Application Note 8
Precision Micro Joining Solutions
Sensor Manufacturing - Micro welding applications
Sensor manufacturing Technologies
The aim of this application note is to present a number of current sensor manufacturing methods that
benefit through the application of micro welding techniques. In terms of technology application, the
sensor market is extremely broad, ranging from high value, low volume hand made devices through to
highly cost sensitive mass produced items.
As manufacturers look for competitive advantage, more often than not, quality and robustness from
the assembly process is now as much a key selling point as is the functionality of the device itself.
Equally, performance and operational expectations delivered in aerospace and medical manufacturing
are now becoming common place in general industrial and FMCG applications, thereby putting
greater demand on sensor design and production engineers to seek the best practice assembly
methods.
Micro joining methods in sensor manufacturing - overview
Solder : In the aftermath of the lead free solder drive a few years ago, most soldering processes have
now been modified and adapted for new temperatures and lead free solders.
Surprisingly perhaps, compared with a single spot welded joint, a solder joint can sometimes offer
greater overall strength as a consequence of the physical solder material mechanically surrounding
and supporting the joint. Similarly, solder is still often used as the interconnection method between
sensor and wire, especially beneficial in acting as a consolidator for flexible stranded wire.
The downsides to using solder for high integrity joining can include the difficulty in determining long
term joint integrity, relatively high temperature stresses at and around the joints, difficulties in process
monitoring, fluxes, fumes and limitations in process speeds to name but a few.
Thus, when evolving new sensor designs, it is not surprising that manufacturers nowadays tend to
avoid soldering wherever possible, looking instead to other methods and design techniques.
Micro Resistance Welding : Micro resistance welding is a very common
assembly method in sensor manufacture and peripheral wire attachment.
There are many sensor products and designs that have existed for years in
much the same state and that employ one or more resistance welded joints.
It is surprisingly common place to find mature product still being made with
old weld heads and old capacitor discharge or AC welders, often with little
knowledge or documentation about the real process in place.
As quality demands are pushed up or perhaps as a consequence of
production line relocation, a once little considered part of the operation can
suddenly become headline news for a manufacturing company. A push for
greater quality or higher yield involving a resistance welding process will
inevitably require process measurement over time, along with a thorough
understanding of the equipment employed.
Above : Wire consolidation with
resistance welding.
Below : Wire and component
resistance weld joining
Micro resistance welding is not just used to make joints. As with soldering,
the process can be used to consolidate flexible stranded wires into a solid
block in order to remove stray strands and to provide a solid base for the
next assembly stage.
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Application Note 8
Precision Micro Joining Solutions
Sensor Manufacturing - Micro welding applications
Micro
methods in
sensor manufacturing - overview
Sensorjoining
manufacturing
Technologies
Micro Arc Welding : Less prevalent than micro resistance welding but nevertheless equally effective,
micro arc welding finds it’s place in sensor assembly generally where two components are bound or
pushed together allowing provision for the two parts to amalgamate when heated by a contact less
electric arc.
Common applications include the termination of fine wires to component pins, for example inductive
sensors or the mounting of semi conductor package pins onto lead frames. The process requires the
provision of a common electrical return path by which a controlled arc current can pass back to the
power supply. While there is an inherent need for relatively high voltages to be be present in
establishing the micro arc, careful consideration of product jigging and current paths will usually protect
sensitive components from possible damage.
As with resistance welding, the process is usually very fast (< 0.5 seconds) and unlike soldering, high
temperatures are localised at the joint.
Right: 32µm
wire joining
Left: 50µm
wire joints
Above: Semi conductor lead to support frame
Stranded Wire consolidation
In some applications it is useful to consolidate wire strands in order to
form a solid mass. Typically additional processes such as DC
resistance welding are then used to weld the consolidated material to
other materials or components forming highly reliable solder free joints.
Wire balling techniques such as this maybe used for ultra small micro
joints with wire sizes down to 30µm.
