Hi Nicholas:
Just so you know, SMTA members are entitled to free reprints of SMI papers
(e.g. the Hunt paper David H. mentioned). The alternative is to purchase a
reprint for $5/page. Just call 612-920-7682 and Amy will be able to help you.
Regards,
-David
At 07:18 PM 5/15/98 -0500, you wrote:
>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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