Carbon Ink PCB: Conductive Printing for Contacts and Jumpers

Carbon ink is one of the quiet workhorses of a board. It appears as a black rectangle on the contacts of a keypad, as a short link across two nets that must be joined without a resistor, and as the resistive element of a simple trimmer. It is printed, cured and forgotten, and when it is specified properly it works for the life of the product. When it is not, the contact resistance climbs and the unit starts to miss key presses.

Where Carbon Ink Is Used

The most familiar application is the conductive pad on a membrane switch or a rubber keypad, where the ink forms the fixed contact that a conductive rubber pill shorts when a key is pressed. The same material is used for the carbon jumper that crosses one trace over another on a single sided board, and for the resistive track of a low cost potentiometer or a heating element.

The common feature is that the conductor does not have to be soldered. Carbon ink is printed onto copper, cured, and left exposed as a contact surface. That makes it chemically stable, resistant to oxidation and to many cleaning agents, and, unlike bare copper, suitable for a surface that will be touched thousands of times.

How the Ink Is Applied

The process is screen printing, one of the oldest forms of conductive printing. A stencil defines the pattern, the ink is forced through the mesh onto the board, and the board is then cured in an oven or with infrared or ultraviolet energy depending on the formulation. Thickness is controlled by the mesh, the emulsion and the squeegee, and it typically falls in the range of ten to thirty micrometres, which is enough to give a predictable resistance.

Curing is where the product is won or lost. An under cured film has poor adhesion and a resistance that drifts; an over cured film becomes brittle and cracks when the board flexes. The profile is specific to the ink, and the fabricator’s process window has to be verified rather than assumed, particularly when the same panel carries inks from two suppliers.

Contact Resistance and Wear

The useful property of carbon ink on a contact is not its bulk resistivity but its contact resistance against the counter surface, and that value depends on the pressure, the geometry and the surface finish of both parts. A printed pad with a rough surface makes better contact than a smooth one, which is why the ink is often printed in a pattern of small dots or lines rather than as a solid rectangle on the contact area.

Wear is the second consideration. Each key press moves the rubber pill slightly, and the carbon surface abrades gradually. Ink formulations with a higher binder content last longer but present a higher resistance, and the two properties have to be traded against the number of operations the product must survive and the resistance the circuit can tolerate.

Carbon Ink as a Jumper

The carbon crossover jumper is a way to route signals on a single sided board without a plated through hole or a wire link. A copper trace is interrupted, a dielectric layer is printed over it, and the carbon ink is then printed across the gap to complete the second net. The carbon provides the connection without touching the copper beneath it.

The jumper carries only a small current, and it has a resistance of its own that must be accounted for in the circuit. Where the current is larger, or where the resistance matters, a copper jumper or a link is the correct answer, and the calculation that decides how much copper a given current needs is set out in trace width and current calculation.

Design Rules for Printed Carbon

Give the carbon plenty of overlap onto the copper it connects to, because the bond between the ink and the copper is the weak point and a small contact area means a contact that fails first. Keep the minimum feature size within what the printer can hold, and remember that the screen printing process has a wider tolerance than the imaging used for copper.

Keep the carbon away from solder pads unless the assembly process is controlled, because solder will not wet the carbon and a printed pad that extends into a solderable area will cause an open joint. The clearance rules and the pad geometry that apply generally are described in PCB pad design standards, and the same margins apply to a printed feature.

Interaction with Solder Mask and Finish

Carbon is normally printed after the solder mask, so the mask must be opened where the ink is to contact copper. Mask that overlaps the carbon contact area can lift during curing or during assembly and take the ink with it. On a flex or membrane construction, the coverlay openings must line up with the printed pads closely enough that the counter surface lands on carbon rather than on laminate.

The finish on the rest of the board also matters. A board that carries both printed carbon and soldered components has to be assembled with a process that does not damage the ink, which usually means keeping the printed areas away from any cleaning or flux operation that would attack the binder. The choice of a soldered finish and its implications are discussed in lead free versus leaded solder.

Choosing the Ink

Conductive carbon inks differ in resistivity, adhesion, flexibility and curing behaviour, and the datasheet values are measured under conditions that may not match the application. Where the board will be flexed, a flexible formulation is required; where the resistance must be low, a formulation with a higher carbon loading is used, at some cost in adhesion and in printing resolution.

The thickness also affects the result. A thicker print gives a lower resistance and a rougher surface; a thinner print is easier to define but has less material to absorb wear. Since the process tolerance is wider than most designers assume, the specification should state a resistance range for the finished feature rather than only a thickness.

Verification and Life Testing

Verification begins with a measurement of the resistance and the thickness on a coupon, and continues with an adhesion test on a sample. For a keypad, the meaningful test is a life test in which the switch is operated through the required number of cycles and the contact resistance is measured before and after, and for a product with a flexing area, a bend test that follows the intended motion.

Environmental testing follows the end product: humidity and thermal cycling for a product that will see both, and a salt spray or a chemical exposure test where the surface will be handled or cleaned. The results should be recorded against the ink batch, because a change of supplier or of formulation is the most common reason for a product that worked in qualification to fail in production.

FAQ

Can carbon ink be soldered? No. Solder does not wet it and will not form a joint. It is a contact surface and a conductive link, not a solderable pad.

How much current can a carbon jumper carry? Only a small current, and its resistance must be included in the circuit. Where more current is needed, use a copper link or a plated through hole.

Why do keypad contacts use a dotted pattern? Because the dots concentrate pressure and break through surface films, giving a lower and more stable contact resistance than a flat printed area.