1
,
Alternatives to Cadmium Coatings
For EIectricaElectronic Applications
.
Although cadmium is widely used
because of its unique Properties, it's
on the "hit list" as one of 17
chemicals in the Industrial Toxics
Project of the U.S. Environmental
ProtectionAgency (U.S. EPA)for
voluntary use reduction. A survey is
provided of candidate alternative
coatings systems, with emphasis on
nickel, tin and zinc alloys. Because
the properties of some of these
candidate coatings compare
favorably with cadmium, this
informationshould be of-use to
those who have to find
replacements for cadmium.
Cadmium is widely used in engineering
applications because of its unique
properties. The coatings are adherent,
smooth and ductile, and may be applied
to a wide range of metals. For electrical
and electronic applications, which make
up about one-quarter of the market for
cadmium coatings, ease of solderability
and high electrical and thermal
conductivity, along with low contact
resistance, are important properties. In
addition, if corrosion does occur, the
corrosion products are not voluminous
and do not cause the types of problems
seen with zinc coatings, for example.'
Whenthese characteristicsarecoupled
with the good lubricity (low coefficient of
friction) cadmium exhibits, it is not
surprisingto find cadmiumcoatings used
for switches, contacts, sockets, relays
and connectors, as well as bonding
surfaces. Table 1 lists some of the
applications of cadmium (and cadmium
compounds) in the electricaUelectronic
products industry.Table 2 describestypes
ofcadmiumcoatingsaccordingtoFedera1
Specification QQ-P-416.Cadmium is
typically plated on ferrous substrates,
but is sometimes used on nonferrous
substrates, such as aluminum or
titanium alloys.
Environmental Concerns
The Occupation Safety and Health
Administration (OSHA) has set very low
limitsforworker exposureto cadmium. In
addition, cadmium (including its
compounds) is one of 17 chemicals
February 1993
~ L 82L D
Eric W, Bmoman
Table 1
Some Uses of Cadmium & Its Compounds
In Electrical & Electronics-RelatedIndustries
~
Application
~~~
Component
Brush- commutators,slip rings
Electrodes for n i c k e l c a d " and mercury-cadrnium
cells reference cells
Holders, contact$ sockets, connectoT$ bonding
Motors
Batteries
Battery packs
SurfaCcS
Printed h i t boards
Solder (fatigue &stant), bipolar transistors, holders,
COXUlectoIS
Latches, hinges, fasteners,chases, EMI shielding
Contacts
Semiconducting materials, contacts
cabineq housings
S w i ~ erelays
s
Photovoltaic cells
targeted by the U.S. EPA for voluntary programstofind substitutes. The Defense
reduction. The goals are a 33 percent Construction Supply Center has funded
reduction in cadmium releases to the one study?while the Defense Electronics
environmentfor 1992,and 50 percent by SupplyCenterplanstoreduceoreliminate
1995.The baseline for these reductions cadmium under its Hazardous Materials
is the data from the 1988 Toxic Release Minimization Plan. In Europe and
Scandinavia, several countries have
Inventory (TRI)?
To compound the finishers' problem, passed legislation restricting the import
about 90 percent of cadmium coatings or export of cadmium-containing
are applied by electrodeposition, usually products. The European Economic
from a cyanide-based plating bath. Community (ECC) has passed Council
Cyanide is a reactivecompound that can Directive 91/338/EEC,which prohibits
generatea toxic compound if not handled the use of cadmium-platedproducts by
properly. It's also includedon the hit list of June 30,1995.4
Industry is also taking a more
17 chemicals.
Another dimension to the problem can aggressive role in reducing the use of
be seen upon inspection
of Table 2, which shows
that for improved
corrosicin=ref$st&ce in
aggressive environ
ments, a chromate
classification
Description
coating is specified,
Type 1
As plated deposit
meaning that
an
As plated plus chromate coating
Type II
additional toxic metal
As
plated plus phosphate coating
Type
(chromium) has to be
treatedbeforedischarge.
The U.S. govemment,
0.W5 inch thick deposit
Class 1
particularly the Departclass2
0.0003 inch thick deposit
ment of Defense, has
Class 3
0.0002 inch thick dewsit
identified cadmium
plating as an environNOTE: Type IL Class 2 will provide a corrosion resistance
of 96 hr, minimum, in the AsIu B-117salt spray test.
mentally unacceptable
process, and has funded
-
29
!
'end of pipe" solution. solutions. One commercialprocq$s has
The problem is only been available for a number of years, 50
nbie 3
controlled, not elimin- and, more recently, a new bath has been
Candidate Substitute Coatings for Cadmium
ated.)
developWforthe platingof leadframes.
As Published in the Literature
2. Process modifica- Forsomeapplications, especially aircraft
tion, efficiencyimprove- components, an ion vapor deposition
Qpc of Coating
candidate
Ref-(*.)
ment. (The amount of (IVD) processhas been commercialized,
(1) Metals
Al
UlWW.
toxic waste tobetreated and this is an acceptable substitute for
M,44,45
is reduced, but not cadmiuminsome militaryspecifications.20
4
44
IVDisavacuum-basedcoatingtechnique,
eliminated.)
