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February 1999

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Subject:
From:
Bruce Stacy <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 17 Feb 1999 16:08:35 -0500
Content-Type:
text/plain
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text/plain (82 lines)
Immersion gold deposits by their nature are porous.
   If the deposit looks uniform after plating, but after going through
   several rinses becomes discolored it would be wise to check the purity
   of the rinses.  Perhaps something is being absorbed during subsequent
   processing.
   If the deposit looks discolored after plating, indeed the possibility
   exists of the gold bath being at its upper limit for nickel or copper
   concentration and these metals are codepositing.  If these levels get
   high enough it will have an adverse effect on solderability and/or wire
   bonding.
    Another thing to check is if the deposit is of uniform thickness across
   all pads. If the discoloration occurs on thinner gold deposits after the
   plated boards have been in storage: this could be due to migration of Ni
   or Cu from beneath the gold surface. This too affects solderability and
   wirebonding (solderability less if using active fluxes.





doug smith <[log in to unmask]> on 02/17/99 03:17:48 PM

Please respond to "\"TechNet E-Mail Forum.\" <[log in to unmask]>,        doug
      smith" <[log in to unmask]>

To:   [log in to unmask]
cc:    (bcc: Bruce Stacy/ECSDOMAIN)
Subject:  [TN] Immersion Gold contaminated with Copper




I am observing a variation in the color of gold immersion finished boards.
My theory is the gold finish is being contaminated by subsequent cleaning
processes prior to assembly.

Does anyone have any experience with copper contamination of immersion
gold? I know that nickel is used as a barrier between the copper SMT pad
and the gold finish. Without the Nickel barrier the copper would migrate
into the gold creating an unsolderable finish. So there is a chemical
reaction possible between copper and gold.

So what if the gold immersion finish is subjected to subsequent processes
in the fabrication facility that have copper in solution. Is the copper
attracted to the gold surface out of solution? Can the concentration of
copper in solution be so high as to make the gold finish unsolderable? Is
there an acceptable limit of copper contamination in an immersion gold
finish from an assemblers solderability viewpoint?

Doug Smith
Technical Support Engineer
Yamamoto Mfg. (USA), Inc.

(408)944-8308
[log in to unmask]

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