Kelly,
The greater is the difference in the cathode polarization (on high/low
current density points of the cathode) -- the more uniform is the current
distribution.
It is really difficult to properly explain this statement without going
into the details and definitions of primary/secondary current distribution,
overpotential and polarization, which without any doubt would bore most
TechNetters to death.
To put it simplistically, high polarization on high current density points
means that actual current density on them is lower then it otherwise would
have been if this polarization did not exist.
If you would like to discuss this subject any further, please contact me
off line.
Eric Yakobson
Kelly Kovalovsky <[log in to unmask]> on 11/09/98 07:21:58 AM
To: [log in to unmask]
cc: (bcc: Eric Yakobson/AlphaPCFabUS/Cookson)
Subject: Re: [TN] Copper plating & Copper thickness
Your note is interesting and educational for me and I appreciate your time
and
effort in providing this detail.
I have a follow on question that maybe you can help me with. If I may
restate
your second paragraph I would say:
At high average current density the cathode polarization difference between
low
and high current points is more uniform (To me, less difference means more
uniform) and therefore the current distribution is less uniform.
I may not understand cathode polarization correctly, but I assume from your
statement that there is an inverse relationship between the uniformity of
cathode polarization and the uniformity of current distribution. Please
confirm.
Kelly Kovalovsky
[log in to unmask] on 10/31/98 08:41:39 PM
Please respond to [log in to unmask]
To: [log in to unmask]
cc:
Subject: Re: [TN] Copper plating & Copper thickness
Michael,
With all due respect, I'm afraid I have to disagree with your explanation.
While it is true that cathode current efficiency for a typical acid copper
electrolyte is close to 100%, this is exactly the reason why it does not
have any effect on current distribution. The key to the explanation of
this phenomenon is cathode polarization (sorry for the electrochemical
terminology, but it would be rather difficult to explain this without using
it).
At low average current density the difference between cathode polarization
on lower/higher current density points of the cathode in most electrolytes
(including acid copper) is normally much greater than at high average
current density. Therefore, the current distribution is more uniform at
low average current density.
In most alkaline electrolytes, where current efficiency is much lower than
100% (the rest of the current is consumed for hydrogen gas evolution), it
does affect the current distribution favorably. Since at higher densities
more current is consumed for hydrogen evolution, the metal distribution
generally is more uniform then in acid solutions.
Low current density will result in higher throwing power in both systems.
One of the few practical exceptions to this rule (low current density/high
throwing power) is hex-chromium electrolyte, in which current efficiency is
directly proportional to current density.
As to the minimum copper thickness, historically (by trial & error I guess)
it was determined that 1 mil (25 micron) of copper is a minimum thickness
that will reliably withstand multiple solder floats. From my practical
experience, 0.5 mil cracks much more readily, which is understandable,
considering the fact that copper tensile strength is proportional to the
thickness of the deposit.
Best regards.
Eric Yakobson
Alpha PC Fab
mcarano <[log in to unmask]> on 10/31/98 07:30:36 AM
To: [log in to unmask]
cc: (bcc: Eric Yakobson/AlphaPCFabUS/Cookson)
Subject: Re: [TN] Copper plating & Copper thickness
Dear Francis,
The best explanation for any type of electroplating in an acid medium has
to
do with plating efficiency. Generally, acid plating process like the acid
copper, acid tin etc are close to 100% efficient. In other words, the
plating favors the high current dednsity areas of a pwb(the surface as
opposed to the hole) By lowering the cathode current density, you now tend
to favor the low current areas somewhat, and thus improve throwing power.
This works just the opposite with alkaline based plating solutions.
With respect to high frequency and plating unformity, I am not sure.
Generally, the more thickness inside the hole, the better the hole wall
plating is able to stand up to thermal excursions.
Hope this helps.
-----Original Message-----
From: Francis Lai <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Saturday, October 31, 1998 9:30 AM
Subject: [TN] Copper plating & Copper thickness
>Dear Sir :
>
>I work inn a high frequency PCB manufacturing company and I have the
>
>following questionns :
>
>1/ Why is it for copper plating (acid copper plating) the lower the
> current, the more the throwing power ? If I start to plate copper
> on the PTH walls with high current density after PTH, what will
> happen ?
>
>2/ Our customer always specify to have a certainn thickness innside the
> PTH walls for copper. Why is that so for high frequency PCB ?
> What happen if copper thickness is thinner than the specified
> thickness at high frequencies ?
>
>Thanks for your help !!!!!!!
>
>my e-mail : [log in to unmask](this is only text base, pls do not
send
> graphics)
>
>Thamks again!!!!!!!
>
>Francis
>
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