Abstract:
A substantially lead-free solder composition having the composition (weight %): Sn, 76.0-83.9%; Ag, 8.0-12.0%; Sb, 8.0-10%; Cu, 0.1-2.0%. In some embodiments the solder composition contains 12 wt % Ag, 8 wt % Sb, 0.1 wt % Cu, the remainder being Sn. The solder can be formed into a wire, a ribbon or sheet, a solder paste or preform, or a powder, for example. Fillers, such as Si or Ag spherical fillers, with a particle size maximum at 35 μm, can be added to any of the various solder forms during alloy fabrication. Also, a semiconductor package having a die attached to a support using the substantially lead-free solder composition.

Description:
CROSS-REFERENCE TO RELATED APLICATIONS  
       [0001]     This application s\claims the benefit of U.S. Provisional Application No. 60/627,929, filed Nov. 13, 2005, titled “Lead-free solder system”, which is hereby incorporated by reference. 
     
    
     BACKGROUND  
       [0002]     This invention relates to semiconductor die attach materials and, particularly, this invention relates to lead-free soft solders that are useful as die attach materials in semiconductor packages that may be subject to high thermal stress.  
         [0003]     Interconnect technology of the first and second level of electronic packaging is based on a tin-lead (SnPb63%) alloy, which provides a cheap and reliable solder joint for the semiconductor industry for the past few decades. Since the early 1990s, concern about lead exposure as toxic effluent to the environment has brought public attention to eliminating the use of lead in consumer electronic products. Various legislations have been proposed, drafted, approved and/or enforced to eliminate lead or to regulate its use to a minimum level. An unprecedented focus towards total lead free manufacturing by January of 2008 is outlined clearly in WEEE Draft IV on 27 th  Jan. 2003 by the European Commission (EC) Board of Directives. Huge efforts have been made by researchers worldwide to develop new formulations of lead free solders so as to accommodate the great demand from the industry. The Institute for Printed Circuits (IPC) and the National Electronics Manufacturing Initiative (NEMI) from the United States have called for a global solution to the voluntary reduction or elimination of lead by electronics interconnection industry.  
         [0004]     First level interconnect materials development remains scanty, and development is immature in terms of meeting die attach processibility and reliability performance compared to the second level interconnect. Industry well-known lead free board level materials include SnAg4Cu0.5% from NEMI and SnAg3Cu0.5% alloys from the Japan Electronic Industry Development Association (JEIDA).  
         [0005]     To be suitable for use in a particular environment, a die attach solder must satisfy a number of requirements, and materials that meet one or some of the requirements often fail to meet one or more others.  
         [0006]     Most board level interconnect materials require solder melting temperatures in the range 183° C. to 228° C., and this requirement is met for example by the well-known ternary eutectic Sn—Ag—Cu alloy which melts at 217° C. Other wave or hand soldering application interconnect materials also have low liquidus temperatures, such as the quaternary Sn97Cu2Sb0.8Ag0.2 alloy, which melts at 228° C. (Kester, SAF-A-LLOY). Lead-free interconnect materials for die attach application in power packages require a higher alloy liquidus point in order to survive board level reflow at a peak temperature of 260° C. It will be appropriate to have alloy melting temperature in the range of 245° C. to 280° C. so as to ensure the integrity of the die attach material (soft solder remains solid or semi-solid during the peak reflow temperature so as to hold the die) in regards to the whole semiconductor package stability and robustness.  
         [0007]     Other than the aforementioned requirement of the relatively high alloy melting point, a lead free soft solder for die attach in power packages should not be brittle or possess high shear modulus. Soft solders such as Alloy J (SnAg25Sb10%) or BiAg12Ni2% are too brittle (1-4% of elongation performance) to be suitable for large die placement in automotive power packages, for example (strain and creep increases incrementally with die effective area). Due to the fact that automotive packages are constantly exposed to severe ambient temperature cycles and vibration, intermetallic grain growth with brittle characteristics will result in cyclic thermal strains at solder joint and likely will result in cracking in areas of high stress concentration. Brittle solder materials possess lower impact resistance, inferior to direct shear upon thermal strains loading. As more emerging automotive and avionic applications require packages having high impact resistance, there is a need to develop a lead free soft solder with intermediate high mechanical strength and resistance to intermetallic creep, and which is ductile with 45% to 60% of elongation performance.  
