Abstract:
A circuit pattern forming method is provided which can reduce a possibility of undesired short-circuits being produced in the circuit by satellites formed when fabricating a conductive pattern and thereby can form a highly reliable printed circuit board. To that end, this invention overlappingly draws a conductive pattern and an insulating pattern of a predetermined thickness by scanning a liquid ejection head and a substrate relative to each other a plurality of times while ejecting droplets of a conductive pattern forming solution and an insulating pattern forming solution. When forming the conductive pattern and the insulating pattern that adjoin each other on the substrate, the step of forming the insulating pattern of at least one scan is executed between the conductive pattern forming steps that are executed the plurality of times until the conductive pattern has a predetermined thickness.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a circuit pattern forming method and a circuit pattern forming device to form circuit patterns by ejecting a conductive pattern forming solution and an insulating pattern forming solution onto a substrate.  
         [0003]     2. Description of the Related Art  
         [0004]     In forming patterns on printed circuit boards, a subtractive method has been in general use. The subtractive method, however, requires a large number of fabrication processes and its cost occupies a large percentage of an overall manufacturing cost. To deal with this problem, a so-called liquid ejection method, which requires a smaller number of steps and is suited for small-lot production of a wide variety of products, has been proposed in recent years. This liquid ejection method draws a pattern on a substrate by ejecting a pattern forming solution onto the substrate and is disclosed in US Patent No. 20040000429 (Japanese Patent Application Laid-open No. 2003-309369).  
         [0005]     The US Patent No. 20040000429 discloses a method for forming wiring patterns by moving a substrate and a liquid ejection head relative to each other and causing the head to eject liquid droplets as they are moved. An overview of this pattern forming method will be explained by referring to  FIG. 10A  to  FIG. 10K .  
         [0006]      FIGS. 10A  to  10 K are cross-sectional views of a multilayered printed circuit board during the manufacturing process.  FIG. 10A  shows a drop of a conductive pattern forming solution containing fine metal particles being ejected from a liquid ejection head  101  onto a substrate  102 . Here, designated  104  are dots formed by the ejected droplets  103  landing on the substrate  102 . Landing intervals of the dots  104  are such that they do not contact each other. After the conductive pattern is formed, a solvent of the conductive pattern forming solution is dried to a state shown at  105  of  FIG. 10B . The dots  105  that are formed after the solvent is evaporated is thinner than before they were dried.  
         [0007]     Next, as shown in  FIG. 10C , dots  106  are formed between adjoining dots  105  by ejecting droplets of the conductive pattern forming solution onto positions between the previously formed dots. Then, they are dried by evaporating the solvent to form a pattern  107  with all its dots connected as shown in  FIG. 10D . Further, to form the conductive pattern to a desired thickness, the above ejection and drying steps are repeated to form first layer conductive pattern  108 , as shown in  FIG. 10E .  
         [0008]     Next, on the already formed conductive patterns, the conductive pattern forming solution is ejected to form an interlayer conduction post  109  for electrically connecting the first and second layer (see  FIG. 10F ). Then, the patterns are heated to have metal particles in the interlayer conduction post  109  electrically contact one another, making the pattern  108  and the interlayer conduction post  109  an integral pattern (see  FIG. 10G ).  
         [0009]     Then, as shown in  FIG. 10H , a solution  110  to form an interlayer insulation film is applied to a thickness such that the interlayer conduction post  109  slightly protrudes from the insulation film. Then, the patterns are heated to remove the solvent and harden the insulating material, with the result that the thickness of the interlayer insulation film  110  is reduced to half, as shown in  FIG. 10 . Here, the insulation pattern forming solution is applied again ( FIG. 10J ) and subjected to a heating step to form an almost flat insulation film as shown in  FIG. 10K .  
         [0010]     In forming a second layer, the above pattern forming procedure is repetitively performed over the first layer of  FIG. 10K  to form a multilayered circuit.  
         [0011]     As described above, with the liquid ejection type circuit forming method that uses the liquid ejection head, the number of steps required is relatively small and thus the printed circuit board can be constructed inexpensively. However, when ejecting liquid droplets from nozzles of the liquid ejection head, small droplets called satellites are also sprayed along with main droplets, causing problems to the circuits being formed.  
