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Fluxless Chip Joining (Soldering) in Vacuum

IP.com Disclosure Number: IPCOM000036577D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 1 page(s) / 11K

Publishing Venue

IBM

Related People

Darekar, VS: AUTHOR [+2]

Abstract

Interconnections between circuit components on a printed circuit board are typically formed by the use of a soldering process at temperatures in the range of 200oC to 365oC in normal atmosphere. A major problem which arises in this soldering is the tarnishing (oxidation contamination) of copper. Although flux may be used to offset this problem the use of flux creates other problems. The most serious problem is that of multiple small voids (MSVs) produced by outgassing. These MSVs have a profound impact on the thermal fatigue life of controlled collapse chip connector (C4) joints. A soldering technique which prevents oxidation and subsequent occurrence of MSVs due to fluxing is accomplished by soldering in a vacuum, without flux, in the following manner.

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Fluxless Chip Joining (Soldering) in Vacuum

Interconnections between circuit components on a printed circuit board are typically formed by the use of a soldering process at temperatures in the range of 200oC to 365oC in normal atmosphere. A major problem which arises in this soldering is the tarnishing (oxidation contamination) of copper. Although flux may be used to offset this problem the use of flux creates other problems. The most serious problem is that of multiple small voids (MSVs) produced by outgassing. These MSVs have a profound impact on the thermal fatigue life of controlled collapse chip connector (C4) joints. A soldering technique which prevents oxidation and subsequent occurrence of MSVs due to fluxing is accomplished by soldering in a vacuum, without flux, in the following manner.

The technique consists of cleaning the copper pads of a substrate, typically by hydrogen or nitrogen cleaning. A chip containing solder balls is placed on the substrate chip area. The module is placed in a vacuum oven and the vacuum is drawn to the desired pressure (1-10 microns). The temperature is raised to approximately 40oC above the melting point of the solder alloy.

The temperature is held constant to allow for normal dwell time (3-5 minutes). After heating, the vacuum oven is allowed to cool to approximately 160oC and is then vented to permit unloading of the module.

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