Soldering to gold and silver metallizations

Discussion in 'Electronic Design' started by Chris Carlen, Jun 20, 2005.

  1. Chris Carlen

    Chris Carlen Guest

    Hi:

    As I understand, Ag and Au may be leached from surfaces when using
    standard Sn63Pb37 and Sn60Pb40 type electronic solders.

    Why is this to be avoided? Does it result in "intermetallic compounds"
    near the interface that compromise mechanical properties?

    Is this enough of a concern that typical gold plated component leads and
    silver plated hook-up wire in general shouldn't be soldered with
    tin-lead, but rather some non-leaching solders such as indium lead
    alloys for gold, or SnPbAg alloys for silver platings?

    Or is it only relevant to specialized applications that place
    substantial mechanical stress or thermal fatigue cycles on soldered joints?

    What is the difference between the issues involved in soldering Ag and
    Au platings, versus thick or solid Ag or Au surfaces?


    Thanks for input.

    Good day!





    --
    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA

    NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.
     
    Chris Carlen, Jun 20, 2005
    #1
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  2. Chris Carlen

    Chris Carlen Guest

    Chris Carlen wrote:
    > Hi:
    >
    > As I understand, Ag and Au may be leached from surfaces when using
    > standard Sn63Pb37 and Sn60Pb40 type electronic solders.
    >
    > Why is this to be avoided? Does it result in "intermetallic compounds"
    > near the interface that compromise mechanical properties?
    >
    > Is this enough of a concern that typical gold plated component leads and
    > silver plated hook-up wire in general shouldn't be soldered with
    > tin-lead, but rather some non-leaching solders such as indium lead
    > alloys for gold, or SnPbAg alloys for silver platings?
    >
    > Or is it only relevant to specialized applications that place
    > substantial mechanical stress or thermal fatigue cycles on soldered joints?
    >
    > What is the difference between the issues involved in soldering Ag and
    > Au platings, versus thick or solid Ag or Au surfaces?
    >
    >
    > Thanks for input.




    I got some info from Indium Corp about this:



    Eric: Rates of dissolution affect whether issue or not, and vary with
    temp. At 200C, electronics SnPb alloys will dissolve 35u-in/s/unit area
    of gold. For Ag, about 21u-in/s. Au reacts with Sn to form a brittle
    intermetallic. Flash Au plate isn't thick enough to make enough SnAu
    intermetallic to be a problem.

    But for Ag platings, the Ag3Sn intermetallic is actually stronger but
    also less ductile. So it depends. Since the SnPbAg solders contain
    silver, duh, they simply reduce the rate of leaching of Ag from the
    plated surface.

    It would seem that the AuSn intermetallic is generally to be avoided,
    and InPb alloys should be used on gold.






    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA

    NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.
     
    Chris Carlen, Jun 20, 2005
    #2
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  3. Chris Carlen

    Ted Edwards Guest

    Chris Carlen wrote:
    > As I understand, Ag and Au may be leached from surfaces when using
    > standard Sn63Pb37 and Sn60Pb40 type electronic solders.
    >
    > Why is this to be avoided? Does it result in "intermetallic compounds"
    > near the interface that compromise mechanical properties?


    Back when Tektronix scopes were built with components soldered into
    ceramic tie strips with silver plated U's for the leads, a small roll of
    silver bearing Sn-Pb solder was mounted inside the scope for use in
    replacing any failed components. Apparently the molten Sn-Pb solder
    would disolve the Ag but a relatively small Ag addition would prevent
    this. Sorry, I don't recall the % added.

    Ted
     
    Ted Edwards, Jun 20, 2005
    #3
  4. Chris Carlen

    CF Guest

    > As I understand, Ag and Au may be leached from surfaces when using
    > standard Sn63Pb37 and Sn60Pb40 type electronic solders.
    >
    > Why is this to be avoided? Does it result in "intermetallic compounds"
    > near the interface that compromise mechanical properties?
    >
    > Is this enough of a concern that typical gold plated component leads and
    > silver plated hook-up wire in general shouldn't be soldered with
    > tin-lead, but rather some non-leaching solders such as indium lead
    > alloys for gold, or SnPbAg alloys for silver platings?
    >
    > Or is it only relevant to specialized applications that place
    > substantial mechanical stress or thermal fatigue cycles on soldered
    > joints?
    >
    > What is the difference between the issues involved in soldering Ag and
    > Au platings, versus thick or solid Ag or Au surfaces?
    >>I got some info from Indium Corp about this:


    >Eric: Rates of dissolution affect whether issue or not, and vary with

    t>emp. At 200C, electronics SnPb alloys will dissolve 35u-in/s/unit area
    >of gold. For Ag, about 21u-in/s. Au reacts with Sn to form a brittle
    >intermetallic. Flash Au plate isn't thick enough to make enough SnAu
    >intermetallic to be a problem.


    >But for Ag platings, the Ag3Sn intermetallic is actually stronger but
    >also less ductile. So it depends. Since the SnPbAg solders contain
    >silver, duh, they simply reduce the rate of leaching of Ag from the
    >plated surface.


    >It would seem that the AuSn intermetallic is generally to be avoided,
    >and InPb alloys should be used on gold.


