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GOLDELECTRODEPOSITION
WITHIN THEELECTRONICS INDUSTRY
Jan R. Christie & Brian P. Cameron
GEC-Marconi LimitedHirst Research Centre, Hertfordshire
England
The use of gold within the electronics industry is widespread
for
domestic, commercial, aerospace and defence equipment.
The combination of good electrical conductivity coupled with
high
resistance to corrosion has led to its widespread adoption as
the stand-
ard material for contacts, bonding, joining, and high
performance high reliability conductor applications. In part due
to
its high intrinsic cost, gold is usually employed in the form of
a thin
layer and the performance of this relatively thin film is
critical to the
correct functioning of many devices. This article deals with
the
production of these thin gold layers by electrodeposition
processes
which play an important part in processing technology.
12 C' Gold Bull., 1994, 27 (1)
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The electrodeposition of gold is in itself not a newprocess, the
first record of gold electroplating being in1802 by Luigi V.
Brugnatelli [1]. However, it was notuntil 1840 that H. and G.R.
Elkington [2] patentedthe forerunner of modern gold electroplating
formu-lations based on the double salt gold potassium cya-nide.
From 1845 the process was commercially avail-able for decorative
gold electroplate. However, theunderstanding of the process was
poor and henceprocessing was somewhat unreliable, much of the
suc-cess being dependent on operator skili. It remainedrelegated to
the production of decorative items untilits re-emergence as part of
the electronics industry inthe 1940s. This new industrial interest
grew rapidlyand electroplating became the major user of gold forthe
electronics industry. For example, in 1990 thetotal usage of gold
for the European electronics indus-try was estimated as 18,097 kg
of which 13,482 kgwas electrodeposited [3]. For 1990, world
consump-tion of gold within the electronics industry was142,800 kg
[4]. This would indicate that approxi-mately 110,000 kg was
electrodeposited.
ELECTROPLATING FORMULATIONS
Two major requirements for the electronics industryexist which
are satisfied by electroplated gold. One isa high purity gold
primarily used for bonding or join-ing operations or where highest
electrical conductivityis at a premium and the second where a hard
wearresistant coating is required in contact and
connectorapplications. The chemistry behind the two processes,
whilst both based on gold potassium cyanide, is some-what
different and requires some knowledge and on-derstanding of
electrodeposition processes.
Pure Gold
Soft pure gold needs to be electrodeposited from elec-trolytes
containing the minimum of impurities andideally should not contain
any foreign element ororganic material which could
co-electrodeposit withthe gold. Hence, a need for high purity
electrolytechemicals and electroplating conditions which do
notfavour co-electrodeposition. Most formulations con-sist of the
gold salt, as the source of metal ions, a buffermaterial to ensure
that the pH of the solution remainseffectively constant over an
extended time period, apH adjuster and, in some cases, salts to
increase elec-trolyte conductivity.
Typical formulations and operating conditions aregiven in Table
1. Note that to obtain high quality elec-trodeposits the plating
cathode current density is keptlow and hence deposition rates are
low.
Replenishment of gold is carried out via the addi-tion of gold
as gold potassium cyanide. An inert anodeis used, usually
platinised titanium rather than solublegold anodes. There are
several reasons for this practice.The electrolyte is not 100 %
efficient with respect togold deposition, whereas anodic
dissolution of themetal is much closer to 100 % and the use of
solublegold anodes would soon result in high electrolyte
goldconcentration and the balance of the solution wouldbe adversely
effected. Also the tost of gold anodes ofany size would increase
inventory costs. To ensure the
production of consistently
Bath composition (gil) andoperating conditions
1. 2. 3.
Gold as KAu(CN)2 10 - 30 I5 14Potassium dihydogen 60 -
82phosphatePotassium citrate 60 85 60Ethyl dihydrbgen phosphate -
45 -pH 5.5 -8 6-8 5-6.5Operating temperature, °C 60 70 45 -
10©Cathode current density,A/dm 2 0.1 - 1.5 0.1 - 0.3 0.1 - 0.4
good quality deposits the elec-trolyte has to be regularly
ana-lyzed and deficienties due tochemical consumption or dragout
losses made good. Over an
Table 1
Pure soft gold electrolytes
( Gold Bull., 1994, 27 (1) 13
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Table 2
Acid golds Bath composition (gil) andoperating conditions
1. 2. 3.
