Abstract:
In designing a printed circuit board, first and second copies of views of first and second major surfaces of the board respectively, are created. The first and second copies are positioned to contact each other. First and second segments on a side of the printed circuit board and normal to a first element in the first copy and a second element in the second copy respectively are obtained. A third segment joining ends of the first and second segments is obtained, and a point dividing the third segment according to a ratio of a distance from the first element to the side and a distance from the second element to the side is calculated. Distances from the first element to the point and from the point to the second element are calculated, and a creepage distance, a sum of the two distances and a thickness of the board, is obtained.

Description:
[0001]    This is a divisional of application Ser. No. 11/448,960, filed Jun. 6, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a method for computer aided design, the system therefore and a medium storing the method, more particularly relates to the method, the system, and the medium which provide calculating distances between elements mounted on each surface of a printed circuit board. 
         [0004]    2. Description of the Related Art 
         [0005]    A calculation of a distance between elements mounted on each surface of a printed circuit board (PCB), can make it possible to evaluate influences of noise caused by a possible creepage discharge. The distance is measured along the surfaces, the distance is referred to as the creepage distance hereinafter, and the distance should be evaluated whether to agree or not with requirements in specifications and/or standards. 
         [0006]    The layout of elements formed or arranged on PCBs is usually designed with a computer aided design system, hereinafter referred to as CAD system. In case of use of the CAD system, the creepage distance can be calculated by a function of the CAD system, where the creepage distance is a distance measured along a surface of a PCB between an element mounted on a front surface of a PCB, the element is referred as to the front element hereinafter, and other element mounted on a rear surface of the PCB, the other element is referred as to the back element. And the resultant creepage distance is judged or evaluated whether it agrees or not with the requirements. 
         [0007]    A conventional measurement is explained in detail with referring to  FIGS. 19A and 19B , where  FIG. 19A  is perspective view and  FIG. 19B  is a side view. A distance LA between a front element  710  and a side  712  of a front surface  702  of a PCB  700 , a thickness LC of the PCB  700 , a distance LB between a back element  720  and the side  712  are measured respectively. And then a creepage distance between the two elements is gained by summing the distances LA and LB and the thickness LC. However, a following consideration on the path to measuring the distance is necessary in case of particular locations of the elements. 
         [0008]    As shown in  FIGS. 20A and 20B , a pair of elements  710  and  720 , each of them being arranged on both surfaces of the PCB  700 , normal lines NA and NB can be overlapped as shown in  FIG. 20B . In this case of the locations of elements  710  and  720 , the creepage distance can be gained as the method described above. However, in case of locations of elements  710  and  730 , there is no common normal line being normal to the side  712 . In this case, an operator must estimate a point α which gives the minimum sum of the distances DA and DB, where DA is a distance between the element  710  and the point α and DB is a distance between the element  730  and the point α. The selection of the point α is subjectively estimated and selected by the operator. The creepage distance, in this case, is given by the sum of DA, DB and the thickness LC of the PCB  700 . Thus on the CAD system, a function to calculating a distance of two points selected by the operator is provided. The operator can gain the creepage distance by use of the function. 
         [0009]    Other method of calculating of the creepage distance is disclosed in a Japanese publication of unexamined application 2005-10835. The method in the publication is disclosed as a function in an insulation-evaluating system. The system itself comprises a process transforming a three dimensional model to a correspondent approximate polyhedron model, a process of topologically recognizing the polyhedoron model, a process of recognizing a relation with adjacent parts, a process of recognizing and creating data of voltage system, a process of extracting a pair of voltage systems including a problem by rough check, a process of calculating “a creepage distance” between both the voltage systems, a process of checking the creepage distance by the information in a specification for insulation, and a process of outputting the information of the voltage systems including problems. 
         [0010]    In the prior art shown in  FIGS. 19A ,  19 B,  20 A, and  20 B, the creepage distance is obtained as the summation of the thickness of the PCB and the distances calculated by a computer between a turning point and points on the front and back elements, where these three points should be indicated by the operator. Thus till obtaining a result, the method in the prior art requires frequently operation by the operator, and imposes on the operator the serious burden and requires a long time. Further, the resultant creepage distance is not guaranteed as the path having the minimum distance between both elements, because the turning point and the points on the front and back elements are subjectively selected by the operator. Therefore, the method in the prior art involves a problem that the resultant distance as a creepage distance has serious error and a low reliability. 
         [0011]    Furthermore, it is necessary to obtain all creepage distances for every possible combinations of a plurality of elements arranged on the both surfaces of PCB. Therefore, it will be vast time to calculate the every creepage distances even if the turning point and the points on the front and back element are set automatically instead of the operator, because the combination of the points for calculating the creepage distances are enormous. Subsequently to detecting front and back element not compatible with the requirement in the specification or a standard, the similar calculation must be repeated after the rearrangement of the front elements by the operator so that the elements become to be compatible with the requirement. Therefore, the prior art has problems such that the method imposes serious burden on the operator and requires a vast time for a whole design process. 
