Chapter 1: Plating Overview
1.1 Electrolytic Plating
Electrolytic plating is one of many metal finishing processes. What is metal finishing? Metal comes from the Latin word "metallum", which means to mine a mineral. Finishing comes from the Latin word "finire", which means the end. Electrolytic and chemical plating is commonly called "plating".
We apply metal coating on the product for the following reasons:
1 To Increase Wear Or Scratch Resistance
Nickel and chromium plating on appliances, electroless nickel on aluminum and hard chromium
on molds.
2 To Beautify The Product
Brass plating on appliances, nickel/chromium on household hardware, and gold plating on watch cases.
3 To Prevent Rusting
Zinc on steel and nickel/chromium on car bumper
4 To Provide Good Reflection
Gold plated sensor and chromium plated light reflector
5 To Improve Solderability
Tin and tin lead on electronic components
6 To Prevent High Temperature Oxidation
Copper on carbon steel for case hardening
7 To Decrease Electrical Resistance
Gold and silver on connectors and switches
8 To Increase Lubricity
Cadmium on friction surfaces
9 To Increase Bonding to Rubber
Brass on steel
0 To Salvage Worn Out Machine Part
Heavy chromium on shafting and electroless nickel on molds.
1.1.01 Rack Plating
The parts are secured on a fixture called a rack and processed through the plating cycle. The plating rack is insulated with a chemical resist coating, except at the current contact points. A small rack can hold a
few pieces and the largest rack can hold hundreds of pieces and require a hoist to operate. Unlike barrel plating, the parts are neither smeared nor scratched during processing.
1.1.02 Barrel Plating
As the name implies, the parts are loaded into a
perforated barrel and carried throughout the plating process. There are two kinds of plating barrels;
horizontal and oblique.
The horizontal barrel is easy to handle and can plate a larger volume of parts. The electrical contact in plating barrel can be a center rod, danglers or button contacts.
The oblique barrel has the parts immersed in a barrel with plating solution in tilted position. Limited
volumes of parts can be processed at one time. The
oblique plating barrel is slowly phasing out, except
in mechanical plating when tumbling is essential.
There is another kind of barrel, called the “open ended barrel”. The parts are continuously fed at one end and travel through a rotating helix in the barrel with current contact. The plated parts are discharged at the other end.
1.1.03 Brush Plating
Brush plating is used extensively to touch up bare spots or rebuild worn parts, such as dies and molds, valves, printing press rolls, etc. Nickel and chromium are principally used.
The process uses a pen with a pad containing plating solution to plate onto selected areas without need for masking and plating tanks. The pad contains the anode soaked with plating solution. Different processes can be performed by changing the pad with different plating baths. The brush can also be used for cleaning and activation. Brush plating eliminates masking, and equipment downtime.
1.1.04 Continuous Plating
Continuous plating is also referred to as reel-to-reel plating. The strip or wire in a coil travels through a series of processing tanks. The material is plated and rewound at the other end. The current density ranges from 30-80 A/dm2 (300-800 A/ft2). The coil travel from 30 to 200 feet per minute. The machine can plate selective strip or depth or spot, multiple strips, or
all over. Gold, palladium, copper, nickel, tin and
tin-lead are used for most electronic applications.
Continuous zinc plating, commonly called electro-
galvanizing, is widely used as a corrosion resistant coating for automotive stamping and hardware products.
1.1.05 Electroforming
This plating process uses a mandrel or master as a “mold” to form the deposited product. Such operations require several hours to complete. Electroforming is used when alternative manufacturing processes are too difficult or expensive. Copper and nickel sulfamate
are used because of their low stress.
The major copper electroforming application is for printing rollers and copper foil for printed circuit boards. Nickel electroforming is used on compact disk molds, very large molds, tiny and complex parts and
mesh products like shaver screens and filters.
1.1.06 Pulse Current Plating
This process uses a variety of interrupted pulse current waveform instead of the standard continuous direct current for metal deposition.
Pulse plating can enhance the grain structure and other properties of the deposit. Pulse plating is gaining acceptance in electronic plating. The square wave pulse current is commonly used. The on-off time can be monitored to make the best use of the plating bath chemistry and cell geometry.
“It does not take lot of strength to hang on.
It takes a lot strength to let go.”
1.1.07 Vibratory Plating
Here, the parts are loaded into a conductive basket. The basket rotates slowly with vibration during
plating. The parts are not subjected to abrasion or smearing. This is an excellent method for plating
small delicate parts with uniform precise deposits. Watch components, tiny spring and screws are plated
in vibrating baskets.
There is another kind of vibration plating equipment called "Vibrobot". It is used to plate large volumes
of parts in one chamber without other processing tanks. The vibrating chamber is a computerized
controlled plating system. The parts are loaded into
a chamber. Different processing solutions are intro- duced into the chamber one at a time with thorough rinsing after each treatment. The whole plating process is done inside the chamber. No labor and no other process tanks are required. The system needs minimum tank space and minimum equipment maintenance.
1.2 Chemical Plating
This is a process of depositing metal by chemical reduction. No electric current is involved. Gold, cobalt, copper, nickel and palladium can be chemically plated.
1.2.01 Electroless Plating (Auto-catalytic)
The plating process involves chemical reduction of metal onto a base metal. There is no electric current required. Unlike immersion plating, it does not depend on oxidation or electro-potential differences between the metal to be plated and the part. Electroless
plating is auto- catalytic and produces a continuous
and relatively heavy thickness. There are numerous
electroless baths available: nickel, gold, copper,
palladium and cobalt. Electroless copper and nickel
are most widely used. Nickel with phosphorus or boron is used for electroforming metal molds, mesh products like shaving screens, and many aluminum component parts. Composite nickel plating is gaining acceptance. The composite can contain diamond powder, ceramics,
chromium carbide, silicon carbide, aluminum power or Teflon.
Electroless copper is mostly used in metallizing
plastic and through-hole plating in printed circuit boards. Electroless gold and palladium have found
applications in many electronic devices. Electroless cobalt is used on magnetic disc and other memory
storage devices.
Metal Reducing Agent
Cobalt Sodium hypophosphite,
amine borane
Copper Formaldehyde, amine borane
Gold Potassium or sodium borohydride
Nickel Sodium hypophosphite,
dimethyl amine borane,
sodium borohydride
Palladium Amine borane,
Sodium hypophosphite
Remember that the reducing agents are toxic. Proper
handling and ventilation are required.
1.2.02 Immersion Plating
Immersion plating is plating process using metal that has lower oxidation electro-potential than that of the
substrate metal. Once the base surface is fully dis- placed and coated, the plating will stop. Immersion plating is used to plate copper inside steel tubing
or on steel wire to act as a lubricant in drawing (reducing the diameter through a set of dies).
Basically any metal can be immersion plated, as long as the base substrate has lower electro-potential.
(See Table on page 8.7)
Base Substrate Immersion Metal
Aluminum Brass, cadmium, copper, tin,
zinc
Copper Cadmium, gold, palladium, tin, platinum, silver, ruthenium
Nickel Palladium, platinum, gold
Steel Bronze, copper, nickel, tin
Zinc Copper, nickel, silver, tin
1.2.03 Mechanical Plating
Mechanical plating is often referred to as contact
or peen plating. It is a plating process that uses kinetic energy instead of electrical energy or a
chemical reducing agent. Unlike immersion plating,
it does not depend on the difference in electro-
potential between substrate and metal to be coated.