Ni
14.223
however,that has difficultycoatinginside
3.
Materials
subSn
5.67.ltI2193
stitution, partial or total. tubes or deep recesses. Like mechanical
7
(Appliedatthefront-end plating and other commercialprocesses
Zn
of the process, where for aluminum, the deposits tend to be
substitution can lead to porous and only exhibit maximum
(2) Alloys
Ni-Ca
partialor full elimination corrosion protection after being given a
Ni-Fc
of the toxic waste chromate-type of conversioncoating. The
Ni-P
deposits are ductile and exhibit good
problem.)
adhesion.
The fatigue properties are
This
paper
is
conNi-Zn
better
than
those of cadmium.
cemed
only
with
the
SnO
thirdapproach. Itshould
Aluminum does provide sacrificial
Sn-In
be emphasized, how- protection-like cadmium-to ferrous
Sn-Pb
ever, that because substrates, and hydrogenembrittlement
Sn-Ni
cadmium exhibits such of the substrate is not a problem. The
Sn-Pd
a wide range of corrosionproductsare similar to those of
Sn-Zn
desirable properties, it cadmium, but the tribological properties
may not be possible to are worse, and the aluminum coatings
Zn-AI
findasuitablesubstitute are much moredifficultto solder because
Zn-co
for some applications. of the thin oxide film that forms on
In these cases, the first exposure to the atmosphere. Corrosion
Zn-Fe
andsecondapproaches
resistance in humid, acid or marine
ZPI-Mn
must
be
employed,
(chloride)
environmentsis similar to that
Zn-Ni
preferably with a non- of cadmium, while it is more resistant in
cyanide bath, or an sulfur-containing atmospheres. AlumiNi + BN,PTFE, Sic
(3) Mixtum
SiN or diamond
alternative deposition num coatings can also be usedat higher
technique such as operating temperatures than cadmium
4 3 + snoz
mechanical plating, coatings. Overall, however, aluminum
Sn + Zn
vacuum evaporation, does not appear to be a strongcandidate
Zn+Al
ionplatingor~puttering~for electricaVelectronic applications.
Zn + Si02
49
should be used.
(4) Compounds TiN
4454
Nickel
Silicates
P
Finding Substitute Nickel may be applied by electrolytic or
electroless plating techniques. Nickel
Coatings
(a) Not a Comprchenrivc listing:a few examples arc Listed when
Data from product coatings are useful because they avoid
there is reference to clcaricayelcar~nicapplications
bulletins, unclassified the problem with Whisker" growth and
and non-proprietary electromigration found to some extent
reports, technical arti- with cadmium or zinc coatings, and to a
. 4.,.... cadmium. Companies serving the cles, scieniic papers and patents have much greater extent with tin and silver
aerospace industry and providingmilitary been review"Pidd,'wilhparticular emphasis coatings. Such coatings are re&tively
hardware (Pratt & Whitney, Eoeing)have onelectrodepositedcoatingsforelectrical hard and do provide good wear and
active projects underway and are and electronic applications. Table 3 abrasion resistance.
Although they exhibit relatively good
beginning to generate specifications for (References 6-55) summarizes the cansubstitutecoatings. FordMotorCompany didate substitute coatings referred to in corrosionresistance,especiallyinalkaline
has said it will sell only cadmium-free the literature. The "best" replacement environments, nickel coatings tamish
products by the 1995 model year. candidatesresulting from this survey will quite easily, especially in sulfur-containing environments. They provide corroCaterpillar has a similar objective, except be discussed here.
sion protection by forming a physical
its target date was January 1993.
barrier on the surface of the substrate.
Metals
Nickel does not give sacrificial protecReduction Options Available
Aluminum
There are three general approaches to Aluminumisnoteasilyappliedasacoating tion, like cadmium, and the degree of
toxic or hazardous waste reduction and/ by electrodeposition. The plating baths protectjondepends on the integnty of the
or elimination:
must be non-aqueous because of the coating and the aggressiveness of the
1. Waste treatment, recovery and reactivity of the metal and the environment. Nickel coatings perform
recycling. (Sometimes referred to as an displacementof hydrogen fromaqueous worse than cadmium in marine, rural and
30
pktingand Suffam Finishing
.
J
' industrialatmospheres."
Electrolessnickelcoatingscanbe heattreated to provide a range of properties,
depending on the phosphotus, boron or
other minor alloying elements present.
Ni-Palloys, either platedor electroless
plated, can be very smooth, bright and
non-magneticaswell. Consequently,they
have been used as an underlayer on
magnetic storage disks for computers.
Nickel used in other applications is more
difficult to solder than cadmium. For
electroless coatings, the difficulty
increases as the phosphorus content
increases, and especially if passivating
films are formed by carrying out heat
treatments in air. A mildly activated rosin
flux can be used. 32 34
Electroless nickel coatings can be
applied to a uniform thickness on
componentshavingcomplicatedshapes.
