The resistive properties of thick film heater resistors are optimised to maximise heat output. They are found in many industrial applications, including test and measurement equipment, printers, enclosure heaters, de-icers, semiconductor equipment and food processing.
Medical applications for thick film resistor heaters include laboratory heating devices, medical diagnostic equipment and blood and IV fluid warmers.
In many of these applications, longevity and zero maintenance are important considerations. In some applications, device failure is a serious issue.
The trade-off between heat output and preventing damage to the resistor is crucial. In this post, we consider the impact of harsh environments on the design and manufacture of thick film heater resistor components.
Understanding Thick Film Heater Resistors
Electric current is the flow of charged particles (electrons). Resistivity quantifies how much a material opposes the flow of electric current. The lower the resistivity, the higher the current flow, assuming all other conditions remain constant.
In conductors, resistivity increases with temperature.Resistance relates to the resistivity of the resistor material, its length and cross-sectional area.
High-resistivity materials deliver efficient heating, but they have high-temperature coefficients of resistance (TCR). Balancing resistivity with TCR delivers a stable resistance value across a temperature range.
Heat transfer from the heater to the target object or medium is the key issue. If heat retention is too high, it can damage the resistive and insulating layers of the resistor device.
Thick film resistor substrates must be able to withstand high temperatures. They must also provide good thermal conductivity to dissipate heat efficiently. Substrate material selection, resistive ink and protective layers affect heat generation and durability.
Resistors In Harsh Environments
Many environmental factors can impact the performance of thick film heater resistors, they include:
- Temperature and temperature cycling.
- Humidity, moisture and salt atmospheres.
- Mechanical stress, shock and vibration.
- Electrical pulse and surge, overload and ESD.
- Dust and particulates.
- Chemical exposure.
- Ionising radiation.
- High altitude.
Harsh environments include automotive applications (particularly under the bonnet), aerospace, industrial and applications with restricted space and/or cooling options.
The potential impact of these factors on thick film resistor performance is discussed in our thick film resistor failure post.
Heater Resistor Design
When designing heater resistors for harsh environments, there are inevitable trade-offs to consider. Engineers must balance performance, durability, reliability and cost. Understanding the specific environment and application is essential to making informed decisions.
The ideal partnership is one between a systems engineer who fully understands the specifications and environment and a specialist heater resistor manufacturer.
Thick film heaters must provide efficient heating while maintaining structural and functional integrity over time.
Electrical Considerations: Higher current ratings generate more heat. This can exacerbate the thermal management issues discussed above and affect longevity.
Designing the resistor for maximum surge survivability impacts the complexity and cost of the resistor.
Material Selection:. Materials that are more resistant to corrosion, thermal cycling and mechanical stress often come at a higher cost. Choosing the right material involves balancing durability and budget constraints.
The use of thinner films and minimal insulation improves thermal efficiency. Unfortunately, this impacts the resistor’s ability to withstand thermal cycling and harsh conditions.
Protective Coatings: Coatings protect against moisture, chemicals and mechanical damage, but compromise heat transfer. Corrosion-resistant coatings often have lower conductivity, affecting the efficiency of the heater.
Mechanical Design: Incorporating redundancy in design can enhance reliability but also increase size and complexity. While a mechanically robust design is more resistant to vibration and shock, it can compromise the application.
Achieving optimal heat output, response time and stability often requires specialist materials, design and manufacture. It requires an understanding of electrical, thermal and mechanical limitations and material properties. Balancing performance vs cost is crucial.
Engineers must trade-off between efficiency, reliability and manufacturability while meeting specific application requirements. A collaboration between systems engineers and specialist resistor manufacturers often delivers the best results.
Although TSEC manufacture products for use in medical equipment, no devices are supplied for use inside the human body.