Phil Ebbert, VP Engineering, Riedon Inc
Resistors will never be as sexy as microcontrollers or other ICs, or even as interesting as their passive relations. This means that when systems are being designed, the performance characteristics of the resistors in the circuit are often overlooked, or left till last. If the characteristics of the resistor do turn out to be important, it can cause issues late in the design cycle. This type of afterthought can often be understandable as 90% of the time, resistor requirements are not too important, and a normal, carbon resistor with a 10% tolerance and ¼ Watt rating may be all that is needed.
The resistor world has actually moved on a lot from the time that carbon composite resistors were the “go to” choice. Newer technologies have come on to the market and carbon film, metal film and thick film resistors are now used much more widely. Thick film resistors, especially, are found in most boards that use surface-mount technology.
So far, so good! But in 10% of cases, the application requires a more specific specification, such as being able to deal with higher temperatures or voltages, or having a tighter tolerance, or even just being stable over time when faced with changing temperatures or stress conditions. Some other areas where more consideration needs to be made are in high power settings, or where there is high levels of moisture. Some specific applications require dedicated resistors, like avoiding outgassing in a vacuum.
This article will use Reidon resistors to demonstrate how these requirements can be met and how any decisions are made.
Parameters and Characteristics
Usually the selection process starts with the more basic criteria - resistance, and mechanical and size constraints. The very first decision, even before the actual resistance value is often through-hole or surface-mount. Luckily both mounting types have a wide variety of values – from milliOhms to TeraOhms. Stage two is making the decision on the resistance value, tolerance and power rating. Since most electronics applications are low voltage and low power, the majority of resistor power ratings are also low power. Thick film surface-mount resistors in 0402 and 0603 sizes have resistor values from 1Ω to 10MΩ and usually can dissipate up to 0.1W at 70oC. In higher temperature applications, derating is often adopted, meaning that a device with a larger power rating, and often physical size, is chosen.
The tolerance required is dependent on the application, but usually the 5% tolerance found in cheap resistors is sufficient. If the application is more demanding, such as a measurement system, then tighter tolerances are often demanded. For tolerances under 1%, completely different technologies, such as wirewound, metal film, or metal foil will need to be considered.
When calculating the tolerance requirement, other factors in the circuit have a real effect. External factors, such as high temperature and humidity, under load and shock may also contribute to variations in resistance. These factors are cumulative, so to ensure a tolerance of ±1% for a product’s lifetime it may require specifying a resistor with a tolerance of ±0.1%.
Another important specification is how stable the resistance is when operating in changing temperatures. The stability is measured as temperature coefficient of resistance (TCR). Resistivity can increase with temperature in conductors, like metals, known a positive temperature coefficient (PTC). It can also decrease with rising heat in other materials, such as carbon or silicon - a negative temperature coefficient (NTC). Some applications exploit TCR, for example thermistors, but most applications require low TCR resistors.
For the majority of conductors, TCR is quite linear and is judged as the average change over a defined temperature range and stated as ppm/oC. Other materials exhibit non-linear characteristics and in that case the TCR is often stated as the worst case scenario between two defined temperatures. The materials used to manufacture resistors achieve different TCR ratings – thin film resistors as low as ±5ppm/oC, metal film ±3ppm/oC and wirewound and metal foil resistors can potentially get as low as ±1ppm/oC.
These are the parameters most generally used, but it is possible to go further with more specialised technologies. High voltage resistors often exhibit a resistance chance with voltage (voltage coefficient of resistance or VCR). Careful design can reduce this figure somewhat. Other types of resistors that can occasionally be specified include resistors that are non-magnetic, or made entirely of non organic materials. Some resistors are manufactured to be non-corrosive, current sensing, flame proof, non-inductive or to withstand pulses.
