Electroplating

Electroplating,Factors in Product Design That Affect Electroplating Introduction to Electroplating This subchapter on electroplating contains information on the design of products for cost-effective, high-quality plating; the advantages and disadvantages of using various substrates: selection of the proper plating materials and processes; and the use of electro-less plating. The various processes and chemistries are definitized, with sufficient detail to permit understanding and application of the many choices. However, consulting with the plater during design of the product and process is highly recommended. 19.3.1 Factors in Product Design That Affect Electroplating Choosing the Right Substrate By choosing the right substrate to produce a part, the design engineer can lower final product cost and help protect the environment by reducing the generation of pollutants and wastes. In general, the fewer processing steps that a substrate requires to achieve the final appearance, the less expensive it will be to process. Also, by avoiding certain metals that require strong or unusual chemicals/acids, the design engineer can help generate less waste (also lowering costs). The designer must be aware of the difficulties imposed on the electroplater by intricate designs and use of metal alloys in the manufacturing of parts that are difficult to plate. The electroplating process does not coat a part uniformly, due to concentrations of electricity that occur at corners and sharp edges. The plater can alleviate this to some extent through use of conforming anodes and shielding. However, this normally drives up costs and increases pollution loading. Exotic alloys create problems for electroplaters in properly cleaning the parts prior to plating (see Subchapter 19.2). The designer should minimize/avoid use of alloys such as stainless steels, inconel. hastalloy, titanium, and combinations of metals, as these are extremely difficult to process for electroplating. If such metals or combinations of metals must be used, the designer should locate and consult with the plater before completing his/her design in order to minimize costs and difficulties. (See also the subsection Design Parameters that Affect Plating Uniformity, p. 892.) The following are typically plated metals and alloys, problems associated with processing them, and the possible reasons for specifying them. Zinc/Zinc Die Castings Zinc die casting is an inexpensive method of manufacturing a part from a low cost basis metal. The die castings are deflashed, polished, and buffed prior to cleaning and plating, adding significant labor costs to the product. The die casting process must be carefully designed (especially the "gating") and controlled (temperature and pressure) to minimize air entrapment, which results in excessive casting porosity. Castings with excessive surface porosity cannot be successfully plated (they will evidence blistering and peeling). Zinc die castings are typically plated first with a cyanide copper "strike." This initial plated layer promotes adhesion of subsequent plated metals. Noncyanide solutions for initially plating zinc die castings are available, but are only in the "trial" phase at this time. If a presently die-casted zinc part can be manufactured using steel, the overall cost of the finished product will normally be reduced due to elimination of manual labor involved in polishing/buffing and increased ease of cleaning and plating. Aluminum/Aluminum Die Castings Numerous aluminum alloys can be used in manufacturing. All aluminum alloys are highly "reactive" when cleaned, forming surface oxides immediately upon contact with air. These surface oxides inhibit adhesion of electroplated metals. Therefore, aluminum alloys are typically cleaned and processed in a manner that will form an initial immersion coating that does not form oxides upon contact with the air. Two such processes are the zincate process and a proprietary process called the Alstan (trademark of Atotech USA) process. These processes form thin layers of zinc (zincate) or tin (Alstan) on the surface of the cleaned aluminum. The electroplated coating is applied over the zinc or tin immersion coating. There are other methods of obtaining adhesion of electroplate on aluminum, including anodizing the aluminum prior to plating, but the immersion coatings are most commonly used and are the least expensive to apply. Problems with aluminum die castings are similar to those discussed above for zinc die castings. If a part can be manufactured from steel instead of aluminum die cast, significant savings can be realized in finishing the part. Stainless Steels All stainless steels have a tendency to form surface oxides upon contact with the air, and therefore pose similar problems to those discussed above for aluminum. The method of processing to obtain adhesion, however, differs in that stainless steel parts are cleaned and first plated in either a sulfamate nickel strike solution or a Woods nickel strike solution. These solutions are specially formulated to created high volumes of hydrogen gas, which removes surface oxides while simultaneously depositing a thin layer of nickel, which can then be plated with other metals. Because stainless steel has much lower conductivity that most other plated metals, the designed