    Yes, gold embrittlement IS a problem.

    We just "wick" the gold off by tinning and solder-wicking the lead
    and then solder it. This removes the gold.

    Carey
     
    CF, Jun 20, 2005
    #4
  5. Chris Carlen

    Chris Carlen Guest

    CF wrote:
    >>As I understand, Ag and Au may be leached from surfaces when using
    >>standard Sn63Pb37 and Sn60Pb40 type electronic solders.
    >>
    >>Why is this to be avoided? Does it result in "intermetallic compounds"
    >>near the interface that compromise mechanical properties?
    >>
    >>Is this enough of a concern that typical gold plated component leads and
    >>silver plated hook-up wire in general shouldn't be soldered with
    >>tin-lead, but rather some non-leaching solders such as indium lead
    >>alloys for gold, or SnPbAg alloys for silver platings?
    >>
    >>Or is it only relevant to specialized applications that place
    >>substantial mechanical stress or thermal fatigue cycles on soldered
    >>joints?
    >>
    >>What is the difference between the issues involved in soldering Ag and
    >>Au platings, versus thick or solid Ag or Au surfaces?
    >>>I got some info from Indium Corp about this:

    >
    >>Eric: Rates of dissolution affect whether issue or not, and vary with

    > t>emp. At 200C, electronics SnPb alloys will dissolve 35u-in/s/unit area
    >>of gold. For Ag, about 21u-in/s. Au reacts with Sn to form a brittle
    >>intermetallic. Flash Au plate isn't thick enough to make enough SnAu
    >>intermetallic to be a problem.

    >
    >>But for Ag platings, the Ag3Sn intermetallic is actually stronger but
    >>also less ductile. So it depends. Since the SnPbAg solders contain
    >>silver, duh, they simply reduce the rate of leaching of Ag from the
    >>plated surface.

    >
    >>It would seem that the AuSn intermetallic is generally to be avoided,
    >>and InPb alloys should be used on gold.

    >
    > Yes, gold embrittlement IS a problem.
    >
    > We just "wick" the gold off by tinning and solder-wicking the lead
    > and then solder it. This removes the gold.



    Very clever!


    --
    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA

    NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.
     
    Chris Carlen, Jun 20, 2005
    #5
  6. Chris Carlen

    Jim Yanik Guest

    Ted Edwards <> wrote in
    news:vnDte.84421$tt5.17111@edtnps90:

    > Chris Carlen wrote:
    >> As I understand, Ag and Au may be leached from surfaces when using
    >> standard Sn63Pb37 and Sn60Pb40 type electronic solders.
    >>
    >> Why is this to be avoided? Does it result in "intermetallic compounds"
    >> near the interface that compromise mechanical properties?

    >
    > Back when Tektronix scopes were built with components soldered into
    > ceramic tie strips with silver plated U's for the leads, a small roll of
    > silver bearing Sn-Pb solder was mounted inside the scope for use in
    > replacing any failed components. Apparently the molten Sn-Pb solder
    > would disolve the Ag but a relatively small Ag addition would prevent
    > this. Sorry, I don't recall the % added.
    >
    > Ted
    >


    In addition,465 and later TEK scopes used gold-plated attenuator PCBs that
    were soldered with ordinary electronic solders.
    (not the 3% Ag stuff,either.That was only used for the ceramic strip
    scopes.)

    --
    Jim Yanik
    jyanik
    at
    kua.net
     
    Jim Yanik, Jun 21, 2005
    #6
  7. Chris Carlen

    Pooh Bear Guest

    Chris Carlen wrote:

    > Hi:
    >
    > As I understand, Ag and Au may be leached from surfaces when using
    > standard Sn63Pb37 and Sn60Pb40 type electronic solders.
    >
    > Why is this to be avoided? Does it result in "intermetallic compounds"
    > near the interface that compromise mechanical properties?
    >
    > Is this enough of a concern that typical gold plated component leads and
    > silver plated hook-up wire in general shouldn't be soldered with
    > tin-lead, but rather some non-leaching solders such as indium lead
    > alloys for gold, or SnPbAg alloys for silver platings?
    >
    > Or is it only relevant to specialized applications that place
    > substantial mechanical stress or thermal fatigue cycles on soldered joints?
    >
    > What is the difference between the issues involved in soldering Ag and
    > Au platings, versus thick or solid Ag or Au surfaces?


    This is totally unrelated to your question ( other than being about using
    silver plating ).

    After the TWA 800 crash, the fuel tank systems of Boeing 747s were closely
    investigated. Early models used silver plated wire for the fuel quantity
    indication system with silver plated crimp lugs mounted to silver plated
    terminal blocks in the fuel tanks.

    It was discovered by examining a few old 747s that had experienced fqis
    problems that sulphur in the fuel combined with the silver plating to form
    semi-conducting silver sulphide that then washed over the terminal block
    coating it with a thin layer.

    One example when tested with a 9V battery actually 'flashed' between terminals.
    I assume the active sensing voltage and energy is lower than this though.

    Nickel plated wire is used now.


    Graham
     
    Pooh Bear, Jun 26, 2005
    #7
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