Editing Electroplating (section)

==Test cells and characterization==

===Throwing power===
''Throwing power'' (or ''macro throwing power'') is an important parameter that provides a measure of the uniformity of electroplating current, and consequently the uniformity of the electroplated metal thickness, on regions of the part that are near the anode compared to regions that are far from it. It depends mostly on the composition and temperature of the electroplating solution.<ref name="Farber 1930" /> ''Micro throwing power'' refers to the extent to which a process can fill or coat small recesses such as [[Through-hole technology|through-holes]].<ref name="McCormick+Kuhn 1993">{{cite journal | last1=McCormick | first1=M. | last2=Kuhn | first2=A. T. | title=The Haring-Blum Cell | journal=Transactions of the IMF | volume=71 | issue=2 | year=1993 | issn=0020-2967 | doi=10.1080/00202967.1993.11870992 | pages=74–76}}</ref> Throwing power can be characterized by the dimensionless [[Wagner number]]:

<math display="block">\text{Wa} = \frac{RT\kappa}{FL\alpha|i|},</math>

where ''R'' is the [[universal gas constant]], ''T'' is the operating [[temperature]], ''κ'' is the ionic conductivity of the plating solution, ''F'' is the [[Faraday constant]], ''L'' is the equivalent size of the plated object, ''α'' is the [[charge transfer coefficient|transfer coefficient]], and ''i'' the surface-averaged total (including [[hydrogen evolution]]) current density.  The Wagner number quantifies the ratio of kinetic to ohmic resistances.  A higher Wagner number produces a more uniform deposition.  This can be achieved in practice by decreasing the size (''L'') of the plated object, reducing the current density {{math|{{abs|''i''}}}}, adding chemicals that lower ''α'' (make the electric current less sensitive to voltage), and raising the solution conductivity (e.g. by adding [[acid]]). Concurrent [[hydrogen evolution]] usually improves the uniformity of electroplating by increasing {{math|{{abs|''i''}}}}; however, this effect can be offset by blockage due to hydrogen bubbles and hydroxide deposits.<ref>Fuller, T. F.; Harb, J. N. Electrochemical engineering; John Wiley & Sons, 2018. ISBN 9781119446583</ref> 

The Wagner number is rather difficult to measure accurately; therefore, other related parameters, that are easier to obtain experimentally with standard cells, are usually used instead. These parameters are derived from two ratios: the ratio {{math|''M'' {{=}} ''m''<sub>1</sub> / ''m''<sub>2</sub>}} of the plating thickness of a specified region of the cathode "close" to the anode to the thickness of a region "far" from the cathode and the ratio {{math|''L'' {{=}} ''x''<sub>2</sub> / ''x''<sub>1</sub>}} of the distances of these regions through the electrolyte to the anode.  In a Haring-Blum cell, for example, {{math|''L'' {{=}} 5}} for its two independent cathodes, and a cell yielding plating thickness ratio of ''M'' = 6 has Harring-Blum throwing power {{math|100% × (''L'' − ''M'') / ''L'' {{=}} −20%}}.<ref name="McCormick+Kuhn 1993" /> Other conventions include the Heatley throwing power {{math|100% × (''L'' − ''M'') / (''L'' − 1)}}, Field throwing power {{math|100% × (''L'' − ''M'') / (''L'' + ''M'' − 2)}},<ref name="Gabe 2002">{{cite journal | last=Gabe | first=David R. | title=Test cells for plating | journal=Metal Finishing | volume=100 | year=2002 | issn=0026-0576 | doi=10.1016/s0026-0576(02)82059-1 | pages=579–586}}</ref>  and Luke throwing power {{math|100% × ''L'' / (''L'' + ''M'' − 1)}}. A more uniform thickness is obtained by making the throwing power larger (less negative), except for Luke's throwing power, which has the advantage of having a minimum of 0 and a maximum of 100, in terms of the less negative value, according to any of these definitions.

Parameters that describe cell performance such as throwing power are measured in small test cells of various designs that aim to reproduce conditions similar to those found in the production plating bath.<ref name="McCormick+Kuhn 1993" />

===Haring–Blum cell===
[[File:Haring Cell.jpg|thumb|Haring–Blum cell]]

The Haring–Blum cell is used to determine the macro throwing power of a plating bath. The cell consists of two parallel cathodes with a fixed anode in the middle. The cathodes are at distances from the anode in the ratio of 1:5. The macro throwing power is calculated from the thickness of plating at the two cathodes when a [[direct current]] is passed for a specific period of time. The cell is fabricated out of [[perspex]] or glass.<ref>{{cite book|last1=Bard |first1=Allan |last2=Inzelt |first2=György |last3=Scholz |first3=Fritz |chapter=Haring–Blum Cell |page=444 |title=Electrochemical Dictionary|publisher= Springer|doi=10.1007/978-3-642-29551-5_8|year=2012|isbn=978-3-642-29551-5}}</ref><ref>{{cite book|page=122|title=Electrochemical Engineering: Science and Technology in Chemical and Other Industries|year=1999|last1=Wendt |first1=Hartmut |first2=Kreyse |last2=Gerhard|isbn=3540643869|publisher=Springer }}</ref>

===Hull cell===
[[Image:Hullcell.jpg|thumb|A zinc solution tested in a Hull cell]]
The [[Hull cell]] is a type of test cell used to semi-quantitatively check the condition of an electroplating bath. It measures useable current density range, optimization of additive concentration, recognition of impurity effects, and indication of macro throwing power capability.<ref>{{Cite book | title = Metal Finishing: Guidebook and Directory. Issue 98 | page = 588 | year = 1998 | volume = 95}}</ref> The Hull cell replicates the plating bath on a lab scale. It is filled with a sample of the plating solution and an appropriate anode which is connected to a [[rectifier]]. The "work" is replaced with a Hull cell test panel that will be plated to show the "health" of the bath.

The Hull cell is a trapezoidal container that holds 267 milliliters of a plating bath solution. This shape allows one to place the test panel on an angle to the anode. As a result, the deposit is plated at a range current densities along its length, which can be measured with a Hull cell ruler. The solution volume allows for a semi-quantitative measurement of additive concentration: 1&nbsp;gram addition to 267&nbsp;mL is equivalent to 0.5&nbsp;oz/gal in the plating tank.<ref>{{cite web|last=Kushner|first=Arthur S.|date=December 1, 2006|url=http://www.allbusiness.com/manufacturing/chemical-manufacturing-paint/3993213-1.html |url-status=dead |archive-url=https://web.archive.org/web/20100313093038/http://www.allbusiness.com/manufacturing/chemical-manufacturing-paint/3993213-1.html|archive-date=March 13, 2010 |title=Hull Cell 101 |website=Products Finishing}}</ref>