Test cells for plating

contro|, analysis, and testing T E S T CELLS F O R P L A T I N G BY DAVID R. GABE INSTITUTE OF POLYMERTECHNOLOGY AND MATERIALSENGINEERING, LOUGHBOROUGH UNIVERSITYOF TECHNOLOGY, LOUGHBOROUGH, LEICESTERSHIRE,U.K.; www.tboro.ac.uk/departments/iptme

Plating cells are employed to define the link between processing conditions and deposit properties and characteristics for electrodeposition. ProcessParameters: Solution concentration Conductivity, pH Temperature Current density Voltage/potential Deposit Parameters: Composition/purity Thickness uniformity Smoothness, brightness Hardness, ductility, strength Grain size

The electroplater needs a rapid and convenient test method to give a good, not necessarily precise, indication of this interaction and, in particular, to indicate any departure From perceived optimum process conditions and then provide the means of correcting that departure. A number of cells have been devised to give such a convenient link. They attempt to correlate the parameters of prime concern-composition and current density-with deposit thickness and thickness distribution. Figure 1 and Table I illustrate this choice. Some of the cells in Fig. i and Table I have very specific applications, e.g., D and K for chromium plating, where the throwing power (TP) is poor and the CrO3:SO4 catalyst ratio is critical. Two (B,F) have been adopted for wider use on account of their versatility. H A R I N G - B L U H CELL

This cell was devised as a TP cell and consists of a long, narrow plating box with Table I. Design and Function of Plating Cells Shown in Fig. 1

Example

Design

Purpose

C,D

Bent cathode cells

Throwing power and recess plating

A,B,I

Hull cells

Current distribution and deposit appearance

J,K

Slot cells

Slot acts as a point anode

F

Haring-Blum cell

Throwing power (TP) and TP index determination

A,C,E,F,G

Variable geometry

Combined cells

H

Current efficiency cell

Uniform deposition for cathode current efficiency measurement 511

\ll

o

A

+

Ill

6

+

+ °

+

-

I

,.

d

!

+

II

Ii I

I

+

Fig. 1. Test cells used in electroplating control.

one central movable anode and two cathodes placed at relative distances, typically of 5:1 from the anode. TP is calculated from a formula, the classic Haring version being: % TP = 100 (L-R)/L where L is the far-to-near cathode distance ratio and R is the ratio of weights of deposits on the cathodes. The experimental principle is well established, but a number of alternative formulas have been used primarily to either give a symmetrical TP scale or to expand the scale at specific ranges. Three other TP index formulas in use are

Healey % T P = 100 (L- M/(L- 1)

Field: % TP = 100 (L- M/(L+M - 2)

Subramanian: % TP = 100 (L- M)/M(L- 1) Of these formulas, that of Field has been most widely used and is incorporated in some standards (e.g., British Standard 205 part 5), with L = 5:1, largely because 512

Table II. Throwing Power Indices for Various Values of M M Haring-Blum Heatley Field

Subramanian

1

80

100

100

100

2

60

75

60

37.5

3

40

50

33.3

16.7

5

0

0

0

0

-100

-125

-38.5

-12.5

-92

-23.75

100

-25

10 100 .

-1,900 .

.

-2,375 .

.

.

the values of TPcan vary from + 100 to -100, with a value ofTP = 0 when the current and metal distribution are equal. This scale gives the poorest TP of[-100% (as } 9t when there is no metal deposited on the far cathode) and the best TP of +100% (for M = 1). If metal distributes according to the length ratio (e.g., M = 5 when L = 1), the TP is 0. Values of the TP index for each formula are tabulated in Table II for the Haring-Blum cell and several others. Chin has proposed a logarithmic index, such that M = 1 when L = 1, which would yield more convenient numbers for the index values. Chin's definition is: Logarithmic TP = Log L/Log M Graphic and computerized methods have also been developed. HULL CELL

The Hull cell provides four basic facilities for the electroplater in plating process control: optimization for current density range; optimization of additive concentration; recognition of impurity effects; and indications of macro-throwing power capability. The c o m m o n type of Hull cell is the 267-ml trapezoidal container utilizing a 4-in. (10.2-cm) cathode panel inclined at 38 ° to the parallel sides and having a 2in. (5.1-cm) anode, which may be corrugated to increase its electrode area_ A solution depth of 2 in. (5.1 cm) gives a volume of 267 ml; a 3-in. (7.6-cm) depth would give a volume of 320 ml. Figure 2 illustrates the form of the standard cell. The dimensions are important to give the following relationships: 267-mi cell: 2 g addition is equivalent to 1 oz per U.S. gal 320-ml cell: 2 g addition is equivalent to 2 oz per Imperial gal

