Flexible printed circuit board

Abstract

A FPCB includes a base layer defining at least one first through hole. A conductive paste block is formed in each first through hole. Each conductive paste block includes a first and a second end portion. The base layer has opposite surfaces, and a first conductive wiring layer is formed on each surface of the base layer. The first end portion at least protrudes from the base layer and is exposed from the first conductive wiring layer. An insulating layer and a second conductive wiring layer are formed on each first conductive wiring layer. At least one second through hole is defined in each insulating layer. The second through hole positioned near the first end portion extends to the first end portion and forms a recess. A conductive via is formed in each second through hole and the corresponding recess, and is electrically connected to the conductive paste block.

What is claimed is: 
     
         1 . A flexible printed circuit board comprising:
 a base layer defining at least one first through hole;   a conductive paste block formed in each of the first through hole, the conductive paste block comprising a first end portion and a second end portion opposite to the first end portion;   two first conductive wiring layers formed on opposite surfaces of the base layer, respectively, the first end portion protruding from the base layer and exposed from a corresponding first conductive wiring layer;   an insulating layer formed on each of the two first conductive wiring layers, at least one second through hole being defined in the insulating layer;   a second conductive wiring layer formed on the insulating layer, a second through hole coaxially aligned with the conductive paste block, the second through hole which is positioned above the first end portion extending to the first end portion and forming a recess in the first end portion; and   a conductive via formed in the second through hole and a corresponding recess, the conductive via being electrically connected to the conductive paste block.   
     
     
         2 . The flexible printed circuit board of  claim 1 , wherein each of the first through hole passes through the base layer and the two first conductive wiring layers; each of the first end portion and the second end portion of the conductive paste block protrudes from the base layer; and the recess is formed in each of the first end portion and the second end portion. 
     
     
         3 . The flexible printed circuit board of  claim 2 , wherein the second conductive wiring layer comprises a second copper foil layer, a conductive layer, and a copper plating layer successively formed on the insulating layer; the conductive layer is further formed on an inner wall of the second through hole, and contacting an inner wall of the corresponding recess; the copper plating layer further fills in the second through hole and the corresponding recess containing the conductive layer, thereby forming the conductive via. 
     
     
         4 . The flexible printed circuit board of  claim 2 , wherein the second conductive wiring layer comprises a second copper foil layer and a copper plating layer successively formed on the insulating layer; the copper plating layer further fills in the second through hole and the corresponding recess, thereby forming the conductive via. 
     
     
         5 . The flexible printed circuit board of  claim 2 , wherein the conductive via is made by conductive paste. 
     
     
         6 . The flexible printed circuit board of  claim 2 , wherein each of the first end portion and the second end portion is flush with the first conductive wiring layer. 
     
     
         7 . The flexible printed circuit board of  claim 1 , wherein the conductive via comprises two third end portions, and each of the third end portions is flush with a corresponding one of the second conductive wiring layers. 
     
     
         8 . The flexible printed circuit board of  claim 1 , wherein the first conductive wiring layer comprises at least one first annular ring, the second conductive wiring layer comprises at least one second annular ring, and each of the at least one second annular ring is coaxially aligned with a corresponding one of the at least one first annular ring. 
     
     
         9 . The flexible printed circuit board of  claim 1 , wherein the first through hole passes through the base layer and one of the first conductive wiring layers; only the first end portion of the conductive paste block protrudes from the base layer; the second through hole which is positioned above the first end portion extends to the first end portion to form a recess in the first end portion; the second through hole which is positioned above the second end portion extends to the first conductive wiring layer to form a recess in the first conductive wiring layer; and the recess in the first end portion has a greater depth than the recess in the first conductive wiring layer. 
     
     
         10 . The flexible printed circuit board of  claim 9 , wherein the first end portion is flush with the conductive wiring layer. 
     
     
         11 . The flexible printed circuit board of  claim 1 , wherein the base layer is made of a polymer selected from a group consisting of polyimide, polytetrafluoroethylene, poly thiamine, poly(methyl methacrylate), polycarbonate, polyethylene terephthalate, and polyimide-polyethyleneterephthalate, and any combination thereof.