01, Mike Fenner ha scritto:
> 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/
>
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