BACKGROUND OF THE INVENTION 1). Field of the Invention Embodiments of this invention relate to a solder composition that may be used in the construction of electronic assemblies. 2). Discussion of Related Art Solder compositions are used for attaching pieces to one another. In the manufacture of electronics works, for example, solder compensations can be used for attaching contacts of a die having a microelectronic circuit formed therein to contacts of a substrate. A solder composition may also be used for attaching a heat spreader or a heat sink to a surface of a microelectronic die. A solder composition typically has to have a relatively low melting temperature, so that the pieces can be attached to one another at a relatively low temperature. Most solder compositions are relatively weak. An electronic assembly that is subjected to thermal stresses may fracture at the locations of the solder compositions. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention is described by way of example with reference to the accompanying drawings, wherein: FIG. 1 is a cross-sectional view of a solder composition according to an embodiment of the invention; FIG. 2 is a perspective view illustrating the direction of forces on a dispersoid particle in a solder matrix material; FIG. 3 is a cross-sectional view illustrating how shear can be reduced by the inclusion of dispersoid particle; FIG. 4 is a graph illustrating the effect of temperatures and volume fraction on creep innovation; FIGS. 5 to 8 are phase diagrams of solder matrix materials that may find applications in the embodiments of the present invention; FIG. 9 are cross-sectional views illustrating one manner of forming a solder composition; FIG. 10 are side views of an electronic assembly that is manufactured utilizing the solder composition of FIG. 9; FIG. 11 is a diagram illustrating the manufacture of a solder composition according to an alternative embodiment of the invention; and FIG. 12 is a block diagram illustrating a computer system or machine in which the electronic assembly of FIG. 10 may find application. DETAILED DESCRIPTION OF THE INVENTION FIG. 1 of the accompanying drawings illustrates a solder composition 10, according to an embodiment of the invention, including a solder matrix material 12 and dispersoid particles 14 in the solder matrix material 12. The solder matrix material 14 has a relatively low melting temperature and the dispersoid particles 14 have a relatively high melting temperature. The relatively low melting temperature of the solder matrix material allows the bulk of the solder composition 10 to be melted and again be solidified at a relatively low temperature to secure first and second pieces to one another. The relatively high melting temperature of the dispersoid particles 14 provide dislocations in a final crystal structure of the solder matrix material 12 after solidification. The dislocations reduce creep and enhance the strength of the solder matrix material 12 and the solder composition 10 as a whole. Dislocation climb/ detachment and Orowan bowing are among key mechanisms identified as direct interaction that can impede dislocation glide and climb. According to the Orowan model, bowing and dislocation climbs/detachment is proportional to:
τ Orow =
Gb
f
〈 r 〉 The variables used in formula (1) are shown in FIG. 2. As can be seen from the above model, the fine and uniform distribution of hard and thermally-stable dispersoid can substantially strengthen the bulk material. If particle size is an order of microns, the direct interaction between dislocation and dispersoid becomes ineffective. Larger particles are associated with a lower Orowan stress for a given volume fraction depending on loading conditions and k value, (r/b) of 10 to 100 can typically be obtained and therefore nano-size particles are key to the success of dispersion strengthening. FIG. 3 illustrates the effect of a dispersoid particle where the dispersoid particle is on a grain boundary or entirely within a granular crystal. In both cases, glide is reduced in a plane of the grain boundary. In both cases, there is also a corresponding reduction in climb for pile up. FIG. 4 illustrates the effect of volume fraction and melting temperature on diffusion creep. The higher the melting temperature of the dispersoids (TM)P relative to the melting temperature of solder matrix material (TM)P, the more inhibition of diffusion creep is achieved. A higher volume fracture of dispersoid particles within the solder composition also results in a greater inhibition of diffusion creep. The volume fraction of the dispersoid particle may, for example, be between 1% and 20%. FIGS. 5, 6, 7 and 8 and In-Ss, Sn—Bi, Bi—In and Bi—Zn phase diagram. The solder matrix material is preferably selected from table 1.
