Techies,
Au-content < =3%. If you are above, AuSn4 intermetalics will form and make the solder joint brittle.
In general there are the following intermetalics: AuSn, AuSn2, AuSn4, Ag3Sn, Cu3Sn, Cu6Sn5, Ni3Sn2, Ni3Sn4, Ni3Sn7 (it is soldering, therefore all contain tin). AuSnx and AgySn are "weak". AuSn can form due to "Ausscheidungen" at the "Korngrenzen" (sorry, perhaps somebody can translate) at room temperature and below the 3% limit. This will also weaken the solder joint, therefore be careful for applications with high reliability requirements. Perhaps Mr. Reliability (Werner Engelmaier) could comment on this and translate the german words.
Dissolution from gold in tin is real quick, at least 1000 times quicker than Ni or app. 30 times faster than Cu dissolves in tin. Therefore the time the solder stays heated shouldn´t be an issue, at least in a reflow process. For wave soldering process the gold from pcb finish will "disappear" in the solder pot.
Gebhard
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-----Ursprüngliche Nachricht-----
Von: TechNet [mailto:[log in to unmask]] Im Auftrag von Jack C. Olson
Gesendet: Montag, 6. Dezember 2004 17:31
An: [log in to unmask]
Betreff: Re: [TN] Gold Plated Board
The amount of gold on an "Immersion" plated board
is only a few microinches think, usually less than 10.
When you solder a component to that surface, some
or all of the gold (depending on how long the solder
stays heated/melted) mixes in with the solder joint.
If the percentage of gold in the solder joint is high
enough the joint will be brittle, but it won't be that
high with such a thin immersion finish.
Trying to solder to a thick electroplated gold surface
is a different story, however, but you can calculate it
yourself by comparing the thickness of gold plating
with the amount of solder paste you are applying to
the lands in question. I can't remember the exact
percentage that means you are getting into the "danger
zone", but someone here will know it, or you can search
the TechNet archives at:
http://listserv.ipc.org/archives/technet.html
good luck,
Jack
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