Patent Publication Number: US-2009217520-A1

Title: Method for forming solder lumps on printed circuit board substrate

Description:
BACKGROUND 
     1. Technical Field 
     The present invention generally relates to printed circuit board packaging technology, and particularly relates to a method for forming solder lumps on a printed circuit board substrate. 
     2. Discussion of Related Art 
     In printed circuit board (PCB) manufacturing, prior to a welding process, a solder lump is formed on a corresponding solder pad which is defined in a predetermined region of a PCB substrate. A typical method for forming the solder lump on the PCB substrate includes the following steps. Firstly, a mask defining a number of through-holes therein is placed onto a surface of the PCB substrate. Each of the through-holes corresponds to a solder pad of the PCB substrate. Secondly, the through-holes are filled with a solder masses using a screen printing process. Thirdly, the solder masses in each of the through-holes are reflowed so that the solder pad is substantially covered by the melted metal masses. Finally, the mask is separated from the PCB substrate, thus obtaining a PCB substrate having a number of solder lumps formed thereon. 
     However, the method described above has the following disadvantages. First, some of the solder massess in the through-holes may be peeled from the PCB substrate during the separation of the mask. As a result, the solder masses accommodated in the through-holes for forming solder lumps is insufficient. Second, precision of the screen printing process is generally in a range from 20 microns to 25 microns, so using the screen printing process for filling the though-holes with metal is not suitable for forming solder lumps on a PCB substrate having a line width less than 0.3 millimeters. 
     What is needed, therefore, is a method for forming solder lumps on a printed circuit board substrate to overcome the above-described problems. 
     SUMMARY 
     One embodiment provides a method for forming solder lumps on printed circuit board substrate. Firstly, a PCB substrate including a number of electrical traces and solder pads formed on a substrate surface thereof is provided. Secondly, a liquid photoresist is applied onto the PCB substrate such that a photoresist layer defining a number of openings therein is formed and each of the solder pads is exposed via the openings. Thirdly, each of the openings is filled with a solder masses. Fourthly, the solder massess are reflowed. Lastly, the photoresist layer is removed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a flow chart of a method for forming solder lumps on a printed circuit board substrate according to an exemplary embodiment. 
         FIG. 2  is an isometric view of a printed circuit board substrate having a number of electrical traces and solder pads. 
         FIG. 3  is a cross-sectional view of the substrate of  FIG. 2 , taken along a line III-III. 
         FIG. 4  is similar to  FIG. 3 , but showing a mask placed on the substrate. 
         FIG. 5  is similar to  FIG. 4 , but showing a photoresist layer formed on the substrate. 
         FIG. 6  is similar to  FIG. 5 , but showing the mask is removed. 
         FIG. 7  is similar to  FIG. 6 , but showing solder masses received in openings of the photoresist layer. 
         FIG. 8  is similar to  FIG. 7 , but showing a reflowing step. 
         FIG. 9  is similar to  FIG. 8 , but showing the photoresist layer is removed. 
         FIG. 10  is a cross-sectional view showing mounting a chip onto the substrate according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates a method for forming solder lumps on a PCB substrate. The method will be discussed in detail with the following exemplary embodiment. 
     Referring to  FIG. 2 , in step  10 , a printed circuit board (PCB) substrate  100  is provided. The PCB substrate  100  has a number of electrical traces  111  and a number of solder pads  112  electrically connected to the electrical traces  111 . The PCB substrate  100  has a substrate surface  110 . The electrical traces  111  and the solder pads  112  are formed on the substrate surface  110 , thus defining a peripheral area  113  between two neighboring electrical traces  111  and the solder pads  112 . The electrical traces  111  have a second surface  114 . In this embodiment, a distance between adjacent electrical traces  111 , i.e., a line width of the PCB substrate  100  is in a range from about 5 micrometers to about 100 micrometers. 
     In step  20 , as shown in  FIG. 6 , a photoresist layer  200  having a number of openings  220  is formed on the PCB substrate  100 , while each solder pad  112  is exposed via a corresponding opening  220 . 
     In detail, referring to  FIG. 3 , a mask  300  and an ejecting device  400  are provided. The mask  300  includes a number of covering portions  320  and a number of through-holes  310  penetrating through the mask  300  between the covering portions  320 . The covering portions  320  are configured for covering the solder pads  112 . The ejecting device  400  is configured for ejecting liquid photoresist onto the second surface (not shown) and the peripheral area  113 . 
     As shown in  FIG. 4 , the mask  300  is placed onto the PCB substrate  100 , and all the solder pads  112  are substantially covered by the covering portion  320  while the electrical traces (not shown) and the peripheral area  113  are exposed via the through-holes  310 . 
     Referring to  FIGS. 4˜5 , a liquid photoresist is applied onto the peripheral area  113  and the electrical traces (not shown) via the through-holes  310  and fills the through-holes  310 . Then the liquid photoresist is quickly solidified, thus obtaining the photoresist layer  200  having a number of photoresist members  230 . A thickness of the photoresist members  230  is more than that of the solder pads  112 . 
     Referring to  FIGS. 5˜6 , the mask  300  is removed from the photoresist layer  200 . Therefore, each two adjacent photoresist members  230  and the solder pad  112  located therebetween cooperatively define an opening  220  for accommodating a solder masses in a following step. 
     In step  30 , referring to  FIGS. 6˜7 , the openings  220  are filled with a solder masses  500  using a typical screen printing method or a known depositing process. In the present embodiment, a screen printing method is used. The metal masses  500  are comprised of tin or other metal which has low molten temperature. 
     In step  40 , referring to  FIGS. 7˜8 , the solder masses  500  are treated using a typical reflow soldering process to obtain a number of solder lumps  600  formed on the solder pads  112 . In this manner, the solder lumps  600  substantially cover the solder pads  112 . 
     In step  50 , referring to  FIGS. 8˜9 , the photoresist layer  200  is removed from the PCB substrate  100  using a chemical etching method, thus obtaining a printed circuit board  750  having a number of solder lumps  600 . 
     In another embodiment, the photoresist layer  200  is formed using an ink injection method. In detail, firstly, an ink jet device with a micro-electro mechanical system (MEMS) is provided. Secondly, positions of the second surface and the peripheral area are stored in the MEMS. Finally, a liquid photoresist is applied onto the second surface and the peripheral area under a controlling signal from the MEMS and quickly solidifies. 
     A method for flip chip packaging using one of the above described methods of forming solder lumps will be described below with an example of packaging a chip onto the PCB substrate  100 . 
     As shown in  FIG. 10 , the flip chip packaging method includes following steps. Firstly, a number of solder lumps  600  are formed on a predetermined region of the PCB substrate  100  using the aforesaid method. Secondly, a chip  700  having a number of solder members  710  is provided. Each of the solder members  710  corresponds to each of the solder lumps  600 . Thirdly, each of the solder members  710  is connected to the corresponding solder performs  600  using a welding process, thus obtaining a package of the chip  700  and the PCB substrate  100 . 
     While certain embodiments have been described and exemplified above, various other embodiments will be apparent to those skilled in the art from the foregoing disclosure. The present invention is not limited to the particular embodiments described and exemplified but is capable of considerable variation and modification without departure from the scope of the appended claims.