ELECTROLESS GOLD PLATING WITH USE OF (EXTERNAL) SOLID NICKEL CATALYST

N. V. MANDICH, CEF HBM Electrochemical & Eng. Co.

2800 Bernice Road Lansing, IL., 60438

ABSTRACT

The object of this study is to find out if "Hypo Gold" ffl.G.1 formulation is a true gold electroless

process and if It can be used to plate gold flash over the selectively plated electronic contacts. The purpose

of gold flash to plate the entire surface of the gold plated contact with thin layer of soft gold for the

purpose of SoIderatMty. Thickness of this gold flash should be from 5-8 microinches. It is found that

addition of solid nickel when barrel plating with H. G. formulatlon, converted this system from immersion

type to true auto cataiytic process.

ELECTROLESS PROCESSES: DEFINITIONS

Because electroless deposition does not involve the passage of externally

applied current to the system, some confusion over the use of the term electroless

has resulted. Electroless deposition has been used synonymously with chemical

deposition which can result from the following processes:

(1) DisPlacement reactions. Depending on its position in the electrochemical

series, a metal higher up in the series may be covered (plated) with the metal lower

down in the series. A well known example is the coverage of iron with copper in an

acidified copper sulphate solution. Two reactions, one anodic and the other cathodic,

take place simultaneously at the surface of the iron.

e svf/Fim 1992, Atlanta, GA

823

AESF Annual Technical Coneerenee SUWF~'N@ 'SDZ

- June 3CSE-Z5,1Cr-221

.Atlanta, CeorgCa .

The American Electroplaters and Surface Finishers Society, Inc. (AESF) is an international, individual- membership, professional, technical and educational society for the advancement of electroplating and surface finishing. AESF fosters this advancement through a broad research program and comprehensive educational programs, which benefit its members and all persons involved in this widely diversified industry, as well as govemment agencies and the general public. AESF dissemi- nates technical and practical information through its monthly joumal, Plating and Surface Finishing, and through reports and other publications, meetings, symposia and conferences. Membership in AESF is open to all surface finishing professionals as well as to those who provide services, supplies, equipment, and support to the industry.

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; .

824

Fe- Fe2 + 2e (Anodic) E" = -0.44V

Cu2 +2e =) Cu (Cathodic) E" = 0.337V

<1>

<2>

Once the iron electrode is fully covered with the thin layer of copper, the process

comes effectively to a halt, and no further thickening takes place. The deposit is

normally thin (< 1 .O /I m); and i ts adhesion is not satisfactory.

(2) Galvanic dissolution reactions. In these, the workpiece (M) is coupled to

a less noble metal M,, and the assembly immersed in the plating solution containing

ions (M,) of a more noble metal M,. In this case the less noble metal M, goes into

solution (anodically) and metal M, deposits onto the work piece.

M,- M,"' +ne

M2"+ +ne 3 M,

The metallization in this case can continue for as long as dissolution of the

sacrificial anode (M,) is possible. There are commercially available gold plating

solutions which are claimed to produce thicknesses in excess of 2.5yin, the preferred

substrates being silver, copper, brass, nickel, tin or mild steel coupled to a zinc or

aluminium wire. The donation and acceptance of electrons is an integral part of the

above processes - as it is indeed with all aqueous plating processes.

To avoid confusion, we should examine the term electroless deposition. Since

electron donation during deposition is essential, we have to rely on methods other

than substrate displacement, galvanic dissolution of the sacrificial anode or supplY of

current via an external power unit (electrodeposition), to achieve this. A chemical

2

"reducing agent" (Red) is the electron donor in all truly electroless plating processes

and the process is catalyzed by the deposited metal.

<5> Red,, OX*o,'" + "= CMaWic sUff.U

where Ox is the oxidation product and Red is reducing agent.

Thus, once the substrate surface becomes covered with the plated deposit

continuation of the process relies on the latter to catalyze further deposition. The

term "auto-catalytic" is therefore used to describe this type of plating. So far, this

term has not suffered from the same confusion as the term "electroless". Before a

gold plating process can be described as autocatalytic one important requirement is

needed, that is, the system in question must be capable of depositing gold on a gold

substrate. In the following discussion the terms electroless and autocatalytic will be

used interchangeably.

