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Film-Resistor Trimming Without Sensing at Module Pins

IP.com Disclosure Number: IPCOM000042381D
Original Publication Date: 1984-May-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 1 page(s) / 13K

Publishing Venue

IBM

Related People

Bartley, GK: AUTHOR [+4]

Abstract

Thick- and thin-film resistors are widely used in electronic packaging schemes. One of the limitations in utilizing these technologies is that the sheet resistance of the materials varies by as much as +50% from nominal. The standard solution is to overdesign the resistor geometry and then laser-trim the resistor back to the desired value. This passive trimming method requires sensing the value of the resistor as the trim is done, usually accomplished by making the resistor contacts available at the external I/O pins of the package. Top surface probing of resistors while trimming is not normally done because the physical requirements of probing interferes with the allowable travel of the laser beam.

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Film-Resistor Trimming Without Sensing at Module Pins

Thick- and thin-film resistors are widely used in electronic packaging schemes. One of the limitations in utilizing these technologies is that the sheet resistance of the materials varies by as much as +50% from nominal. The standard solution is to overdesign the resistor geometry and then laser-trim the resistor back to the desired value. This passive trimming method requires sensing the value of the resistor as the trim is done, usually accomplished by making the resistor contacts available at the external I/O pins of the package. Top surface probing of resistors while trimming is not normally done because the physical requirements of probing interferes with the allowable travel of the laser beam. Resistors used as terminators of logic chip outputs normally only require 5%-15% tolerances, and present simple resistor geometries which are trimmable to a range of values. On a single integrated-circuit chip module, the problem of resistor contact I/O availability is overcome because all chip I/Os will already be available at module I/Os. However, in a multichip module, not all nets are functionally available at module I/Os. Functional I/O requirements of the packaged logic usually consume almost all available I/Os on the multichip module, leaving few or none to spare for use as trim sense points for resistors with contacts that do not naturally appear at module I/Os. It is possible to trim resistors on modules without forcing all the resistor contacts to module I/Os, if (1) the physical locations of the resistors on the substrate relative to the laser beam are known,
(2) sheet resistance of the thin or thick film on the substrate is known, and...