Ed,

     The only spec. I've seen on this subject is a Military Spec.  although
others might be out there.

    MIL-M-38510H specifies that "tin-lead plating shall have in the plated
deposit two percent to 50 percent by weight lead (balance nominally tin)."

    This spec. originated as a way to ensure whisker-free components, as 2%
Pb was found to be the safe minimum percentage to stop tin whisker growth.
Today, most external lead plating operations plate a Sn-Pb composition in
the range of 80-20 to 90-10.

    The thinking behind not plating eutectic Sn-Pb on device leads
originated from the extended storage times which existed between plating and
the board assembly operation - as the plated component ages, some of the tin
on the lead is consumed as copper-tin intermetallic.  If eutectic Sn-Pb were
plated on the leads, you would end up with a Pb-rich component finish after
storage, which is undesirable.  So to be on the safe side, most plating is
done with tin-rich alloys as mentioned above.

    Regards,

    Rob Schetty
    LeaRonal Inc.
    Freeport, NY  USA


-----Original Message-----
From: SEM Lab, Inc. <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Tuesday, March 03, 1998 12:16 PM
Subject: [TN] Tin-Lead Plating Composition on Device Leads


>I would sincerely appreciate some discussion on "preferred" plating
>composition on fine pitch quad flat pack device leads from the
>standpoint of SMT manufacturing (i.e. solderability and yields).  My
>client is using what appears to be an electroplated 90Sn-10Pb
>composition.  My suggestion was to use a composition close to the
>eutecic composition (63Sn-37Pb).  Does anyone know of any IPC (or other)
>specifications that address this topic?
>
>Thanks,
>Ed
>--
>               SEM Lab, Inc.
>Scanning Electron Microscopy and Failure Analysis
>               Snohomish, WA
>               (425)335-4400
>   http://www.eskimo.com/~eriemer/SEMLab.htm
>
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