by Ray Derler, Watlow Electric Manufacturing Company
“Make it portable, smaller and lighter,” comes the demand from the corner office. It’s a constant refrain in the drive of new equipment requirements – and it doesn’t matter if the task is semiconductor processing equipment, scientific instruments, medical devices, automobiles, polymer heat staking or glue dispensing.
The desire by engineers to make it “small yet powerful” drives component manufacturers to scratch that itch. In our world of resistance heating products, this now manifests itself in innovative new approaches such as the subject of this article – advanced ceramic heating technologies.
Not your grandma’s porcelain
Ceramic is a generic term associated with a particular type of chemical composition. In common terms, if a material is not a plastic or a metal, then it’s probably a ceramic. We all recognize clay as a naturally occurring ceramic raw material that, when baked at a high temperature, becomes very hard. As such, it has been used for centuries as baking pots, figurines and floor tiles.
By contrast, “advanced ceramics” are synthetically produced, and whereas plastics are derived from hydrocarbons, ceramics are inorganic compounds of exceptional purity. Examples include alumina (Al2O3), zirconia (ZrO2), silicon nitride (Si3N4) and aluminum nitride (AlN).
One common manufacturing process for advanced ceramics involves coalescing the material in a high-pressure press followed by heating, or sintering, at high temperatures. The well controlled purity and chemistry of the material results in a monolithic, geometrically stable structure that is responsible for mechanical, electrical and thermal properties conducive to application as heater substrates. In the case of Aluminum Nitride, the features include high dielectric strength, low leakage current, low coefficient of thermal expansion, high durability with low mass and high thermal conductivity. The highly organized and near-fully dense microstructure eliminates moisture ingress and subsequent degradation of the dielectric – a unique and powerful attribute for many industrial applications.
The various advanced ceramic materials all are unique with respect to their mechanical, thermal and electrical properties. Depending on the application demands, these properties then become the selection criteria to determine which advanced ceramic material is appropriate for a particular heating requirement.
Watlow’s preferred use of AlN as the ceramic platform is due to its excellent thermal/physical and electrical properties:
- High thermal conductivity – Having a thermal conductivity similar to aluminum provides for rapid heat dissipation and enables the heater to be constructed with a high watt density, giving it the ability to thermally ramp at a rate of 150°C (270°F) per second.
- Clean, non-contaminating material – High temperature sintering produces a heater that is very hard (1100 Kg/mm2) and dense (3.2 g/cc) with virtually no porosity or surface roughness. This means for applications requiring a “clean” heater, AlN is an ideal choice.
- Moisture Resistance- AlN is impervious to moisture unlike many hydroscopic dielectric materials used in conventional heater construction.
- High dielectric strength and high insulation resistance – AlN is an electrical insulator that offers very low leakage current (< 100mA @ 500VAC), a highly sought after characteristic for many applications.
High-performance ceramic heaters, such as the ULTRAMIC™ 600 from Watlow, can be designed with up to 1000 W/in2 and operate at 600°C (1112°F) depending on the application, heater design and process parameters. A unique integrated thermocouple configuration improves the reliability of the sensor/heater interface to ensure control responsiveness during high ramp rate applications. Available in two-dimensional flat or ring forms, these products can be machined to meet specific design requirements found in challenging industrial applications.
Square peg, round hole?
Natural fits for advanced ceramic heaters are those that value one or more of the critical characteristics: ramp rate, uniformity, low leakage current, compact size, cleanliness and chemical compatibility. The packaging segment of the semiconductor processing industry tests integrated circuits by subjecting them to rapid temperature cycling. In this case, the benefits of ramp rate, temperature uniformity and dimensional stability improve equipment throughput. Similarly, die and wire bonding processes benefit from the cycle times that can be achieved. In the plastics packaging market, special high volume sealing needs are met with the same product benefits of thermal speed coupled with the benefit of a non-stick surface.
Some segments of the analytical instrumentation market require that samples are heated by a clean, non-degrading heat source. Advanced ceramic heaters meet that requirement and as such, sophisticated instrumentation techniques like mass spectroscopy, which can detect concentrations as low as part per trillion – consider steroid testing – are excellent candidates for this heating technology. This same cleanliness of operation is valued when heating inert gases in various stages of semiconductor manufacturing. In the medical device market, extremely low leakage current provides system level advantages with respect to overall electrical system design, cost and meeting IEC requirements.
“If it’s not broken, don’t fix it” is often a reasonable engineering strategy. However, without realizing it, a legacy heating solution may contain hidden costs associated with assembly labor, reliability or simply unrealized performance benefits. Should these potential improvements exist, then the benefits of advanced ceramic heaters may be worth exploring.
Ray Derler is a business director with responsibilities that include Watlow’s Advanced Ceramic Heater activities. Derler has over 20 years of management experience in developing new products and services from a diverse business and scientific background in telecommunications, material testing equipment and analytical instrumentation. He holds a Bachelor’s Degree in Chemistry and an M.B.A.