         [0008]     Another criterion for a suitable lead free soft solder is its capability to discharge and retain required heat when the device is turned ON fully for RDSON measurement. A need for a good current containment soft solder with lower thermal junction-ambient (θ JA ) differences is required for high rating power amplifier devices (e.g. NFET, NBJT, NMOSFET, MESFET or rectifier diode). When a power device is used for an amplifier circuit, current forward bias is achieved (VBE is positive &amp; VBC is negative) in an active mode, where the largest current amplification is obtained in a few nano-seconds that may heat up the semiconductor package if heat is not discharged rapidly enough. As for power transistors used in a switching circuit, a fast current switching between the cutoff and saturation is always essential for minimizing parasitic occurrence. As such, during the amplification or switching period, the die attach material, here a soft solder, will function as a heat spreader. Resistance to thermal junction-ambient differences for any soft solder alloy must be low, and should dissipate heat immediately via its conductive elements. The difference between initial current input and output reading is measured in delta VDS/VBE and the total shift should not exceed the industry acceptance level of 20% after subjecting the package to temperature cycling up to minimum 1000 cycles. Accordingly, a high thermal conductivity lead free soft solder having with refined grain structure is greatly desired.  
         [0009]     Various attempts have been made to use alloys having lower liquidus points (200° C. to 220° C.); but these result in exposing the mounted die to molten or liquid phase soft solder even after encapsulation, when reflowing the packages at reflow temperatures up to 260° C. through as many as three repetitions.  
         [0010]     Use of solder compositions having unoptimized Ag constituent % can result in problems stemming from frequent wafer backmetal separation.  
         [0011]     U.S. Pat. No. 5,352,407 describes a lead free bismuth free tin base quarternary alloy with solder composition of 93-98% tin (Sn), 1.5-3.5% silver (Ag), 0.2-2.0% copper and 0.2-2.0% of antimony (Sb) used for board level soldering with solder melting temperature at 210° C. to 215° C., which is too low for a die attach application that requires a 280° C. to 400° C. working condition (acceptable die attach machine heat tunnel temperature range).  
         [0012]     U.S. Pat. No. 5,527,628 describes a lead-free tin-silver-copper ternary eutectic solder. The composition has proportions by weight of 93.6% Sn, 4% Ag and 1.7% Cu, and it has a eutectic melting temperature at 217° C. Eutectic alloy has lower melting temperature and the tendency to expand or contract under temperature cycling condition. Its incapability to withstand harsh working environment thereafter rendering the soft solder limited to wave soldering &amp; all boards level usage only.  
         [0013]     U.S. Pat. No. 5,439,639 describes a ternary tin-silver-bismuth eutectic solder. The composition has proportions by weight of 91.8% Sn, 3.4% Ag, and 4.8% Bi, and it has a melting temperature in the range 202° C. to 215° C. The aforementioned solder does not meet the minimum die attach temperature required for power packages. U.S. Pat. No. 5,352,407 point out that bismuth, which is a by-product mined from lead ores, is brittle and has poor peel strength.  
         [0014]     U.S. Pat. No. 4,170,472 describes a ternary tin-based soft solder, known as “Alloy J”. The composition has proportions by weight of 65% Sn, 25% Ag, and 10% Sb, and has solidus-liquidus points in a wide range from 228° C. to 395° C., eventuating difficulties for die attach operating window optimization. Alloy J is brittle, and can have an elongation less than 4% (inferior to stress and strain induced by thermal cycling). Due to the high Ag constituent %, Alloy J can encounter frequent wafer backmetal separation issue as the entire sputtered Ag layer (wafer backmetal finish) can etch off at high die-attach temperature. The said phenomenon can be seen after die punch verification. Precipitated Ag can form sharp spiral dendrites to increase solder grittiness but its high dissolution rate in the alloy results in over stressing on die backmetal.  
         [0015]     U.S. Pat. No. 5,985,212 describes a lead-free soft solder. The composition has proportions by weight of 75% Sn, 0.01%-9.5% Cu, 0.01 &amp;-5.0% Ga, and indium (In) 6.0% In. This alloy has a high tensile strength (minimum 48 MPa), but it has a low alloy melting temperature owing to the low melting temperature (156.7° C.) of indium. The cost of this solder is high, as indium is a rare earth element.  
         [0016]     U.S. Pat. No. 6,176,947 describes a high strength, high fatigue resistance and high wetting lead-free solder alloy. The composition has proportions by weight of 76-96% Sn, 0.2-2.5% Cu, 2.5-4.5% Au, and 6-12% In. This alloy has a maximum liquidus point at 215° C., which is too low for die attach application in power packages.  
         [0017]     U.S. Pat. No. 5,352,407 describes a lead-free bismuth-free tin-based tin-silver-copper-antimony quaternary alloy for use in board level soldering. The alloy composition has proportions by weight of 93-98% Sn, 1.5-3.5% Ag, 0.2-2.0% Cu, and 0.2-2.0% Sb, and it has a solder melting temperature in the range 210° C. to 215° C.  