The cell can be made of a number of materials, including Perspex, Lucite, polypropylene, or glazed porcelain, depending upon the temperature of usage and the corrosivity of the electrolyte. It is nowadays usual to ANODE FCATHODE "~ ~ 1 7/8" , / supply them as a plain box with incorporated air bubbler or with incorporated thermostatic heater. 2 6 7 ml The standard cell current is 2 SOLUTION A, which gives a current density 2 1/2" I LEVEL range of 2.4-84 A/ft 2 (0.26-9 A/dm 2) corresponding to anodeI to-cathode spacings of 5 to i in., I. -5" .I respectively. The cell is calibrated Fig. 2. Dimensions of the Hull cell.

LL_

513

I

I

I

by using thickness profiles of the cathode panel and assuming appropriate current efficiencies, such data yielding a calibration graph having curves for 1, 2, 3, etc.-A cell currents (see Fig. 3). If preferred, a formula may be derived having the following form:

14 12

3A

tO .<

8

2A

6 4

ill/ = Iappl(a-b log L) 2

2.5

5.0

7.5

10.0

Distance from high current density end, cm.

Fig. 3. Calibration graph for a Hull cell with cell currents of 1, 2, and 3 A.

where ill/ is the current den° sity at a distance L; Iappl is the total cell current employed; L is the length along the panel; and a,b are constants requiring calibration. Using inch and A / f t 2 units, a = 27.7 and b = 48.7; if

cm and A/cm 2 are used, a = 5.10 and b = 5.24. Other sizes of Hull cell, which have been employed for specific purposes and calibrations, would be: 3 x 5-in. panel with 2.5-in solution depth a =18.8; b = 28.3 6.4 x 10.2-cm panel with 5-cm solution depth a = 5.10; b = 5.24 In all cases, extremities of the cathode panel should be neglected and every attempt made to eliminate solution impurities, residues, etc. and to maintain a consistent procedure with a standard time of plating, typically 2 or S min (for S and 2 A, respectively). Means of temperature and agitation control can be built in for the same reason. Interpretation of the cathode panels relates to three pieces of derived information. 1. Establishment of the optimum plating current density range by means of the graph, formula, or various charts. For this purpose, a mid-height line should be taken to minimize stagnation effects at the cell bottom and to avoid emphasizing convection effects at the top meniscus. 2. Establishment of the additive level required to create bright or level deposit zones preferably at the center of the panel. For this purpose, aliquots of 0.5 or 1.0 g are added to the cell, mixed in, and the effect noted. 3. Recognition of atypical appearances attributable to the presence of impurities in solution or to additive degradation products. Experience frequently indicates an appropriate remedy and suppliers of proprietary processes should provide guidance notes. A number of coding and diagrammatic systems have been used for the panel's appearances (see Fig. 4). The panel demonstrates, in zones from left to right, high, medium-, and low-current density appearances corresponding to burned/nodular, semibright/bright/leveled to dull, or no deposit zones. The presence of additives will eliminate extremes of burning and powdery/dendritic deposit formation and ideally widen the leveled/bright zone into a practicable working range of current density at that temperature and solution concentration. 514

BRIGHT P L A T I N G

ADDITION

A.A.

50

AGENT-NONE

% OF" O P T I M U M

W//A A.A.

t----I

OPTIMUM

V/A A.A, DAR...K

BRIG.HT

150%

OF OPTIMUM

CRACKED

A.A. 2 0 0 % OFOPTIMUM GR AY

PITTED

oo oo

NO PLATE

×y

Fig. 4. Diagrammatic system for reporting Hull ceil behavior.

MODIFIED CELLS Each of the basic cells has a number of limitations that have been recognized, but which do not detract from usage as "qualitative procedures to provide quantitative information," however, in a number of specific cases, the limitation could be more serious; for example, temperature control may be poor and metal ion concentrations may change if the test extends over a few minutes. Various modifications have, therefore, been employed, which, for the Hull cell, can be enumerated as in Table III. The most serious shortcoming relaxes to the use of agitation to enhance deposition rates in high-speed electrodeposition processes when a correlation factor of 5 to 50 times may apply between the "static" Hull cell and the "dynamic" plating tanks. This factor can be determined typicallyby comparing bright plating ranges for the Hull cell with those identified for the tank; recently, however, several proposals have been made for current density distribution cells incorporating electrode rotation as a convenient means ofquantitaxive agitation. They utilize rotating cone electrodes; rotating cylinder electrodes; or rotating electrodes in baffled cells. In general, all appear to be reasonably effective, but each requires development work in the context of a particular application to establish its credibility. TEST CELLS AND TROUBLESHOOTING A number of applications for test ceils have been identified, which may conveniently be classified as troubleshooting. In most cases, the most successful use of Table III. Modifications to the HuU Ceil Design

