Hi TechNet - Oh good - a metallurgy question! I'll try to fill in some info
(and add some opinion) that may be useful. Many folks split solderability
into two distinct categories: 1) Surface Oxide Mechanisms, 2) Intermetallic
Mechanisms.  Both of these categories are very heavily dependent on
diffusion for going from a no-problem to a big-problem issue. Diffusion is
a time and temperature variable. If your solderable coating is of
sufficient thickness then you should only be dealing with a surface oxide
mechanism. Some solderable coatings are have better oxide growth resistance
than others (in terms of oxide type and thickness). You always form an
intermetallic layer during soldering. However, if the solderable coating is
too thin then either the solderable coating is consumed (e.g. a thin HASL
finish converts into copper/tin intermetallic) or the sacrificial coating
is breached (e.g. nickel diffusing through a gold finish). Dr. Chris Hunt
of NPL published a good paper containing the formula's for getting a first
order approximation of oxide and intermetallic diffusion growth in the SMI
96 conference proceeds that you may find helpful (Bev -this is who you
couldn't remember!).

WHY all of this info? - because if you would plug in a "cold " storage
temperature into the oxide and/or intermetallic growth equations you will
find that the amount of growth just about the same as  room temperature
storage conditions! I haven't seen anyone bring forth data supporting that
"cold" storage would extend board shelf life. There has been several data
packages and papers that have shown that the amount of moisture (e.g.
humidity) can be detrimental to solderability due to the changes in oxide
formation (e.g. type) that can occur. My recommendation is to focus on a
pwb storage area that is dry and not spend time (and $$) on trying to keep
things cold.

Also, just to add some fuel to the debate that TechNet had on whether
nickel is a diffusion barrier: If you look at the nickel/copper phase
diagram it is very evident that copper can diffuse into nickel. However,
the kinetics parameters need to also be taken into account in addition to
the thermodynamic parameters, so the end results is that nickel does act as
a diffusion barrier (sorry Werner - you and I disagree on this one).

The simple answer is to procure solderable finishes that have sufficient
thickness to avoid intermetallic problems and use storage conditions that
do not promote oxide growth problems. Now if it were only that easy. Hope
this helps. Good Luck.

Dave Hillman
Rockwell Collins
[log in to unmask]




Nicholas Kane <[log in to unmask]> on 05/14/98 08:53:13 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to Nicholas Kane <[log in to unmask]>

To:   [log in to unmask]
cc:    (bcc: David D Hillman/CedarRapids/Collins/Rockwell)
Subject:  [TN] Storing PCB's




I am confused.

For years, I have understood that the best way to store pcb's prior to
assembly is in a heated, relatively dry environment.  I am sure that the
general consensus amongst most manufactureres is that this works best.

Well, just recently I was told by a company setting up a new facility that
they had been advised by a well known consultant to use a cold dry
environment.  They are planning a 12-14 degree Celsius storeroom, once
again relatively dry.

Is there new thinking on this, or have they been badly advised?




[Nicholas Kane]
[Axion Australasia]
[Suite 3, 651 Canterbury Road]
[Surrey Hills]
[Victoria 3127 Australia]
[tel: 613 9899 3511    fax: 613 9899 3811]

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