Abstract:
The electrical and mechanical properties of structures such as lead frames and other electrical/electronic devices containing, during processing, copper/tin interfaces are improved by introduction of nickel to such interface. Typically, a weight percentage of nickel to tin in the range 1 to 12 weight percent yields upon melting of the tin, an intermetallic compound with essentially no occluded, unbound tin. Thus undesirable anomalous structures such as tin needles and substantially non-planar interface compositions are avoided. Advantageously a nickel/tin/copper intermetallic interface that is substantially planar is formed in the substantial absence of needle-like tin structures.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to tin containing interconnects for electronic and electrical devices and in particular to tin containing interconnects 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.  
         [0003]     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 tin. These regions, it is contemplated, are under compressive stress, particularly at the curved sections of the lead frame 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 interconnect metallization structures.  
         [0004]     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 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.  
         [0005]     For similar reasons, use of lead-free solder is also being promoted. Such solders melt generally at temperatures above 217° C., and in application, for process control reasons, are typically used at temperatures above 240° C. Since such temperatures exceed the melting point of tin (approximately 232° C.), concerns about tin whiskers have been mitigated since such whiskers are melted during the soldering process. Accordingly, it would appear that the difficulties associated with whiskers such as inadvertent shorting of lead frames or blocking of optical paths for electro-optic devices need not be a substantial concern.  
       SUMMARY OF THE INVENTION  
       [0006]     Surprisingly it has been found that tin whiskers are present even after soldering with materials having been subjected to soldering temperatures above 232° C. Thus the issues associated with such whiskers remain with lead-free solders. By practice of the invention, such whiskers are essentially totally avoided without the expedient of adding lead to the tin coating of the lead frame. Such results are achieved in one embodiment by forming on the lead frame a nickel/tin composition having from 1 to 12 weight percent nickel relative to tin. Such composition results in formation of an intermetallic compound with copper such that essentially no free tin remains. Additionally, this copper/nickel/tin intermetallic has an essentially planar surface and thus is mechanically quite stable. As a result, tin whiskers are avoided and the resultant solder connection is mechanically robust.  
         [0007]     Additionally, the mechanical stability afforded by use of the invention is useful even in applications where whisker formation is not a consideration. For example, in certain applications, solder regions on an integrated circuit package (regions generally denominated bumps and/or balls) are connected to pad regions on a circuit board. Such connection is accomplished through processes such as by aligning regions on the die to corresponding regions on the package substrate with subsequent cohesion of the aligned regions. In one common lead free application, a tin/silver, tin/copper, or tin/silver/copper solder alloy is used to connect an aluminum pad on an integrated circuit (IC) to the metal pad of a package substrate. To expedite this attachment the aluminum pad on the IC is coated with a solder friendly metal system. Such system includes an adhesion layer, a barrier layer and a solder wettable layer. (Examples of such solder friendly systems include titanium/nickel-doped with vanadium/copper; aluminum/nickel-doped with vanadium/copper; chromium/chromium-copper/copper; electroless nickel/immersion gold; and copper/nickel.) The metal substrate pad is possibly bare copper but is often coated with materials such as electroless nickel/immersion gold or electrolytic nickel/gold. Similarly the substrate is connected to the board with a lead free solder alloy. Exemplary substrate metallization are copper, copper coated with electroless nickel/immersion gold, and copper coated with electrolytic nickel/electrolytic gold.  
         [0008]     The interaction between the tin of the solder and the copper from any of the solder pad metal interfaces (die, substrate, or board) has the potential to produce mechanical instabilities with concomitant reliability issues often characterized by very non-planar interfaces. Through the use of nickel in the solder in the range of 1 to 12 weight percent relative to tin, a robust planar intermetallic compound is formed that substantially reduces the proclivity to induce mechanical instabilities often arising when such an interface is not present. Thus, even in applications where whiskers are not a consideration, but where tin materials promote mechanical instabilities the invention is nevertheless advantageously employed.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIGS. 1 and 2  are illustrative of typical lead frame configurations;  
         [0010]      FIGS. 3 through 5  exemplify structures involved in the invention; and  
         [0011]      FIG. 6  demonstrates results achieved with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]     The presence of nickel in appropriate proportions to tin on a copper containing structure, such as those containing copper and/or copper alloy including brass, or lead frame materials such as alloy  42 , avoids problems associated with whisker formation and substantially mitigates mechanical weakness. To achieve such results, the nickel should be present with tin at an interface with copper so that the weight of nickel relative to the tin is in the range 1 to 12 percent of the weight of tin present. Advantageously, the weight of nickel should be 3 to 7 weight percent to that of tin and most advantageously 4 to 6 the weight percentage of nickel to tin. Although an exact atomistic explanation for the interaction of the nickel, tin, and copper is not presently resolved, it is contemplated that copper and nickel together with tin form an intermetallic compound and that compositions formed by employing tin and nickel in the appropriate proportions yield an essentially complete reaction, i.e. consumption of the tin such that unreacted tin is essentially absent. Thus after reaction there is essentially no excess elemental non-bound tin present at an interface with copper. Since elemental non-bound tin is required to form whiskers, whiskers are precluded. Additionally, the complete reaction of the tin yields a substantially planar surface for the copper/nickel/tin intermetallic compound and thus a stronger mechanical bond.  
         [0013]     For the desired intermetallic composition to form, the tin should be melted, for example, during a soldering process. Since the melting point of copper is 1,083° C. and the melting point of nickel is 1,453° C., melting of these metals for essentially all electronic and electrical applications is not practical. Thus, before melting, the three metals should be positioned geometrically so that their interaction to form an intermetallic compound is possible through diffusion at temperatures near the melting point of tin, i.e. temperatures in the range of about 232° C. to 270° C. where significant degradation of organic substrates and/ or packaging materials is avoided. Since nickel is a slow diffuser in tin, it is typically disadvantageous to have a structure before melting such as shown in  FIG. 3 . As shown in  FIG. 3 , when the tin,  22 , melts, nickel,  23 , must diffuse through the molten tin to interact with the copper,  24 . (If the time is sufficiently long, e.g. in the range 60 to 600 seconds and the temperature sufficiently high, e.g. 232° C. to 300° C., then such a structure is useful but relatively inconvenient to employ.) In contrast the structure shown in  FIG. 4  is preferred since at temperatures around 240° C. copper,  34 , and nickel,  32 , interact with the tin,  33  rapidly forming an intermetallic of the correct composition with a planar surface.  
         [0014]     Generally, for the copper diffusion through and nickel consumption in molten tin to be advantageous in a reasonable period of time, the nickel region between the tin and copper should have a thickness in the range 0.05 um to 1.0 um. Thicknesses less than 0.05 um often contain porosity which promote uncontrolled reactions and non-planar boundaries. Furthermore, such nickel thicknesses require corresponding tin thicknesses that are prone to damage during the trim and lead bending process. Thicknesses greater than approximately 1 um generally yield inadequate nickel/copper interaction with the tin under typical electrical device thermal excursions. Alternatively, it is possible to plate or otherwise form an alloy of tin and nickel in the appropriate weight percentages onto the copper containing structure. (The combination of nickel and tin need not be an alloy. It is possible to use other nickel/tin compositions such as nickel/tin/silver, nicke/tin/silver/copper, or nickel/tin/copper.) The presence of other materials such as those typically used to modify the properties of copper are not precluded. For example, copper is often alloyed with materials such as iron to enhance properties such as mechanical stability. The presence of such modifying materials generally does not preclude the advantages of the invention.  
         [0015]     The method of forming the desired nickel and tin regions is not critical. Techniques such as vapor deposition, physical deposition, electroplating, or paste printing lead to useful results. As discussed, the invention relies on the appropriate presence of tin/nickel/copper at an interface where whiskers are to be avoided and/or mechanical stability is required. Although a composition of nickel/tin having a proportion in the range 1 to 12 weight percent produces the desired result, it is possible to include in the final device structure other regions of tin remote from a copper interface. For example, as shown in  FIG. 5 , an aluminum pad,  41 , is overlaid by a region of nickel,  42 , and copper,  43 . The copper is interfaced with a region,  44 , having the appropriate weight percentage of nickel to tin. This region in turn is overlaid by a larger region of tin-based solder,  45 . Since the nickel is present at the copper interface upon melting, the appropriate proportions for copper/nickel/tin intermetallic having the desired property is maintained by applying a thermal treatment generally involving temperatures greater than 232° C. for times greater than 5 seconds to insure that the proper composition of nickel and tin are provided at the copper interface.  
         [0016]     To produce the desired intermetallic and to avoid excess tin at a copper interface, the structure should be heated to a temperature that allows tin to melt. Typically, temperatures in the range 232° C. to 270° C. are employed. At temperatures below, 232° C. no substantial tin melting occurs, while at temperatures above 270° C, degradation of typical packaging polymers occurs. However, if employed on ceramic, metal, or metal/ceramic packages, temperatures up to 500° C. may be used without unacceptable degradation of the package materials. Typically the desired intermetallic material is formed in a time period between 5 and 120 seconds for temperatures in the advantageous range. Thus, generally, the temperature should be maintained in the desired range for such time periods. Times less than 5 seconds are undesirable since inadequate reaction to form the desired intermetallic is a frequent occurrence, while time periods greater than 120 seconds, although not precluded, are typically not economic.  
         [0017]     Although not required, it is possible to anneal the structure after formation of the desired intermetallic composition. Generally, annealing temperatures in the range 100° C. to 200° C. are useful in conjunction with annealing times in the range 0.5 hours to 8 hours. Subsequent processing of the device after intermetallic formation in the desired annealing range eliminates the need for a specific annealing step. However, generally, processing at temperatures above 270° C. should be avoided with organic packaging materials due to their excessive degradation.  
         [0018]     As discussed, an intermetallic compound is formed at a copper containing interface in an interconnection. This intermetallic material in one embodiment is characterized by a percentage of unbound occluded tin less than approximately  15  weight percent relative to the total free Sn content in the interconnection. (In one advantageous embodiment the occluded tin is essentially absent.) The intermetallic composition is also characterized in one embodiment by a surface having a planarity of at least ÷0.9 um. (Planarity is this context is defined as maximum variation in thickness from peak to valley across the intermetallic interface.) The following example exemplifies processes, conditions, and compositions involved in the subject invention.  
       EXAMPLE  
       [0019]     A 0.25 um thick nickel layer was electroplated onto a copper lead frame. Subsequently a 3 um thick tin layer was electroplated onto the nickel layer. On a weight percentage basis the sample contains approximately 4% nickel and 96% tin. The device was then subjected to a 150° C. 1 hour anneal. Finally the device was subjected to a typical solder reflow process with a peak temperature of 260° C. The time the device was at 260° C. was approximately 18 seconds. A secondary electron image was taken from a focused ion beam cross section of the sample. As shown in  FIG. 6 , the entire tin layer is converted into a planar nickel/copper/tin intermetallic layer  61  on the copper lead  62 . The platinum layer  63  was deposited onto the sample prior to focused ion beam cutting and acts as a reference that defines the surface of the tin/Ni/Cu layer.