Chapter 14 Plating on Plastics

Chapter 14
Plating on Plastics
John J. Kuzmik
The plating of non-conductors has been achieved for many years. Articles
plated were mainly decorative, and adhesion of the plate t o the substrate was
minimal. In the early 1960s' due to technological advances in chemical
processing techniques, plating on plastics began on a commercial level.
Industries that utilize plated plastics include the automotive, plumbing, appliance
and electronics.
One of the early plastics to be plated on a large scale was polypropylene,
which gave adhesion values of over 20 pounds per linear inch. Even though
adhesion was excellent, other problems occurred that ultimately led to the
demise of plating polypropylene. These problems included (1) failure t o pass
thermal cycling due to its high coefficient of linear thermal expansion (i.e., the
amount i n inches per inch the part shrinks in the mold-68 t o 9 5 x lO-'in./in./" c
for polypropylene); (2) brittleness after plating due to notch sensitivity (a
phenomenon whereby a crack in the plate will propagate through a normally
flexible plastic, causing it t o break easily); and (3) "sink" marks caused by the
plastic shrinking i n the mold, especially over bosses and ribs and showing up as
dimples or "sinks" after molding.
Some of the reasons various industries are interested in plating plastics
include (1):
0
Lowercost
No secondary operations (i.e., no deflashing or buffing)
Design freedom (i.e., the ability to mold large and complex parts)
Weight reduction
Weight reduction became a very real benefit to the automotive industry when
the gasoline crunch occurred in the early 1970s.
Many plastics are plated today, including:
0
0
ABS
Polypropylene
Polysulfone
Polyethersulfone
Polyetherimide
Teflon
377
370
ELECTROLESS PLATING
Polyarylether
Polycarbonate
Polyphenylene oxide (modified)
Polyacetal
Urea formaldehyde
Diallyl phthalate
Mineral-reinforced nylon (MRN)
Phenolic
Some typical applications of plated plastics are shown in Figs. 14.1 and 14.2.
Several of the above-mentioned resins have little or no adhesion and/or
unacceptable appearance. Of the resins listed, ABS, acrylonitrile-butadienestyrene, has found the widest acceptance in the plating industry. ABS is a
terpolymer thermoplastic that has an acrylonitrile-styrene matrix with butadiene
rubber uniformly distributed in it. This quality makes it unique for plating, as the
butadiene can be selectively etched out of the matrix (Fig. 14.3), leaving
microscopic holes that are used as bonding sites by the electroless plate. Other
factors influencing the choice are:
Lowcost
Low coefficient of thermal expansion
Ease of molding
Good metal adhesion to the substrate
Good appearance after plate
Because ABS is the most widely plated plastic, it will be the material mainly
referred to in this chapter. Scanning electron micrographs of ABS surfaces are
shown in Fig. 14.4.
Quality plating on plastics involves the following three basic steps:
Molding-converting plastic pellets into the desired part.
Preplate-processing the molded part through an electroless bath in order
to render it conductive.
Electroplating-building additional metal thickness using current.
Because of the importance of molding, which must be done properly to insure
a part with the high quality necessary for plating, it will be advantageous to
mention some general guidelines that should be followed. Molding consists of
transforming the plastic pellets into the shape (part) to be plated. A mold of the
part must be built and in doing so, certain design features must be taken into
account for the finished part. These include (2):
Gates (fill areas) should be put in a non-appearance area.
Integral parts should be used to avoid welded joints.
Ribs and bosses should be designed to eliminate “sink” marks.
Texturing can be used to break up large flat surfaces and hide any defects,
such as scratches.
0
379
Fig. 14.1-Typical applications of plated plastic parts.
Fig. 14.2-More appiicalions of plated plastic parts.
Draft angles should be at least one degree (1") for easy removal of the part
from the mold.
Parting lines (where the mold opens) should be put in a non-significant area
if possible.
Close tolerance fits must include the final plate in the initial part design.
Wall thickness should be sufficient to insure rigidity.
Plate uniformity, which results from current density distribution, must be
considered in the initial design. Use no 90" angles, no V grooves, keep letters
close to the wrface, make angles as large as possible, and crown large flat
surfaces (Figs. 14.5 and 14.6).
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ELECTROLESS PLATING
Fig. 14.4-SEM photomicrographsof ABS: (left) as molded; (right) after etching.
Some other design considerations are: location of drainage holes, no blind
holes, and rack contact areas on the part.
If the above features are taken into account when designing a mold, a part
suitable for plating should be produced. These are not all the considerations, but
some that must be looked at.
Certain molding parameters should also be followed to insure good parts.
Some of these are (2):
Highly polished mold-Poor mold surfaces can cause defects in the molded
part such as pits, which show up in the final plate and cause rejects.
