Screen printing method and screen printing apparatus

Abstract

A screen-printing method according to the present disclosure is for printing cream solder on a substrate using a mask, and comprises: a normal-printing step; and an overlay-printing step in which the normal-printing step is performed a plurality of times on a single substrate. In the normal-printing step, a plate alignment operation of overlaying and aligning the substrate and the mask with each other in a first direction, a printing operation of printing the cream solder on the substrate through openings provided in the mask, and a plate separation operation of separating the substrate and the mask from each other in the first direction are carried out. The overlay-printing step comprises a first normal-printing step and a second normal-printing step. The second normal-printing step is performed, after the first normal-printing step, without moving the substrate from a printing space.

Description

Technical Field

[0001] The present disclosure relates to a screen printing method and a screen printing apparatus.

Background Art

[0002] Conventionally, a screen printing method described in Japanese Patent Application Laid-Open No. 2005-205826 (hereinafter referred to as Patent Document 1) is known. In this screen printing method, in the process of continuing automatic operation, when the stop state of the screen printing apparatus continues for a time longer than a preset time as a reference time required for paste state confirmation, the automatic operation is stopped and a warning is issued to that effect. The operator is prompted by this warning to visually check the state of the cream solder. When it is determined that the cream solder is in a slightly dry state and good filling property into the pattern holes of the mask plate cannot be ensured in normal printing, the operator instructs the screen printing apparatus to perform overlay printing. Here, overlay printing means continuously printing the cream solder on the same substrate a plurality of times in an overlapping manner. The overlay printing is preferably performed by the reciprocating motion of the squeegee.

[0003] By this overlay printing, even when the cream solder is in a somewhat dry state, the decrease in filling property can be compensated by printing the cream solder in an overlapping manner, and printing defects can be prevented. Then, after this overlay printing, the operation state is shifted to the normal automatic operation state, and production can be resumed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the above screen printing method, the steps of checking the condition of the solder paste, determining whether overprinting is necessary, and issuing instructions to the screen printing device must be performed by an operator. Furthermore, depending on the screen printing conditions, it may not be possible to prevent printing defects even if the above screen printing method is followed. [Means for solving the problem]

[0006] The screen printing method of this disclosure is a screen printing method for printing solder paste onto a substrate using a mask, comprising: a normal printing step and a superimposed printing step in which the normal printing step is performed multiple times on one substrate, wherein the normal printing step performs a plate alignment operation to overlap the substrate and the mask in a first direction, a printing operation to print solder paste onto the substrate through an opening provided in the mask, and a plate separation operation to separate the substrate and the mask in the first direction, wherein the superimposed printing step comprises a first normal printing step and a second normal printing step, wherein the second normal printing step is performed after the first normal printing step without moving the substrate from the printing space.

[0007] Furthermore, the screen printing apparatus of the present disclosure is a screen printing apparatus for printing solder paste onto a substrate using a mask, comprising: a substrate transport unit for moving the substrate into a printing space; a mask holder for holding the mask; a substrate holding unit for holding the substrate; a positioning mechanism for moving at least one of the mask holder and the substrate holding unit to bring the substrate and the mask into contact with and separate them in a first direction; a printing unit for printing solder paste onto the substrate through an opening provided in the mask; and a control unit, wherein the control unit performs a normal printing process and, when it determines that predetermined preconditions have been met, performs the same process on one of the substrates. The screen printing apparatus performs a superimposed printing process which involves performing a normal printing process multiple times, wherein the normal printing process includes a plate alignment operation which controls the positioning mechanism to superimpose the substrate and the mask in the first direction, a printing operation which controls the printing unit to print solder paste onto the substrate, and a plate separation operation which controls the positioning mechanism to separate the substrate and the mask in the first direction, and the superimposed printing process includes a first normal printing process and a second normal printing process which is performed after the first normal printing process without moving the substrate from the printing space. [Effects of the Invention]

[0008] This disclosure provides a technology for suppressing printing defects in screen printing equipment. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a perspective view showing the external appearance of a screen printing apparatus according to Embodiment 1. [Figure 2] Figure 2 is a front view of the screen printing apparatus. [Figure 3] Figure 3 is a side view of the screen printing apparatus. [Figure 4] Figure 4 is a perspective view of the mask. [Figure 5] Figure 5 shows the electrical configuration of a screen printing apparatus. [Figure 6] Figure 6 is a flowchart illustrating the process of printing solder paste onto a circuit board. [Figure 7] Figure 7 is a flowchart illustrating the normal printing process. [Figure 8] Figure 8 is a flowchart illustrating the overprinting process. [Figure 9A] Figure 9A is an explanatory diagram illustrating the first plate alignment operation. [Figure 9B] Figure 9B is an explanatory diagram illustrating the first printing operation. [Figure 9C] Figure 9C is an explanatory diagram illustrating the first plate separation operation. [Figure 9D] Figure 9D is an explanatory diagram illustrating the second plate alignment operation. [Figure 9E] Figure 9E is an explanatory diagram illustrating the second printing operation. [Figure 9F] Figure 9F is an explanatory diagram illustrating the second plate separation operation. [Figure 10] Figure 10 is a graph showing the average volume fraction of the solder paste used in Experiment 1. [Figure 11] Figure 11 is a graph showing the average volume fraction of the solder paste used in Experiment 2. [Figure 12] Figure 12 is a flowchart illustrating the overprinting process according to Embodiment 2. [Modes for carrying out the invention]

[0010] [Description of Embodiments in this Disclosure] First, the embodiments of this disclosure will be listed and described. (1) The screen printing method of the present disclosure is a screen printing method for printing cream solder on a substrate using a mask, and includes a normal printing process and a multi - layer printing process of performing the normal printing process a plurality of times on one substrate. In the normal printing process, an alignment operation of overlapping the substrate and the mask in a first direction, a printing operation of printing cream solder on the substrate through an opening provided in the mask, and a separation operation of separating the substrate and the mask in the first direction are executed. The multi - layer printing process includes a first normal printing process and a second normal printing process. The second normal printing process is performed without moving the substrate from the printing space after the first normal printing process. This is a screen printing method.

[0011] According to such a screen printing method, since the multi - layer printing process includes a first normal printing process and a second normal printing process, the amount of cream solder printed on the substrate can be increased by the multi - layer printing process. Therefore, printing defects can be suppressed.

[0012] (2) In the screen printing method according to (1), the multi - layer printing process further includes a cleaning process of cleaning the mask. The cleaning process is preferably performed after the first normal printing process and before the second normal printing process.

