Silver plating

Why silver

Silver conducts electricity and heat better than any other metal. A silver surface keeps contact resistance low at bolted joints and sliding contacts, which is why it is the usual finish for copper busbars and switchgear contacts. See silver plating on busbars.

At radio and microwave frequencies, current flows in a thin layer at the surface of a conductor. Silver plating on a brass or aluminium part puts a highly conductive metal exactly where the current flows, which lowers losses in connectors, filters, cavities and waveguide parts.

Bright, semi-bright or matte

We plate all three finishes. ASTM B700 calls them grades:

If your drawing only says "silver plate", tell us whether appearance matters and we will suggest a grade.

Purity

We plate silver of 99.0% to 99.9% purity, which covers ASTM B700 Type 1 (99.9% minimum) and Type 2 (99.0% minimum). Put the type on your drawing if your application needs a particular purity.

Tarnish

Silver reacts with sulphur compounds in the air and darkens over time. The film matters most on low-force contacts and on parts where appearance is checked at incoming inspection.

We apply a chromate-free anti-tarnish treatment after plating when you ask for it. Treated parts are checked by two tests:

• Ammonium sulphide, IS 1771. A drop of dilute ammonium sulphide on the surface must leave no black or brown mark for at least 5 minutes.
• Sodium sulphide. The part is dipped in a 10% sodium sulphide solution for 3 minutes, then hung in air for 15 minutes. It passes if no black spots appear.

ASTM B700 Class S specifies a supplementary chromate treatment, while our anti-tarnish treatment is chromate-free. If your drawing calls for Class S, tell us in your enquiry so we can discuss it.

Bimetal parts

We also silver plate bimetal parts, such as contacts with a tungsten section joined to a copper or brass body. Tell us both materials in your enquiry, and which surfaces need silver.

Underlayers

Silver is plated over a thin strike so that it bonds to the base metal. On brass, a copper or nickel underlayer stops zinc from moving into the silver. Nickel is also used under silver on parts that run hot, because it slows copper diffusion. Steel parts normally need a nickel underlayer. Aluminium needs its own pre-treatment first; see plating on aluminium.

For RF parts where passive intermodulation (PIM) matters, nickel underlayers are usually avoided because nickel is magnetic. Copper or tri-metal are common alternatives. If your drawing does not specify an underlayer, we will ask before plating.

What to put on your drawing or enquiry

• Base material and grade
• Minimum silver thickness in µm, and the surfaces it applies to
• Finish: bright, semi-bright or matte
• Underlayer, if any, and its thickness
• Areas to mask
• Whether anti-tarnish treatment is needed
• The plating specification and revision, if you work to one
• Quantity per lot and expected yearly volume

Specifications

We plate silver to ASTM B700, the standard for electroplated silver for engineering use. A full callout gives the type, grade, class and thickness, for example ASTM B700, Type 1, Grade A, Class N, 8 µm min. Silver plating to ASTM B700 explains each part of the callout and how we inspect to it.

Other specifications you may see on drawings:

Send the specification and revision with your drawing and we will confirm whether we can meet it.

Thickness measurement

Silver thickness is measured on our Fischerscope X-ray fluorescence (XRF) instrument. XRF measures coating thickness without damaging the part. The method is described in ISO 3497 and ASTM B568.