Abstract:
A tin-rich gold-tin solder is disclosed which is particularly advantageous for self-aligning applications. When utilized with gold-plated bond locations, the out-diffusion of tin from the solder during heating functions to shift the composition of the remaining solder closer to the eutectic value, thus preserving the liquid state of the solder and improving its reflow quality with respect to conventional eutectic solders.

Description:
This application is a continuation of application Ser. No. 07/877,335, filed on May 1, 1992, which is a continuation-in-part of application Ser. No. 47,169, filed on Apr. 12, 1993, with both now abandoned. 
    
    
     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&lt;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Referring now to the drawings, where like numerals represent like parts in several views: 
     FIGS.  1 - 3  illustrate an exemplary sequence of steps utilized to provide self-aligned solder attachment in accordance with the teachings of the present invention; 
     FIG. 4 illustrates in particular the self-aligning ability of the system of the present invention; 
     FIG. 5 contains a Au/Sn phase diagram useful in understanding the process of the present invention; 
     FIG. 6 contains an optically-scanned, computer-generated image of a photograph of bond pad coverage when utilizing a conventional prior art eutectic Au/Sn solder; and 
     FIG. 7 contains an optically-scanned, computer-generated image of a photograph of improved bond pad coverage when utilizing an exemplary tin-rich Au/Sn solder of the present invention. 
    
