Chloride baths for PbSn alloy deposition

Surface Technology, 18 (1983) 77 - 85

77

CHLORIDE BATHS FO R P b - S n ALLOY DEPOSITION

RAJ NARAYAN and G. DEVRAJ* Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016 (India)

(Received August 3, 1982) S u mmar y In this paper, chloride baths of suitable composition for the electrodeposition o f Pb-(5% - 60%)Sn alloys are described. The tin c o n t e n t of the deposit increased with an increase in either the current density or the tin c o n t e n t o f the bath and with a decrease in either the agitation of the bath or the temperature. At higher current densities and plating times the deposits became p o wd er y. The grain size of the electrodeposited alloy increased with increasing bath temperature and decreasing current density. X-ray diffraction studies showed that the alloys were crystalline in nature. The cathode current efficiency was between 64% and 93%, depending on the bath composition and the operating conditions.

1. I n t r o d u c t i o n P b - S n alloys are commercially electrodeposited from fluoborate baths [ 1 ] . It has been claimed t hat these alloys can also be deposited using fluosilicate [2 - 4 ] , sulphonate [5 - 8 ] , sulphamate [9] and p y r o p h o s p h a t e baths [ 1 0 ] . In spite of many inherent advantages, chloride baths have not been extensively tested for the electrodeposition of P b - S n alloys, with the exception of one Russian patent [ 1 1 ] . However, adherent and bright deposits could n o t be produced when a bath prepared in accordance with this patent was tried. The present investigation was undertaken in order to investigate the possibility of depositing P b - S n alloys from chloride baths and to study the effects of the composition of the bath and the operating variables on the composition and properties of the deposit.

2. Experimental details Anodes of the desired composition (Table 1) were prepared by v a c u u m melting the required amounts of lead and tin. Melted ingots were rolled into *Present address: Central Electrochemical Research Institute, Karikudi, India. 0376-4583/83/0000-0000/$03.00

© Elsevier Sequoia/Printed in The Netherlands

78 TABLE 1 Composition of plating baths and Pb-Sn alloy anodes Composition a

A n o d e (% Pb)

Bath 1

Bath 2

Bath 3

Bath 4

Bath 5

Bath 6

46.10

91.73

74.85

59.75

49.85

60.36

Chemicals (g 1- i )

Lead chloride Stannous chloride Boric acid

8.5000 0.9220 1.50

8.2267 1.4753 1.50

6.7113 4.6087 1.50

5.3567 7.3733 1.50

4.4720 4.2000 1.50

3.5780 11.0667 1.50

aThe total metal content of the bath was kept constant at 6.66 g l i while the ratio of lead to tin was varied. sheets 2 m m t h i c k a n d were h o m o g e n i z e d at 120 °C f o r 3 days. T h e c o p p e r s h e e t c a t h o d e s were successively polished with 1/0, 2/0, 3 / 0 and 4 / 0 grade e m e r y papers, w a s h e d with distilled w a t e r a n d degreased with a c e t o n e b e f o r e t h e y were plated. T h e plating b a t h s (Table 1) were p r e p a r e d b y dissolving the r e q u i r e d a m o u n t s o f the chemicals. T h e r a t i o o f t h e area o f the c a t h o d e to the area o f the a n o d e was 1:2. T h e r e q u i r e d b a t h t e m p e r a t u r e was m a i n t a i n e d by circulating h o t w a t e r a r o u n d the b a t h using d o u b l e - w a l l e d P y r e x beakers. A f t e r the c a t h o d e h a d b e e n p l a t e d f o r 2 h, it was w a s h e d with distilled water, weighed and a n a l y s e d gravimetrically. T o judge the quality o f P b - S n alloys over a range o f c u r r e n t densities, Hull cell e x p e r i m e n t s were carried o u t using a cell with a c a p a c i t y o f 267 ml u n d e r the f o l l o w i n g c o n d i t i o n s : t o t a l c u r r e n t in the cell, 6 0 0 m A ; t i m e o f electrolysis, 5 m i n ; t e m p e r a t u r e , 30 °C; p H , 0.75 and 1.5.

