ADVANCEMENTS IN ELECTROLESS NICKEL PLATING
TECHNOLOGY FOR PRINTED CIRCUITS
By Donald W. Baudrand
The printed wiring board (PWB) industry is developing a number of new
manufacturing methods to meet the increasing demand for close dimensional
tolerances found in densely packed circuitry.
Surface-mount devices (SMDs), for example, require fine lines, tiny "vices",
and close spacing; and large-scale integrated circuits require a large number
of leads on tightly spaced centers. Some of the current industry trends are:
1.
Complete automation of the PWB manufacturing process
2.
Composite substrates for leadless ceramic chip carriers
3.
New P.W.B. substrate materials such as extruded or molded
thermoplastics
4.
Permanent resists to protect fragile fine lines
5.
Dry, thermomagnetic resist process
6.
Laser exposure of artwork and resist-covered boards
7.
Multi-layer boards
8.
New processing steps and materials such as additive/semi-additive
circuit production and special electro-/electroless plating materials
This paper will discuss special electroless nickel plating processes which
provide the finish required for surface mount technology, multi-layer P.W.
boards, PWB contact surfaces, and boards exposed to high temperatures.
Surface Mount Technology
In order to provide a surface which can be used for surface mount components
(direct mounting and connecting of microelectronic components), the P.W. board
finish must be receptive to effective wire bonding, die bonding, and soldering.
Very few finishes can provide a suitable surface capable of all three attachment
processes. Gold, for example, which is most widely used, is not ideal for
soldering or wire bonding. Gold in a solder joint causes a dull, weak joint
and will contaminate the solder pot to the extent that it must be discarded
(reclaiming the gold where possible) periodically. (1)
Wire bonding to gold will sometimes have voids under the wire (Kirkendahl
voiding) resulting in a weak bond. (2,3) Die attachment using gold/silicon
pre-forms is excellent, but less satisfactory if epoxy die attaching is used.
Other metals commonly
and tin-lead deposits,
Electrodeposited nickel
Copper solders well if
well.
used for P.W. finishes have similar shortcomings. Tin
for example, solder well but cannot be wire bonded.
does not solder well after aging, even for a short time.
pre-cleaned after aging, but does not wire or die bond
Specially formulated electroless nickel deposits, on the other hand, are capable
of providing excellent bonding characteristics for wire bonding by ultrasonic
or thermalsonic methods, die attachment by gold/silicon or epoxy methods, and
are solderable using RMA fluxes.
Table I shows typical bond strength data for aluminum wire bonding to electroless
nickel-boron deposits.
TABLE I - ULTRASONIC BOND STRENGTH OF .001 IN. DIA. ALUMINUM WIRE
ON ELECTROLESS NICKEL-BORON DEPOSITS
Deposit Thickness p In.
12.5
24.4
49
65
86
114
.3% BORON
Wire bond strength - grams
• 41
6.38 - .• 42
6.51 - .• 38
6.0 - .27
6.51 - .• 34
6.24 - .

6.4-.
26
Solderability was tested on steam aged plated panels using a meniscograph with
a Soltec recorder. The solder was 63/37 tin-lead at 240C. A time of 1 second
is considered excellent with 2.5 seconds satisfactory.
TABLE II - SOLDERABILITY TEST RESULTS FOR NICKEL-BORON ALLOYS
Boron Content %
Bath pH
Treatment
Flux
Average Value in Seconds
To Reach Zero-Force Axis
.2
6.5
Steam aged*
RMA
2.3
.2
6.5
Steam aged
RA
0.73
.2
6.5
Degreased
RMA
1.18
.3
6.5
Degreased
RMA
1.18
3
6.0
Degreased
RMA
1.13
3
6.0
Steam aged
RMA
1.65
3
6.0
Steam aged
RA
0.8
TABLE III - SOLDERABILITY TEST DATA FOR NICKEL-PHOSPHORUS ALLOYS
Phosphorus %
Average Value in Seconds
To Reach Zero-Force Axis
Bath pH
Treatment
Flux
4
9.5
Degreased
RMA
1.98 slidewetting
8
4.8
Steam aged
RA
1.10
8
4.8
Steam aged
RMA
2.7 slidewetting
11
4.8
Degreased
RMA
1.85
11
4.8
Steam aged
RMA
1.75
11
5.2
Degreased
RMA
1.89
The steam aging was conducted for 1 hour in accordance with MIL-STD-202, Method 208.
