Abstract:
In accordance with the invention, a metal substrate is coated with a multilayer finish comprising a layer of tin or tin alloy and one or more outer metal layers. An optional metal underlayer may be disposed between the substrate and the tin. In an exemplary embodiment the metal substrate comprises copper alloy coated with a nickel underlayer, a layer of tin and an outer metal layer of palladium. The resulting structure is particularly useful as an electrical connector or lead frame.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to metal articles coated for solderability and protection from corrosion. In particular, it concerns an article having a multilayer finish comprising a layer of tin or tin alloy and a metal outer layer to inhibit the growth of tin whiskers. The surface finish is especially useful for electrical connectors and integrated circuit lead frames.  
         BACKGROUND OF THE INVENTION  
         [0002]    High quality connectors are increasingly important in a wide variety of products including consumer electronics, household appliances, computers, automobiles, telecommunications, robotics and military equipment. Connectors provide the paths whereby electrical current flows from one device to another. Quality connectors should be highly conductive, corrosion resistant, wear resistant, readily connected by solder and inexpensive.  
           [0003]    Unfortunately no single material has all the desired characteristics. Copper and many of its alloys are highly conductive, but they are subject to corrosion in typical ambients, producing reactive oxides and sulfides. The reactive corrosion products reduce the conductivity of the connectors and the reliability of interconnection. The corrosion products also interfere with the formation and reliability of solder bonds and can migrate to other electronic components which they adversely affect.  
           [0004]    Thin layers of tin have been applied to copper surfaces to provide corrosion resistance and solderability. Tin is easily applied, non-toxic, provides corrosion protection and has excellent solderability. Unfortunately tin coatings are subject to spontaneous growth of metallic filaments called tin “whiskers”. These whiskers have been identified as a cause of short circuit failures in low voltage equipment. Moreover whisker fragments can detach and accumulate within device packages, causing shorts at locations remote from their origin and interfering with electromechanical operation. Accordingly, it would be advantageous to provide metal articles with whisker free coatings of tin.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the invention, a metal substrate is coated with a multilayer finish comprising a layer of tin or tin alloy and one or more outer metal layers. An optional metal underlayer may be disposed between the substrate and the tin. In an exemplary embodiment the metal substrate comprises copper alloy coated with a nickel underlayer, a layer of tin and an outer metal layer of palladium. The resulting structure is particularly useful as an electrical connector or lead frame. 
       
    
    
     BRIEF SUMMARY OF THE DRAWINGS  
       [0006]    The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings:  
         [0007]    [0007]FIG. 1 is a schematic cross section of a metal article coated in accordance with the invention;  
         [0008]    [0008]FIG. 2 is a block diagram showing the steps involved in making the coated metal article of FIG. 1;  
         [0009]    [0009]FIG. 3 shows a substrate for making an electrical connector using the process of FIG. 2;  
         [0010]    [0010]FIGS. 4A and 4B are photographs comparing the invention with a conventionally coated article; and  
         [0011]    [0011]FIG. 5 shows a substrate for making an integrated circuit lead frame. 
     
    
       [0012]    It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale.  
       DETAILED DESCRIPTION  
       [0013]    [0013]FIG. 1 is a schematic cross section of a metal substrate  10  coated with a multilayer finish  11  including, in ascending order from the substrate, an optional metal underlayer  12 , an intermediate layer  13  of tin or a tin alloy subject to whisker formation and an outer metal layer  14  of metal for inhibiting whisker formation. The metal substrate is typically a conductive metal such as copper, copper alloy, iron or iron alloy subject to corrosion in typical ambients. The optional underlayer is advantageously a low porosity metal such as nickel, nickel alloy, cobalt or cobalt alloy. It is preferably an amorphous alloy. Nickel-phosphorus alloy with phosphorus ≧about 10 wt % is satisfactory for substrates comprising copper or iron. Nickel-phosphorus-tungsten or cobalt-phosphorus can also be used. The intermediate layer  13  can be tin or a tin alloy subject to whisker growth such as tin-copper, tin-bismuth, tin-silver, tin-nickel, tin-zinc or tin-copper-silver. The outer layer (or layers)  14  is of a metal or alloy which inhibits whisker formation. It can be palladium, rhodium, ruthenium, platinum, copper, silver, iridium, bismuth or an alloy of one or more of these metals. The optional underlayer  12  can have a thickness in the range of about 0-10 μm. The tin layer  13  typically has a thickness about 0.5-10 μm; and the outer metal layer  14  is preferably about 5-10,000 angstroms.  
         [0014]    The invention can be understood more clearly by consideration of the following specific examples describing the fabrication of coated metal articles in accordance with the invention.  
       EXAMPLE 1  
     Electrical Connector  
       [0015]    [0015]FIG. 2 is a block diagram of the steps in making a coated metal electrical connector. The first step, shown in Block A, is to provide a metal substrate. The substrate can be formed into a desired configuration as by stamping or etching a metal blank.  
         [0016]    [0016]FIG. 3 illustrates a substrate for an electrical connector  30  having a connector body  31  and a mating pin  32 . The connector  31  and the pin  32  are made of high conductivity metal such as copper-nickel-tin alloy No. 725 (88.2 wt. % Cu, 9.5 Ni, 2.3 Sn; ASTM Spec. No. B 122).  
         [0017]    The next step, which is optional, is to coat the conductive substrate  11  with a metal underlayer  12  such as amorphous nickel-phosphorus. The underlayer  12  can have a thickness from 0 to about 5 μm. It can be electrodeposited Ni—P with P content higher than about 10 wt % to assure amorphous structure. A suitable Ni—P amorphous layer can be electrodeposited using the following bath composition:  
                                                           Nickel sulfate   NiSO 4 6H 2 O   100-300 g/l           Nickel chloride   NiCl 2 6H 2 O    40-60 g/l           Phosphorous acid   H 3 PO 3      40-100 g/l           Phosphoric acid   H 3 PO 4      0-50 g/l                      
 