Proper drying of resin-Moisture in the resin can cause "splay" or
delamination on the part, which may result in a blister.
Proper melt temperature-If the melt temperature is too low, stress can be
incorporated in the part, which could cause uneven etching and possible
thermal cycle failure. Consult the resin manufacturer for the proper parameters.
Plating on Plastics
381
i
Fig. 14.5-Design considerationsfor plating thlckness unlformity: (left) 90” angles and V-grooves
result in poor current distribution; (right) suggested design.
Fig. 14.6-Sharp corners(a) result in poor plating uniformity;(b) rounded corners, both Internal and
external,Improvecoverage. Expansive surfaces (c) should be crowned fora convex surtace as In (d).
Proper mold femperafure-If the mold is too cold, “skinning” can occur,
that is, the first material to hit the mold hardens instantly and the hot material
under it flows, causing a surface skin that may cause delamination.
Proper fill speed-A fast fill speed can overpack a mold, thus making it
harder toetch, resulting in adhesion loss. Best resultsareobtained using a slow
fill speed.
These are some of the more obvious parameters t o be considered when
molding a part to be plated. The mold design and molding parameters are
extremely important to insure good parts that are free of stress and other
imperfections. Most plastics can be evaluated for stress. In polycarbonate and
other clear plastics, stress can be detected using polarized light. ABS is
immersed in glacial acetic acid for one minute, rinsed and dried. Any peeling
indicates delamination, white areas depict stressed areas, which usually etch
differently than the rest of the part, causing adhesion and thermal cycle
problems. Cracks or actual breakage of parts indicate severe stress and should
not be plated at all.
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ELECTROLESS PLATING
The type of mold release used can have a profound effect on the molded part.
For example, silicon-type mold releases (materials sprayed on the molded
Surfaceto help in the removal of the molded part) should not be used, as they are
extremely difficult to remove and usually lead to adhesion failures (blisters) or
misplate because they hindered the processing solutions from performing their
functions properly. Molders who mold plastic parts for plating generally do not
use a mold release. If it is essential to use one, a stearate or soap type may be
used sparingly.
The addition of fillers (4) to the resin can create problems when plating of the
parts is involved. In some cases fillers are added to the resins to increase
strength (i.e., glass fibers), to impart color (i.e., carbon black or titanium
dioxide), or for fire retardancy (organic phosphates, antimony oxide). In any
case, problems can arise. Glass, for example, usually gives a rough surface to
the finished part. Titanium dioxide can build up in the processing tanks and
cause a rough surface after plating. Some of the fire retardants and other fillers
can leave a non-adherent fiim on the surface that results in no adhesion after
plating. If the film can be removed prior to catalyst, good results are usually
obtained.
Other fillers such as calcium carbonate are added to facilitate etching. These
fillers are preferentially etched out of the surface to create bonding sites
required for adequate adhesion. If these particles are large, a poor surface
results. Generally, a happy medium is reached where adhesion is adequate and
the surface is acceptable. Mineral-reinforced nylon is one such material.
When parts are molded, they are ready to be plated, as there is no secondary
operation required. Parts are racked and run through the processing solutions,
which include cleaners and/or predips, etching, neutralization, preactivation,
activation, acceleration, and finally electroless plating (see Fig. 14.7). These
steps are known as the preplate cycle (5).
CLEANERS
Cleaners are used for removing light soils such as fingerprints, dirt, and other
debris from the parts. They are usually mild alkaline cleaners. In some cases, a
highly wetted chromic acid solution may be used to insure complete wetting of a
part in knurled or lettered areas prior to etching. This will minimize pinpoint
skips in these areas. Cleaning prior to etching is optional and generally not used
if parts are reasonably clean.
Troubleshooting these materials usually only involves chemical analysis to
insure they are up to concentration.
PREDIPS
Predips, which are generally solvents, such as dimethylformamide for
polysulfone, are used prior to etchants for two types of plastic problems. The
Plating on Plasfics
Fig. 14.7-Process
383
steps in plating plastics.
first use is to help improve the surface of a normally plated but poorly molded,
highly stressed part. This is done on ABS and its alloys by slightly swelling the
surface, allowing the etchant to more uniformly attack the surface. By using a
predip, non-uniform etch conditions are reduced on a stressed part, giving
better overall adhesion. As stated earlier, proper molding can eliminate stress as
well as the need for a predip in the above case.
The second use of predips is to attack or swell the surface of normally hard to
etch plastics. These materials include polypropylene, polycarbonate, polysulfone, and others. This swelling action of the predip enables the chemical
etchant to readily attack the surface of the normally resistant resin. Generally, a
different solvent is needed for each resin in order to be successful.
Troubleshooting the predips involves analysis to insure proper balance.
