Laser Resistor Trimming of Thick- and Thin-Film Hybrid Devices

Active and Passive

Large board thick film resistor trimming is a specialty of Laserod. Board sizes to 24" x 24" or 36" x 24" are easily handled. Upon job completion, Laserod will provide you with Excel® graphs that indicate your initial and final trim values for your comparison.
Large Microwave Circuit Board is Laser Resistor Trimmed.

Laser Trimming Hybrid Microwave Resistors

Fig. 1. 16" x 12" large microwave circuit board is trimmed by YAG laser..

The precision equipment delivers a trim accuracy as high as 0.1%, a measuring accuracy of 100 ppm, and a range between 0.1 ohm to 1 Gigohm.
Passive Trimming

Involves removal of material from a resistor to increase its resistance. The resistance or voltage parameter is monitored across the component contacts.
Active Trimming

Involves removal of material from a resistor to change a circuit parameter. Also called functional trimming, because the circuit is functioning during the trimming process. Parameters monitored are ACV, DCV, ACI, DCI, PERIOD, FREQUENCY.
Laser Trimming Active Circuits

Fig. 2. Setup for active laser trimming of a device under test. Circuit under laser is powered up by cables coming off to the right.

IEEE488 I/F as GPIB to measuring/test equipment.


Laser Parameter Considerations

Spot Size: Our Nd: YAG laser spot size typically ranges from 25 to 75 microns. The spot size should be wide enough to avoid electrical arcing across a narrow trim. Also the spot size should be narrow enough so that the side rays do not "cook" the resistor, producing a heat affected zone and degrading the resistor's long term stability.

Laser Beam Focus: The laser beam focus is not a very critical factor for thick film trimming applications because the optics have very long depths of focus. Typically, the depth of focus can be +/- 250 microns. For thin film it may be a more critical factor. As the beam goes out of focus in the near and far fields, the resistor drift will increase.

Trim Rate: Faster trimming will result in lower accuracies. It may also mean having to use higher repetition rates to achieve satisfactory material removal.