         [0018]    The third step, Block C, is to apply a layer  13  of tin or tin alloy. The layer  13  should have a thickness greater than about 0.5 μm and is preferably about 7 μm. A suitable tin layer can be electrodeposited using the following bath:  
                                                       Tin methane sulfonate    40-80 g/l           Methane sulfonic acid   100-200 g/l           Wetting Agent 300    5-15 g/l           (Harcos Chemicals Inc.)           Anti-Oxidant C 1    1-3 g/l           (Spectrum Laboratory           Products)                      
 
         [0019]    The next step, shown in Block D, is to apply the outer metal layer  14  over the layer  13 . Applicants have found that the proper choice of the outer metal layer will inhibit the growth of tin whiskers. The outer metal layer  14  may be composed of one or more layers of precious metal (palladium, iridium, rhodium, ruthenium, platinum or their alloys such as palladium-nickel or palladium-cobalt). It can also be copper, silver, bismuth or their alloys. The outer layer thickness is in the range about 5-10,000 angstroms. A suitable palladium layer can be deposited from the palladium electroplating solution described in U.S. Pat. No. 4,911,799 issued on Mar. 27, 1990 to J. A. Abys et al, which is incorporated herein by reference. A suitable palladium-nickel alloy with nickel content preferably 10-30 wt % can be deposited from the electroplating solution described in U.S. Pat. No. 4,911,798 issued on Mar. 27, 1990 to J. A. Abys et al., which is incorporated herein by reference. A suitable palladium-cobalt alloy can be deposited as described in U.S. Pat. No. 5,976,344 issued to J. A. Abys et al on Nov. 2, 1999 which is incorporated herein by reference. Plating of rhodium, ruthenium and platinum are described in Metal Finishing, (Guidebook and Directory Issue), Vol. 97, No. 1 (January, 1999).  
         [0020]    To test this new finish, applicants plated onto copper different finishes including tin layers and aged them at a temperature of 50° C. The aged surfaces were then inspected for whiskers using a scanning electron microscope (SEM) with high magnifications (up to 100,000×). No whiskers were detected on the new finish after two months aging. Whiskers were found on the conventional nonovercoated tin finish. FIG. 4A is an SEM picture of the inventive finish after two months aging at 50° C. No whiskers are present. FIG. 4B is an SEM of a conventional nonovercoated tin finish. Whiskers are present.  
       EXAMPLE 2  
     Integrated Circuit Lead Frame  
       [0021]    An integrated circuit lead frame can also be fabricated by the process illustrated in FIG. 2. The only differences are that the substrate is different and the outer coating thickness can be greater (e.g. 0.5-15 μm).  
         [0022]    [0022]FIG. 5 illustrates a substrate  50  formed into configuration for use as a lead frame for an integrated circuit (IC). The substrate  50  includes a paddle  52  on which the IC is to be mounted and the leads  53  on which the IC is to be bonded. Dam bars  54  interconnect the leads before packaging. After the integrated circuit is bonded and a packaging medium has been applied over an area shown in phantom lines  55 , the dam bars  54  are trimmed away.  
         [0023]    The substrate of the lead frame can a copper alloy such as alloy No. 151 (99.9 wt. % Cu, 0.1% Zr) or alloy No. 194 (97.5 wt. % Cu, 2.35% Fe, 0.03% P, 0.12% Zn). Other conductive metals and alloys such as alloy No. 42 (42 wt. % Ni, 58% Fe) can also be used.  
         [0024]    An integrated circuit  56  is mounted and bonded to the substrate, and the substrate is coated by the process illustrated in FIG. 2.  
         [0025]    It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention.