Manufacturing Tools

24
MANUFACTURING TOOLS
TOM RITZDORF
Commercial electroplating began almost two centuries ago.
The manufacturing technology for a long time consisted of
what we consider the minimum elements needed to deposit
metal: open tanks, simple solutions for cleaning, rinsing, and
electrodeposition, soluble or insoluble anodes, and a power
source. There was virtually no ventilation or proper waste
disposal. The entire operation was manual. Electroplating
quality depended greatly on the experience and skill of the
operator. Certain physical properties of deposits were improved after plating by heat treating or buffing [1]. Over the
years the industry grew as an art. Improved processes and
techniques were discovered along the way and many were
kept secret until others heard about them or stumbled on the
technology themselves. Eventually platers began filing patents to protect their rights to discoveries. About the same
time entrepreneurs emerged to make a business of developing
and selling new ideas for better plating solutions, techniques,
and machinery.
Following Edison’s success with his research laboratory,
many industrial companies established their own research
and development laboratories, which included electroplating
sections supporting manufacturing operations. Electroplating manufacturing technology was developed as needed:
(1) to solve production problems, (2) to reduce cost, (3) to
increase production rates, (4) to improve quality of the
product, and (5) to meet new standards set by product
designers and waste disposal regulations. Modern electroplating technologies developed for manufacturing include
solutions with the best chemistry available, machines that
automatically transport the product through all stages of
processing, computerized monitoring, and control systems
that also store and process data, online and offline inspection
facility, rapid chemical analysis systems, and an efficient
waste treatment system. Many developments have resulted
from attempts to reduce manufacturing costs, such as increasing production rates with the same equipment, utilizing
cheaper materials, or eliminating unnecessary process steps
or equipment. Also new technologies originally developed in
non-electroplating industries have been adopted for use in
advanced electroplating. Software for machine automation,
data logging, and data processing are good examples. Equipment designed specifically for electroplating semiconductor
wafers and related microelectronic components have brought
together all of the advanced control concepts and have led to
great improvements in understanding the nature of the electroplating chemistries and processes that have been utilized in
these applications.
Fully automatic electroplating equipment can be justified
only when large volumes of the same or similar product are
processed. Modern automatic tools can be operated with
fewer operators, but these people often must be more skilled
and knowledgeable about instrumentation, sensors, and computers as well as electroplating.
24.1
ELECTROPLATING EQUIPMENT
Automated plating machines were invented to do more costeffective plating through increased worker productivity,
improved quality and reproducibility, and higher plating
speeds. Equipment design and development improved as
time progressed. Better materials, especially special plastics,
minimized solution contamination from the machine. Stronger and more corrosion-resistant construction materials extended machine life while allowing operation at higher
Modern Electroplating, Fifth Edition Edited by Mordechay Schlesinger and Milan Paunovic
Copyright Ó 2010 John Wiley & Sons, Inc.
513
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MANUFACTURING TOOLS
production rates. Modern electronics and sensor technology
have made possible more fully automatic plating machines.
Equipment for electroplating should be designed with the
plating processes in mind. In this regard, care should be taken
to provide mass transfer through solution (or part) agitation,
replenishment of the plating chemistry, and appropriate
distribution of the current density among the parts to be
plated. Additionally, the temperature of the chemistry and
parts and the solution concentrations should be well controlled to provide repeatable results. This may mean paying
close attention to solution drag-in and drag-out, depending
on the type of equipment being considered. It is also important to pay close attention to how electrical contact is made to
the parts to be processed in order to ensure reliable and
repeatable performance.
Much of the development that has gone into providing
efficient automated electroplating of various parts has been to
increase the deposition speed of the electrolytic processes to
improve equipment throughput. This is important in order
to get the best return on the money spent to buy the automated
processing equipment. Since the maximum deposition rate
that is reasonable in a manufacturing process is usually
related to the limiting current density, this effort essentially
consists of boosting the limiting current density for a particular process. Of course, increasing the limiting current
density usually involves one or more of the following three
changes to the process:
1. Increase the metal concentration in the bath
2. Increase the deposition temperature
3. Increase the agitation rate at the workpiece (decrease
the diffusion boundary layer thickness)
While the metal concentration depends more on the
chemistry than the hardware, the equipment may need to be
adapted in order to deal with the high metal concentration by
reducing salt crystal formation or to be compatible with
modified base electrolyte chemistry. An increase in deposition temperature may also require equipment changes to
minimize evaporation or control hazardous fumes or to
handle increased corrosive effects of the plating chemistry.
Typically, increasing the agitation rate near the cathode in a
plating process involves increasing solution flow rates and/or
adding agitation through mechanical means or gas bubblers.
While the cathode surface is usually the focus in order to
increase the flux of metal ions to the surface to enable faster
plating rates, the anodic reactions and adequate supply of
reactants and removal of reaction products may also be
important to avoid limiting the deposition rate due to anode
polarization.
