ABSTRACT
During the 1970s there was a very large increase in the manufacture of thick- and thin-film circuits. However, even with the best processing methods and equipment, it has been impossible to produce resistors with as processed tolerances better than 10 to 20%. The majority of circuits require tolerances much better than this. Analog-to-digital converters and digital-to-analog converters, for example, require tolerances down to 0.01%. Because of this, film resistors are normally processed on the low side and are brought into tolerance by removing part of the resistor material. The old standby in the hybrid industry was air abrasive trimming. In this process the resistor material was removed by "sandblasting" the resistor with a fine stream of particles. Although this process could be semiautomated, it often became the bottleneck in the production process. Precision circuits also required better tolerances than the best air-abrasive process could produce. The air-abrasion process also suffered from problems of downtime due to clogging of jets and pipes, wear on X-Y tables and nozzles, and uncleanliness of the process.
