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WAVY OMEGA STRAP FOR MODULAR INTERCONNECT THERMAL EXPANSION CONSIDERATIONS

IP.com Disclosure Number: IPCOM000007932D
Original Publication Date: 1996-Nov-01
Included in the Prior Art Database: 2002-May-07
Document File: 2 page(s) / 119K

Publishing Venue

Motorola

Related People

Ken Bieda: AUTHOR [+3]

Abstract

The omega straps currently used as interconnects between modules or between modules and circuit boards exist in two forms. The first is the basic shape from which the name is derived, stemming From the Greek letter omega. This strap has a single "hump" between two terminations or feet. The mate- rial is a solid piece of copper with a tin plating on the underside of the "hump" section. Each termi- nation has a single hole centered on each. It was discovered that thermal coefficient of expansion dif- ferences between adjoining modules and circuit boards created shearing forces to which the original omega strap did not acceptably perform. Either the strap would break, tear off the solder connection or rip the circuit loose from the circuit board. If viewed from above the solid omega worked well in the Y and Z directions but not X, Y and Z at the same time. Because of these considerations the second type of omega strap was invented.

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Technical Developments

WAVY OMEGA STRAP FOR MODULAR INTERCONNECT THERMAL EXPANSION CONSIDERATIONS

by Ken Bieda, Jerry McFarlin and Roy Stone

  The omega straps currently used as interconnects between modules or between modules and circuit boards exist in two forms. The first is the basic shape from which the name is derived, stemming From the Greek letter omega. This strap has a single "hump" between two terminations or feet. The mate- rial is a solid piece of copper with a tin plating on the underside of the "hump" section. Each termi- nation has a single hole centered on each. It was discovered that thermal coefficient of expansion dif- ferences between adjoining modules and circuit boards created shearing forces to which the original omega strap did not acceptably perform. Either the strap would break, tear off the solder connection or rip the circuit loose from the circuit board. If viewed from above the solid omega worked well in the Y and Z directions but not X, Y and Z at the same time. Because of these considerations the second type of omega strap was invented.

  The second type of omega strap was basically the identical size and shape as the first except the material from which it is made differs. Instead of being from a solid strip of copper it is fashioned from braided copper strands, similar to solder wick. This material allowed for flexibility in all three direc- tions. The original problem seemed to be solved. However, this new material proved to be quite diffi- cult with which to work and introduced several new, previously non-existing, problems:

  1. Manufacturing the strap is very difficult and still requires careful monitoring. It has yet to come close to a six sigma process. Very many straps arrive from the vendor either in unusable condition or must be formed, by hand, back to the original shape. Those that are unusable are usually coming apart at the ends, or unbraiding.

  2. Preventing solder from wicking into the braid has proven to be much more of a challenge than originally thought because while it must not wick into the braided "hump" section, it must be able to

solder at both terminations. This is an area that con- tinues to be a problem. In fact, it has caused an additional, manual operation to be added to the man- ufacturing process. Solder flux must be dispensed, by hand, using flux dispenser pens. Many times the subsequent manufacturing processes must replace the straps because they have unbraided between steps.

  3. Even ifthe braided straps arrive in usable con- dition and everything in the manufacturing process...