Alloys or metallic compositions – Gold base
Statutory Invention Registration
1995-10-06
2001-01-02
Nelson, Peter A. (Department: 3641)
Alloys or metallic compositions
Gold base
C148S430000
Statutory Invention Registration
active
H0001934
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a gold-tin solder and, more particularly, to a tin-rich, gold-tin solder.
2. Description of the Prior Art
Soldering is a well-known technique for securing metal piece parts in a permanent fashion. Solder material must have the capacity to form metallurgical bonds with the two base metals that are to be joined. The bonding process results in the formation of an alloy in the surface of the base metal characterized by atoms of the soldering composition interspersed between atoms of the base metal.
When the solder is heated to a molten state, it exists as a round droplet as a result of the attraction between the molecules forming the alloy. This attraction is commonly referred to as surface tension. For many technologically advanced applications, such as semiconductor and lightwave technologies, a solder composition of choice is a eutectic gold-tin material which is approximately 80% by weight gold and 20% by weight tin. The eutectic god-tin is often used in soldering components to gold (or gold-plated) bond pad sites.
In a conventional solder process, discrete portions, or preforms of the solder material are disposed or otherwise deposited (e.g., vapor deposition or silk screen paste )on a surface of one of the pieces to be mated (for example, to the bond pad site). The two pieces are joined and heated to a temperature where the solder liquifies (i.e., approximately 315° C. in a nitrogen ambient) and reflows over the mating surfaces to provide attachment upon cooling.
There are many known disadvantages to this procedure. First, the reproducibility of the reflow is questionable and sensitive to process variations, particularly since commonly available eutectic Au/Sn solder may only be guaranteed ±1% around the eutectic 80/20 composition. Additionally, the solid state diffusion of tin into an underlying gold bond pad will shift the preform composition to the gold-rich side of the eutectic and could result in poor reflow by raising the melting temperature of the resulting alloy.
In spite of the above drawbacks, however, Au/Sn remains the solder composition of choice since it is compatible with the materials (in particular, gold) used to plate most device surfaces.
Thus, a need remains in the art for some means of overcoming the various drawbacks associated with the eutectic Au/Sn solder.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by the present invention which relates to a gold-tin solder and, more particularly, to a gold-tin solder suitable for self-aligning applications.
In accordance with the teachings of the present invention, a tin-rich Au/Sn solder (Au
80-x
Sn
20+x
, 0<x≦6) is utilized, preferably in the range of 78/22 to 74/26 (weight % composition). Therefore, if solid state (or liquid state) diffusion of tin into a contacting gold plating layer occurs, the melt temperature will actually drop as the tin is depleted in the solder composition, until the eutectic composition is reached. The use of the tin-rich solder thus insures that the alloy will remain molten during solder reflow.
Further, the soldering process of the present invention may be optimized by heating the solder preform within a reducing ambient such that the further growth of any tin oxide on the surface of the preform (skin oxide) is significantly reduced.
An advantage of the solder composition of the present invention is that self-alignment of piece parts may be achieved. In particular, the utilization of a tin-rich solder allows for the solder to remain liquid and completely cover (i.e., “wet”) both bonding surfaces. Therefore, by controlling the size and location of the bonding surfaces, the liquification of the tin-rich solder will result, through surface tension effects, in registration (i.e., centering) of the device with respect to the underlying bond pad.
Other advantages of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
REFERENCES:
patent: 3340602 (1967-09-01), Hontz
patent: 4214904 (1980-07-01), Kitchen et al.
patent: 4385029 (1983-05-01), Desai
patent: 4670217 (1987-06-01), Henson et al.
patent: 4922322 (1990-05-01), Matthew
patent: 5066614 (1991-11-01), Dunaway et al.
patent: 55-16525 (1980-02-01), None
patent: 0406673 (1973-11-01), None
Metals Handbook, vol. 8 pp. 269 and 347 8th id.*
Koba Wendy W.
Lucent Technologies - Inc.
Nelson Peter A.
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