Use of Electroless Nickel
To Reduce Gold Requirements
By Donald W. Baudrand
The high price of gold and the unpredictability of its
supply make the substitution of less costly metals an
important consideration for the electronics industry.
Electroless nickel alloys are logical choices by virtue of
their solderability, diffusion-barrier effectiveness, and
bondability. These performance characteristics are
discussed, and supportive test data are presented and
evaluated. Also provided are examples of applications
where electroless nickel is being used to reduce gold
requirements.
p
recluding an extraordinary metallurgical break through, the unique properties of gold will ensure
its continued use in modern electronic applications.
But t he c os t o f g old is e x c e p tiona lly h ig h, t he
market volatile, and the supply vulnerable to world
conditions. As a matter of economic survival, the electronics
industry must minimize gold usage. One logical approach is
the selective substitution of less costly metals. Toward this
goal, numerous electroplated metals, each having merit for
one or more electronic applications, have been studied and
reported.' Additional studies indicate that certain
characteristics of autocatalytically plated nickel -alloys
permit the deposits to serve functionally as well as gold. The
electroless nickel deposits also can be used satisfactorily as
an undercoat for a thin layer of gold.
Among the deposit characteristics of importance are: (1)
solderability, (2) diffusion-barrier effectiveness, and (3)
bondability. This paper discusses these characteristics and
presents related test data and evaluations. Examples of
applications for which electroless nickel is being used to
reduce gold requirements also are cited.
Solderability
Gold is used for solderability even though it is well known
that it dissolves in the solder and that soldering takes place
onto the basis metal. A high gold content in solder causes a
weak, dull joint. 2 Thus, a thin gold deposit is better than a
thick deposit when a non-activated flux is used. Considering
these shortcomings, various electroless nickel deposits were
tested to evaluate their solderability.
T he s olde r ab ility te s ts w e re c ond uc te d on c lea ned,
polished-steel Hull cell panels plated in eight diffe rent
electroless nickel baths. Deposit compositions from these
baths* were:
1.
2.
3.
4.
5.
N ic ke l -b or o n ( 0 .2 % B)
N icke l -b or on (0.3 % B)
Nickel-boron (3% B)
Nickel-boron-tungsten (0.3% B, 1% W)
Nickel-boron-tungsten (0.3% B, 3% W)
6. Nickel-phosphorus (4% P)
7. Nickel-phosphorus (8% P)
8 . N ic ke l -p ho s p ho r us ( 1 1 % P )
The following cleaning cycle was used to prepare the test
panels for plating: (1) alkaline soak-clean and scrub; (2)
water rinse; (3) acid dip in 50 percent HCI; (4) water rinse; and
(5) deionized water rinse.
A sufficient number of panels was plated in each bath to
assure an accurate evaluation of the solderability of each
type of deposit. All panels were plated with 5 to 6.3 Am (200 to
250 Ain.) of nickel alloy, using a bath loading of 1.2 dm 2 /L
(0.75 ft'-/gal). Following plating, some of the panels were (1)
treated with a mineral -oil lubricant to try to minimize
oxidation, (2) treated with an organic polymer to try to
eliminate oxidation or (3) plated with 2.5 to 3.8 MO to 150
min.) of bright acid tin.**
To establish the effect of aging on solderability, different
groups of the plated panels received the following attention
prior to solderability testing: (1) no treatment —used asplated; (2) degreasing; (3) degreasing and steam-aging for 1
hr in acc or da nc e w ith MI L -ST D -202 , Method 208; (4)
degreasing and heating to 500° C (932° F) for 5 min.
All panels were solder-tested using a Men iscograph with a
Soltec recorder. The solder was 63/37 tin -lead at 240° C
(464° F). The fluxes employed were Type R (water -white
r os i n) , T yp e R A ( a c ti va t ed r os in) , T y p e R M A ( mi ld l y
activated rosin), and organic intermediate fluxes.
Tables 1 and 2 give test conditions and results. The last
column in each table shows the time in sec to reach zero
force axis—the moment when the wetting force is equal and
opposite to the buoyancy of the specimen. A time of 1 sec or
les s is c ons id er ed ex c ell en t a nd a t ime o f 2 .5 s ec is
considered satisfactory.
Several conclusions emerged from the solderability test,
as follows:
1. All surfaces soldered well using Type RMA flux except
for aged nickel-phosphorus deposits.
