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RELIABLE SCREEN PRINTED RESISTORS WITH PHOTODEFINED DIMENSIONS

IP.com Disclosure Number: IPCOM000009744D
Original Publication Date: 2000-May-01
Included in the Prior Art Database: 2002-Sep-16
Document File: 2 page(s) / 75K

Publishing Venue

Motorola

Related People

Gregory Dunn: AUTHOR [+3]

Abstract

Polymer thick film (PTF) resistors terminated directly to etched copper terminals on a printed circuit board exhibit substantial resistance increase under temperature/humidity stress. The problem becomes acute for low aspect ratio resistors, because the resistance of the copper/carbon paste interface can be of the same order as the bulk resistance.

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MOTOROLA

Technical Developments

RELIABLE SCREEN PRINTED RESISTORS WITH PHOTODEFINED DIMENSIONS

by Gregory Dunn, Min-xian Zhang and Steven Scheifers

PROBLEM

Polymer thick film (PTF) resistors terminated directly to etched copper terminals on a printed circuit board exhibit substantial resistance increase under temperature/humidity stress. The problem becomes acute for low aspect ratio resistors, because the resistance of the copper/carbon paste interface can be of the same order as the bulk resistance.

For example, the resistance of a 0.5 mm-wide, 0.5 square resistor printed with 10 kQ/square ink on 18 jlm-thick copper terminations will increase by several hundred percent after 500 hours at 85 C and 85% relative humidity. This substantial change in resistor value may cause a circuit to fail or perform poorly in the field.

For this reason, frF resistors are conventionally terminated on screened silver paste, which offers excellent stability. However, this solution results in

Motorola. lnc. 2000

resistors with length determined by low-resolution screening rather than high resolution photodefinition, limiting both the precision (or tolerance) achievable for the resistor, and the minimum resistor size.

SOLUTION

Silver paste is screened into photodefined wells in a photodielectric layer on the circuit board substrate, and the excess material is then removed by mechanical planarization. The resistive carbon paste is then screened onto the planar surface.

Printing the carbon paste...