[0013] According to such a screen printing method, by executing the cleaning process between the first normal printing process and the second normal printing process, it is possible to suppress the excessive filling of cream solder into the opening of the mask.

[0014] (3) In the screen printing method according to (1), the second normal printing process is preferably performed continuously immediately after the first normal printing process.

[0015] This screen printing method makes it possible to suppress the drying of the solder paste flux on the inner wall of the mask opening between the first and second normal printing processes. Therefore, it becomes easier to increase the amount of solder paste printed on the substrate in the second normal printing process.

[0016] (4) In the screen printing method described in any of (1) to (3), it is preferable that no inspection of the state of the solder paste printed on the substrate is performed between the first normal printing step and the second normal printing step.

[0017] This type of screen printing method allows for smooth overprinting processes.

[0018] (5) In the screen printing method described in any of (1) to (4), it is preferable that the conditions for the first normal printing step and the conditions for the second normal printing step are the same.

[0019] This type of screen printing method allows for uniform printing quality of solder paste.

[0020] (6) In the screen printing method described in any of (1) to (5), it is preferable that the overprinting step is performed when predetermined preconditions are met.

[0021] With this type of screen printing method, if certain preconditions are met that tend to reduce the filling rate of solder paste, a layer printing process can be performed to suppress printing defects.

[0022] In the screen printing method described in (7)(6), it is preferable that the overprinting step is performed on a predetermined number of substrates after the prior conditions have been met.

[0023] This type of screen printing method makes it easier to ensure print quality in the normal printing process when performing a normal printing process after a layered printing process.

[0024] In the screen printing method described in (8), (6), or (7), the precondition is preferably the replacement of the mask.

[0025] After mask replacement, flux is not supplied to the inner wall of the opening in the new mask, making it difficult for the solder paste to separate from the inner wall of the opening. However, with the screen printing method described above, printing defects can be suppressed by performing an overprinting process.

[0026] In the screen printing method described in any of (9)(6) to (8), the precondition is preferably cleaning of the mask.

[0027] Since flux is not supplied to the inner wall of the opening in the mask after cleaning, the solder paste becomes difficult to separate from the inner wall of the opening. However, with the screen printing method described above, printing defects can be suppressed by performing an overprinting process.

[0028] In the screen printing method described in any of (10)(6) to (9), the precondition is preferably the elapsed time after the normal printing process.

[0029] Normally, after a certain amount of time has passed since the printing process, the flux in the openings of the mask dries, making it difficult for the solder paste to separate from the inner wall of the openings. However, with the screen printing method described above, printing defects can be suppressed by performing an overprinting process.

[0030] In the screen printing method described in any of (11)(6) to (10), the prior condition is preferably the drying of solder paste.

[0031] Once the solder paste dries, it becomes difficult for it to separate from the inner wall of the mask's opening. However, with the screen printing method described above, the overprinting process can suppress printing defects.

[0032] (12) The screen printing apparatus of the present disclosure is a screen printing apparatus for printing solder paste onto a substrate using a mask, comprising: a substrate transport unit for moving the substrate into a printing space; a mask holder for holding the mask; a substrate holding unit for holding the substrate; a positioning mechanism for moving at least one of the mask holder and the substrate holding unit to bring the substrate and the mask into contact with and separate from each other in a first direction; a printing unit for printing solder paste onto the substrate through an opening provided in the mask; and a control unit, wherein the control unit performs a normal printing process and, when it determines that predetermined preconditions have been met, performs a precondition for one substrate. The screen printing apparatus performs a superimposed printing process which involves performing a normal printing process multiple times, wherein the normal printing process includes a plate alignment operation which controls the positioning mechanism to superimpose the substrate and the mask in the first direction, a printing operation which controls the printing unit to print solder paste onto the substrate, and a plate separation operation which controls the positioning mechanism to separate the substrate and the mask in the first direction, and the superimposed printing process includes a first normal printing process and a second normal printing process which is performed after the first normal printing process without moving the substrate from the printing space.

[0033] With this configuration, the overprinting process is executed when certain preconditions are met that tend to reduce the solder paste filling rate. Since the overprinting process comprises a first normal printing process and a second normal printing process, the amount of solder paste printed on the substrate can be increased. Therefore, printing defects can be suppressed.

[0034] In the screen printing apparatus described in (13)(12), the control unit further performs a cleaning process to clean the mask during the overprinting process, and it is preferable that the cleaning process is performed after the first normal printing process and before the second normal printing process.

[0035] With this configuration, a cleaning process is performed between the first normal printing process and the second normal printing process, which prevents excessive filling of the mask openings with solder paste.

[0036] In the screen printing apparatus described in (14)(12), it is preferable that the control unit immediately and continuously executes the second normal printing process after the first normal printing process.

[0037] With this configuration, it is possible to suppress the drying of the solder paste flux on the inner wall of the mask opening between the first and second normal printing processes. Therefore, it becomes easier to increase the amount of solder paste printed on the substrate during the second normal printing process.

[0038] In the screen printing apparatus described in any of (15)(12) to (14), it is preferable that the control unit does not perform an inspection of the state of the solder paste printed on the substrate between the first normal printing process and the second normal printing process.

[0039] This configuration allows for smooth overprinting.

[0040] In the screen printing apparatus described in any of (16)(12) to (15), it is preferable that the control unit determines that the preconditions have been met when the mask has been replaced.

[0041] After mask replacement, flux is not supplied to the inner wall of the opening in the new mask, making it difficult for the solder paste to separate from the inner wall of the opening. However, with the above configuration, printing defects can be suppressed by performing a layer printing process.

[0042] In the screen printing apparatus described in any of (17)(12) to (16), it is preferable that the control unit determines that the preconditions have been met when the mask has been cleaned.

[0043] Since flux is not supplied to the inner wall of the opening in the mask after cleaning, the solder paste becomes difficult to separate from the inner wall of the opening. However, with the above configuration, printing defects can be suppressed by performing the overprinting process.

[0044] In the screen printing apparatus described in any of (18)(12) to (17), it is preferable that the control unit determines that the preconditions have been met when a predetermined time has elapsed after the normal printing process.

[0045] Normally, after a certain amount of time has passed since the printing process, the flux in the openings of the mask dries, making it difficult for the solder paste to separate from the inner wall of the openings. However, with the above configuration, printing defects can be suppressed by performing a double printing process.

[0046] In the screen printing apparatus described in any of (19)(12) to (18), it is preferable that the control unit determines that the preconditions have been met when the solder paste is dry.

[0047] Once the solder paste dries, it becomes difficult for it to separate from the inner wall of the mask's opening. However, with the above configuration, printing defects can be suppressed by performing a layered printing process.

