Patent Publication Number: US-2006011482-A1

Title: Electrocodeposition of lead free tin alloys

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      The present application claims the benefit of provisional patent application Ser. No. 60/587,350 filed Jul. 13, 2004, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD  
      The technology relates to tin alloys, and, more particularly, to electrocodeposition of lead free tin alloys.  
     BACKGROUND INFORMATION  
      Tin-lead (Sn—Pb) solder is used in connections for electronic assemblies. Applications of tin-lead coatings include protective solderable finishes as well as masking of printed boards, soldering of leads, and C4 (controlled collapse circuit connection) or “flip chip” packaging using solder bumps. Sn—Pb solders are electrodeposited in some electronics packaging applications because electroplating provides superior thickness control in comparison to application of the melt, and it is easily scaled down to small pitch sizes. Solder bump technology can allow a bare integrated circuit to be mounted on a next level package, without requiring encapsulation of the integrated Circuit. Moreover, solder bump technology can utilize the entire area of a microelectronic substrate for connection rather than only using the periphery thereof (e.g., using wire bonding).  
      One application area of tin-lead plating is its use in the printed circuit board (“pcb”) industry and for coating electrical and electronic components. In pcb manufacture, tin-lead deposition is a major process stage in the subtractive method of patterning. For all components, an assured solderability is essential  
      One of the processes in pcb manufacture is the metal resist stage. After electrodeposition of the pattern from a copper bath, the tin-lead resist is deposited to act as a copper etch resist. A major application area for tin and tin-lead deposition is the coating of connectors on electronic components. Such coatings are a pre-requisite for reliable soldering in making of interconnections on pcbs.  
      Solders based on tin-lead alloys are used extensively in semiconductor packaging. Electrodeposition is a favored solder-application process in flip-chip packaging because it provides superior thickness control and is easily scaled down to small pitch sizes. However, industrial processing and consumer use of lead-bearing products is a recognized public-health hazard, and the international trend toward regulation is driving demand for lead-free alternatives such as the Sn/Ag (Tm=221° C.) and Sn/Ag/Cu (Tm=217° C.) eutectics.  
      Electrocodeposition, in which metallic or dielectric particles suspended in the plating bath are incorporated into a metal electrodeposit, has been used to produce composite materials with tailored mechanical properties. Chromium particles have been codeposited in a Ni—Fe matrix to produce stainless-steel coatings, also, a colloid has been formed by adding a silver salt to a tin plating solution resulting in cementation of silver. Thus, particle codeposition followed by melting to form an alloy has been described.  
      Lead-bearing solder presents a number of problems. First, lead contains an isotope alpha emitter. The alpha particle can cause significant problems. Lead-bearing solder also represents a disposal problem in finished products. It is well known that lead generates an environmental impact indirectly through a chain of process technologies from refining to final application in plating. Because lead represents a well-documented threat to public health, many governments are seeking to restrict its use in consumer products.  
      Eutectic alloys including tin and silver (e.g., Sn/Ag and Sn/Ag/Cu) have been identified as alternatives to Sn/Pb for the applications described above. Two approaches to electrodeposition of Sn/Ag and Sn/Ag/Cu alloys have been identified: alloy deposition and stack deposition followed by reflow.  
      In alloy deposition, ions of the component metals are present in the plating bath. The more noble metal (silver in the case of Sn/Ag eutectic) is present at low concentration so that it plates at the transport-limited rate. In the tin-silver system, the wide separation of reduction potentials requires that the silver ions be complexed. Even with additives, the baths remain more difficult to stabilize and control than pure-tin baths. Because silver is deposited at the transport limited rate, morphological instability leads to undesirable structures over a range of current densities. A second deficiency is that the alloy composition depends on current density (tin deposition rate) and transport conditions (silver deposition rate), both of which may vary over the substrate. This introduces problems with process control and deposit quality. Moreover, silver ion reacts with tin anodes and precipitates silver metal on them. As a result, insoluble anodes are usually required, introducing additional chemical monitoring and control workload.  
      A second approach is to deposit tin and silver in separate steps from single-metal baths. In this process, Sn/Ag alloy is not plated. Rather, individual layers of silver and tin are plated, and then reflowed to form Sn/Ag alloy solder bumps. However, a thorough rinse is required each time the substrate is transferred between baths, so that this approach introduces mechanical and material complications.  
      Accordingly, a need has arisen in the art for a lead-free alloy technology with an improved method of making that is more reliable and reproducible and that reduces waste and rinse-stream volume.  
     SUMMARY OF THE INVENTION  
      In accordance with one aspect of the present invention, a method of electrocodeposition of lead free tin alloys is disclosed. The method includes the following steps: electrocodepositing tin metal and particles of a noble metal to form a composite; and converting the composite to an alloy by heating the composite.  
