Hi Kevin,
Now my 5 cents.
From my experience, I do not see solderability directly linked to the concentration of phosphorus co-plated with the nickel—I see however, brittle interfacial failures between the Ni-P surface and the Ni3Sn4-intermetallic compound which directly track with P-concentration at that interface. The presence of Ni3Sn4-IMC is clear evidence that there was no solderability issue—it is the concentration of the P at the interface which weakens the interfacial bond.
Because of their superficial appearance, conditions with both too much P and too little P [causes hyper-galvanic corrosion along the grain boundaries of the Ni plating] have been termed 'black pad.'
Having too much P at the interface can be the result of improper Ni-plating, but also the result of too much Ni being dissolved in the Sn of the solder leaving to much P behind—this can happen from too many soldering operations.
Werner
-----Original Message-----
From: Glidden, Kevin <[log in to unmask]>
To: [log in to unmask]
Sent: Tue, Sep 21, 2010 8:52 am
Subject: [TN] Solderability of nickel plating
All,
Can anyone comment on the solderability of the nickel plate, particularly with
regard to type of plating: high phosphor / low phosphor, and also storage
conditions:
I've been gathering info, and seen conflicting information.
From one article: http://corrosion-doctors.org/MetalCoatings/Electroless.htm
A paper submitted at the Electroless Nickel conference of 1989 held in
Cincinnati Ohio, Titled "Solderability Parameters of Elecroless Nickel Bearing
Electronic Finishes" By Louis Kosarek of STB Systems, Inc. report that "An
electroless nickel deposit which contains a concentration of phosphorus ranging
from 0.1% to 3.0% is readily solderable on an "As-plated Basis" per Mil-Std 883c
method 2003. The frequency of solderability tests which fail per Mil-Std 883c
will increase as the phosphorus content of electroless nickel alloy increases
from 3.0 to 7.0% phosphorus. A solderability test conducted per Mil-Std 883c
method 2003 incorporating an as-plated surface finish containing phosphorus in
excess of 7%, the components will consistently fail. The mode of failure is
non-wetting of the surface."
This indicates low phosphorous plating is best for solderability. Many other
papers seem to agree with this.
---------------------------------------------------------------------------------------------------------------------
From another article: http://www.pfonline.com/articles/pfd0024.html
Solderability/Weldability. An important aspect of electroless nickel to the
electronics industry is its solderability. All electroless nickel deposits are
solderable provided the soldering conditions are matched to the condition of the
particular electroless nickel deposit. Lower-phosphorus electroless nickel is
more easily solderable immediately after plating than higher-phosphorus
electroless nickel. However, this advantage disappears after 12 - 24 hrs. At
this point, the ease of solderability depends upon the characteristics of the
passive layer that forms on the surface of the electroless nickel deposit. Those
deposits plated from baths containing heavy metal and sulfur-bearing brighteners
and stabilizers, such as most commercial low- and mid-phosphorus electroless
nickel systems, form a thicker tenacious passive layer than those that do not,
such as most high-phosphorus electroless nickel systems. High-phosphorus
electroless nickel systems tend to be more solderable in aged deposits.
Other important factors influencing the solderability of electroless nickel
deposits include residual contamination left on the surface after plating and
storage conditions after drying. Surface contamination and exposure to
environments containing sulfur dioxide, chlorine, high humidity and high ambient
temperatures will detrimentally affect solderability. Therefore, it is
imperative that parts be thoroughly rinsed in clean DI water, dried and stored
in a cool, dry atmosphere, preferably nitrogen.
This article indicates high phosphorous plating is best for long term
solderability, and also that parts must be stored carefully (cool, dry, sealed,
nitrogen). Other articles seem to state this is a highly stable finish and is
even used as a corrosion inhibitor.
Thanks,
Kevin Glidden
Manufacturing Engineer
Astronics Luminescent Systems Inc.<http://www.astronics.com>
4 Lucent Drive
Lebanon, NH 03766
(603) 643-7766 x3152
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