[0048] [Details of the embodiments of this disclosure] The embodiments of this disclosure are described below. However, this disclosure is not limited to these examples, and is intended to include all modifications within the meaning and scope of the claims, as indicated by the claims.

[0049] [Embodiment 1] Embodiment 1 of this disclosure will be described with reference to Figures 1 to 11. Hereinafter, the left-right direction in Figure 1 will be referred to as the X direction, the front-back direction as the Y direction, and the up-down direction as the Z direction. Note that in the case of multiple identical members, reference numerals may be assigned to only some of the members, while the reference numerals for other members may be omitted.

[0050] <Overall configuration of a screen printing machine> The screen printing apparatus 1 according to Embodiment 1 is a device that prints solder paste onto a substrate P using a mask 12. As shown in Figure 1, the screen printing apparatus 1 is equipped with a box-shaped housing 10. An entrance for loading the substrate P is formed in the right wall of the housing 10. An exit for unloading the substrate P is formed in the left wall of the housing 10. The substrate P is loaded into the housing 10 through the entrance on the right wall, and after a circuit pattern is printed on it with solder paste inside the housing 10, it is unloaded through the exit on the left wall.

[0051] As shown in Figure 2, the screen printing apparatus 1 includes a metal base 11, a mask 12, a mask holder 13, a substrate transport unit 14, a printing table 15 (an example of a positioning mechanism), a printing unit 16, an inspection unit 17, a mask replacement unit 18 (see Figure 5), a cleaning unit 19 (see Figure 5), and the like.

[0052] As shown in Figure 4, the mask 12 comprises a frame-shaped mask frame 12A and a plate-shaped mask body 12C supported inside the mask frame 12A via an elastic member 12B. The mask body 12C is made of aluminum, iron, or the like. The mask body 12C has openings 12D that correspond to the circuit pattern to be printed. The elastic member 12B is made of a plate-shaped resin or rubber or the like.

[0053] As shown in Figure 2, the mask holder 13 is provided between the printing section 16 and the inspection section 17. The mask holder 13 consists of two rail sections 13A that are spaced apart in the X direction and extend in the Y direction (perpendicular to the paper plane). The left rail section 13A has a bottom wall and a side wall that rises upward from the left edge of the bottom wall. The right rail section 13A has a bottom wall and a side wall that rises upward from the right edge of the bottom wall. Both sides of the mask 12 in the X direction are supported by the two rail sections 13A.

[0054] The substrate transport section 14 is equipped with three conveyors (upstream conveyor 14A, main conveyor 14B, and downstream conveyor 14C) arranged in the X direction. The upstream conveyor 14A and the downstream conveyor 14C are provided on the base 11, and the main conveyor 14B is provided on the printing table 15. Each conveyor is equipped with a pair of conveyor belts (not shown) spaced apart in the Y direction and driven in a circulating manner in the X direction, and motors (not shown) that drive these conveyor belts.

[0055] As shown in Figures 2 and 3, the printing table 15 is provided on the upper surface of the base 11. The printing table 15 includes a flat table 20, a ball screw 21 extending vertically, a motor (not shown) for rotating the ball screw 21, a substrate fixing unit 22 for fixing the substrate P (an example of a substrate holder), and a backup unit 23 for supporting the substrate P fixed to the substrate fixing unit 22 from below. The table 20 moves up and down as the ball screw 21 rotates. In other words, the printing table 15 can raise and lower the substrate P fixed to the substrate fixing unit 22.

[0056] As shown in Figure 3, the substrate fixing unit 22 consists of a pair of clamps 22A that clamp and fix the substrate P from both sides in the Y direction, a motor (not shown) that drives the clamps 22A, and the like. The clamps 22A fix the substrate P that has been transported to the printing space above the backup unit 23. The clamps 22A have through holes that penetrate vertically. The lower side of the through holes is connected to an air supply device via an air hose. When the table 20 rises while the substrate fixing unit 22 is fixing the substrate P, the upper surface of the clamps 22A comes into close contact with the lower surface of the mask body 12C of the mask 12, and in this state, negative pressure is supplied to the through holes, causing the mask body 12C to be attracted to the clamps 22A.

[0057] As shown in Figures 2 and 3, the backup unit 23 includes a plate-shaped lower plate 23A, a plate-shaped upper plate 23B positioned above the lower plate 23A, a plurality of backup pins 23C supporting the substrate P from below, a ball screw 23D extending vertically, and a motor (not shown) that rotates the ball screw 23D. The backup unit 23 moves up and down as the ball screw 23D rotates.

[0058] Although not shown in the diagram, the upper plate 23B has multiple rows of holes arranged in a single line in the Y direction and spaced apart in the X direction. Backup pins 23C are inserted into these holes. The position of the holes into which the backup pins 23C are inserted varies depending on the type of substrate P. When the type of substrate P being produced is switched, the arrangement of the backup pins 23C is changed according to the type of substrate P after the switch.

[0059] The printing unit 16 includes a squeegee unit 16A as shown in Figure 2, a pair of guide rails 16B that support the squeegee unit 16A so that it can reciprocate in the Y direction, a ball screw (not shown), a motor (not shown) for rotating the ball screw, and the like. The squeegee unit 16A moves in the Y direction as the ball screw rotates.

[0060] As shown in Figures 2 and 3, the squeegee unit 16A includes a solder supply unit 16C that supplies paste-like solder cream onto the mask 12 (mask body 12C), a squeegee 16E that smooths the supplied solder cream, a motor (not shown) that raises and lowers the squeegee 16E, and a motor (not shown) that rotates the squeegee 16E around a rotation axis that extends in the X direction (perpendicular to the plane of the paper in Figure 3). The squeegee 16E has an elongated shape in the X direction.

[0061] As shown in Figure 2, the inspection unit 17 is provided between the mask holder 13 and the printing table 15. The inspection unit 17 is used to inspect the condition of the lower surface of the mask 12 and the circuit pattern of the solder paste printed on the substrate P by imaging the lower surface of the mask 12 and the upper surface of the substrate P. The inspection unit 17 includes a camera unit 17A, a beam 17B that supports the camera unit 17A so as to be able to reciprocate in the X direction, a ball screw 17C fixed to the beam 17B and extending in the X direction, a pair of guide rails 17D arranged on both sides of the beam 17B in the X direction and extending in the Y direction, a ball screw (not shown) extending in the Y direction, and a motor (not shown) for rotating this ball screw.

