Abstract:
It has been found that composition containing copper, tin, and silver prevents tin whisker formation on an electronic structure while allowing solders to wet such structures during soldering processes. It has further been found that conventional techniques, such as electrolytic plating, electroless plating, wet dipping and vapor deposition, for forming such materials have undesirable limitations and/or characteristics. However, by forming a Ag/Sn precursor on a copper containing electronic structure and inducing a self-limiting reaction between the precursor and the copper of the structure, the advantageous Ag/Sn/Cu material is formed without the undesirable limitations and characteristics associated with conventional techniques.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to tin containing structures for electronic and electrical devices and in particular to tin containing structures that avoid formation of tin whiskers.  
       BACKGROUND OF THE INVENTION  
       [0002]     For electrical and electronic devices, typically electrical connections are made through the expedient of a lead frame or other copper containing structures. For example, in the fabrication of integrated circuits, a silicon body having electronic circuitry is connected to a metal, e.g. copper, lead frame such as shown in  FIG. 1  at  2  with the chip positioned at  3  and connections between bonding pads on the chip and the lead frame shown at  4 . After the chip is bonded to the lead frame, the chip is encapsulated typically in a polymer composition. The strip,  6 , in  FIG. 1  is removed from the lead frame in a process generally denominated trimming. Thus, after trimming, the individual leads,  7 , are no longer mechanically connected on one end. The leads are then bent to facilitate connection to other electronic or electrical bodies such as a circuit board. Generally the bending involves the formation of at least one curve such as shown in  FIG. 2  for typical interconnection of an integrated circuit with a circuit board. (For some packages the copper lead frame leads terminate as land pads protruding from the molded package body. For this type of device no forming operation of the interconnect leads is required.) For many applications, the interconnection between the lead frame and another electronic or electrical entity is formed using a lead/tin solder alloy. However, such alloy does not readily wet copper. Therefore, the copper leads are typically coated, e.g. plated, with a layer of tin to enhance wetting of the leads by solders before trimming. Although the tin layer does in fact facilitate wetting of the copper leads, other problems are generated. In particular, there is a tendency to form long needle-like tin structures generally denominated whiskers. These structures are usually from 20 to 100 μm in length and can grow to as long as 1 mm or more. (The whiskers are most often single crystal structures, but multi-crystal whiskers are also possible.) The exact interaction between the copper and tin producing such crystallites is not precisely known. It has been postulated that copper and tin form an intermetallic material in a manner that leads to regions of excess stress (essentially more tin). These regions, it is contemplated, are under compressive stress, particularly for those packages having lead frame interconnections with curved sections after bending. The combination of excess tin and compressive stress enhances the tendency to form whisker structures. The occurrence of, and thus the problem associated with, whiskers are exacerbated because they also form when Sn is plated on brass, alloy  42  and other commonly used electronic metallization structures. Furthermore, exposure of these types of metallurgical structures to moist environments or other environmental stresses tends to exacerbate the problem of whisker formation in ways that are yet to be completely understood.  
         [0003]     Decades ago it was found that if elemental lead (Pb) is added to the tin coating, whisker formation is essentially eliminated. Thus the issue of whiskers has not imposed reliability risks on electronic devices with Pb-doped tin plated structures such as leads. However, impending legislation particularly in European countries prohibits the use of lead for many applications including some involving electronic and electrical devices. Thus, there has been a substantial impetus to remove lead from the tin coating. Such removal has the potential for renewing whisker formation as an issue to be considered.  
         [0004]     For similar reasons, use of lead-free solder is also being promoted. Exemplary of such solders is an alloy of tin (Sn), silver (Ag), and copper (Cu),—generally denominated SAC. Such solders melt generally at temperatures above 217° C. to 230° C. depending on the exact composition. For process control reasons, the temperatures used for board assembly are typically approximately in the range 240° C. to 260° C. Such temperatures exceed the melting point of tin (approximately 232° C.). Thus, concerns about tin whiskers were initially thought to be mitigated because stress is substantially reduced after the melting of the tin is induced by soldering during the board assembly process. Nevertheless despite expectations, difficulties associated with whiskers have persisted.  
