Thick-film resistor resistance value

Thick-film resistors are available with sheet resistance values in the range from 0.1 to 10 M Q/D. By blending different quantities of conductive material and an electrically insulating glass the resistivity is controlled. For a high sheet resistance formulation the ratio of conductor to glass would be about 70/30. 

The distribution of the conductive particles and the contact between them also determines the resistance of the deposited film. Final resistance values are obtained either by sand blasting, where the thickness of the film is reduced, or by laser trimming, to increase the effective length of the resistor. These procedures would be performed after the resistor has been fired, but before the application of the protective glaze coating. 

Thick-film resistors can be processed with a tolerance of about 25%. Laser trimming increases the resistance value. Therefore, a resistor is designed to a lower value than desired and will be trimmed to its target value later on. Besides the resistance value required, the power dissipation density is required to design a thick-film resistor. The power dissipation density (Pdensiiy in mW/mm ) is a paste property, which is specified in the data sheet. It is typically related to a 50% trim cut (maximum allowable trim length) and application on prefired alumina. For a stable resistor, the minimum area Ag is determined by the maximmn circuit power dissipation requirement, as in Equation 9.3 ... 

Resistor Noise. The random motion of current carriers causes spontaneous fluctuations in electric current, termed electrical noise. Current noise in thick-film resistors is called excess noise, as it is considered the excess noise over thermal noise. Thermal noise is predictable and is a function of resistance and temperature, whereas current noise has no definite relationship. Noise in thick-film resistors depends on the sheet resistance value, thickness of the resistor print, and geometry of the resistor. Figure 8.59 shows a typical plot of measured noise versus resistor area for a 200-kQ/sq resistor. Current noise decreases as resistor area increases. Resistor noise also depends on processing conditions. Generally, noise decreases with an increase in peak firing temperature. The typical noise value for commercial thick-film resistors is shown in Table 8.20. 

Hot column stability testing is done by placing the test substrate with resistors on a hot stage at 400°C for 5 min, with a subsequent quench to room temperature. The shift in resistance values is then recorded. Stable resistors experience minimum change from this test. Stability of resistors after thermal shock is shown in Fig. 8.64. The stability of resistors can also be tested by subjecting them to a thermal cycle test which consists of 5 cycles of 5 min at-65°C, transfer within 10 s to-l-150°C, and a dwell of 5 min before transfer back to -65°C. The stability of commercial thick film resistors is considered acceptable if changes in resistance of less than +0.2 percent result from this test. 

Thick-film resistor pastes are composed of a combination of glass Mt, metal and oxides. Resistor pastes are available in values ranging from 0.2 ohm/sq to 10 M ohm/sq. Firing temperatures 600-850°C. Temperature coefficient of resistance can vary from 40-150 ppm/C. These pastes are used in microcircuits, voltage dividers, resistor networks, chip resistors and potentiometers. 

It is fi quently observed that excess l/f noise is related to the microscopic sample structure and to the manufactural technology. This kind of noise is particularly sensitive to the surface and interface defects (Jones 1994). There are manufactural techniques which give small dispersion of the mean characteristic values, such as resistance or currents. On the other hand these devices can exhibit large dispersion ofthe noise characteristics. So, for example, metal thin film resistors have lower l/f noise than granular film resistors made from cermet thick films or carbon resistors. 

Metal film resistors are used for many applications. Very low-resistance values can be made with films of nickel phosphorous. Such films can be deposited on ceramic by a chemical process and, therefore, it is relatively cheap to make them very thick. They can have a TCR below 50 ppm/°C. 

Materials are also available that can combine mixed values of resistance on one layer. Polymer thick films (PTE) are applied by successive screen print/cure cycles so that aU the desired resistance values can be made on one layer. Also, some sheet capacitance manufacturers laminate a layer of resistor foil as one electrode in their construction. Therefore, a fabricator can mix resistance and capacitance on one sheet type raw material. 

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