ETCHANTS
Etchants are usually strong oxidizing solutions that eat away the plastic Surface
to varying degrees, accomplishing two purposes. First, the surface area is
greatly increased, making the part turn from a hydrophobic (water-hating) to a
hydrophilic (water-loving) material. Second, the microscopic holes left in the
surface of the plastic by the etchant provide the bonding sites for the deposited
metal. These sites are needed for adhesion between the plastic and the metal.
When ABS is exposed to an etchant, the butadiene is selectively removed, thus
leaving small holes or bonding sites. Commonly used etchants for ABS are:
Chromium trioxide-375 to 450 g/L
Sulfuric acid-335 to 360 g/L
This is the so-called “chrome-sulfuric’’ version, or,
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ELECTROLESS PLATING
Chromium trioxide-900+ g/L
This is the ”all chrome” version, which tends to give a finer, more uniform etch.
Both types are used in production. All-chrome etchants require several
milliliters of sulfuric acid per liter of etchant to retard tank deterioration if
chemical lead is used. It is also desirable to have about 40 g/L trivalent chromium
in a new all-chrome bath to avoid over-etching the ABS surface. Etchants are
generally operated at 140 to 160” F for 4 to 10 minutes on ABS. Other plastics
may require higher or lower temperatures and times. Polycarbonate, for
example, uses an etchant at 170” F for 10 minutes, while mineral-reinforced
nylon (MRN) uses one at 105” F for 2 minutes.
As the chrome-based etchants are used, the following reduction reaction
occurs as the plastic is attacked:
Cr+6+ 3e-
-
Cr+
reduction
After a certain build-up of Cr” (about 40 g/L in the “chrome-sulfuric’’ type
etchant), the etchant starts to lose its ability to perform properly. Adding more
Cr+6in the form of chromium oxide extends the life of theetchant for a while, but
it is better to eliminate the excess Cr”. This can be done in two ways:
Decant part or all of the etchant and rebuild it. This method may cause a
waste disposal problem if provisions for the spent material are not made.
The excess trivalent chrome can be electrolytically regenerated. This can
be accomplished by using porous pots containing an electrolyte of 10 to 50
percent byvol HrSO,. Copper or stainless steel cathodes are then inserted in the
pots. The tank or lead straps may be used as the anode. The usual anode to
cathode ratio is 2:l.The voltage used is 9 to 12 Vat a temperature of 100 to 155”
F. The current is 100 to 150 amps for a 15-cm-diameter by 45-cm-long porous
pot. Efficiency of this regeneration process is best when more than 20 g/L of
trivalent chromium are present in the bath. The reaction involved is:
Cr“
- 3e- - Crth
oxidation
at the anode.
Using this method and a flash evaporator for the rinses results in a tight closed
loop system. One problem with this system is that all impurities in the etch such
as dissolved metals and titanium dioxide filler are returned to it. This causes a
build-up of impurities that eventually results in problems such as “stardusting,” a
fine roughness usually occurring on the shelf of a part.
The etchant is the most critical step in obtaining an acceptable finished part.
An underetched part can result in poor adhesion and possible skip plate.
Overetching a part can degrade the surface causing poor adhesion and
cosmetics.
Some problems that may be encountered are shown in Table 14.1.
Plating on Plastics
385
Table 14.1
Troubleshooting Etchants for Electroless Nickel
Problem
Cause
Solution
Shiny parts after electroless
nickel
Underetching
Increase time
Poor adhesion
High Cr’3
Check chemistry
Dull or almost black parts after
Overetching
Decrease time
Decrease temperature
Check chemistry
Stardusting or roughness on
part
TiO, build-up
Install spray rinse after etch
Chemistry good, but parts have
low adhesion and look shiny
out of electroless nickel
Poor molding
Mold release on part
Check parts for stress
Parts warp out of etch
Temperature too high
Too much tension on rack
Lower temperature
Check rack tension and
adjust or change rack
CrOl additions do not show up in
analysis
HISOI too high
CrOI is salting out
Check chemistry and
rebalance bath
Poor adheion at gate area only
Part stressed at gate
Check with glacial acetic acid
to confirm. Reduce etch
time and/or temperature.
Use a predip
Increase temperature
electroless nickel
Poorsurface-low adhesion with
black layer on plastic and
metal
NEUTRALIZERS
After etching, the parts are thoroughly rinsed in water and then put into a
neutralizer. These are materials such as sodium bisulfite or any of the
proprietary products available that are designed to eliminate excess etchant
from the parts and racks, usually by chemical reduction. Hexavalent chromium
( C P ) is harmful in the ensuing steps. Even with excellent rinsing, etchant may
be trapped in blind holes. If this is not rinsed out or reduced to trivalent
chromium (Cri3), “skip plate” can occur as a result of “bleedout” of etchant in
subsequent steps.
