Surface-mount resistors may have thin-film or thick-film resistive elements. The latter are
cheaper and so more often encountered, but the price differential has been falling in recent
years. Both thin-film and thick-film SM resistors use laser trimming to make fine adjustments
of resistance value during the manufacturing process. There are important differences in their
behavior.
Thin-film (metal-film) SM resistors use a nickel–chromium (Ni–Cr) film as the resistance
material. A very thin Ni–Cr film of less than 1 mm thickness is deposited on the aluminum
oxide substrate by sputtering under vacuum. Ni–Cr is then applied on to the substrate as
conducting electrodes. The use of a metal film as the resistance material allows thin-film
resistors to provide a very low temperature coefficient, much lower current noise and
vanishingly small non-linearity. Thin-film resistors need only low laser power for trimming
(one-third of that required for thick-film resistors) and contain no glass-based material. This
prevents possible microcracking during laser trimming and maintains the stability of the
thin-film resistor types.
Thick-film resistors normally use ruthenium oxide (RuO2) as the resistance material, mixed
with glass-based material to form a paste for printing on the substrate. The thickness of the
printing material is usually 12 mm. The heat generated during laser trimming can cause
microcracks on a thick-film resistor containing glass-based materials, which can adversely
affect stability. Palladium/silver (PdAg) is used for the electrodes.
The most important thing about thick-film SM resistors from our point of view is that they do
not obey Ohm’s Law very well. This often comes as a shock to people who are used to TH
resistors, which have been the highly linear metal film type for many years. They have much
higher voltage coefficients than TH resistors, at between 30 and 100 ppm. The non-linearity is
symmetrical about zero voltage and so gives rise to third-harmonic distortion. Some SM
resistor manufacturers do not specify voltage coefficient, which usually means it can vary
disturbingly between different batches and different values of the same component, and this
can have dire results on the repeatability of design performance.
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