         [0012]     The above problem will be explained in detail, as well as a droplet ejection process of the head and how the satellites are formed along with main droplets.  
         [0013]      FIGS. 11A  to  11 F are cross-sectional views showing how a circuit forming liquid (referred to simply as a liquid), such as a conductive pattern forming solution and an insulation pattern forming solution, is ejected from the liquid ejection head. In each figure, portions shown shaded represent the solution. As shown in  FIG. 11A , the circuit forming solution is filled from a liquid chamber  121  through a liquid path  122  up to an opening of a nozzle  123 . Denoted  124  is a heater which is formed in a silicon substrate  125 . The solution present in the liquid chamber  121  advances into the liquid path  122  by a capillary attraction. At the same time, the solution in the liquid path  122  is acted upon by a negative pressure generated by a supply unit such as a liquid tank and tends to be drawn back into the liquid chamber  121 . Therefore, when no solution ejection is performed, these two forces balance holding the solution as shown. At this time, the solution in the nozzle  123  forms a concave meniscus  126  as shown by the negative pressure acting toward the liquid chamber  121 .  
         [0014]      FIGS. 11B  to  11 F show how a bubble is formed, a droplet is ejected and a meniscus is formed as the actual ejection operation is performed, as opposed to the static state shown in  FIG. 11A .  
         [0015]     When a printing operation is started and a voltage is applied to the heater  124 , a thermal energy is generated by the heater  124  to heat the solution in the liquid path  122 , producing a bubble  127  as a result of film boiling. The bubble  127  continues to expand while the heater  124  is energized and the expansion force displaces the solution in the liquid path  122 . That is, the solution near the nozzle  123  breaks the meniscus  126  and protrudes out and the solution present near the liquid chamber  121  moves toward the liquid chamber  121 , as shown in  FIG. 11B .  
         [0016]     Further, when the voltage application to the heater  124  is stopped, with the solution greatly protruding from the nozzle  123  as shown in  FIG. 11B , the bubble  127  contracts withdrawing the solution near the nozzle  123  greatly into the liquid path  122 . At this time, the solution that was protruding outside parts from the solution being withdrawn into the liquid path  122  and, as shown in  FIG. 11C , flies in a direction of arrow. The solution that was sent flying includes a main droplet  128  followed by smaller droplets called satellites  129 , both landing on the substrate.  
         [0017]     The meniscus  126 , withdrawn into the liquid path  122  by the capillary attraction after the bubble has vanished, now moves toward the nozzle  123  again, filling the solution into the liquid path  122  (see  FIG. 11D ). During this refilling process, the solution meniscus changes its state from the one shown in  FIG. 11C  to the one shown in  FIG. 11D  and further moves to an initial state near the nozzle. Because of inertia, the meniscus cannot stop at the initial state but slightly bulges out from the nozzle  123  ( FIG. 11E ).  
         [0018]     With the meniscus slightly bulging, the surface tension of the solution and the negative pressure in the tank combine to pull the meniscus into the nozzle  123 . This oscillates the solution and the oscillation progressively attenuates until it finally stops ( FIG. 11F ).  
         [0019]     One printing operation ejects one main droplet  128  from a nozzle, followed by one or more satellites  129 . It is known that the size of the satellites  129  and their distances from the main droplet  128  vary from one nozzle  123  to another and that the ejection performance also changes from one operation to another even in one and the same nozzle.  
         [0020]     In the above described circuit pattern forming method, there is a possibility that these satellites may cause problems to the circuit operation. How the satellites are formed will be explained in detail by referring to  FIGS. 12A  to  12 C.  
         [0021]     First, a relation between the landing positions of the main droplet and satellites ejected from a nozzle of the head during scanning will be explained by referring to  FIGS. 12A-12C .  FIG. 12A  and  FIG. 12B  illustrate the process of drawing a pattern by moving the ejection head and the substrate relative to each other as the head ejects droplets.  FIG. 12C  shows shapes of droplets formed on the substrate in one ejection operation. The relative movement (scan) is done by holding the substrate  132  immovable and moving the head  131  from left toward right in the figure.  
