Conductive Carbon Ink

In PCB manufacturing, carbon ink is a thermalsetting conductive carbon paste, which can be printed onto PCB board through silkscreen technology to connect adjacent copper features to form a conductor between trace or between traces and component. After curing, the carbon layer will form a protective thick film with some resistance. So, we also call Carbon Ink PCB as Carbon Thick Film PCB, or Printed Carbon Resistors (PCR).

Conductive carbon ink has another name carbon paste, which is ok to print on rigid PCB, flex circuit and rigid-flex board with base materials, including FR-4, PTFE, metal cores, and polyimide.

 

 

Two Main Applications

 

• Printed Carbon Resistors: PCR serves as an alternative for applications requiring the duplication of the desired function on a printed board or designated substrate. Carbon ink printing will apply for any combination of fixed resistor or potentiometer tracks in virtually any size, shape or form to a flat surface such as a PCB.
• Carbon Contacts: Carbon contacts or patterns are printed with a conductive carbon ink, which can be used for keyboard contacts, LCD contacts, jumpers and etc. PCB contacts can be gold plated or carbon screen printed. While, gold plated PCB is more expensive than carbon ink printed PCB. As same as hard gold on PCB board, carbon ink creates a protective contact surface for touch-key contacts.

 

Key Factors for Manufacturing High Quality Carbon Ink PCB

 

• Correct sheet resistance carbon ink selection. (Sheet resistance unit is ohm/□, or Ω/□.)
• Screen printing thickness control.
• Carbon ink baking temperature and time control.
• Resistance tolerance control.

 

Applications and Surface Finishes

 

 

Carbon Ink Spacing, Overlaps & Thickness

 

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

 

If requiring thicker carbon ink thickness more than 0.025mm (1mil), we need to print the carbon ink twice. But the second carbon printed shapes should be smaller than that of the first, and the MI engineer needs to write two sets of tools.

 

Carbon Ink Printing Process

 

The  manufacturing process is somewhat different from standard manufacturing process, but similar to silver ink PCB manufacturing process.

• The operator must wear gloves
• The equipment must be clean, the surface shall not have dust, garbage and other debris.
• Screen-printing speed and back to the ink speed suction pressure control in the best range upon the printing effect as a test.
• Screen stencil, scraper, carbon oil specific requirements in accordance with the requirements of MI engineer.
• 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.
• 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.
• Carbon PCB drying temperature is 150oC at 45 minutes. Carbon ink hole drying temperature is 150oC at 20 minutes.
• After the release form oven, the operator should inform the QA to check the carbon resistance and do the adhesion test.
• 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.

 

Electrical Resistivity vs. Sheet Resistance

 

• Electrical Resistivity, also called Specific Electrical Resistance or Volume Resistivity, and its inverse, electrical conductivity, is a fundamental property of a material that quantifies how strongly it resists or conducts electric current. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek letter ρ (rho). The SI (international system of units) unit of electrical resistivity is the ohm-meter (Ω⋅m). For example, if a 1m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contact is 1Ω, the resistivity of the material is 1Ω⋅m.
• Sheet Resistance is a measure of resistance of deposited films that are nominally uniform in thickness. It is commonly used to characterize materials made by semiconductor doping, metal deposition, resistive paste printing (carbon ink, silver ink or the mix ink of carbon and silver), and glass coating. Examples of these processes are: doped semiconductor region, and the resistors that are screen printed onto the substrates of thick-film hybrid microcircuits. The utility of sheet resistance, as opposed to resistance or resistivity, is that it is directly measured using a four-terminal sensing measurement (also known a four-probe measurement) or directly by using a non-contact eddy-current-based testing device. Sheet resistance is invariable under scaling of the film contact and therefore can be used to compare the electrical properties of devices that are significantly different in size. Click Sheet Resistance to check details of its measurement.

 

How to Measure Sheet Resistance on PCB?

 

A four-point probe is used to avoid contact resistance, which can often have the same magnitudes as the sheet resistance. Typically, a constant current is applied to two probes, and the potential on the other two probes is measured with a high-impedance voltmeter. A geometry factor needs to be applied according to the shape of the four-point array. Two common arrays are square and in-line.

Measurement may also be made by applying high-conductivity bus bars to opposite edges of a square (or rectangular) sample. Resistance across a square area will be measured in Ω/sq. For a rectangle an appropriate geometric factor is added. Bus bars must make ohmic contact.

Inductive measurement is used as well. This method measures the shielding effect created by eddy currents. In one version of this technique a conductive sheet under test is placed between two coils. This non-contact sheet resistance measurement method also allows to characterize encapsulated thick-films or films with rough surfaces.

A very crude two-point probe method is to measure resistance with probes close together and the resistance with the probes far apart. The difference between these two resistances will be of the order of magnitude of the sheet resistance.

MADPCB uses four-probe testing device to measure the sheet resistance on flex circuits and rigid boards with carbon ink, silver ink or their mixed ink printed onto the PCBs. This device can measure semiconductor material’s electrical resistivity ρ, sheet resistance R_□ and volume resistance R.

Nowadays, only very a few PCB manufacturers have the carbon ink printing process, while MADPCB would always provide carbon ink printed circuit board service. Need a quote? Contact MADPCB today!