2,808
edits
Changes
no edit summary
===3.3 Attachment of Single Contact Parts===
The following segments give an overview of the usually applied attachment
technologies for contact parts to carrier components. They include mechanical
as well as brazing and welding methods used for electrical contact assemblies.
===3.3.1 Mechanical Attachment Processes===
Rivet staking and the insertion and forming of wire segments into pre-stamped
carrier parts or strips with punched holes are the most commonly used methods
for the mechanical attachment of contact materials.
Riveting (or staking) for smaller volumes of assemblies is mostly done on
mechanical, pneumatic or magnetically operated presses. For larger volumes
the staking process is integrated into a progressive die for fully automated
assembly. Rivets are fed in the correct orientation through special feeding
tracks into the staking station of the tool. To ensure a mechanically secure
attachment the rivet shank must be dimensioned correctly. As a general rule the
shank length of the rivet should be about 1/3 longer than the thickness of the
carrier material.
For switch-over contacts part of the rivet shank is formed into the secondary
rivet head. To minimize deformation of the contact blade carriers, especially thin
ones, this head forming is often performed by orbital riveting.
The insertion and forming of wire segments can be easily integrated into stamp
and bending multi-slide tooling ''(Fig. 3.7)''. Compared to the use of composite
rivets this process uses more precious contact material but for silver based
contact materials these costs or often offset by higher and more efficient
manufacturing speeds. For the more brittle Ag/SnO<sub>2</sub> materials however close
attention must be paid to the danger of crack formation.
Fig. 3.7: Direct press-insertion of wire segments
===3.3.2 Brazing Processes===
Brazing is a thermal process for the metallurgical bonding of metallic materials in
which a third metal component (brazing alloy or solder) is added. In addition a
flux or processing in a protective atmosphere is applied to eliminate oxidation of
the non-precious carrier. The melting range of the brazing alloy starts at the
beginning of the melting (solidus temperature) all the way to complete liquid
phase (liquidus temperature). This range always is below the melting points of
the two materials to be joined. During the brazing process with solubility of the
materials in each other diffusion processes are thermally activated by which
elements of the base material diffuse into the brazing alloy and elements of the
braze alloy diffuse into brazing alloy. This increases the bond strength and
therefore the mechanical stability of the brazed joint.
For attachment of contact parts to carrier base materials only brazing alloys (as
opposed to solders) are used. The reason is the higher softening temperature
and melting point as well as higher mechanical strength and electrical
conductivity of these alloys. The brazing alloys and fluxes used for electrical
contact attachment are listed in Chapter 4 in more detail. Following the most
frequently used brazing methods are described.
References to the bond quality are given according to the test methods
described in Chapter 3.4.
===3.3.2.1 Flame (or Torch) Brazing===
The simplest way to produce braze joints is the use of a gas torch fueled by a
burning gas and air or oxygen containing gas mixes. For higher production
volumes partial automation is applied. The parts to be assembled are
transported after adding the suitable amounts of brazing alloy and flux through a
series of fixed gas burners on a turntable or belt driven brazing machine.
To limit the amount of flux or gas inclusions it is recommended to slightly move
the contact tips forth and back (also known as puddeling) as soon as the
brazing alloy is liquefied. The bonded area achieved in torch brazing is typically
65 – 90% of the contact foot print depending on the size and geometry of the
contact tip.
===3.3.2.2 Furnace Brazing===
Furnace brazing is usually defined as brazing in a protective atmosphere or in
vacuum. Both processes do not require the use of fluxes.
The protective atmosphere brazing is conducted in batch operation in either
muffle or pot furnaces or as a continuous process in belt furnaces using a
reducing atmosphere of pure hydrogen (H<sub>2</sub>) or dissociated ammonia (H<sub>2</sub>,N<sub>2</sub>).
A vacuum is a very efficient protective environment for brazing but using vacuum
furnaces is more complicated and rather inefficient. Therefore this process is
only used for materials and assemblies that are sensitive to oxygen, nitrogen, or
hydrogen impurities. Not suitable for vacuum brazing are materials which
contain components with a high vapor pressure.
Parts with oxygen containing copper supports should not be brazed in reducing
atmosphere because of their susceptibility to hydrogen embrittlement. Similarly
contact tips containing silver–metal oxide should not be exposed to protective
atmospheres because a reduction of the metal oxide even in a thin contact
surface layer changes the contact properties of these materials.
===3.3.2.3 Resistance Brazing===