         [0018]     Other known quaternary tin-silver-copper-antimony quaternary alloys are known; some examples are listed in Table I.  
                       TABLE I                           Composition, weight %   Solidus/Liquidus   Manufacturer/       SN:Ag:Cu:Sb   Points ° C.   Supplier               93:3:2:2   221/224 (eutectic)   NCMS       96.2:2.5:0.8:0.5   211/226   AIM       95:3:1.5:0.5   216/217 (eutectic)   H-Technologies       93-98:1.5-3.5:0.2-2:0.2-2   210/215   AIM       90-95:3-5:0.5-3:0-5   210/220   Senju                  
 
         [0019]     These tin-silver-copper-antimony alloys all have liquidus points below 245° C. and, accordingly, they are unsuitable for die attach applications in power packages.  
       SUMMARY  
       [0020]     This invention is directed lead free soft solder formulations, and particularly to ductile quaternary alloys based on a Sn—Ag—Sb—Cu composition, having a liquidus point in the range 255° C. to 263° C. The invention is particularly useful for die attach in power packages which require die attach materials having intermediate to high mechanical strength and high impact resistance, which are relatively ductile, have improved resistance to intermetallic growth, and provide superior thermal and electrical conductivity, all at reasonable cost.  
         [0021]     Solders according to the invention can be provided in any of a variety of forms as needed for particular soldering applications, especially for use as a first level interconnect material. The solders of the invention can be fabricated for example as solder wire, solder sheet/ribbon, solder powder/paste and solder ingot by conventional solder manufacturing techniques to a very high purity and compositional accuracy alloy.  
         [0022]     The solders according to the invention provide for easy fabrication, and have low brittleness, so that they are especially useful for solder ingot and wire formation.  
         [0023]     In some embodiments of the invention, the solder is formulated particularly for, but not limited to, die attach in automotive semiconductor packages; the solder may be useful as well for die attach in other commercial power packages, particularly those requiring thermal conductivity of 11 W/mK at a minimum, for high heat dissipation capability.  
         [0024]     In general, the invention features a redefined and improvised lead free tin based soft solder, which can be used particularly for metallurgical bonding of die with backmetal to a semiconductor leadframe that possess a heat sink (die paddle). The composition of the solders according to the invention are quaternary alloys consisting essentially of Sn, Ag, Sb and Cu in the ranges shown in Table II:  
                           TABLE II                                   Element   Constituent, %                           Tin (Sn)   76.0-83.9           Silver (Ag)   8.0-12.0           Antimony (Sb)   8.0-10           Copper (Cu)   0.1-2.0                      
 
         [0025]     The solder compositions according to the invention provide intermediate to high mechanical strength (65-80.43 MPa), and elongation/ductility (50-56.96%), high thermal conductivity (55-59 W/m K) and electrical properties (5.50-5.93×10 −6 W −1 m −1 ), as well as wetting angles less than 45° for the formation of good wettability on Ni-plated or bared Cu lead frame die paddle. The solder compositions according to the invention show a uniformly distributed Sn—Ag matrix in Scanning Electron Microscopy (SEM) micrographs, less planar stress due to less Ag sharp spiral dendrites and enhanced intermetallic formation with Cu element presence so as to resist grain growth after temperature cycling for achieving higher soft solder impact resistance.  
         [0026]     In one general aspect the invention features a substantially lead-free solder composition having the composition (weight %): Sn, 76.0-83.9%; Ag, 8.0-12.0%; Sb, 8.0-10%; Cu, 0.1-2.0%. In some embodiments the solder composition contains 12% Ag, 8% Sb, 0.1% Cu, the remainder being Sn.  
         [0027]     The solder according to the invention can be formed into a wire, a ribbon or sheet, a solder paste or preform, or a powder, for example. Fillers can be added to the solder according to the invention, such as Si or Ag spherical fillers, with a particle size maximum at 35 μm by high pressure gas atomization (HPGA). The filler particles can be incorporated into any of the various solder forms during alloy fabrication.  
         [0028]     The solder is used in die attach for assembly of semiconductor packages and, particularly, for assembly of packages used in environments of high mechanical and/or heat stress, such as power packages.  
         [0029]     Antimony (Sb) present up to 10%, serves in the composition to prevent transformation of beta tin (Sn) into alpha Sn at subzero temperatures of 13° C. to 18° C., which can generate “striated needles”, termed whiskers. Beta Sn matrix is redefined with the presence of Sb to form a uniformly deformed tetragonal structural with reduced grain size. An Sb constituent present at more than 10% by weight can decrease the area of spread upon solder spanking during die attach. Also, as Sb increases, there is a tendency to dross Al or other impurities increase.  