Neutralizers are generally cheap to make up and are usually dumped and
remade on a regular basis. They are usually run at 70 to 110” F for 1 to 3 minutes
with air agitation. Troubleshooting is relatively simple, as seen in Table 14.2.
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ELECTROLESS PLATING
Table 14.2
Troubleshooting Neutralizers for Electroless Nickel
Problem
Cause
Solution
Cr'' seen dripping off parts
coming out of neutralizer
Bleed-out of blind holes
on rack tips
Bath spent
Check chemistry
Check bath color-if
orangish, dump
Increase air and/or
temperature
Check rack tips
Drip skip noted after
electroless nickel
Bleed-out of hexavalent
chromium
As above
PREACTIVATORS
After neutralization and rinsing, a preactivator may be employed for certain
resins, such as polypropylene or polyphenylene oxide. These proprietary
materials are designed primarily to enhance activator absorption. Generally
they do not reduce hexavalent chromium. Preactivators help make otherwise
unplateable resins plateable by conditioning the resin surface by forming a film
or changing the surface charge. One must be careful in selecting a preactivator,
as excessive conditioning can lead to resist overplate and rack plating. Typical
parameters for these baths are 70 to 120" F for 1 to 3 minutes.
if the partsarefullycovered out of the electroless plating bath, the preactivator
is doing its job. Troubleshooting guidelines for preactivators are given in Table
14.3.
ACTIVATORS
Activators or catalysts, as they are sometimes called, are materials that in most
cases contain some precious metal such as palladium, platinum or gold.
The early version of activation was a two-step procedure. Step 1 was a
stannous chloride/hydrochloric acid solution in which the stannous ion (Sn")
was adsorbed onto the surface. The part was then rinsed well. Step 2 was a
palladium chloride/hydrochloric acid solution which, when the part with the
stannous ion was immersed in it, caused the Pdt22 ion to be reduced to Pd"
according to the following reaction:
Sn"
+ Pd''
- Sn"
+ Pd"
PI
Plating on Plastics
387
Table 14.3
Troubleshooting Preactivators for Electroless Nickel
Problem
Cause
Solution
Skip or no plate out of
electroless bath
Bleed-out of Cr'6
Preactivator out of spec
Use better rinsing
Increase air
Check chemistry of bathif spent, dump
Rack or resist overplate
Temperature too high
Lower temperature and/or
time
Check chemistry
Concentration too high
The Pd sites formed the catalytic surface needed to deposit the chemical
nickel. Present day catalysts are essentially the earlier two step version
combined. In other words, the palladium chloride, stannous chloride and
hydrochloric acid are in one solution. What we get is a palladium-tin hydrosol,
which is a solution of complex ions and colloidal particles whose activity and
stability depend on the chloride and stannous ion concentrations (6,7).
A typical working activator bath today would consist of the following:
Stannous chloride-6 g/L
Palladium (metal)-20 to 100 ppm
Chloride ion-2.5 to 3.5 N
The chloride ion may be maintained with hydrochloric acid or sodium chloride.
When processing a part, the first noticeable change will be evident after the
activator. The part acquires a tan to brownish color. This coloration, unless the
part is black, can easily be seen and is an indication the activator isworking. The
absence of a coloring on the part usually indicates a problem that can result in
skip plate and possibly low adhesion.
The primary purpose of activators is to provide catalytic sites on the plastic
surface. The usual operating conditions for the activator bath are 80 to 95" F for 2
to 5 minutes. Because this is the most expensive bath in the system, care is taken
to analyze regularly. Even when the bath is not being used, analysis for tin is
required, as tin is continually oxidized to the stannic ion (Sn")). When almost all
the stannous is oxidized, the bath "falls out".
Troubleshooting tips for the bath are given in Table 14.4.
388
ELECTROLESS PLATINQ
~~
~~
Table 14.4
Troubleshooting Activators for Electroless Nickel
Problem
Cause
Solution
Silver mirror seen on bath
surface
Low stannous ion
Reolenish stannous ion
Roughness on parts
Particulates in the bath
Batch filter to the bath.
(Expect to lose 2-3% due to
absorption on the filter
Raise stannous level
Install spray rinse after
activator
Stannous may be low
Solution clear or tea
colored
Bath fell out due to low
stannous, impure HCI, or
an addition of HNO)
Make up a new bath
Rack plate and/or stop-off
overplate
High temperature
High concentration
Lower temperature
Check chemistry
Skip plate after electroless
bath
Low temperature
Low concentration
Raise temperature
Check chemistry
Blotchy surface (light and dark
areas
Stress in Dart
Confirm with glacial acetic
acid
Parts very dark
High temperature
Lower temperature
Check chemistry
Parts very light
Low temperature
Raise temperature
Check chemistry
Agitate parts
ACCELERATORS
After rinsing following theactivator, metallic palladium is present on the surface
of the part surrounded by hydrolyzed stannous hydroxide. Theexcess stannous
hydroxide must be removed from the part before the palladium can act as a
catalyst.