      Some embodiments of this aspect of the invention include one or more of the following. Where the noble metal is selected from the group consisting of silver, gold, and copper. Where the heating is in a temperature range of from 170° C. and 240° C., and/or, where the heating is in a temperature range of from 200° C. and 240° C. Where the composite volume percentage of the noble metal is that volume percentage where the composite melting temperature is in the range of the eutectic melting point. Where the composite includes between 2.5% and 4.5% volume percentage silver.  
      In accordance with one aspect of the present invention, a method of electrocodeposition of lead free tin alloys is disclosed. The method includes the following steps: electrocodepositing tin metal and silver particles to form a composite; and converting the composite to an alloy by heating the composite.  
      Some embodiments of this aspect of the invention include one or more of the following. Where the heating is in a temperature range of from 200° C. and 240° C. Where the composite includes between 2.5% and 4.5% volume percentage silver.  
      In accordance with another aspect of the present invention, a method of electrocodeposition of lead free tin alloys onto a conductive substrate. The method includes the following steps: forming a mask on the surface of the substrate; electrocodepositing tin metal and particles of a noble metal to form a composite on the surface; removing the mask; and converting the composite to an alloy by heating the composite.  
      Some embodiments of this aspect of the invention include one or more of the following. Where the noble metal is selected from the group consisting of silver, gold, and copper. Where the heating is in a temperature range of from 170° C. and 240° C., and/or, where the heating is in a temperature range of from 200° C. and 240° C. Where the composite volume percentage of the noble metal is that volume percentage where the composite melting temperature is in the range of eutectic melting point. Where the composite includes between 2.5% and 4.5% volume percentage silver.  
      In accordance with another aspect of the present invention, a method of electrocodeposition of lead free tin alloys onto a conductive substrate. The method includes the following steps: forming a mask on the surface of the substrate; electrocodepositing tin metal and particles of silver to form a composite on the surface; removing the mask; and converting the composite to an alloy by heating the composite.  
      Some embodiments of this aspect of the invention include one or more of the following. Where the heating is in a temperature range of from 200° C. and 240° C. Where the composite includes between 2.5% and 4.5% volume percentage silver.  
      In accordance with another aspect of the present invention, a method of electrocodeposition of lead free tin alloys for protective solderable finishes. The method includes the following steps: electrocodepositing tin metal and particles of a noble metal to form a composite; and converting said composite to an alloy by heating the composite.  
      In accordance with another aspect of the present invention, a method of electrocodeposition of lead free tin alloys for masking printed boards. The method includes the following steps: electrocodepositing tin metal and particles of a noble metal to form a composite; and converting said composite to an alloy by heating the composite.  
      In accordance with another aspect of the present invention, a method of electrocodeposition of lead free tin alloys for controlled collapse circuit connection packaging using solder bumps. The method includes the following steps: electrocodepositing tin metal and particles of a noble metal to form a composite; and converting said composite to an alloy by heating the composite.  
      These aspects of the invention are not meant to be exclusive and other features, aspects, and advantages of the present invention will be readily apparent to those of ordinary skill in the art when read in conjunction with the following description, appended claims and accompanying drawings, all of which illustrate the principles of the technology, by way of example only. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other features and advantages of the present invention will be better understood by reading the following detailed description, taken together with the drawings wherein:  
       FIG. 1  includes  FIGS. 1A-1C  and show an illustration of the method of making a solder bump according to one embodiment of the present invention;  
       FIG. 2  is an image of Sn/Ag composite bumps formed on a patterned wafer according to one embodiment of the present invention; and  
       FIG. 3  is a graphical representation showing DSC traces representing the melting temperatures of various composites. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The invention includes a method of electrocodeposition of lead free tin alloys. The method includes the steps of electrocodepositing tin metal and particles of a noble metal to form a composite, followed by converting the composite to an alloy by heating the composite. In the preferred embodiment, the electrocodepositing of tin metal and the particles of the noble metal is done simultaneously. The resulting composite has a eutectic melting point.  
      The heating, also termed reflow, step transitions the tin metal and noble metal particle composite to an alloy. Although the noble metal particles diffuse rapidly into the tin metal during the solid state, the diffusion is complete by the time the tin metal and noble metal particle composite reaches eutectic.  
      The present invention has many applications. One of those is the formation of lead free solder bumps, made from a Sn/Ag alloy. The low melting temperature of Sn/Ag alloy is advantageous to many industries and applications. The present invention can be used as a substitute for Sn/Pb solder in any application. The Sn/Ag solder has many advantages, even over Sn/Au and Sn/Cu solder. First, the Sn/Ag alloy has chemical characteristics that are simple and stable. Unlike Sn/Au alloys, the noble ion does not have to be stabilized. Second, in a bath of Sn and Ag, Ag can easily be filtered out of the Sn, thereby allowing for reclamation of Ag in particle form. This is both economically and environmentally advantageous.  