[0062] The camera unit 17A has a mask imaging camera 17E that images the lower surface of the mask 12 and a substrate imaging camera 17F that images the upper surface of the substrate P fixed to it. The camera unit 17A moves in the XY direction within a predetermined range of motion by the rotation of each ball screw. The inspection unit 17 may be configured to move the camera unit 17A by a linear motor. The inspection by the inspection unit 17 may be performed on each individual substrate P, or on multiple substrates at a time. Alternatively, the inspection may be performed at regular time intervals, or when a specified time is reached.

[0063] As shown in Figure 3, the squeegee unit 16A is equipped with a detection sensor 16F for measuring the volume of solder paste (solder roll R1) on the mask 12. The detection sensor 16F is mounted with its detection surface facing downwards. The detection sensor 16F is, for example, a reflective optical sensor, and detects the presence or absence of solder roll R1 supplied to the mask 12 based on the level of the received signal obtained by receiving reflected light from the surface of the mask 12. Specifically, the width (Y-direction dimension) of the solder roll R1 can be detected using the detection sensor 16F, and the volume of the solder roll R1 can be measured.

[0064] Although not shown in Figures 1 to 3, the screen printing apparatus 1 includes a mask replacement unit 18 and a cleaning unit 19 (see Figure 5). The mask replacement unit 18 is a mechanism for automatically replacing the masks 12. The mask replacement unit 18 is composed of multiple magazines capable of accommodating masks 12, a motor for raising and lowering the magazines, and the like. Replacement masks 12 are stored in the magazines. The mask replacement unit 18, for example, returns a mask 12 held in the mask holder 13 to an empty magazine and moves another mask 12 stored in a magazine to the mask holder 13.

[0065] The cleaning unit 19 includes, for example, a sheet roll around which a cleaning sheet such as paper or cloth is wound, a sheet roll shaft that rotatably holds the sheet roll, a winding roll for winding up used cleaning sheets, a winding shaft that rotatably holds the winding roll, a motor that rotates the winding shaft, a cleaning head positioned between the sheet roll shaft and the winding shaft, and a cylinder that raises and lowers the cleaning head.

[0066] The cleaning sheet is stretched from the sheet roll to the winding roll. When positive pressure is supplied to the cylinder of the cleaning unit 19, the cleaning head rises and the cleaning sheet is pressed against the underside of the mask 12. In this state, the winding roll rotates, cleaning the underside of the mask 12. The mask 12 may be cleaned after each substrate P, or after multiple substrates. Alternatively, the cleaning may be performed at regular time intervals, or when a specified time is reached.

[0067] <Electrical configuration of a screen printing machine> Referring to Figure 5, the electrical configuration of the screen printing apparatus 1 will be described. The screen printing apparatus 1 includes a control unit 30, a display unit 31, and an input unit 32. The control unit 30 includes a CPU 30A, RAM 30B, and a storage unit 30C. The storage unit 30C is a storage device having a non-volatile storage medium such as a hard disk. Various programs and data executed by the CPU 30A are stored in the storage unit 30C. The control unit 30 is connected to a display unit 31, an input unit 32, a board transport unit 14, a print table 15, a print unit 16, an inspection unit 17, a mask replacement unit 18, a cleaning unit 19, and the like. The display unit 31 is a display device such as a liquid crystal display. The input unit 32 is an input device such as a touch panel, mouse, or keyboard.

[0068] <Printing solder paste onto circuit boards> The screen printing apparatus 1 prints solder paste onto the substrate P according to the flowchart shown in Figure 6. First, the substrate P is transported to the printing space by the substrate transport unit 14 (S10). Here, the printing space is the space located above the backup unit 23, and the normal printing process or overprinting process described later is performed in this printing space.

[0069] In this embodiment, if at least one of the pre-set conditions (S20, S30, S40, S50) is met, the overprinting process (S70) is executed. If none of the preconditions are met, the normal printing process (S60) is executed. The overprinting process is a process in which the normal printing process is repeated at least twice on the same substrate P (details will be described later). The preconditions are conditions that have been empirically known to be prone to poor solder paste printing on the substrate P. A poor solder paste printing state is, for example, a state in which the amount of solder paste printed on the substrate P through the opening 12D of the mask 12 is insufficient. By executing the overprinting process when the preconditions are met, defects in solder paste printing on the substrate P can be suppressed.

[0070] One of the preconditions may be, for example, that the substrate P to be printed is the N (≤n) substrate P after the replacement of the mask 12 (S20: Yes). Here, n is a predetermined natural number, and N is any natural number less than or equal to n. n can be set by the operator or control unit 30 according to the type of mask 12, substrate P, solder paste, printing conditions, past production results, etc.

[0071] One of the preconditions may be, for example, that the substrate P to be printed is the M (≤ m) substrate P after cleaning the mask 12 (S30: Yes). Here, m is a predetermined natural number, and M is any natural number less than or equal to m. m can be set by the operator or control unit 30 depending on the type of mask 12, substrate P, solder paste, printing conditions, past production results, etc. m may be the same as or different from n.

[0072] One of the preconditions may be, for example, that a predetermined time has elapsed since the most recent printing (S40: Yes). By the time elapsed, it can be determined that the solder paste is dry without actually checking its condition. Here, the predetermined time can be set by the operator or the control unit 30 according to the type of solder paste, printing conditions, past production results, etc.

[0073] One of the preconditions may be, for example, that the solder paste is dry (S50: Yes). As described above, it is possible to determine whether the solder paste is dry after a predetermined time has elapsed, but if the temperature or airflow in the production environment changes due to, for example, a malfunction in the air conditioning, the solder paste may dry before the predetermined time has elapsed. Whether or not the solder paste is dry may be determined by the control unit 30 or by the operator. For example, the screen printing apparatus 1 is equipped with a viscometer capable of measuring the viscosity of the solder paste, and the control unit 30 may determine that the solder paste is dry when the measured value of the viscosity of the solder paste measured by the viscometer exceeds a predetermined threshold. Alternatively, the operator may visually check the condition of the solder paste and determine whether or not it is dry.

[0074] The parameters related to the above preconditions (such as n, m, predetermined time, viscosity threshold, etc.) are stored in the memory unit 30C. The control unit 30 can determine whether or not the above preconditions are met based on these parameters and various production data stored in the RAM 30B.

[0075] After a normal printing process (S60) or overprinting process (S70) is performed on the substrate P, the printing state of the substrate P may be inspected by the inspection unit 17 (S80). In the inspection of the substrate P, for example, the pattern of solder paste printed on the substrate P is captured by the substrate imaging camera 17F of the inspection unit 17, and the captured image is processed by the control unit 30 to determine whether the amount and position of the solder paste printed on the substrate P are appropriate. The inspection of the substrate P may be performed in the printing space, or at a location separate from the printing space within the screen printing apparatus 1.