         [0005]     A variety of approaches have been attempted to reduce the whisker problem by introducing additional layers to the electronic structures. In one such configuration, a tin layer is plated onto a copper alloy structure and then given a post plating anneal (above about 150° C. but below the melting point of tin) for at least 1 hour. Unfortunately whiskers still tend to develop on such structures (See for example “NEMI Sn Whisker Project Status,” M. Williams presented at 2 nd  Whisker Joint Meeting 2004, Japan; “Tin Whisker Mitigation Application of Post Mold Nickel Underplate on Copper Based Lead Frames and Effects on Board Assembly Reflow,” J. W. Osenbach et.al. SMTA Conference Proceedings, 2004, “Lead Free Packaging and Sn Whiskers,” J. W. Osenbach et.al. Proceeding ECTC, 2004, and the NEMI web site for Tin Whisker Work Group www.inemi.org/cms/projects/ese/tin whisker_activities.html). Alternatively, some tin-whisker mitigation has been reported with in a structure having a Sn layer plated on a continuous Ag layer that is, in turn, deposited on a copper structure. Presumably the Ag layer is a continuous barrier layer of metal between the Cu alloy and the final Sn and thus inhibits Cu/Sn intermetallic reaction induced Sn whiskers from occurring. In spite of these added material layers, tin whiskers are still possible. (See “Assessment of Pb-Free Finishes for Lead Frame Packaging,” F. W. Wulfert and N. D. Vo, 9 th  Electronic Circuits World Convention and the NEMI Sn Whisker Test Group, and the NEMI web site for Tin Whisker Work Group www.nemi.org/projects/ese/If hottopics.html.) Given the consequences of whisker formation such as inadvertent shorting of lead frames or blocking of optical paths for electro-optic devices, Sn-whiskers continue to be a potential reliability problem requiring further investigation.  
       SUMMARY OF THE INVENTION  
       [0006]     It has been found that use of a particular material system, tin/silver/copper of specific composition range, overlying an electronic structure effectively eliminates whisker formation as a concern. However, it has proven to be difficult to form a suitable Sn/Ag/Cu Region (SACR) on an electronic structure. In particular electroplating a Sn/Ag/Cu ternary alloy system onto an electronic structure such as a lead frame is impractical in a controllable manufacturing process. Electroplating a controlled composition relies on differences in electrochemical potential among species in an electrolyte plating solution. Silver and to a lesser extent copper are significantly more electrochemically noble than tin. Possibly for this reason during the electroplating process, Ag and Cu tend to precipitate from the plating bath or to form at the counterelectrode unacceptably altering the composition ultimately plated. Thus, the formation by electroplating of a controllable and suitable material is very difficult.  
         [0007]     Other conventional techniques tend to be unacceptable for low cost high volume production of the desired SACR material. For example, hot dipping—the immersion of the structure to be coated in a liquid bath of the alloy of interest—typically does not produce acceptable results for structures with fine pitches, i.e. structures where solder coated regions are separated by uncoated regions having widths less than approximately 1 mm. Sputtering or other vapor phase deposition techniques of the ternary composition also leads to difficulty in controlling the relative weight percentages of the three constituents. Additionally, vapor phase processes such as sputtering are typically substantially more expensive than techniques such as electroplating or hot dipping. Finally vapor phase techniques often are plagued with step coverage and masking issues. That is, imperfect coatings of the entire structure in the former case and/or unwanted removal or deposition in the latter occurs. Thus, conventional techniques are generally not well suited for a SACR structure.  
         [0008]     Despite the limitations of conventional techniques, it is possible to form a suitable composition by techniques that do not substantially change present device fabrication practices and that are economically acceptable. In particular, the composition of SACR for typical formation temperatures that should advantageously be formed contains from 0.1 weight percent to 10.0 weight percent silver, preferably 1.0 weight percent to 4.0 weight percent silver, most preferably, 2.0 to 4.0 weight percent silver, from 0.1 weight percent to 2.0 weight percent copper, and having the remaining material tin. Material in this compositional range is producible with excellent control using a self-limiting reaction between Sn, Ag and Cu from the underlying structure. Specifically a silver/tin precursor composition is formed on a copper containing structure such as a lead frame structure by conventional techniques e.g. electroplating. A reaction between the precursor and the copper of the structure is induced by energy such as by heating to an elevated temperature. The range of acceptable precursor compositions for producing the desired ternary SACR structure upon heating is relatively wide resulting from the thermodynamics of the self-limiting reaction. Thus, excellent control of the SACR composition in an economic process is achievable.  