3. Results and discussion T h e a d d i t i o n o f gelatin, p e p t o n e , ~ - n a p h t h o l and resorcinol to the plating b a t h s h o w e d t h a t bright a n d a d h e r e n t d e p o s i t s were o b t a i n e d o n l y w h e n gelatin was used (Table 2). F u r t h e r e x p e r i m e n t s w e r e t h e r e f o r e carried o u t using this a d d i t i o n agent. T h e a p p e a r a n c e of Hull cell panels d e p o s i t e d f r o m various b a t h s s h o w e d t h a t bright and a d h e r e n t d e p o s i t s c o u l d be o b t a i n e d at l o w and i n t e r m e d i a t e c u r r e n t densities w h e n gelatin was i n t r o d u c e d into the b a t h ; the o p t i m u m a d d i t i o n was 1.11 g 1-1 (Fig. 1). A r e d u c t i o n in the p H o f the b a t h f r o m 1.5 to 0.75 with a gelatin a d d i t i o n o f 1.11 g 1-1 r e d u c e d the range o f c u r r e n t d e n s i t y over which bright a n d a d h e r e n t d e p o s i t s c o u l d be o b t a i n e d . F u r t h e r w o r k was t h e r e f o r e carried o u t at p H 1.5 and with 1.11 g 1 1 o f gelatin. An increase in the tin c o n t e n t o f the b a t h resulted in an increase in the tin c o n t e n t o f the c o a t i n g at all c u r r e n t densities and t e m p e r a t u r e s (Fig. 2). All the curves lie b e l o w the c o m p o s i t i o n r e f e r e n c e line AB, indicating t h a t

79 TABLE 2 Effect of additions to the bath on the plating quality Addition agent (1 g l - l )

Deposit quality

Tin ( q u a l i t a t i v e test)

Gelatin

Bright, good adherence Grey, poor adherence No deposit Powdery deposit, poor adherence

Present

Peptone ~-naphthol Resorcinol

Present -Present

Bath 1;pH 1.5; temperature, 30 °C.

1

0.3

Baths 3

2

1.8

0.3

1.8

0.3

Current d e n s i t y ,

4

1.8

0.3

6

1.8

0.3

1.8

A/drn 2

Fig. 1. Appearance of Hull cell panels plated in different baths at different pH values and with various quantities of gelatin (temperature, 30 °C; time, 5 min): m, powdery ; [], satin; B, semibright; o, bright. chloride b a t h s behave as regular plating baths. Similar results have b e e n obtained b y B l u m a n d H a r i n g [12] a n d R a u b and B l u m [13] f r o m f l u o b o r a t e b a t h s and also b y Piontelli and C a n o n i c a [14] f r o m s u l p h a m a t e baths. Furt h e r m o r e , t h e curves in Fig. 2 a p p e a r to be f l a t t e n e d w h e n t h e b a t h has a tin c o n t e n t o f b e t w e e n 20% a n d 40%. Q u a l i t a t i v e l y , this m e a n s t h a t the increm e n t of t h e less n o b l e m e t a l a d d e d t o t h e b a t h w h i c h c o n t a i n e d m o r e n o b l e m e t a l p r o d u c e d less change in t h e c o m p o s i t i o n o f t h e d e p o s i t at 40 a n d 50 °C t h a n it did at 30 °C. Like f l u o b o r a t e baths, these chloride b a t h s do n o t s h o w a n y limiting b a t h c o m p o s i t i o n a n d P b - S n alloys c o u l d be d e p o s i t e d o n t o the c a t h o d e w i t h tin c o n t e n t s o f 5% - 60% in the b a t h . An increase in t h e c u r r e n t d e n s i t y resulted in an increase in t h e tin c o n t e n t o f t h e c o a t i n g ; at 30 a n d 40 °C this e f f e c t b e c a m e m o r e p r o n o u n c e d w h e n t h e b a t h h a d a higher tin c o n t e n t . A t 50 °C the o p t i m u m e f f e c t ap-

80

50

~

L

[

F

-

-

~

FT

~

50 ~

30

'

T ~ T ~

30L

10

~

(a) ~.

I0

_o-----

(a) ~

~: °° 5 i

0

o~ E 50

=~ 30

~

,

~ 30-

(b) ~

(b) = cJ

5

0

, (e)

[ ~

0

~

y

.

