Carbon Ink PCB

Carbon Ink PCB, also known as Carbon Conductive PCB, is the printed circuit board with carbon ink printed on to the copper pads, traces or PCB edges. The carbon ink will be cured to form a carbon film on the PCB board, and the carbon will be a conductor between two traces, and resistor between the traces or traces and components. For building a high-quality carbon PCB, they key is to well control the carbon ink printing, baking and carbon resistance tolerance.

Of course, carbon ink can be printed onto rigid PCBs and flexible circuits. Carbon ink PCB always has two application purposes. One is applied for Conductive Rubber Keypad, another is applied for Printed Carbon Resistors (or called Printed PCB Resistor). MADPCB is known for the fine art of delivering the very low resistance tolerances at very high volume needed to keep pace with automotive innovation in electronics, and we are professional on every aspect of the printed PCB resistor process, eliminating program risk as we deliver around the world.

  • Rotary, linear and stepped tracks can be freely printed.

  • Sizes and shapes can be tailored according to your constraints and automatic production requirements.

  • Full sensor assemblies can be delivered based in PFC and special features such complex gearing for multiturn rotary or linear position sensing and special connector configurations.


Carbon Keypad Design Guidelines

The conductive carbon ink is used to create a protective contact surface for touch-key contacts. Carbon ink applied onto PCB with screen-printing technology is an economy surface finish, which sometimes can replace the expensive surface finish hard gold. The carbon ink PCB manufacturing with crossover copper conductors creates the printed resistances (/resistors). The application of carbon ink PCBs includes electro-mechanical keyboards, foil keyboards, switch contacts, low voltage circuits, shield areas and heating elements. Nowadays, you can see such PCBs in telephones, toys, TV remote controls, computer keyboards, jumpers and etc. Bother flexible circuit and rigid board can be carbon ink printable.

Application

Typical Stroke (mm)

Actuation Force (g)

Life Cycle (x 1,000)

Automotive

1.0-2.5

150-300

50-1,000

Calculator

0.2-3.5

30-80

300-1,000

Computer

2.0-4.0

40-90

5,000-10,000

Measuring Equipment

0.3-1.6

30-120

1,000-3,000

Musical Equipment

0.7-3.5

30-100

1,000-3,000

Sound Equipment

0.3-1.5

70-225

1,000-3,000

Telephone

1.0-3.5

70-250

1,000-3,000

Transmitter

0.3-1.5

50-150

30-1,000

TV & VCR

0.6-1.5

30-120

300-1,000

Typewriter

3.0-4.0

40-70

5,000-10,000

 

Applications of Printed PCB Resistor

Automotive

Home Appliances

Industrial

Climate control (Temperature, Air flow, Distribution)

Headlight positioning control

Mirror position sensor

Headlight position sensor

Power sunroof controls

Fuel tank level sensor

Pedal sensors

Instrument panel controls

Dimmers for lighting

Seat position sensor

Steering column controls

Light dimmers

Speed control for power tools

Sewing machine controls

Rotary switches

Heavy-duty equipment

Material handling equipment

Marine grade sensors

AG and farm equipment

 

Carbon Ink Spacing, Overlaps &Thickness

Considering of the tolerance and carbon ink leakage and other factors, carbon ink PCB design should be accordance to below parameters.

Min Carbon Gap (Carbon to Carbon Spacing)

0.4mm (16mil)

Min Carbon Trace Width

0.3mm (12mil)

Min Carbon on Copper Overlap

0.2mm (8mil)

Min Carbon on Solder Mask Overlap

0.125mm (5mil)

1st Printing Thickness

0.0125mm (0.5mil) – 0.025mm (1mil)

2nd Printing Thickness (after 1st printing cured)

0.025mm (1mil) – 0.05mm (2mil)

If requires carbon ink thickness on your printed circuit board (PCB) more than 0.025mm (1mil), you need to print the carbon ink twice. But please note the second printing of carbon ink film should be smaller than the first screen-printing. So, the MI engineer needs to write two sets of tools.


Carbon PCB Resistance Measurement

Carbon ink is printed on PCB surface as a conductor of PCBs. For carbon ink PCB, the most important thing is to ensure the quality and resistance of the carbon film, meanwhile, the immersion silver and immersion tin PCB cannot be printed carbon ink, because they are Oxidizing.

