2,808
edits
Changes
no edit summary
of deposits which makes them in most cases not suitable for contact
applications.
Table 7.7: Comparison of different coating processes
The main differences between the coating processes are found in the coating
materials and thickness. While mechanical cladding and sputtering allow the
use of almost any alloy material, electroplating processes are limited to metals
and selected alloys such as for example high-carat gold alloys with up to .3
wt% Co or Ni. Electroplated and sputtered surface layers have a technological
and economical upper thickness limit of about 10μm. While mechanical cladding
has a minimum thickness of approx. 1 μm, electroplating and sputtering
can also be easily applied in very thin layers down to the range of 0.1 μm.
The properties of the coatings are closely related to the coating process.
Starting materials for cladding and sputtering targets precious metals and their
alloys which in the case of gold and palladium based materials are vacuum
melted and therefore exhibit a very high purity. During electroplating, depending
on the type of electrolytes and the deposition parameters, some electrolyte
components such as carbon and organic compounds are incorporated into
the precious metal coating. Layers deposited from the gaseous phase however
are very pure.
===7.4 Hot (-Dipped) Tin Coated Strip Materials===
During hot-dip tinning pre-treated strip materials are coated with pure tin or tin
alloys from a liquid solder metal. During overall (or all-around) tinning the
stripsthrough a liquid metal melt. For strip tinning rotating rolls are partially
immersed into a liquid tin melt and transport the liquid onto the strip which is
guided above them. Through special wiping and gas blowing procedures the
deposited tin layer can be held within tight tolerances. Hot tinning is performed
directly onto the base substrate material without any pre-coating with either
copper or nickel. Special cast-on processes or the melting of solder foils onto
the carrier strip allow also the production of thicker solder layers
(> 15 μm).
The main advantage of hot tinning of copper and copper alloys as compared to
tin electroplating is the formation of an inter-metallic copper-tin phase (Cu<sub>3</sub>Sn,
Cu<sub>6</sub> Sn<sub>5</sub>) at the boundary between the carrier material and the tin layer. This thin
(0.3 – 0.5 μm) intermediate layer, which is formed during the thermal tinning
process, is rather hard and reduces in connectors the frictional force and
mechanical wear. Tin coatings produced by hot tinning have a good adhesion to
the substrate material and do not tend to tin whisker formation.
A special process of hot tinning is the “Reflow” process. After depositing a tin
coating by electroplating the layer is short-time melted in a continuous process.
The properties of these reflow tin coatings are comparable to those created by
conventional hot tinning.
Besides overall tin coating of strip material the hot tinning can also be applied in
the form of single or multiple stripes on both sides of a continuous substrate
strip.
*'''Typical examples of hot tinned strip materials'''
bild
*'''Materials'''
Coating materials: Pure tin, tin alloys
Substrate materials: Cu, CuZn, CuNiZn, CuSn, CuBe and others
*'''Dimensions and Tolerances'''
Width of tinning: > 3 + 1 mm
Thickness of tinning: 1 - 15 μm
Tolerances (thickness): + 1 - +3 μm depending on tin thickness
*'''Quality Criteria'''
Mechanical strength and dimensional tolerances of hot tinned strips are closely
related to the standard for Cu and Cu alloy strips according to DIN EN 1652 and
DIN EN 1654.
Quality criteria for the actual tin coatings are usually agreed upon separately.
7.5 Contact Lubricants
By using suitable lubricants the mechanical wear and frictional oxidation of
sliding and connector contacts can be substantially reduced. In the electrical
contact technology solid, as well as high and low viscosity liquid lubricants are
used.
Contact lubricants have to fulfill a multitude of technical requirements:
£ <span style=" color: #151616;">They must wet the contact surface well; after the sliding operation the lubrication film must close itself again, i.e. mechanical interruptions to heal</span></p><p class="s6">£ <span style=" color: #151616;">They should not transform into resins, not evaporate, and not act as dust collectors</span></p><p class="s6">£ <span style=" color: #151616;">The lubricants should not dissolve plastics, they should not</span></p><p>be corrosive to non-precious metals or initiate cracking through stress corrosion of plastic components</p><p class="s6">£ <span style=" color: #151616;">The specific electrical resistance of the lubricants cannot be so low that wetted plastic surfaces lose their isolating properties</span></p><p class="s6">£ <span style=" color: #151616;">The lubricant layer should not increase the contact resistance; the wear reducing properties of the lubricant film should keep the contact resistance low and consistent over the longest possible operation time