         [0012]    In the method disclosed in the Japanese publication of unexamined application 2005-10835, the calculation of the creepage distance requires the process of transforming the three dimensional model to a correspondent approximate polyhedron model and so on. The calculation in the process of transforming and the like are too vast to obtain the creepage distances between the front and back element on the PCB. The disclosed method in the application is suitable for analyzing a object having a complicate structure. In case of an object having a simple structure such as a rectangular contour of PCBs, it is desired to obtain in a short time the distances between elements on the PCB by a simple calculation such as summation of a distance from an element to a side of the PCB, the thickness of the PCB, and the distance from the side to other element. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention is provided to solve the problems described above. The present invention is capable of detecting accurately the elements between which a creepage distance should be calculated, and of improving the time for processing the calculation of the distances. Furthermore, the object of the present invention is to provide a computer aided design system capable of accurately calculating the creepage distances in a short time. 
         [0014]    According to one aspect of the present invention, there is provided a method for supporting design of a printed circuit board for mounting a plurality of elements disposed on both the major surfaces of the printed circuit board, respectively, the method comprising, a step of creating a copy and a mirror copy of plan views of the major surfaces of the printed circuit board, respectively, a step of performing a calculation for obtaining a shortest straight line distance between one of the elements appearing in the copy and another of the elements appearing in the mirror copy when the copy and the mirror copy are placed side by side across a null thickness of the printed circuit board; and a step of obtaining a creepage distance between the two elements, the creepage distance being equal to a summation of a value of the thickness and a value of the shortest straight line distance. 
         [0015]    According to another aspect of the present invention, there is provided an apparatus for supporting design of a printed circuit board for mounting a plurality of elements disposed on both the major surfaces of the printed circuit board, respectively, the apparatus comprising: means for creating a copy and a mirror copy of plan views of the major surfaces of the printed circuit board respectively, the copy and the mirror copy being placed side by side across a null thickness of the printed circuit board; means for calculating a creepage distance between one of the elements appearing in the copy and another of the elements appearing in the mirror copy, the creepage distance being equivalent to a summation of a distance and a thickness of the printed circuit board, the distance being a shortest straight line distance, the shortest straight line distance being between the one of the elements appearing in the copy and the another of the elements appearing in the mirror copy; and means for extracting the one of the elements appearing in the copy, the one of the elements appearing being within the creepage distance from the another of the elements appearing in the mirror copy, the creepage distance being less than a predetermined distance. 
         [0016]    According to still another aspect of the present invention, there is provided, a method for computer-aided design concerning this invention comprising a mirror copy procedure around a side of the PCB, where the PCB board has the width component, depth component, and board thickness component, and the procedure of calculating the shortest distance in a straight line between the back target element to be calculated on the back surface of the PCB and the element which is a mirror-copied, where it is supposed that there is nothing about the component of the direction of board thickness. And a calculating procedure for obtaining the creepage distance by the summation of the shortest distance in a straight line and the thickness of the PCB. Thus, the distance in a straight line between the back target element and the mirror copied element is obtained under the condition of the thickness od the PCB equal to zero, and then the creepage distance is obtained by adding the thickness disregarded of the PCB to the distance. Accordingly the creepage distance can be obtained correctly and quickly by calculating a creepage distance as a distance on a straight line. 
         [0017]    According to yet another aspect of the present invention, there is provided a method for computer-aided design including a mirror copy procedure around a side of the PCB, where the PCB board has the width component, depth component, and board thickness component, and the procedure of calculating the shortest distance in a straight line between the back target element to be calculated on the back surface of the PCB and the element which is a mirror-copied, where it is supposed that there is nothing about the component of the direction of board thickness. And the method includes a procedure in which a creepage distance is obtained as the summation of the shortest distance in a straight line and the thickness of the PCB. The method further includes a procedure in which elements having a creepage distance within a predetermined distance are extracted. Accordingly, in the present invention, since the calculated creepage distance between a front element and a back target element is compared with the predetermined distance and the elements apart within a creepage distance less than the predetermined distance are extracted, the elements arranged incorrectly automatically and quickly can be specified. 
         [0018]    According to yet still another aspect of the present invention, there is provided a method for computer-aided design comprising; a mirror copy procedure for mirror copying symmetrically a target element about a side of a board of a PCB to be calculate, the target element being on a front surface of the board, where the board has the width component, depth component, and board thickness component; a procedure of detecting a back element on a back surface of the board in a distance from a point on the mirror copied element, the distance being a distance subtracted the board thickness from a predetermined allowable distance, supposing that there is nothing about a board thickness component; a procedure of calculating a shortest distance in a straight line of the back element and the mirror copied element, and of adding the thickness to the shortest distance to obtain a creepage distance, supposing that there is nothing about the board thickness component; and, a procedure of extracting element within a distance less than the predetermined allowable distance. Thus, the creepage distance is found by using only the back target element which is in the distance subtracted the board thickness from the predetermined allowable distance from the point on the mirror copied element, supposing that there is nothing about the board thickness component. Accordingly, procedure for finding the creepage distance is performed only for specified elements not for all the back elements. Therefore, the method can avoid unnecessary calculation and extract quickly elements in incorrect arrangement. 