Authors

Hanging cell to be suspended in a tank

Mohler ~ Skwirzynski and Huttley~

Flat cathode but curved anode to yield a linear current density to L relationship

Gilmont and Walton ~

Circulating electrolyte pumped along cathode plate at controlled velocity

Dimon 4

Nonvertical cathode to counter natural convection

Esih 5

Segment of a concentric cell with cathodes at either end and variably positioned anode

Teraikado and Negasaka6

515

the cells is when the solutions or processes in question have been fully characterized when known to be in a first-class working order. This clearly suggests that each solution should be fully studied when first used and a set of reference panels and data prepared for regular use. (It is usually advisable to lacquer such panels or otherwise protect them in sealed polymer envelopes in order to provide adequate shelf life.) It is also worth noting that each metal and solution type has its own character, which experience will define only too well. The test cells, particularly the Hull cell, can be used to identify noncharacteristic behavior under several categories. 1. Establishment of the optimum current density range. 2. Recognition of the effect of variation of temperature, pH, presence of impurities, changes in solution concentration, especially critical components such as conductivity salts or catalysts in hexavalent chromium solutions. 3. Defining the level of brighteners and their breakdown products. This will indicate the need for activated carbon/filtration treatment and the need for brightener replenishment. 4. The effects of agitation must be carefully noted because they may be related to shortcomings of the cell itself. Cells may also be adapted for other purposes. For example, it need not be necessary to have both a Hull cell and a Haring-Blum cell in regular use, because several workers have shown how TP can be measured using the Hull cell by measuring deposit thickness across the regular panels. In particular, Watson 7 and more

~

Cathodic ~

Anodic f

~Dmq41

e

m

........

I~d ~ dllml I mliml I~ nlol D................

~bNdOEEImIN41

!

,i M a g n e t i c stirrer

_Ll

5ram

1 O0

mm

~]

Fig. 5. Schematic form of the Assaf throwing power ceLL.

516

recently Monev and Dobre 8 have obtained good comparable data. A cell designed to assess throwing power in small/narrow recesses has been described by Assaf) and utilizes a rectangular (40 X 40 mm) or round (40 mm diameter = 0.25 dm 8) coupon as cathode placed 5 mm in front of a dummy perspex cathode sheet (see Fig. 5). The anode-cathode spacing is not specified but is typically 150 ram. The throwing power is expressed as the ratio of deposit thickness on the rear compared to the thickness on the front and as it can be suspended over the installed agitator it can overcome uncertainties of the effect of agitation. Its use for assessing solutions for printed circuit board production has been described and its use in assessing the effects of pulsed current emphasised: throwing power ratios for pulsed current of 0.66 to 0.75 for DC plating compared to 1 for pulsed currents have been reported thus demonstrating its sensitivity.10

Aclenowle~nent This chapter has drawn on some aspects of a similarly titled article by J.B. Mohler included in earlier editions of the Metal Finishing Guidebook. Readers requiring more information may care to refer to those volumes.

1. Mohler, J.B., Metal Finishing, 32:55; 1959 2. Skwirzynski, J.K. and M. Huttley, Journal of the ElectrochemicalSociety, 104:650; 1957 3. Gilmont, R. and R.F. Walton, Proceedings of the American Electroplating Society, 4:239; 1956;Journal of the ElectrochemicalSociety, 103:549; 1956 4. Dimon, 1LA.,Proceedings of the American Electroplating Society, 35:169; 1948 5. Esih, I., Metalloberflache, 20:352; 1966 6. Teraikado, R. and I. Negasaka, Metal Finishing, 77(1):17; 1979 7. Watson, S.A., Transactions of the Institute of Metal Finishing, 37:28; 1960 8. Monev, M. and T. Dobrev, MetalFinishing, 90(10):50; 1992 9. Assaf, Y., Plating, 67(10):12; 1980 10. Gabe, D.R. et al., Plating & Surface Finishing, 88(5): 127; 2001

FurtherReading Gabe, D.R. and G.D. Wilcox, Transactionsof tbe Institute of Metal Finishing, 71(2):71;

1993

Nohse, W., The Hull Cell, Robert Draper, Teddington, London, 1966 The Canning Handbook, E.F. Spon Ltd., London, 1982, pages 939-943

517