Ni-B coatings have good electrical
conductivity and are hard, and so find
applicationon electricalcontacts.38Nickel
coatings, however, do have some utility
as a replacement for cadmium in some
applications.
Overall, the electrical properties of
nickel coatings are similar to those of tin
coatings. Like aluminum, the cost of
applying nickel coatings-especially
electroless nickel-is greater than for
cadmium.
Table 4
Main Features of Plating Baths
For Candidate Nickel Alloy Coatings
Alloying
Element
cobalt
Iran
Bath
w
Conc.FZangc,
96
5-64
5-8
sulfamatelehloride
SulfaWchbride
acid bath, 25-40 C; low cd.
"le,
Zinc
Silver
Silvercoatingsaresoft anddull, as plated,
and like nickel, may tarnish easily,
especially in sulfur-containing environments. With the appropriate soldering
technique, however, they are relatively
easy to solder. Silver is not anodic to
ferrous substrates; therefore, it affords
corrosion protection by acting as a
physical barrier.
Silver exhibits excellent electrical
conductivityandcan be usedon contacts,
but because of problems with
electromigration in use-leading to soft
shorts-a bar$ir.coating is required as a
post-treatment. The cost of silver may
prohibit its use in some applications.
some other metals. It is used as a
substitute for cadmium onsome electrical
chasses.5
Polished tin is more widely used in
Brazil as a replacement7because of its
good appearance, ductility, solderability,
low contact resistance and acceptable
corrosionresistance. While resistanceto
outside environmentsis good, there is a
problem with exposure to high humidity
in closed spaces and to some volatile
fatty acids, such as may be found in
packaging and some wood products?
Chromate-typeconversioncoatingsare
notrequired,andtincoatingsexhibitgood
tamish resistance compared to the other
metals discussed.
Tin coatings may be applied by
techniques other than electrodeposition
(mechanical plating, electroless plating
and ion implantation or mixing). The
choice of technique will depend on the
application and cost constraints.
Nevertheless,tin coatingsarebeingused
for protection from electromagnetic
interference, in selected areas, and as a
replacementfor gold on connectors. Tin
can be reflowedto give a smooth, porefree coating, but it easily cold-welds to
itself. Seizing will occur if two tin-coated
surfaces come into contact. While the
contact resistanze-isnot as low as&tfor
freshly deposited zinc, o v e r ? m h G
contact properties remain more stable.
Tin
Tin "whiskers" can also cause soft shorts
if they grow during use, and these are
particulariyaproblemwith brightdeposits.
Itis said,'however, that whiskerformation
is less likely than.with cadmium or zinc.
Bright deposits also present a problem
with soldering because the occluded
organic materials interfere with the
soldering process.
Tin also forms a barrier coating, and
electrodeposited coatings present less
of a hydrogenembrittlementproblemthan
Zinc
Zinc, like aluminum, is anodic to iron and
provides sacrificialcorrosion protection.
It is not as good, however, as cadmium
in high humidityor marine (high chloride)
environments for extended periods. lo.=
Zinc is reportedto withstandexposure to
SO, environmentsbetterthancadmium.s
The corrosion products are very
voluminous, and can cause a problem
where tolerancesare small or appearance
is important. In less aggressive
environments,zinc tamishes readily. For
htbmty 1993
Comments
Sulfamatdchloride pH 35 to 4,4845 C; low cd
ChloriWuIfat~, pH 2 10 3 , 2 5 4 C; bar to medium
18-35
cd
maximum protection or for preservation
of their bright appearance, zinc coatings
must be given a chromate-type
conversion coating or anodized.
Zinc coatings exhibit good adhesion,
and the ductility depends on the type of
plating bath used and the operating
conditions. Deposits can be quite hard
and deform less than their cadmium
counterparts, resulting in poorerelectrical
contacts.17Lubricityis poor, but abrasion
resistanceis betterthanthat of cadmium.=
Soldering,althoughnot difficult forfresh
deposits, can be problematical because
a galvanic cell can be set up betweenthe
solderjointandthe adjacentzinc coating,
and the latter corrodes. If soldering must
bedone, it shouldbe on a nonchromated
surface, using a mildly activated rosin
flux.7
Zinc may be applied by a number of
commercially available processes, such
as mechanicalplating, electroplatingand
hotdipping(galvanizing).All are relatively
lowcost techniques, and the latter gives
thick, ductile coatings. Mechanically
plated coatings tend to be thicker than
electrodepositedcoatings to provide the
same degree of corrosion resistance.
Barrel plating can be used for small
components, and non-cyanidebaths are
available for electroplating.