[0076] After inspection of the substrate P (S80), the substrate P is transported out of the screen printing apparatus 1 by the substrate transport unit 14 (S90). The transported substrate P is then sent to, for example, an inspection device or a component mounting device.

[0077] <Normal printing process> Figure 7 is a flowchart illustrating the normal printing process. In the normal printing process, a plate alignment operation (S61) is performed in which the substrate P and the mask 12 are superimposed in the vertical direction (an example of the first direction) (see Figure 9A). In this embodiment, the plate alignment operation is performed by the movement of the substrate P relative to the mask 12 held in the mask holder 13. Specifically, the substrate P, fixed to the substrate fixing unit 22, rises due to the rotation of the ball screws 21 and 23D of the printing table 15 and is pressed against the lower surface of the mask 12.

[0078] After the plate alignment operation (S61), a printing operation (S62) is performed to print solder paste onto the substrate P through the opening 12D of the mask 12 (see Figure 9B). On the surface of the mask 12, a solder roll R1 is formed by solder paste supplied from the nozzle 16D of the solder supply unit 16C. The solder roll R1 is rod-shaped and extends in the X direction. The control unit 30 lowers the squeegee 16E onto the mask 12 and moves it from one side to the other in the Y direction (front-back direction). At this time, as the squeegee 16E slides along the surface of the mask 12, the solder roll R1 spreads out on the mask 12, and the opening 12D formed in the mask 12 is filled with solder paste. This makes it possible to print solder paste onto the substrate P superimposed on the lower surface of the mask 12.

[0079] After the printing operation (S62), a plate separation operation (S63) is performed to separate the substrate P and the mask 12 in the vertical direction (see Figure 9C). In this embodiment, the plate separation operation is performed in the opposite manner to the plate alignment operation, by moving the substrate P downward relative to the mask 12 held in the mask holder 13. With the above steps, the normal printing process is complete.

[0080] <Overprinting process> Figure 8 is a flowchart illustrating the overprinting process. In this embodiment, the overprinting process is performed twice. That is, the overprinting process includes a first normal printing process and a second normal printing process. In the first normal printing process, a first plate alignment operation (S71, see Figure 9A), a first printing operation (S72, see Figure 9B), and a first plate release operation (S73, see Figure 9C) are performed. In the second normal printing process, a second plate alignment operation (S74, see Figure 9D), a second printing operation (S75, see Figure 9E), and a second plate release operation (S76, see Figure 9F) are performed on the substrate P after the first normal printing process. The second normal printing process is performed after the first normal printing process without moving the substrate P from the printing space. Specifically, between the first normal printing process and the second normal printing process, the substrate transport unit 14 does not move the substrate P in the X direction.

[0081] In the first printing operation, the squeegee 16E moves from one side to the other in the Y direction, whereas in the second printing operation, the squeegee 16E moves from the other side to the one side in the Y direction. In other words, the direction of movement of the squeegee 16E is opposite in the first and second printing operations. Except for the direction of movement of the squeegee 16E in the printing operation, it is preferable that the conditions for the first normal printing process and the second normal printing process are the same. These conditions include, for example, the speed at which the squeegee 16E moves, the angle between the squeegee 16E and the mask 12 (attack angle), and the pressure applied when pressing the squeegee 16E against the mask 12 (printing pressure). By standardizing these conditions, the print quality can be made uniform.

[0082] In the overprinting process of this embodiment, no extra processing related to the substrate P, such as inspection of the printing state of the substrate P, is performed between the first normal printing process and the second normal printing process. Furthermore, the second normal printing process is performed immediately after the first normal printing process.

[0083] <Experiment 1> Next, Experiments 1 and 2, conducted to verify the effects of this disclosure, will be described. In Experiment 1, a mask that had been left to stand for 20 minutes after cleaning was used to perform a continuous overlay printing process on five substrates. For comparison, normal printing and reciprocal printing were also performed under the same conditions. Here, the reciprocal printing process is a process in which a plate alignment operation, a first printing operation, a second printing operation, and a plate release operation are performed in sequence. In the reciprocal printing process, the plate release operation is not performed between the first printing operation and the second printing operation.

[0084] For each substrate obtained through the above process, the average volume fraction of solder paste was calculated. Here, the volume fraction of solder paste is a numerical value representing the ratio of the volume of solder paste actually printed on the substrate through the opening to the internal space of the opening in the mask. The average volume fraction of solder paste is the average value taken over all the openings in the mask.

[0085] In Experiment 1, a Yamaha Motor Co., Ltd. demo board CUK-B1311-000 was used as the substrate, a Sun Kogyo Co., Ltd. printability evaluation mask CUK-M1311-100 was used as the mask, and Senju Metal Industries Co., Ltd. M705-GRN360-K2V (Type 4) was used as the solder paste. The squeegee speed was set to 50 mm / sec, the printing pressure to 60 N, and the attack angle to 55°.

[0086] <Experiment 1 Results> Graphs A, B, and C in Figure 10 show the results of Experiment 1, corresponding to overprinting, normal printing, and reciprocating printing, respectively. As shown in Graph B, when solder paste was printed using the normal printing method, the average volume fraction of solder paste was low on the first and second substrates. On the other hand, for the third and subsequent substrates, the average volume fraction of solder paste remained stable at around 100%.

[0087] As shown in Graph C, when solder paste was printed using a reciprocating printing process, the average volume fraction of solder paste on the first substrate was slightly lower.

[0088] As shown in Graph A, when solder paste is printed using the overlay printing process, the average volume ratio of the solder paste is approximately 100% on both the first and second circuit boards, indicating better print quality compared to normal printing or reciprocating printing processes.

[0089] <Discussion on the results of Experiment 1> The results of Experiment 1 will be discussed with reference to Figures 9A to 9F. Solder paste consists of liquid flux and granular solder balls. In Figures 9B to 9F, the flux of the solder paste (solder roll R1) is schematically shown in the shaded area, and the solder balls are shown in the white circles within the shaded area.

[0090] As shown in Figure 9A, it is thought that no flux adheres to the inner wall of the opening 12D of the cleaned mask 12. When solder paste is printed with insufficient flux on the inner wall of the opening 12D (see Figure 9B) and the stencil release operation is performed, the solder paste is difficult to separate from the inner wall of the opening 12D. Therefore, it is thought that some of the solder paste adheres to the inner wall of the opening 12D (see Figure 9C). Consequently, as shown in Graph B of Figure 10, it is thought that in a normal printing process, the average volume percentage of solder paste was small for the first and second substrates after cleaning. For the third and subsequent substrates, it is thought that the average volume percentage of solder paste stabilized to about 100% because the inner wall of the opening was sufficiently wetted with flux.