2. Electroless nickel -boron deposits exhibited good
solderability when Type RMA flux (or stronger) was
used, even after steam aging. As a point of information,
a sulfide aging test (not covered in this paper) was
conducted (per French National Specification NF -C90"
NIKLAD 752, 750, 740, 6000, 6000W, 776, 795, and 1000, respectively.
NIKLAD is a tradename of Allied-Kelite, Des Plaines, IL.
*Vulcan Bright Acid Tin, Allied-Kelite, Des Plaines, IL.
3.
4.
5.
6.
550) under test conditions similar to those in this paper.
The results of this previously run test showed that
sulfide exposure of nickel-boron deposits did not
adversely affect their solderability.
Steam-aged nickel-phosphorus deposits had marginalto-poor solderability with Type RMA flux, but soldered
well with Type RA flux.
The solderability of all samples heat-aged at 500° C for
5 min was not acceptable.
No appreciable difference in solderability was noted as
a result of pH variations in baths of the same
formulation.
The organic polymer coating did not improve
solderability and did not withstand steam-aging.
Diffusion Barrier Effectiveness
-
Copper and electrodeposited nickel readily diffuse or
migrate through a gold overplate. Oxides of these metals on
the gold surface can cause poor solderability and increased
contact resistance.' Preventing this degradation is especially
important to the electronics industry. To this end, several
investigators have studied and reported on the effectiveness
of various metal underplates as barriers to diffusion.
According to Turn and Owen," "Pure hexavalent
chromium, and nickel with 8 to 10 wt percent phosphorus,
are the most effective barriers. A thickness of 2 pm of these
yield negligible copper penetration after 12 hr at 550° C."
They further state, "Nickel with 2 to 3 wt percent phosphorus,
nickel with 1 wt percent boron and pure nickel are effective
barriers."
In another report,' the authors state that, "The Co and Co-5
wt percent P barriers are the most effective of those
investigated in this work, and are comparable to the nickel-8
wt percent P barrier investigated by Turn.' Chromium and
tin-nickel failed to significantly impede copper penetration,
and are similar to the precious metals in this regard."
Table 1
Solderability Test Results for Nickel-Boron Alloys
Average
value in
sec to reach
Boron
content, %
pH
Pretest
treatment
Flux
zero-force
axis
0.2
0.2
0.2a
6.5
6.5
6.5
6.5
Degreased
Steam-aged
Steam-aged
As-received
RMA
RMA
RA
RMA
1,18
2.30
0.73
1.25
0.2"
6.5
Degreased
R
1.98
0.2 h
6.5
Degreased
RMA
1.15
0.3
3.0
3.0
3.0
6.5
6.0
6.0
6.0
Degreased
Degreased
Steam-aged
Steam-aged
RMA
RMA
RMA
RA
1.18
1.13
1.65
0.80
0.3'
0.3'
0.3'
0.3d
6.0
6.0
6.0
6.0
Degreased
Steam-aged
Steam-aged
Degreased
RMA
RMA
RA
RMA
1.10
2.45
0.75
0.72
0.2
Bath
'Coated with organic polymer. Differences were noted in the results
obtained with these panels.
"Tin plated.
`These deposits also contained 1% W.
° This deposit also contained 3% W.
The results of our investigation of diffusion barriers
(using a 100-hr steam-aging test) was supportive of the
findings of the studies cited above. In summary, the results
showed that:
(1) an electroless nickel-phosphorus barrier (underplate)
virtually prevents copper diffusion into the gold overlay and
(2) because of its superior surface properties, a nickel/3%
boron deposit plated on a nickel-phosphorus deposit
provides the best overall performance. This combination not
only serves as an effective barrier, but remains solderable
and retains its desirable electrical properties under more
severe exposure conditions than any other barrier material
tested.
Bondability
Electroless nickel-phosphorus and nickel-boron deposits
exhibit excellent bondability. As a result, they are finding
increased use in two process areas of electronic
packaging-wire bonding and diode attachment.
Wire Bonding-Ultrasonic bonding of aluminum wire to
pads and header pins plated with Ni -B has proved
economical and reliable, and a number of manufacturers use
this method. No special preparation is necessary and the
process produces these favorable results: (1) gold plating on
the pads and header pins is eliminated; (2) gold wire is
eliminated in favor of aluminum wire; and (3) defects
(Kirkendall voidine7) caused by the bonding of aluminum to
gold are eliminated.
Table 3 is a tabulation of data from a typical ultrasonic
bond test. With this test, the bond strength of aluminum wire
was determined for different thicknesses of nickel-boron
plate. The wire diameter was 25.4 pm (0.001 in.) and the
nickel-boron deposit contained 0.3 to 0.35 percent boron.
