Carbon Inks

Coates carbon inks for flexible applications

This article describes two applications for carbon inks used for flexible circuits and gives details of Coates inks that meet the requirements for these applications.

FLEXIBLE APPLICATIONS:
FLEXIBLE CIRCUITS
Traditionally, flexible circuits are manufactured by imaging and etching copper-clad polyester or polyimide. This method is necessary when the high current carrying properties of copper are required. An alternative approach is to create conductive tracks by using PTF (polymer thick film) conductive inks; this approach is suitable for lower current carrying applications. Conductive inks offer the following advantages:

Cost The simple additive print and bake process is considerably
cheaper than the traditional subtractive etch process.
Substrate These are not limited to polyester or polyimide; a wide
range of cheaper materials are available; e.g. polycarbonate
Speed The simple additive process permits a faster throughput than
the traditional subtractive etch process.

Carbon inks must have good printing properties for adequate print resolution. The cured ink must have good adhesion to the flexible substrate, enough flexibility to withstand folding, and adequate electrical resistance. Carbon can be used in combination with silver inks, according to the resistance requirements. FIGURE 1 shows the underside of a circuit on polyester where the silver circuit is overprinted with green dielectric and carbon inks.

MEMBRANE SWITCHES
Membrane switches are increasingly replacing mechanical switches for low
power consumer, domestic and industrial applications. FIGURE 2 (right) shows a schematic plan of a membrane switch; Coates supplies all the inks used in the manufacture.

Membrane switches offer the following advantages over mechanical switches(2):

Cost - Ink costs per switch are low when a large number of keypads can
be made with a single print. A keyboard with 40 mechanical keys can be
replaced by a membrane at a fifth of the cost.
Appearance Switches can be overprinted by a vast range of colours
and designs, enabling fast and easy recognition of functions.
Reliability As there are no moving parts, a membrane switch is
extremely reliable and could last longer than the equipment that it
controls.
Wipe-clean The flat surface is easy to clean and is impervious to
dust and moisture.

FIGURE1 - FLEXIBLE CIRCUIT

Carbon inks must have good screen stability to give continuous circuits
and adhesion to the polyester or polycarbonate substrate. The other
properties are as for flexible circuits already described. For more
critical requirements, silver ink provides better conductivity but at higher cost. Frequently, blends of carbon and silver ink are used to provide an optimum between cost and resistance constraints so the two inks must be compatible. FIGURE 3 shows carbon used as part of a membrane assembly.

FIGURE 3 - MEMBRANE ASSEMBLY

COATES CARBON INKS

PRODUCTS
Coates carbon inks 26-8203 and XZ351 are screen printing inks used in the fabrication of flexible circuits and membrane switches on a variety of substrates. They are suitable for automatic and semi automatic printing machines. XZ351 is a lower resistance and slower drying version of
26-8203, to give a longer screen life. It can also be blended with Coates silver inks XZ250 and XZ251.

For details see APPENDIX 1 (product and pack codes) and APPENDIX 2 (ink properties).

Viscosity will vary according to conditions; temperature, viscometer, sample size. This information is provided for guidance only and does not form a specification

SUBSTRATES
26-8203 and XZ351 are suitable for application onto a wide variety of substrates, including untreated and treated polyesters, which are commonly used for membrane switch assemblies. Treated polyester is particularly recommended to ensure good adhesion of subsequent inks such as UV dielectrics and graphic inks.

XZ351 can also be used on other heat sensitive substrates such as PVC and ABS. Before starting a print run it is recommended that print tests for compatibility with a given substrate are carried out.

INK ADJUSTMENT
For many applications, 26-8203 and XZ351 can be used as supplied, but where viscosity reduction is required, the minimum quantity of Retarder XZ42 should be used. It is recommended that a maximum of only 5% is added, as any addition may reduce dry film weight and increase surface resistively.

PRINTING AND DRYING
Resistance is governed to a large extent by print thickness. Therefore meshes should be selected to give the required dry film thickness and surface resistivity.

Monofilament meshes of 49 - 90T/cm. (125 - 230T/inch) are recommended. Finer meshes give thinner prints and higher actual (as printed) resistance values. The smaller mesh hole area is unable to allow as much ink through. APPENDIX 3 shows typical surface resistance values obtained for XZ351 using different polyester screens with 23mm emulsions. For optimum results a polyurethane squeegee of 65° Shore hardness was used. All prints were dried at 120oC
(248oF) for 30 minutes in a fan convection oven. Adequate drying is essential to achieve optimum conductivity and film properties

BLENDING WITH SILVER INKS
Carbon inks 26-8203 and XZ351 can be mixed with silver inks XZ250 or XZ251 to give inks with intermediate resistance. APPENDIX 4 shows mixtures for XZ351. The values shown are typical values and should be used as a guide only. Users are recommended to ensure the required value is obtained before commencing a print run.

REFERENCES
1. Leich, R.H. and Pierce, R.J., Printing Ink Manual, 5th ed., (10.13). 2. Joannou, G., "Touching on Membrane Switches", Point of Sale & Screeenprinting, July (1984).