The silvering processo for decorative purposes

The silvering process is widely used, as well as for decorative purposes, in many applications in the electronic, space, defense, microelectronics and telecommunications sectors in general.
The use of silver is particularly indicated in high-frequency transmissions
 (for example waveguides), thanks to its high conductivity which minimizes the current leakage offering an excellent transmission of the electrical signal, from the RF to the HF the order of tens of GHz.
The silver is sensitive to the presence of sulfides in the air and tends to blacken over time (a phenomenon which is known as "tarnishing"; that, in addition to alter the aesthetic aspect, increases the contact resistance: The "tarnishing"; phenomenon can be inhibited by a final passivation which largely exceeds the test QQ-S-365, it does not affect either the weldability or the electrical properties and conforms to the European RoHS directive.

Power filters in coaxial cavity for spatial applications, UHF band - Thales Alenia Space Italy

Power filters in coaxial cavity for spatial applications, UHF band - Thales Alenia Space Italy

High Power coaxial cavity filters for space applications (UHF) – Thales Alenia Space Italy

 

Choice of the substrate or barrier layer:

  • When the corrosion resistance is an important requirement it is necessary to apply a substrate of electrolytic nickel or chemical nickel (preferable for the greater uniformity of thickness), with a thickness of 10 microns minimum.
  • The silver on copper and its alloys can be deposited directly, without sublayers, as long as the treated articles must not be soldered (brazed) or have an operating temperature exceeding 149 ° C (there is a loss of adhesion due to the formation of a light intermetallic copper-silver layer); otherwise you need a nickel barrier layer of the order of 3-5 microns.
  • On aluminum and its alloys the ideal substrate is the chemical nickel.
  • On ferrous alloys, zinc and its alloys must be provided a first layer of copper followed by nickel.
 


Parts for telecommunications equipment - silver plating on brass base

Parts for telecommunications equipment - silver plating on brass base

 

 

Main silver features are:

  • Density: 5.10 g / cm3
  • Melting point: 962 ° C
  • Good brazeability
  • Excellent electrical and thermal conductivity (the highest among all the metals)
  • Low contact resistance
  • Good corrosion resistance (depending on the base material, the intermediate layer and the thickness)
  • Good anti-seizure characteristics (for static seals, bushings, ...)
  • Reflectance to visible light, the highest among all metals (optimized value with gloss finish)

 

 

Two types of silver are available:


> SEMI-GLOSS SILVER

Telecommunications sector - Tuning silver

Telecommunications sector - Tuning silver
  Electrolytic deposit of high purity silver obtained by galvanic bath with special additives to refine the grain and give it an appearance between the semi-gloss and gloss without negative interference in the RF-HF applications. 

This treatment is widely used in the telecommunications sector, in military and space applications and wherever high electrical performance are required, combined with good wear resistance.


This type of silvering meets the requirements of standards:

      • QQ-S-365 (Type II - Grade A or B
      • ASTM B700-97 (Tipo I – Grado D – Classe N o S)

 


Characteristics: 

  • Semi-glossy / polished appearance function according to the type of finish of the surface that must be treated and the applied thickness;
  • Hardness: 90 ÷ 130 (Knoop)
  • Purity: 99.9%
  • Good wear resistance (for example sliding contacts, with minimum recommended thickness of 20- 25 microns)
  • Highly polished for mirror finish

 

>PURE/MATT SILVER

Electrolytic deposit of silver obtained by galvanic bath free of any type of additive, which guarantees the absolute absence of co-deposition of organic substances: this treatment is particularly suitable for vacuum applications and in the semiconductor industry. The use of special current rectifiers allows to refine the grain of the deposit and to improve the thickness distribution.

This type of silvering meets the requirements of standards:

      • QQ-S-365 (Type I - Grade A or B)
      • ASTM B700-97 (Type I - Grade A - N or S Class)

Special features:

  • White-opaque appearance (or slightly semi-gloss depending on the type of finish of the surface that must be treated and the applied thickness)
  • Hardness: max 90 (Knoop)
  • Purity: 99.9% min
  • Good weldability for "bonding"; (thermal compression)
  • High ductility and absence of internal tensions which make it suitable for high thicknesses (up to 150 microns).

 

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