      The invention can be used in countless applications, including, but not limited to: making protective solderable finishes, masking printed boards, soldering leads, and controlled collapse circuit connection (“C4”) “flip chip” packaging using solder bumps. The term “printed boards” is a general term meaning any printed writing board, completely processed printed circuit or printed wiring configurations. This term includes single, double-sided, and multi-layer boards, both rigid and flexible. Included in the general term “printed boards” are printed circuits and printed wiring boards.  
      This invention is not to be limited only to the examples and applications described herein. The electrocodeposition of Sn/Ag alloy has many applications that will be understood by those of ordinary skill in the art.  
      The size of the metal particles used to make the composite can be any size; the size can be consistent or variable. The metal particle can be any noble metal including, but are not limited to, silver, gold and copper. Furthermore, the composite can include a plurality of novel metals (e.g., Ag/Cu). In the preferred embodiment, silver, Ag, is used.  
      The percentage volume of noble metal to tin varies, but in all instances, the percentage volume required is equal to the percentage volume needed for the composite to reach eutectic. In the preferred embodiment, the composite includes 2.5% to 4.5% silver and 97.5% to 95.5% tin. In one embodiment, the alloy includes 3.5% silver and 96.5% tin.  
      The heating step includes applying heat so as to heat the composite to a temperature in the range of 170° C. to 240° C. In the preferred embodiment, using silver as a particle metal, the heating range is 200° C. to 240° C.  
      Referring now to  FIG. 1 ,  FIGS. 1A-1C  show an illustration of the successful steps of one embodiment of the present invention where the method is used to form a solder bump ( FIG. 1C, 18 ) on a surface of a conductive substrate ( FIG. 1A, 16 ).  FIG. 1A  shows electrocodeposition of metal tin  12  and particle noble metal, silver  12 .  FIG. 1B  shows the alloy after the silver has diffused through the tin.  FIG. 1C  shows the alloy solder bump  18  after heating and reflow.  FIG. 1C  illustrates that the alloy bump  18  has been formed and the silver has become completely diffused in the tin.  
      Referring again to  FIG. 1A , a mask  14  is formed on a substrate  16 . When making solder bumps, the tin can be deposited within the confines of the mask  14 . The noble metal is occluded in the tin to form a composite, shown in  FIG. 1A . In the preferred embodiment, the noble metal and the tin are codeposited to form the composite. As shown in  FIGS. 1B and 1C , in some embodiments, the composite shown in  FIG. 1A  is a precursor material, which is then converted to an alloy bump ( FIG. 1C, 18 ) by solid-state diffusion (e.g., rapid interstitial diffusion) of the noble metal into the tin followed by heating composite to reflow. In some embodiments, the mask  14  is removed prior to heating the composite so that the composite reflows to form the alloy bump  18 . The mask  14  can be patterned with a photoresist with a thickness ranging from about 2 μm to about 500 μm. In the preferred embodiment, the alloy bump  18  includes a eutectic alloy.  
      In all embodiments, the substrate is conductive. In some embodiments, the substrate is a microelectronic substrate or a semiconductor wafer. The substrate can be a silicon wafer, and/or can include an under bump metallurgy for bump formation. The UBM is deposited prior to plating, and acts as the electrode for electroplating and as the base of the alloy/solder bump. In various embodiments, Sn/Ag solder bumps can be formed on silicon wafers with Ni, NiV or Cu seed layers in a commercial plating machine. The solder bumps can have a pitch in the range of about 10 μm to about 300 μm. In the exemplary embodiment, the pitch is about 200 μm.  
      In one embodiment, lead-free solder bumps can be fabricated using a single operation by electrocodeposition of tin and particle noble metal suspended in a tin-plating solution. The electrocodeposition tin-silver electroplating bath can also be used to form codeposits of tin-silver plated film, and can be used to monitor and control the silver particle distribution in the bath and the silver particle distribution on the substrate, which is described in more detail below.  
      In various embodiments of the method, solder bumps are made onto silicon wafers. In these methods, the plating apparatus is a machine for solder bumping wafers and suspends and circulates fine metal particles, e.g., smaller than about 10 μm in size. In some embodiments, the solder bumps are about 10 μm to about 40 μm in size.  
