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Au corrosion

2007

Hi there,

I am plating a stamped terminal with Au at 0.5-0.68um over Ni 1.5-3um with phosphor bronze as my base material. After 3-4 days of storage in room ambience, I can see black holes and green patches on some of the plated parts.

I know very little about porosity but I had the assumption that higher Au thickness on top of a thick Ni barrier would prevent the base material from diffusing upwards to the surface. I read somewhere stating that the min thickness of 1.3um Ni should do the trick as pore corrosion inhibitor.

I am facing this problem on this particular part only although I do plate other terminals at the same Au and Ni thickness or at even at lower Au thickness. Funny thing is the black holes/green patches did not appear immediately after porosity test with HNO3. They only surface after keeping in room temperature for 3-4 days.

My questions are:

1) Does setting a high current density onto the terminals to meet the high Au thickness encourage porosity in the Au deposit due to the "poorer deposit structure" compared to if they are plated under a lower current density?

2) And is there any external agent that would have aggravated Cu diffusion through the Au and Ni layers?

3) If the base material itself is having minute holes on the surface before plating, would this contribute to the Au corrosion problem post-plate? Would higher etching rate during cleaning process to "clean" the so-called holes help solve the problem?

4) If some chloride from the NiCl2 in Nickel Sulphamate bath got trapped in some nook and cranny on the surface, would this contribute to Au corrosion post-plate?

Hope someone can help shed some light on this issue. Thanks.

Jen Teh
Process Engineer - Penang, Malaysia

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2007

Hi Teh,

Since you are facing problem on this product only, I would suggest you to concentrate on stamping department.It may be due to residue/minute bur stuck on the surface while stamping and some of it go through the entire process. It falls of during further operations after plating such as cutting and packing, leaves a minute unplated pore/voids.After 3-4 days, this portion can turn to green/black.Best way to examine your terminals is to through 100- 200 x magnification before plating.Make sure you examine the parts before it is handled rough.
If you find this problem, then there are two areas you have to concentrate
1.Check and correct stamping machine
2. Involve your metallurgical department or send your terminal sheet for analysis
3. Consult your terminal sheet supplier and discuss quality.

please do this and let me know.
Regards

T.K. Mohan
plating process supplier - Mumbai, India

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2007

Hi Mohan,

If there's indeed some minute parts fall off due to flaws after stamping, would vigorous cleaning (both anodic and cathodic) help eliminate this problem?

Thanks.

Jen Teh
- Penang, Malaysia

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Hi Teh,
I am afraid, this will not be of much help. You know you are giving air agitation in your Nickel bath and good solution movement in gold bath also.
I was once involved in investigating a similar situation and our research finally lead to rectification in stamping machine defect and to an extend changing the metallurgy of sheet supplied.
Regards,

T.K. Mohan
plating process supplier - Mumbai, India
2007

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2007

HI Teh,
The problem you stated is looking like porosity problem, to ensure/make sure you can do sulfur dioxide test and confirm where exact the problem is hot water rinse of around 70-80 °C for five minutes and then bake in the oven will help in solving the problem.
Hope this will help you in diagnosing the problem.
Good luck
Mukesh

Panjala Mukesh
fashion jewelry mfgr.
Hyderabad, India