When measuring carbon resistance on carbon PCB, we use a special measuring equipment which can directly display the resistance read (value) and is generally valid for any square surface. With square surfaces, the resistance of the carbon ink -at a given coating thickness approximately 0.025mm (1mil), which is a material constant. This quantity is called layer resistance and expressed in the unit ohm/square.

 

Why Choose Carbon Ink Printed on PCB?

  • Replacement for Gold Fingers

Traditionally, Nickel/Gold plating has been applied over copper tracks used for keyboard contacts or edge fingers. This provides a conductive, corrosion resistant, environmentally stable coating. Replacement of gold plating by carbon ink offers the following additional advantages:

  1. Cost -Replacing gold with carbon reduces materials cost and replacing the electrolytic bath with a simple screen-printing process reduces process costs.

  2. Robustness -Tests have shown that a typical carbon ink is hard enough to withstand >1 million pushbutton operation (keypad) or 100 insertion operation with a 75g force (edge connector) without showing wear or increased resistance.

  3. Resistance -The above advantages are gained without a significant increase in loop resistance of the closed circuit using the graphite pill. The thin print of carbon ink (typically 15μm) has a low resistance, less than the resistance across the graphite pill.

Carbon inks must have good viscosity stability for warmer climates and require good printing properties to give even coverage over the copper pads. The cured carbon ink must be hard and must withstand subsequent processing steps such as solvent cleaning and soldering without loss of adhesion and conductivity. Peelable solder mask is often used to protect keypads and edge connectors during solder against flux contamination and solder pick-up; the carbon ink must resist these materials.

  • Crossovers

The trend towards more complex and smaller circuits has created a challenge for the single-sided manufacturer who need to fabricate more complex boards. The answer is to add a second layer of circuitry that can cross over each other on the single-sided PCB. This second layer of conductive tracks can be printed over the first layer of copper tracks using carbon ink, with contacts to the underlying copper pads at each end. An insulating layer of dielectric ink separates the copper and conductive ink layers. Two prints of dielectric ink may be required to eliminate rejects due to shorts between the conductor layers arising from small pinholes, and also to give a smooth surface on which to print the carbon ink. During the cure of the dielectric layer, the copper pads will oxidize so will require cleaning to ensure low contact resistance and good adhesion of the carbon ink. Finally, a further dielectric layer can be printed over the carbon to ensure optimum environmental stability. Crossovers have the following advantages:

  1. Cost -Materials and process costs for carbon ink and screen printing are much lower than the alternative of drilling and through-hole plating needed to produce double-sided or multilayer PCB boards.

  2. Proven method -PCBs for telephones, toys, television remote controls, computer keyboards, etc have been produced by this method.

Carbon inks must have good adhesion to copper and dielectric and be unaffected by the subsequent processing steps described above for the replacement of gold plating.

 

Carbon Ink Process in Carbon PCB Manufacturing

The conductive carbon ink printing process in fact is use the screen-printing technology, printed carbon ink in the specified location of the rigid PCB or flex circuit, after oven cured and tested, formed a carbon film which with certain resistance to instead of the original resistance components. In fact, the carbon ink PCB production process is the same with the silkscreen, to insist on what is the difference, that is, carbon ink has better conductive performance, the silkscreen is only played soldering identification role.

  1. The operator must wear gloves

  2. The equipment must be clean, the surface shall not have dust, garbage and other debris.

  3. Screen-printing speed and back to the ink speed suction pressure control in the best range upon the printing effect as a test.

  4. Screen stencil, scraper, carbon oil specific requirements in accordance with the requirements of MI engineer.

  5. Carbon ink must be mixing evenly before use, with a viscometer to detect the viscosity within the required range, the ink needs timely closure after the use.

  6. Before printing, all PCB boards must be cleaned plate grease, oxide and other pollutants, all carbon plate must be confirmed by the QA before the official production.

  7. Carbon PCB drying temperature is 150oC at 45 minutes. Carbon ink hole drying temperature is 150oC at 20 minutes.

  8. After the release form oven, the operator should inform the QA to check the carbon resistance and do the adhesion test.

  9. Each carbon ink screen version uses max 2,500 printing times, and must be returned to the network room re-drying the new version when up to 2,500 printing times.


At the moment, only very a few PCB manufacturers have the carbon ink printing process, but MADPCB would always provides carbon ink PCB service for not only for high volume quantity, but also for carbon PCB prototype.