part should allow for a larger number of electrical contact points (see Subsection 19.6.2). Common Steel Common, mild, low-carbon steel is the most easily cleaned and electroplated substrate. However, these steels can be subjected to machining, stamping, drilling, and heat treating, which can significantly alter the ease of plating. Manufacturing operations that use lubricants and corrosioninhibiting fluids should use only those types of fluids that are compatible with the electroplating process. Especially to be avoided are any lubricants/fluids that contain silicones. Corrosion-inhibiting products can also create problems for the plater. Products such as calcium stearate or calcium sulfonate are not easily removed and lead to adhesion problems. Heat-treating steel causes the formation of heat-treat scale, which must be removed through mechanical means to prevent adhesion problems after plating. Heat treating can also alter the structure of the steel, making it much more difficult to prepare for plating, especially case hardening. Case-hardened alloys must be cleaned in a double cycle or plated with a Woods nickel or sulfamate nickel strike in order to obtain adhesion. Case-hardened steels and steels hardened above Rockwell C 40 should be stress relieved at 400°F for l hr prior to processing for plating. Avoid use of leaded steels, as such alloys cause considerable adhesion problems for a plater. Leaded steel alloys must be pickled in acid containing fluoride salts to remove surface oxides. Cast Iron Cast iron can contain enough graphite (carbon) to be impossible to plate in a cyanide zinc plating solution (due to low hydrogen overvoltage potential on carbon). Such parts will require a copper deposit first, or an acid zinc plating process can be used. Cast iron also contains silicon as an alloying element. These alloys will require acid pickling in acid containing fluoride salts or hydrofluoric acid. Copper/Copper Alloys Copper and copper alloys develop an adherent tarnish over prolonged storage that is extremely difficult to remove. Tarnished parts are typically "bright dipped" to remove the oxide/tarnish present on these alloys. The bright dipping process produces a significant amount of toxic nitric oxide fumes, and the solution itself must be periodically disposed of, increasing the cost of plating such parts. Parts manufactured from copper/copper alloys should be coated with a tarnish-preventative material such as a chromate or organic antitarnish product. Heat-treated copper alloys should be carefully treated to create as little oxide scale as possible. Tellurium (0.5%) is added to copper to increase machinability. Beryllium (0.5 - 2.5%) is added to yield high hardness upon heat treatment. Avoid manufacturing parts from tellurium or beryllium containing copper alloys if at all possible. as these alloys are extremely difficult to clean and prepare for plating. Brass often has lead added to enhance machining properties. Leaded brass must be processed through fluoboric acid in the electroplating line, to obtain adhesion. Bronze alloys often contain silicon or aluminum. These alloys require the plater to use a nitric acid dip containing ammonium bifluoride. to promote adhesion of plated deposits. Titanium Parts manufactured from titanium alloys require an electroless nickel deposit that is diffused at 800°F for 1 hr, in order to obtain adhesion. Few platers are equipped to prepare and plate onto titanium parts, so a premium can be expected on the price for plating them.Nickel Silver Nickel silver actually contains no silver (copper 55-66%, nickel 15-30%, balance zinc). Some alloys of nickel silver contain high concentrations of lead. Such alloys must be pickled in acid containing fluorides to remove lead oxide from the surface. Special treatments, such as cathodic charging in sulfuric acid or a Woods nickel strike are also commonly required. Powder Metallurgy Products Avoid the plating of parts fabricated by the powder metallurgy process, as such parts yield high numbers of rejects due to adhesion problems originating from high levels of porosity, which traps processing solutions. If powder metallurgy must be used, the parts should be vacuum impregnated prior to processing for plating. Design Parameters That Affect Plating Uniformity A part that is to be electroplated should be designed to allow for uniform