Pb/Sn solder corroding gold bond wires
Pb/Sn solder corroding gold bond wires
(OP)
While I might end up reposting this in the metallurgy forum, I decided I'd start with the EEs who might have experienced it:
Can anyone explain the mechanism that tin-lead (Pb/Sn) solder corrodes gold bond wires if the materials are in contact?
Galvanic corrosion doesn't explain it; Pb/Sn is about -0.3 on a galvanic chart and gold is +1.42. That indicates gold should be corroding the Pb/Sn. Or is there a secondary effect that goes on? Or do they still have a nasty reaction because they are too far apart on the galvanic chart (1.72)?
The gold bond wires are only 99.99% gold. Could the other 0.001% be reacting? The wire has 3-7 ppm Beryllium (-1.7 on the galvanic chart) so there could be a reaction there. I'd be surprised if there is enough reaction to corrode almost completely through the wire.
Can anyone explain the mechanism that tin-lead (Pb/Sn) solder corrodes gold bond wires if the materials are in contact?
Galvanic corrosion doesn't explain it; Pb/Sn is about -0.3 on a galvanic chart and gold is +1.42. That indicates gold should be corroding the Pb/Sn. Or is there a secondary effect that goes on? Or do they still have a nasty reaction because they are too far apart on the galvanic chart (1.72)?
The gold bond wires are only 99.99% gold. Could the other 0.001% be reacting? The wire has 3-7 ppm Beryllium (-1.7 on the galvanic chart) so there could be a reaction there. I'd be surprised if there is enough reaction to corrode almost completely through the wire.





RE: Pb/Sn solder corroding gold bond wires
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I don't suffer from insanity. I enjoy it...
RE: Pb/Sn solder corroding gold bond wires
Googling leaching leads me into some interesting articles. Especially when going lead-free for RoHS...
A star for you!
RE: Pb/Sn solder corroding gold bond wires
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Pb/Sn solder corroding gold bond wires
RE: Pb/Sn solder corroding gold bond wires
However, it can form a variety of eutectics. Should also do a search for "purple plague," which was a common malady of gold bond wires in the 60's and 70's.
TTFN
RE: Pb/Sn solder corroding gold bond wires
Why do you want gold on the board? Are you using pcb edge connectors? If it isn't cost-critical or a compatibility problem, use a mating pair of connectors instead!
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I don't suffer from insanity. I enjoy it...
RE: Pb/Sn solder corroding gold bond wires
I assumed it was to reduce the ratio of gold to tin/lead in the final solder joint.
RE: Pb/Sn solder corroding gold bond wires
Thanks all for your help as I move into this new world.
RE: Pb/Sn solder corroding gold bond wires
TTFN
RE: Pb/Sn solder corroding gold bond wires
Is it mass-market thick film hybrids, power semiconductor modules, or chip & wire? All interesting stuff!
A few of words of advice -
If you are designing hybrids, ask if you can spend a month or two on the shop floor with production and maintenance where you can learn the processes and equipment, and their limitations. Some of these phases won't apply to you, but many will: Print & Fire, Laser Scribe, Die Attach, Wirebond, Pick & Place, Reflow, Leadframe Attach, Functional Test. You need to understand all of them, at least to the point where you can discuss the design with the people from those departments. Your designs will be much better once you know understand the practicalities. I moved from manufacturing to design, and most of my designs were buildable from the outset because I knew what was within the capability of the plant - some of my design colleagues had the opposite approach and said 'find a way to build it' which used to cause needless friction between design and manufacturing.
Talk to the R&D guys. Do them little favours like knocking out a prototype design for them in a quiet moment, or printing out a life-size photo of a pretty girl on the roll-fed printer for their office.
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I don't suffer from insanity. I enjoy it...
RE: Pb/Sn solder corroding gold bond wires
My favorite quote so far is from an assembly tech who said "our worst designs come from the guys who never operated a bonding machine before". Experience is invaluable and the manufacturing guys really have it.
With my first hybrid design at the fab, I'm working on setting up some design rules so we can get a bit more consistent. I'm also working through our RMAs to see what issues are making it to the customer.
(insert some insightful closing statement here) Good night!