Patent Publication Number: US-2006013722-A1

Title: Lead-free solder pastes with increased reliability

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to solder pastes, particularly to lead-free solder pastes based on SAC.  
      Solder pastes, for example SnAg4Cu0.5, tend toward void formation and consequently to reduced stability relative to lead-containing solder and thus reduced reliability of the solder joints.  
      According to U.S. Pat. No. 5,527,628, in addition to an eutectic composition Sn93.6Ag4.7Cu1.7, other non-eutectic compositions of this three material tin-silver-copper (SAC) system or compositions of two material tin-silver and tin-copper systems are used in an alloy in order to generate a melting range for the soft solder. Such technical solder compositions, particularly SAC solders, exhibit intermetallic phases (IMP). In general IMP with stoichiometric compositions (e.g., Cu3Sn, Cu6Sn5) have low crystal symmetry and are usually very brittle. Compounds that are stable over a large range (e.g., Ag3Sn) in general do have a higher symmetry and are usually also more ductile, but nevertheless can reduce the stability of a solder structure.  
      Thus, IMP are undesirable in soft solders that have several compositions. These so-called critical intermetallic phases are therefore undesirable, because they are preserved during the soldering process, grow there, and thus impart brittleness and break-off points in the final solder joint, which negatively affect its mechanical stability. Intermetallic phases are also formed during soldering at the contact surface, but other intermetallic phases, which are present anyway, are imported by the solder. In turn, these phases increase during the soldering process and then continue to enlarge over time and therefore can lead to intercrystalline fracture or grain deterioration.  
     BRIEF SUMMARY OF THE INVENTION  
      The problem of the present invention is to further improve the quality of solder pastes and in particular to reduce the tendency toward void formation.  
      To solve this problem, the introduction of undesired intermetallic phases (IMP) is reduced. For this purpose, solder pastes are prepared comprising a metal powder mixture that generates a melting range of at least about 5° Kelvin and preferably less than about 30° Kelvin. A suitable mixture here is a metal powder mixture, for which the melting points of the metal powder components are separated from each other more than about 5° Kelvin, particularly more than about 10° Kelvin, preferably more than about 15° K, and optionally less than about 30° Kelvin, particularly up to about 25° Kelvin. However, metal powder mixtures are also possible, for which the melting point spread can reach about 100° Kelvin and more, if the higher melting component dissolves in the melt below the melting point, so that the melting range can be held under about 30° Kelvin.  
      With the solder pastes according to the invention, undesired intermetallic phases can be held nearly arbitrarily low. In addition, the void formation according to the void test can be reduced considerably. In particular, the void formation of tin, silver, copper (SAC) alloys can be reduced by up to an order of magnitude. Consequently, these pastes enable more stable and therefore, in turn, more reliable solder joints, which are of great interest for electronics assembly. Particularly important applications result for printed circuit board construction and wafer bumping. The final alloy of the solder joint is first realized at least partially during soldering. Therefore, with the powder mixture, fewer critical intermetallic phases (IMP) are introduced during the soldering process. If IMP were to be fed to the soldering process with a final alloy (target alloy) of 100%, then IMP would be reduced by the powder mixture according to the portion that first forms during the soldering as the final alloy.  
      In preferred embodiments:  
      the powder mixture is meltable in a temperature range, which exceeds the temperature of an eutectic of this system by less than about 30° Kelvin;  
      the sum of the metal powders corresponds to a ternary, quaternary, or higher metal system;  
      the metal powders differ in terms of their particle size distribution, wherein it is particularly preferred that the finest powder exhibit the highest melting point or have the lowest mass fraction;  
      the lead-free solder paste has powders of differing metal composition, wherein at least two metal powders are produced in terms of one alloy component apart from the main component, such that one powder has a higher content of this component and the other metal powder contains less to none of this component;  
      the solder paste is a tin-silver-copper solder paste;  
      the main component is an SAC alloy, particularly with an alloy percentage of 2 to 5% silver and 0.2 to 1% copper, preferably 3 to 4% silver and 0.3 to 0.8% copper; and/or  
      the SAC powder mixture has a total of 2 to 5 wt. % silver and 0.2 to 1 wt. % copper, preferably 3 to 4 wt. % silver and 0.4 to 0.8 wt. % copper.  
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:  
       FIGS. 1   a  and  1   b  are micrographs of SnAg4Cu0.5 showing the IMP in the alloy.  
       FIG. 2   a  is a micrograph of Sn99Ag1 wherein hardly any IMP can be detected.  
       FIG. 2   b  is a micrograph of Sn99Cu1 wherein hardly any IMP can be detected.  
       FIG. 3  is two micrographs showing the low IMP of Sn97Cu3.  
       FIG. 4  is two micrographs of Sn95Ag5 wherein no IMP can be detected.  
       FIG. 5  is a graph showing the melting range of the powder mixture of Example 1.  
       FIG. 6  is a graph showing the melting point of the alloy produced from the powder mixture of Example 1.  
       FIG. 7   a  is a bar graph comparing Examples 1 and 2 with a standard, wherein the left side shows the portion of small voids and the right side shows the portion of large voids.  
       FIG. 7   b  is a bar graph comparing a quality analysis of Examples 1 and 2 with a standard. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      For manufacturing pastes according to the invention, at least two metal powders with different melting points are mixed with a fluxing agent. For this purpose, the known manufacturing methods are suitable, e.g., for preparing, stirring, kneading, and homogenizing pastes.  
