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Created page with "Besides the selection of the most suitable contact materials the design and type of attachment is critical for the reliability and electrical life of contact components for el..."
Besides the selection of the most suitable contact materials the design and type
of attachment is critical for the reliability and electrical life of contact
components for electromechanical switching devices. The materials saving use
of high cost precious metals and the most economic manufacturing method for
contact parts are most important factors.
There are two basic manufacturing solutions available: One can start with single
contact parts such as contact rivets or tips which then are attached
mechanically or by brazing or welding resp. to carrier parts. In the second case
a base material coated or clad with the precious contact metal - for special
applications also clad with another non-precious material – in the form of strips
or profiles is manufactured as a semi-finished pre-material from which the
contact components are then stamped and formed. Besides mechanical
cladding other processes such as electroplating and deposition from the gas
phase are utilized.
Which of the following manufacturing processes is finally chosen depends on
the final application of the contact components in their respective switching
devices or electromechanical components. Other considerations such as the
required number of electrical operations, the most economical use of precious
metals and the anticipated volumes of parts are also important for the process
selection.
===3.1 Manufacturing of Single Contact Parts===
The group of single contacts includes contact rivets, contact tips, and formed
parts such as weld buttons. Contact spheres (or balls) are today rarely used
because of economical considerations.
===3.1.1 Contact Rivets===
===3.1.1.1 Solid Contact Rivets===
Solid contact rivets are the oldest utilized contact parts. Their manufacturing
requires a ductile contact material and is done without scrap on fully automated
special cold heading machines. A wire slug is cut off and the rivet head is
formed by pressing and hammering. This way contact rivets with various head
configurations such as flat, domed, spherical, or pointed can be manufactured
depending on the final application and switch or relay design.
*Typical Contact Shapes of Solid Contact Rivets
Bild
*Contact Materials
Bild
*Dimensional Ranges
Bild
The respective parameters cannot be chosen independently of each other.
They mainly depend on the ductility of the required contact material. Before a
final decision on the dimensions we recommend to consult with the contact
manufacturer.
*Qualitätsmerkmale und Toleranzen
Bild
===3.1.1.2 Composite Contact Rivets===
Clad rivets for which only a part of the head (composite or bimetal rivets) or also
the shank end (tri-metal rivets) are composed of contact material – with the
balance of the body mostly being copper – have replaced for many applications
solid rivet versions because of economical considerations. The cost savings
depend on the contact material and its required volume for a specific
application. These composite rivets are also produced scrap-less from wire
material on special machinery with two process variations utilized.
During ''cold bonding'' and heading the bond between the contact material and
the copper is achieved without external heat energy by high plastic deformation
at the face surfaces of the two wire segments ''(Fig. 3.1)''. The bonding pressure
must be high enough to move the lattice components of the two metals within a
few atom radii so that the adhesion forces between atoms become effective.
Therefore the head to shank diameter ratio of 2:1 must be closely met for a
strong bond between the two metals.
Fig. 3.1: Cold bonding of bimetall rivets (schematic)
During ''hot bonding'' the required heat energy is applied by a short term electrical
current pulse ''(Fig. 3.2)''. In the case of Ag and Cu a molten eutectic alloy of
silver and copper is formed in the constriction area between the two wire ends.
When using metal oxide containing contact materials the non-soluble oxide
particles tend to coagulate and the bonding strength between the component
materials is greatly reduced. Therefore the cold bonding technology is preferred
for these contact materials. The during cold bonding required high surface
deformation ratio can be reduced for the hot bonding process which allows the
head to shank diameter ratio to be reduced below 2:1.
For composite rivets with AgPd alloys as well as alloys on the basis of Au, Pd,
and Pt the above methods cannot be used because of the very different work
hardening of these materials compared to the base material copper. The
starting material for such composite rivets is clad strip material from which the
contact rivets are formed in multiple steps of press-forming and stamping.
Similar processes are used for larger contact rivets with head diameters > 8 mm
and Ag-based contact materials.
Fig 3.2. Hot bonding of bimetal rivets (schematic)
*Typical contact shapes for composite rivets
bild
*Contact materials
bild
*Base materials
bild
*Dimensional ranges
bild
These parameters cannot be chosen independently of each other. They depend
mainly on the mechanical properties of the contact material. Before specifying
the final dimensions we recommend to consult with the contact manufacturer.
*Quality criteria and tolerances
bild
*Typical contact shapes of tri-metal rivets
bild
*Contact materials
bild
*Base materials
bild
*Dimensional ranges
bild
*Standard values for rivet dimension
bild
===3.1.1.3 Braze Alloy Clad Contact Rivets===
For special cases, especially high surrounding temperatures with high thermal
and mechanical stresses during switching operations, a full metallurgical bond
between the contact rivet and the contact carrier may be required to prevent a
loosening of the connection and early failures of the device. To accomplish this
superior bond a thin layer of brazing alloy is added to the underside of the head
and the rivet shank. During assembly a thermal treatment is added after the
mechanical staking.
