TUT
Department of Electronics
Projects - Design

Kankaanpää Unit - Projects

Design Rules for Flexible Electronics


  • Work done in 2006-2008
  • Extent of work: Diploma thesis work, Research of the effects of component layout in flexible printed circuits
  • Co-operation with Pori Unit
  • Results: Design rules for flexible electronics
  • Contact: Lasse Hahto

The first objective of this research was to find out existing design rules for flexible electronics and material selection. The second objective was to confirm some of the rules by computer modeling and to form new design rules concerning component layout at bending zones by computer modeling and bending tests. These new design rules are needed for example in the applications of wearable technology.

Research methods included Finite Element Method (FEM) modeling and bending tests for flexible printed circuits (FPC). Several FPC designs and layouts were modeled using Comsol Multiphysics to test existing design rules and to create new ones. To test the rules in practice bending equipment was designed and manufactured. Rolling test generally used for FPC materials was selected as the operating principle of the bending machine. Seven different test boards were designed for the bending tests to study the effect of component rotation, distance between components, teardrops and the type of ground plane to flexural endurance of FPCs. Theoretical calculation of flexural endurances of the test boards was impossible due to countless variables caused by the structure of the test boards. Therefore calculation of stresses and strains in the test boards was skipped and the focus of this research was on the design and realization of bending tests and on effects of circuit board design at component level.

Based on the results from modeling and bending tests a simple model can be used in comparing different FPC structures and obtaining indicative results. For example the results of the effect of component rotation to flexural endurance were parallel from both modeling and bending tests. Flexural endurance of FPCs is greatly affected by the component rotation relative to bending zone; especially with components having legs on the two opposite sides of the casing. Increasing the distance between two components enlarges the bending radiuses in FPCs, which leads to smaller strains in conductors and component joints. The bending test results confirmed that only a few millimeter increase in the distance between two components multiplies the flexural endurance of FPCs. A simple method to enhance flexural endurance is to use teardrops in the component pads. The use of teardrops, even small ones, on FPCs is very desirable since the flexural endurance of test boards was tens of times bigger compared to FPCs with no teardrops. It should be noted that even better flexural endurance was achieved by making the teardrops larger. The tests comparing different types of ground planes confirmed the design rule of using crosshatched planes if possible. The tests also confirmed the assumption that better flexural endurance is achieved with coarser crosshatching.

Links and other info:


Updated , Hokkanen Mirja