![]()
1 | Plating of Electrical Equipment |
1.1 ELECTROPLATING FOR CONTACT APPLICATIONS
1.1.1 SILVER PLATING
Silver (Ag) plating has many different uses in an industrial setting. It can be used as an engineering coating owing to its superior conductivity and corrosion resistance. When used in plating, silver’s conductivity allows for extensive use in electronics and semiconductor industries. It is also used extensively in the aerospace, telecommunications, military, and automotive industries.
1.1.1.1 Physical Properties of Silver Plating
Silver plating is considered to be one of the most highly conductive plated surfaces. It is widely applied to copper conductors of any kind, including wires.
In electrical power distribution, a bus bar—a thick strip of copper or aluminum—conducts electricity within a switchboard, distribution board, substation, or other electrical apparatus. Bus bars may be connected to each other and to electrical apparatus by bolted or clamp connections. Often, joints between high-current bus sections have matching surfaces that are silver plated to reduce contact resistance (CR).
Offering conductivity and corrosion resistance, silver plating creates a surface that can be soldered and exhibits low electrical resistance. It can be used as engineering coating as well as for bearing surfaces and antigalling applications. Silver plating should conform to Mil QQ-S-365D and ASTM B 700 standards, as well as to ISO 4521, “Metallic Coatings—Electrodeposited Silver and Silver Alloy Coatings for Engineering Purposes.”
Silver resists oxidation by air but is attacked by compounds containing sulfur. Industrial and urban atmospheric environments as well as certain materials contain sulfides. Under these conditions, the tarnishing of silver becomes inevitable. Tarnishing can have various degrees of severity. For more about silver corrosion, see Chapter 3.
Silver and silver-plated components can yellow slightly and sometimes do not discolor any further. The electrical conductivity of silver is not affected by a light yellowing.
In other cases, tarnishing can lead to a dark brown or black color. This discoloration can be partial or total, depending on the conditions of storage or use (finger marks, opened packing, etc.). In addition to aesthetics, the effects of excessive tarnishing at the electric level may be significant (see Section 3.3). Silver sulfides are unstable with a rise in temperature.
Technical characteristics of silver layers:
• Specific electrical resistance: 16–18.8 × 10–9 Ω
• Electrical conductivity: up to 62.5 × 106 Ω–1 m–1 (at 20°C)
• Hardness: 70–160 high voltage (HV)
• Melting point: 960°C
• Coefficient of linear expansion: 19.3 μm°C–1 m–1
1.1.1.2 Silver Plating Thickness for Electrical Applications
The thickness of plating strongly depends on the application and environment to which the silver will be exposed. It was found that at thicknesses <2 μm silver plating is porous and provides no proper protection of base metal from corrosion. For industrial applications, when electrical equipment is serving in a corrosive environment, the thickness of the silver plating should be in the range of 2–40 μm. The lowest thickness, ~2 μm, may be applied only for bolted contacts assembled in a factory and inaccessible to customers. A thin silver plating such as this is usually used in plating copper. For contacts assembled on site, the thickness of the silver plating should be no less than 5 μm. This is also the minimum thickness of the silver plating on aluminum and aluminum alloys, as well as on ferrous alloys.
1.1.1.3 The Use of a Nickel Underplate for Silver Plating
It is recommended that a nickel underplate (a minimum of 1.25 μm) be used whenever possible in plating silver. The nature of the tarnish film will change significantly if copper alloy elements from the substrate reach the surface of the silver. This can occur through mechanisms such as diffusion or corrosion creep at breaks in the silver electrodeposit. At higher temperatures, oxygen will diffuse through silver to the copper alloy interface at a relatively high rate and can lead to blistering if no nickel underplate is used. A nickel underplate will also prevent a relatively weak layer of silver–copper intermetallics from forming at temperatures greater than 150°C, which could lead to adhesion problems [1].
In most separable contact interface applications that use a nickel underplate, silver plating thickness is typically in the range of 2 μm or greater. What silver thickness is appropriate depends on application factors such as environmental severity, the time at a specific temperature, durability requirements, nickel underplate, and surface treatment. If no nickel underplate is used, a greater thickness of silver may be required to prevent substrate corrosion products from getting to the surface [2]. These higher plating thicknesses also provide more silver material between the atmosphere and the substrate material(s), possibly leading to more wear cycles before any substrate material is exposed.
Silver may be used in plating various base metals. In many cases, the application of underplating prior to plating silver is necessary to provide proper adhesion and corrosion resistivity [3]. Depending on the base metal, different underplating metals should be applied and the thickness of underplating varies (Table 1.1). The data presented in Table 1.1 are a summary of information given in multiple recourses.
On the other hand, constant evolution in a silver plating’s appearance is a sign of the presence of sulfur in the immediate vicinity of the electrical apparatus. In such conditions, antitarnishing treatment of the silver plating is recommended. For electrical applications, only those antitarnishing compounds or techniques which do not contain lacquer and do not experience discoloration should be considered. These compounds should be easily stripped without any damage, should provide antitarnishing protection, and most importantly, must not affect electrical conductivity.
TABLE 1.1
Underplating Types and Thicknesses of silver Platings for Various Base Metals
Base Metal | Underplating Type | Underplating Thickness (μm) |
Copper | None | — |
Brass (CuZn alloy) | Cu | 4 |
| Ni | 5 |
Bronze (CuSn alloy) | Cu | 4 |
Ferrous alloys | Cu | 8 |
Aluminum and aluminum alloys | Cua | 8 |
| CuSn (bronze)b | 3 |
| CuSn | 2 |
| Ni (electroplating) | 5 |
| Ni (electroless) | 5 |
It is important to note that antitarnish and passivation treatments on silver coatings should be compliant with Reduction of Hazardous Substances (RoHS) requirements, which prohibit the use of hexavalent chromium (CrIV). Chromium-free solutions exist and are appropriate for the antitarnish protection/treatment of conductive parts.
1.1.1.4 Types of Silver Platings
ASTM B700 Standard [5] establishes the requirements for electrodeposited silver coatings that may be matte, bright, or semibright finishes. Silver plating is usually employed as a solderable surface and for its electrical contact characteristics, as well as for its high electrical and thermal conductivity, thermocompression bonding, wear resistance on load-bearing surfaces, and spectral reflectivity.
Coatings shall be classified into types according to minimum purity, grade according to surface appearance (bright, semibright, or matte), and class according to whether any surface treatment has been applied. Silver coatings shall undergo preplating operations such as stress relief treatment, strike, and underplating, as well as postplating embrittlement relief.
Silver plating, therefore, may be from white matte to very bright in appearance. Corrosion resistance may depend on the base metal. Hardness vari...