[0091] In the overprinting process, a second plate alignment operation (see Figure 9D), a second printing operation (see Figure 9E), and a second plate release operation (see Figure 9F) are performed on the substrate P in the state shown in Figure 9C. This is thought to supply flux to the inner wall of the opening 12D, improving the leakage of solder paste from the opening 12D, and supplying the substrate P with the amount of solder paste that was insufficient in the first normal printing process. As a result, as shown in Graph A of Figure 10, the average volume fraction of the solder paste is thought to have improved.

[0092] While the reciprocating printing process is similar to the overprinting process in that it involves two printing operations, it differs in that there is no plate separation operation between the first and second printing operations. Although the detailed mechanism is unknown, this difference is thought to make it easier to supply flux to the inner wall of the opening in the overprinting process than in the reciprocating printing process.

[0093] As shown above, it has been demonstrated that printing defects can be suppressed by performing a double printing process after cleaning the mask. Furthermore, when replacing the mask with a new one, or when the solder paste is dry, it is likely that there is insufficient flux on the inner wall of the mask's opening. Therefore, in such cases as well, it is considered preferable to perform a double printing process.

[0094] <Experiment 2> In Experiment 2, after cleaning, the mask was left to stand for 10 minutes. Overprinting was performed on the first substrate, and normal printing was performed on the second to fifth substrates. For comparison, normal printing was also performed on all five substrates under the same conditions. The average volume fraction of solder paste was then calculated for each substrate.

[0095] In Experiment 2, a Yamaha Motor Co., Ltd. demo board CUK-B1311-000 was used as the substrate, a Sun Kogyo Co., Ltd. printability evaluation mask CUK-M1311-100 was used as the mask, and Harima Chemicals Co., Ltd. CLR-B was used as the solder paste. The squeegee speed was set to 50 mm / sec, the printing pressure to 60 N, and the attack angle to 55°.

[0096] <Experiment 2 Results> Graphs D and E in Figure 11 show the results of Experiment 2. Graph D shows the average volume percentage of solder paste when overprinting was performed only on the first substrate and normal printing was performed on the second to fifth substrates. Graph E shows the average volume percentage of solder paste when normal printing was performed on all five substrates. As shown in Graph E, when solder paste was printed using normal printing, the average volume percentage of solder paste was lower on the first substrate. On the other hand, the average volume percentage of solder paste remained stable at around 100% for the second and subsequent substrates.

[0097] As shown in Graph D, when solder paste is printed using a normal printing process after the overprinting process, the average volume ratio of the solder paste remains stable at approximately 100% for the 1st to 5th circuit boards.

[0098] Under the conditions of Experiment 2, it was shown that if the overprinting process was performed only on the first substrate after cleaning the mask, printing defects could be suppressed for subsequent substrates by using the normal printing process. Thus, by performing the overprinting process under conditions where printing defects are particularly likely to occur (in this case, the first substrate after cleaning the mask), and performing the normal printing process otherwise, it is possible to suppress printing defects while minimizing the decrease in production efficiency.

[0099] [Effects of Embodiment 1] As described above, the screen printing method of Embodiment 1 is a screen printing method for printing solder paste onto a substrate P using a mask 12, comprising a normal printing step and a superimposed printing step in which the normal printing step is performed multiple times on one substrate P, wherein the normal printing step performs a plate alignment operation to overlap the substrate P and the mask 12 in a first direction, a printing operation to print solder paste onto the substrate P through an opening 12D provided in the mask 12, and a plate separation operation to separate the substrate P and the mask 12 in a first direction, wherein the superimposed printing step comprises a first normal printing step and a second normal printing step, wherein the second normal printing step is performed after the first normal printing step without moving the substrate P from the printing space, and is a screen printing method.

[0100] According to this screen printing method, the overprinting process comprises a first normal printing process and a second normal printing process. Therefore, the amount of solder paste printed on the substrate P can be increased by the overprinting process. Thus, printing defects can be suppressed.

[0101] In the screen printing method of Embodiment 1, it is preferable that the second normal printing step is performed immediately after the first normal printing step.

[0102] This screen printing method makes it possible to suppress the drying of the solder paste flux on the inner wall of the opening 12D of the mask 12 between the first and second normal printing processes. Therefore, it becomes easier to increase the amount of solder paste printed on the substrate P in the second normal printing process.

[0103] In the screen printing method of Embodiment 1, it is preferable that no inspection of the state of the solder paste printed on the substrate P is performed between the first normal printing step and the second normal printing step.

[0104] This type of screen printing method allows for smooth overprinting processes.

[0105] In the screen printing method of Embodiment 1, it is preferable that the conditions for the first normal printing process and the conditions for the second normal printing process are the same.

[0106] This type of screen printing method allows for uniform printing quality of solder paste.

[0107] In the screen printing method of Embodiment 1, it is preferable that the overprinting process is performed when predetermined preconditions are met.

[0108] With this type of screen printing method, if certain preconditions are met that tend to reduce the filling rate of solder paste, a layer printing process can be performed to suppress printing defects.

[0109] In the screen printing method of Embodiment 1, the overlay printing step is preferably performed on a predetermined number of substrates P after the preconditions have been met.

[0110] This type of screen printing method makes it easier to ensure print quality in the normal printing process when performing a normal printing process after a layered printing process.

[0111] In the screen printing method of Embodiment 1, the precondition is preferably the replacement of the mask 12.

[0112] After replacing the mask 12, flux is not supplied to the inner wall of the opening 12D of the new mask 12, making it difficult for the solder paste to separate from the inner wall of the opening 12D. However, according to the screen printing method described above, printing defects can be suppressed by performing an overprinting process.

[0113] In the screen printing method of Embodiment 1, the precondition is preferably cleaning of the mask 12.

[0114] Since flux is not supplied to the inner wall of the opening 12D of the mask 12 after cleaning, the solder paste becomes difficult to separate from the inner wall of the opening 12D. However, according to the screen printing method described above, printing defects can be suppressed by performing an overprinting process.

[0115] In the screen printing method of Embodiment 1, the precondition is preferably a predetermined time elapsed after the normal printing process.

[0116] Normally, after a predetermined time has elapsed since the printing process, the flux in the opening 12D of the mask 12 dries, making it difficult for the solder paste to separate from the inner wall of the opening 12D. However, with the screen printing method described above, printing defects can be suppressed by performing an overprinting process.