         [0009]     The self-limiting Cu content in the SACR alloy is approximately that given by the liquidous composition of the alloy at the temperature employed. (See “Experimental and Thermodynamic Assessment of the Sn—Ag—Cu Solder Alloys,” K. W. Moom, W. J. Boettinger, U. R. Kattner, F. S. Biancaniello, and C. A. Handwerker, J. Electron. Materials, 29, 112-1236 (2000). Thus by forming a tin/silver precursor region of appropriate composition on a copper structure and heating the tin/silver region to allow diffusion of silver through the tin and interaction with the underlying copper structure a compositionally suitable SACR region with particularly efficacious properties is produced. Indeed, concerns relating to whisker formation are essentially eliminated in a structure that does not contain a continuous elemental silver region adjacent the electronic structure by techniques that allow the use of conventional, economically viable techniques. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIGS. 1 and 2  are illustrative of typical lead frame configurations;  
         [0011]      FIGS. 3 through 6  demonstrate steps in the formation of Ag/Sn/Cu regions, and;  
         [0012]      FIG. 7  relates to compositions of precursors and Sn/Ag/Cu materials. 
     
    
     DETAILED DESCRIPTION  
       [0013]     As discussed the invention involves the formation through a self limiting reaction of a suitable region including silver, tin, and copper (SACR) on a copper containing structure such as a copper containing lead frame. Specifically, the SACR is produced by first forming a precursor region including tin and silver on a copper containing structure. (A copper containing structure is one that has sufficient copper available to react with the precursor to yield a Cu to Sn weight percent ratio of at least 0.1. For example, if the Cu-material contains 95% Cu and the Sn thickness is 10 um where the Sn covers the entire surface area of one side of the Cu-material, then the Cu-material is advantageously at least 0.009 um thick. If the Cu-material had 5% Cu then an advantageous thickness would be at least 0.16 um.) The silver/tin region is producible by forming either individual layers of Ag (continuous or discontinuous) and Sn on a copper containing structure or by forming a layer of an alloy containing Sn and Ag onto a Cu containing structure. When individual silver and tin regions are employed, the silver is generally formed before the tin region. In this manner melting the tin region is facilitated since it is not covered by the silver region. Additionally if the silver layer is discontinuous, such layer is generally surrounded by the overlying tin layer which upon melting of the tin promotes dissolution of the silver and diffusion of silver and tin to react with the copper structure. (For such discontinuous silver layers not all regions of silver need be dissolved into the SACR material.) For a continuous silver layer dissolution in the overlying tin, although not as efficient, is still relatively fast. Thus for normal silver layer thicknesses, dissolution by the overlying tin and reaction with the underlying copper structure prior to completion of the thermal process nevertheless occurs. (Although not preferred, it is possible to form a similar structure by forming the tin layer on Cu and the Ag layer or layers on Sn. For such configuration or for a continuous silver layer underlying a copper-free, tin layer it is preferable to limit the silver layer thickness to promote essentially complete dissolution of silver in the tin.) It is also possible for the precursor to be a multiphase composition such as a binary material or individual regions or layers of silver and of tin. It is also acceptable to introduce other materials such as bismuth or copper to the precursor. (The combined binary mixture or the combined silver layer and tin layer for purposes of this invention will both be referred to as the silver/tin precursor region.)  
         [0014]     The silver/tin precursor region is formed either before or after the die is encapsulated in polymer on the copper containing structure. For example in the formation of an integrated circuit package, a lead frame including a die paddle is employed. Typically Cu-alloy lead frames are produced with a patterned, non-continuous, Ag layer. These lead frames are then used as the basis for the formation of a package. The die, ( 31 , in the plan view of  FIG. 3 ) is attached to the paddle,  32 , using conventional adhesives. Electrical connection between the leads,  35 , and the die,  31 , are made as previously discussed using a wire bonding process. The wire bond,  38 , to the leads,  35 , is made by attaching one end of the wire to a region,  39 , containing silver on the lead. After bonding both to the lead and to the die, the region  33  enclosed by dotted line  34 , (including the paddle, die, and wire bonds) is encapsulated in a conventional polymer material. However, typically, the currently available encapsulating polymers do not bond as well to silver as they do to copper. Therefore, with the current state of the art of encapsulants, the region where the encapsulating material meets the lead should preferably be free of silver. It is possible to accomplish this result either by producing a patterned silver or silver/tin portion of the precursor region before encapsulation or by forming the precursor after encapsulation.  
