Patent Publication Number: US-2006006533-A1

Title: Motherboard structure for preventing short circuit

Description:
BACKGROUND OF THE INVENTION  
     FIELD OF THE INVENTION  
      The present invention relates to motherboard structures, and particular to a motherboard for preventing shot circuit between two adjacent tin balls of an IC device during solder reflow process.  
     GENERAL BACKGROUND  
      It has been a trend in the field of making semiconductor devices to increase the packing density of a silicon wafer. IC designers are continuously tempted to scale down the size of each device and increase chip level integration at an even faster pace. Further, the manufacturers of the devices are striving to reduce the sizes while simultaneously increasing their speed. A plurality of ball-grid array tin balls is formed on the rear surface of a conventional BGA IC device. Since the BGA IC device is used in coordination with surface-mount technology (SMT) where tin balls are utilized for connection with a printed circuit board (PCB) instead of pins, requirements for packaging become stricter. The SMT is a method to mount a BGA IC device on a PCB. Then, a nitrogen solder reflow treatment is performed to melt the tin balls beneath the BGA IC device, making the BGA IC device and the PCB knitted together to form a functional system. At this time, if solder spills out from the connection points, a short circuit between two adjacent tin balls will occur.  
      Referring to  FIGS. 1 and 2 , a motherboard includes a PCB  100  and an IC device  500 . The PCB  100  has an array  30  of a plurality of pads  400  for mounting the IC device  500  thereon. The IC device  500  is a BGA-type IC chip with a plurality of tin balls  700  formed on a bottom surface.  FIG. 1  shows a part of the pads array  300  of the PCB  100 , in fact, the number of the pads is more than shown. For example, an Intel&#39;s Pentium 4 processor has 478 tin balls and the PCB has the matching pads to accept them.  
      The IC device  500  is mounted directly onto the PCB  100  using standard surface mount technology equipment. A nitrogen solder reflow treatment is performed to melt the tin balls  700  to make the tin balls  700  of the IC device  500  and the pads  400  of the PCB  100  knitted together. The PCB  100  is manufactured with a solder mask  800  that covers foils except in the places where the designers do not want them to be soldered. A silk screen  200  is printed on a top of the solder mask  800  and has a similar height to pads  400  extending from the upper face of the PCB  100 . Text and symbols are printed on the PCB  100  to label the locations for the different components that are to be mounted. The solder mask  800  is an insulating and protective coat that protects thin copper wires and prevents solder from attaching outside the connection points. Sometimes if the solder overflowed  720  from the connected point beyond the limit that the solder mask  800  resist, a short circuit between two adjacent tin balls  410 ,  430  of the IC  500  will occur.  
      U.S. Pat. No. 6,239,383 disclosed a ball-grid array BGA IC device for preventing short circuits. A plurality of supporting pads is formed on a rear surface of a BGA IC device where tin balls are located. The melting point of the supporting pads is higher than that of the ball-grid array tin balls. The supporting pads are made of aluminum with a higher melting point and good heat conducting. The BGA IC device can be supported by the supporting pads when the tin balls are melted during a nitrogen solder reflow treatment, thereby preventing short circuits. Moreover, due to the supporting pads with a higher melting point than the tin balls, the supporting pads are never melted during the nitrogen solder reflow treatment. However, packaging technology demands that the ICs equip with more tin balls for connection, and meanwhile, available space for mounting those additional tin balls is shrinking, it is therefore difficult to form the supporting pads on the surface of the BGA IC device.  
      What is needed is a motherboard structure for preventing shot circuit during solder reflow process.  
     SUMMARY  
      A motherboard for preventing short circuit includes an IC device and a PCB. The IC device has a plurality of tin balls, and the PCB has matching pads with the tin balls of the IC device. The tin balls and the pads knitted together to mount the IC device to the PCB. A silk screen is printed on a surface of the PCB, and an additional silk screen is printed on the silk screen where there is no pads formed for preventing short circuit by forming retaining holes above pads to retain the tin balls.  
      It is of advantage that printing silk screen is a necessary step of printed circuit board manufacturing, so the motherboard for preventing short circuit has no need to add additional step to printed circuit board manufacturing. The additional silk screen can retain more solder than conventional PCB when the tin balls are melted during a solder reflow treatment, thereby preventing short circuits.  
      Other objects, advantages and novel features will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a top view of pad arrays of a conventional PCB; and  
       FIG. 2  is a section view showing a short circuit between tin balls of a conventional IC;  
       FIG. 3  is a bottom view of a PCB in according with a preferred embodiment of the present invention with an additional silk screen formed thereon; and  
       FIG. 4  is a section view of the PCB of  FIG. 3  together with an IC mounted thereon. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT  
      Referring to  FIGS. 3 and 4 , a motherboard includes a printed circuit board (PCB)  10  and an electronic device like an integrated circuit (IC) device  50 . The PCB  10  has a plurality of pads array  30  for mounting the IC device  50  thereon. The IC device  50  is an IC of BGA-type with a plurality of solder means, for example tin balls  70 , formed on a bottom surface thereof. A solder mask  80  that covers foils except in the places where the designers do not want it to be soldered is formed on the PCB  10 . A non-solderable screen, for example a silk screen  90 , is printed on a top of the solder mask  80 . Among the pads array  30 , an additional part of the non-solderable screen, i.e. an additional silk screen  92 , is printed in the places where there are no pads  40 . The additional silk screen  92  also has the features of insulating and resisting solder. A total thickness of the solder mask  80 , the silk screen  90  and the additional silk screen  92  is greater than the solder mask and the silk screen of a conventional PCB so that a significant retaining hole is formed right above each pad  40  of the PCB  10  due to difference of the total thickness. When the IC device  50  is mounted on the PCB  10 , if a little of solder is overflowed from the connected point, the additional silk screen  92  can retain more solder than the conventional PCB as shown in  FIG. 2 . That is, The PCB  10  has the additional silk screen  92  formed on the top of the silk screen  90  according to the embodiment of the present invention can provide additional protection function to the PCB  10  for preventing short circuits.  
      In addition, silk screen printing is accomplished with traditional silk-screen technology. This process is necessary in making a PCB, so it is no need to add additional making process to make the motherboard.  
      It is believed that the present invention and its advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.