plating thickness over the part geometry, low liquid retention upon withdrawal from the processing tanks. ease of racking, good electrical contact between the part and the rack, and ease of handling. Following are some guidelines in this regard. Part Geometry Electricity concentrates along sharp edges, ribs, corners, and other points. Conversely, recesses, deep troughs, slots, and other depressed areas are deficient in electric current. The amount (thickness) of plating obtained is directly proportional to the amount of electricity that a specific area of the part obtains during the plating process and thickness is normally directly related to corrosion resistance. One can therefore expect excess thickness at high current densities, and

thickness deficiency (poor corrosion resistance) at low current densities. Sharp edges and points should be reduced as much as practical. Gently curving surfaces, grooves, and serrations yield more uniform plating. Edges should be beveled/rounded to a radius of at least 1/64 in. (0.5 mm), 1/32 in. (I mm) being preferred. The inside/outside edges of flat-bottomed grooves should be rounded off and their depth should be limited to 33% of their width. Avoid V-shaped grooves. Other indentations should also be limited to a depth of 33% of their width. The depth of "blind holes" (holes that do not go all the way through the part) should also be limited to 33% of their width. Avoid blind holes with very small diameters [less than >/4 in. (6 mm)|. Apply countersinks to drilled and threaded holes. The height of fins and other projections should be reduced as much as possible, with rounding at the base and tips by at minimum 1/16 in. (1.5 mm) radius. Avoid manufacturing a part from different types of metals or metals with distinctly different treatments. For example, if a steel stamping is made from mild steel with a case-hardened steel rod attached, during processing, the mild steel will need to be treated with harsh chemicals to be able to plate the rod, and severe etching of the mild steel or poor plating on the rod will result. Influence of Manufacturing Processes on Electroplating Certain methods of manufacturing a product can cause trouble for a plater, which translates to higher costs, rejects, and waste generation. Welding Welding should be performed using welding material that matches the basis metal as closely as possible. The weld must be pore free. Avoid lap welding, unless the lap can be completely pore free. Pores in welds and porous laps will trap processing liquids. contaminating the process solutions and yielding adhesion and appearance problems. Parts that are welded at high temperatures can develop a scale that will require blasting or pickling to remove. Weld spatter must be avoided, as these spots will have a reduced amount of corrosion resistance. Spatter should be removed by grinding/sanding. Brazing Brazing yields the same basic problems as welding, except that the creation of dissimilar metals cannot be avoided. In such cases, the plater must be informed as to what was used to braze the components together, so that he or she can adjust the preparatory cycle accordingly or make other modifications to the cleaning of the pari. Soldering Soldering should be performed after plating as much as possible. If soldering must be performed before plating, the operation should be carefully controlled in terms of temperature and fluxing, to yield as pore free a joint as possible. Avoid use of silver solder, if possible, as this requires extra preparatory steps by the plater. Remove excessive amounts of flux from the parts before sending them to the plater. Drawing The drawing operation utilizes lubricants that can be either easy or difficult for the plater to remove. Consult with the plater you intend to use to determine which cause problems. Drawing at the wrong speed, with poorly maintained equipment, or without adequate lubrication can create surface fissures that trap plating chemicals or can force lubricant deep into surface defects, yielding blistering. Annealing Annealing at the wrong temperature or in the wrong atmosphere can leave oxides on the surface that are very difficult to remove. Case Hardening Case hardening yields a high-carbon surface that yields large amounts of smut upon cleaning and pickling. Careful control of the carbon content of the case to the minimum that will still yield the desired case will reduce plating problems. Shot Peening Use of shot media that leave as little residue on the surface of the part is very important to the plater. Glass beads leave residual glass on the surface that can be very difficult to remove. Cast iron can leave graphite residues on the surface, which can be difficult to remove. Steel, ceramic, or stainless steel shot usually results in a surface that is easier for the plater to prepare for plating.