At the beginning of the line, the end of one coil is welded to the start of the next coil. Then there are two basic methods for continuously galvanizing sheet which differ in the way that the strip is cleaned before galvanizing-chemically or by thermal treatments. Coils of annealed cold reduced sheet may be fed directly to the galvanizing line, or alternatively, coiled sheet is continuously heat treated in the pretreatment line. After leaving the galvanizing bath, in which strip only stays for a few seconds, the surface is wiped to remove excess zinc and may be further treated to after the surface appearance, composition, smoothness or mechanical properties.

The steel sheet electroplating process utilizes the same basic principle as that for conventional decorative finish electroplating. However, the steel sheet process differs in that the electroplated coating is applied by passing the strip at high speeds through a series of plating cells, building the coating thickness by a small amount each time the strip passes through an individual cell. This continuous process for electroplating steel strip requires necessary equipment to transport the strip at high speeds (150 to 200 meters per minute and higher) through a series of individual plating cells, and is not as simple as it sounds.

An Electroplating Cell

The simplest electroplating cell is shown in the sketch where the plating solution bath is zinc sulfate.

The common schema of the electroplating cell.
This simple plating cell illustrates the actions during the plating process. At cathode (steel, for example), zinc ions dissolved in the zinc sulfate solution combine with two electrons and form elemental zinc, which deposits onto the cathode surface. At anode, water is converted to oxygen and hydrogen ions to maintain electrical balance. The oxygen forms a gas and nothing is deposited on the anode surface. The plating solution carries the current between the cathode and anode.

Plating of Steel Sheet in a Continuous Process

There are many types of anode arrangements. Some are horizontal, others are vertical, and one process utilizes a radial cell wherein the strip passes around large diameter rolls inside each plating cell, and the anodes have a radial design to match the diameter of the large rolls submerged into the plating solution. Each type of anode arrangement and design has advantages and disadvantages; thus, it is easy to see why different manufacturers use different methods. Each requires very close control of the anode-to-strip spacing to achieve efficient plating, avoiding arc spots and other defects in the coating.
Modern Continuous Electroplating Line.
Maintenance of the large volume of plating solution that is contained in all the cells is a science unto itself. Whether the plating solution for electrogalvanizing is based on zinc sulfate or zinc chloride chemistry, maintenance of the proper ranges of zinc ion concentration and solution pH are important control features. Besides plating zinc, some manufacturers have the ability to deposit alloy coatings. This requires, at a minimum, at least one more level of control of the plating solution. For example, producing a zinc/nickel alloy coating requires close control of the concentrations of both the dissolved zinc and nickel in the solution. Solution control has to be accomplished on a dynamic basis since these lines operate continuously.

Power Requirements

The electroplating process requires a large amount of electric power to deposit a metallic coating. The total power requirement is a direct function of the coating thickness that is needed to meet the customer’s specification. For example, the power required to deposit a zinc coating mass of 80 g/m2 is approximately twice that required to deposit a coating of 40 g/m2. A typical line that has the capability to process 70 to 120 tons/hour with a coating mass of 50 g/m2 will consume hundreds of thousands of amperes during this one hour of processing time. It is easy to see why power costs are major cost component for a facility that processes large quantities of electroplated sheet product.

Product Types

The most common electroplated coating for steel sheet products is zinc. Electrogalvanized zinc coatings are used by a number of automotive companies for exposed car-body panels, where the typical coating mass ranges from about 50 to 80 g/m2 per side. These coatings are considerably thicker than the electrogalvanized coatings typically used for non-automotive applications, so the lines built to make products for automotive applications usually have a large number of plating cells. Also, each automotive customer has their own specific coated-product specification.

Another attribute associated with the use of electrogalvanized coatings for automotive applications is excellent surface finish that is attainable with the electroplating process. Twenty-five years ago, when automotive companies began using large amounts of galvanized sheet for exposed body panels to improve corrosion protection, one of the few coated sheet products that could meet the demanding surface quality requirements was electrogalvanized. Hot-dip galvanized was, and still is, used for unexposed body parts. As the surface of hot-dip products improves, they continue to replace electrogalvanized sheet for exposed automotive body panels.

Other zinc electroplating lines have been built through the years to make thinner coatings. Typically, the sheet that is made on these lines has a coating mass of less than 25 g/m2. The applications for this product are often indoors; applications where the environment is not very corrosive. Many applications involve painted products. These coating lines often have the ability to apply paint pre-treatment so that the customer can paint directly without additional in-house treating.

A second type of electroplated coated-steel sheet being manufactured today has a coating composed of a zinc/nickel alloy. Typically, the nickel content is 10 to 16 percent with the balance being zinc. The unique feature of this process is that the zinc and nickel ions are co-deposited to make a true alloy coating. It is not composed of alternating layers.

The application for this product has been limited primarily to a few automotive companies. These companies have developed in-house product design and manufacturing processes to take advantage of the unique characteristics of the zinc/nickel coating. For these automotive applications, the metallic coating is often coated with a special corrosion-resistant thin organic coating on top of the zinc/nickel. The zinc/nickel alloy coating is covered by ASTM Specification A 918.