The role of an accelerator is just that. It is to remove the excess tin from the
part while leaving the palladium sites intact for the deposition of the electroless
bath. Tin will inhibit the action of the electroless bath, resulting in skip plate.
This step is important, as too short a time in bath, as well as too long a time, can
cause skip plate. These baths are usually organics or mineral acids.
The biggest problem with accelerators is the effect of metallic contamination.
Most proprietary baths contain a built-in reducer to eliminate any hexavalent
chromium. Chromium may be dragged through the line in blind holes or bad
Plating on Plastics
389
rack tips. These contaminants, such as hexavalent chromium, iron, and other
metals, can cause the accelerator to become over-aggressive. This causes the
accelerator to remove not only the stannous tin, but also palladium. When this
occurs, skip plate can be the result. Some materials are designed to minimize the
effect of these contaminants.
Accelerators are usually run at 110 to 140" F for 2 to 5 minutes. Some
problems that may occur are as shown in Table 14.5.
Table 14.5
Troubleshooting Accelerators for Electroless Nickel
Problem
Cause
Solution
No plate on edges of part
Over-acceleration
High temperature
Check chemistry
Lower temperature
Lower time
Parts have no plate in major areas
or slow takeoff in the
electroless that goes from the
edges inward
Under-acceleration
Low temperature
Check chemistry
Raise temperature
Increase time
Agitate parts in solution
No plate at all out of electroless
bath
Contamination that causes
aggressiveness
When parts come out of the
activator brown, but are
clean after the accelerator,
contamination and/or high
temperature may be the
problem. In this case, lower
the temperatureor make up
a new bath. If parts are
brown out of the accelerator, under-acceleration is
probably the cause. In this
case, increase the time,
temperature, and/or
agitation
High temperature
Check chemistry
ELECTROLESS PLATING
After rinsing, the parts are put into the final step in the preplate cycle. This is the
electroless bath, which deposits a thin, adherent metallic film, usually copper or
nickel, on the plastic surface by chemical reduction. This is accomplished by
using a semi-stable solution containing a metal salt, a reducer, a complexor for
the metal, a stabilizer and a buffer system. When idle, the baths are stable, but
ELECTROLESS PLATING
390
when a palladium-bearing surface is introduced into the solution, a chemical
reduction of metal occurs on the palladium sites, and through autocatalysis,
continues until the part is removed. The basic reactions for copper and nickel
are:
CU"
+
2HCHO + 40H- J2HC00- + 2H:O + Cu" + H:
[41
The electroless nickel used is an alkaline bath, usually partially complexed
with ammonium hydroxide. It is operated at 80 to 100" F for 5 to 10 minutes at a
pH of about 9.0. Nickel thickness can range from 8 to 12 pin. and should have a
resistance of 50 to 20 ohmdin. The pH is controlled with ammonium hydroxide.
If an electroless copper is used, it is usually reduced by formaldehyde and
complexed by various materials, depending on the supplier. Copper baths are
operated at 100 to 140' Fat a pH of 12 to 13. The thickness ranges from 20 to 50
pin.
For most applications, electroless nickel is adequate, as the primary function
of the electroless bath is to render the surface conductive. The automotive
industry has done studies that show that electroless copper tends to exhibit less
of a tendency to blister in a humid, corrosiveenvironment (7,8). Becauseof this,
most exterior automotive parts are processed through electroless copper.
Nickel baths are relatively easy to control, while copper baths generally
require automatic analysis for control. Copper baths are more susceptible to
problems than nickel. Some troubleshooting hints are given in Tables 14.6 and
14.7.
In all cases, if an automatic controller is being used to analyze and add the
necessary chemicals to the bath, the first thing to check is the controller if
something is out of spec. Be sure the pumps are working and the reagents
and/or replenishment solutions are not used up. When the controller is
operating properly, over-the-side additions of chemicals are rare.
Following the electroless deposition, the parts are generally run through an
electrolytic copper or Watts nickel strike (9). The purpose of the strike is to build
up the thin electroless deposit prior to subsequent electroplating. This build-up,
usually to 0.0001 in., prevents "burn-off", the loss of contact between the part
and the rack tips.
The electrolytic strike is usually followed by ar: electrolytic bright acid copper
plating solution, which is normally a proprietary product. This step will build a
copper thickness to about 0.0005 to 0.001 in. and give a bright surface. Copper,
being ductile, will act as a buffer layer between the plastic substrate and the final
plate as the part sees large thermal changes. This buffer layer helps prevent
blisters and/or cracking of the plated deposit.