SURVEY OF KNOWN PROCESSES

A large number of "electroless" gold plating bath formulations exist in the

literature, both in the form of technical papers and patents. Several of these

formulations have been reviewed by Okinaka'. Ali and Christie'have tested some of

these baths, based on hypopho~phite~", hydrazine4i8, thiourea7 and the results were

disappointing. No gold plating was achieved on a gold substrate, and it was

concluded that these baths were not auto-catalytic. In the case of the thiourea bath'

they were unable to deposit gold onto a gold substrate despite the claim of Okinaka'

that it was possible to do so from this solution.

Although the Brookshire hypophosphite bath' was not tested by them, it is

3

,

interesting to note that Okinaka' found that the plating of gold on nickel occurred

whether hypophosphite was present or absent (though no plating rate was indicated).

This is in spite of Brookshire's claim that hypophosphite was a necessary ingredient,

and that decreasing of its content reduced the plating rate. What is more surprising

in Brookshire's work is his claim that the plating rate was completely independent of

the pH of the solution. An electroless gold bath which has been widely tried and

reported upon in the recent literature is Okinaka's borohydride bath'. Plating was

reported to take place on copper, nickel, cobalt, iron, palladium, platinum, Kovar,

Permalloy and steel. It was also reported to occur on gold and thus demonstrating

that the process was truly autocatalytic. In their later work, Okinaka et allo concluded

that in their borohydride bath, Ni (11) decreased the plating rate and increased bath

instability. Gany and Mahapatre," in an excellent review, stated that no formulation

has been developed that provides a reliable, stable, fast-building process for

widespread application.

Okinaka bath, although truly autocatalytic, has several limitations on their own,

such as stability and intolerance toward contaminants and inability to plate on several

desirable substrates. Several i n v e s t i g a t o r ~ ' ~ ~ ' ~ improved Okinaka formulation but

limitations are still present.

C. 0. lacovangelo and K.P. Zarwich" worked with hydrazine based bath which

used hydrazine as source of electrons, but like one truly auto catalytic bath, it would

not plate over gold. However it did plate over nickel and they used the term

"substrate catalyzed". In their subsequent work" they used hydrazine and

dimethylamine borane as combo reducing agent and they obtained truly autocatalytic

process capable of plating on a wide variety of substrates including Au, Ni and Pd.

4 826

EXPERIMENTAL PART

A. EXPERIMENTAL DATA:

In order to evaluate the HG gold solution, we used selectively gold plated

contacts and (1" x 1 ") copper coupons . The intent was to plate the contacts all over

with electroless gold rather than using the standard barrel electroplating process.

Base Metal: Cd-Copper (about 2-4% Cadmium)

Gold Thickness on Contact Pad: 25pin minimum; Surface Area/contact:.l 25in2

1. Cleanina Stem

For the preparation of the surface area for electroless gold plating, the

perchlorethylene vapor was used to remove the remaining ink from the contacts prior

to plating, followed by commercial alkaline cleaner and acid salts immersion with

rinses in between and after.

I I . Electroless Formulation (H.G.)

Potassium Gold Cyanide (67% gold)

Ammonium Chloride

Sodium Citrate

Sodium Hypophosphite

PH

Operating Temperature

2 g/l

75 g/l

50 g/l

10 g/l

7.0 - 7.5

190-21 0°F

5 027

B. EXPERIMENTAL PROCEDURE

1. Electroless Gold Plating

A number of parts were plated with barrel gold using the above HG formulation

and thicknesses of gold deposits were measured to be about 1 microinch, within the

experimental error. This indicates that HG gold has more characteristics of an

immersion gold solution, rather than catalytic.

I I . Electroless Gold + Nickel "Catalvst"

An approach to use of the "catalyst" to boost the thickness of gold deposition

was explored with pure nickel wire (.022" dia.). Results proved to be encouraging.

It is interesting to note that nickel ions must be present in the hypophosphite - reduced electroless comer solution in order to get continuous copper deposition. It

was found that nickel deposit can catalyze the oxidation of hypophosphite". As far

as we know, our work is the first application of nickel ions in electroless gold

deposition using hypophosphite as a reducing agent.

Next, five sets of experiments were performed in order to investigate some

of the basic electroless plating variables and, at the same time, to determine optimum

conditions for plating of electronic contacts:

Amount of electroless gold solution vs. number of contacts;

deposition vs. plating time, operating temperature, amount of Ni-catalyst, and

electroless gold deposition over electroplated gold or copper.

Electroless gold

a28

1. Amount of Electroless Gold Solution vs Number of Contacts

The contacts were plated always using the same amounts of HG solution and

6

nickel "catalyst". Plating time (t) was set at 40 minutes, enough time to plate out

practically all gold from these solutions. Number of contact varied to the extreme of

1 :6 ratio.

