Plating bath composition for electroless plating of gold and a method for depositing a gold layer

Abstract

An electroless aqueous gold plating bath, comprising at least one source of gold ions and at least one reducing agent for gold ions, characterized in that it comprises at least one ethylenediamine derivative as plating enhancer compound according to formula (I) wherein the residues R 1 and R 2 comprise 2 to 12 carbon atoms and are selected from the group consisting of branched alkyl, unbranched alkyl, cycloalkyl or combinations thereof wherein the individual residues R 1 and R 2 are the same or different and a method of depositing of gold. The electroless aqueous gold plating bath is suitable to provide soft gold layers useful for wire bonding and soldering applications which are required for electronic components.

1 . An electroless aqueous gold plating bath, comprising at least one source of gold ions and at least one reducing agent for gold ions, characterized in that the bath comprises at least one ethylenediamine derivative as plating enhancer compound according to formula (I) 
       
         
           
           
               
               
           
         
         wherein the residues R 1  and R 2  comprise 2 to 12 carbon atoms and are selected from the group consisting of branched alkyl, unbranched alkyl, cycloalkyl or combinations thereof wherein the individual residues R 1  and R 2  are the same or different. 
       
     
     
         2 . The electroless aqueous gold plating bath according to  claim 1  characterised in that the residues R 1  and R 2  in formula (I) comprise 2 to 8 carbon atoms. 
     
     
         3 . The electroless aqueous gold plating bath according to  claim 1  characterised in that the residues R 1  and R 2  in formula (I) are the same. 
     
     
         4 . The electroless aqueous gold plating bath according to  claim 1  characterised in that the residues R 1  and R 2  in formula (I) are free of any further amino moieties and/or any hydroxy moieties. 
     
     
         5 . The electroless aqueous gold plating bath according to  claim 1  characterised in that the residues R 1  and R 2  in formula (I) are branched alkyl residues having 3 to 6 carbon atoms. 
     
     
         6 . The electroless aqueous gold plating bath according to  claim 1  characterised in that the concentration of the at least one plating enhancer compound according to formula (I) ranges from 0.001-1 mol/L. 
     
     
         7 . The electroless aqueous gold plating bath according to  claim 6  characterised in that the concentration of the at least one plating enhancer compound according to formula (I) ranges from 10 to 100 mmol/L. 
     
     
         8 . The electroless aqueous gold plating bath according to  claim 1  characterised in that the at least one reducing agent for gold ions is selected from the group consisting of aliphatic aldehydes, aliphatic dialdehydes, aliphatic unsaturated aldehyde, aromatic aldehydes, sugars having an aldehyde group and precursors of formaldehyde. 
     
     
         9 . The electroless aqueous gold plating bath according to  claim 1  characterised in that the molar ratio of reducing agent to plating enhancer compound according to formula (I) ranges from 0.8 to 3. 
     
     
         10 . The electroless aqueous gold plating bath according to  claim 1  characterised in that the pH of the electroless aqueous gold plating bath ranges from 5 to 9. 
     
     
         11 . The electroless aqueous gold plating bath according to  claim 1  characterised in that the concentration of gold ions ranges from 0.1 to 10 g/L. 
     
     
         12 . The electroless aqueous gold plating bath according to  claim 1  characterised in that the electroless aqueous gold plating bath further comprises at least one complexing agent selected from the group consisting of carboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, aminophosphonic acids or a salt of the aforementioned. 
     
     
         13 . A method for depositing a gold layer onto a substrate, comprising, in this order, the steps
 (i) providing a substrate   (ii) contacting at least a portion of the surface of the substrate with the   electroless aqueous gold plating bath according  claim 1     and thereby depositing a gold layer onto the at least a portion of the surface of the substrate.   
     
     
         14 . The method for depositing a gold layer onto a substrate according to  claim 13  wherein the at least a portion of the surface consists of a metal or metal alloy and the metal or metal alloy is one or more selected from the group consisting of nickel, nickel alloys such as nickel phosphorous alloys, nickel boron alloys, cobalt, cobalt alloys such as cobalt phosphorous alloys, cobalt molybdenum phosphorous alloys, cobalt molybdenum boron alloys, cobalt molybdenum boron phosphorous alloys, cobalt tungsten phosphorous alloys, cobalt tungsten boron alloys, cobalt tungsten boron phosphorous alloys, palladium, palladium alloys such as palladium phosphorous alloys, palladium boron alloys, copper and copper alloys and gold or gold alloys. 
     
     
         15 . (canceled)