Gold as I e(CN)2 8 4 12Citrk acid. 40 120 105Sodiurn. citrate 40
- -Tetraethylene pentamine - 20 -Phosphori acid 5 - 12.5 miIn (as
Iri.( 4»3) 5 - -Ni (as Ni3(C6Hs07» - 2.5 -Co (as. CoKz EDTA) - -
1Temperawre, °C 22 - 25 40 35Current density, Aldm 2 5 - 20 20
5
extended time period mostelecrrolytes build up decom-position
products which haveto be removed, usually bytreatment with active
charcoal.After such a treatment a fullanalysis of chemical
composi-tion is carried out followed bytrials with test
coupons.
Hard Gold
The second most important electrolyte used by theelectronics
industry is one designerf to produce tran-sition metal hardened
deposits. Pure gold electro-plated from neutral or alkaline
electrolytes is oftenused — because of its softness — for its
ability to ther-mocompression bond but this very property
excludessuch deposits for applications where contacts and
con-nectors are employed. The pure gold deposit is capableof
welding to itself when contact is made, resulting ina permanently
closed contact.
To overcome this problem, transition metal hard-ened gold
electrodeposits are employed, producedfrom electrolytes operating
in the acid pH range. Thepresence of such metals as nickel, cobalt
or iron insuch electrolytes results in codeposition of a
gold/tran-sition metal alloy. Codeposited with the transitionmetal
is organic material, probably deriving from cya-nide polymerisation
[5] which acts as a lubricantwithin the gold film. Inclusion of
these materials con-siderably alters the propertjes of the deposit
giving asignificant rise in deposit hardness and wear resis-tance.
The presence of these impurities reduces thetendency of the gold
layer to weld by friction to anextent that makes this material very
attractive for con-nectors, sliding contacts and contact
applications, andit is widely used throughout the industry.
Some examples of formulations employed areBiven in Table 2 and
the two most important differ-ences in the formulations and
operations of the elec-trolyte are the inclusion of a chelated
metal complexof, usually, cobalt or nickel and the pH operation
ofthe electrolyte, usually in the range 3.5 - 4.5. Despitethe acid
nature of the electrolyte, cyanide decomposi-tion occurs only
slowly but the bath is unlikely to re-tam n any free cyanide as
this will be evolved as thecomplex decomposes during
electrodeposition ofgold.
The composition of the deposit from these acidgold electrolytes
contains up to approximately0.7 % of the transition metal. The
complex co-or-dinating ligand employed to complex the
transitionmetal salt in solution is important as this controlsthe
amount of free transition metal in the electrode-posit. If the
complex is chemically too strong, lowtransition metal content
alloys are deposited, withreduced wear resistance and hardness. If
the com-plex is too weak, then high transition metal
contentdeposits are produced which result in stress crackingand, in
severe cases, exfoliation of the electrodepo-sit. Indeed, with
these baths care has to be takenthat no transition metals are
allowed to contaminatethe solution via drag-in or in the case of
iron bychemical corrosive attack.
The prime applications for these acid hard goldelectrolytes are
within the connector industry. Hence,a mass produced item with
selective deposition of gold
14 (' Gold Bull., 1994, 27 (1)
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Table 3
Exemples of eteetrochemiealpotentials
Au/Au+ 1.68
PtJPta+ F.20
Ag/Ag+ 0.799
Cu/Cu2+ 0.337
Fe/Fe3+ -0.036
Ni/Niz+ -0.25
Fe/Fe2+ -0.44
Zn/Zn2+ -0.763
requires the highest deposition rates to reduce processtime to a
minimum. This is usually achieved by rapidagitation, for example,
by jetting the electrolyte ontothe selectively masked substrate and
hence, for theseapplications, high gold content electrolytes are
em-ployed. Typical examples are given in Tabie 2.
lmmersion Gold
In many applications an immersion gold solution isemployed to
deposit a very thin (sub micron) layeronto the surface of
electronic components. For exam-ple, transistor leads are often
immersion gold coatedto aid solderability of the wires; the gold
retards theoxidation of the underlying nickelelectroplate. In
recent years, goldimmersion deposits have becomeimportant in
printed circuit applica-tions where an immersion coatingimproves
the performance of printedpressure contacts and acts as an
etchresist.