If good corrosion and abrasion resistance is desired, as for an exterior
automotive part, the next step will be an electrolytic semi-bright nickel plating
Plating on Plastics
391
Table 14.6
Troubleshooting Electroless Nickel Plating Baths
Problem
Cause
Solution
Skip plate
Low reducer
Low temperature
Contamination
Check chemistry
Raise temperature
Check for contamination
Roughness
Particulates in bath
Filter bath
Install spray rinse after bath
Agitate work
Burn-off in strike
Electroless coating too thin
Increase time
Increase temperature
Check bath rate
Check chemistry
Check racks
Bad racking. not enough
contacts
Bath overactive
Bath sluggish
High temperature
High reducer
Palladium contamination
Low complexor
Tank plate-out
Low stabilizer
Lower temperature
Check chemistry
pH out of range
High stabilizer
Low reducer
Low temperature
Contamination
Check chemistry
Clean and strip tank
Check stabilizer
Raise temperature
Check for contamination
solution. Normally 0.0003 to 0.0008 in. of semi-bright nickel are plated, followed
by an electrolytic bright nickel plating solution, which will deposit about 0.0002
to 0.0004 in. of metal. The deposit from the latter bath is generally very specular.
For the best corrosion resistance, a layer of about 0.0001 in. of microporous
nickel can be electrolytically deposited on the bright nickel, followed by an
electrolytic chromium deposit from either a hexavalent or trivalent bath. This
layer,onlyabout5to 10millionthsofan inch thick, isthefinalstepinproducinga
finished part. Be aware that if the surface of the substrate is pitted, scratched,
dented, or marred in any way, electroplating will enhance these blemishes. Care
must be taken when handling and racking plastic parts to avoid damaging them
(1). Parts are usually plated to meet various service conditions, which are:
SC7-Mild. Includes indoor exposure to normally dry, warm atmosphere
and subject to minimal wear and abrasion.
SCP-Moderate. Indoor exposure where moisture condensation occurs.
Examples are kitchen and bathroom fixtures.
ELECTROLESS PLATING
392
Table 14.7
Troubleshooting Electroless Copper Platlng Baths
Problem
Bath slow-dark
deposits
Bath overactive-dark,
deposits
grainy
Roughness on parts-precipitate
forms when additions are
made
Cause
Solution
Low copper
Low caustic
Low formaldehyde
High stabilizer
Low tern perature
Too much air
Check chemistry, plating rate
High caustic
High formaldehyde
Low stabilizer
High temperature
High loading factor
Tank plate-out
Low complexor
Increase temperature
Decrease air
Check chemistry, plating rate
Add stabilizer
Lower temperature
Decrease work in bath
Clean and strip tank
Increase air
Check chemistry
SC3-Severe. Includes frequent wetting by rain or dew, and in some cases
strong cleaners and salt solutions. Examples are outdoor furniture, bicycle
parts, and hospital furniture.
Sc4-Very severe. Includes likely damage from dents, scratches, and
abrasive wear in addition to being in acorrosive environment. Examples are boat
parts and exterior automotive parts.
The various plate thicknesses required for each type of service are given in
Table 14.8.
Chromium is not theonlyfinish that can beapplied to plated plastics. Once the
electroless is put down, any plate combination can be used. Final finishes can be
brass, gold,, silver. or any of the other finishes put on plated metal articles.
After plating to thedesired specification, most platers perform quality control
testing on plated parts. These tests include:
Adhesion (Jacquet Test)
The part, usually one with a flat surface, is plated with 1 mil of bright acid copper.
A 1-in.-wide strip is cut and the plate is separated from the plastic at one end. The
plate is then pulled at 1 in./min at 90" to the surface. Values obtained in pounds
per linear inch are recorded. ABS, for example, has from 5 to 15 Ib/linear in. pull.
393
Plalmg on Plasfics
Table 14.8
Minimum Plating Thickness Requirements
Under Various Service Conditions
Total
Service
Copper
nickel
Chromium
SCl
sc2
0 6 mils
0 fi mils
0 6 mils
0 6 mils
-03 mils
10 pin
10 pin
10 Uin
sc3
sc4
0 6 mils
0 9 mils
1 2 mils
10 p i n
Thermal Cycling
Because various plastics have various coefficients of thermal expansion, some
do not readily pass this test, even though adhesion is good. To counteract this,
more copper may be plated on the part to act as a cushion.
I n thermal cycling, the part is first put into an air-circulating oven for 1 hr at
180" F. The part is then removed and held at room temperature for 15 min. The
part is then placed into a cold chamber at -20' F for 1 hr. This is repeated a
minimum of three times. Loss of adhesion, blisters, or cracking indicate a failure.
Part design and molding can also affect thermal cycling. This test was designed
for ABS and does not necessarily apply to other substrates.
C.A.S.S.