Modern electroplating manufacturing technology is
concerned with mass production, automation, quality, and
flexibility. Mass production is important for low-cost
manufacturing. It requires machines that will process parts
at a high rate while meeting all the specified requirements [2].
Automation helps to minimize labor costs and produce a
uniform product. Quality is required to meet market acceptability and product life. Flexibility provides the ability of
plating machines to process a variety of parts without extensive machine modification. It can significantly reduce the
capital cost of accommodating multiple part types. It is
important, at the same time, to consider the type of parts
being plated and their particular requirements in order to use
equipment that is optimally designed for those parts. Plating
machine types include barrel, vibratory, rack, edge board,
strip and wire platers, and wafer platers.
24.2
BARREL PLATERS
Barrel plating began in the post–Civil War years. Barrels
were the first plating device to dramatically increase productivity [3]. There is no racking and unracking of parts, and
the operation corresponds well with other bulk treatment
operations such as barrel polishing, burnishing, deburring,
phosphating, and oxide coating. Barrel plating machines
usually require less space, and they are easily automated,
minimizing operator attention.
Plating barrels come in many sizes and designs, but there
are two basic types: horizontal and oblique. Horizontal
barrels are usually hexagonal with perforated cylinder walls
and have a removable panel through which parts are loaded
and unloaded (Fig. 24.1). They may be partially or entirely
immersed and rotated via plastic gears. Oblique, or 45 ,
barrels may have solid walls, open at the top, and contain
plating solution, anodes, and the parts being plated or they
may have perforated walls and be partially or totally immersed in a tank with outside anodes. A cross section of an
automatic barrel plating machine is shown in Figure 24.2.
FIGURE 24.1
Rotating plating barrel fixture.
RACK PLATERS
515
FIGURE 24.3 Vibratory plating unit where all processing is done
in one place moving solutions in and out using different anodes.
FIGURE 24.2
Cross section of automated barrel plating machine.
Considerable work has gone into plating barrel design and
evaluation of construction materials [3]. Construction of a
typical barrel plating machine is concerned with (1) the
barrel which is rotated so that parts inside are tumbled to
expose different areas to plating, (2) a tank that holds the
plating solution, anodes, support for the barrel assembly, and
means for connecting the barrel to a current source, (3) a
gearing system to transfer motor power to barrel rotation, and
(4) a means of making electrical contact to parts with flexible
probes called danglers fed in through the rotating bearings or
with metal studs fastened to the inside barrel walls. To
achieve the optimum in barrel plating performance in plating
speed and deposit distribution, careful consideration must be
given to part size, weight, shape, and type of plating to be
applied [4]. Deposit thickness is made more uniform by the
following processes:
1.
2.
3.
4.
Decreasing the deposit thickness
Increasing the plating time
Increasing the rotation speed of the barrel
Increasing the amount of barrel periphery open to
plating solution
5. Decreasing the size of the load
6. Decreasing the size of the barrel
Plating current densities are usually low-l0 to 20 mA
cm2 [or 1–2 amperes per square foot (ASF)]. Since the
number of piece parts plated in a barrel is very large, the
productivity of plating is high even though the plating rate is
slow. Barrel plating machines are automated using lift arm or
elevator-type barrel carriers. They are operated by indexing
or stop-and-go movement. In more recent developments of
automatic barrels, the cylinder and superstructure assembly
is mounted in a carriage, permitting up-and-down movement
and a motor drive to rotate and elevate the cylinder. The
carriage moves from tank to tank along rails either overhead
or on the tanks.
24.3
VIBRATORY PLATERS
Vibratory plating is one of the newer developments in plating
machines. It is used to plate small parts in bulklike barrel
plating, but instead of rotating, it relies on vibrational energy
applied to the bottom of the processing cell to move parts
about [5]. More uniform deposits are claimed for the vibratory plater compared to barrel plating. The technique is able
to handle fragile parts without distortion or damage. At
present there are two types of vibratory platers: (1) where
all processing is done in one place with solutions moved in an
out for the plating sequence (Fig. 24.3) and (2) where the
processing moves a basket with a vibrator attached from tank
to tank in a sequence like a conventional barrel line [6, 7].
Both vibration frequency and amplitude are adjustable. This
is necessary to compensate for process solution viscosity
differences to achieve optimum agitation conditions.
24.4
RACK PLATERS
Rack plating consists of attaching parts to an insulated frame
and either manually or automatically moving the racks of
parts through all plating and rinsing steps [8]. Figure 24.4 is
a cross section of a typical rack plating hoist assembly for
printed circuit boards.
Good rack design and construction are important to
successful rack plating. The following factors must be
considered:
1. Size and shape of parts to be plated
2. Size and shape of rack based on tank size and other
equipment
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MANUFACTURING TOOLS
FIGURE 24.4
Cross section of a typical rack plating hoist assembly for printed circuit boards.