Prior to bonding, the nickel-boron plate was baked at 120° C
(248° F) for 1 hr in air. Testing equipment consisted of a
Lindberg 1101 Ultrasonic Wire Bonder and a Tempress
12965 bonding tool. Bond load was 15 g, power setting 5.5,
Table 2
Solderability Test Data for Nickel-Phosphorus Alloys
Phosphorus Bath
content, %
pH
Pretest
treatment
4'
8
8
8
8
8
9.5
4.8
4.8
5.2
4.8
4.8
Degreased
Steam-aged
Steam-aged
Degreased
Degreased
Steam-aged
8
8
8
4.8
4.8
5.2
d
8°
8
d
11
11
11
11
11
Average
value in
sec to reach
zero-force
Flux
axis
1.98
RMA
RA
RMA
RMA
RMA
RMA
1.10
2.70
0.73
1.38
3.13
Steam-aged
500° C
As-received
RA
All
RMA
0.90
4,4
4.4
4.8
4.8
4.8
4.8
Degreased
Degreased
As-received
As-received
Degreased
500° C
R
RMA
1.88
0.90
1.65
0.90
1.09
4.8
4.8
4.8
4.8
5.2
As-received
Degreased
Steam-aged
500° C
Degreased
RMA
RMA
RMA
All
RMA
'Slight dewetting.
'Some dewetting.
No wetting.
'Tin plated.
`Dewetting on one edge.
'Slight wetting with acid flux.
RMA
RMA
All
1.93
1.85
1.75
2.63
1.89
Table 3
Ultrasonic Bond Strength of Aluminum
Deposit thickness
pm
0.31
pin.
12.5
0.74
1.23
1.63
2.15
2.85
24.4
49.0
65.0
86.0
114.0
Table 4
Effect of Post-Treatments on Ultrasonic Wire Bondability Wire on
Electroless Nidcel-Boron•
Wire bond
strength, g
Number of
bonds made
12
6.24± 0.41
6.38 ± 0.42
6.51 ± 0.38
6.0 ± 0.27
6.51 ± 0.34
6.4 ± 0.26
The deposit contained 0.3% B. The wire diameter
12
12
12
12
396
was
25.4-um (0.001-in.).
and time setting 5.
The test data indicate that lead-wire bond strength was
excellent and was not significantly affected by deposit
thickness. (The lower limit accepted by industry for a bond of
this type is 2.5 g.) These results reinforce those obtained by
Estep, who reported that. "Aluminum ultrasonic wirebonding to the Ni-B layer has been accomplished with bond
strengths approaching 11 g for 1-mil wire."
The reliability and stability of the wire bonds were
evaluated in the second phase of the bond test. The wire
bonds were exposed to simulated environmental effects and
to prolonged baking that reflected conditions that might
occur during repair. Then, using the original test conditions,
new bond strengths were measured. As shown in Table 4,
bond strength was not adversely affected by exposure to
steam, heat or passivation. The exposure to prolonged
baking did result in a decrease in bond strength, but the value
still exceeded the industry's requirement of at least 2.5 g.
Diode Attachment-Preform processing using electroless
nickel-boron has replaced gold spot-plating for diode
attachment in numerous applications. Gold/silicon preforms
bond well to nickel-boron, as do conductive and nonconductive epoxy diode-attach systems. In fact, there is a
broad choice of electroless nickel alloys that exhibit
satisfactory diode-bonding characteristics. These include
nickel/low phosphorus, nickel-boron (0.2-3% B), and nickel
polyalloys such as Ni-Mo-B, Ni-W-B, and Ni-Sri-B. The
final selection depends on solderability, diffusion-barrier
effectiveness, wire bondability or electrical conductivity.
Cox and Dean Nreported difficulties in diode bonding to
Alloy 194 copper lead frames with an electrodeposited nickel
barrier. They additionally noted that, "No problems were
experienced with electroless nickel-plated frames." In their
analysis, they considered two possible causes of the diode
bond failure: "First is the presence of a foreign metallic
impurity which can cause a low surface free energy layer.
