Flexible Printed Circuit Boards Design

A total of six different PCBs were designed for this project with similar flexible PCB properties. The flexible PCBs consist of a 25 µm polyimide core, two 18 µm copper layers (0.5 oz), and a 27.5 µm yellow overlay (Polyimide incl. adhesive), which sum up to a total PCB thickness of 116 µm. Exposed pads have a surface finish of 1U” immersion gold (ENIG) to improve solderability and connectivity of connecting pads, as highlighted in Fig. 6d. In general, traces have a minimum width of 0.254 µm (10 mils). The width is decreased for heating patterns that have a trace width of 76 µm (3 mils). The trace minimum spacing for all PCBs is 76 µm (3 mils). To prevent a potential fracture of the traces while bending, all traces were perpendicularly oriented to the bending edges.
The PCBs in Fig. 6 have 4 LEDs that were connected in parallel, and each leading resistance was chosen such that the circuit could be connected to a 5 V power source. The heating pattern dimensions were designed for a maximum current of 300 mA, at which the temperature rises to 60 °C, which is also the maximum operating temperature for the magnets used. The total resistance mainly depends on the length of the 3 mm traces. Therefore, the letter “E” from Fig. 7 has the highest total resistance of 14.5 Ohm, followed by the letter “T” with 11 Ohm and the letter “H” with 10.5 Ohm. All resistances were measured at room temperature.