While zinc isoften suggested in$Kefal
terms as a replacement for cadmium,
especially on fasteners and small
stampings, the only referencefound for a
specific electricaUelectronic application
was the use of a chromated, electrodeposited zinc coating for record player
mechanism^.^
Alloys
Alloy deposition offers the opportunity of
improvingthepropertiesof the basismetal
or combining the best features of two or
more metals. The alloy coatings
referencedinthe liieratureprimarilyhave
been applied by electrodeposition, al31
Table 5
Main Features ofPlating Baths
For Candidate TiAUoy Coatings
Alloying
Element
cobalt
Lead
ConcRangc,
Bath
%
n
u
2
20
2-10
Sulfste
Alk.bath,incrtpmdes
Fluoborate,
Acid bath operated at mom temp;
low CS; Sn-Pb anodes
-te,
Blllonic
Nickel
33-35
chlorideltluoride pH U,70 C; knv cd,insoL or
pyrophosphate
aolubk anodcq good throwing
Zinc
l5-30
StannatuZincate
f
m
though some zinc "alloys" can be put
. down by mechanical plating techniques
or by hot dipping. Tables 4-6summarize
the main features of the plating baths
usedto depositzinc, tin and nickel alloys.
Comments aboutthealloy coatingsfollow.
B-AI
Adding five percent aluminum to zinc
improves the resistance of the latter to
chloride, SO, and general atmospheric
attack. The non-uniform distribution of
the aluminum oxide on the surface,
however, results invariable solderabili.5'
Applied by hot dipping, the coating is
bright, smooth and ductile, and exhibits
good adhesion. It can be spot-welded,
and if soldering is desirable, then an
appropriatecleaningstep with a NaOHor
rosin flux shouldprovide a good joint. ZnAI coatings have been applied to wire.
Zn-Co
Another sacrificial coating is a Zn-Co
alloy, containingeither below 0.9 percent
or above 1.5 percent cobalt.50Most applications to date have used between 0.2
and 0.6 percent cobalt, followed by a
chromatetype treag9nLfor maximum
corrosion' protection. The corrosion
products include black spots resulting
from the presenceof the alloying element.
B-Fe
ZN-Fe deposits can provide sacrificial protection up to about 85 percent imnl An alloy
in the range of 15-25 percent iron gives
the best corrosion resistance, however.
3-Mg
To obtain manganesecontaining alloys
with improvedcorrosion resistance (A0
percent manganese), high current
densities are needed. Corrosion
protectionis the resultof the formation of
manganous oxide on the surface.
32
power
Alk. bath with brighteners pH
125-1313:60-75 C bw cd.
the lubricity of such coatings. freshly
deposited coatings can be soldered.
easily,' while a chromated surface on a
Sn-22Zn alloy provides a low, stable
contactresistance.sSn-Znalloy coatings
exhibit good friction and wear
characteristics, especially when
lubricated, as well as good ductility.
Electricalresistivityis relativelylow,= but
increases as the zinc content increases.
Alloys containing more than about 25
percentzinc becomeanodicwith respect
to iron, and therefore can provide some
sacrificialprotection. Sn-Zn alloyscan be
spot-welded. They are resistant to SO,
and high humidity environments. One
application is the coating of electrical
chasses.=
Sn-Co
Sn-Co alloys, with about 20 percent
Zn-Ni
Much more interest has been shown in cobalt, can be deposited from a mildly
Zn-Ni (or Ni-Zn) alloy coatings, with nickel alkaline, sulfate-basedplatingbath using
contents typically in the range of 5-20 inert anodes. Specific applications were
percent. These alloys exhibit better not identified.
corrosion resistance than pure zinc In
salt spray tests.s. lo, The ferrous Sn-In
substrates are also less likely to be Although not listed in Tables 4-6, these
alloys find limited use because of their
affected by hydrogen embrittlement.=
As an example, a similarly prepared resistance to corrosion in salt water and
Zn-lONi coating had about twice the ease of soldering. Brush plating, for
resistance to salt spray as did cadmium, example, is used to coat small areas for
and five times that of zinc. For maximum subsequent lowtemperaturesoldering of
corrosion protection, the plated small compOnents.l6
components are given a hydrogen-relief
bake, followed by a chromate-type Sn-Pb
conversion coating. Above about 20 The application of Sn-Pb coatings is
percent nickel, the coatings are brittle relatively well-known, particularly in the
and do not provide sacrificial protection printedcircuit board (PCB) industry. The
alloy coatings are less expensive than
to ferrous substrates.
Good ductility is obtained in the range pure tin, are ductile, have similar
of 5-15 percent,a while the 15 percent appearance and corrosion resistance,
alloy exhibits good resistanceto whisker and can be solderedeasily. The coatings
growth. Zn-Ni coatingsusuzllyhave good do tamish, easily, however, and like tin,
adhesion, are abrasion-resistant andcan are susceptibleto attack by some volatile
be used in environments up to 400 OF. organic compound^.'^ If the coatings are
They have been used on fasteners for used on PCBs, all plastic components
electrical transmission structures and on must first be Drooerlv
. . cured. Alloys with
television coaxial cable connectoV#=TlO
percent lead are preferred for
electronic applications, becausewhisker
instead of cadmium.g
growth is avoided.
a-Sn
Applications include small electronic
A pymphosphatdsulfatebath has been components; over electrolytic nickel on
usedto obtain Zn-Sn alloys, as opposed lead frames, and occasionally on
to Sn-Znalloys, which are moreprevalent, contacts. Contact resistance is not very
and for which commercialprocesses are low on aged deposits, and worse than for
available. Zinc is present normally in the pure tin. For "make and break" sliding
range of 15-20percent, and the deposits tabs on PCBs, however, the coatings are
have an appearance similar to that of satisfactory.
cadmium5 when electrodeposited. The
alloys may also be appliedas coatingsby Sn-Ni
Tin with 33-%percent nickelcorresponds
mechanical plating.
Some tamishing occurs in marine to the intenetallic compound in that
environments, butthesalt spray behavior system. Consequently, the deposits are
is comparable to that of cadmium, as is hard and brittle, and subject to impact
.
Plating and Sutiace Finishing
I
'damage. Wear and abrasive resistance,
on the other hand, are good.
As would be expected, corrosion
protection is by a barrier mechanism.
The coatings do not tamish easily, nor
are they susceptible to whisker growth.
Sn-Nicoatings find applicationas an etch
resist on PCBsbecauseof their resistance
to strongacids, and as a low-costcontact
layer under gold on connectors."Contact
resistance is stable over time.
Freshly deposited coatings can be
solderedwithoutdifficulty, buttheretends
to be some variability in the results
obtained.= Other tin alloys are easier to
solder and have lower contactresistance.
Other applications of Sn-Ni alloy
coatings are on fuse caps, lugs, coaxial
cable connectors, magnetic memory
tapes (with a rhodium overlay),
connectors and electrical switch gear.
Applicationtemperature is limitedto about
650 O F , above which decomposition of
the intermetallic compound occurs. The
commercially available bath has good
throwingpower, and can be usedfor rack
or barrelplating. Dependingon the surface
finish of the substrate (bright or matte),
the deposit can have a similar appearance
to chromium or stainless steel.
---.-
-2-
Table 6
Main Features of Plating Baths
For Zinc Allov Coating Candidates
~
Alloying
ConcRange,
Element
%
Cobalt
Bath
%=
03-2
Chlaridc,
15-25
sulfate
Chloride,
Iron
sulfate
Manganese
>40
sulfatc/citrate
Nickel
5-m
Chloride,
sulfate/sulfonate,
sulfate/chloridc,
Tin
2060
Comments
'
pH range 4 2 to 55.10-50
c.d. except sulfate bath
c;low
pH range 1 5 to 3.0, 15-50C for
sulfate, higher for &on&
high
cd. ,
acid bath; high Cd needed to wdeposit the Mn at these high
concentrations
pH range 1.0 to 6.8, except for alk.
bath, 20-90 C; low to medium cd
except sulfatc/dfonate bath
uaucaline" (zincate)
PYrophoJphaW
sulfate
acid bath (compare with Sn-Zn
allw bath Table 9
-
this survey was conducted; details are
therefore not available.
A proprietary process exists whereby
phosphates and silicon compounds are
Ni-Zn
Zn-Nialloyshavealreadybeendiscussed. occluded in zinc electrodeposits." An
One reference, however,did describe an acid bath is used at a low current density
alloy of nickel with 5-8 percent zinc, to give coatings with good corrosion
deposited from a sulfatekhloride bath at resistance. A chromate treatment further
low current density. It is not clear what improves the corrosion resistance.
Nickel Alloys
Ni-Co
benefits might arise from adding such a Adhesion is good, galling resistance is
relatively low, and the coatings render
Limiteddata were found on nickel alloys. small amount of zinc.
the ferrous substrates less susceptible to
(Ni-P and Ni-B electroless deposits were
pitting attack and stress-corrosion
discussed under nickel in the section on Mixtures
metals.) At one time, Ni-Coalloy coatings Reference 8 states that a silver-plus-tin cracking. The coating might be used on
found some utility as "engineering" oxide coating may be used as a substi- fasteners, small stampings and chasses.
Electroless nickel deposits have been
coatings, and for electroforming. The tute for a silver-plus-cadmium oxide
deposits are hard and wear resistant, but coatingfor low voltage, electromechani- discussed. It is possible to codeposit
have a low internalstress. More recently, cal switch gear. A translation of the particles such as boron or titanium niporous Ni-Codeposits,impregnatedwith reference was not available at the time tride, silica, alumina or solid lubricants.
a solid lubricant, have been promotedfor
applications requiring a combination of
goodwear and abrasion resistance,good
Table 7
corrosion resistance,anda lowcoefficient
of friction.= These coatings have a high
Summary of Some Properties
surface resistivity, and a dielectric
Of Metal Coating Candidates
--.-?'
.
constant 'and low diss~paiionfa~or"fh~t~Salt spray
may be useful in some applications.
Max - Service
Electrical Resistivity,
Resistance,
p ohm-cm
h(a)
Special treatments make the coatings
Metal
Temp, F
conductive enough to provide anti-static
cd
450
6 1 as
2150
.
properties.=
gested" were cores for PCBs, microwave guides and laser housings.