3. Method of supporting and making electrical contact to
parts
4. Insulation of metal parts of rack other than contacts
5. Dielectric shielding to control plating distribution on
parts
Rack plating machines are of two types: straight line or
return. Straight-line machines may be loaded at one end and
unloaded at the other, or the process controller can be
programmed to bring the rack carrier to the starting point
where the same operator can unload and reload the carrier.
Return machines are designed to have the carriers always
move in the same direction and the tanks are laid out so that
racks circle back and the unload station is next to, or the same
as, the load position.
Rack carrier movement through the machine can be stepand-repeat or continuous. Step-and-repeat machines move
carriers a fixed distance on a preset time cycle. When racks
are moved from one tank to another, they are raised before
carrier movement, then lowered. If racks are in a long plating
tank, they are not raised until they reach the end of the tank.
Plating thickness is determined by cathode current density
and dwell time in the tank. Machines with continuous rack
movement automatically raise and lower racks to move from
tank to tank. As with step and repeat, dwell time at each
process step is controlled by line speed.
Electroplating equipment is designed to deposit metal
uniformly on the parts to be plated. Current distribution
within plating tanks is usually controlled by incorporating
dielectric baffles as current shields and by proper design of
anode size and placement within the tank. In some cases,
bipolar electrodes may be used to improve current distribution. Solution agitation in rack plating equipment may be
accomplished in one or more of several methods. The most
common methods of solution agitation include mechanical
agitators, bubble agitation (usually air), or forced-solution
eductors (jet pumps) that mix the solution and direct the flow
within the tank across the cathodes to be plated. Ultrasonic
agitation has also been used in some cases to provide
agitation at the electrode surfaces.
Electroless plating is typically a slow deposition process
but can deposit uniform films and can deposit on nonconductive substrates. Electroless plating solutions rely on a
reducing agent to supply the electrons. The choice of pH,
temperature, reducing agent, metal ion, and their concentrations determines the electroless deposition rate. The slow
plating rate can be compensated by plating many parts at one
time. Plating equipment for electroless plating is similar to
that for electroplating but different in several important
respects. There is no power supply. This provides advantages
in that the equipment is simpler, with no electrical connections. The operating cost may be increased due to higher
bath temperatures, though. Dwell time in the plating tank is
usually long, often 24 hours for a “full build” of copper on
printed circuit boards (PCBs). Continuous solution filtration
is essential to remove particulates that could cause spontaneous metal deposition which depletes metal ions from
solution and potentially “crashes” the bath. Also, an additional tank containing an acidic stripping solution may be
necessary for periodic removal of metal that has plated out
on the equipment. The manufacture of PCBs using fully
electroless copper is a viable technology to produce fine-line
conductors and high-aspect-ratio through holes [9]. Other
electroless processes that are in industrial use include
nickel for decorative coating of plastics or wear-resistant
coatings and electroless nickel/immersion gold (ENIG) for
pad finishing of microelectronic components. Many of
the pad finishing processes that have historically been
used also contained palladium as a barrier between the
nickel and gold, and increasingly only nickel and gold are
being used.
24.5
STRIP PLATERS
Most strip platers can be divided into two groups: (1) electrotinning and electrogalvanizing mill steel platers and (2)
STRIP PLATERS
strip platers for electronic parts. Both types of strip platers
are usually operated reel to reel continuously with splicing
“on the fly.”
24.5.1
Electrotinning
Prior to 1937 all commercial tin plate was manufactured by
the hot-dipping process. Electrolytic tin plate on steel became more economical than the hot-dipping process when
continuous cold reduction mills came into operation in the
early 1930s [10, 11]. By 1935 small experimental plating
machines were designed and built that were capable of
electroplating tin on steel strip at high speeds. Electrolytic
tin plate became a commercial item in 1937. Electrotinning
could produce a thinner, more uniform tin coating than the
hot-dipping process [12–14].
Three plating processes were developed: (1) alkaline
stannate, (2) ferrostan [registered trademark of US Steel]
(polysulfonate), and (3) Halogen (fluoride and chloride).
Plating baths for electrotinning are designed for very high
speed tin deposition. Most commercial tin plating lines in the
world use the ferrostan process, which is based on a sulfonic
acid electrolyte. Figure 24.5 shows schematic arrangements
of three types of handling and processing machines. These
are big machines designed for high-speed strip travel at over
11 m s1 (2000 ft min1). The entry end “pays off” strip into a
looping tower that stores a large amount of strip. When the
payoff strip approaches its end, it is stopped and quickly
spliced by welding to a new roll of strip. While the payoff
speed is below normal line speed, strip in the looping tower
keeps it going until the payoff strip catches up and restores
FIGURE 24.5
517
the volume of strip in the looping tower. A steady tension is
put on the strip after the looping tower.