,

~

-

-

~ s0 I

.-~

t •

L

L__!~

10 20 30 40 50 60 Tin content in the both, wt %

(c)

I m~

02

__

04 06 08 10 Current density, A/dm2

Fig. 2. Effect of the tin content of the bath on the tin content of the coating at different bath temperatures (current density: o, 0.3 A din-2; A, 0.6 A din-2; m, 0.9 A din-2): (a) bath temperature, 30 °C; (b) bath temperature, 40 °C; (c) bath temperature, 50 °C. Fig. 3. Effect of the current density on the tin content of the coating at different bath temperatures (amount of tin in the bath: ~, 5%;A, 8%; LI, 25%; O, 40%;A, 50%; m, 60%): (a) bath temperature, 30 °C; (b) bath temperature, 40 °C; (c) bath temperature, 50 °C. p e a r e d w h e n the tin c o n t e n t o f the b a t h was 40% (Fig. 3). A c c o r d i n g to the diffusion t h e o r y o f alloy d e p o s i t i o n , t h e rate o f d e p o s i t i o n o f a m e t a l has an u p p e r limit which is d e t e r m i n e d b y the rate at which its ions m o v e t h r o u g h t h e c a t h o d e diffusion layer. A t a given c u r r e n t d e n s i t y the rate o f d e p o s i t i o n o f the m o r e n o b l e m e t a l is relatively closer to its limiting value t h a n t h a t o f t h e less n o b l e m e t a l is. An increase in c u r r e n t d e n s i t y t h e r e f o r e m u s t result in the d e p o s i t i o n o f the less n o b l e metal. Similar b e h a v i o u r was s h o w n b y D u R o s e and H u t c h i n s o n [15] f o r P b - S n alloys d e p o s i t e d f r o m f l u o b o r a t e baths. Up to a tin c o n t e n t o f 50% in the b a t h an increase in t h e plating b a t h t e m p e r a t u r e led to a decrease in the tin c o n t e n t o f the coating; h o w e v e r , at a tin c o n t e n t o f 60% in t h e b a t h the e f f e c t s e e m s to be reversed (Fig. 4). An increase in the stirring rate o f the b a t h also resulted in a decrease in the tin c o n t e n t o f the c o a t i n g (Table 3). A c c o r d i n g to B r e n n e r [ 1 ] , if the c o m p o s i t i o n o f an alloy is c h a n g e d c o n s i d e r a b l y by agitating the b a t h , t h e n alloy d e p o s i t i o n occurs u n d e r diffusion c o n t r o l . D u R o s e and H u t c h i n s o n [15] have o b s e r v e d b o t h a slight increase in t h e tin c o n t e n t o f the alloy with an increase in t e m p e r a t u r e a n d a slight decrease with an increase in agitation for P b - S n alloys p l a t e d f r o m f l u o b o r a t e baths. K a d s o n and K a n e [16] have ob-

81

T

1oo -

(a)~ l

O

~

~

(a)

lOO 30~

A

i

ll.

•

6O

o

u

I

8[?

t

c 50

I

F--

I

2 80,

(b) -= ~ lo L _ -~' ~- -- ~- -_- _- ~~~" '-------"-"-"- 0- ~=

c~

iii

iii

III

u

~ 50

i

30 - ~

c~

t

x=

8O

to(c)

~

60

(c)

30 40 50 Beth temperature,°C

L__ I 0 I0

"

•

_A__L 20 30

40

I__L__ 50

60

Tin content in the both, wt %

Fig. 4. Effect of the bath temperature on the tin content of the coating at different current densities (amount of tin in the bath: ©, 5%;A, 8%; G, 20%; e, 40%;A, 50%; I, 60%): (a) current density, 0.3 A dm-2; (b) current density, 0.6 A dm-2; (c) current density, 0.9 A dm -2. Fig. 5. Effect of the tin content of the bath on the cathode current efficiency at different bath temperatures (current density: e, 0.3 A dm-2; A, 0.6 A dm-2; m, 0.9 A din-2): (a) bath temperature, 30 °C; (b) bath temperature, 40 °C; (c) bath temperature, 50 °C. TABLE 3 Effect of the stirring rate of the electrolyte on the tin content of the coating Stirring rate