         [0015]     In the pre-encapsulation approach, the die  42  is attached to paddle  43  and a silver region  44  is formed by conventional techniques such as blanket electroplating with subsequent patterning or by plating through a mechanical mask (conventionally referred to as spot plating) on the lead  45  in the region where wire bonding is to occur. (See  Microelectronics Packaging Handbook , ed. R. R. Tummala and E. J. Rymaszewski, Van Nostrand Reinhold, N.Y., 1989, pages 552-554.) The wire  46  is bonded both at the die and at the silver region  44 . When the wire bond region  44  of silver is deposited, it is convenient to also deposit a silver region  48  for the precursor leaving a region  49  where silver is not present. This pattern of silver regions is producible, as previously discussed, using conventional lithographic techniques or mechanical mask plating techniques. The region of silver  48  ultimately employed in the silver/tin precursor region need not be continuous. Thus as shown in  FIG. 6 , before encapsulation of die  61  paddle  62  and wire bond  68 , precursor silver regions  64  are formed. The die with its wire bonds is then encapsulated by polymer denoted  47  in  FIGS. 4 and 67  in  FIG. 6 . In this way, the polymer contacts the lead in a region where silver is not present so that adhesion between the encapsulating polymer and the lead is not degraded. After encapsulation, tin region  401  in  FIG. 4  or  601  in  FIG. 6  is formed on the silver. (It is possible for this region to extend over the silver and adjoin the edge of the encapsulating material.) Any gap present between the silver/tin precursor region and the package body is generally filled by material flow upon heating to produce the SACR region. Although the current, commercially available encapsulates do not adhere well to silver, encapsulants that have adequate adhesion to Ag make a continuous layer of silver advantageous from a cost and ease of manufacture perspective. In another embodiment, the silver/tin precursor region is formed after die encapsulation. Thus, as shown in  FIG. 5 , a silver region,  51 , is formed on the lead  52  adjacent to the encapsulated die  53 . After the silver region or regions are formed, a complementary tin region  54  is formed to complete the Ag/Sn precursor.  
         [0016]     Although it is acceptable to form the ultimate SACR region over the entire lead, the SACR and thus the silver/tin precursor region need not cover the entire lead before the lead is attached to the circuit board. In many applications some portion of the lead is encapsulated with the solder used to attach it to the board. In this encapsulated area, the device is protected from whiskering by the lead free solder. Thus in such structures to avoid whiskers a SACR need only be formed on such portions of the structure that are not coated with the solder. Accordingly, in one embodiment, precursor (continuous or discontinuous) is formed in the region of the structure that is not encapsulated with the board solder when the board is assembled.  
         [0017]     In the preparation of the silver/tin precursor, silver regions are typically formed by electroless or electrolytic plating. (These processes for silver plating are described in standard electrochemical plating texts such as  Tin and Solder Plating in the Semiconductor Industry , A. C. Tan, Chapman &amp; Hall, New York, 1993.) Tin formation is also generally accomplished using electrolytic or electroless plating as described in Tan supra. It is also possible rather than forming separate regions of silver and tin to form initially a binary precursor region. This binary region formation is accomplished, for example, by plating Sn—Ag binary alloy in one step. Furthermore, it is possible for the tin region to contain other compositions such as other platable solder alloys, e.g. Sn—Bi, Sn—In and Sn—Cu.  
         [0018]     In the final compound, for those reaction temperatures, e.g., soldering operations, that occur at temperatures at or below 260° C., the weight percentage of silver should be from 0.1 weight percent to 10.0 weight percent, preferably 1.0 weight percent to 4.0 weight percent, most preferably 2.0 to 4.0 weight percent. The weight percent of copper should be in the range of 0.1 to 2.0 weight percent. (For those applications where SACR is used in only the portion of the lead that is not covered by solder, these compositional ranges are for the SACR formed by self-limiting reaction and not for the solder itself.) Tin provides the remaining material of the SACR. Such compositions are achievable at the indicated reaction temperatures of 260° C. or below. It is possible to incorporate higher levels of Ag and Cu generally up to 20 and 10 weight percent respectively at higher temperatures for those applications and/or devices that are capable of withstanding temperatures above the peak typical electronic soldering temperature of 260° C. If a soldering temperature above 260° C. is employed the composition is advantageously modified to account for the weight percentage of Ag, Cu and Sn that fall on the liquidous at the temperature at which the SACR formation is induced. It is also possible to introduce other materials to modify the properties. For example, bismuth or indium is introducible to change the melting point of the alloy or to modify its mechanical properties. Generally materials other than silver, copper and tin should be limited to approximately 15 more preferably 5 weight percent of the SACR material.  
