Patent Publication Number: US-2009236136-A1

Title: Printed circuit board assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Taiwanese application no. 097106827, filed on Feb. 27, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a printed circuit board assembly, more particularly to a printed circuit board assembly including a solder paste formed on a portion of a soldermask, which is formed on a portion of a solder pad. 
     2. Description of the Related Art 
     Electronic components can be classified into two types: plated through-hole components and surface mount components. The plated through-hole components usually have pins for extending respectively into holes in a printed circuit board so that the pins are soldered to a back side of the printed circuit board after passing through a solder oven. The surface mount components have end caps for mounting directly on a surface of a printed circuit board. 
       FIG. 1  illustrates a conventional printed circuit board  9  including a substrate  93 , a soldermask  94  formed on the substrate  93  and formed with a recess  941 , a pair of solder pads  92 , which are received in the recess  941 , a pair of conductive traces  95  each extending from a respective one of the solder pads  92  and each having a first end portion  951  covered by the soldermask  94  and a second end portion  952  that extends between the first end portion  951  of the conductive trace  95  and the respective solder pad  92 , and that is exposed to the atmosphere, and a lead-free solder paste  91  applied on the solder pads  92  for connecting leads  961  of an electronic component  96  thereto. 
     During soldering, the leads  961  of the electronic component  96  are attached to the lead-free solder paste  91  on the solder pads  92  of the printed circuit board  9 , and the assembly is subsequently disposed in a reflow-soldering oven so as to conduct soldering. Since the lead-free solder paste  91  has a high surface tension, the lead-free solder paste  91  contracts so as to minimize the surface area thereof, which results in undesired exposure of a portion of each of the solder pads  92  to the atmosphere after soldering, which, in turn, results in problems, such as open-circuit in the solder joint, naked copper, cold joint, and generation of ineffective soldering area for the solder pads  92 . In addition, the joint area between each solder pad  92  and the respective conductive trace  95  tends to corrode after a period of use. 
     Taiwanese Patent No. 249213 discloses a printed circuit board including a pair of solder pads formed on a substrate, a soldermask formed on the substrate and a portion of each solder pad, and a lead-free solder paste applied on the remainder of each solder pad by screen printing techniques. However, the problems of generation of ineffective soldering area and cold joint are still present. 
     SUMMARY OF THE INVENTION 
     Therefore, one object of the present invention is to provide a printed circuit board assembly that can overcome the aforesaid drawbacks associated with the prior art. 
     Another object of this invention is to provide a method for making the printed circuit board assembly. 
     According to one aspect of the present invention, the printed circuit board assembly comprises: a substrate; a soldermask formed on the substrate and formed with a recess to expose a pad-forming portion of the substrate; a solder pad having a first end portion received in the recess and formed on the pad-forming portion of the substrate, and a second end portion extending from the first end portion; and a solder paste having a first end portion formed on the first end portion of the solder pad, and a second end portion extending from the first end portion of the solder paste and formed on a portion of the soldermask, which is formed on the second end portion of the solder pad. 
     According to another aspect of this invention, there is provided a method for making the printed circuit board assembly, comprising: (a) forming solder pads on a substrate; (b) covering the substrate and a portion of each of the solder pads using a soldermask; (c) applying a solder paste on a portion of the soldermask, which is formed on the portion of each of the solder pads, and the remainder of each of the solder pads; (d) attaching leads of an electronic component to the solder paste; and (e) reflowing the solder paste on the solder pads. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments of this invention, with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a conventional printed circuit board; 
         FIG. 2  is a perspective view of the first preferred embodiment of a printed circuit board assembly according to this invention; 
         FIG. 3  is a perspective view of the second preferred embodiment of a printed circuit board assembly according to this invention; 
         FIGS. 4 to 7  are perspective views to illustrate consecutive steps of a method for making the printed circuit board assembly according to this invention; and 
         FIGS. 8 to 11  are fragmentary sectional views of  FIGS. 4 to 7 , respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Before the present invention is described in greater detail with reference to the accompanying preferred embodiments, it should be noted herein that like elements are denoted by the same reference numerals throughout the disclosure. 
