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Browse Prior Art Database

NEW MEASUREMENT TECHNIQUE FOR PROFILING VARIABLE CAPACITOR CAPACITANCE

IP.com Disclosure Number: IPCOM000006070D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2001-Nov-30
Document File: 5 page(s) / 217K

Publishing Venue

Motorola

Related People

Michael Ambrose: AUTHOR

Abstract

Beginning in August 1989, an automated assembly pager production line began experiencing a massive product failure rate for page sensitivity and intermediate frequency shifting out of specification at the customer quality audit (CQA). Also, an increased fallout at final test automated tuning had been observed, the failure mode being traced to the inability to automatically tune the trimmer capacitors utilized on the receiver assemblies.

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MOTOROLA INC. Technical Developments Volume 12 April 1991

NEW MEASUREMENT TECHNIQUE FOR PROFILING VARIABLE CAPACITOR CAPACITANCE

by Michael Ambrose

   Beginning in August 1989, an automated assembly pager production line began experiencing a massive product failure rate for page sensitivity and intermediate frequency shifting out of specification at the customer quality audit (CQA). Also, an increased fallout at final test automated tuning had been observed, the failure mode being traced to the inability to automatically tune the trimmer capacitors utilized on the receiver assemblies.

  The problem was linked, through a capacitance profiling technique, to an altered silicon dielectric/ lubricant used in the trimmer capacitors, starting in date code 6/89. The silicon was being destroyed by the reflow process. An insufficient amount of silicon will cause the dielectric constant to be unstable as well as increase friction between rotor and stator during the tuning process. This affect will cause a very steep delta-capacitance per degree-of-rotation slope characteristic in some silicon leached regions between rotor and stator, inhibiting a stable capacitance setting in those areas.

  A method to accurately construct a variable capacitor's capacitance profile, plotting capacitance values at small rotational degree increments, was determined necessary to verify trimmer capacitor malfunction, the suspect root cause of CQA failures and retunes in the factory. Previous capacitance tests performed, analyzing only minimum and maximum values, were limited in their ability to detect characteristics between these values. Since the trimmer manufacturer could not offer the capability to measure a detailed capacitance profile, a new test had to be realized.

  The measurement setup consists of rotating the trimmer using an Airpax 1.8 degree stepper motor mounted to a Chatillon force gauge, assuring constant rotation while at the same time controlling the downward force applied. The trimmer is securely mounted in a hold down fixture compatible with the

0 Motorola, Inc. ,991

HP4192 impedance analyzer. An HP9121 PC is used to remotely control both the stepper and the HP4192, synchronizing data measurements with each stepper increment (fig. 1). Capacitance and degree of rotation data are output to a printer in both graphic as well as numerical form (fig. 2). Only in this way can a relationship between delta-capacitance and degree-of- rotation be analyzed on a level critical for automated tuning.

  The experiment consisted of analyzing the capacitance profiles of trimmers from failed CQA pagers and making a comparison to trimmer profiles from pagers which passed CQA. Also, a profile comparison was made between trimmers dated 6/89 and trimmers dated 6/88, before reflow, after reflow, and then after freon wash. In addition, trimmers of a second vendor were profiled before and after the automated assembly process and compared to the trimmers which were currently...