Patent Publication Number: US-7719124-B2

Title: Printed wiring board for mounting components

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority under 35 U.S.C. §120 from U.S. patent application Ser. No. 11/372,087, filed on Mar. 10, 2006, which claims the benefit of priority under 35 U.S.C. §119 (a)-(d) from prior Japanese Patent Application 2005-078496, filed on Mar. 18, 2005; the entire contents of all of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a technique for printing solder paste with use of a printing machine and mounting a component with use of an automatic mounter onto a printed wiring board including a board recognition mark, and particularly relates to a technique for properly mounting components on a printed wiring board including a component land covered with resist (hereinafter also referred to as an over resist type component land) and a component land not covered with the resist (hereinafter also referred to as a normal resist type component land) even if resist misalignment occurs. 
   2. Description of the Related Art 
   In recent years, printed wiring boards with “0603 size” small chip components mounted thereon have been increasing. The “0603 size” small chip components were developed in response to demand for components with smaller size and higher density, and have a size of 0.6 mm in length and 0.3 mm in width. 
     FIG. 8A  shows a related art printed wiring board, wherein normal resist type component lands  5   a  and  5 ′ are exposed through resist openings  8   a  and  8 ′ a  having sizes greater than the component lands  5   a  and  5 ′ a , respectively. Conductive patterns  17   a  and  18   a  are extending from the component lands  5   a  and  5 ′ a , respectively. 
   If a resist printing mask (not shown) used for forming resist  2   a  is positioned rightward relative to a predetermined location, an area S 2   a  of the conductive pattern  18   a  exposed through the right resist opening  8 ′ a  is larger than an area S 1   a  of the conductive pattern  17   a  exposed through the left resist opening  8   a . That is, the substantial area of the component lands  5   a  is not equal to the substantial area of the component land  5 ′ a.    
   Referring to  FIG. 8C , when solder pastes  19   a  and  20   a  printed on such component lands  5   a  and  5 ′ a  are heated and melted by a reflow machine, a chip component  15   a  as small as the “0603 size” chip components might be pulled toward the component land  5 ′ a  having the larger substantial area and be partly lifted. 
   Referring then to  FIG. 8B , to avoid such mounting failure, component lands  6   a  and  6 ′ a  having the same area by covering peripheries of conductive foils  22   a  and  22 ′ a  with the resist  2   a  are formed with an over resist method. The component lands  5   a  and  5 ′ a  formed by a normal resist method and the component lands  6   a  and  6 ′ a  formed by the over resist method are present on the same printed wiring board. 
   When a normal resist type component land and an over resist type component land are present on a printed wiring board, printing misalignment and mounting misalignment might occur for the following reasons. 
   The reasons of the occurrence of the printing and mounting misalignment are described with reference to a printed wiring board  1   a  of  FIG. 5  including a board recognition mark  3   a  formed with the normal resist method, wherein rightward and downward misalignment of the resist  2   a  exists. 
   A resist printing mask (not shown) is placed on the printed wiring board  1   a  including the board recognition mark  3   a , the normal resist type component land  5   a , and the over resist type component land  6   a . The resist  2   a  is applied onto the printed wiring board  1   a  through the mask. 
   The resist opening  8   a  for the normal resist type component land  5   a  has a size large enough to accommodate the maximum resist misalignment resulting from misalignment between the printed wiring board  1   a  and the mask, so as to prevent the component land  5   a  from being covered with the resist  2   a . Accordingly, the center point of the component land  5   a  is not affected by the resist misalignment, so that the positional relationship between the center point of the component land  5   a  and a center point  10   a  of the board recognition mark  3   a  is consistent with the design value of the printed wiring board  1   a.    
   On the other hand, since the center point of the over resist type component land  6   a  is defined by the center point of a resist opening  9   a , the center point of the component land  6   a  is displaced from a predetermined center point of the component land  6   a  by an amount corresponding to the resist misalignment. 
   This displacement is equal to the displacement ΔX, ΔY between the center point  10   a  of the board recognition mark  3   a  and a center point  11   a  of a resist opening  4   a.    
