Plating on aluminium

Why aluminium needs its own preparation

Aluminium is light, easy to machine and cast, and conducts electricity well, which is why it is used for RF filter housings, cavities, lids and many electrical parts. Its surface is the difficulty. Aluminium forms a thin, hard oxide film within seconds of meeting air, and plating does not bond to that film. Aluminium also reacts quickly with most plating solutions.

So aluminium parts go through their own pre-treatment before the first layer of metal is plated, and the rest of the coating is built on that.

How aluminium is prepared for plating

The usual method is the zincate process:

Clean and etch. Oils are removed, and a light etch takes off the natural oxide.
Desmut. The etch leaves alloying elements such as silicon, copper and magnesium on the surface as a dark film, which is dissolved off.
Zincate. The part is dipped in a zincate solution, which replaces the oxide with a very thin layer of zinc.
Strike. A first layer of electroless nickel or copper is plated onto the zinc, and the specified finish is plated over it.

Finishes we plate on aluminium

Finish	Usual build-up on aluminium	Typical parts
Silver	Over copper or electroless nickel	RF filter housings, cavities, lids, busbars
Gold	Over nickel or electroless nickel	Connector bodies and RF parts
Electroless nickel	Directly after zincate	Housings and lids, and as a base for silver or gold
Tin	Over copper or nickel	Busbars, switchgear parts, solderable areas
Nickel	Over copper or electroless nickel	Parts needing wear and corrosion resistance
Tri-metal	Over copper	RF connector parts
Copper	Directly after zincate	Underlayer, and conductive coating

Each finish has its own page with thicknesses, variants and specifications: silver, gold, electroless nickel, tin, nickel, tri-metal and copper. Electroless nickel on aluminium has its own page: electroless nickel plating on aluminium.

RF parts and passive intermodulation

On RF filters, diplexers and cavities, magnetic layers in the coating can cause passive intermodulation (PIM). Ordinary nickel is magnetic, so RF parts are usually built up with copper under the silver, or with high-phosphorus electroless nickel, which is non-magnetic as plated. If your part has a PIM requirement, put it on the drawing.

Wrought, cast and die-cast aluminium

Machined parts from wrought alloys usually plate well. Cast and die-cast parts contain more silicon and can have fine porosity, which can trap solution and cause blisters after plating. They need an adjusted etch and careful rinsing. Tell us the alloy and how the part is made, so we can plan the pre-treatment.

What to put on your drawing or enquiry

Aluminium alloy and temper, and whether the part is machined, cast or die-cast
Final finish and its thickness in µm
Underlayer, if your drawing specifies one, and any PIM or non-magnetic requirement
Areas to mask, such as threads, tapped holes or mating faces
Quantity per lot and expected yearly volume

Thickness measurement

Coating thickness on aluminium parts is measured in-house on our Fischerscope XRF instrument, which reads up to three layers in one measurement, for example silver over copper. We supply an XRF thickness report with each batch.