The process relies on the abilityof noble metals in solution to
replacebase metals, a simple example beingthe replacement of iron
by copperwhen iron is immersed in aqueouscopper sulphate solution.
This dis-placement will occur depending onthe value of the
electrode potential ofthe metals concerned. Examples ofelectrode
potentials are given in Ta-ble 3, but note that the electrode
potential can be con-siderably altered by complexing the metal ion.
Nor-mally a metal in this series will replace any metalbelow it on
the list. However, if the potential differ-ence is too great,
spongy non-adherent deposits areformed due to the speed of reaction
being too great.Most of the immersion solutions available are
proprie-tary but all contain gold as the double cyanide and
areusually alkaline. However, one of the authors hasdemonstrated
that acid golds can produce acceptabledeposits on electroplated
nickel with good adhesionand of bright appearance.
In general the deposits are very thin and affordonly marginal
protection from corrosion. Nonethe-less, the rapidity of the
process, usually completewithin two minutes of immersion, coupled
with the
lack of requirements for precision electroplating andthe
improved appearance to the component, makessuch deposits popular
substitutes for thicker gold elec-trodeposits. They are not to be
recommended for anyhigh specification electronic components or
serviceenvironments likely to encounter aggressive atmos-pheres.
However, their main usage as a protective filmduring storage for
subsequent hot dip soldering hasproved beneficial.
Autocataiytic or Electroless Gold
The re-emergence of immersion gold as a productionprocess has
prompted new research into autocatalytic
gold deposition as, for some appli-cations, deposits of greater
thick-ness than those available from goldimmersion processes (where
the lay-ers are sub-micron in thickness) arerequired. For example
in the proc-essing of complex waveguide struc-tures the use of a
uniform relativelythick (two micron) gold layer overthe surface,
including recesses,would be advantageous. The golddeposition occurs
not by the pas-sage of a current but by the actionof a reducing
agent.
The solutions are carefully for-mulated so that the gold
depositiononly occurs at the metal surface tobe plated and do not
lead to the for-
mation of free gold in the bulk solution. Solutions ofthis type
are widely used for the deposition of copperand nickel and the same
principles are now being ap-plied to gold deposition.
PROPERTIES OF GOLDELECTRODEPOSITS
One unique feature of all electrodeposited coatingsincluding
gold is the extremely fine grained nature ofthe electrodeposited
film. All thermally producedmetals in bulk exhibit larger gram
sizes. This meansthat the metallurgical properties of
electrodeposits
' Gold Bull., 1994, 27 (1) 15
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Table 4
Properties ofgold electrodeposits
often differ markedly froin thenormally cited bulk
properties
Gold Contactcontent Hardness Resistivity resistance
(KV N25) (jt.Q.cm) (me)
Pure gold deposit 100 40 - 90 2.4 0.3
Cobalt gold 99.5+ 120 - 250 15.0 0.6Nickel gold 99.3+ 160 - 200
11.0 0.3
of gold and gold alloys.This in turn gives these
electrodeposited coatings
applications not normally expected for gold metal.Table 4
illustrates values obtained on hardness
and electrical properties of pure gold and alloyed hardgolds. As
can be seen, the presence of trace quantitiesof foreign metal
considerably increases electrical resis-tivity but the effect on
contact resistance is consider-ably less. The latter measurement is
in part a measureof the surface cleanliness of the contacting
surfacesand also a measure of the applied force.
Grain size of all the electrodeposited golds is ex-tremely fine
being sub-micron and difficult to resolveby optical methods. This
is in stark contrast to goldand gold alloys metallurgically
prepared via conven-tional routes and is reflected by the
relatively highhardness values for electrodeposited gold. In the
caseof the cobalt and nickel gold alloys, the presence ofentrapped
organic material co-deposited with the gold viacyanide
polymerisation cangive a layer-type structure withthe polymeric
material depos-ited as a discontinuous filmthrough the structure.
This, ineffect, reduces the specific grav-ity of the deposit.
Figure 1
Gold metallised ceramicpackage
APPLICATIONS
In the electronics industry gold is electrodeposited inorder to
malse use of its excellent characteristics interms of its
electrical, chemical and optical properties.It is convenient to
consider the applications utilisingthis unique combination of
characteristics accordingto the type of deposit/process.
