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by
Al Koster, Pat Mentone and Jody West
Two applications will be described
where the combination of forward and reverse square wave
current pulses were used to significantly improve the thickness
distribution of electroformed nickel. Tests were performed
using both a Watts bath and a sulphamate bath. Compared
to DC current, the variation in nickel thickness across
the parts was reduced between 30 and 67 percent. The larger
reduction was obtained when electroforming molds with deep
recesses. The smaller reduction was obtained when electroforming
nickel on flat mandrels. Detailed rectifier settings are
recommended.
Much has been written and published
regarding the use of pulse plating for gold and other metals.
They typical benefit for all metals is a finer grain structure
which reduces the porosity and increases the density and
tensile strength of the deposit. For precious metals, this
means that a lower thickness of precious metal is required
to meet a functional specification for pulse plated deposits
than for DC plated deposits.(1)
Typically on flat parts, plated with
a high efficiency bath such as a pure gold, nickel or copper,
pulsing will make the thickness uniformity worse than DC
plating. For the same number of amp minutes, pulsing will
increase the thickness in the high current density areas
compared to DC plating. The thickness distribution gets
worse as the duty cycle decreases from 100% (DC) to 1%.
The most uniform distribution on flat parts is obtained
with DC .(2)
What we hypothesized is that the reverse
of this effect should also be true. Using anodic pulses,
it should be possible to remove the plating faster in the
high current density area than in the low current density
areas. A pulse periodic reverse (PPR) cycle using DC in
the forward direction and anodic pulses in the reverse direction
should level out the plating. Discussions with Darrell Engelhaupt
from the applied optics lab at the University of Alabama
at Huntsville suggested that this approach should work with
nickel and that the overall cycle time should be approximately
100 milliseconds. Figure 1 shows the PPR current wave form
and Figure 2 shows the Kraft-Dynatronix DPR20-30-100 that
was used to generate the PPR wave form.
This PPR current wave form was tried
on parts electroformed from both a Watts and a sulphamate
nickel bath. The thickness distribution was improved in
both cases. The same wave form was tried on barrel plated
connector pins with no change in distribution of the nickel.
The electroforming applications are described in the following
sections.
At Buckbee-Mears in St. Paul, the
PPR wave form was used to electroform nickel mesh from a
Watts bath.
Bath
Conditions
Ni: 78 g/l
Cl: 15 g/l
H3BO3 30 g/l
pH: 4.7
Temp: 47 deg C
C Anodes: roll depolarized nickel ovals
Agitation: none
Current density: 15 ASF
DC amps: 6
DC Plating time: 20 minutes
The nickel mesh is used in radar storage tubes and also
to make very accurate sieves to size industrial diamonds
and other types of particles. Figure 3 shows a piece of
nickel mesh after it has been removed from the mandrel.
Figure 4 shows assembled sieves made from the nickel mesh.
The sieves are guaranteed to be plus or minus 2 microns
of nominal hole size. The only way Buckbee-Mears could meet
this specification for hole sizes larger than 50 microns
was to use PPR.
The nickel mesh is electroformed on
a reusable nickel mandrel that is coated with non-conductive
resist so that the mandrel surface is exposed where the
mesh must plate. After removing the mesh from the mandrel,
plating is continued on both sides of the mesh to increase
its thickness and strength. Plating is continued till the
desired hole size is reached.
A 330 line per inch mesh pattern was
used for both tests. The electroformed lines on this mesh
pattern are approximately 0.0006 inches wide. The measure
of uniformity used is percent light transmission as measured
by a transmission densitometer. Wider lines lower the open
area of the mesh and decrease the percent light transmission.
The following table summarizes the
densitometer readings for pieces of mesh electroformed on
the same mandrel. The readings are in percent transmission.
Bath
Conditions
Ni: 90 g/l
Br: 0.5 g/l
H3 BO3: 30 g/l
pH: 4
Temp: 42.5 deg C
Anodes: R nickel rounds in titanium basket
Agitation: vertical cathode rod
Current density: 15 ASF
DC amps: 5 DC
Plating Time: 72 hours
With DC plating using shields, 0.005 inch thick nickel on
the critical bottom surface requires 5 amps at 72 hours.
The nickel thickness varies from 0.005 inches on the bottom
surface to a high of 0.050 inches on the top side surfaces
of the mold.