         [0019]    According to above aspect of the present invention, there is provided the method comprising; when the element in the creepage distance less than the predetermined allowable distance is extracted, an intersection of the straight line from the mirror copied element to the back target element and a side of a board of a PCB is calculated as a turning point, and a procedure of outputting two line segments as a path to means for displaying, the two line segments comprising a first line segment from the target element to the point and a second line segment for the point to the back target element as a path; and, a procedure of displaying the path. Thus, the turning point is obtained by use of the mirror copied element, and the path is outputted to means for displaying, the path comprising the two line segments from the target element to the point and the point to the back target element, and the target element and the back target element are also displayed. Accordingly, a user grasps which element is incorrect arrangement easily, even can grasp the path associated with the incorrect arrangement. When a plurality of paths are displayed, the element in severe incorrect arrangement can be grasped especially as incorrect arrangement. 
         [0020]    The present invention also provides a computer-aided design system comprising; means for obtaining an element mirror-copied from a first element, the first element disposed on one of two surfaces of a printed circuit board, the element mirror copied and the first element being symmetrical about a selected side of the printed circuit board; means for calculating a creepage distance between the first element and a second element disposed on other of the two surfaces, the creepage distance being equivalent to a summation of a distance and a thickness of the printed circuit board, the distance being a shortest straight line distance under a condition, the shortest straight line distance being between the element mirror-copied and the second element, the condition being a null thickness of the printed circuit board; and means for extracting the second element, the second element being within the creepage distance from the first element, the creepage distance being less than a predetermined distance 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  shows schematically a block diagram showing the configuration of a printed circuit board design support apparatus of the present invention; 
           [0022]      FIGS. 2A to 2F  show data table structures concerning the embodiment of the present invention; 
           [0023]      FIG. 3  shows a diagram of selection of the front element concerning the embodiment of the present invention; 
           [0024]      FIG. 4  shows a diagram of a mirror coping of the front element concerning the embodiment of the present invention; 
           [0025]      FIG. 5  shows a diagram of selection of the back target element concerning the embodiment of this invention; 
           [0026]      FIG. 6  shows a diagram of the creepage distance calculation concerning the embodiment of the present invention; 
           [0027]      FIG. 7  shows a diagram of the path of the element group concerning the embodiment of the present invention; 
           [0028]      FIG. 8  shows a diagram for deriving the path of the element group concerning the embodiment of the present invention; 
           [0029]      FIG. 9  shows a flowchart which shows an example of extracting processing of the incorrect arrangement of elements; 
           [0030]      FIG. 10  shows a flowchart which shows an example of extracting processing of the incorrect arrangement of elements; 
           [0031]      FIG. 11  shows a flowchart which shows an example of confirming processing of the incorrect arrangement component after relocation concerning the embodiment of this invention; 
           [0032]      FIG. 12  shows a diagram showing the front element moved by edit concerning the present invention; 
           [0033]      FIG. 13  shows a diagram of recalculation of the creepage distance; 
           [0034]      FIG. 14  shows a diagram of re-derivation of the path; 
           [0035]      FIG. 15  shows an outline diagram of all the processes centering on operation of the user of the present invention; 
           [0036]      FIGS. 16A and 16B  show diagrams of an example the circuit network concerning of the present invention; 
           [0037]      FIG. 17  shows a diagram of calculation of creepage distance in case a hole exists in the board concerning the present invention; 
           [0038]      FIG. 18  shows a diagram of another method of calculation of creepage distance concerning the present invention; 
           [0039]      FIGS. 19A and 19B  show diagram of calculation of the creeping distance in prior art; and 
           [0040]      FIGS. 20A and 20B  show diagram of calculation of the creeping distance in prior art. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0041]    The preferred embodiments are explained with reference to FIGS. The present invention can be realized in various embodiments. Therefore, the present invention should not be recognized within the following embodiments. The explanation is directed to the system, while the present invention can be embodied also as a program to be able to be used in a computer or as a method by, so-called, the person skilled in the art. The present invention can be embodied as hardware, software, or combination of software and hardware. The program can be stored in a hard disk, a compact disk ROM, a digital video disk ROM, an optical magneto storage device, magnetic storage device, and a computer readable medium. Further, the program can be stored other computer via a network. 
         [0042]    A preferred embodiment of the present invention is shown in  FIG. 1  as a computer-aided design system (CAD system)  1 . In  FIG. 1 , essential parts of the CAD system  1  are shown as a block diagram, and the system  1  may be embodied with a digital computer such as a personal computer or a work station. The CAD system  1  comprises a standard CAD system  100 , an input device  10 , a means for selecting a front element  20 , a means for mirror-copying  30 , a means for selecting back element  40 , a means for calculating a distance  50 , a means for selecting a pair of elements in incorrect layout  60 , a means for displaying the pair in incorrect layout  70 , an output device  80 , a means for detecting a path on elements  91 , a means for detecting element defining the path  92 , a means for confirming element to be edited  93 . The standard CAD system  100  can perform functions of enlargement, reduction, rotation, and the like which are used in designing a PCB, and the explanation on the standard CAD system  100  is abbreviated, because it is well-known, and is properly realized by various technologies. 
         [0043]    The input device  10  reads out data for designing a figure, such as a board, elements disposed on the PCB, dimensions of the board, from a storage device such as a device for storing temporal data  96 , a database  97 , or the like. And the input device  10  can receive data, such as data for designing a figure, from the standard CAD  100 . And then the input device  10  sends the data to the means for selecting a front element  20 , the means for mirror-copying  30 , the means for selecting back element  40 , the means for calculating a distance  50 , the means for selecting a pair of elements in incorrect layout  60 , the means for displaying the pair in incorrect layout  70 , the means for detecting a path on elements  91 , the means for detecting element defining the path  92 , the means for confirming element to be edited  93 . 
         [0044]      FIG. 2  A to  FIG. 2F  show schematically formats of data stored in a database  97  for the CAD system  1  for designing a PCB in the embodiment.  FIG. 2A  shows the format of common data  310 , such as the thickness of the PCB  312 , the predetermined allowable distance  314 , and the attribution  316  of storage location of the data such as the thickness and the allowable distance.  FIG. 2B  shows the information of the board  320  to be calculated. The format of the information of the board  320  includes each attribution of the identification code  322  assigned to a shape of the board of the PCB to be calculated, the identification data  324  for identifying contour or hole, and the shape data  326  of the board. The “contour” in the attribution of the identification data  324  shows a side in the external shape, and the hole shows the side of the hole formed within the board.  FIG. 2C  shows the information regarding a front element to be calculated concerning to a side. The format of the information  330  includes each attribution of the identification code or the code  332  for discrimination, the identification code of the mirror-copied element  334 , the name of net in circuit  336 , and the shape data of the mirror-copied element  338 . The database includes a table for all front elements, the table has each attribution of the identification data, the name of net in circuit, and the shape data of front element. 
         [0045]      FIG. 2D  shows the information for the back elements  340 , as objects for calculating, which are detected on the basis of the shape of the mirror-copied element. The information includes each attribution of the identification code of the back element  342 , the name of net  344  in a circuit, and the shape data of the back element  344 . The database includes also a table for all back elements, the table has each attribution of the identification data of the back element, the name of net in circuit, and the shape data of back element. In this embodiment, the table for resultant calculation as shown in  FIG. 2E  includes each attribution of the identification codes of front elements  352  and back elements  354 , the calculated creepage distance  356 , and the path information  358 . The table  360  shown in  FIG. 2F  includes the attribution of the initial points  362  and the terminal points  364  and the geometrical data of the calculated path  366 . 
         [0046]    The means for selecting the front element  20  has a function which selects element possibly not to be compatible regarding the creepage distance with a requirement in a specification or a standard, where the element has been arranged on the surface as the front one selected from the both surfaces of the board. In detail, one side of the board is selected, and then the elements located within an modified allowable distance normal to the side are detected, where the modified allowable distance is the allowable distance minus the thickness of the board  312  in  FIG. 2A .  FIG. 3  shows a diagram for explanation of selecting a front element based on the embodiment. The area within the modified allowable distance L′  420  in normal direction to a side  410  of the board  400  is shown as a rectangle  428  drawn with a dash-single dot line. Among the front elements  422 ,  424 ,  426 , the front element  422  is selected as the front target element by the means for selecting the front element  20 , because only the front element  422 , hereinafter referred as to the front target or target element  402 , is located within the rectangle  428 , where the front target element  422  is enclosed by an ellipse drawn by a dash-single dot line. 
         [0047]    The means for mirror-copying  30  has a function creating an image of a turned over element associated with the front element around a side of the board as a symmetrical axis for turning over. The image, hereinafter referred as to the turned over element or the mirror copied element, is symmetrical with the front target element  422  around the side.  FIG. 4  shows a diagram for explanation of mirror-copying a front target element  422  based on the embodiment. The shape design data sent from the input device  10  ( FIG. 1 ) includes the shape data of the board  400  and the element shape data of front target element  422 . By the function of the means for mirror-copying  30 , the element shape data of the turned over element (mirror copied element  434 ) is obtained based on the target element shape data selected by the means for electing the front element  20 . In the  FIG. 4  the front target element  422  and the mirror copied element  434  are apart LM from the side  410  and L′ shows the modified allowable distance. 
         [0048]    The means for selecting back element  40  has a function which selects the back element, which are arranged on the back surface of the board, possibly not to be compatible regarding the creepage distance with a requirement in a specification or a standard. An area including back target element to be detected is obtained, because the means for mirror-copying  30  specifies the mirrored target element and the modified allowable distance L′ is known.  FIGS. 4 and 5  show diagrams for explanation of selecting a back element based on the embodiment. The area  435  to be searched, which is drawn with a dash-single dot line in  FIG. 4 , is ranging from the mirror copied element  434  to the modified allowable distance L′ in directions of west, east, and in direction toward to the board  400 . Further for more accurate area, the area  433  drawn with the dash-double dots line can be selected as the area to be searched. The means for selecting back element  40  detects as the back target elements back elements  440 ,  442 , and  446 , which are cross-hatched and enclosed with each ellipse drawn with broken line, since they have an intersection with the area  435  as shown in  FIG. 5 . 
         [0049]    The means for calculating a distance  50  has a function which calculates the minimum distance from the mirror copied element  434  to the back target element.  FIG. 6  shows a diagram for explanation of calculating a creepage distance based on the embodiment. Each minimum distance between one mirror copied element  434  to three back target elements  440 ,  442 ,  446  is shown with the arrows respectively. The means for calculating a distance  50  calculates each distance shown by the arrows  436 ,  436 ′,  436 ″ and adds the value of the thickness of the board  400  to the each value of the distance to obtain the respective creepage distance. Each element such as  440 ,  442 ,  446 , and  446  has in detail a predetermined area. For example, in case of the conductive pattern for use of electrical connection, the area can be defined with two straight lines and two circular arc of a half circle. The algorithm to obtain the minimum distance between two area having each region described above is well know to design and realize a method to calculate the minimum distance between two area by a skilled person in the art. However the present embodiment uses the method described below, from the view point of a high speed processing. And the method is not well known. At first, the area of one of the elements is transformed into a segment. And the minimum distance between the other area and the segment is calculated by use of the well known algorithm. Or the area regarding two element are transformed into each segment, and the minimum distance between the two elements is calculated by use of the well known algorithm. Furthermore by use of the well known algorithm, it is possible to obtain the minimum distance between each end of the segment and the area of the other element or the segment obtained by transforming the other area. 
         [0050]    The means for selecting a pair of elements in incorrect layout  60  has a function which compares the allowable distance and the creepage distance obtained by the means for calculating a distance  50 . Comparing the two distances, the front target element and the back target element is decided as a pair of elements not compatible with requirement in a specification or a standard if the creepage distance shorter than the allowable distance. 
         [0051]    The means for displaying the pair in incorrect layout  70  has a function which obtains a path between a pair of the target element and back target element detected by the means for selecting a pair of elements in incorrect layout  60 .  FIGS. 7 and 8  show diagrams for explanation of deriving a path in the embodiment. In detail, the intersection of the path  436  and the side  410  of the board  400  is obtained as a turning point  450 , where the path  436  corresponds to the minimum straight line distance of the mirror copied element  434  and the back target element  440 . And the point on both the path  436  and the mirror copied element  434  is obtained. And next, an intersection of the straight line  451  and the front target element  422  is obtained, where the straight line  451  crosses the point on the mirror copied element  434  and is normal to the side  410 . The intersection also can be analyzed by the calculation for mirror copying the front element around the side  410  as a symmetrical axis, because the intersection is corresponding to the point on the mirror copied element  434 . As a result, the line is obtained as a path  452  in  FIG. 8 , where line crosses the point on the back target element  440 , the turning point  450 , and the point on the front target element  422 . 
         [0052]    The output device  80  outputs the data of the object side, such as the side  410 , the front target element, such as  422 , the back target element, such as  440 , and the path, such as the path  452 , which are obtained with the means for selecting a front element  20 , the means for mirror-copying  30 , the means for selecting back element  40 , the means for calculating a distance  50 , the means for selecting a pair of elements in incorrect layout  60 , the means for displaying the pair in incorrect layout  70 , the means for detecting a path on elements  91 , the means for detecting element defining the path  92 , the means for confirming element to be edited  93 . The data is output to the means for displaying data, such as the display  98  in  FIG. 1 , a main memory device (not shown in  FIG. 1 ), other storage device, such as the device for storing temporal data  96 , or a printing device (not shown in  FIG. 1 ). 
         [0053]    The means for detecting a path on elements  91  has a function which searches figure data of the path to obtain the path passing the coordinate expressing the element. The means for detecting element defining the path  92  has a function which searches figure data to obtain the element indicated by the path. The means for confirming element to be edited  93  has a function which decides whether the element relocated by an operator is an element in incorrect layout or in proper layout. 
         [0054]    The front and back surfaces of the board are apart each other at a distance of the thickness of the board. Therefore, each element arranged on the both surfaces has the different coordinate respectively in the direction of the thickness of the board. Usually, the reference of the direction of thickness is given at the back surface of the board, so the coordinate in the direction of the thickness of the back target element is zero, and the coordinate of the element on the front surface equals to the thickness of the board. Therefore, the neglect of the coordinate equivalent to the thickness of the board brings that each element arranged on the both surfaces of the board is considered on the same plane, such as the same surface of the board. Then the creepage distance between the front element and the back target element can be obtain as the summation of the distance between the mirror-copied element of the front element and the back target element and the thickness of the board. If the coordinate of one of the front or back surfaces is not zero but the front and back surfaces are parallel, it is possible to obtain the creepage distance can be obtain in the same manner described above. 
         [0055]      FIG. 9  shows an example of a flowchart of a process for selecting a pair of elements in incorrect layout. With referring to  FIGS. 1 and 9 , the process is explained. In the present embodiment, the process starts at Step  200  and begins substantially a process at Step  201 . At Step  201 , the means for selecting a front element  20  detects a front element placed within a region which occupies area within a distance equal to the allowable distance minus the thickness from the selected side of the board. When a plurality of the front elements are detected, the each identification code corresponding to the each front element is stored as a front target element in the device for temporarily storing data  96  in  FIG. 1 . And in Step  202 , the means for mirror-copying  30  mirror-copies the front target element around the selected side as a symmetrical axis, where one of the front target elements is select in the case of the resultant plural front target elements. In Step  203 , the means for selecting back element  40  specifies the mirror copied element and a region to be detected within the modified allowable distance from the mirror copied element. In Step  204 , the means for selecting back element  40  detects as a back target element a back element within the specified region. When a plurality of the back target elements are detected, the each identification code corresponding to the each back target element is stored as the back target element in the device for temporarily storing data  96  in  FIG. 1 . In Step  205 , the means for calculating a distance  50  calculates the shortest straight line distance from the mirror copied element to the back target element, where one of the back target elements is selected for the calculation if the back target elements are plural. And in Step  206 , the means for calculating a distance  50  sums the thickness of the board and the shortest straight line distance to obtain the creepage distance. In Step  207 , the means for selecting a pair of elements in incorrect layout  60  compares the obtained creepage distance with the allowable distance and decides whether the creepage distance is shorter than the allowable distance or not. In Step  207 , the creepage distance is longer than the allowable distance, the process jumps to Step  211 , while the output device  80  outputs the pair of the element to the memory in Step  208  in the case of the creepage distance being shorter than the allowable distance in Step  207  the process. And the process goes to Step  211 . 
         [0056]    In Step  211 , it is judged whether the back target element is last the last one or not. When it is judged, in Step  211 , the back target element is not the last in the case of the back target element detected in Step  204  being plural, other back target element is taken into as the back target element in Step  212 , and then the process returns to Step  205 . These steps can obtain every creepage distance for the all back target elements detected in Step  204  and can detect any pair in incorrect layout. 
         [0057]    When in Step  211  it is judged that the back target element is the last one, the process goes to Step  221 , where it is judge whether the target element processed in the Step  221  is the last one or not. When in Step  222  it is judged that the target element is not the last one in the case of the plural target elements being detected in Step  201 , other target element is taken into as the target element and the process returns to Step  202 . These steps can obtain every creepage distance for the all target elements and back target elements detected concerning a side of the board, and can detect any pair in incorrect layout. 
         [0058]    When in Step  221  it is judged that the target element is the last one, the process goes to Step  231 , where it is judged whether the side is the last one or not. Usually the board has at least four sides, then it is necessary to check whether there are any pair of the front and back elements in incorrect layout concerning to each of the sides. Therefore, in Step  231 , when it is judged that the side is not the last one, other side is taken into as the side and the process returns to Step  201 . 
         [0059]    All the pairs of front and back elements in incorrect layout about the relation between the front and back surfaces on the PCB are extracted at the above Step  232 . Next, it searches for one path at a time from these pair of front and back elements. First, the intersection of the target side of the board and the shortest straight line from the mirror copied element and the back target element, where the shortest straight line is one used to obtain the creepage distance, is obtained as a turning point (Step  241 ). Next, the means for displaying the pair in incorrect layout  70  identifies the point on the target element corresponding to the intersection of the shortest straight line and the intersection on the mirror copied element in Step  242 . In Step  243 , the output device  80  outputs the each data of the point on the back target element, the turning point, and the intersection on the target element. In Step  244 , it judges whether it is the last pair of elements, if it is the last, the process will end, and if it is not the last, the process will return to Step  241  for the following pair of elements. 
         [0060]    The information on the elements which are derived from the above process, such as the mirror copied element, the back target element, the creepage distance, the path exists on memory, and the output device  80  stores the information in the database. Although the derivation time of each information is sufficient as the timing of storing, it is more efficient to store the information on memory collectively. 
         [0061]    The CAD standard means  100  receives the output of the pair of elements and a path, and specifies and displays on the PCB the front element, the path, and back element, where the a pair of the front and back elements is in a incorrect layout. Changing a color and displaying the front and the back elements which serve as incorrect layout compared with the elements used as correct layout, are displayed for example. Therefore a user can recognize that to display the path shows the pair of the elements in incorrect layout. Although the elements and the path is displayed on the display through the CAD standard means  100 , the output device  80  can also display directly them on a display to specify them on the screen. Furthermore, the pair of elements in incorrect layout and the path concerning not all sides but a side which is specified by the user can be displayed on the screen to decrease the load to display them on the screen from a view point of processing in the output devices  80 . Furthermore, the back target element in incorrect layout and the path concerning a target element specified by the user can be displayed, and the reduction of incidence of a display process also can be performed. When the user actually rearranges the surface element in incorrect layout, as described later, and just a display in the required range is enough. 
         [0062]      FIG. 11  is the flow chart which shows an example of the flow of the pair of elements in incorrect layout confirming processing after relocation in the embodiment of this invention. The user is able to edit an element, where the said pair of elements in incorrect layout component group and the associated path are displayed on the screen. In response to the editing operation from the user, the CAD standard means  100  performs a display change processing for migration of the element and CAD data changing. With treatment of the CAD standard means  100  in regard to the editing operation to the element, the creepage distance calculation is performed again, and the process is performed to confirm that the element remains in incorrect layout or not by a comparison with the allowable distance. Specifically, it is as follows. That is, the front element which serves as an editing element by the CAD standard means  100  in response to the editing operation starting in Step  250  by the user for the front element is specified first (Step  251 ). The means for detecting a path on elements  91  detects a path crossing the coordinate of the specified element (Step  252 ). ID of the element which the detected path indicates is detected from the graphic data of a path by the means for detecting element defining the path  92  (Step  253 ). In response to a user&#39;s editing operation, the CAD standard means  100  performs editing processing of the front element as an editing object (Step  254 ). For example, a front element  422  moves from the display of the state of  FIG. 8 , and it changes to the moved element  460  of the state of  FIG. 12 . ID of the target element moved by the means for confirming element to be edited  93  is judged whether the ID is in agreement with ID of the target element obtained at Step  253  (Step  255 ). When there is a plurality of element IDs, the process for the judgement is repeated. 
         [0063]    The process ends when in Step  255  the means for confirming element to be edited  93  judges that the ID is not in agreement with ID of the target element obtained at Step  253 . In the case of the target element crossing the path associated with other target element, the ID of the element obtained in the process of Step  253  may not agree with the ID of target element. 
         [0064]    When the means for confirming element to be edited  93  judges in Step  255  that the ID agrees with ID of the target element obtained, the mirror copying means  30  creates the mirror copied element  470  of the front element  460  centering on the side  410  (Step  261 , and see  FIG. 13 ). For example, it becomes as it is shown in FIG.  13  by the mirror copy of the front element  460  edit. The distance calculation means  50  calculates the shortest straight line distance  472  from the mirror copied element  470  to the back target element  440 , where one back target element is chosen when there are a plurality of back element to be consider (Step  262 ). The distance calculation means  50  adds board thickness to the found shortest straight line distance, and finds creepage distance (Step  263 ). The creepage distance obtained by the means for selecting a pair of elements in incorrect layout  60  is compared with the allowable distance. In Step  264  it is judged whether the creepage distance is smaller than the allowable distance or not. In Step  264 , if the means for selecting a pair of elements in incorrect layout  60  judges the creepage distance is shorter than the allowable distance, the process goes to a path update process which has been defined (Step  270 ). However, in Step  264 , if the means for selecting a pair of elements in incorrect layout  60  judges the creepage distance is longer than or equals the allowable distance, the process goes to the path elimination procedure (Step  280 ) which has been defined. 
         [0065]    In a path update process (Step  270 ), treatment of said step  241  or Step  243  is made, a new path  476  is generated, and an old corresponding path is deleted. If change is reflected in a display through these treatment, it will become as it is shown in  FIG. 14 . In a path delete process (Step  280 ), an old corresponding path and a corresponding component group are deleted. The path on memory and a component group are updated, and also the information in a database is updated by the outputting part  80 . 
         [0066]    After termination of Step  270  or  280 , it will be judge whether the back target element processed is the last back target element or not (Step  291 ). When it is judged that the back target element is the last one, the process ends and shifts under control of the CAD standard means  100 . When it is judged at said step  291  that the back target element is not the last one, the process returns to Step  261  for the following back target element. 
         [0067]    The path and the element specified by the path is not read from a database at Step  252  and Step  253 , but the path and the element are specified from graphic data, because searching them from graphic data can process at high speed rather than obtaining through access to a database. However, the method realized by access to a database depending on the environment of the computer system to build may be able to process at high speed. 
         [0068]      FIG. 15  is an outline diagram of all the processes focusing on operations of a user. First, the user designs by performing an editing task, displaying graphic data  500  on the display  98  using the CAD standard means  100  (( 1 ) in  FIG. 15 ). The user directs measurement of creepage distance, and judgment of incorrect arrangement to the feature section of this invention after termination of the design (( 2 ) in FIG.  15 ( 2 )). Creepage distance is found through the process which the feature section of this invention described above, the component of incorrect arrangement is extracted, and the path of the component of incorrect arrangement is calculated. And the path of the component of incorrect arrangement is displayed. To the display of the path, the user checks a resultant measuring and judges the need for revision (( 3 ) in  FIG. 15 ). In this task, when it is judged that there is no necessity for revision, the task ends. If there is the necessity for revision, the editing task of the PCB will be performed (( 4 ) in  FIG. 15 ). Usually, the position of parts, such as an electro-conductive pattern and an electric component, is changed, or the shape of an electro-conductive pattern is changed. The user directs re-measurement of creepage distance, and re-judgment of incorrect arrangement to the feature section of this invention after termination of design revision (( 5 ) in  FIG. 15 ). Accordingly, creepage distance is found through the process which the feature section of this invention described above, the component of incorrect arrangement is extracted, and the resultant update path of the component of incorrect arrangement is displayed. To the resultant display on the screen, the user reconfirms a resultant measuring and re-judges the need for revision (( 6 ) in  FIG. 15 ). The design of the PCB is completed by repeating the work of ( 4 ) to ( 6 ) in  FIG. 15  if needed. 
         [0069]    [On the target element] Also in the incorrect arrangement component confirming processing after edit, Step  203  and Step  204  can also be performed, and the creepage distances between the target element and all the associated back components can be found, and it can also be judged whether there is incorrect arrangement or not. In this embodiment, the creepage distance and the judge for the incorrect or correct arrangement only for the component indicated already by the path are performed from the viewpoint of the response performance of a system. 
         [0070]    [Composition Other than the Composition Called if Needed by the Additional Function of the CAD Standard Means  100 ] 
         [0071]    Although the system configuration that the feature section is called to the CAD standard means  100  is explained in this embodiment. But it is not necessary to compose a module division clearly to the CAD standard means  100  and a feature section, and the feature section of the present invention completely included in the CAD standard means  100  as well as the another module which constitutes the CAD standard means  100  can be realized. 
         [0072]    [In the Case of the Surfaces of the PCB being not Parallel to the x-z Plane] 
         [0073]    The system adopts the coordinate (x, y, z), where x component corresponds to the width component of the PCB, z-component corresponds to a depth component, and y-component corresponds to a board thickness, the surface of the PCB is parallel to the x-z plane. In this coordinate, the creepage distance can be obtained by neglect of the component of the thickness as described above. In the case of the x-z plane and the surface of the PCB being not parallel each other, the creepage distance can be similarly obtained by re-configurating in the xyz space which uses the width component of the PCB as x component for the component used for calculating the creepage distance, uses a depth component as z component, and uses a board thickness component as y component. The creepage distance calculation also with same otherwise also carrying out parallel projection of the component used for the creepage distance calculation to the surface or the back of the printed circuit board is applicable. 
         [0074]    [An Element, the Side, a Circuit Network, and Limitation of the Range] 
         [0075]    The target element is detected in this embodiment using the side and the distance which subtracted board thickness from the allowable distance, and the back target element is found by using the distance from the mirror copied element, where the distance is one subtracted the board thickness from the allowable. Furthermore, an element and the side can be limited. That is, the user specifies the element and the side to be checked, and the creepage distance can be found corresponding the element and the side, and it can be judged whether the element is in incorrect arrangement or not. In detail, the user can specify a certain front element, a certain back element, or a pair of a certain front and back elements, and a certain side. Thus, the user can search quickly the result by performing the each operation described above only for the elements and the side specified by the user without performing unnecessary operation. 
         [0076]    An element and the side can be specified and also the user can also specify a circuit network. The circuit network is formed when carrying out the logical design in which each of the contacting portions of parts, such as an electric element, is connected by wiring or electro conductive pattern. The user can decide a circuit net name. The potential within the wiring or the electro conductive pattern between the contacting portions of parts is same. A text file called a net list (there are various formats) may be created and employed. The net composition on a circuit is shown in  FIG. 16A , where  530 ,  540 ,  550  show each circuit network. In a CAD system for designing a PCB, wiring between actual parts is carried out based on the information of logical connection between the parts designed by a CAD system for designing a circuit.  FIG. 16  B shows the net composition on the PCB. Compared with  FIG. 16A , physical composition understands easily by  FIG. 16B . (b). In this  FIG. 16B , the electro-conductive patterns  533 ,  535 ,  543 ,  544 ,  553  and the pads  532 ,  534 ,  536 ,  542 ,  545 ,  552 ,  554  are illustrated as elements. Moreover, only the elements within an area, for example a rectangle area, specified by the user can be calculated for the creepage distance, and it can also be judged whether the element is located in incorrect arrangement. 
         [0077]    [In the Case of the PCB Having a Hole] 
         [0078]    The side constituting the board is not restricted to the border of the outside of the PCB. As shown in  FIG. 17 , for example, the side  621  of a hole  630  formed within the PCB  600  is included in the sides to be considered. The creepage distance with the back element  620  detected as a back target element will be calculated by performing said each process of this embodiment to the front element  614  arranged near the hole  630 . In this case, although the back component  620  and the back component  622  are arranged as the back elements near the hole, the back element  620  may become a back target element. This is because in the case the detection area of detecting the back target element is the range of the distance which is a distance L′ of the allowable distance minus the thickness of the board in the direction of this side shown with an arrow A on the basis of the mirror copied element  616 . By setting the area drawn with a dash-single dot line for detecting in this way, the incorrect calculation of the shortest distance in a straight line between a back element  622  and the mirror copied element  616  is not carried out, therefore incorrect extraction of a pair of the element in incorrect arrangement is not performed. When the allowable distance is large, the back element  622  may turn into a back target element, but it can prevent calculating the mistaken creepage distance by comparing the distance to a target element and a back target element with the distance from the front element to be mirrored to a back target element in such a case. Here, when finding the distance of a target element and the back target element w, the distance is found by adding the thickness of the board to the shortest distance in a straight line not including the thickness without using a mirror copying. 
         [0079]    [How to Find Another Creepage Distance] 
         [0080]    The board thickness is added to the shortest distance in a straight line of the mirror copied element to the back target element to find the creepage distance in this embodiment. However, as shown in  FIG. 18 , when a target element  660  and the back target element  672  do not exist on the same vertical line of the side of the PCB, it is possible to calculate the creepage distance as a following way. Neglecting the thickness of the board, the turning point α is selected, where the point α divides the segment between a segment into the ratio (A:B=a:b) of the distance A from the target element  660  to the side  652  of the PCB  650  and the distance B from the back target element  672  to the side  652  of the PCB  650 , the segment is a straight line connecting the each end of the projected line segment  662  of the target element  660  to the side  652  and the projected line segment  674  of the back target element  672  to the side  652 . The distance A 1  from the target element  660  to the point α and the distance A 2  from the point α to the back target element  672 , where the thickness of the board also is neglected. And then the sum of A 1 , A 2 , and the thickness equals to the creepage distance. When the target element  660  and the back target element  672  on the back surface of the PCB  650  is normally projected on the side  652  and each of the line segments  662 ,  674  projected overlaps each other, the creepage distance is found by the method performed conventionally. 
         [0081]    Although said each above embodiment explains the present invention, it is possible for the technical scope of this invention not to be limited to the range given in an embodiment, but to add various change or improvement to each embodiment. The embodiment including such change or improvement is also contained in the technical scope of this invention. This is clear also from claims.