Applying Carbon Contacts to PCBs

What are Carbon Contacts for PCBs?

Carbon contacts are conductive pads or traces made from carbon-based materials that are applied to printed circuit boards (PCBs). These contacts provide electrical connections between components on the PCB and enable the flow of current through the circuit.

Benefits of Using Carbon Contacts on PCBs

Carbon contacts offer several advantages over traditional metal contacts for PCBs:
– Lower cost
– Corrosion resistant
– High temperature stability
– Non-magnetic
– Biocompatible

The following table compares some key properties of carbon contacts versus common metal contacts used on PCBs:

Property	Carbon Contacts	Copper Contacts	Gold Contacts
Cost	Low	Moderate	High
Corrosion Resistance	High	Low	High
Max Operating Temp	400°C	180°C	200°C
Magnetism	Non-magnetic	Diamagnetic	Diamagnetic
Biocompatibility	Excellent	Poor	Good

As shown, carbon contacts provide an attractive combination of low cost, high corrosion and temperature resistance, non-magnetic behavior, and biocompatibility compared to metals. This makes them well-suited for applications like medical devices, high-temp electronics, and low-cost consumer products.

How Carbon Contacts are Applied to PCBs

Carbon Ink Printing

One common method of applying carbon contacts to PCBs is through printing conductive carbon ink, similar to how metal traces are often screen printed. The carbon ink, made from graphite or carbon nanoparticles dispersed in a polymer binder, is printed onto the PCB substrate through a patterned stencil or screen.

The printed carbon traces are then thermally cured to evaporate solvents, leaving behind solid conductive pathways. Typical curing temperatures are in the range of 100-200°C for 10-60 minutes.

Compared to screen printing, other printing methods like inkjet and aerosol jet can enable finer resolution carbon traces down to 10 microns wide. However, screen printing remains the most common and lowest cost approach.

Carbon Film Lamination

Another way to apply carbon contacts is by laminating carbon-based films onto the PCB. The film, which consists of a carbon-loaded polymer on a carrier sheet, is cut to size, positioned on the PCB, and bonded using heat and pressure.

Lamination allows carbon contacts to be applied over larger areas of the PCB more easily than printing. The film can also be patterned by laser-cutting or photolithography prior to lamination.

Carbon films with different resistivities (surface resistance) are available to tailor the electrical properties of the contacts. Typical values range from 10-1000 ohms/square.

Applying Carbon Contacts to Flexible PCBs

Carbon contacts are especially attractive for flexible electronics because the carbon material can bend and flex with the PCB without cracking or delaminating like metal films. To leverage this advantage, carbon inks and films have been developed with elastomeric binders like silicone rubber that can withstand high strain.

Applying carbon to flexible PCBs follows the same basic printing and laminating processes, but may require cooler cure/lamination temperatures and the ability to handle moving flexible webs. The end result is a flexible circuit with durable carbon contacts that can bend and stretch with the device.

Designing PCBs with Carbon Contacts

Carbon Contact Patterning and Routing

Laying out carbon contacts on a PCB follows many of the same design rules and guidelines as for metal traces, with some key differences:

• Minimum carbon trace width and spacing is typically larger than for copper (e.g. 8 mil vs 4 mil) due to the lower conductivity and printing resolution of carbon. Finer traces require specialized carbon inks and printing processes.

• Carbon contacts can typically carry less current than metal traces of the same size. Wider carbon traces or multiple traces in parallel may be needed for higher current pathways.

• Carbon has higher resistivity than metals, so maximum trace lengths are shorter to limit voltage drop. Voltage-sensitive signals should be routed with shorter, wider carbon traces.

• Vias between carbon contact layers require conductive carbon or metal paste to be printed/dispensed into drilled via holes. Carbon films cannot be electroplated like copper to fill vias.

• Angular bends in carbon traces should be avoided to prevent cracking or delamination, especially for contacts on flexible PCBs. Rounded trace corners and serpentine bends are preferred.

Printed carbon also has a lower maximum thickness than copper traces (around 25 microns vs 35 microns for 1 oz copper). Thicker carbon traces require multiple print-and-cure cycles.

Soldermask and Silkscreen with Carbon Contacts

Soldermask, which insulates metal traces on a PCB, is not used with carbon contacts because carbon does not wet with solder (which is an advantage for avoiding solder bridging).

Text and graphics are typically printed directly onto the exposed carbon contacts using either conductive or non-conductive inks. Alternatively, labels can be applied over carbon contacts since the low surface energy prevents good adhesion of inks.

Connecting Carbon to Copper

Interfacing carbon contacts with metal traces or components on a PCB requires special considerations. While carbon and copper have similar work functions and can exchange charge, a metal-carbon junction creates a galvanic cell that accelerates corrosion in humid environments.

To mitigate this galvanic corrosion, a non-metallic conductive adhesive (such as silver-filled epoxy) can be used to bond carbon to metal. The adhesive provides a conductive bridge while preventing direct metal-carbon contact. Applying conformal coating or encapsulant over the junction also helps prevent moisture intrusion.

Soldering wires or components to carbon is very difficult due to carbon’s low surface energy and high temperature resistance. Conductive epoxy or mechanical crimp/spring contacts are preferred for making electrical connections to carbon contact pads.

Applications of Carbon Contacts on PCBs

Medical Devices

Carbon contacts are increasingly used in PCBs for medical devices because of their biocompatibility, flexibility, and MRI compatibility (non-magnetic). Examples include:

• Wearable monitors with carbon contact electrodes
• Implantable sensors with carbon contact pads
• Disposable test strips with printed carbon traces

The low cost of carbon also makes it attractive for single-use medical electronics that would be prohibitively expensive with gold contacts.

High Temperature Electronics

The high temperature stability of carbon (up to 400°C in non-oxidizing atmospheres) enables PCBs with carbon contacts to operate in extreme environments where solder connections would melt, such as:

• Downhole drilling equipment
• Automotive engine controls
• Aerospace & military systems

While ceramics are used as PCB substrates for very high temperature applications (>400°C), carbon contacts can provide a lower cost solution for electronics up to 400°C.

Flexible & Stretchable Electronics

Carbon’s crack resistance and compatibility with elastomers make it a good choice for contacts in flexible and stretchable electronics, including:

• Flexible displays and touchscreens
• Flexible solar cells
• Conformable and wearable sensors
• Soft robotics

Compared to metal films that quickly fatigue and crack under repeated bending or stretching, carbon contacts can provide more robust connectivity for flexible circuits.

Low Cost Devices

Replacing metal contacts with carbon is an effective way to reduce the cost of high-volume, disposable electronic devices like:

• RFID tags
• Smart cards
• Disposable sensors
• Novelty electronics

For these applications, the per-unit cost savings of carbon over metal can be significant due to the large production volumes. Combining carbon contacts with low-cost additive manufacturing methods like inkjet printing enables fully printed electronics.

Frequently Asked Questions

How do carbon contacts compare to metal contacts in terms of conductivity?

Carbon contacts have lower conductivity than metals, typically in the range of 10-100 S/cm compared to 58,000 S/cm for copper. However, this conductivity is sufficient for many applications, and the conductivity can be increased by using special carbon materials like graphene or carbon nanotubes.

Can carbon contacts be used for high frequency signals?

Yes, carbon contacts have been used for high frequency applications up to several GHz. The skin effect resistance of carbon is lower than metals at high frequencies, which can be advantageous for RF and microwave circuits. However, the lower conductivity of carbon may require wider traces or ground planes which can limit the maximum frequency.

Are there any environmental concerns with using carbon contacts?

Carbon contacts are generally considered environmentally friendly and non-toxic. Unlike some metals like lead and beryllium, carbon is not a heavy metal and does not pose the same health risks. Carbon materials are also readily available and renewable compared to scarce and expensive metals like gold and palladium. However, care should be taken to properly dispose of any solvents or other chemicals used in carbon ink printing or lamination.

How do you specify carbon contacts on a PCB?

When designing a PCB with carbon contacts, the type and thickness of carbon material should be specified, along with any requirements for resistivity, printing resolution, or Via Filling. The choice of carbon ink or film and the corresponding cure/lamination conditions will depend on the specific application and substrate material. It’s best to work with an experienced PCB manufacturer to develop a suitable process for your design.

What are the challenges of using carbon contacts on PCBs?

Some of the key challenges include:
– Lower conductivity requires wider traces and limits current carrying capacity
– Higher resistivity limits trace length and voltage drop
– Carbon inks require thermal curing which can be a bottleneck in production
– Laminated carbon films can be difficult to pattern with fine features
– Soldering to carbon is not feasible

Designers must account for these limitations and adapt their PCB layout and assembly process accordingly when using carbon contacts. However, for many applications the benefits of carbon outweigh the challenges.

Conclusion

Carbon contacts offer a compelling alternative to traditional metal contacts for PCBs, providing lower cost, higher durability, and unique properties for medical, high-temperature, flexible, and low-cost electronics. As carbon materials and printing processes continue to improve, the use of carbon contacts on PCBs will likely grow.

While carbon is not a drop-in replacement for metal, designers who understand the strengths and weaknesses of carbon can leverage its advantages to create innovative PCB designs. With the right application and manufacturing process, carbon contacts can enable new possibilities for the future of printed electronics.