The parts to be plated are loaded into an oblique
barrel. The electrolyte contains promoters, metal
powder and glass beads. As the barrel rotates, the tumbling action of the glass beads and parts impact
the metal powder onto the substrate surface and form
an adherent uniform metal layer. Thicknesses of 127
um (5 mil) can be obtained.
The mechanical-plated parts do not have hydrogen embrittlement and have very good corrosion resistance. An excellent application is for hardened steel parts; (over Rockwell C-32) and other spring steel. Mechanical plating is used for tin, lead, copper,
cadmium and zinc coatings.
1.2.04 Metallizing Plastic
Here, one applies a conductive coating on plastic or other non-conductive material followed by electro- plating. The non-conductive part (plastic) is cleaned, etched and sensitized with palladium or tin colloids. It is then coated with electroless copper and electro- plated to a specified thickness. Porous materials like wood, leather, flower and even insects must be sealed with lacquer or copper conductive paint. Then the
workpiece is metallized and electroplated to a speci- fic thickness.
The most commonly plastics used are acrylonitrile butadiene styrene (ABS), polyimides, polysulfones and poly-carbonates. Ceramics like aluminum oxide and beryllium oxide components can be coated with this process. Other applications are metallizing printed circuit boards, buttons, appliance parts and automo- tive hardware.
1.3 Physical Vapor Deposition (PVD)
This is a metal deposition process that does not require an electrolyte. In this process, metal atoms
vaporized from a solid or liquid source (condensed) onto the part in a vacuum or low pressure gaseous chamber.
PVD can deposit almost any metal. Titanium and
zirconium nitrides are commonly employed on many
industrial tools like drill bits, saw blades, or
cutting blades. Silicon dioxide,titanium carbide,
tungsten silicide, can also be deposited.
1.3.01 Vacuum Metallizing
Vacuum metallizing is performed in an air tight
chamber. The chamber is held at low pressure. The
metal to be coated is heated to its boiling point and
evaporated. As the pressure increases, it causes the metal to condense on the parts and the inner wall of the chamber. The coating is very thin and bright.
Aluminum is primarily used in vacuum coating. The applications are high quality automotive parts, hand bag accessories and novelties. The aluminum coating can be protected with clear or dyed lacquer. Many
plastic parts are vacuum metallized and coated with
a clear lacquer top coat.
1.3.02 Sputter Deposition
Sputter deposition is a non-thermal process where the atoms are ejected by momentum transfer of gaseous ion acceleration. Many elements, alloys and compounds can be sputter deposited. This application is widely used to deposit thin films on semiconductor material. Typical uses are as reflective coatings on compact disks, architectural glass, magnetic films and other wear resistant coatings.
“If you are not criticized, you may not be doing much.”
1.3.03 Arc Vapor Deposition
This process uses a low-voltage high current arc to
vaporize the metal or composite in a low pressure chamber. The material to be vaporized is secured to the cathode or anode arc.
Examples of cathode arc deposition are TiN (gold color) or ZnN (brass color). Many plumbing fixtures, hardware, industrial drills and saw blades are processed with this method. Examples of anode arc
deposition are chromium and diamond-like carbon on industrial tools and high wear-resistant optical
coatings.
1.3.04 Ion Plating
Ion plating uses a concurrent or periodic energetic bombardment of ions to form a thin film on the part. The ion is usually argon which is extracted from a plasma or vacuum environment in a vacuum chamber. Ion plating is used to deposit metal and hard coatings, and high density coatings for lenses.
1.3.05 Vacuum Impregnation
This is a process using vacuum and pressure to seal porous metal casting with plastic resin. Die casting like aluminum, zinc, bronze, iron and powdered metal are porous and must be sealed prior to plating or
chemical processing. Vacuum impregnation also helps
to prevent internal corrosion.
Automotive and military applications use vacuum impregnation to improve part performance. Submarine periscopes, oil drills and air compressor components are examples where this process is used.
1.4 Surface Treatment
1.4.01 Anodizing
Unlike plating, the parts are positively charged in
a weak acid solution containing no metal. They are oxidized to form a dense, porous hard film. The parts can then be sealed and dyed with different colors. Aluminum products are commonly anodized. However,
magnesium, zinc, titanium and tantalum alloys can also be anodized.
1.4.02 Antiquing
This process is very similar to blackening (see Black Oxide on section below), except that the finished
product is highlighted or burnished to reveal partially the original base metal. Wear resistance is dependent on the lacquer applied. The typical process involves cleaning, activation, oxidizing, highlighting, lacquer and drying. Ferrous parts require copper plating prior to antiquing.
1.4.03 Blackening (Black Oxide Coating)
Blackening is an oxidation process which produces a metal sulfide coating by heat treatment, chemical dip or electrolysis. Usually an oil coating is added to enhance the corrosion resistance.
Blackening improves the corrosion resistance many
times and is relatively inexpensive. Many hardware items such as screws and nails are black oxide-coated. Blackening is basically the same as plating, except
the cost is much less. The steps are: cleaning, acti- vating, rinsing, blackening, rinsing, sealing, drying.
1.4.04 Chromate Conversion
Chromate conversion coating using hexavalent and/or trivalent chromium. Zinc and cadmium-plated parts and anodized aluminum product are usually chromated. The coating is usually produced by immersion, but to a lesser extent by spray or electrolytic processes. During immersion, about 12.7 um (0.0005 uin.) of deposit is dissolved to form the chromate coating.
The color of a chromate film can be clear bright, bluish, yellow iridescent to olive drab and black. Chromate coatings are simple to produce and have the following features:
1 Enhance physical appearance
2 Increase corrosion resistance
3 Increase hardness and scratch resistance
4 Increase bonding to paint and other organic
finishes
5 Extend corrosion resistance on electrical
contacts
The latest trend is to use a more environmentally
safe non-chromate process.
1.4.05 Electropolishing
Electropolishing is a surface finishing process using electrical energy instead of mechanical abrasion. The metal part to be polished is connected to the positive terminal of the rectifier (anode). Normally 316 stain- less is used as the cathode. With the passage of
electric current, the surface of the part dissolves,
polishing rate depend on the base metal, current
density, electrolyte chemistry and bath operating
temperature. Portable electropolishing units are
also available.
Advantages:
1 Uniform smooth and bright finish with no
directional polish lines
2 Passivity by formation of an oxide layer
3 Uniform finish, impossible with mechanical
polishing
4 Allows inspection of cracks and fissures on
the part
5 Low labor cost
6 Non-stress polished surface
7 Releases stress on fabricated parts
8 Superior corrosion resistance
9 Hygienically clean (required for all surgical
instruments)
1.4.06 Electro-Chemical Machining (ECM)
ECM is a high speed electropolishing process. Unlike most electropolishing solution which uses acid, ECM uses a neutral pH sodium chloride or sodium nitrate solution. The salt solution is force sprayed onto the part (anode) at high voltage and high current density. The metal removal is much faster than in the electro-
polishing. ECM uses a conforming cathode to control current flow on the part to obtain the final shape and finishing.