===3.1.1.4 Contact Rivets with Brazed Contact Material Layers===
For certain applications contact rivets with non-ductile or brittle materials such
as tungsten, silver–tungsten, or silver–graphite are required. Rivets with these
contact materials can only be fabricated by brazing. Small round tips are brazed
to pre-fabricated copper or steel bases using special brazing alloys in a
reducing atmosphere.
===3.1.2 Contact Tips===
Flat or formed contact tips, welded or brazed to contact carriers, are frequently
used in switching devices for higher power technology. Depending on the
contact material and specified shapes these tips are produced by various
manufacturing processes. The most frequently used ones are:
*Stamping from strips and profiles
*Cutting from extruded rods
*Pressing, Sintering, and Infiltrating
*Pressing, Sintering, and Re-Pressing
*Pressing and Sintering
For stamping sufficiently ductile semi-finished materials are needed. These are
mainly silver, silver–alloys, silver–nickel, silver–metal oxide, and silver–graphite
(with graphite particle orientation parallel to the switching surface). silver–metal
oxides and silver–graphite need an additional well brazable or weldable silver
layer on the underside which can be bonded to the bulk of the contact material
by various processes. To further facilitate the final attachment process strips and
profiles are often coated on the brazing underside with an additional thin layer of
brazing alloy such as L-Ag 15P (CP 102 or BCuP-5).
For Ag/C with the graphite orientation perpendicular to the switching surface the
brazable underside is produced by cutting tips from extruded rods and burning
out graphite in a defined thickness.
The press-sinter-infiltrate process (PSI) is used mainly for Ag/W and Cu/W
material tips with tungsten contents of > 50 wt%. A silver or copper surplus on
the underside of the tip later facilitates the brazing or welding during final
assembly.
The press–sinter–re-press method (PSR) allows the economic manufacturing of
shaped contact parts with silver or copper contents > 70 wt%. This process also
alloys parts pressed in two layers, with the upper being the contact material and
the bottom side consisting of pure Ag or Cu to support easy attachment.
Press–sinter processes are limited to smaller Ag/W contact tips with a Ag
content of approximately 65 wt%.
*Contact materials
bild
*Typical contact shapes of tips and formed contact parts
bild
*Dimensional ranges
bild
Because of the wide variety of shapes of contact tips and formed contact parts
the user and manufacturer usually develop special parts specific agreements
on quality and tolerances.
===3.1.3 Weld Buttons===
of attachment is critical for the reliability and electrical life of contact
components for electromechanical switching devices. The materials saving use
of high cost precious metals and the most economic manufacturing method for
contact parts are most important factors.
There are two basic manufacturing solutions available: One can start with single
contact parts such as contact rivets or tips which then are attached
mechanically or by brazing or welding resp. to carrier parts. In the second case
a base material coated or clad with the precious contact metal - for special
applications also clad with another non-precious material – in the form of strips
or profiles is manufactured as a semi-finished pre-material from which the
contact components are then stamped and formed. Besides mechanical
cladding other processes such as electroplating and deposition from the gas
phase are utilized.
Which of the following manufacturing processes is finally chosen depends on
the final application of the contact components in their respective switching
devices or electromechanical components. Other considerations such as the
required number of electrical operations, the most economical use of precious
metals and the anticipated volumes of parts are also important for the process
selection.
===3.1 Manufacturing of Single Contact Parts===
The group of single contacts includes contact rivets, contact tips, and formed
parts such as weld buttons. Contact spheres (or balls) are today rarely used
because of economical considerations.
===3.1.1 Contact Rivets===
===3.1.1.1 Solid Contact Rivets===
Solid contact rivets are the oldest utilized contact parts. Their manufacturing
requires a ductile contact material and is done without scrap on fully automated
special cold heading machines. A wire slug is cut off and the rivet head is
formed by pressing and hammering. This way contact rivets with various head
configurations such as flat, domed, spherical, or pointed can be manufactured
depending on the final application and switch or relay design.
*Typical Contact Shapes of Solid Contact Rivets
Bild
*Contact Materials
Bild
*Dimensional Ranges
Bild
The respective parameters cannot be chosen independently of each other.
They mainly depend on the ductility of the required contact material. Before a
final decision on the dimensions we recommend to consult with the contact
manufacturer.
*Qualitätsmerkmale und Toleranzen
Bild
===3.1.1.2 Composite Contact Rivets===
Clad rivets for which only a part of the head (composite or bimetal rivets) or also
the shank end (tri-metal rivets) are composed of contact material – with the
balance of the body mostly being copper – have replaced for many applications
solid rivet versions because of economical considerations. The cost savings
depend on the contact material and its required volume for a specific
application. These composite rivets are also produced scrap-less from wire
material on special machinery with two process variations utilized.