A third type of electroplated coating is zinc/iron alloy coating. The attributes of this specialized coating are somewhat like those of hot-dip galvannealed product. Like zinc/nickel alloy, zinc/iron coating is co-deposited as an alloy coating. Iron is uniformly deposited throughout the coating thickness. Also, like zinc/nickel coating, zinc/iron coating is used predominantly by the automotive industry. The attributes of electroplated zinc/iron is that it is relatively easy to weld and paint if the proper electro-priming equipment is available to the automotive manufacturer. Also, the coating is very hard, making it is less susceptible to scratching during stamping and handling. This is the important feature since the zinc/iron alloy coated-sheet product is being used almost exclusively for exposed car-body panels.

Corrosion Resistance of Electroplated Coatings

Concerning the corrosion behavior of electrogalvanized versus hot-dip galvanized coating, it is important to note that it is essentially equivalent for identical coating masses. A coating mass of 100 g/m2 will provide essentially the same amount of corrosion protection whether it is a hot-dip galvanized or electrogalvanized coating.

The reason that the automotive companies can successfully use a coating mass in the 50 to 80 g/m2 range is because they apply additional treatments on top of the metallic coating, including a zinc phosphate coating, an electro-deposited organic-based coating, a primer, and multiple-layer finishing paint coatings. Clearly, the corrosion resistance needed to protect a car body panel for over 10 years is more than that afforded by the metallic coating alone. Application of the above coatings over the electroplated metallic layer results in a synergistic system, whose corrosion resistance is more than the sum of its individual components.

Taking Electroplate Orders
2. Commercial electroplating companies vary in their specialties. Some focus on the automotive industry, some the computer industry, and some are large enough to take on many industries and use a full range of plating technologies. In the process of writing an order, directions or specifications for the process are outlined. The electroplating company and its employees must understand the specs and fulfill the order accurately. Required delivery times, transportation methods and payment dates are agreed upon and signed for. Delayed orders or orders delivered on time or before the agreed date may be further defined by payment penalties or additional incentives.

Managing Electroplating
3. Commercial electroplating companies have the option of working with tool manufacturers and suppliers to enhance their production capabilities. From the management of the waste from electroplate processes—metals must be worked with and disposed of properly by law—to the need for hoists, material handling and purchasing, commercial electroplate businesses have a need for strong management. The scheduling and timely completion of several large orders simultaneously require close attention.

Conductive Electroplating Process
4. Traditional electroplating methods require the substrate (object) to be conductive or able to take a negative electrical charge. Once charged, the object is placed into a bath solution of salt and the chosen metal. The positive ions of the salt solution are attracted to the negatively charged object. When the two connect, the ions return to a metallic state and are now adhered to the substrate (object). Before being electroplated the object must be cleaned of any debris, oils or waxes, or the areas of dirt will not receive the process. Dimples or rough spots must be polished smooth or the plating will take on the irregularities.

Non-conductive Electroplating Process
5. Plastics have presented an interesting aspect to the plating process. Plastics are insulators, not conductors, which means charging them with a negative charge is not possible–at least not in the context of electroplating. This has given rise to all manner of processes designed to adhere metals to various forms of plastic. From using friction to developing special paints, this portion of the commercial electroplating industry is in constant discovery and development.

Application of FPC
FPCs are employed in a wide variety of applications due to the nature of their characteristics. Examples of applications for FPCs include cell-phone liquid crystal display enclosure, hinge parts, keypad, battery enclosure and interface components. FPCs are also used in optical pickup and device interfaces inside hard disk drives, digital still cameras and digital camcorders. Desired performance characteristics are: 1) wiring within small spaces; 2) wiring connection accompanied by mechanical functions within working part/device and motherboard; and 3) high density interconnect resulting from denser and narrower features.

FPCs fall into three broad categories: single-sided flexible printed wiring boards, double-sided flexible printed wiring boards and multilayer flexible printed boards. Single sided and double sided FPCs are widely employed for personal computers, optical pickup (OPU), HDD and cell phones. When calculated based on substrate area, half of these are single sided and the remainder are double sided. Multilayer FPCs are mainly used in cell phones, OPU, portable music players and DSC/DVC. However, multilayer FPCs only represent approximately 3 to 4% of the total FPC production by finished board area base. This is because there are relatively few large volume applications for multilayer FPCs.

FPC Materials
Polyimide is a crucial material which provides key features to FPCs and is used in almost all FPCs. In general, FPC is manufactured with a flexible copper clad laminate (FCCL) or one of many combinations. FCCL may be broadly grouped into the following four types: 1) material made from single polyimide and copper clad sheets connected with epoxy adhesive; 2) material laminated using polyimide adhesive (laminate); 3) material made using polyimide film and a sputtering/plating method; and 4) material made by coating polyimide varnish on copper foil (casting) followed by a curing step.

Today, the dominant films for FPC applications are 12.5 to 25 microns thick, with the industry trend being toward ever thinner materials. Two major types of copper foil are used for FCCL–electrolytic copper foil and rolled copper foil. Electrolytic copper foil is typically 18 or 12 microns in thickness, and rolled copper foil 18 microns thick. Both types of copper foils are moving to thinner dimensions. Generally, rolled copper foils demonstrate flexural properties superior to those of electrolytic copper foil. HDD applications, in particular, require high flexibility and reliability, so rolled copper foils dominate this segment. In recent years, flexural properties of electrolytic copper foil have been much improved and these foils are being increasingly used for optical pickup applications.

FPC Manufacturing Process
As mentioned previously, FPCs fall into three broad categories; single sided, double sided and multilayer. Each type of FPC has a series of required manufacturing process steps, examples of which are provided below. In particular, multilayer FPCs have a wide variety manufacturing processes based on the specifics of desired structures and performance characteristics.