What is a metal film resistor?Applications and uses of metal film resistors
Metal film resistors are widely used and are superior to metal oxide film resistors in many aspects, and of course early carbon film resistors and early carbon composition resistors.
The term metal film resistor is generally applied to axially leaded components, although thin film surface mount chip resistors use the same technology to manufacture the resistors. As such, metal film resistors are the most widely used technology for resistors.
In terms of its performance, metal film resistors offer better stability, accuracy, reliability, and it produces much less noise than some other types, especially carbon composition, carbon film, and even metal oxide film resistors.
What is a metal film resistor?
As the name suggests, metal film resistors are made by depositing a thin layer of metal on a ceramic former. Metal films act in the same way as resistor wire, and because thickness, width, and length can be precisely controlled, high-tolerance metal film resistors can be produced.
Additionally, long-term stability is good because metal film resistors can be pre-aged and the metal is protected to ensure it does not degrade over time.
Metal film resistors are a popular choice for leaded axial resistors today, so they are used in large quantities.
Metal Film Resistor Fabrication and Manufacturing
Metal film resistors are made by vacuum depositing a metal layer onto a high-purity ceramic cylindrical rod.
Generally, the thicker the deposited metal film, the more stable the resistance value will be. Additionally, the thickness of the material has a significant impact on resistance. Obviously, the thicker the material, the lower the resistance, although other factors can have a significant impact, including the resistivity of the material used and the width of the helical cuts made late in the process.
Typically, film thickness is between 50 and 250nm, as material thickness in this region tends to provide higher levels of long-term stability. Many different techniques can be used for this, but one of the more common is sputtering. The metal deposited is typically nickel-chromium, nickel-chromium, but other metals, including platinum-containing or tantalum nitride, are available for special applications.
Once the film is deposited, metal end caps are pressed against the deposited metal. This is in contact with the resistive film and contains the leads. These metal caps make contact with the resistive elements and are connected to leads leaving the entire structure, allowing electrical connections to be made.
Structure diagram of metal film resistor
The next stage of fabrication is to trim the resistor values to the desired numbers. This is usually achieved by using a laser to cut spirals into the metal film. This extends the length of the metal elements and also reduces the width of the current-carrying tracks, while keeping the thickness of the deposited material within an optimal range for long-term stability.
The precision of laser trimming means metal film resistors can be manufactured to very tight tolerances. Tolerances of 0.1%, 0.25%, 0.5% as well as 1% and 2% are available. 1% and 2% are the most widely used, given their tight tolerances, which allow the circuit to maintain good repeatability from one unit to the next.
Metal film resistors also maintain a good temperature coefficient of resistance, typically between 50 and 100 ppm/°K.
The final stage of metal film resistor manufacturing is the addition of protective coatings and markings. Typically, protective coatings consist of resins that are added in several layers that are baked individually. Finally, mark rings are added to indicate the value and other relevant characteristics of the metal film resistor.
Surface mount resistors use the same basic technology, metal film, but use a different form of manufacturing process that takes into account different mechanical considerations.
Applications and uses of metal film resistors
Metal film leaded resistors are pretty much the definitive standard for leaded low power general purpose resistors today. Cost, reliability, and the availability of close-tolerance products mean that they are almost universally used for general-purpose leaded resistors.
Of course, surface mount resistors are used in much greater quantities as mass production technology has evolved. However, when leaded electronic components are required, metal film resistors are often used.
These resistors are highly tolerant and offer good long-term stability and a good temperature coefficient of resistance and can be used with confidence in most applications.
Metal film resistors also have good voltage stability performance levels. This is a parameter that measures the resistance of an electronic component to an increase in applied voltage.
When using these resistors, it is best to run them below their maximum rating. This is standard procedure for any design, typically they can operate at 50% to 60% of maximum rating.
Due to their size and construction, metal film resistors are not suitable for applications where surge transients may be encountered. Carbon composition resistors are more suitable as they can withstand transients better.
Also, these resistors may be slightly inductive, given that the resistance is adjusted by spiral cutting. While hey are good for most applications, their performance may suffer, but when used in the microwave area, helical cutting can cause inductance. However, for these frequencies, surface mount resistors are more likely to be sued in these applications.
Another advantage of metal film resistors is that due to their construction and technology, they produce low levels of noise.
Typical Metal Film Resistor Specifications
Metal film resistors are specified using many of the same parameters as other types of resistors. Values, accuracy, power consumption, noise generated, etc. all apply to metal film resistors.
Typical performance data for metal film resistors are given below as a performance guide
Metal Film Resistor Performance Guide
Metal film resistor parameters
Metal Film Resistor Performance
Typical tolerance availability
Load life (percentage change above 1000h)
Maximum noise (μV/V)
Temperature coefficient (ppm/°C)
±50 - >±100
Voltage coefficient (%/V)
Maximum resistance temperature (°C)
Metal film resistors are the most widely used form of axially leaded resistors today, almost replacing other types of resistors, even metal oxide film resistors. In most cases, they offer better performance and cost less than other types. Therefore, they are almost universally adopted except for certain specific applications.
Resistors come in a variety of power consumption formats, quarter watt, half watt, one watt, etc. Different manufacturers may have different dissipation levels, so it's always worth referring to the specifications.
In terms of their appearance, it is difficult, if not impossible, to distinguish between metal film resistors and their closely related metal oxide films. Fortunately, the performance of both types is very similar and won't matter in most cases. Today, most leaded resistors will be metal films rather than metal oxide films.
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