During ''cold bonding'' and heading the bond between the contact material and
the copper is achieved without external heat energy by high plastic deformation
at the face surfaces of the two wire segments ''(Fig. 3.1)''. The bonding pressure
must be high enough to move the lattice components of the two metals within a
few atom radii so that the adhesion forces between atoms become effective.
Therefore the head to shank diameter ratio of 2:1 must be closely met for a
strong bond between the two metals.
Fig. 3.1: Cold bonding of bimetall rivets (schematic)
During ''hot bonding'' the required heat energy is applied by a short term electrical
current pulse ''(Fig. 3.2)''. In the case of Ag and Cu a molten eutectic alloy of
silver and copper is formed in the constriction area between the two wire ends.
When using metal oxide containing contact materials the non-soluble oxide
particles tend to coagulate and the bonding strength between the component
materials is greatly reduced. Therefore the cold bonding technology is preferred
for these contact materials. The during cold bonding required high surface
deformation ratio can be reduced for the hot bonding process which allows the
head to shank diameter ratio to be reduced below 2:1.
For composite rivets with AgPd alloys as well as alloys on the basis of Au, Pd,
and Pt the above methods cannot be used because of the very different work
hardening of these materials compared to the base material copper. The
starting material for such composite rivets is clad strip material from which the
contact rivets are formed in multiple steps of press-forming and stamping.
Similar processes are used for larger contact rivets with head diameters > 8 mm
and Ag-based contact materials.
Fig 3.2. Hot bonding of bimetal rivets (schematic)
*Typical contact shapes for composite rivets
bild
*Contact materials
bild
*Base materials
bild
*Dimensional ranges
bild
These parameters cannot be chosen independently of each other. They depend
mainly on the mechanical properties of the contact material. Before specifying
the final dimensions we recommend to consult with the contact manufacturer.
*Quality criteria and tolerances
bild
*Typical contact shapes of tri-metal rivets
bild
*Contact materials
bild
*Base materials
bild
*Dimensional ranges
bild
*Standard values for rivet dimension
bild
===3.1.1.3 Braze Alloy Clad Contact Rivets===
For special cases, especially high surrounding temperatures with high thermal
and mechanical stresses during switching operations, a full metallurgical bond
between the contact rivet and the contact carrier may be required to prevent a
loosening of the connection and early failures of the device. To accomplish this
superior bond a thin layer of brazing alloy is added to the underside of the head
and the rivet shank. During assembly a thermal treatment is added after the
mechanical staking.
===3.1.1.4 Contact Rivets with Brazed Contact Material Layers===
For certain applications contact rivets with non-ductile or brittle materials such
as tungsten, silver–tungsten, or silver–graphite are required. Rivets with these
contact materials can only be fabricated by brazing. Small round tips are brazed
to pre-fabricated copper or steel bases using special brazing alloys in a
reducing atmosphere.
===3.1.2 Contact Tips===
Flat or formed contact tips, welded or brazed to contact carriers, are frequently
used in switching devices for higher power technology. Depending on the
contact material and specified shapes these tips are produced by various
manufacturing processes. The most frequently used ones are:
*Stamping from strips and profiles
*Cutting from extruded rods
*Pressing, Sintering, and Infiltrating
*Pressing, Sintering, and Re-Pressing
*Pressing and Sintering
For stamping sufficiently ductile semi-finished materials are needed. These are
mainly silver, silver–alloys, silver–nickel, silver–metal oxide, and silver–graphite
(with graphite particle orientation parallel to the switching surface). silver–metal
oxides and silver–graphite need an additional well brazable or weldable silver
layer on the underside which can be bonded to the bulk of the contact material
by various processes. To further facilitate the final attachment process strips and
profiles are often coated on the brazing underside with an additional thin layer of
brazing alloy such as L-Ag 15P (CP 102 or BCuP-5).
For Ag/C with the graphite orientation perpendicular to the switching surface the
brazable underside is produced by cutting tips from extruded rods and burning
out graphite in a defined thickness.
The press-sinter-infiltrate process (PSI) is used mainly for Ag/W and Cu/W
material tips with tungsten contents of > 50 wt%. A silver or copper surplus on
the underside of the tip later facilitates the brazing or welding during final
assembly.
The press–sinter–re-press method (PSR) allows the economic manufacturing of
shaped contact parts with silver or copper contents > 70 wt%. This process also
alloys parts pressed in two layers, with the upper being the contact material and
the bottom side consisting of pure Ag or Cu to support easy attachment.
Press–sinter processes are limited to smaller Ag/W contact tips with a Ag
content of approximately 65 wt%.
*Contact materials
bild
*Typical contact shapes of tips and formed contact parts
bild
*Dimensional ranges
bild
Because of the wide variety of shapes of contact tips and formed contact parts
the user and manufacturer usually develop special parts specific agreements
on quality and tolerances.
===3.1.3 Weld Buttons===