Figure 24.6 shows the entry end of an electrolytic tinning
line. The steel is cleaned and acid pickled before entering the
plating section. Tin anodes need to be replaced often. The
procedure is illustrated in Figure 24.7. Note the size of the bus
bars that carry current to the cathode contact cylinders. Highspeed strip travel requires very high currents to deposit the
required tin. The typical electrical power capacity for a
ferrostan line is 100,000 A at 24 V.
Solution drag-out control is important since it can amount
to 30 gal h1. After rinsing, the tin-plated strip passes a reflow
tower that heats the strip above the melting point of tin. This
gives the tin a brilliant luster typical of hot dipping. The fused
and quenched tin is given a chemical treatment, rinsed, and
dried. Finally, a lubricant oil film such as dioctyl sebacate is
applied to improve its handling properties in succeeding
operations. This is usually an electrostatic process using a
high potential between the strip and a fixed electrode while a
mist of the oil is produced in between the surfaces.
The strip next enters the unit which supplies traction
power to pull it through the machine. A tachometer generator
is attached to the drive motor which produces the signals to
regulate the plating and melting currents in relation to strip
speed.
24.5.2
Electrogalvanizing
Electrogalvanizing strip plating machines were developed
after electrotinning machines [10]. They are similar in
design, as shown in the schematic arrangement of a line at
Schematic arrangement of handling and processing units of three types of electrotinning lines.
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MANUFACTURING TOOLS
FIGURE 24.6
Entry end of electrotinning line with one coil being paid off and another in reserve position—splice by wleding.
FIGURE 24.7 Tin anodes being placed in tank. Note the size of
the bus barscarrying current to cells.
United States Steel (Fig. 24.8). A looping unit is employed at
both ends to enable splicing while the machine is running at
full speed. Machines for plating on both sides use vertical
cells like those used for electrotinning with either soluble or
insoluble anodes and a zinc sulfate electrolyte.
FIGURE 24.8
The auto industry requires one-sided zinc-coated steel.
United States Steel developed the radial cell system for
single-side zinc deposition illustrated in Figure 24.9. The
design achieves efficient plating power usage by reducing the
distance between anodes and strip. The conductor rolls are
2.4 m (8 ft) in diameter. The total plating pass length is about
67 m (220 ft) for all 18 cells. The zinc for electrolytic
replenishment is provided from cast zinc anodes supported
on conducting bridges.
The anodes are moved across the bridges at a rate determined by the line control computer. Approximately one-half of
the anode is consumed as it moves across the bridge. It is then
removed and remelted for casting into new anodes. The plating
current at each cell is 28,000 A for the 18 cells. The maximum
plating voltage is 12 V. Strip widths on the line range from
91.4 to166.4 cm (36 to 65.5 in.). Line speed is computer
controlled to maximize production with the available current.
Maximum line speed is 183 m min1 (600 ft min1).
After leaving the plating section, the strip is rinsed, dried,
and moved to the main pulling station. An X-ray coating
gauge continuously monitors the coating thickness and provides a feedback signal to the computer for line speed
adjustments to maintain the specified zinc thickness.
Electrogalvanizing one side of strip up to 6 ft in width at
600 ft min1 is a commercial process. Machines for doing
this generally operate 24 h a day seven days a week and stop
only for maintenance. Splices are made by welding “on the
fly,” and the plating thickness is continuously monitored by X
ray with the output signal fed to a plating current controller to
Schematic arrangement of one-sided electrogalvanizing line.
STRIP PLATERS
FIGURE 24.9
519
Schematic arrangement of conductor roll and plating unit CAROSEL for one-sided electrogalvanizing.
FIGURE 24.10 Schematic of belt cell for one-sided electrogalvanizing steel strip: (1) metal belt, (2) rubber bonded to top of metal belt,
(3) continuous moving steel strip, (4) electrical contact to metal belt, (5) insoluble anode, (6) solution into cell, and (7) solution exit.
maintain a constant deposit thickness. The large expense of
these machines can only be justified if there is a large volume
of product to be plated over several years.
Another commercial electrogalvanizing plating cell design developed for depositing zinc on only one side of sheet
steel is shown schematically in Figure 24.10 [11, 15]. The
“belt cell” uses solution flow between an insoluble anode and
a cathode 6 mm apart at 4 m s1 to achieve turbulent flow.
Steel strip moves through the cell in the opposite direction to
solution flow at speeds up to 200 m 1. Dense, coherent zinc
deposits are obtained from sulfate solutions at high current
densities up to 350 ASD [16, 17]. Cathode efficiencies are as
high as 95%, which is attributed to the high solution flow rate.
24.5.3
Strip Plating for Electronics
The explosive growth of the electronics industry began about
40 years ago, and it continues today. It led to a tremendous
demand for contacts, lead frames, wire leads, chip carriers,
and many other components requiring electroplating of tin,
solder, nickel, silver, gold, palladium, or palladium–nickel.