         [0030]     Silver (Ag) and copper (Cu) serve as electrical conductive elements and to enhance alloy conductivity level and solder grittiness due to formation of at least two intermetallic compounds, one with Sn and Ag (Ag 3 Sn) and another one with Sn and Cu (Cu 6 Sn 5  and Cu 3 Sn). Both intermetallic compounds are non-embrittling with the said constituent percentage by weight respectively.  
         [0031]     Copper (Cu) present, limited to 2 wt % reduces the tendency of copper oxides formation, minimizing the rate of grain coarsening due to temperature cycling (directly increasing creep resistance), and prevents the liquidus point of the said solder from residing at temperature higher than 400° C.  
         [0032]     Silver (Ag) present, limited to 12 wt % serves to prevent further dissolution rate of Ag in the said solder. Precipitated Ag (not in the intermetallic layer) remains semi-molten at 400° C., creating high planar stress that has direct impact to “etch off” wafer the backmetal Ag finish layer from the conventional stacked Ti—NiV—Ag layers.  
         [0033]     In another general aspect the invention features a semiconductor package comprising a die attached to a support using a substantially lead-free solder composition having the composition (weight %): Sn, 76.0-83.9%; Ag, 8.0-12.0%; Sb, 8.0-10%; Cu, 0.1-2.0%. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]     The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings(s) will be provided by the Office upon request and payment of the necessary fee.  
         [0035]      FIG. 1A  is a diagrammatic sketch in a perspective view showing a die attached to a support using a solder according to the invention.  
         [0036]      FIG. 1B  is a diagrammatic sketch in a sectional view showing a die attached to a support using a solder according to the invention.  
         [0037]      FIG. 1C  is a diagrammatic sketch showing a portion of a semiconductor package according to the invention in a sectional view as in  FIG. 1B , enlarged.  
         [0038]      FIG. 2A  is a diagrammatic sketch showing a tetragonal lattice structure of pure tin.  
         [0039]      FIG. 2B  is a diagrammatic sketch showing a uniformly deformed tetra-rhombohedral lattice structure of Sn—Sb.  
         [0040]      FIG. 3  is quaternary phase diagram showing proportions of Sn—Ag—Sb in compositions according to the invention.  
         [0041]      FIG. 4  is a plot showing results of a differential scanning calorimetric analysis of a solder composition.  
         [0042]      FIG. 5  is a photomicrograph showing the surface morphology for a prior art solder alloy (Alloy J).  
         [0043]      FIG. 6  is a photomicrograph showing the surface morphology of a solder alloy according the invention (SnAg12Sb8.5Cu0.1%).  
         [0044]      FIGS. 7 and 8  are a diagrammatic sketches comparing the surface morphology of a spanked soft solder according to the prior art ( FIG. 7 ) and according to the invention ( FIG. 8 ).  
         [0045]      FIGS. 11A and 11B  are scanning electron micrographs taken to show wetting angles of solder performs on Ni-plated Cu ( FIG. 11A ), and bare Cu ( FIG. 11B ) lead frames.  
         [0046]      FIGS. 12A and 12   b  are images of showing results of a destructive die punch test demonstrating wettability of solder compositions according to the invention.  
         [0047]      FIG. 13  is a stress vs strain diagram for a solder composition according to an embodiment the present invention in a test carried out at a room temperature of 27.8° C.  
         [0048]      FIG. 14  is a plot of the coefficient of thermal expansion for a solder composition according to the invention.  
         [0049]      FIG. 15  is a series of images showing solder spread of a solder composition according to the invention on a nickel plated lead frame.  
         [0050]      FIG. 16  is a series of images as in  FIG. 15 , showing die mounted on the die attach solder.  
         [0051]      FIG. 17  is a series of X-ray images as in  FIG. 16 .  
         [0052]      FIG. 18  is a scanning electron microscope image showing a die mounted on a substrate using a die attach solder according to the invention.  
         [0053]      FIG. 19  is a transmission electronic microscope image showing bond line thickness using a die attach solder according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0054]     The invention will now be described in further detail by reference to the drawings, which illustrate alternative embodiments of the invention. Except for machine plots and images, the drawings are diagrammatic, showing features of the invention and their relation to other features and structures, and are not necessarily made to scale. For improved clarity of presentation, in the FIGs. illustrating embodiments of the invention, elements corresponding to elements shown in other drawings are not all particularly renumbered, although they are all readily identifiable in all the Figs.  
