Sensor Manufacturing - Micro welding applications

Application Note 8 Precision Micro Joining Solutions Sensor Manufacturing - Micro welding applications Sensor manufacturing Technologies The aim of ...
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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. © 2013 MacGregor Systems AN8 Rev1.0

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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 [email protected]. 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.

US OFFICE MACGREGOR SYSTEMS LTD 7959 Silverton Avenue, Suite 1021, San Diego, CA 92126-6355 Tel: +1 858 586 9000, Fax: +1 858 586 9291 E-mail: [email protected] Web: www.macgregorwelding.com BENELUX OFFICE MACGREGOR WELDING SYSTEMS LTD Vivaldilaan 5, 5707 RA Helmond, The Netherlands Tel: +31 492 538 597 Fax: +31 492 538 597 E-mail: [email protected] Web: www.macgregorsystems.com

Approval Certificate N0:910599 Design and manufacture of precision welding equipment and sub-contract micro-welding services

© 2013 MacGregor Systems AN8 Rev1.0

UK OFFICE MACGREGOR WELDING SYSTEMS LTD 69 Fred Dannatt Rd, Mildenhall, Suffolk, IP28 7RD, U.K. Tel: +44 (0)1638 510011 Fax: +44 (0)1638 510329 E-mail: [email protected] Web: www.macgregorsystems.com

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