   With reference to  FIG. 6 , in a process of mounting chip components  15   a  and  16   a  onto the printed wiring board  1   a  having the configuration as described above, solder pastes  13   a  and  14   a  are printed onto the component land  5   a  and  6   a , respectively, of the printed wiring board  1   a  through a screen mask (not shown) with use of a printing machine. 
   More specifically, the printing machine captures an image of a recognition mark (not shown) of the screen mask with use of a CCD camera or the like so as to detect the center point of the recognition mark. The printing machine then captures an image of the board recognition mark  3   a  of the printed wiring board  1   a  so as to detect the center point  10   a  of the board recognition mark  3   a.    
   The misalignment between the center point of the recognition mark of the screen mask and the center point  10   a  of the board recognition mark  3   a  of the printed wiring board  1   a  is corrected, so that openings of the screen mask are aligned with the corresponding component lands  5   a  and  6   a . Then, the solder pastes  13   a  and  14   a  are printed onto the component lands  5   a  and  6   a , respectively, through the corresponding openings of the screen mask. 
   The openings of the screen mask corresponding to the component lands  5   a  and  6   a  are formed based on design data of the printed wiring board  1   a . Therefore, although the solder paste  13   a  is aligned with the normal resist type component land  5   a , the solder paste  14   a  is misaligned with the over resist type component land  6   a  by ΔX, ΔY. 
   Accordingly, printing position correction is required to equalize the amount of printing misalignment of the over resist type component land  6   a  to the amount of printing misalignment of the normal resist type component land  5   a.    
   The following briefly describes a process of mounting the chip components  15   a  and  16   a  onto the component lands  5   a  and  6   a , respectively, with use of an automatic mounter with reference to  FIG. 7 . 
   Before mounting the components  15   a  and  16   a , the automatic mounter captures an image of the board recognition mark  3   a  of the printed wiring board  1   a  with use of the CCD camera or the like so as to detect the center point  10   a  of the board recognition mark  3   a.    
   The automatic mounter then mounts the component  15   a  in a position spaced apart from the center point  10   a  by a distance (L 1 , W 1 ), and the component  16   a  in a position spaced apart from the center point  10   a  by a distance (L 2 , W 2 ). 
   The distance L 1 , W 1  is from the center point  10   a  of the board recognition mark  3   a  to the midpoint between conductive foils forming the normal resist type component lands  5   a  and  5 ′ a , while the distance L 2 , W 2  is from the center point  10   a  of the board recognition mark  3   a  to the midpoint between the conductive foils  22   a  and  22 ′ a  forming the over resist type component lands  6   a  and  6 ′ a . These distances L 1 , W 1  and L 2 , W 2  are defined in a design specification of the printed wiring board  1   a , and are registered in advance in a database together with the corresponding components  15   a  and  16   a.    
   Therefore, although the component  15   a  is aligned with the normal resist type component lands  5   a  and  5 ′ a  without misalignment, the component  16   a  is misaligned with the over resist type component lands  6   a  and  6 ′ a  by ΔX, ΔY. 
   Accordingly, mounting position correction for equalizing the amount of misalignment of the over resist type component  16   a  and the amount of misalignment of the normal resist type component  15   a  or mounting position correction for correcting the misalignment of the over resist type component  16   a  is required. 
   Patent Document 1 discloses a technique for avoiding such problems. According to Patent Document 1, in addition to a recognition mark formed of conductive foil, a second mount recognition mark covered with resist is formed when the resist is applied for partially covering an over resist type component land. The first mount recognition mark formed of the conductive foil is used for mounting a component onto a normal resist type component land, and the second mount recognition mark covered with the resist is used for mounting a component onto the over resist type component land. Thus, the components are mounted on the corresponding component lands without misalignment. 
   However, because only one of the mount recognition marks can be detected during a solder paste printing process, one of the component lands is misaligned with the solder paste. Therefore, printing position correction is required. 
   &lt;Patent Document 1&gt; Japanese Patent Laid-Open Publication No. 11-40907 
   As can be understood from the above description, with the above-described related art techniques, if a component land covered with resist and a component land not covered with the resist are present on the same printed wiring board, misalignment between solder pastes and corresponding component lands and misalignment between components and the corresponding component lands due to resist misalignment cannot be avoided. 
