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Editorial
12/09/2019

Sensor Manufacturing

Micro welding techniques improve sensor manufacturing 

Whether manufacturing sensors for use in high-value, low-volume handmade devices or for highly cost-sensitive mass-produced items, manufacturers are experiencing increased pressure to deliver higher quality than in the past. For many, taking advantage of modern micro welding techniques can offer the assembly quality and robustness manufacturers desire. 

Micro welding methods and design techniques 

Micro resistance welding 
Micro resistance welding has been used for years for making joints and consolidating flexible stranded wires into a solid block. More recent quality demands and the desire for higher yield means manufacturers have to pay greater attention to process measurement when using this method. This means replacing traditional weld head and capacitor discharge technology and upping their game when it comes to process knowledge and documentation.

Micro arc welding 
Micro arc welding is usually used in sensor assembly where two components are bound together. Common applications include termination of fine wires to component pins, for example inductive sensors or the mounting of semiconductor package pins onto lead frames. The process is usually very fast and high temperatures are localised at the joint. 

Controlled percussive arc welding 
With this method, 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. It is particularly useful when joining stranded wires axially onto sensor pin devices, producing extremely high integrity joints with little or no prior wire preparation. 

Sensor manufacturing equipment considerations 

In most cases, a single welded joint provides an electrically high integrity joint. It is critical to carefully define 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. The finer the work, the tighter the process control must be to ensure consistency in energy transfer into the joint, guarantee quality, and ensure production yields. This is especially true for sensors used in medical or aerospace products, which generally have to meet higher weld verification standards. 

Special considerations for fine wire sensors – process control is key

Typically, small wires and pins of less than 1 millimetre (mm) are resistance welded at levels below 1000A DC. To achieve reliable consistent welding, we need the best possible level of control coupled with the best possible method of checking each weld process. 

When micro welding below 1000A DC, the best equipment choice is usually a DC linear resistance welding power supply, which gives a pure DC current, usually from a few amps to 1000A, preferably in fine incremental 1 amp steps. For volume manufacturing, look for high duty cycle ratings that can keep up with speed demands. Opt for fully closed-loop-controlled units, which measure and store the uncontrolled electrical signals that occur during the welding process. 

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. 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. Process window analysis allows operators to fully document the process and note correct pressures so they can easily return to a working process. Manufacturers can utilise the window analysis feature from Amada Miyachi Europe to determine the appropriate weld head pressure for an application.

Statistical process control for improved quality and yield

Statistical process control (SPC) and analysis of welding data is a very useful production engineering feature, especially in networked mass manufacturing lines. Most good welding power supplies provide a way of communicating data. In the past, the data collection/analysis process was extremely tedious, but nowadays most good manufacturers provide their own proprietary SPC software analysis packages. 

Built-in weld SPC functionality is a great tool for production and process welding engineers to easily record and analyse their welding cell information. With SPC, skilled operators can monitor their own work and determine when corrective action is needed, well in advance of serious yield or quality production problems. 

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, operators can more readily detect, assess and compensate for the effect of such changes. Process engineers can also more easily assess the pros and cons of any production line changes. 

With the data on hand – and the ability to analyse it – skilled machine operators can make judgements that would otherwise be reserved for more expensive engineers.


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