EMI-Suppression Design in Flexible Printed Circuit Design in Small Electronic Devices

In this work electromagnetic interference (EMI) issues on flexible printed circuit (FPC) design are covered in physical interconnection design, and also signaling point of view. Mechanical requirements towards light and flexible interconnection designs is sometimes conflicting against design principles in electric design of interconnects with adequate shielding properties and immunity from the noise signals from the system itself and surroundings. Therefore mechanical and electrical design criterias must be full-filled, and this makes compromising between electrical and mechanical properties a challenging task. In this work one-two- and three-layer FPC structures are used as a sample cases. All the structures are similar in impedance point of view, so strip widths, and thickness of the layers are set to match the impedance requirements. The focus on this study is on the field distributions around the structure in all of the sample geometries. In this work the near-field attenuation from the signal traces are compared in the studied structures, and also afar-field emission characteristics are calculated and compared to existing EMC regulations for far-field emissions. Single-ended, and differential signaling techniques are used for sample geometries, and the effects of the signaling for the emission properties is included in the analysis. The modeling methodology is described briefly, and it consists of time-domain full-field electromagnetic analysis in which it is possible to excite several signal traces simultaneously, and evaluate the field concentration, and field attenuation around the interconnection structures. Commercial CST Microwave Studio electromagnetic analysis tool is used for electromagnetic analysis of the cases. There are little cases published, where mechanical, and material properties of the physical structures are presented together with the used signaling techniques. However, it is important to treat mechanical and electrical properties together - to get a picture of the EMI-properties of the interconnection (Gravelle et al., 1992). Electromagnetic analysis part is the only part in the modeling phase where the field distribution information is accessible