A success story

Rigid-flex PCBs replace complex wiring

Connectors, electronic drives and sensors can be scaled to smaller dimensions than ever before due to increasingly sophisticated development and production methods in electronical engineering, electronics and precision engineering. Nevertheless, there remains a sticking point: complex wiring needs space - which is often not available. The solution has arrived: rigid-flex PCBs. They are the result of a creative interdisciplinary collaboration between the drive manufacturer WITTENSTEIN in Igersheim and Würth Elektronik in Niedernhall (both in Baden Württemberg).

High precision servo motors are used in different industries and need to comply to the specifications of their respective industries, in particular in the aerospace, medical or automotive sector. In many applications, the predictable reliability of the components used is important. For motor-transmission units, both the mechanics and the control electronics must be optimized or intelligent sensors need to be integrated. Yet the question of precision and robustness ultimately decides on the interplay between control electronics and mechanical components.

All three dimensions of the housing volume of the electromagnetic assembly were used as the flexible part of the board was bent into every angle.

Especially in vibrating mechatronic units, there is a particular weak point: the cable connections. Contact failure and loss of function or even weaknesses due to long-time use can occur.

“At a connector where two electronic modules are joined with a cable, impurities may be deposited or the contact springs of the connectors produce insufficient connections” explains Michael Matthes, an expert on new electronic technologies at WITTENSTEIN electronics GmbH, a subsidiary of WITTENSTEIN. “Such impurities and temporary contact problems can, for example, lead to changes in the contact resistance between the modules and subsequently change the electrical characteristics of the tracks in the long-term or even lead to control and measurement errors.”

For this reason, WITTENSTEIN started early exchanges with PCB manufacturers to find better solutions. This resulted in an innovative combination of employees from WITTENSTEIN and Würth Elektronik, who dealt with the question of how connections in confined spaces can be successfully achieved. The solution: a rigid-flex PCB to replace the conventional cable and connectors.

“We had the progressive miniaturization of electronic components in mind during the development,” explains Andreas Schilpp, Product Manager at Würth Elektronik. “This requires considerations for the efficient use of the housing volume in all three dimensions. Whether and how this idea can be implemented, of course, was a creative challenge, but we were happy with our solution.” The result is a combination of a stable rigid PCB with a flexible film of polyimide and LCP (Liquid Crystal Polymer); the so-called rigid-flex printed circuit boards.

The interdisciplinary team from drive manufacturer WITTENSTEIN in Igersheim and Würth Elektronik in Niedernhall: (f.l.t.r.): Michael Matthes, WITTENSTEIN electronics, Andreas Schilpp, Product manager rigid-flex and Jürgen Wolf, R & D (both Würth Elektronik).

The advantages of the rigid-flex printed circuit boards are not just obvious to Michael Matthes: “The risk of using error-prone components, such as connectors and connection cables, is eliminated immediately. The signal transmission is also improved because conductor cross-section changes disappear. Generally speaking, the reliability of the overall system is improved. Without doubt, this is not just an important factor for us.”

Andreas Schilpp assures: “Malfunctions are excluded in the rigid-flex printed circuit boards. Aging effects on the connection, for example transition resistance, no longer occur over time. The copper flexible cables are directly protected by solder mask or coverlay from external influences”. He follows up with further arguments: “The flexible part can be bent at any angle and is still robust. So even difficult contacts are possible without the need for additional space. It leaves free space in the motor housing or mounting area for other things.”

In particular, the flexibility of this board required some thought beforehand. “Based on the results of the development work with WITTENSTEIN, we have developed a design guide for rigid-flex printed circuit boards for our customers,” explains Andreas Schilpp. In this hand-out, not only fundamental considerations of the different variants of the rigid-flex printed circuit boards can be found, but also a checklist for system requirements.” This checklist provides our customers with the reflections on the printed circuit board design and helps us in the development of the final product.”

However, despite the handy design guides for both partners, close communication between manufacturers and developers remains an important building block for success. The earlier the exchange between PCB manufacturer and designer begins, the higher the chance of success. “Particularly in complex systems, the dialogue can not begin early enough. For many specifications, which arise from interference contours of the device mechanics, have an impact on the complexity of the circuit board. This, in turn influences the specific design rules and the implementation for mass production,” says Matthes.

The continuing miniaturization of electronic components was always in focus: the innovative team of employees from WITTENSTEIN and Würth Elektronik.

Meanwhile, for both reasons of cost and in terms of constant production quality, rigid-flex PCBs have provided a real alternative to cable, because the required connectors are directly on the modules by flexible printed circuit board areas. “The cost savings during installation of the plug connections outweigh the small additional cost in the production of printed circuit board,” says Andreas Schilpp. Michael Matthes from WITTENSTEIN conducted a cost comparison of the technologies: “We have compared the two systems, standard PCB technology with cables and connectors against rigid-flex printed circuit boards and tested for their efficiency and cost effectivenes. The result has been positive for the transition to rigid flex.”

Meanwhile, the cooperation between WITTENSTEIN and Würth Elektronik has borne more fruit: As part of the 20th FED annual conference in Dresden they were commended with a PCB Design Award for their work in creating a common printed circuit board solution for a medical application in the form of an active implant in the category 3D/Build Space.

The use of this implant in the human body necessarily required the greatest possible miniaturization of the electronics assembly. Since the circuit in the installed state is no longer, or at least only through a surgical procedure on a patient, accessible, they must also have low probability of default and very long life. Here, each saved component reduces the space requirement and at the same time improves the reliability of the assembly.

Due to the latter reason, and also because of their space requirements, WITTENSTEIN has eliminated any type of connectors and designed all circuit boards using the rigid-flex platform. The implementation of the design was only possible through the close collaboration between the two companies over the entire design phase. Again, the overall view is extremely positive.

Both partners are sure that the rigid-flex technology will play an important role in the future. Andreas Schilpp promises: “We remain committed to the future for sure!”

About WITTENSTEIN

WITTENSTEIN AG develops, produces and distributes high-precision planetary gearboxes, complete electromechanical drive systems, AC servo systems and motors. Applications include robots, machine tools, packaging technology, materials and process engineering, paper and printing presses, medical and aerospace. WITTENSTEIN sees itself as a partner for intelligent components and controllable servo systems in the field of mechatronic drive technology. www.wittenstein.de

About Würth Elektronik Circuit Board Technology (CBT)

Würth Elektronik Circuit Board Technology was founded in 1971 and has established itself as one of the leading PCB manufacturers in Europe. Developers are able get standard circuit boards, new and innovative technologies and even complete system solutions all from one source. Here at Würth Elektronik, we cover the entire product life cycle, from the initial idea for a design, for example during early R&D projects, to the production of prototypes on the online shop WEdirekt and finally, manufacturing of medium and large volumes. Knowledgeable specialists are not just located at our German plants. Internationalization is an important strategic aspect for Würth. We have numerous sales teams set up across many European countries.

Every day more than 120 new PCB designs enter our production. We have over 4,000 customers, ranging from large corporations to one-man designers. In addition to the personal care provided through our dense network of over 100 internal and external sales people, customers also have the option to purchase printed circuit boards online through the easy to use online shop, WEdirekt.

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