These tests show that electroless nickel-boron deposits exhibit good solderability
using RMA flux, or stronger, even after steam aging.
Die bonding - pre-form processing using electroless nickel-boron is replacing
spot gold plating for die attachment in numerous applications. Gold-silicon preforms bond well to nickel-boron deposits, as do conductive and non-conductive
epoxy die attachment compounds.
Cox and Dean (4) reported difficulty in die bonding to alloy 194 copper lead
frames with electrodeposited nickel. "No problems were experienced with
electroless-nickel plated frames." The mode of failure from electroplated nickel
was de-wetting of the gold-silicon eutectic from the lead frame. No de-wetting
was observed when 50 micro in. of electroless nickel was plated onto the copper
alloy.
MULTI-LAYER PRINTED WIRING BOARDS
The requirement for small hole diameters and a large number of layers creates
exacting demands on present plating interconnect processes. The limitation of
current distribution, causing low current density inside small diameter holes
(which are also deep due to the several layers stacked) results in either thin
copper deposits or voids. The voids result from either incomplete electroless
copper deposition or burn-out which occurs when current is applied but no
electroplated copper is deposited. The thin electroless copper deposit is
dissolved by the copper electroplating solution.
Certain electroless nickel systems of low phosphorus or boron content can be
plated easily to greater thickness and with complete coverage than with most
electroless coppers and are capable of surviving the plating conditions, allowing
electroplated copper to deposit. In some cases the electroless nickel deposit
can be the entire interconnect conductor. For example, multilayer printed wiring
boards have been successfully produced which had twenty layers with holes of
.010" diameter, others with 40 layers and holes of .020" diameter. These are
used in high frequency circuits where low background noise is required.
Electroless copper was not only incapable of interconnecting all the holes in
the PWB, but the few successful connections resulted in high noise (low signalto-noise ratio).
The electrical resistivity of electroless nickel is higher than that of electrodeposited copper (8-12.i ohm/cm for electroless nickel and 1.7-4 p ohm/cm for
copper). However, for low current, high frequency applications the difference
is negligible.
PWB CONTACT SURFACES
Contact surfaces such as plug-in "fingers" require good wear resistance, low
electrical resistance, corrosion resistance, and heat resistance. Electroless
nickel-boron and some low-phosphorus deposits meet these requirements either
alone or overplated with a thin (up to 5 micro inches) layer of gold.
HIGH TEMPERATURE EXPOSURE
Because electroless nickel deposits are good diffusion barriers compared with
electrodeposited nickel and most other metals, electroless nickel-plated copper
circuits and contacts can withstand high heat environments such as "burn-in"
products. "Burn-in" cycles are typically run from room temperature to 93.3 C
(200F) alternately for from 1 day to 1 week.
Isolated copper circuit areas can be plated electrolessly by using a special
electroless nickel poly-alloy strike followed by more conventional electroless
nickel deposits. (Copper is not considered catalytic to electroless nickelphosphorus deposition.) This unique property provides great versatility in
the use of electroless nickel for printed wire boards.
CONCLUSION
As the printed wiring board industry develops smaller, higher density, and
more complex circuits with tiny vias, hole pads, and surface mounting areas
in close proximity and with small diameter through-holes, the performance
requirements of plated deposits will become more critical and have tighter
tolerances. Electroless nickel formulations of low phosphorus or boron content
can fulfill all of these requirements, often without the need of any additional
plated deposits.
On PWB's requiring copper deposits, an overplate of these electroless nickel
alloys will provide the multiple properties of solderability and wire and
die bondability that are prerequisites for successful surface mounting of
components. The versatility and excellent deposit characteristics of electroless
nickel deposits will continue to meet the needs of the advancing state of the
art.
REFERENCES
1.
H. Manko, Solders and Soldering, 2nd Ed
McGraw-Hill Book Co., New York, N.Y. 1969; p76
2.
E. Philofsky, Solid State Electronic, 13, 1391 (1970)
3.
C.W. Horsting, Proceedings of the 10th Annual Reliability Physics,
IEEE, 155 (1972)
4.
B.T. Cox and S.W. Dean Jr., "Analysis of Die Bond Failures On Alloy
194 Lead Frames," Proc. of Technical Program of NEPCON, 1976, pp 11-118.