The Electroforming Process

Electroforming is very similar to conventional electroplating as far as facilities and electrolytes are concerned. However, the controls are more stringent, because the process consumes much more time and the product must be mechanically sound and have low internal stress for dimensional acceptance. With long deposition times, high current densities at edges and surfaces closer to the anodes result in significant buildup, leading to nodules and uncontrolled growth. This results in further current density variations that can seriously affect the mechanical properties of the deposit.

In electroforming nickel, cobalt, or iron there is significant hydrogen codeposition that, if not removed, causes pits in the deposit surface. Pumping filtered electrolyte through sprays over the surfaces being electroformed will minimize the problem and aid in maintaining a smooth deposit. Areas of high current density showing excessive and rough buildup can be corrected by using nonconducting shields as baffles to improve the current distribution. Where recessed areas exist, low current density will be experienced. Undesired trace metal impurities will codeposit in such locales, leading to inferior mechanical properties and surface appearance. Auxiliary or bipolar anodes may be necessary to overcome the low-current problem.

Electroforming solutions may be used with one or more additives to control stress, brightness, leveling (smoothness), and microstructure. When mechanical properties (including high ductility) or good electrical or thermal conductivity are important in the deposit, it is advisable to use nonadditive electrolytes. Because most additives are organic compounds, they are subject to decomposition if the deposit is subjected to elevated temperatures.

Stress-reducing agents are often used in nickel, iron, and cobalt plating baths to produce neutral or compressive residual stresses. Such agents are usually grain-refining compounds also. These deposits are generally harder, have higher yield strength, and exhibit less ductility than conventional deposits of the same metal. Advantages in neutral or compressively stressed deposits are ease of removal of electroforms from mandrels and inhibition of growth of cracks in deposits should they occur from impact. A problem with stress reducers in nickel is that sulfur codeposits form when the agent reacts at the cathode, because most stress reducers contain sulfur. Brazing or welding such deposits causes sulfur to react with nickel to form a nickel sulfide liquidus in the range of 483 °C (901 °F) to about 650 °C (1200 °F). This leads to the effect known as "hot shortness" experienced in wrought nickels. Such deposits can be alloyed with as little as 1500 ppm Mg to counter the problem.

Copper Electroforming. Acid sulfate electrolytes are the industry standard for copper electroforming. Additives are usually employed for grain refining, leveling, and brightening. The mechanical property improvements achieved are mostly a result of grain refining. Organic compounds capable of reducing copper oxides at the cathode may also be used to produce an oxygen-free, high-conductivity copper equivalent (<10 ppm oxygen). Decomposition products from copper bath additives will codeposit to degrade ductility. Without additives, acid sulfate baths produce copper with grain size increasing proportionally to deposit thickness. Intergranular voids are created that seriously degrade mechanical properties. A plating technique known as periodic current reversal will promote deposition of a copper deposit having uniform grain size and excellent mechanical properties for thicknesses of 0.5 cm (0.2 in.) or greater. This procedure requires plating in a conventional direction for a given period of time, followed by a reversal of current direction for a lesser period of time. Although the process results in a slow rate of deposition, the benefits of good mechanical properties, relatively smooth deposit surfaces, and ability to plate dense, thick deposits make this technique most useful.

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