         [0055]     Turning now to  FIG. 1A , there is shown in a diagrammatic overview an assembly  10  including die  102  affixed using a die attach solder  104  according to the invention to a support  106 , which may be a leadframe, for example. A portion of the assembly  10  of  FIG. 1A  is shown in a diagrammatic sectional view generally at  12 . The assembly includes a die  122  affixed passive side downward on a substrate  126  such as a leadframe, using a die attach solder  124  according to the invention. A portion of a semiconductor package in a sectional view generally as in  FIG. 1B  is shown enlarged at  14  in  FIG. 1C . Here the substrate is shown as a including a patterned copper sheet  136  plated with nickel  137 . The die  132  is mounted upon the plated leadframe using a die attach solder  134  according to the invention. The dies may be electrically interconnected with leads on the leadframe using wire bonds (not shown), and the wire bonded assembly is encapsulated with an encapsulant  138  for mechanical protection as well as for protection from the environment in use.  
         [0056]     The various embodiments of the solder of the invention are tin based lead free alloys consisting tin (Sn), silver (Ag), antimony (Sb) and copper (Cu), having majority of tin (maximum 83.9%) in the alloy formulation. Each of these elements, together with the other elements, contributes to the properties of the composition according to the invention.  
         [0057]     Sn is identified as the most promising element for solder base material in regards to various previous arts finding. Based on the Hume-Rothery rules, Sn forms a stable solid solution over a wide range of compositions with minimal differences in the eletronegativities, atomic radii and crystal structures. The element has a tetragonal lattice structure, as shown diagrammatically in  FIG. 2A ; this has a high ionization potential and fairly (average) high thermal and electrical conductivity performance. However, Sn can not be used alone, as it has a tendency to form alpha tin at subzero temperature of 13° C. to 18° C. Alpha tin generates “striated needles”, also known as “whiskers”, and this imparts a poor mechanical strength if used for die attach.  
         [0058]     In order to prevent beta Sn from transforming into alpha Sn, Sb is added to restructure the solder lattice binding. A uniformly deformed tetragonal structure with tetra-rhombohedral lattice bonding is achieved, having higher mechanical strength to withstand direct shear loading, as shown diagrammatically in  FIG. 2B . However, Sb as a constituent more than 10% by weight decreases the area of solder spread, resulting in high solder viscosity and poor electrical performance. Sb also has a tendency to dross Al or other impurities to be precipitated on top of the spanked solder, forming a thin film with inhibited impurities that prevent good wettability of the soft solder.  
         [0059]     The soft solder of the invention provides high electrical and thermal conductivity to serve as interconnect material for die attach owing to the availability of the element Ag element in the composition. The element Ag is ranked top in electrical conductivity, of the 114 earth elements, having electrical conductivity higher than gold (Au) by 39.4%. Ag by its natural form having impurities of 2%, assists in increasing solder spread, strength of the solder and grittiness in excess of solubility, and serves as a superior conductive element in the solder compositions of the invention. The presence of Ag also enhances intermetallic formation of Ag with Sn to form Ag 3 Sn, which is non-embrittling. At a constituent % more than 12%, however, precipitated Ag can form a sharp dendrites matrix that results in high dissolubility of the solder compositions.  
         [0060]     Cu enhances the integrity of the present invention, with Cu forming another phase of intermetallic layer, the so-called globular Cu—Sn intermetallic phases with oval round shape of Cu 6 Sn 5  and Cu3Sn intermetallic compounds. A small percentage of Cu in compositions according to the inventions enhanced the creep resistance, and compensated for the needle-like Ag—Sn intermetallic or precipitated Ag dendrites so as to withstand higher impact resistance. Cu is known also for its high electrical and thermal conductivity, ranked second after the superior Ag in transmitting current and heat. However, the constituent % of Cu must be limited to below 2% in order to avoid oxide formation during die mounting at elevated temperatures in the range 300° C. to 400° C. A high percentage of Cu can increase the liquidus temperature, making solder more viscous and sluggish. Sluggish preform is to be avoided, as it results in difficulties of controlling bond line thickness.  
         [0061]     A ternary phase diagram of Sn—Ag—Sb as shown in  FIG. 3  is used as being illustrative for the invention, as a quaternary phase diagram of Sn—Ag—Sb—Cu shifted temperature range (more than 400° C.) with limited information to be secured. Projection from the phase diagram of Sn—Ag—Sb shown in  FIG. 3  delineated the composition area for solder compositions according to the invention. As the compositional percentage of Cu is small in the compositions according to the invention, Cu is treated as an additive, to refine the ternary phase structure, and particularly to increase the solder impact resistance to creep. Additives added in small percentage are often not modeled in phase diagrams.  
