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Resistors are a type of electrical component which are widely used in electrical circuits. In fact, they are one of the most widely used and most important components of an electrical circuit. In short, resistors implement an electrical resistance into the electrical circuit by restricting the amount of current that can flow through it. Resistors are often governed by either precision or power; and the choice of which one is implemented into an electrical circuit is dependent upon the tolerance of the resistor and the electrical properties of the circuit. As far as the structure of resistors go, the two most common types either employ thick film materials or thin film materials. In this article, we look at the latter.
What are Thin Film Resistors?
Thin film resistors are a type of resistor that possess a thin resistive layer sat on top of a ceramic base. The principle difference between thick and thin film resistors is the thickness of this resistive layer, with thin films possessing a thickness of around 0.1 microns. By comparison, thick film resistors can be up to 1000 times thicker (usually around 100 microns). Whilst this is the main categorical difference, the vast differences in the thickness also cause thick and thin film resistors to have different properties and different fabrication methods. As a rule of thumb, thin film resistors are usually more accurate, precise, and stable, which enable them to be used in higher precision technologies. However, this does make them more expensive than their thick film counterparts.
Manufacturing Thin Films
One of the key differences between a thin and thick film resistor is in the fabrication process. To create a thin film resistor, a dense, uniform metallic alloy layer is deposited onto the ceramic base under a vacuum (other less-frequently used base materials can include silicon or glass). This metallic layer acts as the resistive layer. The metal alloy is usually composed of chromium and nickel, with nichrome being the most common choice.
As fair as deposition techniques go, there are two that are used. The first is thermal deposition under a ‘hard’ (conventional) vacuum, and the second is through physical vapour deposition (PVD) using a ‘softer’ vacuum, i.e. a vacuum that is backfilled with argon to increase the pressure. Of these, PVD techniques are more widely used, and sputtering is often the PVD method of choice for many manufacturers.
After the metallic layer is deposited, it is usually patterned using photolithography and subtractive processes. The subtractive process is often used to trim the resistance value as per the intended application. The etched patterns also help to increase the resistive path within the metallic layer and can help with calibrating the resistance value of the resistor. As per any resistor manufacturing process, a protective coating, base contacts and termination sections are also incorporated into the resistor.
How Thin Film Resistors Differ to Thick Film Resistors
Aside from their obvious difference in thickness, there are some key differences between thin and thick film resistors. Whilst they have a similar external appearance, they are made using different fabrication processes, with thick films often being manufactured using screen and stencil printing approaches. The different fabrication process also yields a difference in the materials used; as mentioned, thin film resistors often use a uniform metallic film such as nichrome as the resistive layer, whereas thick film resistors are made up a paste composed of either ruthenium oxide or a platinum-based alloy.
As far as the properties of the resistors go, there are differences. Thin film resistors have lower resistance values and a narrower tolerance range, whereas thick film resistors are more versatile over a wider resistance range. Thin film resistors can also operate in a wider voltage range (and up to higher voltages) and possess a lower current noise than thick film resistors. Thin film resistors also possess a lower parasitic inductance and capacitance, but this can be negated in some thick film resistors depending on the geometry of the resistive material.
However, the one area that thin films have a significant disadvantage against thick film resistors is in their resistance to moisture – thick film resistors have a glassy-type structure which makes them more resistant to moisture. Even though the cost of thin film resistors is greater than their thick film counterparts, advances in mass manufacturing methods have brought the price of these units down in recent years.
Thin film resistors often have fewer applications than their thick film counterparts. However, thin film resistors are usually used for applications where a higher precision is required, such as for measuring and monitoring equipment within the medical and aerospace sectors, audio applications, computer chips, power supply converters, telecommunications, radiofrequency (RF) applications, HVAC systems and precision control applications.
Sources and Further Reading
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