      According to the invention, a soldering process can be designed, in which a metal powder is subjected to a re-melting process to form a solder joint, with reduction of undesired IMP. The decisive factor for this process is the coating with a paste, which has powders with different alloy compositions, optionally also pure metal, which, in turn, have in general a different melting range or melting point. The composition of the solder joint is here predetermined by the mixture ratios of the metal powders, but first realized during the soldering process, i.e., upon melting of the paste. Here, IMP can be essentially controlled in terms of quality and quantity.  
      Furthermore, it is possible to combine technical standard alloys as metal powder mixtures, such that the composition of the solder paste corresponds to an individual target alloy.  
      Preferably, the solder pastes of the present invention contain as the main component one of the elements of tin, copper, silver, gold, or platinum. The other components are selected so that the powder composition generates a melting range in the vicinity of an eutectic. The metal powders mixed in the solder paste have, besides different compositions, preferably also different particle sizes. Here, it has proven effective when the powder with the lowest melting composition, for example of an eutectic composition, is the main component of the metallic powder and preferably has the largest powder particles on average. The particle sizes should differ at least by the factor 0.7, preferably up to an order of magnitude, wherein, however, even two orders of magnitude can be advantageous.  
      This is particularly the case with a relatively high melting alloy and particularly when the high melting alloy must be dissolved by the melt. Therefore, the melting range need not absolutely agree with the melting range of a phase diagram. Essential for the melting range is that the metal powders form a melt in this range and all powders melt, at least to a significant degree, within a melting range of less than about 30° Kelvin, particularly up to about 25° Kelvin, or dissolve in the melt. In terms of the melting range, both the lower and the upper melting range limit can differ from the respective lower or upper value of another alloy or the melting point of a pure metal. The individual metal powders can be pure metals or alloys, but must be in the condition, as a powder mixture, to form an alloy, preferably a ternary or higher alloy, within a melting range of less than about 30° Kelvin, particularly a maximum of about 25° Kelvin. Thus, the powders must be in condition to form a melt, which contains at least two elements, preferably at least three.  
      With the lead-free pastes, the new legal regulations in many countries should be taken into consideration. As a limit for the lead content, 1% is provided (Elektronik Praxis Marktreport Bleifrei [Electronic Practice Market Report Lead-free]—June 2004, page 6).  
      With the powder compositions the fraction of critical intermetallic phases (IMP) can be reduced by more than an order of magnitude. This is associated with significantly increased stability of the solder joint.  
     EXAMPLES  
      Solder pastes with the target alloys SnAg4Cu0.5 and Sn96.5Ag3Cu0.5 were manufactured according to a conventional manufacturing process for solder pastes, with the difference that here various alloys are mixed into one powder mixture. The powders correspond to powder grain size 3 according to EN 61190-1-2:2002. Clearly recognizable IMP were detected only in the SnAg4Cu0.5 powder and the Sn96.5Ag3Cu0.5 comparison powder.  
      SnAg4Cu0.5 (melting point 217° C.)  
      Sn96Ag4 (melting point 222° C.)  
      Sn99Cu1 (melting point approximately 227° C.)  
      Sn95Ag5 (melting range 221 to 240° C.)  
      Sn99.15Cu0.85 (melting point approximately 229° C.)  
      Target alloy SnAg4Cu0.5  
     Example 1  
      80% SnAg4Cu0.5 and 16% Sn95Ag5 and 3% Sn97Cu3 and 1% Sn99Cu1  
     Example 2  
      60% SnAg4Cu0.5 and 32% Sn95Ag5 and 6% Sn97Cu3 and 2% Sn99Cu1  
     Example 3  
      50% SnAg4Cu0.5 and 15% Sn100 and 10% Sn80Cu20 and 25% Sn99Cu1  
      Target alloy Sn96.5Ag3Cu0.5  
     Example 4  
      50 wt. % SnAg4Cu0.5, 25 wt. % Sn96Ag4, and 25 wt. % Sn99Cu1  
     Example 5  
      50 wt. % SnAg4Cu0.5, 20 wt. % Sn95Ag5, and 30 wt. % Sn99.15Cu0.85  
     Example 6  
      50 wt. % Sn95.5Ag4Cu0.5 and 20 wt. % Sn95Ag5 and 30 wt. % Sn99.15Cu0.85  
      The solder pastes are each soldered to a copper joint on a printed circuit board and the solder joints are examined.  
      The solder joints produced from the powders according to the invention contain significantly fewer IMP than those from the alloys SnAg4Cu0.5 (Comparison example 1) and Sn96.5Ag3Cu0.5 (Comparison example 2). The mixture with 80% SnAg4Cu0.5 lay approximately between the comparison examples and the other examples according to the invention, which exhibited only approximately half as many IMP as the comparison examples.  
      The mechanical loading capacity in terms of sensitivity to shock and vibration is improved in the solder joints manufactured according to the invention, as well as the resistance to temperature fluctuations.  
      The melting range of the powder of Example 1 was determined according to  FIG. 5  by differential thermal analysis (DTA). The subsequent DTA ( FIG. 6 ) shows that the alloy produced from the powder mixture is nearly eutectic in view of the sharp melting point.  
      According to the void test described in the article “Avoiding the Solder Void,” by Richard Lathrop, Heraeus Circuit Materials Division, Philadelphia, Pa., the pastes according to the invention are improved relative to the standard alloys. In  FIG. 7   a,  the measured voids are shown. According to the invention, the large voids could be considerably reduced. From these results, a quality analysis according to  FIG. 7   b  was produced, in which the compositions according to the invention from Examples 1 and 2 with 68 and 64 points achieve a clear increase in quality relative to the prior art (“Standard”)(Comparison example 1).  
      It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.