         [0022]     When a droplet  134  is ejected from the nozzle  133  of the head  131 , since the head  131  is moving toward right, the droplet  134  flies down diagonally toward right from the ejected position as shown in  FIG. 12A . Then, as shown in  FIG. 12B , the main droplet  134  is accompanied by one or more satellites  135  as it is ejected. This satellite  135 , too, falls down diagonally toward right. The satellite  135 , since it is formed following the main droplet  134 , lands at a position slightly shifted to right (in the scan direction) from the main droplet  134 . Conversely, when the scan is executed in the opposite direction, i.e., toward left, the satellite  135  lands at a position to the left of the main droplet  134  (not shown). A distance D between the main droplet  134  and the satellite  135  varies depending on a head scan speed, a droplet ejection speed, a droplet volume, components of a droplet, and a distance from the nozzle to the substrate.  
         [0023]     Next, the process of drawing a pattern by continuously ejecting droplets will be described.  FIG. 13A  to  FIG. 13C  show the pattern drawing process,  FIGS. 13A and 13B  being cross-sectional views and  FIG. 13C  a plan view. It is assumed that the ejection head scans from left to right.  
         [0024]      FIG. 13A  shows a pattern  140  drawn on the substrate  132 , with a satellite  141  accompanying the ejected droplets shown to have landed near the pattern  140 .  FIG. 13B  shows the pattern thickened by applying droplets to the same positions again. Although a satellite  142  is also formed at this time, since the distance between the landing positions of the main droplet and the satellite often varies from one ejection operation to another, as described above, the satellite  142  that has landed in the subsequent scan often does not land on the preceding satellite  141 . When the droplet application is repeated until the pattern reaches a desired thickness, there are cases where satellites may contact one another, as shown enclosed by a one-dot line in the plan view of  FIG. 13C , depending on the landing state of the satellites. In these cases, two patterns may get connected or short-circuited, resulting in an abnormal circuit operation.  
       SUMMARY OF THE INVENTION  
       [0025]     The present invention has been accomplished to overcome the problems experienced with the conventional technology. It is an object of this invention to provide a circuit forming method which can reduce a possibility of undesired short-circuits being produced in the circuit by satellites formed when fabricating a conductive pattern and thereby can form a highly reliable printed circuit board.  
         [0026]     To achieve the above objective, the present invention has the following construction. According to a first aspect, the present invention provides a method of forming a circuit pattern having a conductive pattern and an insulating pattern of a predetermined thickness, comprising steps of: performing, a plurality of times, the step of forming a conductive pattern of one scan while scanning a first head and a substrate relative to each other, the first head ejecting onto the substrate a first solution for forming the conductive pattern, until the conductive pattern formed on the substrate has a predetermined thickness; and performing, a plurality of times, the step of forming an insulating pattern of one scan while scanning a second head and the substrate relative to each other, the second head ejecting onto the substrate a second solution for forming the insulating pattern, until the insulating pattern formed on the substrate has a predetermined thickness; wherein, when forming the conductive pattern and the insulating pattern that adjoin each other on the substrate, the step of forming the insulating pattern of at least one scan is executed while the conductive pattern forming steps are executed the plurality of times.  
         [0027]     According to a second aspect, this invention provides a printed circuit board comprising: a conductive pattern formed by a first solution ejected onto a substrate with predetermined thickness, while scanning a first head and the substrate relative to each other, the first head repeatedly ejecting a first solution onto the substrate; a insulating pattern for at least one scan, the insulating pattern being formed by a second solution ejected onto a substrate while scanning a second head and the substrate relative to each other, the second head ejecting a second solution onto the substrate.  
         [0028]     According to a third aspect, this invention provides a device to form a circuit pattern having a conductive pattern and an insulating pattern of a predetermined thickness, the device comprising: a first head to eject a first solution to form the conductive pattern; means for performing control to execute an operation of forming the conductive pattern of one scan a plurality of times until the conductive pattern on a substrate has a predetermined thickness, the operation of forming the conductive pattern of one scan being performed by scanning the first head and the substrate relative to each other while causing the first head to eject the first solution onto the substrate; a second head to eject a second solution to form the insulating pattern; and means for performing control to execute an operation of forming the insulating pattern of one scan a plurality of times until the insulating pattern on the substrate has a predetermined thickness, the operation of forming the insulating pattern of one scan being performed by scanning the second head and the substrate relative to each other while causing the second head to eject the second solution onto the substrate; wherein, when forming the conductive pattern and the insulating pattern that adjoin each other on the substrate, the operation of forming the insulating pattern of at least one scan is executed between the conductive pattern forming operations that are executed the plurality of times until the conductive pattern has a predetermined thickness.  
