Microelectronics Laser Trim Resistors
When resistors are printed, they are printed using decade inks, 1O, 10O, 100O, 1kO etc up to 100MO. For example, one printed square of a 1kO ink will have a value of 1kO. 3 squares long will have a value of 3kO and 3 squares wide will have a value of 333O. Thus by varying the geometry of the resistor, any value can be achieved.
Resistors are always designed to fire around 15-20% low of target value. This is because you can only laser adjust upwards in value.
The laser-trimming machine incorporates a probe ring connected to a highly accurate bridge measuring system. Each resistor is probed and the value is measured by the bridge. Using pre-programmed co-ordinates, a NdYg laser is fired at the resistor in a series of overlapping pulses which vaporises the resistor material in a 0.001" wide cut. All the time, the bridge is measuring the increasing resistance value until the required value is reached and the laser is cut off. The laser trimmer machine is capable of trimming 10,000 resistors per hour.
There are several different types of cut, a straight plunge cut, a serpentine cut for large changes, an L shaped cut which approaches the final value more slowly and minimises resistor hot-spots and an edge-scrub, which is best for minimising resistor noise, but takes much longer to trim.