The huge number of components required makes it necessary
to develop electroplating technology capable of high speed
and sometimes selective processing. Loose parts such as pins
can be plated in high volume in barrels and vibratory
machines. Gold and other precious metals often must be
deposited selectively for economic reasons and strip platers
are best suited to the job. These are usually reel-to-reel
plating machines designed to process one or more part
shapes. Modern technology has provided strip plating machine designers with a vast array of chemically stable materials, electrical and electronic devices, and subsystems.
High-speed reel-to-reel platers can be divided into four
segments: (1) strip transport system, (2) a table with small
plating cells and larger solution reservoirs, (3) electrical
power sources and controls, and (4) a display of plating
conditions. An example of such a machine is shown in
Figure 24.11. The machine was designed to electropolish,
nickel plate, and gold plate telephone modular cord plug
blades. All cells and reservoir tanks were covered to minimize contamination from the environment and make fume
exhaust more efficient. It has four separate plating lines each
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MANUFACTURING TOOLS
FIGURE 24.11 Four-line automatic strip plater for telephone
modular cord blade plugs.
running at 12 ft min1. Figure 24.12 is a schematic of the strip
plater.
Since the blade being processed in the equipment shown
in Figure 24.11 was formed by punching, the contact area had
a shear/break surface and needed considerable electropolishing. Dwell time in the electropolishing cells required electropolishing selectively at a current density of about 4000 A
ft2 to achieve the desired finish. A high current to a strip can
lead to several problems: (1) electrical arcing at the contacts
can eventually cause a poor contact, (2) excessive heating of
the strip, and (3) excessive heating of the solution. The
arcing problem was solved by using tungsten/silver alloy
contacting material.
Electropolishing quality degrades with solution heating.
Strip heating is minimized by keeping the cell short and
electrically contacting strip at both ends to minimize resistive
heating. Solution heating is also controlled by keeping
anode–cathode spacing as close as possible and cooling the
phosphoric acid electropolishing solution in the tank. Another problem occurs when electropolishing copper alloy
strip. The solution becomes saturated with copper and even-
tually starts depositing a powder at the cathode. The copper
powder bridges the cell, causing an electrical short so electropolishing stops. The powder is easily removed manually,
but an automatic method is more economical. Two methods
can be used: (1) a rotating cylindrical cathode with a scraper
to remove the powder or (2) periodically vacuuming off the
solution with powder next to the cathode. The solution is
returned to the tank after the powder is separated out.
Good rinsing between chemical and electrochemical steps
is very important in all electroplating systems. Spray rinsing
is very effective. Nickel plating cells use small titanium
baskets with bags and nickel pellets inside. The entire strip
receives a gold strike followed by a selective hard gold plate
just at the contact edge. Sensors detect when a payoff reel
empties or a take-up reel becomes full. That particular line
automatically stops until the operator changes reels and
restarts the line.
Narrow strip platers are used mainly to process parts for
electronic applications. These are reel-to-reel platers and
often deposit metal selectively. Considerable development
has gone into these machines in recent years [18]. They are
highly automated, plate at high speeds, and deposit metal
with excellent precision. Typical applications are connector
terminals and semiconductor lead frames. Gold was often
used in these applications, but recently palladium and
palladium–nickel alloy with a gold flash has replaced most
of the gold [18]. Specially designed selective plating systems
have been developed to minimize the usage of precious
metals [19].
Strip may be punched or unpunched. Plating unpunched
strip is referred to as stripe on strip, with punching done
afterward. Most connector terminal plating is done on
punched strip with terminals held together with tie bars
which are removed during connector assembly.
Development of high-speed strip platers began just at the
time small low-cost microprocessors became available for
machine control. This led to programmable logic controllers
(PLCs) being used for overall process and machine control of
strip platers. More recently, small computers such as PCs
FIGURE 24.12 Schematic of strip plating facility. Each cell is a miniature chemical processing tank.
STRIP PLATERS
have been used but not in direct machine control. PCs are
very good for operator interface and reporting functions.
PLCs are better for direct control [20, 21]. Often both
electronic devices are used in combination for plating machine monitoring and control [22]. A main plant computer
can be programmed to watch over plating operations and alert
engineers of problems as they occur. The machine shown in
Figure 24.12 utilizes a PLC for process monitoring and
control with an interface PC for data logging and analysis.
Customers often request data on the processes used to
electroplate their product and will pay extra for the information. A central console and process display panel
(Fig. 24.13) allows the operator to quickly see the status of
machine operation. Each process segment of the machine is
designed to run in either the manual or automatic mode. This
is desirable when a segment may not be functioning properly
in the automatic mode.
Another type of strip plater is used in the printed circuit
board industry to electroform a thin continuous sheet of
copper which is bonded to a plastic dielectric. Copper is
plated on the outside of a large rotating polished stainless
steel drum. The drum is treated so the copper is removed
easily in a continuous sheet. The outside surface of the sheet
is purposely plated with a slightly rough copper surface to
improve adhesion when it is bonded to a plastic dielectric.