         [0029]     With this invention, when forming a circuit pattern by ejecting the conductive pattern forming solution and the insulating pattern forming solution onto the substrate, satellites which are formed accompanying the ejections of the conductive pattern forming solution and land on the insulating pattern are covered with the insulating pattern forming solution. It is therefore possible to greatly reduce a chance of undesired short-circuits being formed in the conductive pattern. This in turn allows highly reliable circuits to be formed, improving a yield in fabricating printed circuit boards.  
         [0030]     The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.  
         [0031]     The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]      FIG. 1  is a perspective view schematically showing an outline construction of a circuit pattern forming device of one embodiment;  
         [0033]      FIG. 2  is a block diagram schematically showing a control system in the circuit pattern forming device of this embodiment;  
         [0034]      FIG. 3  is a flow chart showing a drawing procedure in the pattern forming method in a first embodiment of this invention;  
         [0035]      FIGS. 4A  to  4 F are schematic diagrams showing a process of drawing conductive and insulating patterns on a substrate in the first embodiment of this invention;  
         [0036]      FIG. 5  is a flow chart showing a drawing procedure in the pattern forming method in a second embodiment of this invention;  
         [0037]      FIGS. 6A  to  6 F are schematic diagrams showing a process of drawing conductive and insulating patterns on a substrate in the second embodiment of this invention;  
         [0038]      FIG. 7  is a flow chart showing a drawing procedure in the pattern forming method in a third embodiment of this invention;  
         [0039]      FIGS. 8A  to  8 H are schematic diagrams showing a process of drawing conductive and insulating patterns on a substrate in the third embodiment of this invention;  
         [0040]      FIGS. 9A  to  9 K are schematic diagrams showing a process of drawing conductive and insulating patterns on a substrate in a fourth embodiment of this invention;  
         [0041]      FIGS. 10A  to  10 K are cross-sectional views showing a process of fabricating a conventional multilayered printed circuit board;  
         [0042]      FIGS. 11A  to  11 F are cross-sectional views showing how a droplet of solution is ejected from the head;  
         [0043]      FIGS. 12A  to  12 C show a positional relation between a main droplet and a satellite; and  
         [0044]      FIGS. 13A  to  13 C show a circuit made faulty by satellites. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0045]     Now, embodiments of this invention will be described in the following order.  
         [0046]     1. A description of an outline construction of the circuit pattern forming device and a description of a control system for the device  
         [0047]     2. A description of a material used in the circuit pattern fabrication  
         [0048]     3. A description of the circuit pattern forming method  
       1. Outline Construction of Circuit Pattern Forming Device And Construction of Control System  
       [0049]     First, as one embodiment of this invention, let us explain an outline construction of the circuit pattern forming device used to form a circuit pattern made up of an insulating pattern and a conductive pattern formed on a substrate.  
         [0050]      FIG. 1  is a perspective view schematically showing an outline construction of the circuit pattern forming device in this embodiment.  
         [0051]     The circuit pattern forming device shown here has a carriage  109  that reciprocally travels in the scan direction (X direction) and a stage  103  on which a substrate  1  to be formed with a circuit pattern is mounted. In this carriage, a liquid ejection head  2  for ejecting an insulation pattern forming solution onto the substrate  1  and a liquid ejection head  3  for ejecting a conductive pattern forming solution are arranged side by side in the X direction. Also mounted on the carriage are two tanks (not shown) that supply the insulation pattern forming solution and the conductive pattern forming solution to the liquid ejection heads  2 ,  3  respectively. Each liquid ejection head has arranged in a direction crossing the scan direction (X direction) a large number of nozzles to eject the liquid supplied from the associated tank. These nozzles form a nozzle array. This nozzle array should preferably extend in a Y direction perpendicular to the scan direction. Each head may have two or more nozzle arrays. The liquid ejection heads  2 ,  3  are repetitively scanned over the substrate while ejecting the conductive pattern solution and the insulating pattern solution onto the substrate to form a circuit pattern.  
         [0052]     While in the above description, the liquid ejection head is moved over the substrate, the effect of this invention can also be produced by holding the head stationary and moving the substrate.  