The C.A.S.S. ( l o ) , or Copper Accelerated Acetic Acid Salt Spray Test, checks
the corrosion resistance of the final plate. This test is generally performed in a
cabinet, according to ASTM 6-368. Requirements are as follows:
SC1-None
SC2-1 8-hr cycle
SC3--2 16-hr cycles
SC4-3 16-hr cycles
Failure is indicated by white or green corrosion.
The above are the most widely used tests. Others include:
N.S.S. (10)-5 percent neutral salt spray, which is also used in another
corrosion test-ASTM 6-1 17.
Plate thickness-Checking plate thicknesses by either a destructive or
non-destructive method.
Outdoor exposure-Setting industrial samples in environments they will be
normally exposed to.
394
ELECTROLESS PLATING
C.S.A. 8-125, LRP-15 (11)-A Canadian test for plumbing goods that
includes 450 cycles of dipping the parts in alternating hot (175' F) and ambient
(70' F) water baths for 40 seconds. One cycle is 40 seconds in the hot water, and
40 seconds in the ambient water.
New technologies in plating on plastics are leaning toward plating engineering
resins for printed circuits. Resins with high strength and heat distortion are also
being looked at to replace metals i n functional as well as decorative roles.
ELECTROMAGNETIC INTERFERENCE SHIELDING
EM1 shielding is another area where plating-on-plastics technology can be
utilized. Electromagnetic interference is electrical "noise" generated by a piece
of electrical or electronic equipment that causes a problem of interference with
the operation of another piece of electrical or electronic equipment. An example
is the annoying lines on a TV set caused when an electric razor or other small
appliance is in use. Sources of EM1 are shown in Fig. 14.8.
Some of the more common sources of EM1 are:
0
0
0
0
0
0
0
Airplanes
Computers
Electric motors
Ignition systems
Power lines
Radio and television transmitters
Videogames
Lightning
Some equipment affected by EM1 "noise" includes:
Radio and television receivers
Computers
Telephones
In the early days of electronic packaging, €MI radiation emitted from
equipment was not a major concern. Today, these emissions are subject to
controls, it has become necessary to redesign and effectively shield electronic
equipment. The electromagnetic radiation spectra, which covers a wide range of
frequencies, is shown in Fig. 14.9.
Metal enclosures were used for many years as the main shielding material.
This provided structural integrity as well as a shielding function. Some
drawbacks of metal were weight and design limitations. As housings for
electronic equipment changed from metal to plastic, it became necessary to
apply shielding to protect the equipment. Several types of technical approaches
have been used in an attempt to effectively solve the shielding problems of
plastic housings. One such method could be the use of electroless-plated
Pfafrng on Plastics
Fig. 14.8-Sources
395
of EM1 and RFI.
Fig. 14.9-Electromagnetic radiation spectrum.
copper and/or nickel. The process used would be similar to that presently used
to plate on plastics. Steps would include:
1.
2.
3.
4.
5.
Pretreatment (if necessary)
Rinse
Etch
Rinse
Neutralize
396
ELECTHOLESS PLATING
Fig. 14.10-Attenuation as a function of thickness of metal deposits.
6. Rinse
7. Preactivate (if necessary)
8. Rinse
9. Activate
10. Rinse
11. Accelerate
12. Rinse
13. Electroless copper
14. Rinse
15. Activate
16. Rinse
17. Electroless nickel
18. Rinse
19. Dry
The electroless copper (Step 13) generally would be deposited to a thickness
of 40to60millionthsof an inch.Thecopperisaveryeffectiveshieldby itself, but
because of its corrosive nature, an electroless nickel (Step 17) topcoat of about
10 to 20 millionths of an inch is applied to retard the corrosion. Nickel can be
used by itself, but itsshielding effectiveness is much lower than that of copper or
copper/nickel. It is also an excellent base i f the article is to be painted.
Electroless shielding is applied i n general to both sides of a plastic enclosure,
which assures the best protection against the transmission of electromagnetic
radiation (Fig. 14.11). Still, techniques have been developed to apply the
electroless coatings only to the inside of the enclosure, to allow the use of
color-molded finishes.
Shielding effectiveness is the ability of the method used to absorb or reflect
the unwanted "noise" signal, typically over a frequency range of 14 K H z to 1
Plating on Plasfics
397
Fig. 14.1 1-Double-sided shielding
Table 14.9
EM1 Shielding Effectiveness
Attenuation
Shielding
effectiveness
0 to 10 dB
10 to 30 dB
30 to 60 dB
60 to 90 dB
90 to 120 dB and above
Very little
Minimal
Average
Above average
Maximurntostate-of-the-art
GHz. This is called attenuation, which is a function of theelectrical conductivity
of the shield used, measured in decibels (dB). Attenuation as a function of the
thickness of the metal deposits is shown in Fig. 14.10. Shielding values for
various attenuation levels are shown in Table 14.9.