Using PPR, after 24 hours of plating,
the nickel on the bottom surface was 0.007 inches thick
and the nickel on the top side surface of the mold was 0.014
inches thick. This is a dramatic change in how the nickel
is distributed in the mold and allows the plating time to
be reduced by 67 percent.
These tests show that PPR can be used
to even out the thickness distribution normally seen with
nickel. The above cycle is based upon the experience of
many users and can be used for any type of part wether it
be rack plated or electroformed using nickel. The recommended
procedure is to optimize the DC plating conditions using
shields, masking, directed agitation, etc and then use the
following formulas to calculate the current settings for
PPR.
Forward direction
80 msec
Forward on time 80 msec
Forward off time 0 msec
Forward peak current 1.55 x DC current
Reverse direction
20 msec
Reverse on time 1 msec
Reverse off time 3 msec
Reverse peak current 3 x Forward peak current
The ratio of forward current to reverse
current is 18.75. This means that compared to DC an additional
18.75% of metal is plated during the reverse current portion
of the cycle. In one complete cycle the current is on in
the forward direction 80% of the time, in the reverse direction
5% of the time. The current is off 15% of the time. These
conditions might not be the optimum for each part, but they
are a good starting point to use when evaluating the use
of PPR for nickel plating or electroforming.
| |
DC |
PULSE PR |
| X bar |
61.4 |
62.1 |
| Sigma |
1.0 |
0.7 |
| High |
63.8 |
63.7 |
| Range |
4.5 |
2.5 |
The results are very
favorable. PPR reduced the standard deviation by 30 percent
and the range by 44 percent. PPR should also increase the
strength of the mesh and this was shown to be true. A 6
percent increase in tensile strength was obtained.
For acid copper baths, pulse plating has been very effectively
used to plate high aspect ratio holes in printed circuit
boards. The acid copper brighteners need a minimum current
density to work effectively and this can be obtained more
easily in the holes using pulsed current than with DC. This
has lead to plating speed increases of up to 300 percent
inside the hole. Such speed increases are not possible on
flat parts(3). One recent paper has shown
that by using a combination of forward and reverse current
pulses, it is actually possible to plate more copper in
the holes than on the surface of the circuit board.(4)
This lead us to try the PPR cycle on electroformed nickel
molds with deep cavities.
At United Technologies Automotive
in Iowa City, the PPR wave form was used to electroform
a nickel mold. The piece cast in the mold becomes the hand
pull on the inside door panel of an automobile.
Figure 5 shows the fiberglass mandrel
and the electroformed mold after it has been separated from
the mandrel. Figure 6 shows an idealized cross section of
the mold.
The critical surface is at the bottom
of the mold. The bottom surface of the mold is a replica
of the surface of a piece of real leather.
List of Figures
Figure 1 PPR wave form
Figure 2 KDI DPR20-30-100
Figure 3 Nickel mesh at 750X
Figure 4 Assembled electroformed sieves
Figure 5 Fiberglass mandrel and electroformed mold
Figure 6 Idealized cross section of electroformed mold
About the Authors
AL KOSTER
is in charge of the Electroforming Department at Buckbee-Mears.
He has worked as a supervisor and engineer in electroforming
for over 25 years.
PAT MENTONE
is a consultant for electronic plating and electroforming.
He received his PhD in chemistry from the University of
Minnesota and has developed many processes for the selective
plating of lead frames. He can be contacted at 612/699-3119.
JODY WEST,
CEF, is an engineer with United Technologies Automotive
in Iowa City. He has been electroforming nickel parts for
over 12 years.
1. Jean-Claude Puippeand Frank Leaman, "Theory
and Practice of Pulse Plating", American electrolplaters
and Surface Finishing Society, 1986.
2. D.T. Chin and M.K. Sunkara, Selective
Pulse Plating of Gold and Tin-Lead Solder, SUR/FIN 88 Proceedings.
3. W.W. Pschoda and A.E. Walker, Production
Plating of Thin Film Circuits from an Acid Copper Bath,
Electronic Packaging and Production, June 1975, p74-78.
4. Peter Leisner and Per Moller, Throwing
Power in Pulse Reverse Plating from an Acid Copper Bath,
SUR/FIN 92 Proceedings.
