Flux for soldering, soldering method, and printed circuit board

Abstract

A flux for soldering used when soldering is performed to a board subjected to electroless nickel plating, the flux containing resin having film forming ability, activator, and solvent, and further containing metallic salt in an amount of 0.1 to 20% by weight of the total amount of flux. The use of this flux suppresses diffusion of nickel solder in soldering portions, and prevents concentration of phosphorous, thereby improving the bonding strength of soldering.

1 . A flux for soldering used when soldering is performed to a board subjected to electroless nickel plating, said flux containing resin having film forming ability, activator, and solvent, and further containing metallic salt in an amount of 0.1 to 20% by weight of a total amount of flux.  
   
   
       2 . The flux according to  claim 1 , wherein said metallic salt is salt of metal selected from gold, silver, copper, lead, zinc, bismuth, indium, antimony, and nickel.  
   
   
       3 . The flux according to  claim 1 , wherein said metallic salt is metallic salt of organic acid, and said activator is organic acid that is same as organic acid composing said metallic salt of organic acid, or organic acid having a lower acidity than that.  
   
   
       4 . The flux according to  claim 2 , wherein said metallic salt of organic acid is copper salt of organic acid.  
   
   
       5 . The flux according to  claim 3 , wherein said metallic salt of organic acid is metallic salt of saturated fatty acid whose hydrocarbon group except for carboxyl group, has a carbon number of 7 to 21.  
   
   
       6 . The flux according to  claim 3 , wherein said activator is rosin.  
   
   
       7 . The flux according to  claim 1 , wherein said resin having film forming ability is rosin or acrylic resin.  
   
   
       8 . The flux according to  claim 1 , wherein said resin having film forming ability is rosin, and said rosin also functions as said activator.  
   
   
       9 . The flux according to  claim 1 , containing no halogen compound.  
   
   
       10 . A soldering method including: 
 printing a flux for soldering according to  claim 1  on a board having on its surface copper lands subjected to electroless nickel plating;    mounting solder balls on said lands; and    heating to have said solder balls reflow and connect onto said lands.    
   
   
       11 . The soldering method according to  claim 10 , wherein said solder balls are lead-free.  
   
   
       12 . A soldering method including: 
 printing paste that is a mixture of a flux for soldering according to  claim 1  and solder powder, on a board having on its surface copper lands subjected to electroless nickel plating; and    heating to have said paste reflow so as to form solder alloy on said lands.    
   
   
       13 . A printed circuit board in which solder is connected by a soldering method according to one of claims  10  to 12.