[0117] In the screen printing method of Embodiment 1, the precondition is preferably the drying of the solder paste.

[0118] Once the solder paste dries, it becomes difficult for it to separate from the inner wall of the opening 12D of the mask 12. However, according to the screen printing method described above, printing defects can be suppressed by performing a layer printing process.

[0119] The screen printing apparatus 1 of Embodiment 1 is a screen printing apparatus 1 that prints solder paste onto a substrate P using a mask 12, and comprises a substrate transport unit 14 that moves the substrate P to the printing space, a mask holder 13 that holds the mask 12, a substrate holding unit (substrate fixing unit 22) that holds the substrate P, a positioning mechanism (printing table 15) that moves at least one of the mask holder 13 and the substrate holding unit to bring the substrate P and the mask 12 into contact and separate them in a first direction, a printing unit 16 that prints solder paste onto the substrate P through an opening 12D provided in the mask 12, and a control unit 30, wherein the control unit 30 controls normal printing processes and predetermined The screen printing apparatus 1 performs a superimposed printing process, in which a normal printing process is performed multiple times on a single substrate P when it is determined that certain preconditions have been met. The normal printing process includes a plate alignment operation, in which a positioning mechanism is controlled to superimpose the substrate P and the mask 12 in a first direction; a printing operation, in which the printing unit 16 is controlled to print solder paste onto the substrate P; and a plate separation operation, in which the positioning mechanism is controlled to separate the substrate P and the mask 12 in a first direction. The superimposed printing process includes a first normal printing process and a second normal printing process, in which the second normal printing process is performed after the first normal printing process without moving the substrate P from the printing space.

[0120] With this configuration, the overprinting process is executed when certain preconditions are met that tend to reduce the solder paste filling rate. Since the overprinting process comprises a first normal printing process and a second normal printing process, the amount of solder paste printed on the substrate P can be increased. Therefore, printing defects can be suppressed.

[0121] In the screen printing apparatus 1 of Embodiment 1, it is preferable that the control unit 30 immediately and continuously executes a second normal printing process after the first normal printing process.

[0122] With this configuration, it is possible to suppress the drying of the solder paste flux on the inner wall of the opening 12D of the mask 12 between the first normal printing process and the second normal printing process. Therefore, it becomes easier to increase the amount of solder paste printed on the substrate P during the second normal printing process.

[0123] In the screen printing apparatus 1 of Embodiment 1, it is preferable that the control unit 30 does not perform an inspection of the state of the solder paste printed on the substrate P between the first normal printing process and the second normal printing process.

[0124] This configuration allows for smooth overprinting.

[0125] In the screen printing apparatus 1 of Embodiment 1, it is preferable that the control unit 30 determines that the preconditions have been met when the mask 12 is replaced.

[0126] After replacing mask 12, flux is not supplied to the inner wall of the opening 12D of the new mask 12, making it difficult for the solder paste to separate from the inner wall of the opening 12D. However, with the above configuration, printing defects can be suppressed by performing the overprinting process.

[0127] In the screen printing apparatus 1 of Embodiment 1, it is preferable that the control unit 30 determines that the preconditions have been met when the mask 12 has been cleaned.

[0128] Since flux is not supplied to the inner wall of the opening 12D of the mask 12 after cleaning, the solder paste becomes difficult to separate from the inner wall of the opening 12D. However, with the above configuration, printing defects can be suppressed by performing the overprinting process.

[0129] In the screen printing apparatus 1 of Embodiment 1, it is preferable that the control unit 30 determines that the preconditions have been met when a predetermined time has elapsed after the normal printing process.

[0130] Normally, after a predetermined time has elapsed since the printing process, the flux in the opening 12D of the mask 12 dries, making it difficult for the solder paste to separate from the inner wall of the opening 12D. However, with the above configuration, printing defects can be suppressed by performing a double printing process.

[0131] In the screen printing apparatus 1 of Embodiment 1, it is preferable that the control unit 30 determines that the preconditions have been met when the solder paste is dry.

[0132] Once the solder paste dries, it becomes difficult for it to separate from the inner wall of the opening 12D of the mask 12. However, with the above configuration, printing defects can be suppressed by performing the overprinting process.

[0133] [Embodiment 2] Embodiment 2 of this disclosure will be described with reference to Figure 12. Note that the configuration of Embodiment 2 is the same as that of Embodiment 1, except for the procedure for the overprinting process; therefore, redundant explanations will be omitted. Hereinafter, components similar to those in Embodiment 1 will be denoted by the same reference numerals as in Embodiment 1.

[0134] <Cleaning process> Figure 12 is a flowchart showing the overprinting process (S100) according to Embodiment 2. In the overprinting process (S100), mask cleaning is performed between the first normal printing process and the second normal printing process. In the first normal printing process, the first plate alignment operation (S101, see Figure 9A), the first printing operation (S102, see Figure 9B), and the first plate release operation (S103, see Figure 9C) are performed. After the first normal printing process, a mask cleaning process (S104) is performed on the mask 12. After the cleaning process, the second normal printing process is performed. In the second normal printing process, the second plate alignment operation (S105), the second printing operation (S106), and the second plate release operation (S107) are performed on the substrate P after the first normal printing process. The second normal printing process is performed after the first normal printing process without moving the substrate P from the printing space.

[0135] In the overprinting process of this disclosure, the same substrate P is normally printed two or more times. However, according to the overprinting process of Embodiment 1, depending on the type of mask 12 and substrate P, the amount of solder paste filling the opening 12D of the mask 12 may be excessive. In this embodiment, a cleaning process is performed between the first and second normal printing processes of the mask, which is thought to remove an appropriate amount of solder paste adhering to the inner wall of the opening 12D of the mask 12 and to wet the inner wall of the opening 12D with flux. Therefore, it is thought that in the second normal printing process, excessive filling of the solder paste into the opening 12D can be suppressed, while improving the leakage of the solder paste from the opening 12D.

[0136] Furthermore, the cleaning process performed during overprinting may be excluded from the preconditions for the overprinting process.

[0137] [Effects of Embodiment 2] In the screen printing method of Embodiment 2, the overprinting step further comprises a cleaning step for cleaning the mask 12, and it is preferable that the cleaning step is performed after the first normal printing step and before the second normal printing step.

[0138] With this screen printing method, by performing a cleaning step between the first normal printing step and the second normal printing step, it is possible to suppress excessive filling of the opening 12D of the mask 12 with solder paste.