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TABLE 1
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Composition range of “non-eutectic” Sn—In—Bi—Zn alloys
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Sn (wt. %) In Bi Zn Liquidus range (C.) |
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42-19 0-25 58-56 0 138-79 |
48-20 52-48 0-32 0 118-59 |
0-19 33-25 67-56 0 110-79 |
0-20 67-48 33-32 0 72-59 |
48-46 52-52 0 0-2 118-107 |
0 33-33 67-66 0-1 110-108 |
0 33.4-52.2 66.3-47.4 0.3-0.4 108-86 |
0 52.2-66.8 47.4-32.7 0.4-0.5 86-68 |
0 66-66.8 34-32.7 0-0.5 72-68 |
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Note:
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you will get 100 wt. % by adding each element's composition in sequence (e.g., 1st row, Sn(42) + In(0) + Bi(58) + Zn(0) = 100, Sn(19) + In(25) + Bi(56) + Zn(0) = 100, etc.)
Ideally, the solder matrix material is a eutectic because of the lower melting temperature of a eutectic. A eutectic composition may be selected from table 2.
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TABLE 2
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Compositions of “eutectic” Sn—In—Zn alloys
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Lowt Tm Interlayer Eutectic Melting Example of bonding temp |
(wt. %) temp. (C.) (10 C. above Tm.) (C.) |
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In—48Sn 118 128 |
Bi—22In 110 120 |
Bi—33In—0.3Zn 108 118 |
In—46Sn—1.5Zn 107 118 |
In—47Bi—0.4Zn 86 96 |
Bi—25In—19Sn 79 89 |
In—34Bi 72 82 |
In—33Bi—0.5Zn 68 78 |
In—32Bi—20Sn 59 69 |
In—35Bi—16Sn—0.4Zn 58 68 |
As can be seen from the above tables, the solder matrix material may have a melting temperature below 150° C., more preferably below 125° C. Optionally, the composition may have a small amount of precipitation-forming alloying elements so that after reflow, fine precipitation will be formed throughout the baseline alloy matrix. For precipitation, alloying elements should form an intermetallic compound with the baseline constituents at room temperature. Based on the binary phase diagrams in FIGS. 5, 6, 7 and 8, the following elements can be added for precipitation: Cu, Ni, Ag, Ti, Mn, Co, Au or Fe. The weight percentage of the precipitation-forming alloying elements is preferably between 0.1% and 10% of the composition. The dispersoid particles should preferably have a high melting temperature, typically above 1000° C., be non-shearable, and should preferably be non-soluble in the solder matrix material. Based on the above criteria, the oxides and carbides in table 3 may be used as dispersoid particles.
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TABLE 3 |
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Dispersoid Tm (C.) Dispersoid Tm (C.) |
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SiC >2700 In2O3 1920 |
W2C >2800 Yb2O3 2315-2413 |
WC >3000 Y2O3 2356-2453 |
ZrC 3540 TiO2 1830-1850 |
TiC 3170 ZrO2 2700 |
B4C 2450-2723 Cr2O3 2300 |
Cr3C2 1250 MgO 2800 |
Cr7C3 1665 SiO2 |
Cr3C6 1890 WO3 1473 |
Al2O3 2000 |
Optimum sizes for dispersoid particles are 10 to a few hundred nm, e.g. 200 nm. The larger the sizes of the dispersoid particles, the weaker the strengthening effects. The higher the volume fraction of the dispersoid particles in the composition, the higher the strengthening effects. FIG. 9 illustrates one method of making the solder composition. Solder particles 16 are mixed with the dispersoid particles 14. The composition 10 is then reflowed. During reflow, the composition 10 is heated to a temperature above the melting temperature of the solder particles 16, so that the solder particles 16 melt, and the composition 10 is then allowed to cool so that the material of the solder particles 16 form the solder matrix material 12. The dispersoid particles 14 are located between portions of the solder matrix material 12 that formed the original solder particles 16. FIG. 10 illustrates how the composition 10 can be used in the construction of an electronic assembly 20. The electronic assembly 20 includes a substrate 22 and a die 24 having a microelectronic circuit formed therein. Contacts 26 and 28 are formed on the substrate 22 and the die 24 respectively. Either the contacts 26 or the contacts 28, or all the contacts 26 and 28 can be formed from the composition 10 on the left in FIG. 9. The contacts 26 and 28 are then brought together. During reflow, the contacts 26 and 28 combine with one another to form interconnects 30 between the substrate 22 and the die 24. What should be noted from FIG. 9 is that the