   SUMMARY OF THE INVENTION 
   The present invention may solve at least one problem described above. A preferred embodiment of the present invention may properly mount components onto a printed wiring board including a component land covered with resist and a component land not covered with the resist even if resist misalignment occurs, thereby improving quality of the printed wiring board as well as increasing production efficiency of the printed wiring board. 
   According to an aspect of the present invention, there is provided a printed wiring board that comprises a board recognition mark formed of a conductive foil, a first component land covered with resist, and a second component land not covered with the resist, wherein the board recognition mark is defined by an area of the conductive foil exposed from a resist opening having the same shape and size as the conductive foil. The image of the board recognition mark may be captured by an imaging device such as a CCD camera so as to find the center point of the board recognition mark. Solder paste printing and component mounting may be performed with reference to the center point. Thus, it is possible to equalize the misalignment amount of the solder paste printing position and the component mounting position with respect to the first component land covered with the resist to the misalignment amount of the solder paste printing position and the component mounting position with respect to the second component land not covered with the resist if resist misalignment occurs. 
   According to the above described aspect of the present invention, it is possible to properly print solder pastes and mount components onto a component land covered with resist and a component land not covered with the resist on a printed wiring board even if resist misalignment occurs. Therefore, the need for printing position correction and mounting position correction can be eliminated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram illustrating a printed wiring board according to an embodiment of the present invention; 
       FIG. 2  is a schematic diagram illustrating the printed wiring board of  FIG. 1  with solder paste printed thereon; 
       FIG. 3  is a schematic diagram illustrating a first example of the printed wiring board of  FIG. 1  with components mounted thereon; 
       FIG. 4  is a schematic diagram illustrating a second example of the printed wiring board of  FIG. 1  with components mounted thereon; 
       FIG. 5  is a schematic diagram illustrating a relationship between component lands and board recognition marks of a related art printed wiring board with misaligned resist; 
       FIG. 6  is a schematic diagram illustrating solder paste printed on a related art printed wiring board with misaligned resist; 
       FIG. 7  is a schematic diagram illustrating components mounted on a related art printed wiring board with misaligned resist; and 
       FIGS. 8A through 8C  are schematic diagrams illustrating a problem with a relate art printed wiring board resulting from resist misalignment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The following describes preferred embodiments of the present invention with reference to the accompanying drawings.  FIG. 1  is a schematic diagram illustrating a printed wiring board  1  according to an embodiment of the present invention.  FIG. 2  is a schematic diagram illustrating the printed wiring board  1  of  FIG. 1  with solder pastes  13  and  14  printed thereon.  FIG. 3  is a schematic diagram illustrating a first example of the printed wiring board  1  of  FIG. 1  with components  15  and  16  mounted thereon.  FIG. 4  is a schematic diagram illustrating a second example of the printed wiring board  1  of  FIG. 1  with the components  15  and  16  mounted thereon. 
   Referring to  FIG. 1 , the printed wiring board  1  includes conductive foil  7  for a board recognition mark  3 , a resist opening  4  for the board recognition mark  3 , the board recognition mark  3  being defined by an area of the conductive foil  7  exposed through the resist opening  4 , a normal resist type component land  5 , and an over resist type component land  6 . The conductive foil  7  and the resist opening  4  have the same shape and size. 
   The components  15  and  16  (see  FIGS. 3 and 4 ) are to be mounted on the component lands  5  and  6 , respectively. 
   Resist  2  (the shaded area in  FIGS. 1-4 ) is applied through a resist printing mask (not shown) on the printed wiring board  1  including the conductive foil  7  for the board recognition mark  3 , conductive foil  21  for the component land  5 , and conductive foil  22  for the component land  6 . The conductive foil  22  has a size large enough to accommodate the maximum resist misalignment. 
   The resist printing mask used for the printed wiring board  1  of this embodiment is designed to: 
   (a) form a resist opening  8  having a size large enough to accommodate the maximum resist misalignment so as to prevent the conductive foil  21  from being covered with the resist  2 ; and 
   (b) to form a resist opening  9  such that the area of the conductive foil  22  exposed through the resist opening  9  has the design size of the component land  6  while the resist  2  covers across the circumferential area of the conductive foil  22 . 
   The area of the conductive foil  21  exposed through the resist opening  8  and the area of the conductive foil  22  exposed through the resist opening  9  define the component land  5  and the component land  6 , respectively. 
   The components  15  and  16  are mounted onto the printed wiring board  1  as follows. It is to be noted that the following describes a case where rightward and downward misalignment of the resist  2  exists. 
   First, solder paste printing is performed as follows. With reference to  FIG. 2 , before printing the solder pastes  13  and  14  onto the corresponding component lands  5  and  6 , a printing machine captures an image of a recognition mark of a screen mask (not shown) using a CCD camera or the like, and detects a center point of the recognition mark. 
   Then, the printing machine captures an image of the board recognition mark  3  of the printed wiring board  1  so as to detect a center point  12  of the board recognition mark  3 . The printing machine corrects misalignment between the center point of the recognition mark of the screen mask detected by the CCD camera or the like and the center point  12  of the board recognition mark  3 , determines the positions of the screen mask openings (corresponding to the areas of the solder pastes  13  and  14 ) with respect to the component lands  5  and  6 , and then prints the solder pastes  13  and  14  onto the component lands  5  and  6 , respectively, through the screen mask openings. 
   If the resist is misaligned, since the board recognition mark  3  of the printed wiring board  1  is defined by the conductive foil  7  and the resist opening  4  having the same size and shape, the midpoint between a center point  10  of the conductive foil  7  and a center point  11  of the resist opening  4  defines the center point  12  of the board recognition mark  3 . 
   Accordingly, the resist misalignment is distributed such that the solder pastes  13  and  14  are printed with the same degree of misalignment with respect to the corresponding component lands  5  and  6 . It is to be noted that the printing machine performs the above-described printing operation according to a program stored therein in advance. 
   The following describes a process of mounting the components  15  and  16  onto the component lands  5  and  6 , respectively, of the printed wiring board  1  with the solder pastes  13  and  14  of  FIG. 2  printed thereon with use of the automatic mounter, with reference to  FIGS. 3 and 4 . 
   First, a mounting method for a printed wiring board is described according to an embodiment of the present invention with reference to  FIG. 3 . Referring to  FIG. 3 , before mounting the components  15  and  16 , the automatic mounter captures an image of the board recognition mark  3  of the printed wiring board  1  with use of the CCD camera or the like so as to detect the center point  12  of the board recognition mark  3 . 
   The automatic mounter then mounts the component  15  in a position spaced apart from the center point  12  by a distance (L 3 , W 3 ), and the component  16  in a position spaced apart from the center point  12  by a distance (L 4 , W 4 ). These distances are defined in a design specification of the printed wiring board  1 , and are registered in a database of the mounter in advance together with the corresponding components  15  and  16 . 
   In this embodiment, the conductive foil  7  and the resist opening  4  have the same shape and size, and the midpoint between the center point  10  of the conductive foil  7  and the center point  11  of the resist opening  4  define the center point  12  of the board recognition mark  3 . Therefore, when resist misalignment exists, the resist misalignment is distributed such that the component lands  5  and  6  are mounted with the same degree of misalignment with respect to the corresponding component lands  5  and  6 . The solder paste printing positions are aligned with the component mounting positions. 
   Next, a mounting method for a printed wiring board is described according to a modified embodiment of the present invention with reference to  FIG. 4 . With reference to  FIG. 4 , the printing machine captures an image of the board recognition mark  3  defined by the area of the conductive foil  7  exposed through the resist opening  4  with use of the CCD camera or the like, so as to find the center point  12 . The printing machine then measures the size (X 2 , Y 2 ) of the board recognition mark  3  so as to calculate the difference X 3 , Y 3  from the design size X 1 , Y 1  of the conductive foil  7 . 
   The printing machine divides the difference X 3 , Y 3  by two to obtain X 4 , Y 4 , and adds or subtracts X 4 , Y 4  to the X coordinate and the Y coordinate of the center point  12  of the board recognition mark  3  in accordance with resist misalignment direction so as to find the center point  10  of the conductive foil  7  and the center point  11  of the resist opening  4 . 
   A distance (L 5 , W 5 ) and a distance (L 6 , W 6 ) from the center point  12  of the board recognition mark  3  to the mounting positions of the components  15  and  16  are defined in the design specification of the printed wiring board  1 . The component  15  is mounted onto the normal resist type component land  5  (not covered with the resist  2 ) with reference to the center point  10  of the conductive foil  7 , whereas the component  16  is mounted onto the over resist type component land  6  (covered with the resist  2 ) with reference to the center point  11  of the resist opening  4 . Thus, the components  15  and  16  are mounted in correct positions even if resist misalignment exists. It is to be noted that the automatic mounter performs the mounting operation for the printed wiring board  1  described with reference to  FIGS. 3 and 4  according to a program downloaded therein in advance. The automatic mounter selectively uses the reference points depending on the component lands according to this program. 
   As described above with reference to  FIGS. 1-4 , the printed wiring board  1  includes the component land  6  covered with the resist  2  and the component land not covered with the resist  2 . The conductive foil  7  and the resist opening  4  have the same shape and size. The board recognition mark  3  is defined by the area of the conductive foil  7  exposed from the resist opening  4 . The image of the board recognition mark  3  is captured by an imaging device such as the CCD camera so as to find the center point  12  of the board recognition mark  3 . The center point  12  is used as a reference point. It is therefore possible to equalize the misalignment amount of the printed solder paste  14  and the mounted component  16  with respect to the component land  6  covered with the resist  2  to the misalignment amount of the printed solder paste  13  and the mounted component  15  with respect to the component land  5  not covered with the resist  2  if resist misalignment occurs. 
   In the process of printing the solder pastes  13  and  14  onto the printed wiring board  1 , the image of the board recognition mark  3  defined by the area of the conductive foil  7  exposed from the resist opening  4  is captured by the imaging device such as the CCD camera so as to find the center point  12  of the board recognition mark  3 . Then the solder pastes  13  and  14  are printed with reference to the center point  12  so as to equalize the misalignment amount of the solder paste  14  with respect to the component land  6  covered with the resist  2  to the misalignment amount of the solder paste  13  with respect to the component land  5  not covered with the resist  2 . Accordingly, the need for printing position correction can be eliminated. 
   In the process of mounting the components  15  and  16  onto the printed wiring board  1  with use of the automatic mounter, the image of the board recognition mark  3  defined by the area of the conductive foil  7  exposed from the resist opening  4  is captured by the imaging device such as the CCD camera so as to find the center point  12  of the board recognition mark  3 . Then the components  15  and  16  are mounted with reference to the center point  12  so as to equalize the misalignment amount of the component  16  with respect to the component land  6  covered with the resist  2  and the misalignment amount of the component  15  with respect to the component land  5  not covered with the resist  2 . Accordingly, the need for mounting position correction can be eliminated 
   In the modified embodiment, in the process of mounting the components  15  and  16  onto the printed wiring board  1  with use of the automatic mounter, the image of the board recognition mark  3  defined by the area of the conductive foil  7  exposed from the resist opening  4  is captured by the imaging device such as the CCD camera so as to find the center point  12  of the board recognition mark  3 . Further, as described with reference to  FIG. 4 , the size (X 2 , Y 2 ) of the board recognition mark  3  is measured so as to calculate the difference X 3 , Y 3  from the design size X 1 , Y 1  of the conductive foil  7 . The calculated difference X 3 , Y 3  is divided by two to obtain X 4 , Y 4 , and. X 4 , Y 4  is added or subtracted to the X coordinate and the Y coordinate of the center point  12  of the board recognition mark  3  in accordance with the resist misalignment direction so as to find the center point  10  of the conductive foil  7  and the center point  11  of the resist opening  4 . Then the component  15  is mounted onto the component land  5  not covered with the resist  2  with reference to center point  10  of the conductive foil  7 . On the other hand, the component  16  is mounted onto the component land  6  covered with the resist  2  with reference to the center point  11  of the resist opening  4 . Accordingly, the components  15  and  16  are properly mounted even if resist misalignment occurs. 
   It is to be understood that the present invention is not limited to the preferred embodiments illustrated herein, and variations and modifications may be made without departing from the scope of the present invention.