Printed wiring board having metal layers producing eutectic reaction

Abstract

A printed wiring board includes a Cu wiring pattern formed on a substrate. A first metal layer is formed on the Cu wiring pattern. A second metal layer is formed on the first metal layer. The first metal layer has a less reactivity with Cu than the second metal layer. The first metal layer and the second metal layer together cause an eutectic reaction.

What is claimed is: 
     
         1 . A printed wiring board comprising:
 a substrate;
 a Cu wiring pattern formed over said substrate; 
 a first metal layer formed on said Cu wiring pattern; 
 a second metal layer formed on said first metal layer, 
 wherein said first metal layer has a less reactivity with Cu than said second metal layer, and said first metal layer and said second metal layer together cause an eutectic reaction. 
   
     
     
         2 . The printed wiring board according to  claim 1 , wherein said second metal layer forms an intermetallic compound with Cu, and said first metal layer does not form an intermetallic compound with Cu. 
     
     
         3 . The printed wiring board according to  claim 1 , wherein said eutectic reaction is produced in a temperature range from 139° C. to 150° C. 
     
     
         4 . The printed wiring board according to  claim 1 , wherein said first metal layer is made of a metal element selected from a group consisting of bismuth, lead, indium and silver or an alloy containing one of bismuth, lead, indium and silver as a major component, and said second metal layer is made of a metal element selected from a group consisting of tin and gold or an alloy containing one of tin and gold as a major component. 
     
     
         5 . The printed wiring board according to  claim 1 , wherein said Cu wiring pattern includes a first interface area along an interface between said first metal layer and said Cu wiring layer, the first interface area containing the metal element forming said second metal layer; and said second metal layer includes a second interface area along an interface between said first metal layer and said second metal layer, the second interface area containing Cu. 
     
     
         6 . A manufacturing method of an electronic device, comprising:
 placing said electronic device on a printed wiring board according to  claim 1  so that terminals of said electronic device are brought into contact with said second metal layer; and   reflowing said first and second metal layers to join said terminals of said electronic device to said Cu wiring pattern.