Pb/Sn solder corroding gold bond wires 1

[zappedagain]

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.

 

[ScottyUK]

Is this reacting by contact you're describing, or the leaching of gold into molten solder? The latter is quite well known - it leads to brittle joints if too much gold gets into the alloy IIRC - it has been a long time since I worked in hybrids, so I might have to dig back through my notes.

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I don't suffer from insanity. I enjoy it...

 

[zappedagain]

Ah, could be the leaching... I haven't seen this yet, I've only been warned to design around it.

Googling leaching leads me into some interesting articles. Especially when going lead-free for RoHS...

A star for you!

 

[MacGyverS2000]

Scotty beat me to the punch... it is most likely leaching you're talking about. Gold-plated boards that are then wave-soldered are most prone to this. Proper manufacturing reduces the problem to practically nil for all intents and purposes, but improper manufacturing will lead to a nightmare months down the road when boards start failing. Make sure they place the proper thickness of gold on the boards, too much and you'll have leaching issues, too little and the solder could wash it away almost immediately.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[zappedagain]

I've heard that nickel plating under the gold helps too. Any truth to that? My first thoughts are yes, thinner gold (so less leaching) and thicker metal to bond to.

 

[IRstuff]

Gold, of course, doesn't technically corrode.

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

 

[ScottyUK]

Soldering direct to nickel requires a fairly active flux and reasonable heat, but it is quite acceptable in it's own right or as a substrate for a gold flash.

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...

 

[zeitghost]

When I worked in the semiconductor industry, we used to tin dip gold plated leads on transistors and such for hi rel customers.

I assumed it was to reduce the ratio of gold to tin/lead in the final solder joint.

 

[zappedagain]

ScottyUK - I'm moving into the hybrid world. My dielectric is alumna and the conductive layer is Nickel/Gold. No more FR-4 and copper for me!

Thanks all for your help as I move into this new world.

 

[IRstuff]

Wirebonding requires considerable amounts of stress concentrated in a teensy area and volume. These stresses can accelerate otherwise ineffective processes.

TTFN

 

[ScottyUK]

zappedagain,

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. We were dumped in the crap by our maverick R&D department who had illicitly taken a development product through to customer approval, at which point we found that their hand-assembled prototypes couldn't be made on our existing plant. Politically it was messy, and could have been avoided if the 'old guard' in Design would have talked to their equally pig-headed counterparts in R&D.

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I don't suffer from insanity. I enjoy it...

 

[zappedagain]

ScottyUK, thanks for the advice. Chips & wire, by the way.

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!