       FIG. 2  illustrates the first preferred embodiment of a printed circuit board assembly  1  according to this invention. The printed circuit board assembly  1  includes: a substrate  10 ; a soldermask  11  formed on the substrate  10  and formed with a recess  111  so as to expose a pad-forming portion  101  of the substrate  10 ; a pair of solder pads  22  each having a first end portion  220  received in the recess  111  of the soldermask  11  and formed on the pad-forming portion  101  of the substrate  10 , and a second end portion  221  extending from the first end portion  220  of the solder pad  22 ; and a lead-free solder paste  13  having a first end portion  131  formed on the first end portion  220  of each of the solder pads  22 , and a second end portion  132  extending from the first end portion  131  of the lead-free solder paste  13  and formed on a portion  112  of the soldermask  11 , which is formed on the second end portion  221  of the solder pad  22 . 
     In this embodiment, the printed circuit board assembly  1  further includes a pair of conductive traces  21  each of which is formed on the substrate  10 , covered by the soldermask  11 , and connected to the second end portion  221  of a respective one of the solder pads  22 . 
     In this embodiment, the recess  111  of the soldermask  11  has a peripheral edge  113 , which has a segment  114  dividing each of the solder pads  22  into the first and second end portions  220 ,  221 . The segment  114  is linear in shape. 
     Preferably, the first and second end portions  220 ,  221  of each of the solder pads  22  are opposite to each other in a direction along the length of the solder pad  22 . The second end portion  221  of each of the solder pads  22  has a length along the direction ranging from 4 to 6 mil. 
     Preferably, each of the conductive traces  21  and the solder pads  22  is made from copper. 
       FIG. 3  illustrates the second preferred embodiment of the printed circuit board assembly  1  according to the present invention. The second preferred embodiment differs from the previous embodiment in that the segment  114  is curved in shape. 
     This invention also provides a method for making the printed circuit board assembly  1  with at least one electronic component  3  soldered thereto. The method includes: (a) forming the solder pads  22  on the substrate  10  (see  FIGS. 4 and 8 ); (b) covering the substrate  10  and the second end portion  221  of each of the solder pads  22  using the soldermask  11  (see  FIGS. 5 and 9 ); (c) applying the lead-free solder paste  13  on the portion  112  of the soldermask  11  (see  FIGS. 5 and 9 ), which is formed on the second end portion  221  of each of the solder pads  22 , and on the remainder, i.e., the first end portion  220 , of the solder pads  22 ; (d) attaching leads  31  of the electronic component  3  to the lead-free solder paste  13  (see  FIGS. 6 and 10 ); and (e) reflowing the lead-free solder paste  13  on the solder pads  22 (see  FIGS. 7 and 11 ). 
     Preferably, the method further includes forming the conductive traces  21  on the substrate  10 , each of which is connected to the second end portion  221  of the respective solder pads  22  and is covered by the soldermask  11 . 
     Note that the lead-free solder paste  13  contracts toward the leads  31  of the electronic component  3  during reflowing due to its surface tension. By virtue of forming the lead-free solder paste  13  on the portion  112  of the soldermask  11 , the lead-free solder paste  13  can completely cover the first end portion  220  of each of the solder pads  22  after reflowing so as to prevent generation of the ineffective solder area or occurrence of naked copper and cold joint for the solder pads  22 . 
     By forming the lead-free solder paste  13  on the portion  112  of the soldermask  11 , which is formed on the second end portion  221  of the solder pad  22  of the printed circuit board assembly  1  of this invention, the aforesaid drawbacks associated with the prior art can be eliminated. 
     With the invention thus explained, it is apparent that various modifications and variations can be made without departing from the spirit of the present invention. It is therefore intended that the invention be limited only as recited in the appended claims.