Pure Gold
Pure gold is often referred to as soft gold becauseof its low
hardness (approximately 40-60vhn). Con-siderable use is made of its
relative softness for bond-ing. A compression bond can be made
simply bybringing together two gold plated surfaces (or com-monly a
gold wire and a gold plated surface) and
16 (' Gold Bull., 1994, 27 (1)
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applying pressure. The absence of an oxide layer onthe surface
coupled with high diffusion rates resuits ina rapid joining
process. Additionally, heat may beapplied.
Use is made of compression bonding or thermalcompression bonding
to make connections in semi-conductor devices, to seal packages
such as semi-con-ductor cans and in a variety of joining
applications.This process is carried out on a wide variety of
silicondevices to make a connection berween the chip andthe
substrate leads (Fig. 1). In many cases the deviceis subsequently
packed in a hermetically sealed can,again using the
thermo-compression bonding processto seal the can. The device
itself may have a large num-ber of connections berween the chip and
the externalunit, as illustrated in Figure 2. In long
productionruns there may be millions of joints to be
processedrequiring a high degree of reliability and consistencyin
operation.
An alternative process, known as TAB (for tapeautomated bonding)
removes the need for a goldwire between the silicon chip and the
lead frame.The lead frame is selectively plated to apply goldonly
to those locations to be bonded. The compo-nents are in the form of
a tape or strip to allow acontinuous process. The chip itself is
also selectivelyplated. By compression bonding of the lead frameto
the chip, good joints are made with the need foronly one connection
rather than the two requiredfor wire bonding connections.
Pure gold is widely used in the fabrication of mi-crowave
integrated circuits for communication, aero-
space and military applications. Here the dielectricproperties
of conventional printed circuit laminate areinadequate and use is
made of high purity alumina orquartz as the substrate material. The
substrate is firstmetallised by vacuum deposition of a
nickel-chro-mium alloy followed by a thin (2000A) layer of
gold.Conductivity of the gold layer is then enhanced to therequired
level by the electrodeposition of pure gold tothicknesses of up to
twenty-five microns. Subsequentmasking and etching produce a
suitable pattern oftracks and bonding pads on the substrate [6],
exam-pies are shown in Figures 3, 4 and 5. An alternative isto
vacuum metallise and electroplate only those areasrequired.
Plated gold layers also provide readily solderablesterfaces.
Here again the absence of an oxide layer con-tributes to the ease
of processing. As gold can formbrittle inter-metallics with tin
when the gold contentexceeds approximately 4 %, the amount of gold
goinginto, say, a Pb-Sn joint must be carefully controlled
oralternatives, such as indium based solders, used.
Hard Gold
As discussed previously, hard gold alloys are
employedparticularly in those sections of the industry where
thecontact could be subjected to wear by make and breakor by
insertion and witlidrawal. The unique combina-tion of properties
combining high corrosion resistancewith good lubrication (and hence
low wear rates) hasmade hard golds the main contender for the top
coatof contacts. Alternatives, such as palladium and palla-
dium-nickel, have been pro-posed but gold is by far themost
widely employed mate-rial.
Figure 2
Gold electroplated lead formbonded to metallised chip(note the
large number ofinterconnects)
GoldBull., 1994, 27 (1) 17
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Figure 3
Cerarnic package bonded toceramic substrate showing gold
metallised trucks leading toconnector couplings
As gold is an expensivemetal and as many millions ofcontacts are
required to beplated, methods have been de-veloped to limit the
applica-tion of the metal to only thoseareas where contact has to
bemade. These procedures arecollectively termed
selectivedeposition.
The simplest of these is to only partly immersethe connector in
the gold electrolyte, thus reducingthe area plated. However, as the
thickness of the goldlayer can be relatively high (typically 0.8 to
2.5µm)excess metal is still deposited as (a) both sides of
theconnector are plated whereas often contact is onlymade on one
side and (b) it is difficult to control theexact immersion depth,
leading to difficulties in con-trolling deposit thickness.
A more satisfactory method is to mask both sidesof the
connector, one side fully and the other so as toallow solution
access only to the area which requiresplating. This type of process
is capable of great preci-sion with respect to gold placement as
welt as thick-ness control but is more expensive in terms of
plantand the variety of masks required to accept a widerange of
connector types.
