Fill,
I'm glad you ask (despite you are a professionell ya'self). I've asked this myself. If you do searching on this topic, you will find a lot of scientific articles about atomic behaviour in various solutions, matrix properties, copper and nickel behaviour in a thousand situations, but no exact answer on a direct question like yours. As far as I understand you must, first of all, ask yourself what the board is made for: wire bondable? Yield? Cost? Shelf life? Leadfree? Environment? Life expectancy?..etc. which you have done, of course.
Now, specs and thumb rules use to say a minimum of 1 micrometer nickel, and in practice some PWB makers recommend little thicker. The diffusion of copper through nickel starts early in the board's life, namely when you solder mount your components. I'm not sure following is correct, but found copper diffusion to be about 0.0005 micrometer per second at 215 Centigrades, resp 0.005 at 250 Centigrades. Means that approximately a micrometer or so of copper has diffused into the nickel when at top temperature during soldering. According to Arrhenius law, diffusion at lower temperatures is very slow, takes probably years to make on micrometer at room or even at rather high operating temperature.
Reality then. Nickel plating can look very different. The HP report I mentioned once , illustrated numerous faces and lots of variants speaking impurity levels. From what I've seen, diffusion through ideal nickel is one aspect, diffusion through porous nickel, or impure nickel, banded nickel, columnar nickel is something else. I remember something Werner E said about ten times faster diffusion, or even higher speed. So, one has to face reality rather than academic and ideal tests.
As follows from the above, I use to tell people here, that 3 micrometers is minimum nickel thickness and 5 micrometers ought to be typical value. Hence, one supplier has stated 3-7 micrometers of nickel. This makes a reliable and longlasting and robust board.
Interesting topic..
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]
Sent: den 3 maj 2005 22:28
To: [log in to unmask]
Subject: [TN] minimum nickel thickness`
hi,
forgetting about ipc spec requirements, what is the minimum nickel plating thickness over copper on a printed wiring board that you would feel comfortable with as a barrier against copper migration.
thanks,
phil
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