         [0019]     A controlled sample is useful in determining the appropriate weight percentage of silver and tin in the silver/tin precursor region to yield after reaction a SACR region having an appropriate composition. However, the graph in  FIG. 7  displays the composition of a Ag/Sn/Cu ternary at the liquidous point and is employable to determine a silver/tin precursor region composition that generally yields an appropriate SACR composition. The final SACR composition depends on the temperature used to form the material as well as the initial composition of the silver/tin precursor material. Thus as shown in  FIG. 7  if a temperature of 260° C. is employed, a composition with 1.0 weight percent silver represented by the indicated graph line, will have approximately 1.48 weight percent copper. Thus for an intended temperature and desired copper percentage a suitable silver/tin precursor region composition is determinable. (It should be noted that the initial weight ratio of silver to tin in the precursor region before heating will not be substantially different than that ultimately in the SACR region. Therefore in determining by, for example, a controlled sample, a desired composition for the precursor silver/tin region the final weight percentage ratio between silver and tin in the SACR composition is appropriately used. However, for more exact calculation all of the elements within the alloy should also be taken into account.)  
         [0020]     It is generally advantageous to control the relative weight ratio of silver to tin in the silver/tin precursor region by 1) establishing the necessary ratio in a binary Ag/Sn precursor, or 2) controlling the thickness of individual silver and tin regions. (This thickness determination is made for each discrete final SACR by considering the thicknesses of the precursor regions ultimately forming such discrete region.) Since the density of both silver and tin are well established, the volume of tin employed relative to the volume of silver employed yields through a simple calculation the relative weight percentages of each. In turn, since the surface area for the silver and tin deposits are generally determined by the considerations discussed above, the relative weight percentage between the silver and the tin is controlled by suitably controlling the thickness of these individual regions. Generally the thickness ratio of Sn to Ag less than approximately 500 to 1 do not substantially limit the desired reaction. However, at 260° C. or lower for a continuous silver/layer and a Sn to Ag thickness ratio of less than approximately 10, interaction of the overlying tin with the underlying copper tends to be limited. Thus it is advantageous in such situations to use discontinuous silver regions. In this case the Ag regions should be spaced such that no more than 10 mm exist between one Ag layer and the mold cap body of the package or 5 mm between two adjacent Ag regions and more preferably no more than 5 mm between the mold cap and the Ag region or 1 mm between two adjacent regions. Formation of binary precursor regions have been discussed in some detail in Y. Zhang, et.al, “Lead-Free Bumping and its Challenges” International Wafer Level Packaging Congress (IWLPC), San Jose, Calif., Oct. 10-12, 2004.  
         [0021]     The temperature used to induce reaction should be higher than the melting point of tin when an individual Sn region is used. Temperatures in the range 232° C. to 260° C. are advantageous for devices that use standard encapsulant materials. Alternatively, binary alloys of Sn and Cu or Sn and Ag are useful to replace all or a part of the tin layer. In such case temperatures required for the self limiting reaction as low as 220° C. are useful for compositions in the range of 3.4-3.8 Ag, 0.5-0.9 Cu and the balance of Sn. Higher temperatures as previously discussed are employable if the device and/or accompanying board are not unacceptably damaged by enhanced temperatures. For precursors with individual tin regions temperatures below the tin melting point are undesirable because they require reliance on unacceptably slow solid state diffusion for the reaction and not on relatively rapid liquid phase dissolution and diffusion. Temperatures above 260° C., although not precluded, often lead to substantial degradation of the polymeric materials used in the encapsulant and/or board. Heating is typically continued for a period of 3 seconds to 2 minutes. Time periods less than 3 seconds tend to produce incomplete reaction. Time periods greater than 2 minutes, although not precluded, are generally unnecessary. The source of heat is not critical. The use of a furnace is typical, but other heat sources such as hot air and light absorption are useful with no detrimental effects on the SACR.