Richard Barber, engineering manager, Amada Miyachi UK
Whether manufacturing sensors for use in high-value, low-volume handmade devices or for highly cost-sensitive mass-produced items, manufacturers are experiencing more pressure to deliver higher quality than in the past. Performance and operational expectations delivered in aerospace and medical manufacturing are becoming increasingly commonplace, even in general industry applications. In response, sensor design and production engineers must re-evaluate current methods and seek to integrate assembly best practices. Indeed, as manufacturers look for a competitive advantage, sensor quality and robustness gained from the assembly process is now as much a key selling point as device functionality. For many manufacturers, taking advantage of modern microwelding techniques can offer a range of benefits to sensor assembly, including tight process control.
Sensor manufacturing technologies
Of course, microwelding is not the only technology available for sensor manufacturing. Microsoldering remains a popular option, but the downsides are many. Determining long-term joint integrity presents a major challenge, as does process monitoring. Furthermore, the method entails relatively high temperature stresses at and around joints. The presence of fumes as well as limitations on process speed also make the technology less than ideal.
Of the microwelding technologies, resistance microwelding is the more common. Resistance microwelding has been used for years for making joints and consolidating flexible stranded wires into a solid block. Still, more recent quality demands and the desire for higher yield means manufacturers have to pay greater attention to process measurement when using this method. Alternating current (AC) and capacitor discharge (CD) welders remain suitable for a number of applications, but some manufacturers with older weld heads and older AC or DC equipment are investing in newer technology, particularly when legacy gear fails to accommodate appropriate process measurement.
Less prevalent, but equally effective is arc microwelding, whereby a contactless electric arc heats parts to cause amalgamation. Arc microwelding is predominantly used to bind two components together. A common application is the termination of fine wires to component pins. A controlled arc is struck between the wire end and the pin end to create a focused heating zone, and in the same instant, the two parts are brought together, fusing and consolidating the two ends as a butt joint. This method is particularly useful for joining stranded wires axially onto sensor pin devices, producing extremely high integrity joints with little or no prior wire preparation. Arc microwelding can be used for inductive sensors or mounting semiconductor package pins onto lead frames. It offers two notable benefits over microsoldering, namely the process is usually very fast (less than half a second per weld) and high temperatures are localised at the joint.
Sensor microwelding equipment considerations
Microwelding allows effective joining of extremely small sensor parts, typically ranging from around 3 mm down to around 10 μm. General rules of thumb apply; typically, small wires and pins of less than 1 mm are resistance welded at levels below 1,000 A direct current (DC), but at this scale, it is not enough simply to define the weld heat and electrode configuration. Quality and yield depend on equipment that enables close control of the welding process and continuous monitoring. This is especially true for sensors used in medical or aerospace products, which generally have to meet higher weld verification standards. To achieve reliable consistent welding, manufacturers need the best possible level of control coupled with the best possible method of checking each weld process. When microwelding below 1,000 A DC, the best equipment choice is usually a DC linear resistance welding power supply. This gives a pure DC current, usually from a few amps to 1000 A, preferably in fine incremental 1-amp steps. For volume manufacturing, manufacturers should implement units with high duty cycle ratings that can keep up with speed demands.
Microwelding equipment for high-value process control
For the greatest value, manufacturers should opt for fully closed-loop controlled units that measure and store the uncontrolled electrical signals that occur during the welding process. The resolution and speed of such measurements will determine the welding unit’s 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 set up and record a process analysis window for such parameters. Process window limiting can detect potential problems to a much more precise degree than basic limit checks, making it an essential functionality for competitive sensor manufacturers.
For example, a manufacturer utilised the process window analysis feature of Amada Miyachi’s DC1013-T linear DC resistance welding power supply to determine the appropriate weld head pressure for an application. Weld head pressure and 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. 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. Process window analysis allows operators to fully document the process and note correct pressures so they can easily return to a working process.
Statistical process control for sensor microwelding
Using equipment able to achieve a very high degree of accuracy, repeatability and process control is a prerequisite for sensor manufacturers today. However, more and more manufacturers that use microwelding as an assembly process are also implementing mass production statistical techniques as a means of continuous process improvement.
Most processes tend to follow mathematically defined curves. Statistical process control (SPC) and analysis of welding data compare field process data to these standard curves to predict, with a high degree of accuracy, outcomes such as likely reject rates. In the past, this data collection and analysis process was extremely tedious, but now welding unit manufacturers such as Amada Miyachi provide SPC analysis software packages.
Built-in weld SPC functionality is an exceptional tool for enabling production and process welding engineers to easily record and analyse their welding cell information. Skilled operators can use SPC to monitor their own work and determine when corrective action is needed, well in advance of serious yield or quality production problems. For example, electrode or product materials are changed either accidentally or deliberately, but with a welding process tied closely to real-time local SPC monitoring, operators can more readily detect, assess and compensate for the effect of such changes.
Microwelding process control for high-quality, high-yield sensor manufacturing
Ultimately, quality sensor assembly, whether for mass market or custom devices, depends largely on process control, which is best served by sensitive and measurable microwelding equipment. SPC and SPC analysis software offer production engineers comprehensive and useful soldering and welding data.
Of course, the right equipment must be effectively integrated into an engineering culture that embraces the newly available data in order to benefit an organisation in a meaningful way. Engineers should check equipment regularly to ensure wear does not impact weld quality, make process changes that data support and remain aware of the latest developments in microwelding and microweld monitoring for sensors.
Amada Miyachi Europe
www.amadamiyachi.eu
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1a & b: A butt joint produced using Amada Miyachi’s PAW-200 programmable 200 A percussive pulsed arc linear TIG welding power supply.
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1a & b: A butt joint produced using Amada Miyachi’s PAW-200 programmable 200 A percussive pulsed arc linear TIG welding power supply.
Amada Miyachi’s DC1013-T linear DC resistance welding power supply is automation ready, affording SPC data collection and reporting capabilities via the ethernet.