[0139] In the screen printing apparatus 1 of Embodiment 2, the control unit 30 further performs a cleaning process to clean the mask 12 during the overprinting process, and it is preferable that the cleaning process is performed after the first normal printing process and before the second normal printing process.

[0140] With this configuration, a cleaning process is performed between the first normal printing process and the second normal printing process, which prevents excessive filling of the openings 12D of the mask 12 with solder paste.

[0141] [Other embodiments] (1) In the above embodiment, two normal printing processes were performed in the overprinting process, but three or more normal printing processes may be performed in the overprinting process.

[0142] (2) In the above embodiment, the plate alignment operation and plate separation operation were performed by raising and lowering the substrate P relative to the fixed mask 12, but the mask may be raised and lowered relative to the substrate.

[0143] (3) In the above embodiment, the control unit 30 automatically performs the overprinting process when at least one of the preconditions S20, S30, S40, and S50 is met, but the preconditions may be different from those described above. Alternatively, the preconditions may not be set, and the overprinting process may be performed by an operator issuing instructions when necessary.

[0144] (4) In the above embodiment, the screen printing method of the present disclosure was performed by the control unit 30 of the screen printing apparatus 1. However, for example, the screen printing method of the present disclosure may be performed by an operator or by the control unit of a control PC that is communicatively connected to the screen printing apparatus. [Explanation of symbols]

[0145] 1: Screen printing device 10: Housing 11: Base 12: Mask 12A: Mask frame 12B: Elastic member 12C: Mask body 12D: Opening 13: Mask holder 13A: Rail section 14: Circuit board transport section 14A: Upstream conveyor 14B: Main conveyor 14C: Downstream conveyor 15: Printing table 16: Printing section 16A: Squeegee unit 16B: Guide rail 16C: Solder supply section 16E: Squeegee 16F: Detection sensor 17: Inspection section 17A: Camera unit 17B: Beam 17C: Ball screw 17D: Guide rail 17E: Mask imaging camera 17F: Circuit board imaging camera 18: Mask replacement unit 19: Cleaning Department 20: Table 21: Ball screw 22: PCB fixing unit 22A: Clamp section 23: Backup unit 23A: Lower plate 23B: Upper plate 23C: Backup pin 23D: Ball screw 30: Control section 30A: CPU 30B: RAM 30C: Storage section 31: Display section 32: Input section P: Circuit board R1: Solder roll

Claims

1. A screen printing method for printing solder paste onto a circuit board using a mask, The normal printing process, The process includes a superimposed printing process in which the normal printing process is performed multiple times on a single substrate, In the aforementioned standard printing process, A plate alignment operation in which the substrate and the mask are superimposed in a first direction, A printing operation in which solder paste is printed onto the substrate through an opening provided in the mask, A plate separation operation is performed to separate the substrate and the mask in a first direction. The overprinting process comprises a first normal printing process and a second normal printing process. The second normal printing process is performed after the first normal printing process without moving the substrate out of the printing space. A screen printing method in which, when the mask is cleaned, the overlay printing process is performed only on the first m substrates after the mask has been cleaned, as an initial group of substrates until the opening state of the mask stabilizes immediately after cleaning, and the normal printing process is performed on the m+1 and subsequent substrates.

2. The aforementioned overprinting process further includes a cleaning process for cleaning the mask, The screen printing method according to claim 1, wherein the cleaning step is performed after the first normal printing step and before the second normal printing step.

3. The screen printing method according to claim 1, wherein the second normal printing step is performed immediately and continuously after the first normal printing step.

4. The screen printing method according to any one of claims 1 to 3, wherein no inspection of the state of the solder paste printed on the substrate is performed between the first normal printing step and the second normal printing step.

5. The screen printing method according to any one of claims 1 to 4, wherein the conditions for the first normal printing process and the conditions for the second normal printing process are the same.

6. The screen printing method according to any one of claims 1 to 5, wherein the overprinting step is performed when predetermined preconditions are met.

7. The screen printing method according to claim 6, wherein the overprinting step is performed on a predetermined number of substrates after the prior conditions have been met.

8. The screen printing method according to claim 6 or 7, wherein the precondition is the replacement of the mask.

9. The screen printing method according to any one of claims 6 to 8, wherein the precondition is the elapsed time after the normal printing process.

10. The screen printing method according to any one of claims 6 to 9, wherein the aforementioned precondition is drying of solder paste.

11. A screen printing apparatus that prints solder paste onto a substrate using a mask, A substrate transport unit that moves the substrate to the printing space, A mask holder for holding the aforementioned mask, A substrate holding portion for holding the substrate, A positioning mechanism that moves at least one of the mask holder and the substrate holding portion to bring the substrate and the mask into contact with and separate from each other in a first direction, A printing unit that prints solder paste onto the substrate through an opening provided in the mask, It comprises a control unit and, The control unit performs a normal printing process and an overlay printing process in which it performs the normal printing process multiple times on a single substrate when it determines that predetermined preconditions have been met. In the aforementioned normal printing process, A plate alignment operation that controls the positioning mechanism to superimpose the substrate and the mask in the first direction, A printing operation that controls the printing unit to print solder paste onto the substrate, The positioning mechanism is controlled to perform a plate separation operation that separates the substrate and the mask in a first direction, In the aforementioned overprinting process, The first normal printing process and the second normal printing process are performed. The second normal printing process is performed after the first normal printing process without moving the substrate from the printing space. The control unit determines that the preconditions have been met when the mask has been cleaned, and as an initial group of substrates until the opening state of the mask stabilizes immediately after cleaning, it performs the overprinting process only on the first m substrates after cleaning the mask, and performs the normal printing process on the m+1 and subsequent substrates.

12. The control unit, in the overprinting process, further performs a cleaning process to clean the mask. The screen printing apparatus according to claim 11, wherein the cleaning process is performed after the first normal printing process and before the second normal printing process.

13. The screen printing apparatus according to claim 11, wherein the control unit immediately and continuously executes the second normal printing process after the first normal printing process.

14. The screen printing apparatus according to any one of claims 11 to 13, wherein the control unit does not perform an inspection of the state of the solder paste printed on the substrate between the first normal printing process and the second normal printing process.

15. The screen printing apparatus according to any one of claims 11 to 14, wherein the control unit determines that the preconditions have been met when the mask has been replaced.

16. The screen printing apparatus according to any one of claims 11 to 15, wherein the control unit determines that the preconditions have been met when a predetermined time has elapsed after the normal printing process.

17. The screen printing apparatus according to any one of claims 11 to 16, wherein the control unit determines that the preconditions have been met when the solder paste is dry.

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