Chip resistors are individually adjusted to value by laser trimming, a relatively fast process given that they are made as a closely packed array on a sheet of ceramic, so that probes and laser can be quickly ‘stepped’ across a number of parts in sequence. The adjustment process is to tight tolerances, so that they leave the machine with a very tight spread of values. Subsequent glazing and handling increases this spread, and may move the mean value somewhat. However, most chip resistors are 2% parts, even if the nominal selection tolerance is looser.
By comparison, what determines the value of a chip capacitor is the number of layers, the average thickness of the dielectric, and the dielectric constant of the material. Whilst the number of layers is fixed and under control of the manufacturer, both the thickness and the dielectric constant will vary with the process. Of these two factors, thickness is the easier to control, though how this is done depends on the process. However, the dielectric constant of the fired material depends on the original formulation supplied, its crystal structure, and crucially upon the firing conditions, both time/temperature and atmosphere. In consequence, whilst chip capacitors are targeted to give maximum yield round the nominal value, there is some spread of values, so that close tolerance parts have to be selected from a larger batch, and command a higher price.
Author: Martin Tarr
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