Controlled Percussive Arc Welding : A further application of micro arc welding in sensor
manufacture is controlled percussive arc. This is a particularly useful technique when joining stranded
wires axially onto sensor pin devices, producing extremely high integrity joints with little or no prior wire
preparation. (Not to be confused with basic cap discharge spark and squash applications).
Put quite simply, a controlled arc is struck between the pin end and wire end to create a focused
heating zone. In the same instant, the two parts are brought together, fusing and consolidating the
part two ends as a butt joint.
The photograph to the right shows an end to
end butt joint between standard high flex
equipment wire and a gold plated sensor pin.
Note the small localised heating zone.
Right : Produced with an MWS PA200-PP with built in process
monitoring.
© 2013 MacGregor Systems AN8 Rev1.0
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Application Note 8
Precision Micro Joining Solutions
Sensor Manufacturing - Micro welding applications
Micro
equipment
in sensor manufacturing
Sensorwelding
manufacturing
Technologies
Resistance welding power supplies and heads
This section discusses basic equipment facets generally worth appreciating when considering
resistance welding as a process for sensor manufacture. For detailed in depth technical information, a
free guide book can be obtained upon request to sales@macgregorsystems.com.
Understand the joint requirements
Typically with sensor welding, a single welded joint will provide an electrically high integrity joint which
if needs be, can be traced and tested in terms of metallurgy. Similarly, if configured correctly, the joint
should be mechanically as strong as the weakest part being joined, typically a connection wire.
Micro welding in sensors and similar applications typically implies parts from say around 3mm down to
around 10µm. Some smaller examples are shown below :Left: 13µm wire drawn, spot welded and cut
across sensor 0.1mm diameter sensor pins.
Right: 8µm Tungsten wire spot welded in place
The basic principle is quite simple. An electric current is passed through the two parts to be joined
and the resultant heat generated from the electrical resistance of the parts will be proportional to
Ohms law i.e. P=I²R.
By considering the finer end of the assembly scale in the photos above, one can start to appreciate
the importance of a real definition of welding heat in terms of the adjustment and repeatability
resolution required from the assembly equipment involved, as well as the physical definition,
determined by the weld head and electrode configuration.
Clearly, the finer the work, the tighter the process control needs to be to ensure consistency in energy
transfer into the joint.
Similarly, and this is a point often overlooked, exactly the same principle applies to physically larger
joints where it is desirable to improve the quality. i.e. Demand for greater yield, fewer rejects etc...
If a welding process is not properly under control by virtue of the equipment AND it is not monitored
continually AND is not analysed continually, then as with any other process, changes in quality
demands and production yields, desired (or otherwise), can quickly become a real issue.
This is especially true for high integrity products like sensors and medical or aerospace products
which generally have to meet higher standards of weld verification.
© 2013 MacGregor Systems AN8 Rev1.0
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Application Note 8
Precision Micro Joining Solutions
Sensor Manufacturing - Micro welding applications
Fine
wire
sensors - subTechnologies
1000A micro welding
Sensor
manufacturing
Typically, small wires and pins of less than 1mm are resistance welded at levels below 1000A DC.
The exact threshold depends very much upon materials and physical fit up, but for now we’re
considering the finer end of the sensor scale.
To achieve reliable consistent welding, we need the best possible level of control coupled with the best
possible method of checking each weld process. Overall, if we want the best level of long term quality,
then we need to fully understand the process over time and the factors that can effect it.
Power, resolution and duty requirements :
When micro welding below 1000A DC, the best equipment choice is usually a DC Linear resistance
welding power supply. Such supplies are designed to give a pure DC current, usually from a few
amps all the way to 1000A, preferably in fine incremental 1 amp steps. For volume manufacture, look
for high duty cycle ratings in order to keep up with speed demands.
Such welding units typically replace older technologies such as capacitor discharge and AC welders
when such processes no longer satisfy modern manufacturing needs.