Slow Medium Fast

A m o u n t o f tin in the coating (wt.%) Bath 1

Bath 3

Bath 6

3.97 3.40 3.24

17.50 15.64 14.47

56.68 53.14 45.08

served t h a t an increase in the t e m p e r a t u r e and the agitation o f the bath increases the tin c o n t e n t o f the deposit, an e f f e c t w h i c h is the o p p o s i t e o f t h a t expected. A t all t e m p e r a t u r e s , an increase in the tin c o n t e n t o f the b a t h h a d alm o s t n o e f f e c t on the c a t h o d e c u r r e n t e f f i c i e n c y at a c u r r e n t d e n s i t y o f 0.9 A dm -2, whereas at l o w e r c u r r e n t densities there was first a marginal reduction in the e f f i c i e n c y and t h e r e a f t e r , at a tin c o n t e n t o f a b o u t 15% in the bath, the e f f i c i e n c y b e c a m e c o n s t a n t (Fig. 5). A c a t h o d e c u r r e n t e f f i c i e n c y

82 of up to 90% could be achieved with all baths. An increase in the bath temperature also led to an increase in the cathode current efficiency, indicating that the concentration polarization decreased with increasing temperature. Filter paper soaked in 2% potassium ferrocyanide solution was placed over the deposit and the num be r of blue spots was taken as a measure of the porosity. Since n o t a single spot was observed, it appears that the deposits obtained from all the baths were absolutely free of pores. A metallographic examination showed that the grain size of the coating increased with increasing temperature and decreasing current density. X-ray diffraction studies of electrodeposited and cast alloys of similar composition showed (Figs. 6 and 7) almost no differences, indicating that the P b - S n alloys were deposited in a crystalline form similar to that in the cast condition.

4. Coating thickness The coating thickness was calculated by weighing the cathode before and after the application of the coating. It was observed that the coating thickness increased with increasing plating time up to 2 h; the plating rate was approximately 23 pm h -l (Fig. 8).

Pb

Pb

Pb Sn

Pb

Sn

S

I >. Pb

Pb

Pb

8 c

Pb PbsnS (b) ~

~ 56

5/.

~

t

52

1,8

I ,4.6 K.,O 36 28, degree

t

I

J

I

3/.

32

30

28

Fig. 6. X-ray diffraction pattern of alloys (composition: lead, 93%; tin, 7%): (a) cast alloy and (b) electrodeposited alloy.

83

Pb Sn

Pb

Sn

(a>l

loo[ n

~, 6o

Sn

Pb

(b) 48

46

44

42

40 38 36 2 0 , degree

f

J

b

_ ~ 34

J 32

I 30

_

28

0

. ~

0

I

~

1 2 3 4 5 Ploting time+ hour

6

Fig. 7. X-ray diffraction pattern of alloys (composition: lead, 51%; tin, 49%): (a) cast alloy and (b) electrodeposited alloy. Fig. 8. Effect of plating time on the coating thickness: o, without buffing; e, with intermediate buffing.

Powdery deposits were obtained when plating was continued b e y o n d 2 h and there was a slight decrease in the coating thickness. It was observed t h a t the deposits became dull after 2 h of plating and it appeared that the c a t h o d e surface was n o t favourable for the further deposition of a coherent coating. In order to check this, the surface of the cathode was lightly buffed after it had been plated initially for 2 h. With intermediate buffing at 1 h intervals, it was possible to deposit a coating 82 pm thick in 5 h; the deposit rate for subsequent coating was a ppr oxi m a t e l y 12 pm h -1 (Fig. 8). Cathodic polarization curves for P b - S n alloys in baths 2, 3, 4 and 6 were measured. The cathodic polarization curves were also determined for lead and tin in each case, using the equivalent concentrations of their chlorides. However, because of the low metal c o n t e n t and the high hydrogen discharge rate, the potential of the tin in baths 2 and 3 and of lead in bath 6 could n o t be measured. The curves indicated that very little polarization occurred u n d er the conditions studied. Since in the present investigation P b - S n alloys were plated at above the limiting current densities for lead and tin (the

84 TABLE 4 Effect of the bath composition, the temperature and the current density on the ratio R d o f l e a d t o t i n in t h e d e p o s i t a n d o n t h e i r r a t i o R s in t h e b a t h a f t e r d e p o s i t i o n Temperature ('C)

