Carbon Contacts and Resistors in Printed Circuit Boards - a Practical Overview

Embedding passive components in and on the printed circuit board as well as integrating functions into the boards by using standard PCB production steps have great potential for saving space and cost by coeval increased functionality. This paper describes experiences with the replacement of expensive gold coatings for sliding contacts and keypad applications by carbon layers in mass production. It is possible to produce carbon layers with equivalent properties and high reliability regarding to life-time and reducing the PCB cost significantly. 150000 switching cycles for sliding contacts can be obtained absolutely reliable and 500000 cycles with acceptable results. The boundary conditions and cost aspects for this technology will be introduced. Replacing passive discrete components like resistors through embedded ones, opens a lot of possibilities for the designer to put more circuitry into smaller spaces and save costs for the complete electronic device. Polymer thick film (PTF) resistor technology with resistor inks is very similar to carbon contact technology. Therefore trials to implement these technology by using carbon contact production environment has been undertaken. The tolerance behaviour of different inks in dependence on layout has been tested and variations through PCB process steps has been investigated. So it was possible to release design rules for PTF resistors. To reduce resistor tolerances to use them as component resistor trials with laser trimming has been done. It was possible to obtain resistors with tolerances of plusmn1% for trimmed ones and withplusmn 25-30% for untrimmed ones