Carbon Print on PCB

PCBs can be treated with different coatings in addition to solder resist and screen print. Carbon print is one of such additional PCB coatings.

Contacts on PCBs

To use carbon as PCB surface for switches, PCB contact pads are printed with carbon paste.

Advantages versus hart gold surface are

  • no expensive gold necessary
  • PCB layout without electrical connection for galvanic deposit
  • no selective deposit with additional resists necessary

 

Design for PCB contact

  • Width of carbon track >0.4 mm, gap >0.6 mm
  • copper should be overprinted 0.1 mm around
  • Solder resist should be carbon print + 0.2 mm  around

 

PCB Resistor Print

Reisitors can be directly printed with the carbon paste. Resists are equal the area resist [Ohms per square], multiplied by the ratio of length to width:

Widerstandsdruck auf Leiterplatten

There are carbon pastes from 15 Ohms/sq to 1000 kOhms/sq. To obtain different resists it is recommended to use different paste geometries (i.g. meandrian). Tolerances are ca. 30%.

Potentiometers on PCBs

A printed strip of carbon resist can work as potentiometer as well.

PCB wiper

If a conductor strip is designed under the carbon strip, this structure can work as wiper.