Patent Publication Number: US-2017365759-A1

Title: Plated lead frame including doped silver layer

Description:
FIELD OF THE INVENTION 
     The invention relates to lead frames for the assembly of semiconductor devices, and in particular to lead frames that are adapted for the assembly of light-emitting device (“LED”) related products. 
     BACKGROUND AND PRIOR ART 
     In lead frames used for the assembly of LED-related products, a bright silver plating is typically plated on a copper lead frame, or a bright silver plating is plated on a nickel plating layer such that the nickel plating layer is located between the bright silver plating layer and the copper lead frame. 
       FIG. 1  illustrates prior art plating schemes for the manufacture of bright silver LED devices. A first typical prior art plating scheme for a lead frame  100  comprises a base copper substrate  104  which is plated with copper to form a copper layer  106  over the base copper substrate  104 . The copper layer  106  is further plated with a bright silver layer  108  comprising silver. 
     A second typical prior art plating scheme for a lead frame  102  comprises a base copper substrate  104  which is plated with copper to form a copper layer  106  over the base copper substrate  104 . Nickel is then plated over the copper layer  106  to form a nickel layer  110  on top of it. Finally, the lead frame  102  is plated with a bright silver layer  108  on top of the nickel layer  110 . 
     For both plating schemes used to fabricate the respective lead frames  100 ,  102 , copper plating is applied on the surface of the base copper substrate  104  prior to conducting bright silver plating or nickel plating onto the lead frame surface. 
     In the typical plating schemes described above, it has been found that silver or silver ion migration occurs on the bright silver layer  108 . Such silver or silver ion migration is a major problem because it causes tarnishing of the bright silver layer  108 , leading to a decrease in the lumen or emission of visible light from the LED device. The foregoing events deteriorate LED performance in the final product. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the invention to seek to improve the LED performance in LED devices by retarding migration of silver or silver ions to the surface of the lead frame. 
     According to a first embodiment of the invention, there is provided a method of manufacturing a lead frame, comprising the steps of: providing a substrate comprising copper; plating a layer of bright silver onto the substrate; and thereafter plating a layer of doped bright silver over a top surface of the layer of bright silver. 
     According to a second embodiment of the invention, there is provided a lead frame comprising: a substrate comprising copper; a layer of bright silver; and a layer of doped bright silver over a top surface of the layer of bright silver. 
     It would be convenient hereinafter to describe the invention in greater detail by reference to the accompanying drawings which illustrate specific preferred embodiments of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of plating processes in accordance with the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates prior art plating schemes for the manufacture of bright silver LED devices; and 
         FIG. 2  illustrates plating schemes for the manufacture of bright silver LED devices according to the preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
       FIG. 2  illustrates plating schemes for the manufacture of bright silver LED devices according to the preferred embodiment of the invention. One embodiment of a lead frame  10  fabricated according to the plating scheme comprises a base copper substrate  14  which is plated with copper to form a copper layer  16  over the base copper substrate  14 . Thereafter, a bright silver layer  18  comprising silver is plated onto the copper layer  16 . Thereafter, a doped bright silver layer  20  is further formed over a top surface of the underlying bright silver layer  18 . 
     In another embodiment of a lead frame  12  manufactured according to the plating scheme, a base copper substrate  14  is plated with copper to form a copper layer  16  over the base copper substrate  14 . A layer of nickel is then plated over the copper layer  16  to form a nickel layer  22  on top of it. Thereafter, the nickel layer  22  is plated with a bright silver layer  18  on top of the nickel layer  22 . A doped bright silver layer  20  is then further formed over the underlying bright silver layer  18 . 
     According to the preferred embodiment, the plating to form the copper layer  16  may be performed in an aqueous solution comprising: 10-80% Copper sulfate, 0.1-1% brightener, 0.1-1% leveler and 0.1-1% carrier. The carrier may be made from a polymeric material, such as Polyethylene Glycol (PEG) or Polyalkylene Glycol (PAG). The brightener may comprise organic sulfides, disulfides, thioether or thiocarbamates, and the leveler may comprise quaternary nitrogen compounds. 
     The plating to form the nickel layer  22  may be conducted in an aqueous solution comprising: 30-80% nickel sulfamate, 15-30% nickel chloride and 5-10% boric acid. 
     The plating of the bright silver layer  18  may be conducted in an aqueous solution comprising: 20-60% Potassium silver cyanide, 10-15% potassium cyanide, 0.5-5% brightener, and 20-30% potassium phosphate. 
     The dopant used to form the doped bright silver layer  20  may comprise either palladium or gold. In case palladium is used as the dopant, plating of the doped bright silver layer  20  is conducted in an aqueous solution comprising: 2-20% ammonium tetrachloropalladate, and 40-60% ammonium phosphate. Alternatively, where palladium is used as the dopant, plating may also be conducted in an aqueous solution comprising: 2-20% ammonium tetrachloropalladate, 40-60% ammonium phosphate, and 1-5% brightener. 
     Where gold is used as the dopant, plating is conducted in an aqueous solution comprising: 2-20% potassium gold cyanide and 20-40% potassium phosphate. 
     It should be appreciated that a dopant comprising palladium and/or gold integrated in bright silver layer can improve the performance of LED as the GAM (gamma brightness) of silver is increased and the doped silver retards silver or silver ion migration. The dopant thus promotes the GAM of silver as well as improves GAM stability by minimizing silver or silver ion migration. 
     With the addition of the dopant, the GAM of silver can be improved by 0.2 GAM, and stronger thermal stability of the lead frame  10 ,  12  is achieved since the GAM of silver can thereby be preserved even after heating the lead frame  10 ,  12  at 360° C. for 20 seconds. 
     The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.