1.4.07 Metal Coloring
Various non-ferrous metals can be colored to enhance
appearance and tarnish resistance. Color coating can be highlighted or antique. A variety of colors and shades can be applied by chemical dipping and electrolytic processing. Chemical dipping is the
simplest and most economical coloring process. Many commercial processes are available.
The following are typical baths for such coloring processes:
1 Copper carbonate solution to blacken brass
2 Arsenic oxide solution followed by a copper sulfate solution to blacken tin
3 Ferric nitrate solution to produce green on copper and brass
4 Polysulfide solution to blacken or produce French gray on silver
5 Dichromate solution followed by strong sulfuric acid to blacken stainless steel
(see Blackening on page 1.14)
1.4.08 Phosphate Coating
This is a chemical process to apply a non-metallic conversion coating onto a metal surface. The coating can be applied by immersion or spray. This major
application is to improve paint and powder coating adhesion.
A magnesium phosphate coating is used as a lubricant and to increase wear resistance. Zinc phosphate coat- ings are used to maximize rust resistance of iron
I-beams in building construction.
1.4.09 Painting
Painting is an organic coating used to improve appearance or corrosion resistance. To a lesser extent, painting is done on industrial products.
The paint can be solvent based as in lacquer enamel, or water-based as in latex for household paint.
Painting can be applied by electro-chemical process as on automobile body and electrostatic spray on
automatic painting line.
1.4.10 Powder Coating
This process applies a plastic coating onto a metal part by thermal fusion and chemical reaction. An
electrostatic fluidized bed uses ionized air and
thermal attraction to spray powdered plastic on the part.
The powdered treated parts are then heated to form
a uniformly cured coating. Both thermoplastic and
thermosetting resins are available. Plating racks
and shopping carts are powder coated with PVC resins.
Other applications include dish washer baskets, fan guards and other wire products.
1.4.11 Solid Film Lubricant
This is a process in which the parts are coated with resin bonded lubricant such as molybdenum disulfide, graphite or poly tetrafluoroethylene.
The coating is normally applied by spray or aerosol. Usually, 0.5-1.3 um (20-50 uin) is applied to enhance wear and lubrication performance. Thicker coatings are required for optimum corrosion resistance.
1.5 Plating Basics
1.5.01 Activation
After cleaning the parts, a thin residue film is
usually left behind. This film dissolves easily in dilute acid. The acid dip also activates the surface enable it to accept the subsequent deposit more
readily.
1 Dilute sulfuric or hydrochloric acid (5-10 vol%)
is commonly used for most metals, except for parts containing lead.
2 Dilute hydrofluoboric or hydrofluoric acid
(10-15 vol%) is used for leaded parts, stain-
less steel, chromium alloy and nickel alloys.
1.5.02 Additives (Brightener)
Additives are added to the plating bath to enhance
the grain structure of deposits, cover scratches and pits, and to widen the plating current density range.
The mechanisms explaining how the additives work are not well understood in many cases. Most probably some additives affect the cathode polarization. This causes a cyclic process of differing cathode film potentials that improve the deposit characteristics. Some of the additives may be codeposited with the coating, others may result in breakdown products in the bath.
1.5.03 Bath Classification
Plating solutions are divided into three groups based
on pH or acidity.
1 Acid Bath
The pH of the bath is less than 2. Examples are sulfuric acid copper, Woods nickel, methane sulfonic tin and tin-lead.
2 Neutral Bath
The pH of the bath is between 2 and 8. Watts nickel, acid zinc and most of the gold baths belong to this class.
3 Alkaline Bath
The pH of the bath is higher than 8. Alkaline zinc, brass and all cyanide baths of of gold,
silver, brass, cadmium, zinc and copper belong
to this group.
1.5.04 Cleaning
Good adhesion depends on having a clean and active surface substrate. The part should be free from
solids, oxide films or embedded material.
1 Chemistry
The cleaners are either organic solvent or
aqueous saponification agents (i.e. soaps). Organic vapor solvents are very effective in removing fatty acid buffing compounds and
greases. Pre-cleaning or hot soaking is used
to remove heavy soil and buffing compound
residues prior to regular cleaning processes.
Aqueous cleaners are subdivided into strong
and mild soaps. Strong soap cleaners are used
for steel, stainless steel and copper. Mild
surfactant cleaners are used for brass, zinc
die casting and aluminum.
2 Temperature
Most cleaners must be heated to effectively
dissolve or loosen the soil and buffing compound. The maximum temperature range is 60-70oC (140-160oF). Mild detergent cleaners are usually kept below 49oC (120oF). The base metal can be attacked above 70oC (160oF).
3 Solution Movement
Mechanical solution movement is required to
aid physical removal of surface residues.
a. Cathodic cleaning has twice the gassing and twice the scrubbing action than does anodic cleaning. The only drawback is that metallic contaminant in the cleaner can be codeposited and hydrogen embrittlement may occurs.
b. Anodic cleaning has half the gaseous scrub- bing action compared to the cathodic. The residue on surface is effectively removed. The draw-back is the possible formation of
a passive oxide film which can cause poor
adhesion.
c. Ultrasonic or turbosonic agitation are some- times used to reach cavities or deep
recesses. This is an excellent way to
clean tiny or delicate precision parts.
d. High-pressure spraying is employed for reel- to-reel and conveyor plating.
Cleaning processes are divided into four groups;
vapor degreasing, soak cleaning, electrocleaning
and wet blasting.
1 Vapor Degreasing (Solvent Cleaning)
Organic solvents are used to remove protective coatings or greases in a vapor chamber.
2 Soak Cleaning
The part is soaked in hot alkaline solution with part movement. This is usually done on parts with heavy soils or greases prior to electrocleaning. oak cleaning also is used to protect the more expensive electrocleaner from contamination.
3 Electrocleaning
This is commonly performed in an alkaline cleaner, and to a lesser extent in an acid cleaner.
a. Cathodic or Direct Cleaning
- Parts are negatively charged
- Twice the scrubbing action compared to
anodic cleaning
- Evolves hydrogen, that may cause hydrogen
embrittlement on high carbon steel
- Effectively removes oxide film
- Deposit smut on the part, if the cleaner
is contaminated with copper, zinc or lead
b. Anodic or Reverse Cleaning
- Parts are positively charged
- Evolves oxygen. The scrubbing action is
only half that of cathodic cleaning
- More readily removes smut from the part
- More tolerance to metallic contamination
- Tends to form an oxide film and passivate the part
c. Periodic Reverse Cleaning
- Combines both cathodic and anodic cleaning.
This is desirable for cleaning metals
like zinc die casting
- Normally the part is given a cathodic
cleaning followed by a short anodic cleaning
- The anodic cleaning is usually done in
a separate cleaner
4 Wet Blasting
This process uses high pressure liquid to clean the parts. Wet blasting is commonly used
on sand casting to remove semi-adherent sand and other inclusions.
1.5.05 Covering Power
Covering power is the ability of the bath to plate
into recesses. This is related to the minimum
electro-deposition potential of the metal in the
bath. The bath with higher throwing power usually
has better covering power. (See Covering Power Test
on page 2.46 & 5.16)
1.5.06 Current Density
In rack or barrel plating, the current distribution
on the parts is uneven. This affects brightness,
thickness and ductility. The plating bath must be
able to plate uniformly over a wide current density range.
In general higher metal concentration and the used of organic additive widen the current density range. Proper racking position, the use of current shields and auxiliary anodes are helpful. In barrel plating, the load size, barrel perforations and rota- tion speed play important roles.
1.5.07 Heating
Most acid plating baths are operated at room tempera- ture. Most alkaline and cyanide plating baths are
operated above room temperature. Immersion heaters and heat exchangers are employed.
1 Side mount immersion heaters are recommended for heating heavy sludge generating baths.
2 The heating element should be maintained at least
5 cm (2") below the solution level of the tank.
3 Use heater guards to protect the heater from being hit by the flow of work.
4 Multiple smaller wattage heaters are preferred over a single large wattage heater.
5 Spaghetti TeflonR heat exchangers using steam or hot water are available. They are easy to install and maintain. The best feature is that there is no polarization from the heater and no localized heating zones to break down additives or organic chemical in the bath.
1.5.08 The Hull Cell
The most common and useful test is the Hull cell test.
Hull cell is used to check
plating bath performance and
determine how much additive
replenishment is needed. Hull
Cell was invented by Richard
O. Hull in 1950 (U.S. patent
#2149344). The trapezoidal
geometry generates a current
density profile on the test
panel. The transparent plastic cell allows one to see
what is happening at the cathode and anode during the
test. It also allows one to see that the chemicals added are completely dissolved or well mixed. Refer- ring the picture on opposite page, test panel is inserted along the inclined wall. The anode is placed on the opposite wall. The edge of the panel closer to the anode drawn more current. It is commonly called the high current density region (HCD). Cathode edge farthest from the anode is the low current region (LCD).
Properly interpreted the test panel indicates the following:
- Additive concentration
- Bath chemistry
- Carbon treatment effect
- Metallic impurities
- Organic breakdown
- Suspended particles
- Temperature effects
The main objective of the Hull cell test is to study or evaluate the effect of chemical composition and
different plating parameters.
- Deposit quality over a range of current densities, including burning, dullness,
cracks, etc.
- Leveling power
- Covering power
- Thickness variation
- Alloy composition
- Metallic and organic contamination
- Ductility
1.5.09 Power Supply
A rectifier is used to supply direct current to deposit metal onto the part. The metal ions (positively charged) in the bath are attracted and deposited on the part (negative charge). The anode which is positively charged, is oxidized, and dissolves to form metal ions and replenish the metal ions which were plated out.
In the early day of plating, mechanical DC electrical generators were used. They were very noisy and required regular maintenance. Around 1930 the silicon controlled rectifier (SCR) was introduced which used no mechanical parts (See Rectifier on page 2.7). The rectifiers used today consist of the following parts.
1 Transformer
It reduces the incoming high voltage current;
120-440 VAC to low voltage: 6-12 VAC.
2 Current Converter
It converts the low AC voltage to low DC voltage: 6-12 VDC.
3 Capacitor
It filters the residual AC super-imposed (ripple) in the DC output. Rectifiers with more than 3% AC ripple are not used
in production and not recommended for use
in the laboratory.
4 Power Control
It regulates the voltage and ampere output for plating.
1.5.10 Processing
The plating process varies according to the basis metal
and its functional requirements. The following proce- dures are generally used.
1 Soak clean, rinse (25oC; 77oF)
2 Electroclean, rinse (25oC; 77oF)
3 Acid activation, rinse (25oC; 77oF)
4 Plate
5 Drag out, rinse (25oC; 77oF)
6 Post treat as required, rinse (25oC; 77oF)
7 Dry
1.5.11 Product Design
The part configuration has a significant effect on
current distribution and deposit uniformity. The product design engineer should work with the plater before starting the design. Sharp edges or sharp
cornered parts are easily over plated. Parts with blind holes or closely-spaced slots are difficult to plate. The National Association of Metal Finisher (MFSA) has published eight guides to aid product designer.
1 Chemical Surface Preparation For Electroplated And Metallic Coating 2 Decorative Copper, Nickel, Chromium
3 Decorative Precious Metal Plating
4 Electroless Nickel Plating
5 Hard Chromium
6 Mass Finishing
7 Tin And Tin Alloy Coatings
8 Zinc And Cadmium Coatings
The above publications can be obtain at:
National Association Of Metal Finisher
112-J Elden Street
Hemdon VA 20170
phone 703-709-8299 fax 703-709-1036 www.namf.org
1.5.12 Rinse Water
No cleaning is complete without thorough rinsing. The
rinse water should be clean with less than 100 ppm
of suspended solid and minimal hardness (calcium,
chloride or magnesium ion). Deionized or reverse
osmosis-purified rinse water are recommended after
plating and before drying.
1.5.13 Surface Tension
The cathode (plating) efficiency in most plating bath is less than 100%. This mean that some of the current applied is generating hydrogen at the cathode. The
hydrogen bubbles tend to stick on the part until large enough to escape to the surface. Delaying escape can cause pitting. Surface active chemicals, commonly called wetter, are added to decrease the surface tension of the solution and quickly release the hydrogen bubbles. In all cases, some solution agitation or part movement is recommended. Most of the wetting agents are organic sulfonic acids, and are non-foaming. In some baths, the wetter also acts as carrier brightener.
1.5.14 Throwing Power
In order to have a relatively uniform deposit on large part or on each part on a rack, the throwing
power should be high. Throwing power is the ability
of the bath to deposit uniformity into a recess. Throwing power compares the amount of deposit on a
high current density to the amount on a low current density. Throwing power is affected by cathode polar- ization, conductivity, plating efficiency and current density. Throwing power given should specify the cur- rent density, bath chemistry and parameters in the test. Throwing power only applies to the described conditions of the test and is not an absolute value.
( See Throwing Power Table on page 4.119 )
1.5.15 Viscosity
The viscosity of a plating bath affects the cathode and anode surface films and the cathode polarization during plating. The more viscous the solution, the higher the cathode polarization and throwing power. For example in Watts nickel, increasing the sulfate concentration increases the throwing power. On the other hand, the more viscous the solution, the more one encounters drag out losses. Normally the viscosity of the bath does not change significantly unless there is excessive drag-in or drag-out, leakage in the tank or over filling with water.
1.6 Applications
1.6.01 Brass Plating
Overview:
Brass is an alloy of copper and zinc. Normally, it is plated from a cyanide solution and to a lesser extent from pyrophosphate and hydroxyl aliphatic electrolyte.
Non-cyanide brass is difficult to control. Many shops are still using cyanide brass.
Applications:
Brass is plated over bright nickel for its brilliant finish. Brass has no tarnish resistance and is quite soft. The deposit is normally passivated and top coat- ed with a clear lacquer. Brass plating is applied to household and hardware products, like door locks, drawer knob and hinges. Brass plating steel improves rubber and paint adhesion on steel. Brass plated steel wire can be drawn smoothly to increase the die life. Yellow brass and red brass can be antique finished.
1.6.02 Cadmium Plating
Overview:
Cadmium is mostly applied by electroplating. To a
lesser extent cadmium is also applied by mechanical plating and vacuum deposition. Most of the electro-
lytic baths are cyanide based. Other systems use acid sulfate, fluoborate and neutral chloride chemistry. Mechanical cadmium plating is used to minimize or eliminate hydrogen embrittlement.
Post treatment with dilute nitric acid, chromic acid with sulfuric acid and dichromate, greatly improves corrosion resistance. Heat treatment at 177-304oC
(350-400oF) for three hours or more to relieve hydro- gen and maintain the fatigue strength of steel. Cadmium is very toxic. Once inhaled, it can migrate
to the bone, causing pain and fragile bones. Japan was the first country to ban cadmium plating, followed by the European countries. Many industries have banned the use of cadmium in their products in North America.
There are several cadmium replacements available,
including zinc-nickel, zinc-tin and ion-vapor deposited aluminum coatings. None of the substitutes possess all the positive attributes that cadmium has.
Applications & Attributes:
1 Cadmium is used to provide sacrificial protection
on steel, just like zinc. Cadmium works better
in marine or salt water environments. While zinc deposits are best in industrial and outdoor environments.
2 Cadmium provides good bonding for paint to metal.
3 No galvanic action of cadmium coating on aluminum
parts take place. Compatibility with aluminum made cadmium the No.1 metal specified for aircraft fastener and other aluminum products for military applications.
4 Cadmium is the only metal that will not arc weld
on electrical contacts.
5 Cadmium resists attack by strong alkalis, whereas
zinc can not.
6 Cadmium has excellent lubricity and friction
reducing properties. Fastener need only 5-10 um (0.2-0.4 mil) of cadmium.
7 Cadmium can absorb large volumes of hydrogen
during plating which can lead to embrittlement. Cyanide cadmium deposits are the most brittle. Acid cadmium plating on steel generates less hydrogen embrittlement than the cyanide bath.
8 Cadmium exhibits good solderability and corrosion
resistance. Cadmium is used on relays, chassis
and other electrical components.
9 Cadmium tends to sublime at low temperatures in a vacuum environment. Therefore, cadmium is not recommended for space applications.
1.6.03 Chromium Plating
Overview:
Chromium deposits exhibit a bluish white color and have excellent abrasion, wear, tarnish and corrosion
resistance.Chromium plating is divided into decora- tive, industrial and black chromium.
1 Decorative Chromium, Hexavalent
Most of the commercial chromium baths use hexavalent chromium chemistry. The bath is
simple and easy to use. However, this form of chromium is toxic to humans and environmental life. Waste treatment and proper disposal is required and expensive.
2 Decorative Chromium, Trivalent
Trivalent chromium plating started some 25 years ago and using the same formulation. In the last few years, the U.S. Environmental Protection Agency restricting hexavalent chromium waste
disposal, pushing the advancements of trivalent chromium processes.
The newer chromium deposits have a bluish color closer to that of hexavalent chromium. The
trivalent chromium bath is similar to the Watts nickel bath. It operates at pH 4-5 and uses a secondary additive for brightness and color. Unlike the hexavalent process, trivalent bath uses only 20 g/L (2.7 oz/gal) of chromium metal instead of 120 g/L (16 oz/gal) in hexavalent chromium bath.
Pros:
a. Better throwing power than hexavalent
chromium.
b. Low waste treatment cost.
c. Less corrosive to the parts and surroundings.
d. Less drag out, because it operates at lower chromium content.
Cons:
a. Deposit color is not as bluish or as sharp as hexavalent chromium.
b. Deposit is microporous, and less corrosion resistant.
c. Cannot plate heavy thicknesses, though new developments, using pulsed waveform, are making this possible.
3 Hard Chromium
The basic chemistry is the same as the hexavalent
chromium with chromium trioxide, except it has higher chromium content, operates at higher temperature and higher current density.
4 Black Chromium
Black chrome is a non-homogeneous deposit of
chromium oxide. The black deposit has only 50%
reflectivity of the bright surface. When plated over matte surfaces it becomes a light absorber and is excellent for collecting solar energy. Black chromium has a denser microporosity than does bright chromium. Hence black chromium is better for outdoor corrosion resistance.
5 Other Chromium Deposition
To a lesser extent, chromium can be deposited
by vacuum evaporation, sputtering and plasma
spraying. Many plastic parts are processed by
vacuum evaporation followed by a clear Acrylic
top coating. The plasma sprayed chromium is used on various industrial cutting tools and oil drill bits that require a precise uniform
coating.
Applications:
1 Decorative Chromium
Decorative chromium is generally used over bright nickel in many household appliances, household hardware, drawer knobs and automotive parts. In general the chromium deposit is about 0.25 um (10 uin.) thick.
2 Industrial (Hard) Chromium
Hard chromium is used on shafts, hydraulic cylinders, cutting tools, turbine molds and dies, and rebuilt worn-out or undersize machine parts.
Hard Chromium vs. Electroless Nickel
a. Hard chromium has slightly better wear resistance than electroless nickel.
b. Hard chromium has less corrosion resistance than electroless nickel.
c. Overall cost of hard chromium is more than
that of electroless nickel.
3 Black Chromium
Black chromium has a high light absorption value and highly anti-glazed surface. It is used for
solar energy collectors and optical instruments. Many household hardware and electronic compo-
nents are black chromium plated. Black chromium can be plated only a few microinches thick. Hence it is normally applied as a top coat over a regular chromium deposit.
1.6.04 Copper Plating
Overview:
Copper is a pink, soft metal, used on printed circuit boards and as an underplate in many industrial appli- cations. There are four distinct types; namely acid, cyanide, pyrophosphate and fluoborate copper.
1 Acid copper is bright and can cover substrate defects with minimum thickness. The bath is simple, containing copper sulfate, sulfuric acid and traces of chloride.
Some high speed acid copper baths can operate as high as 26 A/dm2 (250 A/ft2) with vigorous
solution agitation. The solution is pumped through a sparger at 100 cfm onto the part. The copper concentration is much higher here than for a normal bath.
2 Cyanide copper baths have high throwing power and good complexing properties. Minimal clean- ing is required and adhesion is excellent. Due to its toxicity, the U.S. EPA requires that all spent and waste effluent must be treated.
3 Pyrophosphate copper has been used for many years. The deposit is soft and very ductile. The bath has good throwing power and waste
treatment is simple. However, the chemical
components are expensive and the bath is
difficult to control.
4 Fluoborate copper is used in high speed
plating. The chemicals are highly corrosive
and expensive. It is the least commonly used copper bath.
5 Electroless copper contains copper sulfate, sodium tartrate, formaldehyde and a stabilizer. It operates at a pH between 10 to 13. Formal- dehyde is highly toxic and proper exhaust should be used. New commercial baths are using sodium hypophosphite or dimethyamine borane as reducing agents. They are not as toxic, but the chemical cost is higher.
Applications:
1 Bright Acid Copper
Bright acid copper has relatively good micro- throwing power. It is used on steel wire, zinc
die casting, stainless steel cooking ware and
as a heat stop-off treatment on high carbon steel. Acid copper is widely used as an under coating where nickel and chromium are required. Examples are plumbing fixtures, household and many hardware appliance accessories. Copper baths without
leveling additives are used in through-hole plating on printed circuit boards and other
electronic connectors.
2 High-Throw Acid Copper
This bath contains low copper sulfate and high
sulfuric acid. It is used predominantly on printed circuit boards and deposits into holes with high aspect ratios.
3 Alkaline Pyrophosphate Copper
Copper pyrophosphate is more expensive than
copper sulfate. The pyrophosphate ion tends to break down on electrolysis. Pyrophosphate copper is used mostly in electroforming. Some federal agencies specify pyrophosphate copper plating because of its excellent ductility.
4 Cyanide Copper
The bath is simple to use and has good chelating properties. It is excellent for plating on zinc die casting, aluminum and iron casting. It is
used as a stop-off coating on steel for heat treatment. Steel wires are plated with copper as lubricant in size drawing. Bright cyanide copper is used as a base coat to reduce nickel metal cost in copper-nickel-chromium applications.
5 Electroless Copper
Electroless copper is primarily used to metallize the holes in printed circuit boards and is used as conductive coating on plastic for subsequent nickel and chromium over-plating.
High speed heavy copper processes can be used on printed circuit boards to eliminate the elec- trolytic copper over-plate in some cases.
“The real glory is being knocked to your knees and then come back.”
1.6.05 Gold Plating
Overview:
Gold can be deposited via four processes.
1 Electroplating
Baths used are alkaline cyanide, alkaline
sulfite, neutral and acid. The neutral bath is most commonly used. Acid gold is used as
top plate on jewelry to match the desired color.
2 Electroless Plating
Actually this is done by chemical immersion
and not autocatalytic.
2 Thermal Decomposition
This involves thermal decomposition of screen printed gold paste onto the part.
3 Vacuum Deposition
Here are vaporizes an organic gold compound or gold metal onto the part in a vacuum chamber.
4 Mechanical Cladding
This is commonly known as gold filling. Gold
foil is rolled and pressed onto a metal strip.
The product is then punched from the gold strip and gold flashed to cover the raw edges.
Applications:
Gold is the most noble metal on earth. It is the only metal existing as metallic element in the earth’s crust. It does not form any oxide or other compound. Gold is used for jewelry and gold coins because of
its non tarnishing properties.
Most costume jewelry has only a gold flash over nickel. The thickness is about 0.05-0.12 um (2-5 uin.).
Jewelry stamped "gold plated" must have at least
7 uin. of gold. Expensive gold jewelry is made of
solid gold or alloy gold.
The percentage of gold is expressed in terms of karat. 24 karat gold jewelry is 100% gold. 18 karat is 75% gold, and 14 karat is 50% gold. Jewelry of less than
18 karat will tarnish and is usually over-plated with higher karat gold.
Gold has high electrical conductivity, ranking just behind silver and copper. Many electronic components and contacts are gold plated. Gold has high infrared reflectivity and is used in space instruments. It is hard to believe that the industrial gold usage is many times more than that for decorative application and jewelry.
1.6.06 Indium Plating
Overview:
1 Cyanide Bath
The throwing power is very good and deposits uni- formly on complex engine parts. Its disadvantage
is that the cathode efficiency is not stable and drops gradually from a new bath from 90 to 65%. This makes it difficult to plate the required
thickness consistently.
2 Sulfamate Bath
The sulfamate bath uses a soluble indium anode.
The bath is easy to operate and control. Its
efficiency is the same as for the cyanide bath,
90%, except it is stable throughout its useful
life.
3 Fluoborate Bath
The deposit has a very fine grained structure. The throwing power is good. The cathode effi- ciency is low; only 45 to 75%. Fluoborate
chemicals are expensive and corrosive.
Applications:
The main application of indium is as a diffusion alloy on aircraft engine bearings and other high
temperature applications.
1.6.07 Iron Plating
Overview:
Most of the iron baths use ferrous chloride or fer- rous sulfate or a combination of both. The bath
chemistry is simple and easy to control. The deposit from the ferrous ammonium sulfate bath is harder than that from the chloride bath. Fluoborate and sulfamate baths can be operated at higher current densities.
Applications:
Iron plating was first done around 1900. Limited application arises from its poor corrosion resistance and lesser than pleasing appearance. Iron deposits
are highly stressed and brittle due to hydrogen absorption. Heat treatment is required.
1 Iron wets well with solder, yet it does not alloy with the solder. Hence iron deposits are used on copper soldering tips.
2 Iron is used to build up worn parts, glass and
rubber molds, salvaging undersize machine parts, stereotypes and electrotypes. Iron deposits are
relatively ductile compared to hard chromium.
3 Iron’s superior drawing properties find application in forming special wires.
4 Iron deposits can be hardened by cyaniding and nitriding heat treatments. It is used in some industrial tools and molds.
5 Iron-nickel alloy plating has special magnetic properties. It is used for magnetic foils and electric motor cores.
6 High purity iron powder is produced by electro- refining, and is used for scientific research.
1.6.08 Lead Plating
Overview:
Lead deposits are soft and tend to oxidize on aging, from blue-purple to matte gray and ash white. Limited applications call for lead plating. There are three types of lead baths used commercially: fluosilicate, sulfamate and fluoborate systems.
1 Fluosilicate baths cost the least, but do not
plate directly onto steel. The deposit is coarse grained. The throwing power is poor.
2 Fluoborate baths are more expensive. They produce a finer grained, dense deposit, and can plate
directly on steel. Tin can also be codeposited with lead in this system to form various solder alloys.
3 Sulfamate baths were first used as early as 1938.
The sulfamate bath is stable and non-hygroscopic. The bath can be operated over a wide range of current densities. It is easy to operate and
maintain.
4 Hot Dipping is less commonly used. Lead has a low melting point of 327oC (620oF) and can be used in hot dipping. Some tin is added in the hot melt to improve its adhesion onto iron. Depending on the iron composition, the tin content can range from
2.5-20%.
Applications:
Lead metal is fairly resistant to non-oxidizing acids like dilute sulfuric acid and cold hydrofluoric acid. Lead is used in lining brine refrigeration tanks,
storage batteries and in lining metal gas shells. The lead is soft and used as a lubricant film in some machine parts.
1.6.09 Nickel Plating
Overview:
1 Sulfate Nickel: Watts
This bath consists of nickel sulfate, nickel
chloride and boric acid. It also contains a
wetting agent with or without a brightener. This bath is simple and easy to use. High purity nickel salts are readily available and relatively
inexpensive.
2 Sulfamate Nickel
This bath consists mainly of nickel sulfamate,
sulfamic acid,boric acid and a wetting agent to
control pitting. The bath is relatively simple
to use. However, the sulfamate and sulfamic acid break down slowly and need replenishment. The deposit is ductile with low stress and good
elongation properties. Sulfamate nickel is used
on fingers of printed circuit boards and other electronic connectors.
3 Sulfamate Nickel Strike
The high acid sulfamate nickel bath can be used
as a strike to improve adhesion on subsequent
nickel and chromium.
- Has better throwing power than Woods nickel
- High cathode efficiency, 50 to 60%
- Less susceptible to metal contamination,
thus minimal replacement
- More expensive than Woods nickel
4 Woods Nickel
This strong acid bath contains hydrochloric acid and nickel chloride. It is mainly used as a strike to activate stainless and other nickel alloy
substrates.
- Throwing power is not as good as the
sulfamate nickel strike
- Plating efficiency is less than 10%
- No tolerance to metallic contamination
- Costs less than sulfamate nickel strike
5 Fluoborate Nickel
This is a low pH nickel bath using fluoboric acid and nickel chloride. The bath can plate at high
current densities, and is often used for electro forming. The fluoborate chemical is expensive and corrosive. The throwing power is not as good as the Watts nickel.
6 Black Nickel
The deposit is lustrous and has a black or gray color. The bath operates between pH 5 and 6 with little or no agitation. It is used as an over- plate on bright nickel.
7 Electroless Nickel
This is a very bright and hard deposit. On heat
treatment the deposit is as hard as chromium. Electroless nickel involves autocatalytic reduc- tion instead of electrolytic current.
The most commonly used reducing agents in elec- troless nickel baths are following:
Sodium hypophosphite
Dimethyl amine borane
Sodium borohydride
Remember that some of these reducing agents are toxic. Proper chemical handling and ventilation are required.
Applications:
Nickel has a pleasing brilliant pale white color. It
is hard and exhibits good wear resistance.
1 Sulfamate nickel is used to rebuild worn machine parts, and in the electroforming of printing
plates, compact disc molds, mesh products (like shaver screens) and juice extract filters.
2 Nickel deposits are solderable and have excellent magnetic properties. It is used on many electronic devices.
3 Watts nickel deposits without brightener or leveling agents are soft and ductile. This bath
is commonly used in barrel plating and finger
tabs of printed circuit board.
4 Watts nickel baths with brightener offer maximum
brightness and leveling. Nickel is not tarnish- resistant. Therefore it has to be over-plated either with gold, chromium or brass. Nickel is used extensively on automobile and motorcycle bright work, household appliances and hardware.
5 Black nickel is used on solar energy panels and
some select jewelry.
6 Electroless Nickel is extremely bright and hard. It is used for hard disc drives. The phosphorus
content controls the magnetic properties.
7 Electroless Nickel vs. Hard Chromium
(See Chromium Plating on page 1.32)
“Every exit is an entry somewhere else.”
1.6.10 Palladium Plating
Overview:
Palladium plating was first used in the 1850’s, using ammonium hydroxide based baths. In the 21st century,
we are still using ammonium based systems with
palladium chloride. Palladium can also be applied by electroless plating, sintering and vacuum deposition.
Palladium baths are non-cyanide systems, and therefore are less toxic and cost less for waste disposal. The only disadvantage is that palladium readily absorbs hydrogen readily during plating. Hence palladium
plated parts have to be heat treated to relieve the hydrogen.
Summarizing the palladium features:
1 High thermal stability
2 Good wear resistance and hardness
3 Cost less than gold, 38% less metal per unit thickness
4 Good solderability and non-toxic, excellent
replacement for tin lead (toxic)
5 Excellent diffusion properties
6 Less toxic to health
7 Minimum waste treatment cost
Applications:
1 Palladium is a pearl white metal similar in color to silver. The specific gravity of palladium is 12.0 while that of gold is 19.3. For an equal
thickness, palladium weighs 38% less than gold. This saving makes it very attractive to replace gold plating in some applications.
2 Palladium can withstand high temperatures. Its
major application is in automotive and other catalytic converters.
3 Another application is for high wear connectors
and electrical contacts. Palladium is also used
on lead frames for plastic packaged ICs, glass- sealed contacts and ceramic capacitors.
4 A gold flash is usually plated over palladium
to improve its tarnish- and wear-resistance.
5 Palladium-nickel alloys contain 10 to 30% nickel.
The nickel improves hardness and wear resistance, and reduces hydrogen embrittlement.
1.6.11 Platinum Plating
Overview:
The first platinum plating bath patent was granted to
a French chemist, Pilet in 1883 using platinum chlo- ride. Today, the platinum chloride system is still used. Recently a platinum diamino nitrite bath has
been used as a replacement for platinum chloride.
Applications:
Platinum has a pearl like color similar to silver.
Platinum is used in both jewelry and various industrial
applications.
1 Costume jewelry has less than 0.25 um (10 uin) platinum as the top coat. The underlying basis metal is usually nickel or zinc alloy. Jewelry
with the word "platinum" inscribed, must have at least 5% by weight of platinum.
2 Platinum can withstand high temperatures. Its major
application is in automobile exhaust catalytic converters, high temperature contacts and turbine blades.
3 Platinum is inert to most chemicals. Platinum
plated expanded mesh can be used as auxiliary anode for nickel electroforming and chromium plating. Platinum plated titanium and platinum- clad niobium expanded mesh are used as insoluble anodes in gold and palladium baths.
1.6.12 Rhodium Plating
Overview:
Rhodium is a rare and most expensive precious metal.
Rhodium is the whitest metal, a brilliant bluish
white, and is non-tarnishing, hard and wear resistant.
Rhodium plating began in 1915 and found commercial use in 1930. The bath consists of sulfuric acid and
rhodium sulfate. The bath is simple to use and
maintain. Phosphate and ammonium hydroxide baths are also in use.
Rhodium can also be coated by vacuum evaporation. Unfortunately, the metal spray in the vacuum chamber
condenses more than on the part. Hence electro-
plating processes is extensively used.
Applications:
Rhodium is used as a top coat in jewelry in the range of 0.05-0.15 um (2-6 uin). Its high melting point and
stable contact resistance have found many applications in electrical reed switches and high temperature
relay contacts. In some electronic applications, heat treat ment is applied to reduce hydrogen embrittlement.
1.6.13 Ruthenium Plating
Overview:
Ruthenium is a dark gray precious metal that is harder
than rhodium. It is the hardest electrodeposited metal and the least expensive among the platinum group
metals. The ruthenium bath consists of ruthenium
nitrosyl sulfamate and sulfamic acid.
Applications:
Its major application is the ruthenium oxide coating
on titanium mesh. It is used in salt water sacrificial protection. Some jewelry is plated with ruthenium. Ruthenium plated jewelry has a unique dark color. Ruthenium has found some commercial application in platinum replacement.
“What makes something special is
not just what you have to gain,
but what you have to lose.”
1.6.14 Silver Plating
Overview:
The first silver patent was issued in 1838 to Elkington and Barratt in England using a cyanide solution. In 2003, most silver solutions are still based on the cyanide system. The sodium cyanide
formula produces brighter deposits while the
potassium formula has a wider current density range and better throwing power. Hence the combination of both salts are generally employed in the cyanide
system to achieve optimum performance.
The phosphate and organic phosphate silver have found application in high speed and selective spot plating for lead frames and ceramic chips. Non-cyanide silver baths such as the thiosulfate and succinimide systems are also available. These baths are not as easy to operate as the cyanide system and have limited
commercial application.
Applications:
1 Silver is a light pearl white metal that has
a pleasing appearance and is used in jewelry.
2 Silver is highly conductive and is non-arcing
on high voltage contacts. It is excellent for plating switch contacts.
3 Silver is used on lead frames where the silicon chip is attached and wire-bonded.
4 Silver is non-toxic and looks expensive. It is used in tableware and kitchenware.
5 Silver is used as a high temperature lubricant
coating on precision machine parts.
1.6.15 Tin Plating
Overview:
Before 1930, most tin coating was done by hot dipping. After 1930, electrolytic tin plating began to emerge and become dominant. Tin baths use either acidic or alkaline chemistry.
1 Alkaline Tin: Stannate
Alkaline tin used as early as 1843 by Morewood
and Rogers. The modern and improved bath is
quite easy to operate, has excellent throwing power and can tolerate metallic impurities. The only drawbacks are its low deposition rate and
low cathode efficiency. Its deposit is not
brilliant bright in appearance. Power consumption
is higher than for acid tin.
2 Acid Tin: Sulfate
In 1909, Hollis obtained a patent using fluosi-
licate acid. By 1923, sulfate tin followed
and replaced the fluosilicate. Acid tin has an excellent bright current density range, a high deposition rate and low electric consumption. However, its throwing power and deposit ducti- lity are not as good as those of alkaline tin.
3 Acid Tin: Fluoborate
Tin fluoborate is very soluble. These high-metal baths can plate at much higher current densi- ties. Lead can be codeposited with fluoborate tin to form various solder alloy deposits. Fluoborate baths are more expensive than the tin sulfate bath.
4 Acid Tin: Methane Sulfonic Acid
The problem with sulfate tin is that divalent tin slowly oxidizes to tetravalent tin and corrupts the process. When the tetravalent tin reaches its upper limit, the bath has to be replaced. Methane sulfonate tin does not have
this problem. Further the solderability seems
to be more consistent.
Various solder alloys can be deposited from
the methane sulfonate bath. This bath is less
corrosive than the fluoborate tin lead bath.
Applications:
1 Tin is non-toxic and not reactive to many food chemicals. Tin plated steel is used for all food cans. Tin is used extensively in dairy
and other food handling equipment.
2 Tin is soft, ductile and solderable. It is applied in electronic components, diode and
connectors.
3 Tin coatings are employed as bearing surfaces and as break-in lubricants on automotive and other pistons.
“You never get promoted when no one else knows your current job.
The basis for being advanced is to organize yourself
out of every job you’re put into.”
1.6.16 Tin-Lead Plating
Overview:
Tin lead can be plated from pyrophosphate, fluoborate, methane sulfonate (msa) chloride and fluosilicate baths. Until 2001, the pyrophosphate bath was exten- sively used
1 Fluoborate Bath
The bath contains stannous and lead fluoborate, fluoboric, boric acid, additives and wetting agents. The fluoborate tends to break down during electrolysis and periodic carbon treatment is required. Fluoborate is corrosive and waste treatment is expensive.
2 Methane Sulfonic Acid Bath (MSA)
The system uses stannous methane sulfonate,
lead methane sulfonate, methane sulfonic acid
and additives.
- MSA chemicals are expensive - Throwing power is good, and solderability
is consistent. Further, the bath offers a wide current density range
- MSA is stable in acidic, neutral or alkaline solutions. MSA does not hydrolyze readily
- MSA metal salts are highly soluble and
readily available
- Low cost waste treatment is an advantage
Methane sulfonic acid baths are gaining commer- cial success and are replacing the fluoborate bath
3 Sulfamate, chloride and fluosilicate baths have
no significant commercial application.
Applications:
1 Tin-lead deposits are used as a metal etch resist
in printed circuit board manufacture.
2 60-40 tin-lead is used to protect the basis metal and retain the solderability of electronic
components.
3 Typical alloys with 5% to 10% tin and 95% to 90% lead are used as bearing contacts.
1.6.17 Zinc Plating
Overview:
Zinc Metal Facts:
- Zinc is the most abundant metal in the earth’s crust. On average, every ton of earth contains
2.3 ounces of zinc
- Zinc is the cheapest metal to plate for its
high protective properties
- Zinc sacrificially protects iron and ferrous
metals from corrosion
- Zinc is a bluish to pale gray metal
- Zinc is malleable at 38-66C (100-150F)
- Zinc is a brittle solid at room temperature
- Zinc is a good electrical conductor
1 Cyanide Zinc
Cyanide baths was first used in the 1800’s and
are still in use today. Cyanide baths are easy
to maintain and have good throwing power. Plate
adhesion is excellent, even if the parts are not properly cleaned. Cyanide chemicals are toxic to humans and the environment. Around 1980 the U.S. E.P.A. began regulating the waste cyanide effluent. The cyanide waste treatment is expensive. Over time, existing cyanide zinc baths are being converted to alkaline non-cyanide or acid baths.
2 Alkaline Zinc: Non-Cyanide
This system emerged around 1960 using chelating agents to replace the cyanide. However, disposal
of the chelate metal effluent are difficult and expensive. Later, organic additives were used to replace the chelators. The bath is sensitive, and has a narrow current density range. Now, new reac- tive organic products have been developed and are used with higher zinc contents. Potassium salts
have also replaced sodium salts. Alkaline zinc has two weaknesses. First, it can not be plate directly on cast iron and high carbon steel parts. Second, the parts have to be very clean to obtain good
adhesion.
3 Acid Zinc
Unlike the alkaline zinc, acid zinc is brilliant,
has excellent leveling power and is easy to
control. The only disadvantage is that the chloride zinc baths are very corrosive.
4 Zinc Alloy
These alloys have been used to replace the cadmium. Since Japan and European countries were first to
ban cadmium plating, they developed most of the zinc alloy baths. There are many commercial baths available, but only zinc-nickel and zinc-iron are widely used commercially.
Acid Zn-Ni Alk. Zn-Ni
Cathode Efficiency High 40-60%
Electric Power Low High
Nickel Content 12-15% <9%
pH Control Critical Not Critical
Environment Corrosive Friendly
Applications:
1 Zinc deposits are bright, pearl white. A variety of color conversion coatings can be applied and are very economical to use. Zinc deposits provide
sacrificial protection on steel. Most industrial products such as bolts, washer, electrical con- nectors and junction boxes are plated with zinc.
2 Cyanide zinc is the easiest bath to operate. It never has adhesion problems even with inadequate
cleaning. The bath can operate for years and never need replacement. However, cyanide baths are toxic and strictly regulated by the U. S. Environment Protection Agency. The system is now almost non-existent, except in some applications.
3 Acid zinc can plate directly on cast iron, high
carbon and malleable steel. Acid zinc with a clear nitrate conversion coating is very close
in appearance to chromium.
4 Zinc-nickel has excellent corrosion resistance.
It has been used to replace cadmium in many
applications.
5 Zinc-iron is economical, has good ductility and
weldability. Its corrosion resistance is not as good as zinc-nickel, particularly when parts are subjected to elevated temperatures.
