Patent Publication Number: US-9421755-B2

Title: Screen printing system with positional alignment

Description:
FIELD OF THE INVENTION 
     This invention relates to a screen printing system with positional alignment. More particularly, this invention relates a method for aligning substrate with screen mask in a screen printing system. 
     BACKGROUND OF THE INVENTION 
     Screen printing apparatuses are known for applying a material (e.g. a conductive paste) on the surface of a substrate (e.g. a print tape) to form a particular printing pattern defined by a screen mask or mesh. Typically, a squeegee is pulled across the top surface of a screen mask and pushes the material applied on the screen mask passes through the pattern holes of the screen mask onto the surface of a substrate positioned below the screen mask. It is a known problem that each time a new substrate is positioned at the printing position or a new screen mask is installed in the printing apparatus, the substrate and the screen mask may not in a perfect alignment and thus resulted in an imprecise printing pattern on the substrate. Therefore, if a high printing accuracy is desired, the position of the substrate or the screen mask has to be corrected before the printing process so that the substrate and the screen mask are aligned with each other. 
     Many alignment methods have been introduced to improve the printing accuracy and throughput. For example, some recognisable markings are printed on the substrate or the screen mask for facilitating the alignment process. The locations of the markings are measured and compared with an expected location so that positional error can be determined and corrected. Many alignment methods are meant for large scale printing systems and hence their methods are complicated and costly to implement. Although there are simpler alignment methods which can be performed semi-auto or manually, these methods are mostly less accurate and hence not desirable. 
     Therefore, those skilled in art are constantly striving to devise an alignment method for a screen printing system which is simple and able to provide a high printing accuracy. 
     SUMMARY OF THE INVENTION 
     The above other problems are solved and an advance in the art is made by a screen printing system that provides an efficient and accurate method for aligning the substrate with the screen mask. The alignment or compensation values for correcting the position of the substrate are determined based on the offsets between the substrate and the screen mask. 
     A first advantage of the alignment method in accordance with this invention is that the method reduces the setup time and the alignment time of screen mask/substrate, thereby improving the efficiency and productivity of screen printing. This is because the screen printing system and the alignment method of this invention are relative simple and easy to implement with minimum cost. A second advantage of the alignment method in accordance with this invention is that the method enhances the printing accuracy and quality. This is because the alignment method of this invention has less operational errors since it involves simple mathematical calculations. As a precise alignment of the substrate and the screen mask can be achieved with the method in accordance with this invention, some known problems such as short circuit of the conductive lines printed on the substrate can be avoided. 
     In accordance with a first aspect of this invention, a method for aligning substrate with screen mask in a screen printing system is provided in the following manner. The method comprises: providing a screen mask which has first and second registration holes through the screen mask; positioning a substrate below and substantially parallel to the screen mask wherein the substrate has first and second registration holes through the substrate; capturing a first image of the first registration holes of the screen mask and the substrate with a first image capturing device wherein the first image shows the first registration hole of the screen mask superimposed with the first registration hole of the substrate; capturing a second image of the second registration holes of the screen mask and the substrate with a second image capturing device wherein the second image shows the second registration hole of the screen mask superimposed with the second registration hole of the substrate; determining from the first and second images whether the first and second registration holes of the substrate are vertically aligned with the first and second registration holes of the screen mask; calculating offsets between the first registration holes from the first image and offsets between the second registration holes from the second image in response to a determination that at least one of the registration holes of the substrate is not vertically aligned with the respective registration hole of the screen mask; calculating alignment values for correcting position of the substrate based on the calculated offsets; and correcting position of the substrate based on the calculated alignment values so that the substrate is vertically aligned with the screen mask. 
     In accordance with embodiments of this invention, the step of calculating the offsets comprises: measuring differences in distance between the first registration hole of the screen mask and the first registration hole of the substrate in a X axis and a Y axis orthogonal to the X axis wherein the X and Y axes define a horizontal X-Y plane parallel to the screen mask and the substrate; and measuring differences in distance between the second registration hole of the screen mask and the second registration hole of the substrate in the X axis and the Y axis. 
     In accordance with embodiments of this invention, the step of calculating the alignment values comprises modifying the calculated offsets so that the differences in the X axis and the Y axis between the first registration hole of the screen mask and the first registration hole of the substrate are same as the differences in the X axis and the axis between the second registration hole of the screen mask and the second registration hole of the substrate respectively. 
