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===3.4 Evaluation of Braze or Weld Joints===
The switching properties of brazed and welded contact assemblies is strongly
dependent on the quality of the joint between the contact and the carrier. The
required high quality is evaluated through optical methods, continuous control
of relevant process parameters and by sampling of finished products.
===3.4.1 Brazed Joints===
Despite optimized brazing parameters non-wetted defect areas in the braze
joint cannot be avoided completely. These wetting defects can mostly be traced
to voids caused by flux inclusions in the braze joint area. Depending on the
shape and size of the joint areas, the portion of the fully wetted joint is between
65% and 90%. In its final use in switching devices a joint area of 80% is
considered good or excellent if the individual void size does not exceed 5% of
the joint area. Frequently wetted joint areas >90% with voids <3% can be
obtained.
Evaluation of the quality of the joint can be performed either by destructive or
non-destructive methods.
===3.4.1.1 Destructive Testing===
Destructive tests can be performed on a sampling basis in rather simple ways:
*De-brazing
The contact tip is being removed by slow heating and simultaneous application
of force perpendicular to the contact surface area. Visual inspection of the
separated components reveals the non-wetted defect areas as discoloration
from either flux remnants or oxidation of the carrier material.
*Milling Sample
The contact tip is milled off in layers to a depth that makes the joint area visible
for optical evaluation.
*Saw-Cutting
A crossing pattern is cut with a fine saw into the contact tip. Areas that are not
bonded fall off in pieces.
*Metallographic Micro-section
In a metallographic micro-section perpendicular to the contact surface wetting
defects can also be made visible ''(Fig. 3.14)'' which however are only indicative
of the brazing temperature and brazing time.
Fig. 3.14:
Braze joint with voids.
Ag/CdO tip on Cu carrier.
*Shear test
The contact tip is sheared off from the carrier with the required shear force being
a measure for the bond quality. This method is especially suitable for hard and
brittle contact tip materials such as for example tungsten.
===3.4.1.2 Non-Destructive Test Methods===
Typically the non-destructive testing of braze joints requires more elaborate test
equipment. Besides this such test methods have limitations regarding the
shape of the contact tips and/or carriers. The prevalent methods are ultrasound
testing and X-ray analysis.
*Ultrasonic testing
This method is based on the disruption of the propagation of sound waves in
different media. High resolution modern test systems with graphic print-out
capabilities and analytical software are capable to detect even small (<0.5 mm
diameter) voids in the braze joint. The portion of the wetted areas is calculated
as a percentage of the whole joint area. Fig. 3.15 shows an example of different
braze qualities for a Ag/SnO<sub>2</sub> contact tip brazed to a copper carrier and
illustrates the position and size of void areas as well as the final joint quality.
Fig. 3.15: Ultrasound print-out of braze joints between Ag/SnO 88/12 tips and Cu carrier with 2
different degree of wetting (dark areas = voids)
*X-Ray testing
X-ray testing is an additional method for evaluating brazed joints. Using finefocus
X-ray beams it is possible to achieve a sufficient picture resolution. There
are however limitations about the thickness of the contact tip compared to the
size of the void area. This expensive test method is rarely used for contact
assemblies.
===3.4.2 Welded Joints===
Since welded contact assemblies are usually produced in rather high quantities
the quality of the weld joints is monitored closely. This is especially true because
of the high mechanical and thermal stresses quite often exerted on the joint
areas during switching operations. The quality of the joints is dependent on the
process control during welding and on the materials used to manufacture the
welded assemblies.
Despite the ability to closely monitor the relevant welding parameters such as
weld current, voltage and energy, simultaneous testing during and after
manufacturing are necessary.
A simple and easy to perform quality test is based on the shear force.
Evaluations of welding assemblies in electrical performance tests have shown
however that the shear force is only a valid measure if combined with the size of
2 the welded area. As rule of thumb the shear force should be > 100 N/mm with
the welded area > 60% of the original wire or profile cross-sectional area. For
critical applications in power engineering, for example for high currents and/or
high switching frequency, a higher percentage of the joint area is necessary.
During series production every weld is usually probed in a testing station
integrated in the manufacturing line with a defined shear force – mostly 20% of
the maximum achievable force. In this way defective welds and missing
contacts can be found and sorted out. The monitoring of the actual shear force
and size is performed during production runs based on a sampling plan.
Fig. 3.16: Ultrasonic picture of a weld
joint, Ag/C tip on Cu carrier
(ABB-STOTZ-KONTAKT)
Besides destructive testing for shear force and weld area the non-destructive
ultrasound testing of the joint quality is also utilized for welded contact
assemblies ''(Fig. 3.16)''.
===3.4.3 Selection of Attachment Methods===
In the preceding sections a multitude of possibilities for the attachment of
contact materials to their carriers was described. A correlation of these
methods to the switching current of electromechanical devices is illustrated in
Table 3.2. it shows that for the same switching load multiple attachment
methods can be applied. Which method to chose depends on a variety of
parameters such as contact material, material combination of contact and
carrier, shape of the contact, required number of switching operations and last
but not least the required volume of parts to be manufactured.
Based on the end application the following can be stated as general rules:
Electroplated contact surfaces are limited to switching without or under
extremely low electrical loads. In the lower and medium load range contact
rivets and welded contacts are used. For high switching loads brazing,
especially resistance and induction methods, are utilized. For extremely high
loads, for example in high voltage switchgear, percussion welding, electron
beam welding, and copper cast-on processes are preferred.
