Timothy: I beg to differ. Mass transfer under normal (current
density/lateral+air agitation) conditions has nothing to do with it. If plating
current density is in normal 10 to 30 ASF range and use conventional lateral
rack movement and air agitation, your current efficiency is close to 100%, which
means that close to 100% of your current passed through the electrolyte is
converted into equivalent amount of copper. If there are not enough copper ions
available, or in other words the current applied is higher than localized Cu
concentration can support -- the deposit will "burn", i.e. form rough
dendrite-like deposits. Only part of the current -- as much as copper ions can
carry -- will be consumed for Cu deposition, the rest -- for hydrogen evolution
(that's why it "burns", hydrogen evolution disrupts orderly crystallization
stage of Cu ad-atoms). Therefore, mass transfer limitations start to kick in at
much higher current densities. Ohmic resistance and charge transfer, rather
then mass transfer are the limiting factors in high aspect ratio hole plating
under normal current density+agitation conditions.
ASF can't be roughly the same as you said, they HAVE to be directly proportional
to the amount of copper deposited. If copper thickness is non-uniform, that was
caused by the difference in ASF (Faraday still rules). And the PTH in the
middle of a ground plane will plate slower then an isolated pad/PTH due to lower
localized current density (larger plating area), rather then due to the lower
copper concentration, i.e. ground plane shields the PTH or acts as a thief area.
Eric Yakobson
Alpha PC Fab
Timothy Reeves <[log in to unmask]> on 10/20/99 01:14:50 PM
To: [log in to unmask]
cc: (bcc: Eric Yakobson/AlphaPCFabUS/Cookson)
Subject: Re: [TN] ASF info...Super Plater
The ASF is roughly the same regardless of location, but the mass transfer
(of copper ions) is going to be different due to concentration gradients
being different, and that depends on bulk solution concentration, agitation,
copper patterns on the board, hole geometry and probably a few other things
to lesser degree.
The PTH in the middle of a ground plane will plate slower than a isolated
one, due to lower Cu++ concentrations in the area right around the ground
plane hole.
I suppose what you are really asking is "how does one calculate the plating
rate in this hole or that hole?"
I don't think there's a simple formula, though I understand some CAM
software (we don't have) does look at copper distribution and recommend
adding thief areas to boards that need it.
One can reduce the plating variation in acid Cu by operating at the low end
on CuSO4 conc. and the high end on H2SO4 conc., maximizing agitation,
evening out anode distribution, and increasing anode-to-panel distance.
But knowing all that hasn't helped me reduce copper thickness
variation much, so I'll be looking for better answers along with you!
> ----------
> From: Robert Jordan
> Sent: Tuesday, October 19, 1999 7:45 AM
> Subject: ASF info...Super Plater
>
> Hello,
> Once again another question drawing in the vast talent and experience of
> Technet. Is there a way to calculate the ASF inside a through hole in a
> printed circuit board during the electroplate copper process? Is that ASF
> different if the pth is in the middle of a ground plane vs. an isolated
> area? If no formula exists please bend my ear with your opinion.
>
> Thanks,
> Bob Jordan
>
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