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Laser trimming of resistors on PCB



Laser trimming of resistors on PCB | 30 January, 2001

I have an idea of trimming the resistors on a teflon PCB (FR4) after mounting and soldering. Can I use a laser to cut on a smt mounted resistor while the PCB is in functional test? The goal is to balance the resistances for a telecom application.

Have any of you fine people done this?

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Laser trimming of resistors on PCB | 30 January, 2001

You seem to pretty far from shore to begin thinking about boats and floatation devices.

Most laser trimmers are used by: * Semiconductor fabricators * Resistive component fabricators * Hybrid circuit assemblers

Component fabricators and hybrid [MCM whatever] circuit assemblers make resistors by screening a paste [like ruthenium dioxide] onto a ceramic [usually Al2O3] substrate to overlap with conductive pads on the substrate. They fire the paste at 900�C and trim it to the desired value, because these resistors show broad spread both within and between batches. Component fabricators cover the trimmed resistor with a frit [like lead borosilicate], fire the frit 550�C to form a glass for protection, break the ceramic substrate along score lines, add terminations, and package the resistors.

We actively trim circuits during test by taking a file to carbon comp resistors, routinely, but we�re talkin� PTH now.

I imagine you could print polymer thick film resistors on your board and trim them as you suggest.

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