Passive Electronic Components
Different approaches to classification exist:
Properties of resistive element materials
(Foil, Wirewound, Thin-Film)
in "TCR - Ohmic Value" Coordinates
Wirewound resistor is historically the first type of resistors. Its resistive element is made of Cu/Ni or Ni/Cr wire that is wound around ceramic or polymer core. Wire diameter starts from 12 m. Its resistivity is 50-130 cm. Wire ends are welded to the metal terminals (caps, rings, rods). The winding is coated by high temperature polymer or ceramic enamel.
Example of modern precision (TCR 10 ppm/K, tolerance 0.005%) wirewound resistor is Vishay Ultronix wirewound resistor:
Sometimes special bifilar noninductive winding is used to reduce inductance of wirewound resistor. The winding is made using 2 parallel wires. The wires are shortened at the one end of winding. Two wire ends coming out in the second end of winding are separately connected to the resistor terminals.^
Resistive element (see right picture below) is cut from the strip of resistive material and welded to the copper leads. The resistive value is adjust by laser cuts. Then the element is molded into cylindrical plastic body (see left picture below). This construction is used for low-value (less than 1) resistors.
The bulk resistive element ensures the main advantages of carbon composition resistors regardless of the resistance value:
Conventional wirewound and film resistors (carbon film, metal film, metal oxide, metal glaze) with the same power rating generally are not capable to withstand the same energy levels.
Example of pulse load capability of carbon composition resistor
In 1930s appeared Allen-Bradley carbon composition resistors and started to replace wirewound resistors. Carbon composition usage has been declining in1990s due to poor environmental performance and high prices. But in special cases (pulse load) they are very useful. Company Jaro developed surface mount MELF version of carbon composition resistors (see picture below). The RM, (carbon-ceramic) and RO, (carbon-polymer) series of carbon composite resistors are intended for replacing 1 W and 2 W leaded carbon composition resistors.
High-stability and low-noise resistors must have metallic resistive element. But it was impossible
The both problems were solved by Dr. Felix Zandman who developed in 1962 foil resistor and founded Vishay company. Modern Vishay S102C resistor has TCR as low as 0.6ppm/K and tolerance 0.001%. Tracking TCR 0.1ppm/K is available (see paragraph 2.1).
Thin (2.5…5 m) metal foil is glued to ceramic substrate. The difference between thermal expansion of foil and ceramics results in foil compression. Foil compression decreases its resistance and therefore compensates for small positive TCR of the foil.
The serpentine patterns are formed by etching of the foil (see picture below). It increases substantially resistance of the foil element. Trimming of resistance is discrete – by cutting of jumpers between special patterns. So all patterns that conduct a current are not damaged by laser beam and foil intrinsic properties do not suffer.
All above considered resistors have resistive element manufactured from bulk material. Nevertheless, majority of the modern resistors have film resistive element. It is convenient to characterize resistive property of a film by “sheet resistivity” that may be expressed in the terms of film material resistivity and film thickness :
(By the way this parameter is applicable to metal foil too).
The advantages of film resistors are lower price and wider frequency range than in resistors with bulk element. The price is lower because of high productivity of thin- and thick-film technologies. The wide frequency range may be explained by insensibility to skin effect (see the previous lecture). Commonly film thickness is significantly less than thickness of “skin” in wide range of frequencies.
Axial thin-film resistor
Thin-film chip resistor
Thin films are conductive, resistive or dielectric materials deposited on dielectric substrate made of alumina (Al2O3), aluminum nitride (AlN), beryllia (BeO), silicon (Si), silicon carbide (SiC), glass, or quartz. Methods of deposition are:
Film thickness varies typically in the range of 0.01…1 m.
Common thin-film resistive materials are
Resistor cermet inks are based on RuO2 (middle and high resistance values) and Ag/Pd (low resistance values). Conductor cermet inks are based commonly on Ag or Pd/Ag and are used for terminal electrodes. Listed materials are mixed with glass and polymers to form a paste for printing on the dielectric substrate. The thickness of the printed and fired material is usually 5...15 µm. Laser trimming is used for fine adjustments of resistance value. However, the heat generated during laser trimming often causes micro-cracks in the brittle thick-film cermets. It may affect resistance stability.
Common construction of thick-film resistor is a flat chip (see below).
Resistors with nominal values 1mR<1 are mainly used for current sense applications. Sometimes copper pattern in PCB that has resistance in milliohms range is used as a current sense resistor. For example 35 m (1 oz.) copper foil has sheet resistivity
The resistance of the pattern may be calculated multiplying the sheet resistivity by form factor (length-to-width ratio) of the pattern:
Therefore “two-squares” (l/b = 2) 1 oz. copper pattern (see below) has approximately 1 m resistance regardless of its absolute outline dimensions l and b.
The copper has high TCR: 4.3103 ppm/K. When low-ohmic and low-TCR discrete resistor is mounted on PCB the copper patterns that are connected in series may increase significantly total TCR of the circuit. Suppose that 2 squares of 1 ounce copper patterns are added to 1 m resistor that has 3102 ppm/K TCR. Total TCR of the resistor and the copper patterns connected in series may be calculated as the following:
It is approximately 7 times more than intrinsic TCR value of the resistor.
To fix this problem low value resistors are often constructed as four-terminal devices (see pictures below).
Almost every resistor has to withstand the “overload voltage” (commonly it is 2.5 times rated voltage or 6.25 times rated power) for some seconds (5 seconds for example in MIL-PRF-55342H military standard). The shorter is pulse duration the more power resistor can dissipate. See below the graph from CRCW Vishay General Purpose Thick-Film Chip Resistors datasheet that shows maximum permissible pulse load (single pulse).
CRCW (General Purpose Thick-Film Chip Resistors)
CRCW…HP (Pulse Proof, High Power Thick-Film Chip Resistors)
For example, general purpose CRCW1206 chip resistor (3.2mm1.6mm) that is characterized by 0.25W steady state rated power dissipation may dissipate about 60W for 10s. Special CRCW…HP Pulse Proof, High Power Thick-Film Chip Resistors are capable to withstand about 300W for 10s having the same dimensions.
Increase of pulse load capability of resistor is possible by:
Leaded resistor networks.
Through-hole mount, conformal coated (left).
Surface mount, molded (right).
Chip resistor array and network
The pair of resistors manufactured on the common substrate is characterized by tracking tolerance and TCR:
Tracking tolerance and TCR always approximately 5 times less than the same characteristics of individual resistor.
The popular type of chip resistor network is chip attenuator. Standard dimensions of chip attenuators are 1.01.0 and 1.61.6 mm.
An attenuator is an arrangement of non-inductive resistors used in electrical circuit to reduce the signal strength without introducing distortion. Attenuator can be designed to work between equal or non-equal impedances. Hence they are often used as impedance matching networks.
Example: Pi-type attenuator between equal impedances.
Z – impedance, ;
K – attenuation coefficient;
A – attenuation, dB.