Good linear welding units will be fully closed loop controlled offering the ability to measure and store
the uncontrolled electrical signals that occur during the welding process. So for example, a power
supply developing a controlled 500A DC into a 1 milliohm joint, will record an uncontrolled voltage
parameter of 0.5V. The resolution and speed of such measurements will determine the units ability to
adapt and react in real time to process changes. While most linear power supplies provide an ability
to set upper and lower limits around the uncontrolled parameters, few provide a means to setup and
record a process analysis window for such parameters.
Fine wire
sensors - process
windowing
Sensor
manufacturing
Technologies
Screen shots taken from
MWS DC1013T Linear
DC Power Supply.
Material settling
point exceeds
main weld area
level.
Recorded
uncontrolled
parameter trace
is shown in
Recording the weld trace and applying a limiting window allows the key process
yellow.
area to be tightly controlled. Note the basic upper/lower limits are not tight enough.
© 2013 MacGregor Systems AN8 Rev1.0
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Application Note 8
Precision Micro Joining Solutions
Sensor Manufacturing - Micro welding applications
Fine
wire
sensors - process
windowing
Sensor
manufacturing
Technologies
Right : This shot shows how window
tolerancing enables the welding process
engineer to hone the limit checking
around the real process point.
Right : Touch, zoom and scroll to record
and analyse process parameters for QC
setups.
Fine wire sensors - know your limits
Left : Here we see
how process window
limiting is able to
detect potential
problems to a much
closer degree than
basic limit checks.
Here the 1V upper
limit is not exceeded
but clearly the
recorded trace is
outside of the typical
process window.
© 2013 MacGregor Systems AN8 Rev1.0
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Application Note 8
Precision Micro Joining Solutions
Sensor Manufacturing - Micro welding applications
Fine
wire
sensors - Know
your limitations
Sensor
manufacturing
Technologies
The previous illustration demonstrates the advantages of window limiting over basic limiting and was
setup simply by modifying the weld head pressure, (something operators sometimes like to do,
especially on the night shift !).
Weld head pressure and indeed weld head dynamic performance are key criteria for fine component
welding. Quite often, welding processes start to deviate from the norm and begin to fail as a result of
worn out weld heads. Since it is crucial that the weld head ensures the welding electrodes remain in
good contact throughout the weld process, worn out bearings and sticky mechanisms can creep up on
the unsuspecting production engineer.
Moreover, if the process has not been fully documented and the correct pressures noted, it can be very
tedious getting back to a working process, all of which is generally quite costly if not embarrassing.
It should be clear therefore that whether or not you actually use process monitoring to flag potential
rejects, the simple act of recording, storing and visualising the process, is a useful tool when trying to
determine if something has gone wrong.
Fine wire sensors - Anticipating the future
Obviously no one knows what the future holds, but it’s a fair guess that it will be something along the
lines of faster, cheaper, more production, fewer rejects.
So how do we do take our cherished sensor production process that’s been “working” like that for years
and make it better ? How do the more successful businesses and corporations address these real
world commercial demand issues ?
One approach is to look at the methods the Japanese successfully employed thirty years ago in stealing
a march on their European manufacturing counterparts. The application and analysis of measurement
statistics in process control is now a fast growing necessity not only in mass manufacturing, but also in
lower volume, hand assembly shops also.
SPC (Statistical Process Control) can be applied to any process where data relevant to the process can
be collected. Simply put, most processes tend to follow mathematically defined curves e.g. The Normal
distribution curve. SPC analysis compares process data to these curves to provide a measure of
whereabouts on that curve a process might be. By statistically knowing where one is on the curve, one
can predict with a degree of accuracy, the likely reject rates and the likely period before the process
gets out of control.
Most good welding power supplies will provide a
means to communicate their data. Clearly, the
usefulness of this data will depend upon the inherent
repeatability and accuracy of the supply; however,
once out, the data can then be analysed.
In the past, this process has been tedious and off
putting, requiring the use of commercial spreadsheet
programs and data conversions. Attempting to
make this easier, most good manufacturers provide
their own proprietary SPC software analysis
packages.