         [0062]     Information obtained from phase diagram is limited: to lattice face structure, intermetallic phases (if applicable), liquidus projection and eutectic lines. Additional research, beyond what may be seen from the phase diagram, is required to demonstrate the properties of the formulation, and especially the material properties.  
         [0063]     The compositions of the present invention resides on a 300° C. to 400° C. liquidus zone in a beta Sn—Ag rich region, projected to have appropriate enthalpy of heat fusion with minimum Gibbs energy of activation for alloying. Solders fabricated according to the invention are further characterized, for example by scanning electron microscopy, for determination of solder intermetallic formation.  
         [0064]      FIG. 3  shows at  30  a compsition having lquidus projection from 300° C. to 400° C. possessing suitable alloy material characteristics for soft solder fabrication.  
         [0065]     Differential Scanning Calorimetric (DSC) analysis is conducted to arrest phase diagram prediction variabilities. A specimen crafted from the ingot of a solder fabricated according to the invention is placed in the analysis chamber under a flowing N 2  atmosphere, with a heating/cooling rate of 5° C./minute being applied.  FIG. 4  shows a DSC thermogram of a composition according to the invention, having a specific composition as shown in the following Table III:  
                                         TABLE III                                   Element   Constituent %                                        Tin (Sn)   79.4%           Silver (Ag)   12.0%           Antimony (Sb)   8.5%           Copper (Cu)   0.1%                      
 
         [0066]     This DSC thermogram registered a solidus point at 233.2° C. (onset of phase transition point) and a liquidus point at 257.3° C. (peak of the activation curve). The registered liquidus point approximates to the peak reflow temperature of the well known 260° C. for lead free SMT semiconductor packages.  
         [0067]     Other temperature characteristics of a composition of the invention are summarized in the table IV below.  
                                                                                                                         TABLE IV                                       Element Constituent % (Min &amp; Max)   Solidus   Liquidus                Tin   Silver   Antimony   Copper   Point   Point            No.   (Sn)   (Ag)   (Sb)   (Cu)   (° C.)   (° C.)                    1   76   12   10   2   249   263       2   83.9   8   8   0.1   231   252                  
 
         [0068]     Table IV shows minimum and maximum composition ranges for the elements of the compositions of the invention, as characterized by DSC analysis.  
         [0069]     A microstructural (micoromorphological) analysis can be performed to further characterize compositions according to the invention, showing morphology and structure of intermetallic phases.  
         [0070]     For example,  FIG. 5  is a photomicrograph showing the surface morphology of a spanked soft solder according to the prior art (Alloy J).  FIG. 6  shows the surface morphology of a spanked soft solder according to the invention. As the photomicrographs show, a more refined morphology is achieved with at least two intermetallic phases (Ag—Sn and Cu—Sn) according to the invention; that is, a more refined Ag—Sn and Cu—Sn intermetallic matrix can be achieved, with less sharp precipitated Ag dendrites, in the solder compositions of the invention. Fewer protruding sharp dendrites in the soldr compositions according to the invention result in less induced planar stress.  
         [0071]     FIGS.  7  (prior art) and  8  (according to the invention) diagrammatically further illustrate these morphological improvements provided by the invention.  
         [0072]     To verify the intermetallic phases and the grittiness of the present invention to Ni-plated or Cu lead frame, a cross section optical micrograph of a die mounted on the present invention is acquired after encapsulation, as shown for example in  FIG. 9 . A needle-like Ag—Sn intermetallic is present, as well as a globular Cu—Sn intermetallic; here, a SnAg12Sb8Cu0.1% alloy was employed for the solder. Magnification of the micrographs (20×), shown in  FIGS. 10   a  and  10 B, shows that deep diffusion occurred during die mount, with good intermetallic formation between die backmetal and the solder of the invention, and from the solder of the invention to the Ni-plated lead frame surface. The needle-like Ag—Sn intermetallic interface is more obvious compared to the globular Cu—Sn intermetallic interface as the later constituent % is less.  FIG. 10A  is taken from a SnAg12Sb8Cu0.1% specimen formed at a die mount temperature of 360° C.; and  FIG. 10B  is taken from a SnAg12Sb8Cu0.1% specimen formed at a die mount temperature of 380° C. No significant differences are observed between the two temperatures for intermetallic formation.  
