Hi JaMi,
Unfortunately, your explanation is only partially correct and contains an
number of "myths."
>When gold is present somewhere in a joint that is soldered, some of the gold
>goes into "solution" and alloys with the other metals present.
A: Actually, typically all the gold will go into solution with Sn because the
Au-layers are thin relative to the available solder volume. The two limiting
cases are (1) no more Au is available, or (2) no more Sn is available. Over
most of the Sn/Pb alloy-range two alloy-phases, a lead-rich phase with
roughly 80/20 Pb/Sn and a tin-rich phase with roughly 97/3 Sn/Pb, make up the
solder volume. When intermetallic compounds [IMCs] are formed, some of the
tin-rich phase is converted to lead-rich phase and IMCs. Experiments
[Margareta Nylén, Swedish Institute for Metals Research] have been performed
that show that when the Au-layer is on the same order of thickness as the
solder, Au stops going into solution with Sn when essentially the whole
solder volume is lead-rich phase.
>The presence of gold in the alloy of the solder joint causes what is known
>as "gold embrittlement", which I can best describe as a "crystallization" or
>"crazing" of the solidified solder joint that mechanically weakens the
>joint, and can even cause physical and electrical failure of the joint.
>It is this "crystallization" or "crazing" of the solder joint that causes it
to look "very >dull" as you described.
A: Well, it is not exactly a "crystallization" or "crazing". IMCs are
crystalline and typically form needle-like or platelett-like structures. When
these structures come to the solder joint surface, they cause the dull
"crazed" appearance. With the IMCs being crystalline, they are by their very
nature brittle--most of them are strong however, with the exception of Au-
and Ag-based IMCs. Thus, the presence of about 4% by volume of Au or Ag
produces enough concentration of those IMCs to cause the structural weakening
generally referred to as "gold embrittlement."
>The amount of gold that goes into solution is dependant on two basic things:
>First, the amount of gold present in the joint.
>Second, the composition of the solder, i.e.: the specific alloy of the
solder.
>It is this second issue that is being ignored here, and is important for you
>to follow up on, and the reason is as follows:
>Standard solder, such as Sn60 or Sn63, has the ability to "wick" or "suck"
>metals such copper, silver, or gold, into its' own solution or alloy, up to
>about 2 percent of its volume. When gold is wicked into the solution, it
>causes "gold embrittlement".
A: Your first point is correct, your second is not. There is no "wicking" or
"sucking,"
but different metals have different dissolution rates in Sn. These relative
dissolution rates in Sn, pegging Sn in itself at 100 mm/s at 230°C, are: 100
Sn: 2.5 Au: 0.85 Ag: 0.085 Cu: 0.0021 Ni [Ref.: Klein Wassink, Soldering in
Electronics]. There is no limit of "up to about 2 percent of its volume."
>To combat this problem, special solders have been developed which have
>already combined a certain amount of silver in the alloy (about 2 percent)
>which tends to "satisfy" the alloy of the solder so that it will no longer
>will "wick" or "suck" additional metal (such as the gold on the pads of
>your PCB in this case) into "solution".
A: Unfortunately, this a myth. Adding a small amount of Ag to the solder does
not change the dissolution rate of either Ag or Au in the Sn. the is no such
thing as "to "satisfy" the alloy of the solder so that it will no longer will
"wick" or "suck" additional metal."
>So what this boils down to is this: The "gold embrittlement" that your
>customer is experiencing could be caused by your putting more gold (i.e.:
>too much gold) on the PCB, or it could be caused by your customer changing
>his solder from Sn62 (which contains about 2 percent silver) to something
>along the lines of Sn60 of Sn63, either of which will "wick" substantial
>amounts of gold into "solution".
>The point here is that you could actually have less gold on your boards, and
>yet still have more gold going into "solution", if the solder was changed.
A: there is no technical basis for this statement.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com
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