Zinc has been found to be detrimental to the solderability of
copper alloys. There has been no evidence of zinc in this
system, but copper has been found in many of the failed
interfaces. Copper can codeposit with gold or nickel in
electroplating systems. Copper can also diffuse through the
nickel barrier from the 194 Alloy substrate. . The other
possibility for the cause of failure is the presence of oxygen
to form an oxide at the nickel eutectic interface." In
describing the role of the electroless nickel barrier in
minimizing the detrimental effects of both the copper and
oxygen, the investigators noted that, "The phosphorus in the
electroless nickel would tend to deoxidize this system and
thus explain the greater eutectic spread we observed. In
addition, the lamellar grain structure which is developed in
electroless nickel also would minimize grain boundary
diffusion of copper through the deposit." The mode of failure
from electroplated nickel was dewetting of the gold/silicon
eutectic from the lead frame. No dewetting was observed
when 1.25 pm (50 pin.) of electroless nickel was plated on the
copper alloy.
Post treatment
Wire bond
strength, g
Immersion in chromic acid,
then 120° C bake for 1 hr.
6.03 ± 0.21
Immersion in boiling water for 4 hr,
then 120° C bake for 1 hr.
6.03 ± 0.25
Exposure to steam for 4 hr,
then 120° C bake for 1 hr.
130° C bake for 64 hr.
6.09± 0.16
4.5 ± 0.69
Gold Reducing Applications
-
The following applications are typical examples of how
electroless nickel alloys are used to reduce gold
requirements in the electronics industry:
 TO-8 hybrid packages, previously plated with 2.5 to 3.8 pm
(100 to 150 pin.) of sulfamate nickel and then 12.5 to 20 pm
(500 to 800 pin.) of gold, are now plated with 20 ± 5 pm (800±
200 pin.) of electroless nickel/3% boron in place of gold.
Soldering, wire bonding, and epoxy diode attaching are
carried out satisfactorily on this production component.
 Dual inline sockets, previously plated with 5 pm (200 pin.)
of nickel and then 5 pm of gold, are now finished with 2.5 pm
(100 pin.) of electroless nickel/0.3% boron. The basis metal is
beryllium-copper. Using a "burn-in" or "heat-soak" test for
quality control, there was no failure of the parts plated with
nickel-boron after 500 hr at 95° C (203° F), whereas the goldplated parts often would not survive 500 hr. The mode of
failure was an unacceptably high resistance or a complete
loss of contact. In addition, the mating fingers of the PC
board in this application are now plated with 0.25 pm (10 pin.)
of an electroless nickel-tin-boron strike and then with 2.5 pm
(100 pin.) of electroless nickel/0.3°/0 boron. Good results
were obtained from the same QC test used for the sockets.
 Isolated copper circuits are now plated in an electroless
nickel-tin-boron strike solution. This bath deposits directly
onto copper without any special steps such as palladium
activation and without the need to make contact with a
dissimilar metal. The circuits are then plated with 37.5 to 87.5
pm (1500 to 3500 pin.) of electroless nickel-phosphorus or
nickel-boron, depending on requirements. Immersion gold
is sometimes used over the electroless nickel. The ability to
plate onto isolated r.;27per-circuit elements eliminates the
need for connecting bars, on which gold is pl ated
unnecessarily. Presently, circuits for watches and minicalculators are produced using this technique. Other
applications are being evaluated.
 Transformer copper contacts, previously plated with 0.7
pm (28 pin.) of gold, are now plated with 2.5 to 3.8 pm (100to
150 pin.) of electroless nickel/3% boron. The life expectancy
of the transformer was determined by continuously
contacting two transformer turns at the contact point,
imposing a voltage, and measuring heat rise using a thermocouple on the contact. In comparison tests, the gold-plated
contacts generally would fail by "burn out" at about 500 hr,
whereas the electroless nickel deposit showed no failure
after 700 hr, at which time the test was terminated. There
was a fairly constant 4° C (39° F) differential between
the contacts plated with electroless nickel and those plated
with gold until near the "burn out" stage of the goldplated contacts. At that point, the temperature of the goldplated contacts rose suddenly, and was followed by a
loss of contact.
 Thin-film hybrid microcircuit devices employ electroless
nickel/3% boron as an improved finish for a gold/silicon
diode attachment. The metallization process consists of a
sputter-deposited nickel-chromium film on an alumina
substrate. This step is followed by nickel deposition, then a
1.3µm (50 pin.) deposit of electroless nickel/3% boron, and a
final 1.3 pm electroplate of gold. As described by Estep,8 "By
incorporating the Ni-B layer under thin gold, the process
allows nearly complete freedom in the selection of mediumtemperature solders, with primary consideration being bond
strength." (The solder selected was 95/5 tin-silver.) Estep
further states that, "The limited solubility of nickel in tin
permits a good bond between the solder and the Ni-B layer
through the thin gold without conductor failure; rework of
the bonds can be performed several times without conductor
damage. ... Wetting of the conductor is excellent (even after
exposure to the high temperatures used in film heat
treatment) with a maximum exposure of the hybrid to 280° C
for 3 min during reflow."