         [0073]     Wettability of colder compositions according to the invention is verified by determining the wetting angle of a real specimen preformed onto a Ni-plated Cu or bare Cu lead frame, using a modified sessile drop test method performed under forming gas (90% N 2  10% H 2 ) flow rate of 5.5 kg/m 3  in a close-loop heat chamber. In these tests, small spheres of solder compositions of the invention were placed on a Ni-plated Cu or on a bared Cu lead frame die paddle, and heated to a temperature up to 380° C. Calculated wetting angle on Ni-plated lead frame shows lower wetting angle (average 36°) compared to bared Cu lead frame (average 38°) from a sample size of 20 prefroms. It is acknowledged that the surface tension between the present invention to the bared copper lead frame is higher than the Ni-plated lead frame.  FIGS. 11A and 11B  show wetting angles obtained by measurement from real specimens, micrographed by scanning electron microscoy (SEM). Maximum allowable contact angle is 45° in order to achieve good wettability.  
         [0074]     A further demosntration of the wettability of solder compositions according to the invention is provided by a destructive die punch test. Using a preset force of 35 N exerted from 45°, die mounted on the present invention were punched directly. A minimum 50% of the SiO 2  residues could be expected to adhere to the solders of the the invention after die punch.  FIGS. 12A and 12   b  present two images of the punched area. These Figures show images of two die mounted on a solder composition according to an example of the invention (SnAg12Sb8.5Cu0.1% alloy). In these tests, more than 70% of the die residues remained intact after the destructive test, reckoning strong adhesion and good wettability of the present invention.  
         [0075]     Mechanical properties of a die attach solder must also conform to specifications. The mechanical properties of solders according to the invention were determined by conventional load and pull tests, conducted using an Instron Universal Testing Machine in accordance with standard ASTM testing procedures. Frabricated solders according to an example of the invention (SnAg12Sb8.5Cu0.1% alloy) in solder wire form were subjected to a biaxial load frame. Compositions within the range of the invention were tested for ultimate tensile strength (UTS), percentage of area reduction, maximum stress to rupture and maximum strain to rupture.  
         [0076]     The ultimate tensile strength means the maximum intensity of mechanical strength achievable prior to unhomogeneous deformation of the tested specimen towards failure. The percentage of area reduction occurs upon unhomogeneous deformation displayed by tested specimen necking phenomenon. The maximum stress to rupture means stress that concentrated at a certain point of the test specimen (necking zone) causes rupture thereof. The maximum strain to rupture means strain that concentrated at a certain point of the plastic deformed test specimen (necking zone) causes rupture therof. Maximum strain to rupture is equal to the elongation performance of the test specimen which justifies the ductility of the test specimen.  
         [0077]     Solder compositions of the invention, particularly a tested SnAg12Sb8.5Cu0.1% alloy, shows high mechanical strength with UTS achieved at 80.43 MPa. Surprisingly, the ductility of the specific tested compositions is relatively high, registered at 56.96%. Maximum stress to rupture for this example of the invention occurred at 76.91 MPa. Table V, below, sets out the mechanical properties of the specific examle of the invention;  FIG. 13  shows the stress vs strain diagram for the example according to the invention (SnAg12Sb8.5Cu0.1% alloy).  
                                                                                                                               TABLE V                           Mechanical Properties of                Sn-Ag-Sb-Cu Alloy   Ultimate       Rupture            Elements by Weight %   Tensile   Max Stress   Max Strain   Area            Tin   Silver   Antimony   Copper   Strength   to Rupture   to Rupture   Reduction       (Sn)   (Ag)   (Sb)   (Cu)   UTS (MPa)   (MPa)   (%)   (%)                    79.40   12.00   8.50   0.10   80.43   76.91   56.96   83.79                  
 
         [0078]     The thermal conductivity of the die attach solder is also, a useful perfomance criterion. Thermal conductivity of a solder composition according to the invention was measured on 1 mm thick discs of the composition using a laser flash method with a Holometrix Model Microflash instrument. The breakdown of the thermal conductivity of the elements in a particular example of a composition of the invention (SnAg12Sb8.5Cu0.1%) alloy thermal conductivity are listed in Table VI.  
                                                   TABLE VI                                       Thermal Conductivity                Element/Alloy   (W/m K)                            Tin (Sn)   66.80           Silver (Ag)   428.91           Antimony (Sb)   24.41           Copper (Cu)   400.79           SnAg12Sb8.5Cu0.1%   58.91                      
 
         [0079]     The thermal expansion coefficient of the die attach solder is also a useful criterion. The coefficient of thermal expansion (CTE) of a solder composition according to the invention (SnAg12Sb8.5Cu0.1%) was measured by heating a specimen crafted from an ingot to 200° C. in a close-loop chamber. Only an alpha one (α1) reading is taken prior to the speciment reaching its solidus point.  FIG. 14  shows the CTE profile of the specific composition, with CTE registered at 23.89×10 −6 K −1  at 200° C., which is lower than the conventional Pb—Sn—Ag soft solder by 17.6% on average.  