Six different batches of contacts were plated under the same conditions:

HG solution volume (V): 40 ml; temperature (T) 190-201°F; plating time: (t) 40 min;

nickel "catalyst" - wire: (8 in/1000 contacts).

30 contacts from each group was measured for thickness by Betascope. Thickness

variation with number of contacts is presented in Table 1.

Experiment Batch Size Average Thickness Number No. of contacts) (p in.)

1 300 3

2 250 4

3 200 7

4 150 10

5 100 13

6 50 25

TABLE #1 Thickness Distribution vs number of contacts.

Actual thickness corresponds with theoretical calculations of thickness; e.g.

experiment No. 3:

1 liter of HG solution = 1.34 gms of gold/l = .0536 9/40 ml

40 ml of HG solution = .0536 gms of gold

200 contacts plated = 25 in2 of surface area

Thickness = .0536 = .0536 = .000007 in = 7 p i n 298x25 7450

7 829

,

830

Fig. 1 Represents data from Table 1.

2. Gold Thickness vs. Platinq l7me ?

v = 100 ml; 500 contacts; "catalyst": Ni-wire, 5 in long

Samples of 8 contacts each, were plated for 5,10,15,20 and 35 min. Gold

thickness was measured via Betascope and presented in Table 2 and Fig. 2.

Plating Time (Min) Ave. Thickness (p inch)

5 3.2 10 4.5 15 5.2 20 5.7 35 7.0

TABLE 2 Ave. Thickness vs. Plating Time

Practically all gold from the solution was deposited in 35 minutes and less than 2%

of gold remained in plating solution.

3. ODeratinq Termerature of the Solution vs. Gold Thickness

When Ni catalyst (in the form of 2 in long wire) is used with HG

electroless gold plating solution, plating starts at approximately 170°F. However, a t

this temperature, deposition rate is slow. Therefore two higher temperatures were

used: 180°F (Table 3) and 195-201°F (Table 4). As expected, chemical reaction of

deposition is faster a t higher operating temperatures due to higher mobility of ions and

increased free energy.

The original HG electroless formulation requires 198-201°F. as operating

temperature. It is obvious that Ni catalyst initiates and accelerates plating reaction

and is recommended for extended operating temperature of 190-201°F. After both

8

plating experiments all the solutions were tested for remaining gold content (v = 40

ml). Results are presented in Fig.3 which is derived from Tables 3 and 4

Plating Time(Min1 Residual Gold in Sol'n (G/L)

- 1.34 0

5

15

25

35

.925

.483

.275

.048

1.34 0 5 .570

0

15

20

25

30

35

.350

180

.I25

.066

.025

.oo I

TABLE 4 Gold Depletion vs Time, at 195-2oI"F.

9 831

From the results it follows that when recommended operating conditions are

kept, plating time should be 35 minutes. If, for some reason, this time is extended,

the solution will start attacking gold deposit, worsening the color and the grain

structure. When plating time is less than 30 minutes a significant amount of gold will

remain in solution.

4. Influence o f Nickel Catalyst on Thickness

Plating rate depends on the amount of nickel used as catalyst in regards to

number of the contacts or, in other words, to the total surface area exposed.

The following experiments were performed in order to find out the optimum

ratio: surface area of the contacts vs. amount of nickel (wire or shots) used

expressed as Ni surface area.

Five samples of contacts were plated in HG solution under the same operating

and with varying Ni-catalyst surface area.

Data: 250 contacts; V = 40 ml, T = 195-2Ol0F, t = 35 min

No. of Ni-Wire/250 contacts Ni Wire1250 contacts Residual Gold Experim. (linear in .) (in') in solution (G/L)

1.3

1.5

1.7

2.0

2.2

.OS98

. l o36

.1174

.1381

.1519 -

.5495

.2610

.1343

.0013

.0008

TABLE No.5 Nickel Catalyst vs Gold Content

10 832

Samples from experiments 1 to 4 were acceptable, therefore optimum ratio

sample vs. catalyst is 31.25 id.1381 in. = 225:l under given conditions, (Fig.4).

5. Electroless Gold Deposition 0 ver Electroplated Gold

This set of experiments was done to establishwhether HG gold with Ni-catalyst

would plate over electroplated gold.

250 contacts previously selectively gold plated were used. Thirty contacts

from this group were marked with a diamond pen for identification purpose. Selective

gold thickness was tested prior to electroless gold plating.

