Carbon Ink PCB Pads: Design Rules for Keypad and Switch Contacts

Carbon ink on a PCB is a screen-printed, graphite-filled polymer paste that is cured onto copper pads to make keypad and switch contacts without gold plating. On a carbon ink PCB, it is the black pad you see under the rubber buttons of a remote control. The pads that work are simple: print the carbon over the copper before the solder mask, overlap it well past the copper edge (one ink maker’s report gives 1.5 mm as the proven overlap for carbon-only contacts), keep printed tracks at least about 1 mm wide, and specify a sheet resistance with a measuring method rather than “conductive”. Carbon contacts suit rubber-dome keypads and membrane switches; they do not suit edge connectors that are inserted repeatedly.

Values are from Peters Report 148 on carbon-conductive inks, a technical report by the ink maker Lackwerke Peters; the document dates from 2015, so check current product data for your ink.

What Is Carbon Ink on a PCB?

A conductive paste of carbon black and graphite in a polymer binder, printed through a screen onto the board and heat-cured. After curing it forms a hard, abrasion-resistant film that conducts well enough for switching and does not corrode, which is why it replaces plated gold on contact pads.

It is far less conductive than copper. Sheet resistance for carbon inks starts at about 14 Ω per square at a 25 µm film, against a fraction of a milliohm per square for 35 µm copper. That is fine for a keypad contact that only has to switch a logic input, and it is the reason carbon is not used to carry current.

Carbon-filled inks are covered by printed electronics standards rather than PCB plating standards. IEC 62899-202 defines terms and test methods for conductive inks and printed conductive layers (IEC 62899-202:2023), but there is no IPC plating specification for carbon contacts, so the drawing has to carry the requirements.

Where Are Carbon Contacts Used?

Mainly on keypad and switch boards where a conductive rubber pill or dome closes the contact:

• Rubber-dome keypads: remote controls, telephones, instruments and appliance panels. Peters reports that carbon tip contacts exceeded 25 million contact cycles in long-term tests, against a German postal specification of 1 million.
• Sliding contacts and position sensors: potentiometer tracks and selector switches, where the report cites up to 125,000 cycles with optimized processing.
• Printed resistors and jumpers: carbon resistors printed between copper landings, and carbon cross-over conductors on single-sided boards.
• Membrane keyboards: carbon printed on polyester or polyimide film.

Contact fingers are the exception. Carbon overprinted on edge-connector fingers is good for only about 20 insertion cycles in the same report, so connectors that are mated repeatedly need hard gold.

How Should You Design Carbon Contact Pads?

Make every copper area that forms part of the contact fully covered by carbon, and hide every transition under solder mask. Exposed copper at the edge of a carbon pad corrodes and eventually causes intermittent contact.

There are two pad styles:

1. Carbon over copper. The contact pattern is etched in copper and carbon is printed over it. Print the carbon before the solder mask so the copper-to-carbon transition sits under the mask. Use rounded or semicircular contact shapes; meandering interdigitated copper is hard to cover completely with a screen print.
2. Carbon only. The copper track stops under the solder mask, and the contact pad itself is printed in carbon directly on the laminate. This needs less print accuracy and removes exposed copper entirely. Overlap the carbon onto the copper by about 1.5 mm so a misaligned print still makes contact.

For the rubber contact itself, match the pad diameter to the conductive pill with margin for keypad alignment, and leave the area around the pad clear of legend ink and mask so the pill seats flat.

How Do You Specify Carbon Resistance?

Give a sheet resistance or a resistance between two named test points, with a tolerance and the measuring method. Carbon films vary with thickness, so a value without a method cannot be checked. Peters notes there is no standardized method for conductive inks and suggests measuring over a defined area; whichever method you pick, put it on the drawing so incoming inspection and the fabricator measure the same thing.

Design for a sheet resistance above the ink’s best value, because the film will not be uniform across a panel. For a carbon track, resistance is roughly the sheet resistance times the number of squares (length divided by width): a 10 mm long, 1 mm wide carbon jumper at 14 Ω/sq is about 140 Ω. If that is too high for the circuit, widen the track, shorten it, or use a silver-filled ink for cross-overs, which is far more conductive but needs carbon on top to stop silver migration.

For printed resistors, Peters lists pastes from 14 Ω/sq up to 1 MΩ/sq, copper landings of at least 500 × 400 µm, a carbon width of at least 450 µm, and laser trimming where tight tolerances are needed. Most designs now use chip resistors instead, but printed resistors remain in high-volume, low-cost boards.

How Does Carbon Compare With Gold Contacts?

Carbon is cheaper and simpler; gold is lower in resistance and survives insertion wear. Selective gold plating needs masking, electroplated nickel and gold, resist stripping and a second mask step. Carbon needs one screen print and a cure. The saving comes as much from the shorter process as from the material.

On many keypad boards the rest of the board still needs a solderable finish, so carbon is printed over an ENIG or HASL board. Our comparison of HASL and ENIG covers that choice, and the ENIG surface finish guide covers the plating itself.

Can You Solder to Carbon Ink?

No. Solder does not wet carbon, so carbon pads are contact surfaces only. Keep solder pads and carbon contacts as separate features, and if the board is wave soldered, protect the carbon areas: peelable solder masks are made for this, and the SD 2950 peelable mask series from Peters, for example, lists several grades as suitable for use over carbon-conductive inks. Check that the carbon is cured before assembly heat and that the cleaning chemistry does not attack the binder.

When Is Carbon the Wrong Choice?

When the contact carries real current, sees repeated insertion, or needs a precise resistance. Carbon is a poor choice for edge connectors, power contacts, test points probed many times, and analog circuits where a few percent of resistance drift matters. It is also limited on fine features: screen prints spread at the edges, so stay with about 1 mm track width and 0.5 mm spacing unless the fabricator confirms finer printing for your ink.

For dynamic flexing, carbon on polyimide flex works in membrane switches, but bend areas need their own validation; see our flexible PCB page for flex constructions.

What Should the Drawing Include?

How Can We Help?

QueenEMS prints carbon ink contacts on its bare-board lines and reviews the carbon layer in the same DFM pass as the rest of the board, checking overlap, mask coverage and spacing against rules like those above. Send the Gerber files with the carbon layer as its own file and the resistance requirement; our free DFM check flags pads that will leave copper exposed, and you get bare-board pricing within 2 hours. For early keypad samples, rapid prototyping gets boards in hand for fit and feel tests.

Conclusion

Cover all copper, overlap generously, and put a measurable resistance on the drawing. Contact us with the Gerber files, carbon layer, resistance requirement and quantities.

FAQ

What is carbon ink on a PCB? A screen-printed, graphite-filled polymer paste cured onto contact pads, used mainly for keypad and switch contacts in place of gold.

Is carbon black conductive? Yes, but much less than copper; carbon PCB inks reach about 14 Ω per square at a 25 µm film.

How long do carbon keypad contacts last? Published long-term tests show more than 25 million contact cycles for carbon tip contacts.

Can you solder to carbon pads? No. Solder does not wet carbon, so carbon areas are contact surfaces only.

How do you repair worn carbon contacts? Clean them with isopropyl alcohol first; badly worn pads can be recoated with a conductive repair paint, but heavy wear usually means replacing the keypad or board.