         [0080]     The electrical conductivity of the die attach solder is aso a useful criterion. Electrical conductivity is measured through the material/alloy resistivity (1 siemens=1×10 −6  W −1  m −1 ). The electrical conductivity of the elements used in fabricating a solder according to one embodiment of the invention (SnAg12Sb8.5Cu0.1%), and of the alloy itself, are listed in Table VII.  
                                                   TABLE VII                                       Electrical Conductivity                Element/Alloy   (× 10 −6  W −1  m −1 )                            Tin (Sn)   0.0917           Silver (Ag)   0.6310           Antimony (Sb)   0.0288           Copper (Cu)   0.5961           SnAg12Sb8.5Cu0.1%   0.5932                      
 
         [0081]     Processing buyoff performance is also a useful cirterion. Processibility buyoff performance of the solder compositions according to the invention assessed at die attach level show uniform solder spread control, minimum solder voids and no surface oxidation on solder preform, spank and die mount stations at the die attach machine.  FIG. 15  is a series of images showing uniform and well controlled solder spread of a solder composition according to the invention on Ni-plated lead frame (SnAg12Sb8.5Cu0.1% alloy at 380° C.).  FIG. 16  is a series of images as in  FIG. 15 , with die mounted on the die attach solder (SnAg12Sb8.5Cu0.1% alloy with die mounted at 380° C.).  FIG. 17  is a series of X-ray images for detection of voids in the solder (SnAg12Sb8.5Cu0.1% alloy at 380° C.). The average solder void is 3-6%, which is below the industry benchmark control limit of maximum 15% for soft solder applications.  
         [0082]     Bond line thickness is also a useful criterion. Bond line thickness of die attach bonds according to the invention was measured by cross-sectioning the encapsulated specimen to check for solder height beneath the die. A constant reading of 2.9 mils was achieved using a solder accordng to the invention invention with optimized die mounting parameters.  FIG. 18  is a scanning electron microscope image of a die mounted on a substrate using a die attach solder according to the invention (See also  FIG. 1A ); this shows a uniformly spanked solder, with no observable die tilt; and  FIG. 19  is a sectional microspopic image showing the bond line thickness measurements using a die attach solder according to the invention.  
         [0083]     Die attach solders according to the invention can provide significant advantages over conventional sie attach solders. For example, the mechanical strength of solders according to the invention can be as much as 128% higher than that of the Pb—Sn—Ag soft solder which is widely used currently for die attach application in power packages. The die attach solders according to the invention can achieve high ultimate tensile strengths (for example, the SnAg12Sb8.5Cu0.1% alloy according to the invention, achieves an ultimate tensile strength at 80.43 MPa). And, for example, die attach solders according to the invention have lower planar stress with refined morphology to resist grain growth (needle-like Ag—Sn intermetallic structure compensated by globular Cu—Sn intermetallic structure). The maximum strain to rupture (ductility) of solder compositions according to the invention is (at least in some embodiments) 69.5% higher than that of a conventional Pb—Sn—Ag soft solder. And, for example, the thermal conductivity of the SnAg12Sb8.5Cu0.1% alloy according to the invention was recorded at 58.91 W/mK, which is 33.9% higher than the conventional Pb—Sn—Ag soft solder, and the electrical conductivity of the SnAg12Sb8.5Cu0.1% alloy according to the invention was measured at 69.4% higher than the conventional Pb—Sn—Ag soft solder. And, for example, at least two phases of intermetallic structures exist in solder bonds according to the present invention, creating deep diffusion to the die backmetal as well as strong bonding to the Ni-plated or bared Cu lead frame. The solders according to the invention have superior wetting characteristics, the SnAg12Sb8.5Cu0.1% alloy recording a wetting angle at 36° to 38°, so that the solder spreads well and uniformly on Ni-plated and bared Cu lead frame. The SnAg12Sb8.5Cu0.1% alloy of the invention meets all processability buyoff criteria with minimum solder splash issue, uniform bond line thickness, minimum solder voids and no surface oxidation. The solders according to the invention have suitable coefficients of thermal expansion; the CTE value of the SnAg12Sb8.5Cu0.1% alloy of the invention registered at 23.89×10 −6 K −1  at 200° C., which is lower than that of the conventional Pb—Sn—Ag soft solder by 17.6% on average.  
         [0084]     Other embodiments are within the following claims.