    
     DETAILED DESCRIPTION 
     An exemplary process for providing self-aligned solder attachment of a pair of components is illustrated in FIGS.  1 - 3 . The inventive bonding process will be particularly described in association with the self-aligned attachment of an LED to a bond pad. However, it is to be understood that the tin-rich solder of the present invention is ubiquitous in nature and can be utilized within virtually any desired soldering process. Referring back to FIG. 1, a device submount  10 , which may be formed of silicon, is illustrated as including a bond pad  12 . Conventionally, bond pad  12  is formed of a highly conductive material and, in particular, may comprise a tri-layer structure of titanium, platinum and gold. Gold, in general, is often used for bond pads due to its inert nature and relatively high conductivity. A device  14 , such as an LED, is illustrated in FIG. 1 as positioned above bond pad  12 . Device  14  also includes a conductive layer  16  (which may also comprise a tri-level, or gold plated layer) to facilitate bonding. In order to provide alignment of device  14  to submount  10 , conductive layer  16  should be substantially similar in dimension to bond pad  12 . 
     Bonding is achieved, in accordance with the present invention, by utilizing a tin-rich (with respect to the eutectic 80/20) Au/Sn solder preform  18  which is disposed over and brought into contact with bond pad  12 . Initially, solder preform  18  may comprise a solid compound, and exhibit a weight percent composition within the range of, for example, 74/26 to 78/22. To initiate bonding, as shown in FIG. 2, device  14  is brought into contact with solder preform  18  and the combination is heated to a temperature (for example, 400° C.) at which the tin-rich solder will liquify. As the ambient temperature increases, the rate of solid state diffusion of the tin out of solder  18  and into gold layers  12  and  16  will increase, as indicated by the arrows in FIG.  2 . The solid state diffusion is also known to occur with the conventional eutectic solder. In particular, problems arise when utilizing the prior art eutectic in that as the tin diffuses out, the solder becomes gold-rich, and the temperature required for liquifying the solder raises. FIG. 5 contains a Au/Sn phase diagram illustrating the effect of the composition on the phase state of the material. Thus, when soldering at the temperature associated with a “eutectic” solder (e.g., 278° C.), a portion of the solder will remain in the solid state and will not completely coat (i.e., “wet”) the underlying bond pad. 
     In contrast, when utilizing the tin-rich solder of the present invention, the solid state diffusion of tin from solder preform  18  into bond pad  12  and contact layer  16  results in moving the composition closer to the eutectic, as indicated by the arrow in FIG.  5 . That is, as the tin is depleted, the temperature required for liquifying the solder actually drops (following along the line from 419.3° C. toward the limit of 278° C.), and essentially all of the solder will remain in the liquid state. FIG. 3 illustrates the relative location of device  14  with respect to the liquified solder preform  18 . As shown, when completely liquified, solder  18  will completely cover the top surfaces  20 , 22  of bond pad  12  and conductive layer  16 , respectfully. The surface tension created at the outer bound of solder  18  will cause device  14  to center itself, thus resulting in device  14  to be self-aligned with respect to the underlying bond pad  12 . 
     It is known that the soldering operation such as that discussed above will cause a tin oxide layer to form (or an existing layer to increase) at the outer surface of solder  18 . The presence of this oxide may necessarily affect the solder reflow. Thus, in the preferred technique of the present invention, the heating step is performed in a reducing ambient which inhibits the formation of this oxide. For example, formic acid may be used to prevent tin oxide formation. Other reducing environments (indicated as “R.E.” in FIG. 2) include, but are not limited to, forming gas (20%H 2 /80%N 2 ), carbon monoxide, or any suitable liquid flux. 
     As mentioned above, an advantage of utilizing the tin-rich solder of the present invention is the ability to provide automatic alignment of a device to an underlying bond pad. Referring to FIG. 3 in particular, the tin-rich solder allows for a device  14  to be aligned with bond pad  12 . The coverage of liquified solder  18  with respect to surfaces  20  and  22  of bond areas results in alignment since the solder material will naturally move so as to reduce its overall surface area. Reference is made to FIG. 4 which illustrates a device  14 ′ offset with respect to the underlying bond pad  12 . As shown, liquified solder  18  will exhibit an increased surface area with respect to a minimum, as a function of the amount of offset. Thus, the liquified solder will naturally move, in the direction as indicated by the arrows, so as to minimize the surface area. The natural tendency of the solder movement thus results in alignment of the device to the bond pad. Experimentally, it has been found that misalignment on the order of 100 μm can be corrected to within a one μm limit. 
     FIGS. 6 and 7 contain top views of actual bond pad sites including a reflowed solder preform (original preform being circular in shape). Both figures are optically-scanned, computer-generated images taken from actual color photographs. The bond pad of FIG. 6 contains a conventional eutectic 80/20 Au/Sn solder after reflow. As shown, the solder does not completely coat the surface of the bond pad. As discussed above, the out-diffusion of tin into the bond pad results in a gold-rich solder which does not completely liquify and, hence, has poor reflow qualities. In contrast, the bond pad of FIG. 7 contains a tin-rich (76/24) solder of the present invention. As shown, essentially all of the bond pad surface has been coated with the reflowed material. 
     Advantageously, the utilization of a tin-rich solder in accordance with the present invention results in a significant manufacturing achievement. In particular, by virtue of using a tin-rich solder, a bonding hierarchy may be employed, without the need of first employing a soldering hierarchy. That is, as one proceeds through sequential bonding operations, all using the same solder and bonding compositions, previous bonds will not be disturbed (i.e., reflow). 
     As long as the component and/or substrate is conventionally metallized (with Ti/Pt/Au or Ti/Ni/Au, for example), the utilization of the tin-rich solder of the present invention results in the intermetallic compound formation of Pt with Sn (when using Ti/Pt/Au) as well as interdiffusion of Sn into the Au metallization. Both of these processes result in significantly raising the melt temperature of the resultant bond, to above 400° C., for example. 
     Therefore, when the next component is bonded, using the tin-rich solder again, at a nominal temperature of 315° C., the first component will remain fixed, since bonding was carried out at a temperature well below the melt temperature. The process may be repeated time and again, without disturbing any of the previously-formed bonds. Such an advantage, realized by using the tin-rich solder compound of the present invention, is especially critical in optical applications, where alignment of bonded components cannot be subjected to movement subsequent to bonding. 
     It is to be understood that the arrangement described above is exemplary only, and the tin-rich Au/Sn solder of the present invention is applicable to any system which requires the soldering of a component to a gold, or gold-plated bond pad.