I would generally believe that voids are not related to a rack location on the
electroplate copper line,  the normal pattern plate copper related voids are
"bubble" type where air bubbles trapped in the smaller holes prevent your etch
resist (tin or solder) from plating,  this is easily identified by cross section
analysis and is called "etch-out" voids,  they typically show etched side walls
at the edge of the void and occur around the entire circumference of the hole
which electrically separates one end of the hole barrel from the other. Bubble
voids are usually caused by inadequate through-hole agitation or extremely
roough holes.   If you are sure that the voids are truely electroplate related
and do occur only on the bottom portion of the rack,  check your anode film.
Anode film is necessary in a healthy plating cell,  it is formed by the
brightener chemistry and aids in uniform anode corrosion and fine grain
deposits.  The film should be black in color, thick enough to be non-transparent
but not thick enough to sludge.  If the film is grey or white in color,  the
anode is passivated (polarized) and must be etched to remove all films.  Your
rectifier voltage is a good indicator of your anode film condition,  as the
anode becomes more passivated a higher voltage will be required to deliver the
same amps as a well filmed anode.  In extreme cases your voltage will max out on
the rectifier and never allow the required amps to be achieved.  Polarized
anodes will result in poor throw into the smaller holes first,  and the copper
deposit will be very grainy, this grainy deposit will not coat well with your
tin or solder and could result in etch out type voids.  There should also be no
copper sulfate crystals in your anode baskets,  a blue copper crystal indicates
that the anode bags are plugged and are not allowing the solution to pass
freely.  As the anode corrodes,  the copper sulfate concentration will build up
saturation levels if the bag is plugged.

If you would like to talk about the problem please give me a call.  Hope this is
usefull.

Tony King
Sanmina,  Nashua N.H.
603-886-0066

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