&
-
Ni-Fe
Some experimental studies of Ni-Fe
deposition havebeenreported.31,uThese
studies were trying to produce materials
with very low thermal coefficients of
expansion for precisioninstruments and
the like. For high iron contents, the
sulfamatekhloride bath gave the best
results. The deposits were bright and
smooth. Electronic applications sugFebruary 1993
fd
930
28-38
L 150
Ni (E' less)
Ni (E' dep)
m@)
20-60
72 7.7
1.6
> 50
'900
965
Ag
Sn
zn
4
-
-
450
11.4
5 45
500
5.9 75
572
-
(a) ASTM B-117; hours to red rust, 0.0003-incb coating, not chmmated
@) Depends on heat treatment received.
33
.
Table 8
Summary ofSome Properties
Of Alloy Coating Candidates
salt sorav
Ni-Co
Ni-Co
+ PTFE
-
Sn-Pb
c 360
Sn-Ni
660
Sn-Zn
-
Zn-AI
-
Zn-Co
Zn-Fe
Zn-Mn
Zn-Ni
Zn-Sn
D-
I
-400
-
-
13.4
14.7
-
> 700
L
-
300
2
450
-
> 300
-
- lo00
-
> 250
> 750
(a) Values given for optimum composition.
@) ASIU B-117; hours to red rust, O.OOO3-inch coating, not chromated.
(c) 0.001-inch coating.
rities in the deposit. and so on.
The most promising candidates
identified in this survey as altemativesto
cadmium are:
Electrolessnickel (Ni-P, Ni-6)
Electrolytictin
Electrolytic Sn-Co, Sn-Pb, Sn-Zn &
Sn-Ni
ElectrolyticZn-Ni.
The composition and thickness of the
coatings will depend on the specified
requirements. In some cases, posttreatments will be required to maximize
corrosion resistance or to remove
hydrogen. The use of chromate-type
conversioncoatings, however, is another
problem to be addressed if the coating
system truly must be non-toxic or nonpolluting.
Manyof the candidatecoatingsappear
on the list of candidate, but untested,
replacementsfor cadmium in a survey of
manufacturers, suppliers and military
users (in certain stock classes) that was
conducted by the Defense Electronics
Supply Center as part of its "Hazardous
Materials Minimization Program.%
References*
1. R. Marce,rTTConnectswithCadmium,"
flat. 81'sur. Fin., 69,28 (Nov. 1982).
2. B.A. Graves, 'Industrial Toxics Project:
The 33/50 Program," Prod. Fin., 66,132
For applications requiring good corro- coatings with good corrosion resistance,
(Sept. 1992).
sion resistance along with good wear or particularly in acid en~ironments.~~
3. E.W. Brooman et al, "Cadmium Plating
abrasion resistance or lubricity, such Substrates such as aluminum, magneSource Substitution," Final Report to
Defense Construction Supply Center,
sium, titanium, niobium and tantalumadditions might be useful.
Columbus, OH (1989).
Some mechanically plated coatings or their alloys-may be coated by this
4.
F.
Altmayer, "PollutionPrevention: Both
may be considered to be mixtures rather commercial process. Applications are
Sides," Plat. & Sur. Fin., 79, 23 (Aug.
than alloys. Examples are zinc-plus- said to be found in the automotive and
1992).
aluminum and tin-plus-zinc, where the electronic industries, as well as in solar 5. E.H. Lesser, "Altematives to Cadmium
particlesmay be intimately mixed. These panels. Details, however, were not
Plating: Reflections Five Years Later,"
types of coatings are usually applied to provided.
flat. & Sur. Fin., 69, 34 (Nov. 1982).
6. P. Baeyens,N o MoreCadmiumPlating?
fasteners and small stampings, rather
Is There a Substitute Process?",
than being used for electrical/electronic Most Promising Candidates
Surfaces, 17,50 (1978).
applications. Their advantage is that While a number of possible altemative
7. P. Baeyens,"NoMorecadmiumPlating?
the application technique is less likely coatings to cadmium have been
Are There Processes to Replace It?",
to cause hydrogen embrittlementof the suggestedin the literaturefor application
Galvanotechnik,68, 590 (July 1977).
inthe electricaVelectronicindustries, none 8. N. Behrens, "Substitutionfor Cadmium in
substrate.
- __--_--...
4',<,."2
. _ %aninatch all the desirable properties of
.. Low V o l t a g + & + j t c h Apparatus
Engineering," Wllamossag,35,76 (Feb/
cadmium-although for some specific
Compounds
Mar
1987).
Compounds, such as titanium nitride, applications, certain altematives appear
9. CJ.Burkitt,'IsZincPlating GoodEnough
deposited as thin coatings, have a to be acceptable. Tables 7 and 8
for a Safe Earth?",Electr. Rev., 213,28
pleasing appearance. Adhesion is fair, summarize some of the property data
(Sept. 1983).
and the coatings tend to be hard and available.
