Design for Manufacturing of a Bendable Wrist Device Cherie Jing , Antti Salo , Anping Zhao, Quayle Chen, Tom Xue , Leon Xu , Kalevi Ekman Nokia Research Center (Beijing), Helsinki University of Technology 1,21 111121 2 Building 2, No. 5 Donghuan Zhonglu, BDA, Beijing, China 100176 ABSTARCT This paper describes the design for manufacturing of a flexible wrist device demonstrator. The key components such as display, battery, circuit board and covers are all flexible, so the design for manufacture is a new challenge. In this paper, we take a novel approach to form the covers of the device by insert molding FPCB into elastomer. INTRODUCTION Currently, most of the electronic mobile devices are rigid, such as mobile phones, MP3 players, and wrist watches. Even the mobile phone concepts on wrist are rigid, and they are formed simply by combining a small phone with a wrist strap. This approach has limited flexibility and yields to thick form factors. If we can design a bendable and flexible wrist device, it can bring a lot of convenience to users. For example, it would be more comfortable to wear and can be designed to be thinner than existing devices. This paper shows the architecture, the design of main parts based on simulation results and the manufacturing steps of a bendable wrist display device. DESIGN OF THE FLEXIBLE WRIST DEVICE Based on the product design and development process, the demonstrator description was first defined. In our case, it was specified to have the capability of whole device bending, minimized thickness and width. According to the user study result, an average wrist device perimeter of 190 mm was defined and the device thickness should be less than 6 mm. After the study and cooperation with the vendors of key components, we got the following key components specifications: bendable and very thin STN display with 128x160 pixels and 30.55 mm thickness; bendable battery with size 52x46x1.35 mm. Table 1 shows the list of the key components. To realize the flexible engine board, a conventional 4-layer FPCB (flexible printed circuit board) concept was selected, the electrical components were arranged in groups, so that the FPCB can be bended between the components. Components were soldered only on one side of the FPCB to keep the back surface smooth. Smooth surface was needed in order to be able to insert mould the FPCB with elastomer and to add stiffener plates behind the component areas. Figure 1 shows the device geometry and the main dimensions. The industrial design defined shape, the mechanical design, DFA (design for assembly) and DFM (design for manufacture) were restricted by the selected manufacturing method. Since the covers were fabricated by insert molding the FPCB, the manufacturing shape had to follow the molding tool design restrictions and an open shape like shown on Figure 2 (b). Figure 2 (a) shows also the rendered ID with bended shape in actual use situation. Usually screws and snaps are used for assembling the FPCB and covers, but they can’t be used to fix flexible parts. Instead we used insert molding and adhesives to integrate parts together. The window and covers material was selected based on the requirement for flexibility, window’s transparency and good adhesion between the A-cover and window part. LA elastomers 4285 and 2250 can match these requirements and were selected. GE Lexan series PC 1414 with tough property was selected for the lock cover and extension parts. T ABLE 1. KEY COMPONENTS LIST FIGURE 1. DEVICE GEOMETRY AND MAIN DIMENSIONS (a) (b) FIGURE 2. ID RENDERING AND UNWARPED 3D SHAPE Figure 3 shows the FPCB layout, geometry and size. The USB and vibrator were placed in the end of FPCB, to protect them against the molding pressure. The lock covers were used to form a cover for them. FIGURE 3. FPCB MAIN COMPONENTS LAYOUT SIMULATION OF THE BENDING ANALYSIS Like the Figure 2 shows, the bendable wrist device’s manufacturing shape differs from the shape when it’s bended around user’s wrist. This shape change requires good flexibility performance, which was first evaluated by simulations. Detail simulation description was presented in the 2nd reference paper. Figure 4 shows the bending simulation result. The Mises stress distribution shows that the highest stresses and a potential failure is located in the connection place between window and covers. This information was used for optimizing the FPCB component layout, by placing them outside the highly stressed area. This simulation also showed that very good adhesion between the two parts was needed. Adhesion was ensured by selecting the window and A-cover materials from same product family and optimizing molding process parameters. FIGURE 4. SIMULATION MISES STRESS DISTRIBUTION MANUFACTURE OF THE DEVICE Figure 5 shows the wrist device’s assembly process. The first step is to connect the window with display by double sided adhesive tape. Since the flexible display is manufactured flat and the flexible window is curved, the assembly needed to be done in a jig for pre-bending through pressure and elevated temperature. This improved the connection performance and also reduced the stress concentration for the display in bended state. The second step is to join the flexible circuit board with display assembly with double adhesive tape and load the devices program to the MCU. The engine assembly is then placed in the tool for insert molding. After the molding of A-cover on top of the engine assembly; battery and the separately molded B-cover is glued on its back side. Finally the lock covers are assembled to both ends of the device with snaps and screws. Figure 6 shows a photograph of the manufactured final device. FIGURE 5. DEVICE OF ASSEMBLY PROCESS INSTRUCTION FIGURE 6. THE MANUFACTURED DEVICE CONCLUSION We were able to form a functional bendable wrist display device by insert molding the covers on it. This device is an example of a flexible electronics product with all its key components being flexible. The remaining rigid components were arranged in a way that most of the device is still bendable. As a result we achieved to manufacture a very thin and highly integrated device. However the new manufacturing method imposed many restrictions, which still leave room for improving flexibility and reliability in the future. REFERENCES [1] Karl T. Ulrich . Steven D. Eppinger, Fourth Edition “Product Design and Development”, McGraw·Hill International Edition [2] Quayle Chen, Leon Xu, Cherie Jing, Tom Xue, Antti Salo, “Flexible Device and Component Reliability Study Using Simulations”, 2008 International Conference on Thermal Mechanical and Multi-Physics Simulation and Experiments in Micro-Electronics and Micro-Systems, PP303-307.