Left : MWS SPC Software showing Cpk analysis
© 2013 MacGregor Systems AN8 Rev1.0
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Application Note 8
Precision Micro Joining Solutions
Sensor Manufacturing - Micro welding applications
Fine
wire
sensors - Anticipating
the future
Sensor
manufacturing
Technologies
Statistical process control and analysis of welding data is a very useful production engineering feature
especially in networked mass manufacturing lines, where continuous process monitoring and dynamic
process adjustments area possible.
For smaller scale sensor production lines, perhaps one operator to one machine, the business of
central networking and central data collection can represent a difficult to justify capital expenditure.
Sometimes, there simply isn’t the time or resource available to collect and monitor the data.
Fine wire
sensors - Complex
problems, simple solutions
Sensor
manufacturing
Technologies
Given that real-time process information can be monitored and reported by a resistance welding
power supply unit, it makes good sense that for single cell production applications, the information at
the station can be statistically processed and analysed with built in SPC functionality.
Built in weld statistical process control functionality is a great tool for production and process welding
engineers to easily get started with recording and analysing their welding cell information.
Moreover, since high end sensor manufacturers tend to use skilled operators, some basic training with
built in SPC, means that operators can monitor their own work and determine when corrective action
is needed, well in advance of serious yield / quality production problems.
Above: Local process related PDF documents stored in a DC613T
Above: An MWS DC2013T showing built in yield analysis
Fine wire sensor manufacture - When should you clean the electrodes ?
One of the most common questions you hear and one which nicely illustrates the benefit of SPC
analysis. So what is the answer ? Well in automated mass production, this is done automatically either
on a fixed time basis or increasingly as a result of statistical process monitoring and automatic
adjustment.
By contrast, in the single operator cell environment, the answer is commonly “when it stops working,
when it sticks or when we start getting too many rejects”.
Clearly a more scientific approach offers the potential to reduce waste and improve quality.
Electrode wear is a a fact of welding life. It is vastly improved when closed loop power supplies are
used but nevertheless, always represents an undesirable trend in the process.
Being able to assess this trend and make corrections before yield drops can be a significant advantage
© 2013 MacGregor Systems AN8 Rev1.0
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Application Note 8
Precision Micro Joining Solutions
Sensor Manufacturing - Micro welding applications
Sensor manufacturing
manufacturingTechnologies
- A change is not always as good as a rest
Having your sensor welding process clearly defined and monitored at the welding station offers the
chance to detect and even embrace changes, even when you haven’t been told they’re happening.
Sometimes, and without warning, electrode or product materials are accidentally or deliberately
changed. With a welding process tied closely to real-time local SPC monitoring, the effect of such
changes is more readily detected, assessed and compensated for.
Similarly, when a commercial change is impressed upon production, knowing the typical statistical
process parameters that have gone on before, allows the process engineer to easily assess the pros
and cons of any changes required.
While change generally mean more work, actually having the data to hand and the ability to analyse it,
vastly reduces the overall workload and often allows skilled machine operators to make judgements
that would otherwise be reserved for more expensive engineers.
The
trends
in sensor manufacturing
Sensor
manufacturing
Technologies
Peaks and troughs in production
yield and quality are more often
than not explained by analysis and
detection of one or more up stream
changes.
Sometimes these changes are
sudden and unexpected, other
times, they are as a result of a
naturally occurring pattern or trend.
SPC and trend analysis is a
serious tool in detecting and
controlling these processes.
Left : MWS SPC software showing trend data
In terms of industry trend, sensor and other device manufacturers that use micro welding as an
assembly process are increasingly looking towards mass production statistical techniques as a means
of continuous process improvement.
For this to be effective in all sectors, it is vital that production and process engineers are able to quickly
gather and analyse the relevant process data. Equally, a full understanding of the process and
equipment is needed in order to correlate the data to the process.
Clearly, possessing equipment inherently able to achieve
a very high degree of accuracy, repeatability and
process control is a good starting point when looking for
process improvements.
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Design and manufacture
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