dispersoid particles 14 are not located within the volumes of the original solder particles 16, so that these volumes are not strengthened. Referring now to FIG. 11, another process is shown for the manufacture of a solder composition having superior strength to the solder composition of FIG. 9. The solder matrix material 12 is held as a liquid metal in a first container 34 and the dispersoid particles 14 are held in a second container 36. The solder matrix material 12 and the dispersoid particles 14 are then mixed in a third chamber 38. Because the solder matrix material 12 is in liquid form, the dispersoid particles 14 become embedded within the crystals of the solder matrix material 14. The solder composition that results is then allowed to cool. A master ingot is then manufactured utilizing conventional milling and extrusion processes. The master ingot is then subjected to gas atomization, which breaks the master ingot into powder particles 40. Each powder particle may for example be about 1000 nm across. Each powder particle 40 includes some of the metal matrix material and some of the dispersoid particles 14 within the metal matrix material 12. It is the metal matrix material that includes the powder particles 40 that is applied to form the contacts 26 and/or 28 in FIG. 10. Following reflow, the dispersoid particles 14 are held within the original volumes of the powder particles 40. Embedding of the dispersoid particles 14 into the crystal structures of the particles 40 in this manner substantially enhances their strengthening effect. FIG. 12 shows a diagrammatic representation of a machine in the exemplary form of a computer system 1200 within which a set of instructions, for causing the machine to perform any one or more methodologies may be executed. In alternative embodiments, the machine operates as a stand alone device or may be connected (e.g., network) to other networks. In a network deployment, the machine may operate in the capacity of a server or a client machine in a server-client network environment, or as peer machine in a peer-to-peer (or distributed) network environment. The machine may be a Personal Computer (PC), a tablet PC, a Set-Top Box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term (machine) shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. The exemplary computer system 1200 includes a processor 1202 (e.g., a Central Processing Unit (CPU), a Graphics Processing Unit (GPU) or both), a main memory 1204 (e.g., Read Only Memory (ROM), flash memory, Dynamic Random Access Memory (DRAM) such as Synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM), etc.), and a static memory 1206 (e.g., flash memory, Static Random Access Memory (SRAM), etc.), which communicate with each other via a bus 1208. The electronic assembly 20 shown in FIG. 10 may be any one of the components 1202, 1204 or 1206. The computer system 1200 may further include a video display 1210 (e.g. Liquid Crystal Display (LCD) or a Cathode Ray Tube (CRT)). The computer system 1200 also includes an alphanumeric input device 1212 (e.g., a keyboard), a cursor control device 1214 (e.g., a mouse), a disk drive unit 1216, a signal generation device 1218 (e.g. a speaker), and a network interface device 1220. The disk drive unit 1216 includes a machine-readable medium 1222 on which is stored one or more sets of instructions 1224 (e.g. software) embodying any one or more methodologies or functions. The software may also reside, completely or at least partially, within the main memory 1204 and/or within the processor 1202 during execution thereof by the computer system 1200, the main memory 1204, and the processor 1202 also constituting machine-readable media. The software may further be transmitted or received over a network 1228 via the network interface device 1220. While the machine-readable medium 1224 is shown in an exemplary embodiment to be a single medium, the term “machine-readable medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term “machine-readable medium” shall also be taken to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by the machine and that cause the machine to perform one or more methodologies. The term “machine-readable medium” shall accordingly be taken to include, but not be limited to, solid-state memories, optical and magnetic media, and carrier wave signals. While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, that this invention is not restricted to the specific instructions and arrangements shown and described since modifications may occur to those ordinarily skilled in the art. |
