Abstract:
A printed circuit board (PCB) includes a plastic substrate, and a layout formed on the plastic substrate. The layout comprises a first layout and a second layout, the second layout is less dense than the first layout. The second layout comprises a pseudo-layout to prevent the PCB from being bent when heated.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a printed circuit board (PCB), and more particularly, to a PCB remaining unbent even when heated.  
         [0003]     2. Description of the Prior Art  
         [0004]     Every electronic product requires a PCB to carry its electric components. Thus, the quality of the PCB plays an important role in an electronic product. The technology on PCB has been fully developed, as the electronic products become compact and cheaper, the PCB is accordingly required high density, compactness and multi-layers.  
         [0005]     Please refer to  FIG. 1  showing a conventional PCB  10 . The PCB  10  is single-layered, including a substrate  12  and a circuit layout  14  disposed there above. The substrate  12  is in general made of plastic being insulating from electricity and heat. The traces on the PCB  10  form the circuit layout  14 . In general, the circuit layout  14  is made of copper. When manufacturing the PCB  10 , a copper foil is placed on the substrate  12 , and the remaining part after photolithography and etching forms the circuit layout  14 . These traces transmit signals or power to the electric components installed on the PCB 10 . Since the circuit layout  14  and the electric components are on the same side of the substrate  12 , such kind of PCB is called a single-layered PCB. If the circuit layout  14  and the electric components are installed on both sides of the substrate  12 , it is called a double-layered PCB.  
         [0006]     As the circuits of the electronic products became more and more complicated, single-layered and double-layered were no longer suitable. Thus, multi-layered PCB was developed. Please refer to  FIG. 2  showing a multi-layered PCB  20 . The PCB  20  includes a plurality of substrates  12   a - 12   d.  Copper foils cover each substrate  12   a - 12   d  and form the circuit layout  14 s after photolithography and etching. Afterwards, the substrates  12   a - 12   d  are hot pressed together to form the multi-layered PCB  20 . 4 substrates are shown in  FIG. 2 , which is the maximum number possible.  
         [0007]     Please refer to  FIG. 2  as well as  FIG. 3 , which shows the PCB  20  formed by the substrates  12   a - 12   d  shown in  FIG. 2 . According to the requirements of design, the circuit layout  14  on each substrate is not necessarily uniform in density. For clarity, the PCB  20  can be divided into first layouts  18   a - 18   d  and second layouts  16   a - 16   d.  As shown in  FIG. 2 , the second layouts  16   a - 16   d  corresponding to the substrates  12   a - 12   d  are less dense than the first layouts  18   a - 18   d.  Please notice that the density of a layout is determined from a comparative perspective on the whole PCB. Therefore, even if there are traces in region  16   c,  the average density of the first layouts  18   a - 18   d  is larger than that of the second layouts  16   a - 16   d.  Unfortunately, the PCB will undergo several high temperature reflow process and due to the difference of thermal expansion coefficients of the copper-made circuit layout  14  and the plastic substrate  12 , the less dense second layouts  16   a - 16   d  will be bent up compared with the first layouts  18   a - 18   d  as shown in  FIG. 3 . For electronic products, any small deformation may cause problems on assembly. In order to solve the problem of deformation, the circuit needs to be redesigned to be more uniform, which can cause a waste in time.  
       SUMMARY OF INVENTION  
       [0008]     It is therefore a primary objective of the claimed invention to provide a PCB remaining unbent even when heated to solve the problem mentioned above.  
         [0009]     Briefly, a printed circuit board (PCB) includes a plastic substrate, and a circuit layout formed on the plastic substrate, having a first layout and a second layout . . . . The second layout comprises a pseudo-layout to prevent the PCB from being bent when heated.  
         [0010]     The present invention further provides a method for forming a circuit layout on a PCB substrate, the circuit layout comprising signal lines and power lines; and installing a pseudo-layout in the circuit layout to prevent the PCB from being bent when the PCB is heated.  
         [0011]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]      FIG. 1  illustrates a conventional single-layered PCB.  
         [0013]      FIG. 2  illustrates a conventional multi-layered PCB.  
         [0014]      FIG. 3  illustrates the PCB formed by the substrates shown in  FIG. 2 .  
         [0015]      FIG. 4  illustrates a PCB according to the present invention.  
         [0016]      FIG. 5  is an enlarged view of the pseudo-layout. 
     
    
     DETAILED DESCRIPTION  
       [0017]     Please refer to  FIG. 4  showing a PCB  40  according to the present invention. The numberings of elements in  FIG. 4  are the same as  FIG. 2  assuming they have the same functions. The PCB  40  includes a plurality of plastic substrates  12   a - 12   d  and a plurality of circuit layouts  14  formed on the substrates  12   a - 12   d.  In general, the circuit layouts  14  are made of copper.  
         [0018]     As described above, since the PCB requires many reflow process and the high temperature cause the deformation of the PCB, a pseudo-layout  46  is installed, for example, in the region of second layout  16   b  on the PCB  40 . The pseudo-layout  46  is neither for transmitting signals nor power. The reason of deformation is that the average density of the second layouts  16   a - 16   d  is less than the first layouts  18   a - 18   d.  Thus the heat applied on the second layouts  16   a - 16   d  and the first layouts  18   a - 18   d  is not uniform. The pseudo-layout  46  can increase the average density of the second layouts  16   a - 16   d.  Therefore, the pseudo-layout  46  can prevent the PCB  40  from being bent when heated.  
         [0019]     The position of the pseudo-layout  46  can be determined as follows. First, find the regions on the PCB  40  that will be bent after heating through different test processes. Of course, deformation occurs generally on low density second layouts  16   a - 16   d.  Subsequently, install the pseudo-layout  46  in the second layouts  16   a - 16   d.  As shown in  FIG. 4 , region  16   b  has the pseudo-layout  46  with a trace width of 1 mm. Certainly, the pseudo-layout  46  can be installed in any one or two, or even more of the second layouts  16   a - 16   d.  Then manufacture enforcement of the new PCB  40  with the pseudo-layout  46 . Afterwards, subject the new PCB to the deformation test and adjust the amount of the pseudo-layout  46  accordingly. Repeat this cycle of testing and adjusting until the new PCB  40  is found to not bend after being heated.  
         [0020]     Please refer to  FIG. 5  showing an enlarged view of the pseudo-layout  46 . In the present invention, the pseudo-layout  46  has a plurality of parallel pseudo-traces with an interval distance of 5 mil (0.125 mm) and a width of 5 mil (0.125 mm). Certainly, the interval distance and the width of the traces are not limited to 5 mil. The pseudo-traces are the same as the typical traces on a typical circuit layout  14 ; however, they do not transmit signals or power but only spread the heat uniformly. It is an advantage to design the pseudo-traces in netlike circuit layout that not only the bending of the PCB can be improved, but also the netlike circuit layout forms an electric loop to prevent electromagnetic interference (EMI) with other components. Of course, the structure of the pseudo-layout  46  is not limited to a netlike structure; any other structures capable of spreading the heat uniformly also belong to the present invention. Moreover, the area and the length of the pseudo-layout can be adjusted according to the deformation of the PCB. In addition, in the present invention, the pseudo-layout  46  is isolated from the signal traces and the power traces on the PCB  40 . However, the pseudo-layout  46  is not limited to that, it is not necessary to be isolated from the signal traces and the power traces on the PCB  40  if it does not influence the operation of other components on the PCB  40 .  
         [0021]     In contrast to the prior art, the PCB according to the present invention has the pseudo-layout to prevent the deformation caused by a high temperature manufacturing process. The pseudo-layout is designed to be in one or more circuit layouts so that no additional hardware or design is required. Moreover, the pseudo-layout neither cause EMI problem nor influence the operation of other components on the PCB. Therefore, the present invention is very simple and useful.  
         [0022]     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.