     In accordance with embodiments of this invention, the step of modifying the calculated offsets comprises: calculating a X_alignment value by this equation: (X 1 +X 2 )/2 wherein X 1  is the measured difference in the X axis between the first registration holes of the screen mask and the substrate and X 2  is the measured difference in the X axis between the second registration holes of the screen mask and the substrate; and calculating a Y_alignment value by this equation: (Y 1 +Y 2 )/2 wherein Y 1  is the measured difference in the Y axis between the first registration holes of the screen mask and the substrate and Y 2  is the measured difference in the Y axis between the second registration holes of the screen mask and the substrate. 
     In accordance with embodiments of this invention, the step of calculating the alignment values further comprises calculating a relative angle between two intersecting line segments (L, L′) based on the X_alignment value, the Y_alignment value and the calculated offsets wherein a first one of the intersecting line segments is linearly connecting the first and second registration holes of the screen mask and a second one of the intersecting line segments is linearly connecting the first and second registration holes of the substrate. The relative angle is calculated using a trigonometry formula. 
     In accordance with embodiments of this invention, the step of correcting position of the substrate comprises moving the substrate laterally along the X axis by the X_alignment value; moving the substrate laterally along the Y axis by the Y_alignment value; rotating the substrate by the relative angle around an axis perpendicular to the X-Y plane wherein the perpendicular axis is at an intersecting point of the two intersecting line segments (L, L′). The intersecting point is a middle point between the first and second registration holes of the screen mask and a middle point between the first and second registration holes of the substrate. 
     In accordance with embodiments of this invention, the first and second registration holes of the screen mask are being formed at opposite diagonal corner regions of the screen mask. The first and second registration holes of the substrate are being formed at opposite diagonal corner regions of the substrate. In accordance with an embodiment of this invention, the substrate is a low temperature co-fired ceramic tape. 
     In accordance with a second aspect of this invention, a system for screen printing with alignment of substrate with screen mask is provided in the following manner. The system comprises: a screen mask holder for holding a screen mask wherein the screen mask has first and second registration holes through the screen mask; a substrate holder for receiving a substrate wherein the substrate has first and second registration holes through the substrate; first and second image capturing devices; an alignment system has a processing unit and a memory readable by the processing unit; and instructions stored by the memory that direct the processing unit to: position the substrate holder below the screen mask holder so that the substrate being substantially parallel to the screen mask; capture a first image of the first registration holes of the screen mask and the substrate with the first image capturing device wherein the first image showing the first registration hole of the screen mask superimposed with the first registration hole of the substrate; capture a second image of the second registration holes of the screen mask and the substrate with the second image capturing device wherein the second image showing the second registration hole of the screen mask superimposed with the second registration hole of the substrate; determine from the first and second images whether the first and second registration holes of the substrate are vertically aligned with the first and second registration holes of the screen mask respectively; calculate offsets between the first registration holes from the first image and offsets between the second registration holes from the second image in response to a determination that at least one of the registration holes of the substrate is not vertically aligned with the respective registration hole of the screen mask; calculate alignment values for correcting position of the substrate based on the calculated offsets; and correct position of the substrate based on the calculated alignment values so that the substrate is vertically aligned with the screen mask. 
     In accordance with embodiments of this invention, the substrate holder is movable laterally along a first axis relative to the screen mask. The substrate holder is movable laterally along a second axis orthogonal to the first axis relative to the screen mask. The substrate holder is rotatable around an axis perpendicular to the screen mask. The screen mask holder is movable vertically relative to the substrate holder. 
     In accordance with embodiments of this invention, the system further comprises a slidable table on which the substrate holder is mountable wherein the slidable table is movable laterally relative to the screen mask holder. The system further comprises an interface unit in communication with the alignment system wherein the interface unit allows a user to view images and/or enter data. For example, the interface unit is a touch screen. 
     In accordance with embodiments of this invention, the first and second registration holes of the screen mask are being formed at opposite diagonal corner regions of the screen mask. The first and second registration holes of the substrate are being formed at opposite diagonal corner regions of the substrate. The substrate is a low temperature co-fired ceramic tape. 
     In accordance with embodiments of this invention, the instructions to calculate the offsets comprise: instructions to measure differences in distance between the first registration hole of the screen mask and the first registration hole of the substrate in a X axis and a Y axis orthogonal to the X axis wherein the X and Y axes define a horizontal X-Y plane parallel to the screen mask and the substrate; and instructions to measure differences in distance between the second registration hole of the screen mask and the second registration hole of the substrate in the X axis and the Y axis. 
     In accordance with embodiments of this invention, the instructions to calculate the alignment values comprise instructions to modify the calculated offsets so that the differences in the X axis and the Y axis between the first registration hole of the screen mask and the first registration hole of the substrate are same as the differences in the X axis and the Y axis between the second registration hole of the screen mask and the second registration hole of the substrate respectively. 
     In accordance with embodiments of this invention, the instructions to modify the calculated offsets comprise: instructions to calculate a X_alignment value by this equation: (X 1 +X 2 )/2 wherein X 1  is the measured difference in the X axis between the first registration holes of the screen mask and the substrate and X 2  is the measured difference in the X axis between the second registration holes of the screen mask and the substrate; and instructions to calculate a Y_alignment value by this equation: (Y 1 +Y 2 )/ 2  wherein Y 1  is the measured difference in the Y axis between the first registration holes of the screen mask and the substrate and Y 2  is the measured difference in the Y axis between the second registration holes of the screen mask and the substrate. 
     In accordance with embodiments of this invention, the instructions to calculate the alignment values further comprise instructions to calculate a relative angle between two intersecting line segments (L, L′) based on the X_alignment value, the Y_alignment value, and the calculated offsets wherein a first one of the intersecting line segments is linearly connecting the first and second registration holes of the screen mask and a second one of the intersecting line segments is linearly connecting the first and second registration holes of the substrate. The relative angle is calculated using a trigonometry formula. 
     In accordance with embodiments of this invention, the instructions to correct position of the substrate comprise instructions to move the substrate laterally along the X axis by the X_alignment value; instructions to move the substrate laterally along the Y axis by the Y_alignment value; instructions to rotate the substrate by the relative angle around an axis perpendicular to the X-Y plane wherein the perpendicular axis is at an intersecting point of the two intersecting line segments (L, L′). The intersecting point is a middle point between the first and second registration holes of the screen mask and a middle point between the first and second registration holes of the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of this invention will now be described, by way of example only, with reference to the accompany drawings, in which: 
         FIG. 1  is a perspective view of a screen printing system according to an embodiment of the invention; 
         FIG. 2  is an enlarged view of a screen mask holder and a substrate holder in the screen printing system shown in  FIG. 1 ; 
         FIG. 3A  is a plan view of two perfectly aligned registrations holes of the screen mask and the substrate; 
         FIG. 3B  is a plan view two misaligned registrations holes of the screen mask and the substrate; 
         FIG. 4  is a flow chart showing an alignment method of the substrate with the screen mask in according to an embodiment of the invention; 
         FIG. 5A  is a plan view image captured by the first camera showing offsets (X 1 , Y 1 ) in the X axis and the Y axis of first registration holes of the screen mask and the substrate; 
         FIG. 5B  is a plan view image captured by the second camera showing offsets (X 2 , Y 2 ) in the X axis and the Y axis of second registration holes of the screen mask and the substrate; 
         FIG. 5C  showing the offset values are determined by the direction of the measurement. 
         FIG. 6  is an illustration showing the modified offsets of the registration holes of the screen mask and the substrate. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention relates to a screen printing system and a method for aligning the substrate with the screen mask. The alignment values for correcting the position of the substrate are based on the offsets between registration holes of the screen mask and the substrate. 
       FIGS. 1 and 2  show perspective views of a screen printing system  100  in accordance with an embodiment of the invention. Screen printing system  100  has base  101  on which various components are mounted or connected to, including, but not limited to, screen mask station  103 , substrate station  105  and input/output unit  107 . Base  101  could be the housing for certain hardware and software components, for example processing unit and memory unit of an alignment system of screen printing system  100 . 
     Screen mask station  103  is disposed above base  101  and supported by a plurality of legs  109 . Each leg  109  at one end is connected to screen mask station  103  and at the other end is connected to base  101 . All of the legs  109  are movable vertically and simultaneously relative to base  101  so that the height of screen mask station  103  can be adjusted. Screen mask station  103  comprises screen mask holder  113  for holding a screen mask  111 . Screen mask  111  is a metal plate, preferably in a rectangular shape, having mesh holes defining a printing pattern. Two registration holes  115 A,  115 B are formed through screen mask  111 . Preferably, through holes  115 A,  115 B are formed at the opposite diagonal corners of screen mask  111 . Although two registration holes are described in this application, more holes may be formed in screen mask  111 . Through holes  115 A,  115 B are useful for the alignment process which will be discussed later. Each of the registration holes  115 A,  115 B has a circular shape (viewing from top) with a diameter of approximately 3 mm. Other shapes of registration hole are possible without departing from this invention. Screen mask holder  113  is removably mounted to screen mask station  103  by locking means, such as locking plate  117  as shown in  FIG. 1 . Preferably, screen mask holder  113  has a frame-shaped opening in which a screen mask can be attached to. This frame-shaped opening is useful as a screen mask can be installed and replaced easily, without replacing the whole screen mask holder  113 . 
     Substrate station  105  is mounted on base  101  and comprises two components connected to each other, namely substrate holder  119  and slidable table  121 . Substrate holder  119  for holding a substrate is mounted on top of slidable table  121  so that movement of slidable table  121  causes substrate holder  119  to move together simultaneously. In a preferred embodiment of the invention, a low temperature co-fired ceramic (LTTC) tape is used as a substrate. However, substrate can be any materials which are suitable for screen printing process. Two registration holes  125 A,  125 B are formed through substrate  123 . Preferably, through holes  125 A,  125 B are formed at the opposite diagonal corners of substrate  123 . Although two registration holes  125 A,  125 B are described in this application, more holes may be formed in substrate  123 . Similar to screen mask  111 , each of the registration holes  125 A,  125 B has a circular shape (viewing from top) with a diameter of approximately 3 mm. Other shapes of registration hole are possible without departing from this invention. In this invention, the size of registration holes  125 A,  125 B of substrate  123  is substantially same as the size of registration holes  115 A,  115 B of screen mask  111 . 
     Slidable table  121  is movable laterally along a X axis (shown by arrow X in  FIGS. 1 and 2 ) between a loading position and an alignment position. At loading position (see  FIG. 1 ), substrate station  105  is at an access area away from screen mask station  103  so that substrate  123  can be conveniently loaded onto substrate holder  119 . At alignment position (see  FIG. 2 ), substrate station  105  is positioned directly below a screen mask held by screen mask holder  113  so that substrate  123  being substantially parallel and adjacent to screen mask  111 . If registration holes  125 A,  125 B of substrate  123  are precisely coincided with registration holes  115 A,  115 B of screen mask  111  respectively, substrate  123  will be perfectly aligned with screen mask  111 . In other words, circular shapes of registration holes  115 A,  115 B of screen mask  111  are perfectly superimposed on the circular shapes of registration holes  125 A,  125 B of substrate  123  respectively (see  FIG. 3A  for an example). If this is the case, substrate  123  may proceed directly to screen printing process. However, if any one (or both) of the registration holes  125 A,  1258  of substrate  123  is not precisely coincided with the respective one of the registration holes  115 A,  115 B of screen mask  111 , substrate  123  will be misaligned with screen mask  111 . In other words, at least one of the registration holes  115 A,  1158  of screen mask  111  is not perfectly superimposed on the respective one of the registration holes  125 A,  125 B of substrate  123  (see  FIG. 3B  for an example). If this is the case, the position of substrate  123  has to be adjusted to correct the misalignment before the screen printing process. 
     The position of substrate holder  119  is independently adjustable in different directions for aligning substrate  123  with screen mask  111 . Substrate holder  119  is movable laterally along a X axis (shown by arrow X in  FIGS. 1 and 2 ). Substrate holder  119  is also movable laterally along a Y axis (shown by arrow Y in  FIGS. 1 and 2 ) which is orthogonal to the X axis in the same plane. X and Y axes define a two-dimensional horizontal plane which is referred to as X-Y plane, parallel to screen mask  111  and substrate  123 . Further, substrate holder  119  is rotatable (clockwise or anti-clockwise between X and Y axes) in the X-Y plane around an axis perpendicular to the X-Y plane. In other words, substrate holder  119  is rotatable around an axis perpendicular to screen mask  111 . 
     In screen printing station  103 , squeegee  127  and a chamber containing a printing material (e.g. conductive paste) are connected to movable plate  129 . Movable plate  129  is mounted to screen mask station  103  and is movable to bring the attached squeegee  127  to slide across the top surface of screen mask  111 . During printing process, a printing material is dispensed on screen mask  111  and squeegee  127  is made to slide on screen mask  111  to apply the printing material across the surface of screen mask  111 . This causes the printing material to pass through the pattern/mesh holes of screen mask  111  onto the printing surface of substrate  123  positioned below screen mask  111 . 
     Two cameras,  131 ,  132  are attached to screen mask station  103  and disposed above registration holes  115 A,  115 B of screen mask  111  respectively. Preferably, cameras  131 ,  132  are positioned directly above and adjacent to registration holes  115 A,  115 B respectively so that, at alignment position, an image of two superimposed circulars of registration holes ( 115 A superimposed with  125 A;  115 B superimposed with  125 B) of screen mask  111  and substrate  123  can be captured by each camera clearly. These images are useful in positional alignment process in accordance with this invention. Although a camera is described in this application, one skilled in the art will recognise that other image capturing devices can be used. 
     In accordance with an embodiment of the invention, the position of screen mask  111  is fixed which represents the expected position for printing process. Therefore, the position of substrate  123  has to be aligned with screen mask  111  before a printing process can be carried out. If substrate  123  is aligned with screen mask  111 , the image captured by first camera  131  shows two perfectly superimposed (matched) circulars of registration holes  115 A,  125 A; and the image captured by second camera  132  shows two perfectly superimposed (matched) circulars of registration holes  115 B,  125 B. See  FIG. 3A  as an example. If substrate  123  is misaligned with screen mask  111 , the image captured by first camera  131  shows two not perfectly superimposed (mismatched) circulars of registration holes  115 A,  1254 ; and/or the image captured by second camera  132  shows two not perfectly superimposed (mismatched) circulars of registration holes  115 B,  125 B. See  FIG. 4B  as an example. 
     A method for aligning substrate  123  with screen mask  111  automatically by calculating alignment values to correct the position of substrate  123  is described in the following manner and illustrated by a flow chart  400  shown in  FIG. 4 , in accordance with an embodiment of the invention. A screen mask  111  having two registration holes  115 A,  115 B is installed in screen mask station  103 . In step  401 , a substrate  123  having two registration holes  125 A,  1258  is placed on substrate station  105  and positioned below screen mask  111  at alignment position. Substrate  123  is substantially parallel and adjacent to screen mask  111 . In step  402 , first camera  131  is activated to capture an image of first registration holes of screen mask  111  and substrate  123 . The image captured by first camera  131  shows two superimposed circulars of first registration holes  115 A,  125 A. Second camera  132  is also activated to capture an image of second registration holes of screen mask  111  and substrate  123 . The image captured by second camera  132  shows two superimposed circulars of second registration holes  115 B,  125 B. 
     In step  403 , the method determines whether substrate  123  is perfectly aligned with screen mask  111  based on the images captured in step  402 . This step is performed by an image processing system. If substrate  123  is perfectly aligned with screen mask  111 , two perfectly superimposed circulars of registrations holes is observed from each captured image (i.e. no offset between two superimposed circulars). See  FIG. 3A  for an example. Hence, substrate  123  may proceed directly to screen printing process in step  407 . However, if substrate  123  is misaligned with screen mask  111 , at least one of the captured images showing two not perfectly superimposed circulars of registrations holes (i.e. offsets are observed between two superimposed circulars). See  FIG. 3B  for an example. Hence, the position of substrate  123  has to be corrected so that registration holes  125 A,  125 B of substrate  123  are vertically aligned with registration holes  115 A,  115 B of screen mask  111  respectively. 
     In step  405 , positional alignment calculation is performed and alignment values for compensating offsets between the registration holes of substrate  123  and screen mask  111  are determined. The details of step  405  are described in the following manner with reference to  FIGS. 5 and 6 . 
     Differences in distance (i.e. offsets) between two superimposed circulars of first registration holes ( 115 A,  125 A) of screen mask  111  and substrate  123  with respect to X axis and Y axis are determined by an image processing system based on the image captured by first camera  131  in step  402 . These offsets are denoted by ‘X 1 ’ in X axis and ‘Y 1  in Y axis as shown in  FIG. 5A . Similarly, offsets between two superimposed circulars of second registration holes ( 115 B,  125 B) of screen mask  111  and substrate  123  with respect to X axis and Y axis are determined by an image processing system based on the image captured by second camera  132  in step  402 . These offsets are denoted by ‘X 2 ’ in X axis and ‘Y 2 ’ in Y axis as shown in  FIG. 5B . The offset values may be positive or negative depending on the direction of the measurement as shown in  FIG. 5C . For example, offsets X 1  and X 2  are positive values if they are measured in the X-direction from left to right. Conversely, in the opposite direction, i.e. from right to left, X 1  and X 2  are negative values. Offsets Y 1  and Y 2  are positive values if they are measured in the Y-direction from bottom to top. Conversely, in the opposite direction, i.e. from top to bottom, Y 1  and Y 2  are negative values. 
     Based on the determined offsets X 1 , Y 1 , X 2  and Y 2 , mean value of X 1  and X 2  (denoted by ‘X_alignment’) and mean value of Y 1  and Y 2  (denoted by ‘Y_alignment’) are calculated using the following equations:
 
 X _alignment=( X 1 +X 2)/2 , Y _alignment=( Y 1 +Y 2)/2
 
Subsequently, offsets X 1 , Y 1 , X 2  and Y 2  are being modified to X 1 ′, Y 1 ′, X 2 ′ and Y 2 ′ respectively using the following equations:
 
 X 1 ′=X 1 −X _alignment , Y 1 ′=Y 1 −Y _alignment
 
 X 2 ′=X 2 −X _alignment , Y 2 ′=Y 2 −Y _alignment
 
     The purpose of modifying the offset values is to ensure that the offsets in X and Y axes between the first registration holes  115 A,  125 A are same as the offsets in X and Y axes between the second registration holes  115 B,  125 B, i.e. magnitudes of X 1 ′ and Y 1 ′ are same as magnitudes of X 2 ′ and Y 2 ′ respectively. Therefore, if X 1 ′=X 2 ′ and Y 1 ′=Y 2 ′, the line L connecting the first and second registration holes  115 A,  115 B of screen mask  111  and line L′ connecting the first and second registration holes  125 A,  125 B of substrate  123  will intersect at an intersecting point P (see  FIG. 6 ) which is the middle point of line L and line L′. As a result of the offset modification, line L is deviated from line L′ by a relative angle θ in X-Y plane, and θ=θ1=θ2 as shown in  FIG. 6 . A rotation of the substrate around point P in X-Y plane by an angle θ will result in line L coincides with line L′, in which first and second registration holes  125 A,  125 B of substrate  123  are perfectly aligned with first and second registration holes  115 A,  115 B of screen mask  111 . Therefore, positional alignment of substrate  123  is accomplished and substrate  123  is ready for printing process in step  407 . Relative angle θ is determined by the following equation: 
             θ   =       cos     -   1       ⁡     (         l   2     +     l   2     -     z   2         2   ⁢           ⁢   ll       )             
where l is the half distance of L (or L′ since L′=L), i.e. distance between the intersecting point P and the first registration hole or second registration hole of screen mask  111  (or substrate  123 ). l is a predetermined value that can be calculated with the equation √{square root over (a 2 +b 2 )}/2, where a is the distance between the first and second registration holes of screen mask  111  (or substrate  123 ) along the Y axis, and b is the distance between the first and second registration holes of screen mask  111  (or substrate  123 ) along the X axis. z is the distance between the center point of the first (or second) circular registration hole of screen mask  111  and the center point of the first (or second) circular registration hole of substrate  123 . See  FIG. 6 . The value of z can be calculated with the equation
 
√{square root over ( X 1′ 2   +Y 1′ 2 )}(or √{square root over ( X 2′ 2   +Y 2′ 2 ))}.
 
     At step  406 , the position of substrate  123  is corrected based on the calculated alignment values in step  405 . Substrate  123  is arranged to move laterally along the X axis by the calculated X_alignment and move laterally along the Y axis by the calculated Y_alignment. These movements ensure that the offsets in X and Y axes between the first registration holes  115 A,  125 A are same as the offsets in X and Y axes between the second registration holes  115 B,  125 B. After that, substrate  123  is arranged to rotate around an axis perpendicular to X-Y plane by the calculated relative angle θ. As such, substrate  123  will be vertically aligned with screen mask  111 . 
     At the stage where substrate  123  is perfectly aligned with screen mask  111 , screen printing process on substrate  123  will begin in step  407 . An optional step may be implemented between step  406  and  407  to check if the positional alignment of substrate  123  is successful. In this optional step, images of the registration holes will be captured similar to step  402  and determined whether substrate  123  is perfectly aligned with screen mask  111  before proceeding to screen printing process in step  407 . If substrate  123  is not properly aligned with screen mask  111 , alignment process may be activated and steps  405  and  406  are repeated. 
     While preferred embodiments of the present invention have been described and illustrated above, it is to be understood that they are exemplary of the invention and are not to be considered to be limiting. It is expected that those skilled in the art can and will design alternative embodiments that infringe this invention as set forth in the following claims.