John,

In reply to your inquiry, unlike the situation with Pb in Au, the Pb in
Ni will not fully mix with the solder, because the solder dissolves only
a small amount of the Ni in wetting to it. 

To get a perspective of what is involved, consider this picture: A
soldered assembly, with hundreds or thousands of tiny solder
connections, is covered by the RoHS directive and put on the market in
the EU after the deadline. The connections to lands on the board are
typically under the component terminations, but in some cases at least a
little of the land can be seen along side the termination. The land, 10
mils (250 micrometers) wide by 40 mils long, consists, in downward
sequence, solder, Ni, and Cu. Below that is the laminate. The solder
(3.5 percent Ag, 0.7 percent Ag, balance Sn) is not a flat layer, but
averages several mils (several tens of micrometers) thick. It contains a
tiny amount of Pb from the Ni that was dissolved (tens of nanometers)
during wetting. There is a layer of Sn-Ni intermetallic compound (a few
more tens of nanometers) that formed during the wetting. It may contain
some of the Pb that had been in the Ni that reacted with the Sn in the
solder. The rest of the Pb will have accumulated in the Ni below the
intermetallic compound. The Ni is 50 microinches (roughly 1 micrometer)
thick. It has intentionally added Pb at 600 ppm. The Cu is 1 mil (25
micrometers) thick. It will not contain Pb - certainly no intentionally
added Pb. The laminate (typically epoxy-glass, with the - legal -
brominated flame retardant TBBPA added) is a millimeter thick.

Now try to imagine a test (nondestructive or destructive) for the
presence and concentration of Pb in the Ni on this product. Then try to
imagine this test being applied in a practical situation by an
enforcement authority to determine whether a manufacturer has violated
the spirit of the RoHS directive by intentionally adding a trace amount
of Pb to the Ni.

Being in business requires willingness to accept certain risks. I
suspect that most manufacturers who want to employ Ni or Au plating are
not going to agonize over the risk associated with the intentional
addition of trace amounts of Pb to it.

 

Gordon Davy 
Baltimore, MD 
[log in to unmask]

410-993-7399 


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