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. Thepurpose 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
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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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Published by the American Electroplaters and Surface Finishers Society, Inc. 12644 Research Parkway 0 Orlando, FL 32826-3298 Teiep"re: 407L%I -6441 : Fa: "2
Copyright 1992 by American Electroplaters and Surface Finishers Society, Inc. All rights resewed. Printed in the United States of h-~er ica. This publication may not be reproduced. s t p s m 'tted in whole or mrt, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise without the prior written Dermission of AFSF, 12644 Research
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SURIFIN'is a registemd trademark of the American Electroplaters and Surface Finishers Society, Inc.
; .
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: