Patent Publication Number: US-6907824-B2

Title: Screen printing apparatus and method of the same

Description:
This application is a divisional of U.S. patent application Ser. No. 09/907,188, filed Jul. 17, 2001, which has issued a U.S. Pat. No. 6,609,458 on Aug. 26, 2003. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a screen printing apparatus for printing paste, such as cream solder or conductive paste, onto a substrate, and a method of the same. 
     BACKGROUND OF THE INVENTION 
     A screen printing method has been used for printing paste such as cream solder onto a substrate in a parts-mounting-process by a part-mounting-machine. This method works as this: first, overlay a maskplate on a substrate, the mask-plate has pattern holes corresponding to spots to be printed on the substrate. Then supply cream solder on the mask-plate, and slide a squeegee. Cream solder is thus printed onto the target spots through the pattern holes. 
     A highly accurate printing by this printing method would require the following two points: 
     First, a substrate must be accurately positioned with respect to a mask plate. In other words, the substrate must be accurately positioned to the mask plate in a horizontal direction, and the substrate must solidly adhere to the lower face of the mask plate. Therefore, when the apparatus starts on or a model is changed in the mounting line, various adjustments must be done in order to position a substrate accurately to the mask plate. The adjustments include preparing machine-parameters of a moving table of a substrate-positioning-section. 
     Second, the cream solder must be filled up in the pattern holes when the squeegee is slid, and various printing conditions should be set appropriately to an object to be printed. For instance, a squeegeeing speed on the screen mask, a pressure urging the squeegee against the screen mask and other parameters are to be prepared depending on the characteristics of the object to be printed. 
     These printing conditions have been set by skilled workers based on their experience and personal know-how. In other words, before actual printing starts, fill-in condition of the cream solder is inspected with human eyes on a trial printing stage, then the condition is corrected by the skilled worker based on the experience and intuition. The printing conditions are thus manually prepared. 
     However, even if these conditions are adjusted correctly, the substrates are not always positioned exactly with the mask plate at actual printing. There are dispersions on dimensions of the substrates, aged deformation of the mask plate, and looseness of mounting the mask plate. These factors would deviate the pattern holes from the correct positions. Further, gaps would be produced between the substrate and the lower face of the mask plate. 
     In these cases, defects such as “deviation” of a printed spot from the target one or “blur” of the paste from the printed spot are produced. As such, in the conventional printing method, deviation of the substrate from the mask plate tends to occur, and it is difficult to keep a stable printing quality. 
     In screen printing, since the squeegee is horizontally moved while it is urged to the mask plate, an external force is applied to the mask plate so that the mask plate is pulled by the squeegee in the lateral direction. This external force repeatedly works on the mask plate at every printing operation, which could loosen the mounting condition of the mask plate or extend the mask plate. As a result, even if the substrate is correctly positioned, the movement of the squeegee at every printing deviates the mask plate relatively from the substrate, and a printing position cannot be exactly maintained. 
     In these days it becomes difficult to keep the skilled workers. Further, limited production of a variety of products becomes more popular, thus the printing conditions must be changed frequently at every model change. Since the printing conditions are manually prepared, the frequent changes take time and labor, which, as a result, prevents the productivity from improving. 
     Once the printing conditions are set, the fill-in condition of cream solder is not necessarily kept stable because the cream solder changes over the time, so that the printing quality becomes unstable. 
     As such, the conventional screen printing apparatus has problems such as (1) preparing the printing conditions takes time and labor, (2) it is difficult to stabilize the printing quality due to dispersion of the printing conditions. 
     SUMMARY OF THE INVENTION 
     A method of screen printing comprises the steps of:
         (a) detecting a position of a mask plate having a pattern hole relative to a substrate;   (b) determining an extent to which the hole is filled with paste; and   (c) modifying printing on the substrate using the mask based on the determined extent.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a screen printing apparatus in accordance with an exemplary embodiment. 
         FIG. 2  is a lateral view of the screen printing apparatus shown in FIG.  1 . 
         FIG. 3  is a plan view of the screen printing apparatus shown in FIG.  1 . 
         FIG. 4  is a perspective view of a laser measuring device in the screen printing apparatus shown in FIG.  1 . 
         FIG. 5  is a block diagram illustrating a structure of a control system of the screen printing apparatus shown in FIG.  1 . 
         FIGS. 6A ,  6 B,  7 A and  7 B illustrate how a positioned status is detected at screen printing by the screen printing apparatus shown in FIG.  1 . 
         FIGS. 8A and 8B  illustrate how to measure a positional deviation of a mask plate at screen printing by the screen printing apparatus shown in FIG.  1 . 
         FIG. 9  shows data of library of screen printing conditions of the screen printing apparatus shown in FIG.  1 . 
         FIGS. 10A ,  10 B and  10 C illustrate how to detect a fill-in status at screen printing by the screen printing apparatus shown in FIG.  1 . 
     
    
    
     The entire disclosure of U.S. patent application Ser. No. 09/907,188, filed Jul. 17, 2001, which has issued a U.S. Pat. No. 6,609,458 on Aug. 26, 2003, is expressly incorporated by reference herein. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention aims to provide a screen printing apparatus and a method of the same. Positional deviation of a substrate from a mask plate can be prevented and printing conditions can be prepared easily without dispersion by the apparatus and the method. As a result, quality printing can be maintained. 
     The screen printing apparatus of the present invention comprises the following elements:
         (a) substrate-positioning-means for positioning a substrate relatively to a mask plate having pattern holes;   (b) three-dimensional measuring means for detecting the positioned status by measuring the mask plate from its above, the substrate being positioned to the mask plate; and   (c) control means for driving the substrate-positioning-means to correct the positioning based on a result detected by the three-dimensional measuring means.       

     In this apparatus, the mask plate is overlaid on the substrate, and a squeegee head is slid on the mask plate, so that paste is printed on the substrate through the pattern holes. 
     Another screen printing apparatus of the present invention comprises the following elements:
         (a) substrate positioning means for positioning a substrate relatively to a mask plate having pattern holes based on printing position data;   (b) mask position measuring means for measuring a horizontal position of the mask plate to which the substrate is positioned; and   (c) storing means for storing print-position-data corrected based on a measurement result by the mask position measuring means.       

     In this apparatus, the mask plate is overlaid on the substrate, and a squeegee head is slid on the mask plate, so that paste is printed on the substrate through the pattern holes. 
     Still another screen printing apparatus of the present invention comprises the following elements:
         (a) substrate positioning means for positioning a substrate relatively to a mask plate having pattern holes;   (b) fill-in status detecting means for detecting fill-in status of cream solder in the pattern holes by measuring the mask plate from its above in a three-dimensional way;   (c) printing condition modifying means for modifying printing conditions based on a result detected by the fill-in status detecting means;   (d) storing means for storing the printing conditions modified.       

     In this apparatus, the mask plate is overlaid on the substrate, and a squeegee head is slid on the mask plate, so that paste is printed on the substrate through the pattern holes. 
     The methods of screen printing according to the present invention are for printing paste on a substrate through pattern holes by sliding a squeegee head on a mask plate. The mask plate having the pattern holes is overlaid on the substrate. 
     The method of screen printing according to the present invention comprises the steps of:
         (a) detecting a positioned status by measuring a mask plate from its above in a three-dimensional way, the substrate being positioned to the mask plate; and   (b) correcting the positioned status through controlling substrate-positioning-means by control means based on a detection result obtained in step (a).       

     Another method of screen printing according to the present invention comprises the steps of:
         (a) positioning a substrate with respect to a mask plate based on print position data;   (b) measuring a positional deviation of the mask plate due to printing action by measuring the horizontal position of the mask plate before and after the printing action, the substrate being positioned to the mask plate; and   (c) storing the printing position data corrected based on the step (b).       

     Still another method of screen printing according to the present invention comprises the steps of:
         (a) detecting a fill-in status of cream solder by measuring a mask plate from its above with three-dimensional-measuring means, the mask plate being filled up with cream solder in its pattern holes by a squeegee;   (b) correcting printing conditions based on the result obtained in step (a); and   (c) storing the printing conditions corrected into storing means.       

     The present invention, having the structures or steps discussed above, realizes the following advantages:
     (1) The mask plate, to which a substrate is positioned, is measured from its above in a three-dimensional way for detecting the positioned status, and the positioned status is corrected based on the detection result, so that an exact positioning is always carried out, and as a result, quality printing can be kept.   (2) A horizontal position of the mask plate, to which a substrate is positioned, is measured before and after a printing action, so that a positional deviation of the mask plate due to the printing action is measured. Then printing-position-data is corrected based on this detection result, thereby keeping the exact printing position.   (3) A screen mask, of which pattern holes is filled up with cream solder by the squeegee, is measured from its above with three-dimensional means for detecting a fill-in status of the cream solder. Screen printing conditions are corrected based on this detection result, so that the printing conditions can be prepared easily without dispersion.   

     An exemplary embodiment of the present invention is demonstrated with reference to the accompanying drawings. First, a structure of a screen printing apparatus is described.  FIG. 1  is a front view of a screen printing apparatus in accordance with the exemplary embodiment.  FIG. 2  is a lateral view, and  FIG. 3  is a plan view of the screen printing apparatus. 
     In FIG.  1  and  FIG. 2 , substrate-positioning-section  1 , functioning as means for positioning a substrate, is formed by placing θ-axis table  4  on top of a moving table, and further, placing Z-axis table  5  on top of table  4 . The moving table comprises X-axis table  2  and Y-axis table  3 . On table  5 , substrate-holder  7  is provided, which holds upwardly substrate  6  clamped by clamp  8 . Substrate  6 , an object of printing, is fed into substrate positioning section  1  by carry-in conveyer  14  shown in  FIGS. 1 and 3 . The position of substrate  6  can be adjusted by driving substrate-positioning-section  1 . After being printed, substrate  6  is transferred by carry-out conveyer  15 . 
     Screen mask  10  is disposed above positioning section  1 . Mask  10  is formed by mounting mask plate  12  to holder  11 . Substrate  6  is positioned to mask plate  12  by positioning section  1 , and brought upwardly into contact with mask plate  12 . 
     Above mask  10 , squeegee head  13  is disposed such that head  13  can shuttle back and forth in a horizontal direction. While substrate  6  is put under and in contact with the lower face of mask plate  12 , cream solder  9  (paste) is supplied onto mask plate  12 . Squeegee  13   a  of head  13  is placed on mask plate  12 , then slid, so that solder  9  is printed on electrode  6   a  (refer to  FIG. 3 ) formed on substrate  6  via pattern holes  12   a  provided on mask plate  12 . 
     Above mask  10 , laser-measuring-device  20 , functioning as three-dimensional measuring means, is disposed. As shown in  FIG. 3 , laser measuring device  20  is movable horizontally in X, Y directions by X-axis table  21  and Y-axis table  22  as well as movable vertically by lift means  23  (refer to  FIGS. 1 ,  2 ). In other words, device  20  is lowered to a measuring place by driving lift means  23 . X-table  21 , Y-table  22  and lift means  23  function as moving means for moving laser-measuring-device  20 . 
     Laser measuring device  20  has two functions, one is to measure a vertical displacement by irradiating laser beam, and the other is a scanning function of scanning a laser-irradiating-position in X, Y directions.  FIG. 4  is a perspective view of device  20 . As shown in  FIG. 4 , irradiating point P is scanned within measuring range R, thereby detecting sequentially the vertical positions of the object&#39;s surface within range R. As a result, a three-dimensional shape of the object can be detected. 
     Device  20  is moved by the moving means with respect to substrate  6  and mask plate  12 , whereby the shapes of substrate  6  and mask plate  12  in any ranges can be measured from above in a three-dimensional way. Then the detection data obtained is processed, thereby detecting electrode  6   a  (features of substrate  6 ), pattern holes  12   a  (features of mask plate  12 ) and the holes for detecting the position. As a result, a horizontal position of mask plate  12  can be detected. 
     In other words, laser measuring device  20  functions as a mask-position-measuring means for detecting the horizontal position of mask plate  12 . Devices other than laser measuring device  20  (three-dimensional measuring means) can be used as mask-position-measuring means, e.g., features such as recognition marks provided on mask plate  12  can be shot by a camera then the positions of the features can be recognized through Image-processing. 
     Further, substrate  6  undergoing screen printing is regarded as a measuring object, and laser measuring device  20  measures substrate  6  in a three-dimensional way, thereby detecting a shape of cream solder  9  printed on substrate  6  in a three-dimensional way. As a result, the print can be inspected, i.e., a position and a printed amount of solder  9  are determined whether or not they are acceptable. 
     When substrate  6  is positioned with respect to mask plate  12 , i.e., substrate  6  is positioned to mask plate  12  in the horizontal direction and placed under and in contact with the lower face of mask plate  12 , the three-dimensional measuring is carried out from above mask plate  12 . As a result, a positioned status can be detected, which is hereinafter described with reference to  FIGS. 6 ,  7 . 
       FIGS. 6A ,  6 B and  FIGS. 7A ,  7 B illustrate two types of positioning defects.  FIG. 6A  illustrates that the relative position of substrate  6  to mask plate  12  deviates in the horizontal direction. In other words, pattern hole  12   a  is not exactly positioned on electrode  6   a  of substrate  6 , and gap “d” is produced at the edge of pattern hole  12   a.  When this status is measured by laser measuring device  20  in a three-dimensional way, as  FIG. 6B  shows, an extra recess indicating the deviation and marked with an arrow appears on a recessed measuring line which illustrates a cross sectional shape of what is shown in FIG.  6 A. Therefore, when this measuring line is compared with the normal one, a positional deviation in the horizontal direction can be detected. 
       FIG. 7A  illustrates that the relative position of substrate  6  to mask plate  12  deviates in the vertical direction. In other words, electrode  6   a  of substrate  6  is not exactly placed under and in contact with the lower face of mask plate  12 , thereby producing gap “c”. When this status is measure by device  20  in the three-dimensional way, as shown in  FIG. 7B , the depth of recessed measuring line indicating pattern hole  12   a  appears deeper than mask thickness “t” by gap “c”. Therefore, when the stepped measuring line indicating pattern hole  12   a  is compared with the normal dimensions, the positional deviation in the vertical direction can be detected. 
     When substrate  6  is placed under and in contact with the lower face of mask plate  12 , head  13  is moved for squeegeeing. Then the upper face of mask plate  12  is three-dimensionally measured, so that a fill-in status of cream solder  9  into pattern hole  12   a  can be detected. The detection of the fill-in status is detailed with reference to FIG.  10 A through FIG.  10 C. 
       FIGS. 10A ,  10 B and  10 C illustrate a normal fill-in status, an excess fill-in status and an insufficient fill-in status respectively, and also show measuring lines La, Lb and Lc obtained by laser measuring device  20 . 
     When pattern hole  12   a  is filled normally with cream solder  9  as shown in  FIG. 10A , measuring line La indicates the normal height h 0  of the upper face of mask plate  12  including the range of pattern hole  12   a.    
     When pattern hole  12   a  is filled excessively with cream solder  9  as shown in  FIG. 10B , surplus cream solder  9  overflows pattern hole  12   a . In this case, measuring line Lb reflects the surface of solder  9  and becomes different from the normal height h 0 . 
     When pattern hole  12   a  is filled insufficiently with cream solder  9  as shown in  FIG. 10C , measuring line Lc indicates a partial recess within the range of a pattern hole  12   a.    
     As discussed above, when the upper face of mask plate  12  undergone the squeegee operation is compared with the normal shape, the defect types of fill-in can be detected. 
     A structure of the control system of the screen printing apparatus is described with reference to FIG.  5 . In  FIG. 5 , CPU  30  is a controller governing individual sections described here. Program storing section  31  stores various programs such as, an operation program of screen printing, a processing program for detecting a shape of substrate  6  and mask plate  12  from a detecting signal of laser measuring device  20 , a processing program for correcting a positioning of substrate  6  and mask plate  12 , a determining program at a printing inspection, a processing program of preparing a printing condition and the like. Data memory  32  stores various data such as, reference data for determining a process at the printing inspection, printing condition data prepared for respective models together with printing condition library collecting the data necessary for setting the printing condition, printing position data, i.e., axis-driving-data for positioning substrate  6  relative to mask plate  12  by driving respective axes of substrate-positioning-section  1 , reference data indicating three-dimensional measuring data at the normal fill-in of cream solder  9  in pattern hole  12   a.  In other words, data memory  32  functions as storing means for storing the printing position data and the printing condition. 
     Mechanism controller  33  controls operations of respective mechanical sections such as positioning section  1 , carry-in conveyer  14 , carry-out conveyer  15 , X-axis table  21 , Y-axis table  22 . Shape detector  34  processes the detecting signal obtained by laser-measuring-device  20 , so that an electrode placement pattern, an opening pattern showing the shape and placement of pattern hole  12   a  and a shape of cream solder printed on substrate  6  are detected. Thus the positioning status of substrate  6  with respect to mask plate  12  is detected. 
     Shape detector  34  detects a specific pattern hole  12   a,  and CPU  30  calculates the positional coordinates of the detected pattern hole  12   a,  thereby measuring the horizontal position of mask plate  12 . In other words, shape detector  34  and CPU  30  function as mask position measuring means for detecting the horizontal position of mask plate  12 . 
     Printing condition setter  35  prepares screen printing conditions, such as a squeegeeing speed, a printing pressure (an urging value of squeegee  13   a  against mask plate  12 ), a relative releasing speed of substrate  6  from mask plate  12  just after the printing. These parameters are prepared responsive to the characteristics of a printing object. The screen printing conditions are modified or changed based on a fill-in status detection result and a printing determination result, both being detailed later. In other words, printing condition setter  35  functions as printing-condition-modifying means for modifying the printing conditions based on the fill-in status detection result. 
     Position correcting section  36  (position correcting means) corrects a positioned status based on the positioned-status-detection result which has been obtained by three-dimensional measuring of mask plate  12  from its above, where substrate  6  is placed under and in contact with the lower face of plate  12 . The relative positions of substrate  6  to mask plate  12  in both the horizontal and vertical directions are detected, and are compared with the reference data stored, so that deviations are found. Finally, the positioned status is corrected to an exact positioned status. 
     Substrate and mask determiner  37  compares an electrode-placement-pattern detected by shape detector  34  and an opening pattern of mask plate  12  with the reference pattern of the design data, thereby determining whether or not substrate  6  and mask plate  12  supplied to the apparatus are acceptable. 
     Print determiner  38  compares the data about the shape of cream solder  9  with the reference data stored, thereby determining whether or not the print is acceptable. The data has been obtained by measuring substrate  6  undergone the screen print with laser measuring device  20 . 
     Fill-in status determiner  39  compares the three-dimensional data about the upper face of mask plate  12 , on which cream solder  9  has been filled, with the reference data stored in data memory  32 , thereby determining whether or not the fill-in status of cream solder into pattern hole  12   a  is acceptable. Shape detector  34  has detected the upper face of mask plate  12 , where the cream solder was filled in. 
     Next, the preparation of the printing conditions is described.  FIG. 9  shows data content of printing-condition-library. In this library, a combination of the parameters (squeegeeing speed, print pressure, releasing speed, and releasing distance which indicates a relative moving distance) is assigned to respective couples of a typical dimension of an electrode (e.g. width of the electrode) and a typical dimension of the screen mask (e.g. thickness of the mask plate) where pattern holes are provided. These parameters takes different values depending on the physical properties of cream solder  9 . 
     A type of cream solder  9 , substrate  6  to be printed, and screen mask  10  to be used, are designated by the typical dimension of the electrode and the typical dimension of the mask, whereby parameters corresponding to the combination are selected. The parameters selected are automatically prepared. The screen printing apparatus of the present invention is thus structured. 
     A method of screen printing is described hereinafter. A first embodiment of the screen printing method is demonstrated. First, a model is changed in a production line. Screen mask  10  is prepared for the newly introduced model, and the mask is inspected. As shown in  FIG. 1 , this inspection is carried out by three-dimensional measuring, i.e., laser measuring device  20  is moved above mask plate  12  by X-axis table  21  and Y-axis table  22 , thereby measuring mask plate  12 . The thickness of mask plate  12  is measured and an acceptance of the mask is determined through the mask inspection. 
     Next, substrate  6  is inspected. As shown in  FIG. 2 , carry-in conveyor  14  carries substrate  6  onto substrate-positioning-section  1 , then substrate-positioning-section  1  under mask  10  is moved in Y-direction (marked with an arrow in  FIG. 2 ) to a substrate-measuring-position (section  1  and substrate  6  drawn in broken lines in  FIG. 2. ) Then a width and a length of an electrode, onto which a printing is carried out, formed on substrate  6  can be measured. 
     A trial printing is carried out before an actual printing. First, supply cream solder  9  on mask plate  12 , then shuttle squeegee  13   a  back and forth for tempering solder  9  (preparatory squeegeeing.) Second, raise Z-axis table  5  of substrate-positioning-section  1  so that substrate  6  is placed under and in contact with the lower face of mask plate  12 , thereby positioning substrate  6  to mask plate  12 . 
     After the positioning, a positioned status is detected. To be more specific, move laser-measuring-device  20  above mask-plate  12 , then measure the position of a given pattern hole specified as a detecting position. Thus positional deviations of substrate  6  from mask plate  12  in the horizontal and vertical directions are detected, i.e., the positioned status is detected. 
     Based on the positioned status detected, X-axis table  2 , Y-axis table  3  and Z-axis table  5  are controlled and driven by CPU  30  and mechanism controller  33 , so that the positional deviation is corrected, and substrate  6  is exactly positioned to mask plate  12 . CPU  30  and mechanism controller  33  function as controlling means for correcting the positioned status by controlling the substrate positioning means based on the positioned-status detection result. 
     Then carry out squeegeeing for filling solder  9  into respective pattern holes  12   a.  After that, lower Z-axis table  5  to release substrate  6  from mask plate  12 , thereby completing the print of solder  9  on electrode  6   a  of substrate  6 . 
     Then inspect the trial print on substrate  6 . In this inspection, positioning section  1  is moved again from under mask  10  to the substrate-measuring-position, and substrate  6  is three-dimensionally measured from its above by device  20 . When this inspection determines that the print status is acceptable, the printing conditions are determined properly. Then an actual print starts based on the proper printing conditions. 
     A second embodiment of the screen printing method is demonstrated here. First, as same as the first embodiment, a model is changed and a newly introduced model is put on a production line. Screen mask  10  is mounted responsive to the new model. Substrate  6  is positioned to mask plate  12  before a trial print is carried out. This positioning is carried out by driving substrate-positioning-section  1  based on the print-position-data stored in data memory  32 , so that substrate  6  is raised by Z-axis table  5  while being positioned in the horizontal direction, and finally placed under and in contact with the lower face of mask plate  12 . 
     Next, a mask position is measured. As shown in  FIG. 8A , mask plate  12  is three-dimensionally measured from its above by moving laser-measuring device  20  above screen-mask  10  with X-axis table  21  and Y-axis table  22 . Thus the horizontal position of mask plate  12  is measured. This horizontal position can be measured also by using a specific pattern hole  12   a  for position-measuring, or by measuring a position-measuring-dedicated hole. 
     Then printing is carried out. Squeegee  13   a  is urged against mask plate  12 , and squeegee head  13  is moved so that cream solder  9  is filled in pattern holes  12   a,  and as shown in  FIG. 8B , device  20  is moved to measure the pattern hole  12   a  which has been measured as a measuring point before this printing. Thus a horizontal shift of mask plate before and after the printing, i.e., a positional deviation “d” of mask plate  12  due to the printing action is detected. This deviation of the mask is measured with respect to a plurality of squeegees  13   a  in respective sequeegeeing directions. The print position corrected based on the deviation data is stored in data memory  32  as a new print position data. 
     Based on the data about the positional deviation detected, the data other than print position data, e.g., a data contributing to the positional deviation of the mask plate, can be modified. In other words, mask plate  12  is shifted by squeegee  13   a,  thereby producing the deviation discussed above. Therefore, a pressure urging squeegee  13   a  or a squeegeeing speed is changed, thereby restraining the positional deviation. 
     When substrate  6  is re-positioned based on the new print position data corrected, the trial print is carried out again. To be more specific, after the print the same as the previous one, the mask position is measured again. When an exact mask position is confirmed, substrate  6  is released from mask plate by lowering Z-axis table  5 . Thus solder  9  is exactly printed on electrode  6   a  of substrate  6 . 
     Then inspect the trial print on substrate  6 . In this inspection, positioning section  1  is moved again from under mask  10  to the substrate-measuring-position, and substrate  6  is three-dimensionally measured from its above by laser-measuring-device  20 . When this inspection determines that the print status is acceptable, an actual print starts. 
     As discussed above, the second embodiment of the screen printing method includes the following steps: When substrate  6  is positioned to mask plate  12 , the horizontal position of mask plate  12  is measured, thereby obtaining the positional deviation of mask plate  12  due to the printing action. The print position data at actual printing is corrected by that deviation. As a result, the deviations due to deformation of mask plate  12  over time and looseness of mounting the mask plate  12  can be corrected. Quality print without any print deviation is thus obtainable. 
     A third embodiment of the screen printing method of the present invention is demonstrated here. First, a model is changed and a new model is introduced in a production line. Screen mask  10  is mounted responsive to the new model, and the mask is inspected. As shown in  FIG. 1 , this inspection is carried out by three-dimensional measuring, i.e., laser measuring device  20  is moved above mask plate  12  by X-axis table  21  and Y-axis table  22 , thereby measuring mask plate  12 . The thickness of mask plate  12  is measured and an acceptance of the mask is determined through this mask inspection. 
     Next, substrate  6  is inspected. As shown in  FIG. 2 , carry-in conveyor  14  carries substrate  6  onto substrate-positioning-section  1 , then substrate-positioning-section  1  under mask  10  is moved in Y-direction (marked with an arrow in  FIG. 2 ) to a substrate-measuring-position (section  1  and substrate  6  drawn in broken lines in  FIG. 2. ) Then a width and a length of an electrode, onto which a printing is carried out, formed on substrate  6  can be measured. 
     The procedure of the third embodiment up to this point is the same as that of the first embodiment. 
     The typical dimension of the electrode and the thickness of mask plate  12  are found. Then the parameters of printing conditions in printing-condition-library are read based on a type of cream solder  9  already input, data of substrate  6  and screen mask  10  both the data have been measured. The parameters are used in the trial printing. 
     At the trial printing, first, supply cream solder  9  onto screen mask  10 , and shuttle back and forth squeegee  13   a  to temper solder  9  (preparatory squeegeeing). Then raise Z-axis table  5  of substrate-positioning-section  1  so that substrate  6  is placed under and in contact with mask plate  12 . Next, move squeegee head  13  on mask plate  12  to fill solder  9  into pattern holes  12   a.  In this status, move laser-measuring-device  20  above mask plate  12  to detect the fill-in status of solder  9 . 
     Based on this fill-in status detected, the printing conditions are corrected. To be more specific, in the case of an excessive fill-in status, namely solder  9  overflows pattern holes  12   a,  a print pressure should be increased for instance. In the case of insufficient fill-in status, e.g., the print pressure should be lowered. These corrections of the print conditions are made based on experimental values stored as data. The print conditions corrected are stored in memory  32 , thereby updating the print conditions. 
     After the print conditions are updated, the trial print is carried out again. When the fill-in status is determined proper at this trial print, Z-axis table  5  is lowered for releasing substrate  6  from mask plate  12 , whereby solder  9  is printed on electrode  6   a  of substrate  6 . Then substrate  6  undergone the print is inspected. In this inspection, positioning section  1  is moved again from under mask  10  to the substrate-measuring-position, and substrate  6  is three-dimensionally measured from its above by device  20 . When this inspection determines that the print status is acceptable, the parameters of the print conditions read out are determined proper, and these parameters are set for an actual print and stored in data memory  32 . 
     If a defect is found at the inspection after the trial print, feed back process of the print conditions works based on feed-back data stored in the print condition library. In other words, the inspection result outputs numerical data to respective items such as a print area, a print height of each electrode. These numerical data are compared with the reference values prepared as proper values, thereby finding deviations from the reference values. 
     These deviations allow the parameters correlated to be corrected to negative or positive side by a correction amount responsive to the deviations. The data of this correlation between the deviation and the correction amount are collected from the results of the trial prints which have been carried out systematically by changing respective print conditions in various ways. The results of the trial prints have been statistically processed. The data of the correlation are also stored in the print condition library of data memory  32 . 
     After the print conditions are corrected, another trial print is carried out, and the print result is inspected. An actual print starts after the print result is determined acceptable. Throughout the actual printing, the print conditions are fed back at given intervals, namely, substrates  6  undergone the print process are inspected by device  20  on sampling base. 
     The data are collected from the inspections, and deviations from the reference values of respective inspection items are found, then some of the parameters, if necessary, are corrected, thereby changing the print conditions for the actual print. Through this procedure, even if the viscosity of solder  9  is varied due to temperature change, the print conditions can be always kept in a proper range. As a result, quality print can be maintained. 
     As discussed above, the screen printing method of the present invention carries out the following procedure: First, measure mask plate  12  undergone a squeegeeing operation from its above in a three-dimensional way, so that the fill-in status of cream solder  9  is detected. Then based on the detection result, correct the screen print conditions. As a result, a complicated work of preparing the print conditions based on human eyes by skilled workers can be done with ease, and the print conditions can be prepared without dispersion. Thus stable print quality is obtainable. 
     In the embodiments discussed above, an open type squeegee head having board-shaped squeegee  13   a  is used; however, the squeegee head is not limited to this example, and a closed type squeegee head can be used. This head is filled with the cream solder inside thereof, and the head is slid on the mask plate while pressure is applied to the cream solder, thereby filling the cream solder into the pattern holes. 
     In short, according to the present invention, the mask plate, to which a substrate is positioned, is three-dimensionally measured from its above, thereby detecting a positioned status. Based on the detection result, the positioned status is corrected. Therefore, an exact positioning can be always expected and stable print quality is obtainable. 
     A horizontal position of the mask plate, to which the substrate is positioned, is measured before and after a printing action, so that a positional deviation of the mask plate due to the printing action can be measured. Based on the detection result, print condition data is corrected. Therefore, an exact print position is always maintainable. 
     A screen mask, of which pattern holes are filled with the cream solder, is three-dimensionally measured from its above, thereby detecting a fill-in status of the cream solder. Based on the detection result, screen print conditions are corrected. Therefore, the print conditions can be prepared easily without dispersion.