Conductive Ink Heaters
By Dr W.J. Thatcher. Special Projects Manager

The Operational requirements
The dominant factors tend to be the warm up time and the maximum operating temperature. Of course the rate of warm up and the maximum power are constrained by other materials as explained earlier-for example glass will usually crack if a power of greater than 0.15 watts per square cm. is applied, and the cost of substrate material increases dramatically if temperatures over 1200C are required as they may be for example in the case of millenium style electric kettles (which usually use ceramic substrates). It is usually a requirement that the heaters be designed in such a way as to fail safe if blocked. In this respect the PTC heaters offer considerable cost saving potential, combined with simplicity of design. Otherwise a circuit breaker, fusible ink or other cut off device is required.

In many cases the heater needs to be covered for safety or environmental reasons such as humidity protection. PTC inks are particularly critical in this respect and need to be covered or laminated with a material which has the same coefficient of thermal expansion. Simple carbon inks can usually be covered with a polyester film or even coated with a dielectric ink.

Finally the heater needs to be able to withstand multiple cycles operating at the power and frequency dictated by the end use. A car mirror heater for example may make 50,000 cycles or more in its operating life, with temperatures ranging from (minus)10C to 70C. The conductive ink-and its substrate-need to be able to withstand this operational requirement; stress testing is therefore needed for any prospecUtive design, and this can be the most costly and time consuming aspect of the development of any new heater product.

The frst thing to observe is that the ohms per square specification is meaningless unless the thickness is specified. 15 or 25 microns DFT are commonly used as a reference standard but many end users simply expect fo have a resistance related to the film weight they are used to printing. If should be noted that the more electrically resistive the ink is (and the more tightly controlled that resistivity has to be) the more difficult it is to manufacture and consequently the more expensive. Greater fluctuations in current density are always more likely with very resistive inks. The heater manufacturer frequently resorts to blending inks to obtain the desired resistance (this also gives manufacturing flexibility for differenf jobs). Very resistive inks are also more likely to fail due to tracking, where a favoured electrical path is discovered by the current leading to a very high local current density, carbonisation of the path and breakdown. For this reason the heater design may be better modified to avoid the need for such very resistiive materials

Typical resistivities for carbon inks are 25 fo 500 ohms per square at 15 microns DFT. More conductive inks can be made with a blend of carbon and silver inks ranging from 0.05 fo 25 ohms per square, though such inks are necessarily more expensive.

A very recent development in conductive ink materials has resulted in heaters which can be self regulating. When a heater is blocked - that is when the space around it is so enclosed that heat cannot escape. With an applied voltage of 240 volts AC the heater was operating at 1.44 watts per square cm., in a sealed environment. The equilibrium temperature of approximately 1200C was reached in 8 minutes but the heat generated was unable to escape, so the surface temperature continued to rise to nearly 2000C in 30 minutes at which point the substrate was well past its safe operating temperature limit of 1650C and the power was turned off. In a real life situation this could have caused a fire, and the heater would have been designed with a safety switch ( an overload sensor). A 600 ohm heater operating at only 0.5 watts per square cm. reaches a much lower equilibrium temperature and fhe effect of blocking is much less severe, in this case there was sufficient cooling of fhe blocking environment for an equilibrium temperature to be reached af 740C. But a PTC heater regulates it self. Operating at only 0.12 watts per square cm. The resistance of the ink increases as the temperature rises, thus reducing the power density. The example shows that such a heater was completely stable even when held in a blocked state for over 400 minutes

The electrodes

The design and construction of the electrodes affectsthe performance of the heater in several ways. The spacing defines the heater resistance for a given ink, alternatively the ink has to be formulated for a given electrode spacing. The material of the electrodes should be as conductive as possible-usually copper or silver; copper has the advantage in both conductivity and solderability, but silver can be applied as an ink, and may therefore be more cost effective. The temperature gradient between the electrodes is also affected by the spacing; the gradient is greater the larger the spacing i.e. the uniformity of the surface temperature is less.

The construction of heaters using carbon or other conductive inks depends critically on the performance requirement of the heater. The key factors to be considered are:-

1. The dimensions
2. The maximum temperature required
3. The substrate material
4. The limitations imposed by the conductive ink
5. The design and construction of the electrodes (terminals)
6. The operational requirements of the heater eg. On/off cycle Warm up
time
Fail safe requirements
Humidity resistance/other environmental needs
Expected design life.

