Surfaces of Metal Parts

The basis material of the housing consists mainly of steel or iron-nickel alloys. In addition to corrosion resistance, our customers require these components to have other functional characteristics, such as suitability for bonding and soldering. To ensure these characteristics, electroless plating or electroplating is required after sealing the glass.

Plating

SCHOTT Electronic Packaging provides electroless and electro-plating services for all kinds of metal parts and special alloys. In addition, we specialise in the processing of sensitive electronic parts for the automotive industry. This includes the following processes and surfaces:

Nickel Electroplating

Fig. 19 shows the basic principle of the electroplating process, illustrated by the deposition of nickel. When nickel salts are dissolved in water, nickel ions are formed. The nickel is deposited by applying an external current to the cathode. At the same time, metallic nickel is dissolved at the anode.

To make sure that the reactions always take the same direction, direct current must be used. The individual processes at the anode and the cathode always occur simultaneously. Parameters such as the type of metal salt, concentration of the plating solution, temperature, current density and pH value influence the ongoing processes at the anode and the cathode and thus determine the properties of the plating.

Chemical or Electroless Nickel Plating

Chemical or electroless plating involves the exchange of charges between the basis metal and the metal salt solution without an outside source of electricity. Electroless plating means that the electrons needed for the reduction of metal ions are supplied by a reduction agent that is present in the plating solution. This agent sets free electrons while being oxidized. The deposition process and the functional properties of the nickel plating, for example the phosphorus content of the nickel plating, are controlled by the reduction agent as well as the other components in the chemical nickel bath. The characteristics of electroless nickel plating are:

Uniform deposit of nickel on geometrically complex parts
Low coating thickness tolerance
Good suitability for soldering, bonding and welding
Corrosion resistance

Gold Plating

Uniform metal deposits are obtained from plating solutions in which the gold ion is present as a complex. Today, gold-plating solutions generally contain a potassium gold cyanide K [Cu(CN)₂] complex. To improve adhesion, thin pre-gold layers have been used with good results. The pre-gold plating solutions must be compatible with the main gold plating solution. Hard gold platings for plug connections are obtained by codepositing cobalt or nickel.

Combined Plating & Functional Properties of Metal Plating

Figure 20

Combined Plating

Well matched plating combinations can be used to alter the functional properties of the metal surface. Thus it is possible to reduce unwanted diffusion processes and to improve adhesion and corrosion resistance. Figure 20 shows typical combined platings applied by SCHOTT.

Functional Properties of Metal Platings

The plating table gives an overview of the various metal platings and the resulting functional surface properties. The thickness of the plating layer is of decisive importance and an essential criterion for subsequent application.

Plating Table

Surface	Thickness	Functional Properties
Electroless nickel plating	2 - 8 µm	Good corrosion protection Good wire bonding Good solderability with flux Uniform thickness distribution Good weldability Au / substrate depletion layer Solder / gold layer depletion layer
Nickel electroplating	3 - 12 µm	Good corrosion protection Good solderability with flux Au / substrate depletion layer Solder / gold layer depletion layer
Purest - grade gold flash	0.05 - 0.3 µm	Improved solderability Protection layer against tarnish / oxidation
Purest - grade gold	0.5 - 2.5 µm	Good alloying of semiconductors Flawless bonding of connector leads (bonding wire Au or Al) Very good solderability without aggressive flux
Hard gold	0.1 - 2.0 µm	Wear protection: contacts, plug connectors
Silver	4 -12 µm	Very good solderability without aggressive flux High electrical conductivity
Tin, tin / lead	6 -18 µm	Improved corrosion protection, Very good solderability without aggressive flux