Current Bath 1 density (A d m 2) R d Rs

Bath 2 Rd

30 30 30 40 50

0.3 0.6 0.9 0.9 0.9

16.50 15.50 13.48 16.01 19.60

49.25 36.17 23.20 41.50 49.50

46.61 24.50 23.27 33.36 52.19

Bath 3

Bath 4

Bath 5

Bath 6

Rs

Rd

Rs

Rd

Rs

Rd

Rs

Rd

22.20 12.60 16.50 16.82 26.19

7.05 5.88 4.75 5.05 8.80

8.79 6.59 5.12 5.41 6.66

3.21 2.59 1.78 3.32 4.13

3.70 1.86 2.78 1.56 1.74 1.22 3.95 1.50 3.10 1.86

1.92 1.61 1.30 1.61 1.88

0.99 1.16 0.95 1.00 0.77 0.80 0.71 0.82 0.82 0.72

Rs

limiting current densities for lead and tin are 0.021 A dm 2 and 0.138 A dm -2 respectively), the composition of the deposited alloys cannot be predicted using cathodic polarization curves. When the alloy deposition is diffusion controlled the ratio Rd of the two metals in the deposited alloy is less than [1, 17] their ratio Rs in the bath. Table 4 gives the Rd and Rs values obtained by chemical analysis of the deposit and the bath under different plating conditions. It can be seen that the majority of Rd values are lower than the R~ values, indicating that the P b - S n alloy deposition from these chloride baths is diffusion controlled. The deviation in some cases may be due to hydrogen evolution or agitation of the bath.

5. Conclusions (1) Pb-(5% - 60%)Sn alloys can be deposited from chloride baths using gelatin as the addition agent. (2) Chloride baths are similar to fluoborate baths in the effect that the plating variables have on the composition of the electrodeposit; these baths belong to the regular alloy plating system. The cathode current efficiency was between 64% and 93% for different plating conditions. (3) The alloy deposition rate was approximately 17 pm h -1 for the first 2 h and subsequently decreased. However, with intermediate buffing, approximately this plating rate could be maintained for longer periods. (4) A decrease in the bath temperature and a decrease in the current density resulted in an increase in the grain size of the electrodeposited alloy. (5) The structure of the electrodeposited alloy was the same as that of a cast alloy of the same composition.

References 1 A. B r e n n e r , E l e c t r o d e p o s i t i o n o f A l l o y s , V o l s . 1, 2, A c a d e m i c Press, N e w Y o r k , 1 9 6 3 . 2 C. H. C h a n d l e r , U.S. P a t e n t 1 , 3 7 3 , 4 8 8 , 1 9 2 1 .

85 M. S p e i c h e r t , Can. Patent 269,480, 1927. J. W. A n d r e w s , U.S. Patent 2,633,450, 1953. M. S c h l S t t e r , Br. Patent 329,346, 1930. J. R. Stack, U.S. Patent 2,313,371, 1943. N. L. Leek, Br. Patent 555,929, 1943. L. S. Dietz, Jr., U.S. Patent 2,393,239, 1946. F. L. Clifton, U.S. Patent 2,489,523, 1949. J. Vaid and T. L. R a m a Char, J. Sci. Ind. Res., 16A (1957) 324 - 325. N. V. Melnikov and S. N. Sizov, U.S.S.R. Patent 116,524, 1959. W. Blum and H. E. Haring, Trans. A m . Electrochem. Soc., 40 (1921) 287 - 304. E. R a u b and W. Blum, Metalloberfli~'che, 9A (1955) 54 ~ 57. R. Piontelli and L. Canonica, Proc. 3rd Int. Conf. on Electrodeposition, Electrop l a t e r s ' Teeh. S o t . , 1947, pp. 121 - 125. 15 A. H. D u R o s e and D. M. H u t c h i n s o n , Plating, 40 (1953) 470 - 476, 4 9 7 , 6 3 0 - 632. 16 A. E. K a r l s o n and J. M. Kane, Mon. Rev. A m . Electroplat. Soc., 33 (1946) 255 - 260. 17 G. Devaraj, E l e c t r o d e p o s i t i o n o f l e a d - t i n alloys f r o m c h l o r i d e baths, M.Tech. Thesis, Indian I n s t i t u t e o f T e c h n o l o g y , K a n p u r , 1979. 3 4 5 6 7 8 9 10 11 12 13 14