Solutions for High-Performance Applications
There are many ways to search for the most suitable resistor for any application. Riedon (www.riedon.com), and other manufacturers, provide search facilities on their websites for individual part numbers, component parameters of mounting type, resistance value, tolerance, power and TCR. The final section of the article will look at different technologies to narrow down the wide selection of choices.
Wirewound resistors are an older technology that still offers designers superior performance in some areas. They can dissipate high amounts of power, as well as being able to handle pulses and transients. They are also very precise and stable over time because of their construction. Reidon’s UAL series is designed to handle power of up to 300W and features excellent pulse handling and operation over the (-55oC to +275oC) temperature range. Tolerances can be as low as 0.01% and the TCR is very low at ±20ppm/oC. Specific designs are manufactured for specialised applications, such as a four terminal design for current-sensing.
Riedon also offer a surface-mount wirewound range, and a high precision range that can offer tolerances down to ±0.005% with a TCR of ±1ppm/oC. Wirewound resistors can also be wound in a way to ensure they are non-inductive. Because of the way that wirewound resistors are manufactured, they also offer low-noise operation.
Wirewound resistors also have the inherent advantage of low-noise performance compared to technologies that use granular conductive particle structures where the current flow can be affected by thermal stress, especially in high value devices.
Figure 2. In a conventional axially leaded wirewound resistor the resistance wire is wound around a ceramic core and welded to the metal end-caps. It is then encapsulated, usually in silicone, to protect from moisture and physical damage.
Thick film is an ideal technology for producing cheap, miniature surface-mount resistors. Thick film technology can also be optimised to give high performance, with accurate values up to 10TΩ. The devices can be refined further by avoiding some materials in construction. Terminals manufactured in tin or copper have limited working temperatures. Riedon uses silver/palladium as an inner terminal with an outer terminal of silver/platinum to manufacture devices with an operating temperature up to 300oC. Eliminating the magnetic metal, nickel, also has the benefit of enabling thick film resistors to work in applications with strong magnetic fields, such as MRI. There are also other benefits from getting rid of nickel and tin from the resistor. Terminals made from these materials can not be used in assembly with silver epoxy, stopping them being used in high reliability and high temperature applications. Tin is also forbidden from some applications because of the risk of tin whiskers.
Thick film resistors handle high power well, capable of operation to 6,000V, but at high voltage, VCR can become a problem. The factors that cause VCR can be mainly negated by care in manufacture. For example, Riedon manages chip size, layout, termination material and trimming geometry in the company’s offerings. Coatings can also case an issue for thick film resistors, especially polymeric materials, which can outgas in a vacuum. Riedon doesn’t use any organic materials during manufacture, stopping outgassing completely.
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Figure 3. Thick film chip technology provides precise high value resistors that can also operate in tough environments at high temperature and high voltage.
Thin Film resistors allow the cheap manufacture of small surface-mount devices that require accurate, high resistance values (up to several MΩ), with minimal TCR. These resistors are also more stable at high frequencies due to the uniform metal film developed during manufacture, and they also boast low inductance. If the film is formed on a heat conducting alumina substrate, that is then metalized and attached to a heat-dispersing copper plate tab, it creates a cheap, high power capable, film resistor in a small TO-style package.
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Figure 4. Riedon’s low cost TO-style thin film power resistors provide options for both through-hole and surface mounting and can handle up to 140W.
For precise, small form factor, surface mount resistors, metal foil is the preferred technology. The resistor is formed using a foil Nichrome alloy (Ni/Cr) applied to a ceramic substrate and finally photo-etched to the required resistance. The initial value can be tailored by removing shorting patterns from the etch. The pattern itself minimizes capacitance and inductance. A good example of this technology is Riedon’s UHPC series, which has resistances up to 125kΩ and can dissipate 0.75W, with a TCR of ±0.05ppm/oC. The devices are available in sizes from 0805 to 2512. Members of the range can operate from -55oC to 150oC and feature low inductance of <0.08μH, capacitance of 0.5pF and a VCR of <0.1ppm/V.