Data: v = 40 ml, catalyst 2 in wire; T = 195-201" F; t = 35 min

After plating, marked contacts were separated and the composite gold deposit

thickness (electroplated + electroless gold) was measured on the same spots.

Average thickness after HG plating: 31 pin and 26 pin before plating

Electroless gold thickness is given by the difference: 31-26 = 5 pin.

It is obvious from Table No. 6 that HP gold with Ni-catalyst plated satisfactorily

over previously electroplated gold as well over the copper.

11 a33

Electroplated Gold (pin)

Electroplated + Electroless Gold (pin)

Electroless Gold (pin)

56

51

57

67

67

66

66

62

66

76

76

76

10

1 1

9

9

9

10

36

44'

41

45

55

53

9

1 1

12

Ave: 10p in

TABLE No.6 Electroless plating over gold underplate

12 834

CONCLUSIONS

A. HG gold plating solution as is has mostly immersion characteristics. In most cases

thickness of this deposit was in a 2 microinch range.

B. If nickel wire is used as a "catalyst" or "activator," higher deposit rates resulted

when plating over the copper portions of electrical contacts.

C. Our experimental results demonstrated that the final gold flash over previously

gold plated contacts can be applied using the HG electroless gold. Five to ten

microinches thick gold flash will plate over the entire surface of contact, including

selectively plated gold pad and rest of contact area made of copper when using Ni

catalyst.

D. An enclosed plastic barrel needs to be designed for these particular plating

applications. Electroless solution, Ni catalyst and contacts have to be placed inside

of this barrel and the barrel must rotate slowly, 2-3 RPM.

Notes: Purity of HG gold deposit was not tested; "We suspect that there will be

some codeposition of nickel along with gold;" Some gold was deposited over nickel

wire. In most cases this deposit was not uniform and will "brake-off" to allow further

activation effect; Some nickel metal is dissolved in the solution but we don't have

analytical data at present.

It may be of interest to mention two electroless solutions patented long before

Brenner. U.S. Patent 1,207,218 (1918)!) states the following formulas:

13 835

Ni-citrate 10 g/l

Ammonia 10 g/l

AI kaline Hypophosphite 10 g/l

gold cyanide 10 g/l

Na-phosphite 10 g/l

Temp 212°F

It is never too early to invent! Is it?

REFERENCES 1. Y. Okinaka, in "Gold Plating Technology", edited by

E. H. Reid and W. Goldie, Electrochemical Publications, Ayr, Scotland, 1973

2. H.O. Ali, R.A. Christie, Gold Bull. .l7,(4),118,( 1984).

3. S. D. Swan, E.L. Gostin, Met-Finish., =,(4),52,1961.

4. E. L. Gostin, S. D. Swan, U.S.Patent 3,032,436 (1962).

5. T. Ezawa, H. Ito, Jpn-Patent 40,1081 (1965).

6. B. M. Luce, U.S.Patent 3.300,328 (1967).

7. T. Oda, K. Hayashi, U.S.Patent 3,506,462 (1970).

8. R. R. Brookshire, U.S.Patent 2,976,181 (1961).

9. Y. Okinaka, Plating, =,(9),914,(1970)

10. Y. Okinaka, at al, J. Electrochem. SOC. =,(1),56,(1974).

1 1. G.M. Gany, S. Mahapatra, J.Sci.1nd.Res. ,46,(4),154,( 1987).

12. M. El Shazley, K.B. Baker, U.S.Patent 4,337,091 (1982)

13. M. El Shazley, A.A. Halechv, U.K. Patent GB 212,144A

14. M. Matsuoka at al, J.Met.Finish.Soc. of Japan, 38,(2),19,(1987).

15. M. Matsuoka at al, P1at.Surf.Finish. 5,102,(1988).

16. C.D. lacovangelo, K.P. Zarnoch, J.€lectrochem.Soc., 138,(4),983,( 1991 1.

17. Ibid, p.976.

18. A. Hungjord, K. E. Chen, J.€lectrochem.Soc.,1,72,(1989).

14 836

28

27 20 2s 24 23 22 21

20 13 10 17 16 1s 94

13 12 11 10

9 0 7 0 S

4 3 2 1

150 200 250 0 so loo NO. OF CONTACTS

Raure 1. " M A R OF CONTACTS PLATED VS. THICKNESS

t 1.50

1.34

1.25

(w

1.00

.90

.EO

.70

.60

.50

.40

3 0

.20

.10

O\

30 IMW 15 20 25 6 10 0

Figure 3. PLATING TIME VS. RESIDUAL GOLD

I

I G N

838