High Value Resistors with specified properties at small sizes
To analyze very low electronic signals, resistors with extremely high values are required. Based on a long tradition of working in this niche market, SRT Resistor Technology GmbH in Cadolzburg/Germany has succeeded in producing such components with stability better than the best measuring devices available on the market. It is possible to achieve also at very small sizes like 0402 (1×0.5mm) resistance values close to the range of insulators with specified properties for temperature and voltage dependency.
Basics of High Value Resistor Technology
Thick film technology has proved to be the best technology to produce high resistance values with relatively good properties, as well as small sizes. For this technology thixotropic pastes are used, consisting of a mixture of metal/metal-oxide particles with glass and organic binders. The mixture is screen-printed on structured Aluminum-oxide ceramics and fired in the range of 850°C. During this firing process the printed material is transformed to a glassy structure with an electrical conductivity depending mainly on the amount and composition of the metal/metal-oxide particles. The electronic conduction mechanism in such structures is a combination of metallic and semiconductor conductivity. Figure 1 shows a schematic picture of this mechanism, with the metallic conduction inside the particles and the semiconductor portion at the boundaries of the particles. For getting a stable resistor, a sufficient high number of such conduction pads are required.
This combination of conduction mechanism is the reason that such resistors are not more pure ohmic resistors with linear characteristics over voltage and temperature. With the reduction of the metallic particles the temperature coefficient (TCR) and the voltage
coefficient (VCR) have increasing negative values with increasing resistance. These two properties are important for most applications of such resistors, so that the selection of materials and processes have the target to reduce these values as much as possible.
Figure 1: Schematic conduction mechanism
Resistor pastes for the high value range are much more sensitive to the firing conditions than the materials for the standard range. This is the main reason, that most of the chip resistor manufacturers’ don´t offer values much higher than 100 Megaohm in their program. It is an additional problem that at values in the Gigaohm range the measuring techniques from the standard resistance range are not more working satisfactory enough.
Styles of High Value Resistors
Resistors higher than 100Megaohm are available as very small chip resistors like size 0402 (1.0×0.5 mm) as well as in big sizes with axial or radial lead wires. Because the reason that properties of resistors are better at low sheet resistances (resistance of a square area), the resistor design should use a very long and very small structure. With the big sizes of leaded resistors it is possible to get a length-to-width relation of more than 1000, so a very low resistive paste can be used. With the much smaller chip resistors it is only possible with the size 1206 (3.2×1.6mm) to get a meander-structure with a length-to-width-relation higher than 10. Figure 2 shows a relatively large chip resistor size 4020 with the standard design for power resistors and the high value design. With this design and a value of 1 Gigaohm a resistor paste below 10 Megaohms can be used with the results for an excellent data for TCR and VCR.
Trimming and measuring technique of high value resistors
The high value structure of size 4020 has in the middle a range with a broader line for trimming to close tolerances with a laser cut. At high value resistors this standard process is restricted to resistors with sheet resistance up to 100 Megaohms. In the resistor material with higher sheet resistance the high energy of the laser beam causes alterations in the internal structure of the resistor material near the cutting line, so that the resistors do not increase their value as normal and are unstable after this process. A solution for this problem is the usage of air abrasive trimming, which requires longer working time and costs.
Figure 2: Power and high value design of size 4020
For measuring high resistor values the best device available on the market at the moment is the Electrometer K6157 from Keithley Instruments. Best measuring results occur under controlled conditions with an accuracy of about 0.5% in the Gigaohm-range. For this accuracy, it is required that the adapter for the resistor has a shielding against electrostatic and electromagnetic disturbances and the measuring results of the device are continuously checked against external resistor standards.
The software of such devices results in rather long measuring cycles, so they cannot be used in automatic equipment for sorting and trimming. For this purpose own devices have been developed which have short measuring cycles, but also have to be calibrated against external standards. With this equipment it is possible to supply high quantities of chip resistors in the Gigaohm range with tolerances of 1%. Closer tolerances like 0.1% are not offered because of the limited measuring accuracy and the natural instability of thick film materials.
Temperature and Voltage dependency
The TCR, normally given in ppm/K, and the VCR, expressed in ppm/V, are properties of high value resistors, which depend mainly from the resistor material. The suppliers of such thick film materials show almost no data about these properties in their datasheets. So it is necessary that the users of such materials have their own test procedures for the evaluation of the data of each shipment and have to compare these data with earlier shipments. Table 1 shows the data of TCR and VCR for the square resistance RS of 1 and 10 Gigaohm for some suppliers.
The properties of these materials are rather different, so sometimes it needs an adjustment of the used resistor material to the requirements of the customers. If the circuit of the customer has to be used in very high or different temperatures, the material of supplier D with the lowest TCR is the best choice. For application with very low voltages or a broad voltage range the material of supplier C would be the best solution.