Browse Prior Art Database

FORCE LIMITED SPRING WASHER

IP.com Disclosure Number: IPCOM000006977D
Original Publication Date: 1993-Jun-01
Included in the Prior Art Database: 2002-Feb-13
Document File: 2 page(s) / 88K

Publishing Venue

Motorola

Related People

Brian Webb: AUTHOR

Abstract

In many applications of Single In-line Power Pack- ages a screw is inserted through a mounting hole in the heat sink of the package to hold it against an external heat sink. Over torqueing of the screw results in forcing the heat sink away from the plastic encapsulent. This in turn results in device failures from cracked die, wire bond failures and/or corrosion due to moisture ingress. This failure mode may be eliminated by the use of a spring washer which limits the amount of force applied to the device but at the same time insures intimate con- tact with the heat spreader for eff%zient heat dissipation. The novelty of this design includes the incorporation of a self limiting device in the spring washer design to insure foolproof assembly in semiconductor and other sensi- tive applications.

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MOTOROLA INC. Technical Developments Volume 19 June 1993

FORCE LIMITED SPRING WASHER

by Brian Webb

   In many applications of Single In-line Power Pack- ages a screw is inserted through a mounting hole in the heat sink of the package to hold it against an external heat sink. Over torqueing of the screw results in forcing the heat sink away from the plastic encapsulent. This in turn results in device failures from cracked die, wire bond failures and/or corrosion due to moisture ingress. This failure mode may be eliminated by the use of a spring washer which limits the amount of force applied to the device but at the same time insures intimate con- tact with the heat spreader for eff%zient heat dissipation. The novelty of this design includes the incorporation of a self limiting device in the spring washer design to insure foolproof assembly in semiconductor and other sensi- tive applications.

  Spring washers have been utilized to apply constant force to assembled parts in applications such as slip clutches and expansion joints for some time. Common designs include Belleville washers, Curved washers, Fin- ger washers, and Wave washers numerous variations also exist. Increased force is often achieved by stacking mul- tiple spring washers into an assembly.

  Conventional spring washers will achieve the desired effect of applying a controlled amount of force to the Single In-line Power Package, however, if the mounting screw in over torqucd by an inexperienced or ill informed operator the spring will be collapsed resulting in poten- tial damage to the package. Conventional applications eliminate this by the use of special, and expensive, shoul- der bolts to limit the amount of compression on the spring washer.

  These dangers may be eliminated by incorporating a tube into the design of the spring washer....