Most connectors are electroplated using a reel-to-reel process
where the connectors are stamped fromthe base metal, often phosphor
bronze, in the form ofa continuous strip. The strip is subsequently
processedthrough a series of cleaning, etching and electroplat-ing
baths as a continuous reel. This process enableslarge numbers of
connectors to be processed in largevolumes with minimal labour
costs (Fig. 6).
Gold electrodeposits can be plated which containsmall (up to 0.7
%) amounts of nickel, cobalt oriron. This alloying addition
increases the hardnessof the deposit to the region of 80-200
vhn.
This type of gold deposit is more suitable for con-tact
applications. Soft gold deposits would quickly besmeared and
deformed by insertion forces. Gold alloydeposits do not just depend
on their increased hard-ness for their resistance to wear. A small
amount ofco-polymer material from the complexer present inthe gold
plating solution is also incorporated into thedeposit and this
improves the wear resistance by aminute degree of lubrication. With
a contact resistanceof the order of a few milli-ohms hard gold
deposits formthe ideal contact material. Telephone exchanges
arejust one example of the high-volume applications ofgold plated
connectors where good corrosion and veearresistance combined with a
low contact resistance play avital role.
An additional benefit comes from the fact thatgold does not
catalyse polymeric reactions. Palla-dium or palladium alloy
finishes can cause poly-meric material in the atmosphere to be
polynrerisedto form a surface film which increases the
contactresistance.
18 (' Gold Bull., 1994, 27 (1)
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Figure 4Gold metallisedceramic microwave tracks
considerable engineering prob-lems of mask alignment,
devel-opment is currently under wayon developing new
techniqueswhich direct a high velocity jetof electrolyte at the
surface;under the correct depositionconditions gold is only
platedwhere the electrolyte impingeson the surface.
As with many other applications for plated gold,considerable
effort has been directed at the selectivedeposition of gold to
minimise the amount of marerialused and thus the cost. This
selective deposition cantake the form of formulating solutions to
give an im-proved distribution of metal or selective plating by
theuse of masks to prevent deposition. Because of the
Autocatalytic and Immersion Gold
Autocatalytic and immersion golds are used whererelatively thin
films of gold are required. If compo-nents are isolated it is not
possible to make electricalcontact with individual areas and hence
the part can-not be electroplated. Also if any fine line
geometries
are utilised the resistance of theline makes electrodepositionat
best a process yielding willevariations in metal depositthickness.
An advantage of theautocatalytically depositedgold is that the
deposits areuniform in thickness and thusit is possible to evenly
coatvery complex geometries.
Figure 5
Experimental 1-band modules
Gold Bull., 1994, 27 (1) 19
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Figure 6
Gold electroplated contact zone instrip of reel-to-reel
connector
females
Applications for immer-sion gold are betoming moreimportant as
surface mounttechnologies within the indus-try slowly mature. By
using atop coat of immersion goldover topper or nicicel
printedcircuit tracks it is feasible to re-tamn improved
solderabiliry forsubsequent soldering opera-tions. The gold retards
or prevents oxidation of thesubstrate, presenting a good surface
for subsequentoperations. As the process is a simple immersion
tech-nique it is gathering in importante throughout the
in-dustry.
CONGLUSION
Electroplated gold continues to play an importantpart in modern
electronic technology. It is difficult tosee a truly equivalent
substitute due to the uniquecombination of properties of the metal.
As the com-munication and information technologies continue
toexpand it is to be expected that the quantiry of goldused by the
industry will continue to increase.
REFERENCES
1. L.V. Brugnatelli, Ann. Chim. (Pavia), 1802, 21,148
2. G.R. Elkington, British patent 8447, 18403. `Gold 1991',
Goldfield Mineral Services Ltd., Lon-
don 1991
4. `The Consumption of Gold Products by WesternEuropean
Electronics and Electrical Industries',3rd Quarter and whole year,
G.G. WedgewoodServices, Dec. 1991
G.B. Munier, Plating and Surf ice Finishing, 1960,56, 1159
6. `Electroplated Gold in Microwave Integrated Cir-cuits', I.R.
Christie & W. Mazur, Gold Bull.,1986, 19(2)
20 (' Gold Bull., 1994, 27 (1)