10. J.W. Dini and H.R. Johnson,
"Electrodeposition of Zinc-Nickel Alloy
Corrosion data are difficult to interpret
brittle. Such deposits-applied by vapor
Coating,"P m . , WorkshopAlt. Cadmium
deposition techniques-exhibit good because the references do not always
Elechoplat.Met. Fin., 171 (Gaithersburg,
wear and abrasion resistance,buttendto specify if a conversion coating has been
MD, 1979).
be more porous than electrodeposited used. If it has, the type of coating is not 11. F.C. Eversteijn, "Zinc and Tin as
cadmium coatings.=As a result, corrosion always specified. In addition, coating
Altematives for the Use of Cadmium in
resistance may be less than desirable. thickness may not be given. The data in
the ElectronicsIndustry," Constmcteur,
the tables, therefore, shouldonly be used
Such coatings find limited application.
18.34 (Mar. 1979).
Silicates, deposited by an as a guideline to relative performance. 12. F.C.Eversteijn,G. KrijlandA.W. Deursen,
"Zincand Tin as Altemativesfor the Use
electrophoretic technique and sub- The values may vary widely with compoof Cadmium in the Electronic Industry,"
sequently fused, provide hard, glassy sition, heat treatment, presenceof impu-
-
34
Raring and S u d Fmishing
1
c
Belg-Ned. Tijdschr. Oppervlakfetech. 33. 'Nedox Synergistic Coatings,' General
Mater., 23,28 (Feb. 1979).
Magnaplate Corp., Linden, NJ (1989).
13. E. Furka, K. Neemes and R. Kahan, 'Re34. 'Plating EnhancesCorrosion Resistance
placementof Cadmium by Zinc Coatings
and Solderability," Prod. Fin., 42, 34
with Special Reference to Low Tem(Dec. 1989).
perature Resistance," P m . 6th Symp. 35. S.J.Blunden,~n-ZincCoatings."I.T.R.I.
on €lectrt"pat .,96 (Budapest, Hungary,
Leaflet LEO, I.T.R.I. (1990).
1985).
36. J. Henry, A
. New FluorinatedElectroless
14. R. Marce, 'In Cannon Connects with
NickelCodeposit," Met Fin.,88,15 (Oct.
Cadmium," Plar. & Sur. Fin.,69.28 (Nov.
1990).
1982).
37. TnandTin Alloy Coatings," MFSA Queliiy
15. D.E. Muehlberger,PlatingwithAluminum
Metal Finishing Guide (1987).
by Ion Vapor Deposition," SA€ Conf, 38. M. Bayes, 'Electroless Nickelinthe'Ws,"
Proc. (1978).
Shipley Catalyst (1990).
16. M. Rubenstein and A.Rosen,
39. 'Another Coating Dimension," Prod. Fin.,
"ElectrochemicalMetallizingTin and Tin
56,60 (May 1992).
Alloy Applications," Plat & Sur. fin., 75, 40. N.Zaki and E. Budman, 2inc Alloy Plating
34 (Aug. 1988).
Today," Prod. fin., 56,46 (Jan. 1991).
17. E.M. Walsh, "Zinc as a Substitute for 41. Sn-Zn Alloy Electroplates Outperform
Cadmium," finishing, 6,13 (June 1982).
Cadmium Deposits," Adv. Mat. 8 Proc.,
18. P.S. Wilcox, "Uses of Electroplated
140,37 (June 1991).
Cadmium," Proc. Cadmium 86,65 (San 42. D.L. Grimmett, M. Schwarhand K. Nobe,
Francisco, CA, 1986).
'Electrodeposition of Iron-Nickel (Invar)
19. A.H.Chapman, W.6.HampshireandD.J.
Alloys," Plat. & Sur. fin., 75, 46 (June
Maykuth, "Tin Coatings: Present &
1988).
Future," Plat. & Sur. Fin., 70. 40 (Dec. 43. N. Phan, M. Schwarh and K. Nobe, "lron1983).
Nickel, Iron-Nickel-Cobalt Electro20. V.L. Holmes, D.E. Muehlberger and J.J.
deposition," Plat. & Sur. Fin., 75, 46
Rerlly, The SubstitutionofIVDAluminum
(Aug. 1988).
for Cadmium," Final Report, Data Base 44. N.A.G. Ahmed and K. Watkins, 'Ion
HandbookC87-10162, Dept. of Air Force
Assisted Deposition for Surface
(1989).
Engineering," finishing, 15, 23 (June
21. C.V. Lundberg, "Longterm Weatheringof
1991).
OrganicandInorganicCoatingsonSteel 45. W. Kautek. W. Frombergand J.A. deHek,
and Aluminum," Polym. Mat. Sci. Eng.,
"Galvano-Aluminum: Process En58,405 (1988).
gineering
and
Applications,"
22. R.P Tracy and T.W. Bleeks, "Using
Metalloberflache,46,67 (Feb. 1992).
ElectrolessNickelCoatings," Mat.Engrg., 46. J.M. Bowden, 'New Prospects for Tin38 (Nov. 1989)
NickelCoatings," I.T.R.I. PublicationNo.
23. 'Silicate Coating Process-Fast and
720, Metal Bul, Monthly (April 1991).
Tough," lnd. fin., 57,41 (June 1981).
