Letter 3045

Black nickel plating in PCBs [circuit boards]. 

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Anyone familiar with Black Nickel phenomena? It occurs when plating Immersion Gold over electroless Nickel, over copper foil, wherein copper is on a printed circuit board (PWB or PCB). If possible, explain this phenomena, and why is causes brittle, weak solder joints after soldering an electrical component to a gold/nickel/copper pad. Cite any printed references you may have seen on this topic. Thanks

michael alderete
Aerojet - Azusa CA

I suppose U r worried about the weak solder joint and not the colour of the nickel. Forget the colour change and check If u r using the right kind of solder and flux for the gold soldering because the normal type is not recommended it would cause the joint as weak and thin.

M.KHAWAR
- islamabad.Pakistan.

M.KHAWAR- Thanks for your response.

I had understood that the so-called "black nickel" phenomenon is a plating defect which may produce poor solder joints. The physical setup is typically as follows:

-epoxy-glass PWB (printed wiring board)

-Cu foil [as bonded to epoxy glass]

-approx 150 microinches Ni plating

-approx 5 microinches Au plating

The above metallization stackup is assumed to exist on PWB circuit pads which will be soldered to. I'd assume that eutectic solder alloy [183 deg C Eutectic temperature] 63Sn/37Pb would be used to attach electrical components to these Au plated pads. The solder would be in a paste form, comprised of 63/37 solder particles in flux matrix, and the solder paste is screened onto the Au/Ni pads.

Solder attachment is then achieved by reflowing the board assembly in an oven, allowing the solder paste to melt [at >200 deg C], wetting the plated pads and the component leads, and solidifying into a memchanically and electrically robust joint.

IN REFERENCE TO YOUR COMMENTS, "Forget the colour change and check If u r using the right kind of solder and flux for the gold soldering because the normal type is not recommended it would cause the joint as weak and thin."...

Are you advising that a particular solder alloy and flux be used.? If so, what would you recommend? Regards

michael alderete
Sr. Enginer, Elec Packaging
Aerojet - Azusa, CA

Sorry I am late. well u got right ! i:e; u need special type of alloy solder paste for the good non brittle solder joint of the pad with the wire or part.WHY? well when differrent types of metals are soldered together a layer of intermetallic compound is formed which is infact non metal type and becones brittle and any shock or stress can cause failure and gold surfaces are the most vulnerable for this kind of problem .so what u do is increase the thickness of the gold plating to around 35u inches and also use the special kind of paste for it. You can solder the thicker gold deposit with the normal solder paste but the results are not gaurenteed. So please try to get the paste type from some vender as of now I don't know any address .

KHAWAR
- ISLAMABAD.PAKISTAN

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