The phosphorus (or boron) content of the electroless
nickel deposit provides unique properties that permit its
effective use as a barrier layer between the substrate and a
thin gold overplate. The barrier layer virtually prevents
diffusion of copper into the gold plate and the alloy
minimizes the diffusion of nickel through the gold.
Electroless nickel deposits provide generally better
corrosion protection for the basis metal than other barriers,
except when cracking of the deposit occurs. Severe
deformation can be the cause of such cracking.
Wire bonding to electroless nickel-boron deposits is, in all
cases studied by this author, superior to wire bonding to
gold. Eutectic diode bonding to electroless nickel is equal or
superior to eutectic diode bonding to gold. Soldering to fresh
or cleaned electroless nickel-boron and nickel/lowphosphorus deposits is excellent using RMA fluxes.
 TO-5 headers and pins, formerly plated with gold, are
now plated with electroless nickel/8% phosphorus. The
transition from gold to electroless nickel among a number of
manufacturers is interesting. It was customary to plate TO-5
headers and pins with 2.5 pm (100 gin.) of gold to provide
good solderability to the pins and gold/silicon eutectic
bonding to the diode chip. Later, this requirement for gold
thickness was reduced to 1.5 gm (60 gin.), and the next step
was to spot plate only on the areas where attachment was to
be made. A subsequent major change in the evolution of the
finish was to replace all of the gold plate with electroless
nickel/8% phosphorus. The diode is bonded using epoxy,
cured, and then the electroless nickel deposit is activated in
hydrochloric acid and leads are welded. Some components
are soldered.
in the Electronics Industry (Jan. 1979).
2. H. Manko, Solders and Soldering, 2nd Ed., McGraw-Hill
Book Co., New York, NY, 1979; p. 76.
3. M. Antler, Plating, 57,615 (1970).
4. J. C. Turn and E. L. Owen, Plating, 61, 1015 (1974).
5. D. R. Marx, W. R. Bitler and H. W. Pickering, Plating & Surf.
Fin., 64, 69 (1977).
6. E. Philofsky, Solid State Electronics, 13, 1391 (1970).
7. C. W. Horsting, Proc. 10th An. Reliability Physics, IEEE,
155 (1972).
8. G. J. Estep, Elec. Pack. and Prod., 86 (Jan. 1974).
9. B. T. Cox and S. W. Dean Jr., Proc. NEPCON, 111 (1976).
References
1. R. G. Baker and R. Sard, Proc. AES 7th Symp. on Plating
 Metallized ceramic resistors, previously plated with gold,
were plated with electroless nickel/3% boron. Later, the
manufacturer eliminated prior metallization (molybdenummanganese) by plating a very thin electroless nickel/3%
boron strike directly on the ceramic, after appropriate
catalyzation of the surface. This was followed by a heavier
deposit of electroless nickel-phosphorus alloy. Resistor
patterns are evaporated onto the ceramic.
 Silicon wafers, once finished with gold, are now plated
with nickel/0.2% boron from an alkaline electroless solution.
Three years of productio n experience by several
manufacturers has validated this application.
Summary
There is increasing use of electroless nickel by the
electronics industry as a means of reducing or eliminating
gold requirements. For many applications, electroless nickel
offers characteristics that allow it to function as well or better
than gold, or as an excellent barrier underplate for thin gold
deposits.
About the Author
Donald W. Baudrand, CEF, is vice president, Research and
Development, for the Allied-Kelite Products Division of The
Richardson Company, 29111 Milford Rd., New Hudson, MI
48165. Mr. Baudrand graduated from Whittler College with a BA
degree in chemistry and mathematics. Graduate studies at the
University of California at Berkeley consisted of nuclear physics
and chemical engineering. He has served Allied-Kelite since 1966
after their acquisition of Electrochemical Laboratories in Los
Angeles, a firm he founded and served as president for 14 years.
Mr. Baudrand is a member of the AES Detroit Branch.
Reprinted Courtesy of
Allied-Kelite Products Division
The Richardson Company
2400 East Devon Avenue/Des Plaines, Illinois 60018
REPRINTED FROM PLATING & SURFACE FINISHING, DECEMBER, 1981