The electrical conductivity required to satisfy the minimal functional
requirements are expressed in surface resistance (ohmdsquare) as follows:
EM1 shielding-Less than 1 ohm/square
RFI (radio frequency interference)-Less than 10 ohms/square
ESD (electrostatic discharge)-50 to 100 ohmdsquare
This allows performance to be measured in surface resistance as well as dB.
Some of the other technologies presently being used to shield plastic
housings are given in Table 14.10.
ELECTROLESS PLATING
398
Table 14.10
Coating Techniques for EM1 Shielding
Of Plastic Housing
Method
Advantages
Disadvantages
Zinc arc spray
Good conductivity
Hard, dense coat
Effective over a wide
frequency range
Need special equipment
Adhesive problems
Coating cracks
May distort the housing
Good conductivity
Conventional equipment
Resists flaking
Conductive oxide
Easy to apply
Expensive
Conventional equipment
Multiple coats needed for
good conductivity
Thickness problems
Questionable attenuation
Conductive painfs
Silver
Nickel
Resists flaking
Economical
Easy to apply
Copper
Conventional equipment
Resists flaking
Economical
Easy to apply
Multiple coats needed for
good conductivity
Thickness problems
Oxidation reduces
conductivity
Vacuum metallizing
Good adhesion
Good for all plastics
Not limited to simple
designs
Familiar technology
Size limited to vacuum
chamber
Base coat needed
Expensive special
equipment
Low thickness
Cathode sputtering
Good conductivity
Good adhesion
Expensive equipment
Microscopic cracking
May distort housing
High power needed
Foil application
Die cut to part shape
Good conductivity
Good for experimentation
Complex parts are difficult
to coat
Labor intensive
Conductive plastics
No secondary operation
Material expensive
Poor attenuation
Surface usually poor
Silver reduction
Good conductivity
Low initial cost
Tendency to oxidize
Difficult to mask
Multiple-step process
Electroless plating (copper
and/or nickel)
Uniform thickness
Good for all size and shape
parts
Good conductivity
Resists chipping
Can be electroplated for
metallic appearance
Can be painted
Limited to Lertain resins
Plating on Plastrcs
399
As can be seen above, besides electroless plating, numerous other methods of
shielding are available. The success of plating on plastics technology is
dependent on the industry and its requirements.
REFERENCES
1. J.L. Adcock, Electroplating Plastics, American Electroplaters and Surface
Finishers Society.
2. “Design and Converting Techniques for Plating Cycolac Brand ABS,” EP3510, Marbon Division, Borg-Warner Corporation, Technical Bulletin, (May
1967).
3. “Glossary of Terms,” Thomas H. Ferrigno, ed.; Fillers and Additives
Committee, Society of Plastic Industry.
4. “Plating on Plastics 11,” Technical paper-Society of Plastics Engineers,
Connecticut Section.
5. A. Rantell and A. Holzman, “Mechanism of Activation of Polymer Surfaces by
Mixed Stannous Chloride/Palladium Chloride Catalysts,” Division of Metal
Sciences, Polytechnic of the South Bank, London SElOAA (February 12,
1973).
6. C.H. deMinjer and P.F.J. v.d. Boom, “The Nucleation with SnCI?-PdCI
Solutions of Glass Before Electroless Plating,” Phillips Research Laboratories,
Einhaven, Netherlands.
7. R.G. Wedel, Plating, 62(3), 235 (Mar. 1975).
8. R.G. Wedel, ibid., 62(1), 40 (Jan. 1975).
9. Metal Finishing Guidebook-Directory Issue, Metals and Plastics Publications, Inc., One University Plaza, Hackensack, NJ 07601.
10. ASTM 8-117, 15-22; ASTM 8-368, 164-169, Part 9, American Society for
Testing and Materials, Philadelphia, PA.
11. CSA Standard 8-125, LRP-15, Canadian Standards Association, Ontario,
Canada.
12. E.B. Saubestre, Plating, 52(10), 982 (Oct. 1965).
13. E.B. Saubestre, Modern Electroplating, p. 636 (1974).
14. W.P. Innes, Plating, 58(10), 1002 (Oct. 1971).
15. H.Narcus, ibid., 55(8), 816 (Aug. 1968).
16. “Plating of Plastics with Metals,” Chemical Technology Review, Noyes Data
Corporation, Vol. 27 (1974).
17. “Plating on Plastics,” Extended Abstracts (1965-1971), The International
Nickel Company, Inc.
18. James L. Adcock, Products Finishing, p. 51 (May 1982).
19. Gerald Krulik, lndustrial Finishing, p. 16 (May 1983).
20. Gerald Krulik, Products Finishing, p. 49 (Oct. 1983).