Dimensions

These are mostly set by the end user requirement, and are constrained by the physical dimension of the part to be heated and the maximum temperature it is required to reach. Clearly if a very large part has to reach a very high temperature the total power requirements are going to be greater and the power density in watts per square cm. May also need to be greater to cope with the residual heat losses.

Maximum Temperature

Again this is usually an end user defined requirement but will be constrained by the maximum temperature the substrate can reach as well as the conductive ink. Once again the power density may well define the maximum temperature that CAN be reached. The substrate materials.

There are a huge range of materials onto which conductive inks may be printed; but they range enormously in their physical properties such as shrinkage, thermal stability ,flexibility, adhesion, dielectric strength (insulating properties), thermal conductivity, and heat capacity to name but a few. Typical substrates include polyester (heat stabilised or otherwise; cheap, flexible, available in various film thicknesses, treated for improved adhesion or otherwise). Polyllex or similar hybrid materials. Paper phenolic. FR2, FR4, or similar glass epoxy laminates capable of withstanding 1650C. Anodised or otherwise coated metal sheets for very high temperature requirements.

The carbon or other conductive ink

There are to many variations to cover in a single report since many of the constraints are set by the end user requirements. However an awareness of the limitations of such conductive film heaters may be of assistance in their configuration. Carbon inks can be formulated with a wide range of conductivities; these are usually expressed in terms of the electrical resistance of the ink in ohms per square for a stipulated thickness.

HINTS ON THE USE OF PTC INKS

1. PTC inks are very different from conventional screen inks. A basic property of the resins used in them ensures that they are either a very thin free flowing liquid when highly diluted or that they are thixotropic with a tendency to gel. Generally the higher the solids content of the ink the greater the PTC effect they exhibit. Over thinning with solvent will reduce the PTC effect as
well as the coverage.
2. If the ink is gelled in the pot, warming it gently to 30-35C and stirring it will return the ink to its printable condition without harm.
DO NOT add excess solvent in an attempt to achieve this. A maximum of 5% additional solvent only is acceptable.
3. PTC inks are usually based on highly crystalline resins such as fluoropolymers; and solubilised in very powerful solvents; they must be handled with care. Skin contact should be avoided.
4. The resistance properties and PTC performance depend on the print thickness and
the drying conditions. Generally there will be a decrease in the resistance over the first few hours after predry. Film thickness and predry may be used as a means of fine tuning the desired end properties as required. Variations greater than 10%
of the specified process conditions are NOT recommended.
5. Screen cleaning is best achieved with MEK/Butanol.

PERFORMANCE SPECIFICATION PTC 100

RESOLUTION Approx. 200 microns track/gap

ADHESION/TAPE TEST Pass

SOLVENT RUB TEST Poor. 10 double rubs MEK
Material normally requires
a polyester protective layer.

HUMIDITY RESISTANCE As above

FLEXIBILITY Fair, approx 3 cm. radius bend, depending on film weight.

RESISTIVITY 2Kohms/square at 15 microns typical

TYPICAL PTC RATIO 3.5 *

SUBSTRATES FR2, FR4
Semiflex
Polyester
Polyimide
Primer coated glass
Primer coated aluminium

*PTC RATIO = resistance at 100C/resistance at 20C

POSSIBLE AUTOMOTIVE APPLICATIONS
OF PTC HEATER TECHNOLOGY

SEAT HEATERS
OIL SUMP HEATERS (PREHEATING ENGINE)
INTERNAL MIRROR DEMISTER
DOOR PANEL HEATERS - TO REPLACE CONVENTIONAL FAN HEATERS

POSSIBLE MEDICAL APPLICATIONS

OPERATING THEATRE HEATER BLANKETS
DISABLED PERSONS CLOTHING
WHEELCHAIR SEAT HEATERS
WALL PANEL SPACE HEATERS FOR RETIREMENT HOMES

POSSIBLE DOMESTIC/OTHER APPLICATIONS

PLATE WARMERS
ELECTRIC COOKING PANS
WATER STORAGE TANK HEATERS
GREENHOUSE HEATERS