This is called vendor copper and may be applied to one or
both sides of a flexible or rigid dielectric.
FIGURE 24.13 Central console and process display panel.
521
A third type of strip plater manufactures flexible printed
circuits. All chemical and electrochemical processing is done
on wide roll-to-roll strip plating machines. A thin copper
sheet is bonded to one or both sides of a flexible dielectric
material such as mylar. A plating mask is applied to one or
both sides. Rolls of strip, or web as it is called, then are passed
through the plating machine to produce a printed circuit
copper much thicker than the initial copper. Tin–lead solder
may be plated over the copper for solderability and/or as an
etching mask for definition of electrical circuits after the
plating mask is removed. The web moves over wide rollers on
top and down into each tank in the processing line. The end of
each roll is spliced to the next, so the machine sees a
continuous web.
Bandoleer Plating Bandoleer plating involves attaching
individual parts such as connector terminals to a carrier belt
that moves through all processing steps like a strip plater.
Parts that otherwise would be plated all over in a barrel or
vibratory plater can be selectively plated on a bandoleer
machine. A vibrator parts handler orients and moves the parts
to the carrier belt where they are attached. At the end of the
line, parts are detached and the belt circles back to the front of
the machine.
Edge Board Platers Edge board or tab platers are designed
to plate precious metal contacts on printed circuit boards.
These are relatively new machines in the plating industry and
were developed to serve a specific need for rapid selective
plating of gold on contact fingers. Boards are loaded
automatically into the continuous motion transport system
by indexing the board carrier at the proper time so that
moving belts can grasp the board and start it through all
processing steps similar to plating nickel and gold on
connector contacts in a strip plater. Rubber belts also
serve as plating masks for selective plating to eliminate
taping. Transport and plating belts are specially designed
to firmly contact boards with compliant rubber that will not
wear excessively and not stretch. These properties can only
be achieved with a compound belt made up of several layers.
The belt may also incorporate metal inserts for electrical
contact to the contact fingers.
Good solution agitation around the cathode is essential for
high-speed plating which is necessary for rapid movement of
boards through the machine. Plating cell length is another
factor that can determine machine speed. If line speed is
limited by plating rate in a particular cell, any increase in that
cell length can increase line speed. Improving solution
agitation around the cathode area may also allow higher line
speeds.
Good rinsing between chemical treatments is important.
Belts also must be rinsed thoroughly before the belts return
and contact new boards coming into the plating line.
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MANUFACTURING TOOLS
Drying is done with a blast of hot air directed at the wet
areas. Unloading is automatic with boards pushed into board
carriers that are indexed to receive each board as it comes off
the plating line. Most edge board platers are single lines to
avoid being too complicated, especially when belt transport
and masking are used.
Some tab platers do not use belts. Masking is done with
tape that can be done by machine, but this operation is more
costly. In such machines contact to the fingers is made via a
tie bar around the outer edge of the board which is cut off
later. Electric current is brought to the tie bar at either the side
or top. Board transport is by moving belts or an overhead
trolley with fasteners holding boards.
24.5.4
Wire Platers
Wire platers were the first high-speed platers, with plating
current densities up to 2000 ASF [10]. These high current
densities are made possible by moving the wire at high speeds
through plating solution causing rapid wire motion relative to
solution, not only parallel to the pull direction but also
perpendicular as the result of wire vibration as it travels
through the plating cell. Wire is an ideal cathode, usually
being round in cross section and continuously uniform in
shape throughout the plating operation. Wire also lends itself
to multiple line platers, which is important in achieving highvolume production of plated wire needed in a variety of
applications. Tin and tin–lead alloys are plated on copper for
electrical wire. Gold or palladium–gold is plated on copper
wire for connector jacks, copper on steel wire for communication systems, and zinc on steel for fencing and other
outdoor applications.
Classification of various cell designs is done on the basis
of the direction of forced fluid flow which may be axial,
normal/radial, or tangential. These are modified somewhat in
practical cells [23–25]. Conditions which produce turbulent
flow are favored for high-speed plating.
24.6
DECORATIVE AND ENGINEERING PLATING
At one time a thin bright chromium deposit over bright nickel
plating was used extensively on automobile and appliance
parts. In recent times, a combination of product styling
changes and increased cost of the Ni–Cr finish due to tougher
environmental regulations has reduced its use mainly to just
trim. A significant improvement in the cost of hexavalent
chromium disposal or recovery would help bring greater use
of the attractive bright Ni–Cr electroplated finish.
