Computer controlled laser systems are used in mass production for the automated trimming of thick film resistor elements. Current technique uses a pulsed laser beam to notch the resistors. Resistance values can only be increased, in other words, the layout must be designed that all resistances before trimming - taking into consideration of the manufacturing dispersion of the values - should be below their nominal values.

Laser trimming strategy for the resistors of a thick film circuit

A laser trimming systems consists of the following main parts:

- the laser, or laser head, including the most popular Nd:YAG laser crystal, the pumping source, the Q-switch and the beam shaping optics;

- beam delivery system, usually using rotating mirrors driven by galvanomotors, and/or X-Y stages;

- flat-field optics, to focus the rotating beam to the flat substrate;

- probing system, to make the proper connections between the measuring bridge and the actually trimmed resistor, either by using a multi-probe frame or a pair of flying probes;

- bridge type measuring system;

- a control computer. 

A laser trimmer

A circuit under the probes of a laser trimming system

Laser system for trimming and marking

In addition to resistor trimming, computer controlled laser systems are applied in the technology of thick film hybrid circuits for the separation of the substrates, for marking, soldering, welding, etc.

The parts of a laser marking system

A trimming system in operation