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Plating Thickness And UniformityWhat is the difference between an average, range or minimum plating thickness specification and how do I properly call out my thickness requirement on a print? There are five (5) methods most commonly used for defining plating thicknesses. They are average, range, minimum, maximum and customer negotiated. The differences between each are very distinct. The definition of each is provided below: Average An Average deposit thickness requirement is given as a single number i.e. “nickel plate 0.0002”.” An Average deposit thickness provides the target that the mean of multiple thickness readings should be within a 75uin window. In the example given this would be 0.0002” ± 0.000075” (200±75uin). However, it is at the plater’s desecration to choose the location where to measure the product unless the measuring location is indicated on the print. This is a very important distinction as the plating thickness on electroplated products can vary considerably from point to point. The thickness distribution is heavily dependent on part geometry. On products that are drawn out such as rods or pins there can be an extremely wide range of thickness. However, the thickness distribution can be quite minimal on spherical products such as ball bearings. Range A Range deposit thickness requirement is given as a range of numbers i.e. “nickel plate 0.0001-0.0003”.” The range provided is the range within which the mean of the thickness measurements must lie. Similarly to Average thicknesses, it is at the plater’s discretion where to measure the product unless the measuring location is indicated on the print. If the range is ≤ 0.00015” (150uin) it is treated as a single average with the mid point of the range being the target average and the definition of average thickness above applies. Minimum A Minimum deposit thickness requirement is identified with a single number with the word minimum i.e. “nickel plate 0.0002” minimum or min.” A Minimum thickness requirement is defined as all readings measured on the significant surfaces must be greater than the thickness indicated. In the example provided the thickness measured on any significant surface would have to be greater than 0.0002” (200uin). Significant surfaces are generally defined as any portion of a part that can be touched by a 0.75” diameter sphere. However, the manufacturer of a product can identify significant surfaces as required on the blueprint of the product. Maximum A Maximum deposit thickness requirement is given as a single number with the word maximum i.e. “nickel plate 0.0002” maximum or max.” A Maximum thickness requirement is defined as all readings measured on significant surfaces must be less than the thickness indicated. In the example provided the thickness measured on any significant surface would have to be less than 0.0002” (200uin). If a significant surface is not defined all readings shall be less than 0.0002” (200uin) on any area of the part that can be touched by a 0.75” diameter sphere. Customer Negotiated Specifications When a customer has a defined specification or sites commonly referenced specifications i.e. ASTM, MIL, AMS etc. they shall be followed unless both the specification and thickness requirement are provide on the print. In this case, the thickness otherwise specified on the print supersedes the spec referenced. For example, specification QQ-N-290 requires minimum plating thickness requirements based on the grade of the plating (i.e. in QQ-N-290 Grade G is 0.0002” Minimum). However, if a customer were to indicate on a print, “Nickel Plate per QQ-N-290 0.0001”-0.0003”,” this is understood as a range plating thickness requirement because the customer has otherwise specified the thickness desired without indicating the grade. If a customer desires the thicknesses defined within the spec, the specific grades should be cited, i.e. “Nickel Plate per QQ-N-290, Grade G.” Customer-unique specifications can always be developed based on the wishes of our customers but they must be known upfront. Unique specifications developed by a customer will be maintained in our Quality Control department and will be referenced on the appropriate process routing instructions. This document will be the ruling document as to significant surfaces, thickness interpretation and target thicknesses desired. Why is the plating thickness on my product non-uniform? Electroplating is an electrochemical process wherein the metal deposited on a substrate is supplied through the bath chemicals, containing the metal of interest under the application of a direct electric voltage across the anodic and cathodic sides of the plating cell. The generated current through the bath is an ionic current flow, and the current in the external circuit is electronic. Both are direct currents. Direct current always seeks the path of least resistance from the anode (metal source) to the cathode (work piece). The geometric shape of the work piece can shorten the distance to the anode, and thus decrease the electrical resistance (the resistance of the plating solution is directly proportional to the linear distance between anode and cathode). The least resistant path will carry more current and thus deposit more metal. The classic example of this phenomenon is a sharply pointed object (i.e. a rod) with the ends pointing at the anodes. The ends will have dramatically more plating than the center of the piece. Often the shape of a part will restrict the ionic movement over its surface and thus, metal deposition. Dead end holes limit effective solution exchange within the hole, that is they have very little plating if any within the hole. Electrodeposits are notorious for their nonuniformity. Awareness of this property helps the design engineer to build his product such that functional surfaces will not receive diminished coating thickness. Reference the Design for Plating Guide for additional information on how part design affects electrolytic plating distribution. (back to top) Where should I measure the deposit thickness? Thickness of electrodeposited coatings varies from point to point on the surfaces of a product. The thickness is less in interior corners and holes. Such surfaces are normally exempt from the thickness requirement. If the full thickness is required on these surfaces, the electroplater will have to use special techniques that probably will increase the cost of the process. Because of deposit thickness variability, it is necessary to identify the key operational area on the coated article where the deposit must function. This is defined as the "significant surface." It is defined as: "that portion of the surface of a coated article at which the coating is required to meet all of the requirements of the coating specification for that article; significant surfaces are usually those that are essential to the serviceability or function of the article, or that can be a source of corrosion products or tarnish films that interfere with the function or desirable appearance of the article; significant surfaces shall be indicated on the drawings of the parts or by the provision of suitably marked samples." |
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