Engineering electroplating technology generally involves
improving mechanical wear or corrosion protection by applying metal with special physical properties on very small or
very large objects [26]. Outdoor objects such as statues and
building domes are usually made decorative by plating but
the process of doing the plating involves many engineering
factors. The technique of brush plating is used extensively for
applications where the part to be plated is too large or in a
fixed position away from a plating shop. Special plating
fixtures and solutions for substrate cleaning and depositing a
variety of metals have been developed [27]. The technique is
also used to repair worn or poorly machined parts. Electroforming of metal shapes can be another type of engineering
plating [28]. Complicated forms impossible to machine are
examples of ideal applications of electroforming. These can
be very large objects or extremely tiny forms such as inertial
elements in MEMS (microelectromechanical systems) gyroscopes and acceleration sensors or the copper coils and
magnetic yokes used in computer memory disk drive and
tape read–write heads [29, 30]. Another example is the
manufacture of compact discs (CDs) where electroplated
nickel stampers are electroformed with the precise image of
the master audio or video recording [31]. Where these small
devices are plated on silicon wafers or similar substrates, the
equipment used will be wafer plating equipment, as described in the next section.
24.7
WAFER PLATING EQUIPMENT
Electroplating of wafers used in semiconductor or microelectronic manufacturing has matured dramatically over the
last two decades [32–34]. As electroplating has moved from
the thin-film recording head and GaAs semiconductor device
manufacturing areas to mainstream silicon semiconductors
and the manufacturing of numerous MEMS devices, the
understanding of the chemistry, process, and equipment
interactions has grown dramatically. In addition to advances
in chemistry formulations specifically designed to provide
superconformal deposition profiles (>100% step coverage)
or relatively high deposition rates, several advances have
been made to electroplating equipment that is used in the
microelectronics industry. These advances include such
things as current shielding and virtual anode designs
[35, 36], multiple anodes for current distribution control,
independently controlled current thieving, and membrane
solution separation systems. Many advances have also been
made in automated analysis and replenishment systems that
are integrated into or support the plating equipment, as will
be described in the next chapter.
The considerations that are usually the most important in
microelectronics plating include process robustness and
repeatability. Plating processes usually occur late in the
manufacture of microelectronics components, where their
value is greatest, and the batch processing nature of microelectronics manufacturing means that between hundreds and
hundreds of thousands of devices may be at risk on a single
wafer if it is misprocessed. Wafer-to-wafer repeatability
is very important in order to ensure uniform product
WAFER PLATING EQUIPMENT
characteristics, and within-die or within-feature thickness or
alloy composition uniformity may also be critical factors.
Some of these considerations mean that the microscale
pattern density of the substrate may be important in terms
of determining the results on the wafer or substrate [37–39].
Generally, semiconductor wafers and similar microelectronic substrates are plated in machines that are automated.
The automation required in a manufacturing environment for
leading-edge semiconductor devices typically involves
completely automatic unloading of the wafers from the
holder (cassette or FOUP—front opening unified pod), automatic transfer and processing of the wafers through each of
the process steps, and loading the wafers back into the correct
holder after they are rinsed and dried. Complete data logging
of all the process parameters and response data is also typical,
as well as sending these data files to a host computer system
over a network connection automatically to facilitate data
archiving. Most of these aspects are accomplished according
to industry standard specifications for equipment automation, ergonomics, data handling, and general semiconductor
equipment guidelines [40–44]. There are also special regulations that apply to equipment that will be used in the
European Union [45]. All of these special requirements and
specifications, coupled with the understanding that the
value of the product being processed may be much more
than the equipment itself, results in processing equipment
that is relatively complex and costs multiple millions of
dollars to purchase (see Fig. 24.13). Also, wafer plating
systems have sophisticated error recovery schemes in order
to minimize the chance of scrapping wafers when a processing fault occurs. Sometimes semiautomated versions of the
equipment are used for research and development or for
troubleshooting process and integration problems in a
manufacturing line. In these cases, an evaluation of which
of the standard semiconductor equipment guidelines must
be met should be completed prior to manufacturing of the
equipment.
Laboratory facilities are also important to help solve
production problems when the plating process does not
perform as expected. Experimental runs on a small scale
can be set up to determine the cause of a problem and also
how it may be corrected on the production line. Laboratories
are also set up to do routine chemical analysis and plating
tests such as the Hull cell or the hydrodynamically controlled
Hull cell, which can provide insight to manufacturing
problems [46].
Microelectronic plating is done in several types of plating
chamber designs. Cell designs based on paddle agitation
were some of the first configurations used in microelectronic
plating processes. Most often, a fountain plating cell is used
for semiconductor plating applications. Rack plating
systems can also be configured to handle microelectronic
substrates. These plating cells may be configured with
the wafer oriented vertically or horizontally, with different
523
advantages and disadvantages for each particular design and
configuration.
24.7.1
Paddle Cells
Electroplating manufacturing technologies are often developed first in a laboratory. An example is the reciprocating
paddle plating cell which was developed by IBM for uniform
and rapid deposition of NiFe, Cu, and Au on micrometerscale features for electronic and magnetic devices [47–49].
