Rudy
I don't have my EMF tables (nor would I state they are important when put
into an alloy), but what Dave said about Pb-Sn not inducing corrosion makes
sense. Corrosion really needs a strong galvanic push to cause
reactions...and Pb-Sn just don't seem to pop up in discussion much. From an
academic standpoint, the two elements are so similar in electron structure
(yes, the periodic table actually can predict some things) that it is hard
to believe they can drive corrosion. From an historical perspective, you
would have seen Pb-Sn solders falling apart everywhere because there are
certainly many impurities in them (don't think today, think historically
that they have been used for centuries...spears, early electronics). This
has not happened. First hand, I've sat and rubbed a soldering iron until
the darn black (from goop) ball of solder finally scratched into copper.
Those are not far from early electronics.
About pure tin: tin oxide (that is the surface that forms most stably on
tin) is a semiconductor/insulator (I'm more liberal on calling an insulator
a semiconductor). It has excellent properties and is not susceptable to
corrosion. It is used in flat panel displays (your notebook computer
screen) and other LCD applications. As a substrate, it is exposed to harsh
environments including semiconductor manufacturing (i.e., lots of
chemicals).
Corrosion resistance in alloys/mixtures is much more complicated than
internal galvanometric forces. It is a surface phenomenon in conjunction
with atomic migration (typically along grain boundaries). For example, one
of the metal elements may migrate to corrosion susceptible locations and
block corrosion from occuring. I think chrome oxide acts this way in
stainless steels (with small parts of Cr only). From my own hands-on
experience: Aluminum-1%Cu alloy is excellent in semiconductors against
electromigration (the Cu formed precipitates at key grain boundary locations
that then form mini-batteries). But it corrodes like crazy (by galvanic
reactions....) in chlorine (say, from plasma etch...not ablation...joke).
By adding 0.5% silicon to the mixture, its corrosion is stopped. Inspection
of the materials found that the Cu still formed precipitates. The key was
to realize that the silicon migrated to the outer surface at the grain
boundaries to form a protective oxide only at those locations from allowing
hydrogen formation, etc. that is typical in galvanic reactions.
In short, if you find Pb-Sn better than pure Sn, it is probably from
impurities left from the coating process or from the migration of Sn-Oxide
to the sensitive areas (Pb-Oxides tend to continue to oxidize, as compared
to SnOxide which passivates the surface). Lead-tin mixtures are used over
pure tin because: a) pure elements are never pure, b) the alloy controls
(masking other impurities) the properties better, c) they are more stable to
long term degradation (interfacial reactions, metal migration- wiskers,
etc.), and d) ease of manufacturing.
Carey (your long-winded admirer)
-----Original Message-----
From: Rudy Sedlak <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Friday, November 05, 1999 9:22 AM
Subject: [TN] Tin vs. Tin/Lead solder
>Dave:
>
>With all due respect to your exalted position in this area....
>
>I have seen many many problems in the fabrications area caused by having
pure
>Tin, as compared to having Solder, especially if the it is true solder,
i.e.
>that it is the true alloy, which seems to have profoundly better corrosion
>resistance than merely "plated solder", which is nothing other than
discrete
>chrystals of Tin and Lead.
>
>(Should I note that we make money and have products which have a life only
>because they solve this issue... in selected situations:-)
>
>And ignoring any ostensibly authoritative EMF tables, which I have found to
>be effective in academia, and few other places (because they are meaningful
>only when the electrolyte is deionized water), the solder seems to have
much
>better resistance to corrosion than pure Tin, and especially plated Tin,
>which seems especially "active".
>
>If what I am saying is true, that Solder is much more passive than pure
Tin,
>you have the makings for a seriously pumping galvanic cell, which should
need
>only an electrolyte to crank up, and deliver you Tin oxide all over the
pure
>Tin plating...
>
>And remember, free advice is often worth what you pay for it...
>
>Regards,
>
>Rudy Sedlak
>RD Chemical Company
>
>##############################################################
>TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
>##############################################################
>To subscribe/unsubscribe, send a message to [log in to unmask] with following
text in
>the body:
>To subscribe: SUBSCRIBE TECHNET <your full name>
>To unsubscribe: SIGNOFF TECHNET
>##############################################################
>Please visit IPC web site (http://www.ipc.org/html/forum.htm) for
additional
>information.
>If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or
>847-509-9700 ext.5365
>##############################################################
>
##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe: SUBSCRIBE TECHNET <your full name>
To unsubscribe: SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or
847-509-9700 ext.5365
##############################################################
|
