ASTM B488-01 Standard Specification for Gold Electroplating Coatings for Engineering Purposes

The American Society for Testing and Materials (ASTM) standard "ASTM B488-01 Standard Specification for Gold Electroplated Coatings for Engineering Purposes" describes the requirements for electroplated gold coatings containing at least 99,00 mass percent gold and used for engineering applications. Specifically excluded from this standard are autocatalytic, immersion, and vapor deposition gold coatings.

Gold coatings conforming to this standard are used for their corrosion and tarnish resistance (including resistance to fretting corrosion and catalytic polymerization), connectivity, low and stable contact resistance, solderability, and infrared reflectivity. Various types of coatings that differ in gold purity and hardness are covered by this standard.

For the purposes of this standard, the terms and definitions given in ASTM B374 or ASTM B542 apply. In addition,

Important surfaces are those surfaces that are normally visible (directly or by reflection) or are essential to the usability or function of the article. They may be the source of corrosion products or tarnish films that affect the functionality or desired appearance of the article. Important surfaces shall be shown on drawings of parts or by the provision of appropriately marked samples.
Undercoat refers to a layer of metallic coating between the base metal or substrate and the top metallic coating. The thickness of the undercoat is usually greater than 1 µm (as opposed to toast or flashing, which are thinner).

A coating is specified by a combination of:

Type characterizing minimum impurity
Code specifying Knoop hardness
A number indicating the thickness in micrometers

Purity, the publication of this standard establishes a new ASTM type designation identical to the original standard (MILG-45204) established for electrodeposited gold and outlined in the table within the standard. It is generally accepted that a gold purity of 99,7 percent is defined as not more than 0,3 percent of the total coded metallic constituents, excluding potassium and sodium. Similarly, 99,9 percent purity means not more than 0,1 percent of the total coded metallic impurities, excluding potassium and sodium. Gold purity is calculated by subtracting the sum of all nongold metals by mass, excluding potassium and sodium, from 100 percent. The presence of carbon, nitrogen, hydrogen, sodium, and potassium in the electrode bed is not included in the calculation.

Hardness values are specified by ASTM code. Military standard class definition has been added for reference. ASTM code B is 45204-91 hardness level in accordance with MIL-G-129 standard.

The thickness is specified by an Arabic numeral indicating the minimum linear thickness in microns. Examples of commonly specified thicknesses are shown in the table provided in the standard.

The coating requirements are determined as follows:

Nature of the coating: The gold deposit must meet the following appropriate purity requirements:
- Type I coatings shall contain at least 99,70 mass percent gold, excluding potassium, sodium, carbon and nitrogen. For Type I Code A (soft) coatings only, metallic hardening agents such as nickel, cobalt or iron shall be present together in less than 0,05 mass percent, and none of these three elements shall be present together in an amount greater than 0,03 mass percent. All other metals except potassium and sodium are considered impurities and shall not be present together in an amount greater than 0,05 mass percent. For Type I Code B and C coatings, metallic hardening agents such as nickel, cobalt or iron shall be present together in an amount equal to or less than 0,3 mass percent. All other metals are considered impurities and shall not be present together in an amount greater than 0,05 mass percent, and 0,1 mass percent. Gold purity is calculated by determining the content of all nongold metals as a percentage (excluding potassium and sodium) and subtracting this value from 100 percent.
- Type II coatings must contain at least 99,00 mass percent gold, excluding potassium, sodium, carbon, and nitrogen. Type II coatings may contain metallic hardening agents such as nickel, cobalt, or iron, which may be present in amounts greater than 0,3 mass percent. All other metals, except potassium and nitrogen, are considered impurities and must not each be present in amounts greater than 0,1 mass percent. Gold purity is calculated by determining the mass percent content of all non-gold metals (excluding potassium and sodium) and subtracting this value from 100 percent.
- Type III coatings must contain at least 99,90 mass percent gold, excluding potassium, sodium, carbon and nitrogen. Individual metal impurities must not be present in an amount greater than 0,04 mass percent. The combination of iron, nickel and cobalt must be less than 0,05 mass percent and none of these three elements must be present in an amount greater than 0,03 mass percent. All non-gold metals except potassium and sodium are considered impurities and must not be present in an amount greater than 0,1 mass percent. Gold purity is calculated by determining the content of all non-gold metals as a mass percent (excluding potassium and sodium) and subtracting this value from 100 percent.
Hardness: Gold plating should have a Knoop hardness within the specified Koop range when properly tested.
Appearance: Gold coatings should be consistent, continuous and have a uniform appearance as far as the nature of the base metal and good commercial practices permit.
Thickness: Unless otherwise specified, the gold plating shall have a thickness conforming to this standard on the major surfaces. The thickness of the gold plating shall be equal to or greater than the specified thickness over the major surface. When plated parts are used in mating applications, the maximum thickness shall not exceed the tolerance specified on the engineering drawing for the part. It is customary for plated coatings to include the allowance in the part tolerance.
Adhesion: Gold coatings should adhere to the substrate when tested with one of the procedures outlined here.
Integrity of coating: Coatings should be free from visible mechanical damage and similar gross defects when viewed at magnifications up to 10x. For some applications this requirement can be relaxed to allow a small number (per unit area) of such defects, especially if they are outside or around critical surfaces. Almost all coated electroplatings contain some porosity. The amount of porosity that can be tolerated in a coating depends on the severity of the environment the article is likely to encounter in service or storage. If pores are few in number or away from critical surfaces, their presence can generally be tolerated.

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