      For embodiments where the present method is used to make Sn/Ag solder, the solder is formed after reflow with a silver-tin alloy including 2.5% to 4.5% silver and 97.5% to 95.5% tin. In one detailed embodiment, the alloy includes 3.5% silver and 96.5% tin. In one embodiment, the alloy is homogeneous. In various embodiments, the tin is substantially pure. As described above, the technology can also be applied in the formation of Sn/Ag, Sn/Cu, Sn/Au, and Sn/Cu/Ag solder bumps.  
      The composite is prepared by occlusion of metal powder(s) in an electrodeposit matrix during the electrocodeposition process. The composite is heat treated to diffuse the occluded powder(s) throughout the electrodeposit matrix, thereby forming the desired alloy.  
      In this embodiment, an aqueous electrolyte is used and normal electroplating parameters for that electrolyte are maintained. For the metal particle(s) to be occluded during this step, the metal particle(s) is typically relatively inert to the electrolyte. Furthermore, the metal particle can be selected so that it can be dispersed in the electrolyte. Mechanical stirring or other forms of agitation can be used to improve dispersion and to achieve physical contact with the surface being coated.  
      As described above, the composite can be heat treated to diffuse the noble metal particles. A time-temperature combination is typically used so that the alloy formed has a homogeneity comparable to that of the commercial form of the alloy. In some embodiments, a diffusion barrier coating can be used between the composite coating and the substrate to control excessive interdiffusion.  
      In various embodiments suitable temperature ranges for the heat treatment are between about 200° C. and about 320° C. In one detailed embodiment, the composite is heated to about 222° C. prior to reflow. Suitable current densities for the electroplating are between about 10 mA/cm2 and about 800 mA/cm2. In one detailed embodiment, the current density is about 40 mA/cm2.  
      The techniques described above can produce the eutectic alloy consistently over a practical range of bump sizes and pitches. The process variables that can be optimized to produce these ranges include bath composition, current density, mass-transfer coefficient, agitation, particle loading, and particle characteristics. In one embodiment of the technology, bath compositions and process conditions that produce suitable eutectic deposits can be used to select favorable bump sizes and compositions to be formed on substrates.  
     EXAMPLE  
      Sn/Ag composite bumps were formed on a patterned wafer coated with a 50 um thick photoresist with an array of 130 um holes on a 250 um pitch (see  FIG. 2 ). The solution was based on a commercial pure-tin bath, Rohm and Haas Solderon BP Acid, with addition of spherical silver particles (1-3 μm). DSC analysis of deposits formed in a bath without additives showed that the composite melted near the Sn/Ag eutectic melting point of 221° C. (see  FIG. 3 ).  
      The bath volume was 30 mL and the temperature was 24° C. The ethanol and silver particles were mixed to form a slurry, and the slurry was added to the plating solution before the experiment. The purpose of this step is to promote dispersion of the particles. The substrate was a segment cut with a diamond scribe from a demo wafer, Cu blanket on silicon, with a 50 micron thick photo-resist with 130 micron holes on a 250 micron pitch. The segment was attached to a rotating disk electrode (RDE) apparatus and masked to leave a circular disk of 0.4 square centimeters exposed to the solution. The RDE was then placed face down in the solution to a depth of 5 mm and rotated at 1600 rpm. The counter and reference electrodes were pure tin. The applied current density was 60 milliamperes per square centimeter. The film was produced by the same procedure except that the substrate was a polished stainless-steel disk.  
      Referring now to  FIG. 3 , a graph depicting the DSC traces showing the melting temperatures of the various composites is shown. The scan rate was 20° C./min. The first trace  30  is of pure tin deposited without suspended particles on aluminum, which melted at about 234° C. or near the melting point of the pure metal.  
      The second trace  32  is of a composite formed on aluminum, which melted at about 222° C. The third trace  34  is of a composite on copper, which melted at about 219° C. Both composite melting temperatures are close to the eutectic melting temperature of about 221° C. for Sn/Ag. The melting temperature of eutectic Sn/Ag/Cu is about 217° C. The sample deposited on copper may contain some ternary alloy.  
      An electrodeposited composite can provide suitable reflow and wetting behavior for use as a solder. The diffusivity of silver, gold, and copper in tin between ambient temperatures and the tin melting point can be orders of magnitude higher than the tin self-diffusion rate because the noble metals are transported by an interstitial mechanism. As the composite is heated in reflow, interdiffusion of the metals produces alloy zones within the tin matrix. The alloy zones melt first, accelerating interdiffusion. If diffusion is sufficiently fast, the entire composite can melt near the melting temperature of the average composition and produce a bump of uniform composition. In one embodiment, the bump adopts a truncated spheroid shape on the substrate. The factors affecting melt and wetting behavior are particle size and dispersion within the composite and incorporation of other bath components in the composite.  
      While the principles of the invention have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.