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Created page with "==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 a..."
==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
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