These cells were designed to provide cathode agitation and
reproducible alloy composition uniformity on substrates that
could not easily be rotated during the deposition of magnetic
films deposited in the presence of an aligning magnetic field.
Cells based on this original design have been used in the
magnetic recording head industry for the last 30–40 years
(see Fig. 24.14). More recently, the reciprocating paddle
design has been updated to provide more uniform agitation at
the cathode surface along with better control of the current
distribution across the face of the substrate and compatibility
with automated microelectronic processing systems, as described above [50, 51].
24.7.2
Fountain Cells
Most electrochemical deposition on semiconductor wafers or
similar substrates occurs in fountain electroplating cells.
These are cells, or electrochemical reactors, where the
solution flow is generally from the bottom to the top of the
chamber and where the solution exits the main processing
area by flowing over a weir near the edge of the wafer.
Figure 24.15 shows two examples of common fountain
plating cells used in semiconductor processing. Fountain
FIGURE 24.14 Deposition cell designed to provide cathode
agitation.
524
MANUFACTURING TOOLS
FIGURE 24.15 Two examples of common fountain plating cells used in semiconductor processing.
plating cells have the advantage of having a characteristically
uniform diffusion layer thickness from center to edge when
used for plating blanket films on a spinning substrate.
Since automated fountain plating systems were first introduced in the early 1990s, many improvements have been
made to these electroplating cells to adapt them to the
specialized needs of microelectronics plating. Most of these
modifications have allowed more efficient utilization with less
maintenance, lower cost of operation, or improved process
performance and stability. Features such as quick-change
anodes and process kits to support different wafer sizes have
helped to eliminate expensive down-time associated with
these maintenance activities. Incorporation of ion-selective
membrane systems have increased bath life, decreased addi-
tive consumption, and improved chemical stability. Multiple
electrode systems have improved current density control
across the wafer and have helped automated plating systems
respond to variations in incoming material. It is also becoming
more common for microelectronic plating systems to incorporate automated rinsing capability as part of the plating cell,
itself, which minimizes the chance of corrosion and minimizes dragout of plating chemicals.
24.7.3
Rack Platers
Semiconductor and other microelectronic substrates may
also be plated in rack platers. These have already been
described above, in Section 24.4, and function in much the
REFERENCES
same way when used for microelectronic manufacturing.
Substrates may be loaded automatically into racks, or fixtures, by the processing equipment or the substrate may be
loaded into the rack by operators and loaded into the machine
for automatic transport and processing. The racks may be
transported and processed in a vertical position or in a
horizontal position. Mechanical agitators and eductors provide the most common forms of solution agitation for
processing microelectronic components. When using rack
plating equipment, it is always critically important to make
sure to rinse thoroughly to minimize carry-in of chemicals
from upstream processes. These systems typically exhibit
greater solution dragout than other types of wafer plating
equipment.
24.7.4
controllers and sensors to increase the level of automation in
the industry. Technology exists to fully automate most
plating machines. The utilization of this technology depends
on its cost effectiveness and the ability of plating engineers
and operators to handle advanced technology such as computers with complex control software. Plating processes
continue to improve with better understanding and knowledge about the processes and the deposits produced for
various applications.
ACKNOWLEDGMENTS
Figures 24.2 and 24.4 were supplied by NAPCO, Inc.
Figures 24.6–24.10 are reprinted with permission from The
Making, Shaping and Treating of Steel, 10th ed., AISE, 1996.
Ancillary Chambers
Automated wafer processing systems will contain chambers
other than electrochemical deposition (ECD) chambers to
accomplish such tasks as wafer alignment (using flats or
notches), prewetting, surface activation, intermediate rinses,
back-side contamination cleans, final rinse and dry, thickness
measurement, and anneals. In fact, they may even include
chambers to strip photoresist or to etch metal seed and barrier
layers after plating and photoresist stripping. Some of these
chamber types are straightforward to include in a plating
system, while others may require special considerations with
respect to segregation of chemicals and air handling.
The configuration, or layout, of a wafer plating tool with
multiple chamber types can be fairly complicated, especially
if the process times can vary within a particular process
chamber, as in parts with different plated thickness requirements. Multiple chambers can be used in parallel to accommodate longer processes. While more process chambers may
be added to overcome bottlenecks in the process sequence,
complex scheduling software is usually required to allow
efficient use of all the chambers, especially when a mix of
product types must be accommodated. This scheduling
software may also take into account delays between process
steps, which may need to be minimized in order to minimize
surface oxidation between steps and maintain product quality. These scheduling algorithms may be incorporated into
equipment simulators that are used to predict the equipment
throughput in order to configure the tools that will best meet
the expected needs with respect to product mix and fab
capacity requirements [52].
24.8
525
SUMMARY
A wide variety of plating machines have been invented and
developed to serve specific needs for electroplating parts with
increased productivity and better deposit properties and at a
lower manufacturing cost. There is greater use of electronic
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