Jacob,

The thickness distribution of the metal deposited on the cathode depends
on:

1. Geometrical dimensions of the plating cell and anode/cathode placement.
2. Composition of the plating solution and plating parameters.

In order to be able to compare metal thickness distribution in various
plating solutions (electrolytes)  or in the same electrolyte at various
plating conditions Hering and Blum (in 1923) introduced the term "throwing
power". Throwing power can be simplistically described as ability of an
electrolyte to distribute electric current on the surface of the cathode.
Current distribution is usually defined as primary and secondary.  Primary
distribution is dependent on solely geometrical parameters: cell dimensions
and anode /cathode placement.  Secondary (actual) distribution depends on
the nature of the electrolyte and electrolysis parameters, therefore it can
be evaluated by measuring the throwing power of a particular plating
solution at certain plating conditions (temperature, current density,
agitation etc.).

There are not only many techniques to measure the throwing power, but also
many ways to calculate it using various equations.

The original (and probably the least complicated) Hering-Blum method
involves  plating two identical cathodes attached to the short sides of a
long rectangular plating cell with an insoluble perforated anode placed
between cathodes.  The anode is placed closer to one of the cathodes.  The
anode-to-cathode distance ratio is 5 : 1.

     Throwing Power = (5 -- W)/(5 -- 1) x 100%
     W = W1/W2.
     W1 is the first (closer) cathode weight increase after plating, W2 --
second cathode weight delta.
     When W1 = W2, W = 1 (maximum), throwing power = 100%.
     When W1 = 5xW2, throwing power = 0%.

Generally, throwing power is directly proportional to the conductivity of
the electrolyte and inversely proportional to the current density applied.
The throwing  power of an Acid Copper process will increase with higher
sulfuric acid concentration (to a point) and lower copper concentration due
to higher solution conductivity.  It is also dependent on cathode
polarization created by the additive system and all the factors that affect
it (temperature, agitation, concentration etc.), but the relationship is
not linear.

Best regards.

Eric Yakobson
Alpha PC Fab





Jacob Ransborg <[log in to unmask]> on 10/05/98 05:31:44 PM

To:   [log in to unmask]
cc:    (bcc: Eric Yakobson/AlphaPCFabUS/Cookson)
Subject:  [TN] Throwing Power




Hi Tech Netters

I have seen different ways to define the throwing power in a Cu-plating
process.
Can anyone show/tell me the exact definition and explane what throwing
power means.

Regards
Jacob Ransborg

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