47. Tin-Zinc Coatings from the I.T.R.I.
24. G.G. Kraft, T h e Future of Cadmium
Stanzec Process," I.T.R.I. Data Sheet
Eiectroplating," Met. Fin., 88, 29 (July
No. DS9.
1990).
48. Tin-Nickel Coatings: Versatility and
25. R.G. Baker and C.A. Holden,
Performance," I.T.R.I. Data Sheet No.
'Investigation of Zinc-Nickel Alloy
DS10.
Electrodeposits-Part I, Rack Plating," 49. K.W. Riai, N.J. SpiliotisandK.F. Blurton,
Proc. 71st AES An. Tech. Conf. (1984).
'New Zinc ElectroplateFightsBothWear
26. I.V. Kadija, J.C. Fister, J. Winter and A.
andCorrosion," Met. Prog.,129,51 (Feb.
Parthasarathi, Whisker Resistant Tin
1986).
Coating and Baths for Making Such 50. R. Winand, "Continuous Strip Coating:
Coatings," U.S. patent4,749,626 (1988).
New Zinc Alloy Tailored Coatings,"
27. M.W. Bayes, "Electroless Nickel in the
Surface Coatings Tec.,37,65 (1989).
'9Os," Met. Fin., 88,27 (April 1990).
51. F.E. Goodwin, "Galfan Galvanizing Alloy
Yang, S.K. Doss and P. Peterson,
andTechnology,"lKRO Publication,Pnd
.
-_
-.e
- 28. E.
'Corrosion of Electroless Ni-P-Cu Alloy ---- ,Ed.,-tg849*.?-:F& 1
in an Atmospheric Gas Chamber," Plat. 52. 6. Knotkova, E. Boubelova and J.
& Sur. fin., 75, 60 (Dec. 1988).
Mechura, "Alternative to Cadmium
29. M. Matsuoka, S. ImanishiandT.Hayashi.
Coatings," Koroze Ochr. Mater.. 27,25
"Physical Properties of Electroless Ni-P
(Feb. 1983).
Alloy DeposrtsforaPyrophosphateBath," 53. L. Nanova, V. Kozhukharov and V.
Plat. & Sur. fin., 76, 54 (Nov. 1989).
Vetinov, "Zinc, Cadmiumand Cadmium30. T. Kato, T. It0 and M. Maeda. 'Chemical
Zinc Coatings, Part 2-Testing Under
Vapor Depositionof Aluminum Enhanced
Natural Atmospheric Conditions,"
by Magnetron-Plasma." J. Electrochem.
Galvanotechnik,76, 453 (April 1985).
SOC.,135,455 (Feb. 1988).
54. M. Ingle and T. Marchesani. 'Evaluation
31. P.C. Andricacos, C. Arana. J. Tabib, J.
of Environmentally Acceptable
Dukovic and L.T. Romankiw,
Substitutes for Cadmium Plating," SA€
"Electrodepositionof Nickel-IronAlloys,"
Proc. (1991).
J. Electrochem. SOC.,135,1336 (1938). 55. G. Hsu, 'Zinc-Nickel Alloy Plating: an
32. D.M. Rapp, T h e Solderability of ElectroAlternative to Cadmium," Plat. & Sur.
less NickelCoatings," Electroless Nickel
Fin., 71,52 (April 1984).
News, 2,4 (Feb. 1986).
1
i
-
56. H. Simon, 'Alternatives to Cadmium Plating," tnd. Prw! Eng., 9, 40 (July 1985).
57. N. Zaki and E. Budman, 2inc Alloy Plating Today," Prod. Fin., 56, 46, (Nov.
1991).
58. TinandTinAlloyCoatings,"MfSAQuality
Metal Finishing Guide, 1 (1987).
59. D.E. Moore, Defense Logistics Agency,
personal communication (Nov. 1992).
*Editor's Nota: Because Dr. Brooman's literature
sufvey is a valuable one and lengthy, we deviated
hwnPBSFsusualreferencestyleinordertoprovide
Wes of articles. Readers who are interested in
studyingm e of the referencedarticles can thereby
quickly focus on specific areas.
About the Author
Dr. EricW. Broomanis a researchleader
in the Metals and Ceramics Department
(a not-for-profitcontract R&D laboratory)
and manager of the Electrochemical
Technology Group at Battelle,
Commercial & Industrial Technology
Division, 505 King Avenue, Columbus,
OH 43201. Originally from England, Dr.
Brooman has an honors degree in
metallurgy and a PhD from Cambridge
University in metallurgy for electrochemical research. In the early '70% he
was a researchfellow in electrochemistry
at the University of Salford in England.
Dr. Brooman's career has included
serving asaconsultant,universitylecturer
and researcher. His current interests
are materials related, whether for
energy conversion and storage, metal
finishing, or waste minimization and
metals recovery recycling and reuse. __ _ .
".-
Dixie Regional -Technical
Conference '93
April 15-16-17
Ramada Inn-Opryland
Nashville, TN
Sponsored by
AESF Mid-Tennessee Branch
Contact:
Jim Richardson 6151452-3338