Hi Folks,

        A number of you have discussed the pros and cons of Throwing power and
Anode and Cathode current densities. I appreciate many of the comments, but I
just have to put in my two cents worth. Although it is driven by current
density, the throw in the hole is also related to boundary layer conditions on
the panel surface vs. the boundary layer in the hole. Due to the fact that
there is usually a high turbulence of solution at the panel surface, because of
solution and/or air agitation, the boundary layer is relatively thin there.
There is also a relative high concentration gradient of Cu ++. ions across the
boundary layer. This two factors tend to drive the mass transport across this
boundary layer rather high. Within the hole there is little turbulence and
therefore a much thicker boundary layer. Also since there is a large amount of
Cu ++ ions being consumed at the panel surface, there is a smaller
concentration gradient of Cu ++ ions across the boundary layer in the hole.
These two factors tend to impact the mass transport and therefore lower plating
rate within the hole. Thus the Throwing power or ratio of copper in the hole to
that on the surface is low. The lower current densities tend to minimize this
phenomena, but it is still present.
        In years gone by attempts to get more agitation in the hole through
Impingement plating were implemented. The cost of this type of equipment for
most board shops was prohibitive and most of the manufacturers dropped the
technology.
        Electroless copper plating overcomes this Phenomena to a large degree,
due to the fact that air agitation is typically not used and the plating rate
is so slow that the Concentration Gradient across the two boundary layers is
about equal, since the Cu ++ ions can be rapidly be replaced. Of course this
type of plating has its own set of problems.

So there's my two cents worth, maybe it will help someone.

Regards,
Les

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