Table 3.2: Correlation between Contact Joining Methods and Switching Currents
===3.5 Stamped Contact Parts===
Stamped electrical contact parts typically consist of a base carrier material to
which a contact material is attached by various methods ''(Fig. 3.17)''. They serve
as the important functional components in many switching and
electromechanical devices for a broad range of electrical and electronic
applications. On the one hand they perform the mostly loss-free electrical
current transfer and the closing and opening of electrical circuits. In addition the
contact carriers are important mechanical design components selected to meet
the requirements on electrical, thermal, mechanical and magnetic properties.
The increasing miniaturization of electromechanical components requires ever
smaller stamped parts with low dimensional tolerances. Such precision
stamped parts are needed in the automotive technology for highly reliable
switching and connector performance. In the information and data processing
technology they transfer signals and control impulses with high reliability and
serve as the interface between electronic and electrical components.
Fig. 3.17:
Plated and contact
containing pre-stamped strips and
stamped parts for different
applications
===3.5.1 Types of Stamped Parts===
Stamped parts are produced as single pieces, in pre-stamped strip and comb
configurations. Depending on the requirements and application the contact and
base material as well as the coating and attachment technology is carefully
selected.
*Coated stamped parts
Stamped parts can be selectively or completely coated with precious metal
containing materials based on gold, palladium, and silver as well as
non-precious materials such as tin, nickel and copper ''(Fig. 3.17)''. For stamped
parts in high volumes like those used as electrical components in automobiles
the carrier material is mostly coated in a reel-to-reel process starting with either
solid or pre-stamped strips (see also chapter 7.1.1.3). Frequently the prestamped
strip will be used directly in further automated assembly of the finished
functional component. As an alternative finished stamped parts can be
electroplated using barrel and rack plating methods.
Very thin coating layers with tight tolerances are deposited by electroplating. For
many applications the high mechanical wear resistance is advantageous. Since
even very thin layers are mostly pore-free, these coatings also act as an
effective corrosion inhibitor. The type of coatings, the sequence of multiple
layers, and the coating thickness, for example for connectors, are chosen
according to the requirements for the end application.
*Clad stamped parts
For many applications thicker precious metal surfaces or AlSi layers are
necessary. These cannot be deposited by electroplating. Besides meltmetallurgically
produced materials on the basis of gold, palladium and silver,
also powder-metallurgical materials are required frequently. The metallurgical
bond between these contact materials and the mostly copper based substrates
is achieved through various mechanical cladding methods (see also chapter
3.2.1). In this way also aluminum clad strips are manufactured in which the
aluminum layer serves as the bondable surface in the interface between
electromechanical connections and electronic circuits. These clad semifinished
materials can be further fabricated into pre-stamped strips, in comb
form, or single stamped parts ''(Fig. 3.18)''.
Fig. 3.18:
Examples of clad stamped parts
*Welded stamped parts
Welded stamped parts can be fabricated by various methods (see also Chapter
3.3.3). Single contact pieces can be attached to pre-stamped or finished
stamped strips as weld buttons and wire or profile segments by electrical
resistance welding. Contact parts can also be stamped from seam-welded
semi-finished strip. Fitting the end application contact materials based on gold,
palladium and silver. Depending on the contact material and the design of the
finished contact component the contact bottom surface may be consist of a
weldable backing material.
*Brazed stamped parts
Brazed stamped contact assemblies are manufactured by two joining methods
(see also chapter 3.3.2). The contact material is either attached by resistance
or induction brazing to base metal carriers as prefabricated contact tip or they
are stamped from brazed semi-finished toplay strip. It is typical for brazed
contact parts that the contact material consists of silver based contact material
and a good conducting copper base material with larger cross-sectional area
for the usually higher current carrying capacity.
*Stamped contact parts with rivets
Riveted stamped contact parts are manufactured with the use of contact rivets
which are transferred over suitable feed mechanisms correctly oriented into
holes punched into the carrier ''(Fig. 3.19)''. Frequently also wire or wire segments
resp. are used which are subsequently coined and formed into the desired
contact shape (see also chapter 3.3.1). Both attachment methods have their
distinct advantages. Using composite or tri-metal rivets allows limiting the use
of precious metal custom tailored to the volume needed for specific switching
requirements. For wire staking the precious metal usage is usually higher but
the staking can be performed at significantly higher production rates
and the additional rivet making step is eliminated.
Fig. 3.19:
Examples of riveted stamped parts
*Pre-mounted component stamped parts
Components stamped parts consist of a minimum of two carrier parts which
differ in their material composition and geometrical form and the contact
material
''(Fig. 3.21)''. The assembly of these components as single pieces or stamping
progressions is performed in a stamping die by riveting or coining. To increase
the current carrying capacity at the joining area an additional welding step can
be added. Depending on the requirements the different properties of the two
carrier components can be combined. As an example: the high electrical
conductivity of a contact carrier blade is joined with the thermal or mechanical
spring properties of a second material to form a functional component. For this
process both carrier base materials can also be coated with additional
layers of other functional materials.
Fig. 3.20:
Examples of pre-mounted stamped
component parts
Stamped parts which are insert molded into or combined with plastic parts are
used in electromechanical components (see Chapter 10).
===3.5.2 Stamping Tools===
