PCB | July 21, 2009
Cantilever sensors revolutionise applications such as ERP and identification systems
German PCB manufacturer Würth Elektronik integrates cantilevers operating as a sensor, allowing new industrial applications. This technological innovation has been made possible due to an inhouse conductive polymer, i.e. thick film polymer paste, enabling the necessary application intelligence structured into a specific designed printed circuit board.
This solution is characterised by a high potential of technical innovations. It is suited to numerous new and customized applications, e.g. allocation and identification tools in ERP systems. The first serial products are already approved. Currently existing cantilevers are limited to simple functions, e.g. pure force measurement. By means of resistive strain gauges printed on the PCB and integrated sensor and electronics on board, the new cantilever also replaces the weaknesses of traditional capacitive sensors for allocation systems, e.g. insufficient robustness against environmental conditions. Precisely due to this robustness, the cantilever with polymer strain gauge is suitable for applications that are subject to special demands in terms of temperature, humidity, industrial climate and similar. These cantilevers, which can be integrated into any PCB type (i.e. double-sided, multilayer, HDI or flex-rigid), are commonly used as short stroke sensors and force indicating devices, as well as making integrated pressure sensors possible. The special technique of embedding the polymer hybrid as a screen-printed, passive component inside internal layers of a PCB, provides a level of high packing density combined with reliable connections to conductive traces never before experienced. This cantilever can be incorporated into any PCB type (DS, ML, HDI, flex- rigid) and can be applied as, to mention but a few, a short stroke sensor or force indicator, therefore an option for replacing short stroke - and pressure sensors. The crucial factor to this further integration level is the in-house developed and secret conductive plastic paste. By applying new conductive and other polymer types with targeted electrical characteristics, Würth Elektronik succeeded in printing passive components on board and integrating sensors inside the PCB. This technology enables the development of customized solutions, which, among other items, reduce space and volume as well as production efforts at a considerable extent. Development time and costs are reduced while simultaneously improving the end application robustness. Frank Dietrich, head of the product division FLATcomp systems at Würth Elektronik and developer of the specific “smart conductive“-polymer paste, points out: „The customer does not get the paste in the actual sense, but the cured thick film, therefore we use the correct term PTF (Polymer Thick Film). We are tending more and more towards this variation of thick film due to the wide range of resistance it offers; in the future we will have many passive components screen printed on the PCB.” For polymer strain gauges, the thick coating creates an excep-tionally stress-sensitive resistor; achieving output signals 50 times higher than a conventional strain gauge’s signal, and therefore needs only a low level of amplification. With this, Würth Elektronik is able to generate a thick film strain gauge on the PCB so that the integration of sensor and evaluation electronics within the compact design of a mechanically structured PCB becomes possible. The advantages of this integration are outperforming all former approaches for improving sensor technology: The designer enjoys great creative freedom and needs less time for development; the sensor is positioned close to the electronic evaluation unit; the PCB is ca-pable of taking over several functions; the solution gives high reliability and the start-up costs are reduced. Moving forward with integration really pays off here, since more than 80% of all components are passive components, occupying more than 35% of space available and more than 25% of all solder joints in assemblies. Reduction of solder joints means improvement of the products’ reliability and robustness.? Thick Film strain gauge combined with cantilever in comparison Strain gauges usually are glued onto metal cantilevers. Designed as a Wheatstone bridge, a signal corresponding to the proportional value of the force effect on the cantilever is transferred. Com-mercially available strain gauges are available at values between 1 mV/V and 4 mV/V. The practical consequence is, that the signal needs to be amplified by 250 to 1000 times before it becomes fit for purpose. Moreover, side effects like drifting and EMC interferences be-come more likely with a high amplification. On the other hand, the printed polymer strain gauge with struc-tured PCB (applied for patent) utilises the TK-compensation due to the Wheatstone bridge and the matching of FR4 to the conductive polymer. The signal obtained using this method is around 50 times higher than for conventional strain gauges. Conclusion: Only a slight amplification is needed. In many applications, even amplification becomes no longer necessary, resulting in cost savings, higher reliability and less interference. As with printed resistors, the application becomes more robust. This type of sensor may be also used as short stroke sensors or – combined with a diaphragm - as a pressure sensor. Equally, it is possible to replace simple position switches using this PCB element, both cost-effectively and with reduced interference. Peter Tiefenbach, head of the product management Polymer technology FLATcomp at Würth Elektronik, says: „The printed circuit board takes over functions, playing the role of a sensor. That generates an essential difference: The PCB is not applied as backing material for electronic components or as a connecting element; nowadays it is con-sidered a valuable system, able to take on additional tasks. Advancing functionality – this is one of the basic technological principles today. “ Peter Tiefenbach (Polymertechnologie FLATcomp - left) and und Frank Dietrich (FLATcomp Systeme at Würth Elektronik) Technical article: Würth Elektronik