         [0053]     A CR linear motor (carriage linear motor)  101  is provided as a power source to cause the carriage  109  to execute forward and backward scans. As a means for moving the substrate  1  in the Y direction, the stage  103  and a LF linear motor (line feed linear motor)  102  which drives the stage are provided. The LF linear motor  102  is rigidly secured to a bed  108  so that the upper surface of the stage  103  carrying the substrate  1  can be kept parallel to the upper surface of the bed  108  at all times if the stage  103  is moved. The CR linear motor  101  is secured to highly rigid bases  104 ,  105  erected on the bed  108 .  
         [0054]     The carriage  109  reciprocally moves in the main scan direction (X direction) along an upper surface of the bed, i.e., a stage surface. The CR linear motor  101  and the LF linear motor  102  each incorporate a linear encoder  111 ,  112  and an origin sensor  106 ,  107 . The outputs of the linear encoder  111 ,  112  and the origin sensor  106 ,  107  are used as a servo control input for driving the linear motors. Further, the linear encoder  111  on the carriage side is used to generate a solution ejection timing. The encoder has a resolution of 0.5 μm, high enough to form circuit patterns several tens of μm wide.  
         [0055]     Though not shown here, the circuit pattern forming device has a raise/lower mechanism that finely moves the carriage up or down in a direction (Z direction) perpendicular to the upper surface of the stage  103 . This raise/lower mechanism can adjust a gap between the carriage and the upper surface of the substrate or the circuit pattern formed on the substrate.  
         [0056]     Further, the circuit pattern forming device of this embodiment is connected with a personal computer (not shown) as a host device. Based on figure information (circuit pattern forming information) sent from this personal computer, the circuit pattern forming device moves the stage  103  to a predetermined position by the LF linear motor  102  and scans the carriage  109  by the CR linear motor  101  while ejecting the conductive pattern forming solution or insulating pattern forming solution from the head onto a predetermined position on the substrate to form a conductive pattern or insulating pattern. At this time, if the nozzle array length as measured in the Y direction is shorter than the length of the substrate  1  as measured in the Y direction, when the drawing operation of the head in the first scan is completed, the substrate  1  is moved a nozzle array length in the Y direction by the LF linear motor  102  and the head is again scanned to perform drawing. By repeating this scan (drawing) of the liquid ejection head and the feeding of the substrate  1 , one layer of a predetermined conductive pattern or insulating pattern can be formed in the entire circuit pattern forming area of the substrate  1 . Of course, if the length of the circuit pattern forming area is shorter than the head, one layer of circuit pattern can be completely drawn by the first scan of the liquid ejection head. Then, the pattern forming operation is repeated on the entire circuit pattern forming area, forming the conductive pattern or insulating pattern of a predetermined thickness. Details of the drawing operation to obtain a desired thickness of pattern will be described later in [3. Description of Pattern Forming Method].  
         [0057]     To form a circuit pattern on the substrate by the circuit pattern forming device and complete a printed circuit board, a fixing process is required which evaporates the solvent in the solutions drawn on the substrate  1  to fix the circuit pattern on the substrate. Thus, a drying device is necessary in addition to the circuit pattern forming device.  
         [0058]     Next, a control system of the circuit pattern forming device of this embodiment will be explained.  
         [0059]      FIG. 2  is a block diagram schematically showing an overall configuration of a control system in the circuit pattern forming device of this embodiment. A mechanical unit  46  includes the CR linear motor  101  for moving in the main scan direction the carriage  109  carrying the liquid ejection heads  2 ,  3 , and also the LF linear motor  102  for transporting the stage  103  carrying the substrate  1 .  
         [0060]     A main control unit  44  is a central part of the control system that controls the entire circuit pattern forming device of this embodiment including the liquid ejection head and the mechanical unit  46 . The main control unit  44  has a CPU, a ROM in which operation programs are stored, and a work RAM that allows reading and writing of a variety of data.  
         [0061]     The main control unit  44  outputs a control signal to the mechanical unit  46  to perform a mechanical control, for example, on the movement of the carriage  109  and stage  103 . It also transfers signals to and from a head control unit  42 , a memory control unit  50  and a drawing position signal generation unit  41  to control the operation of the liquid ejection head  2 . An I/F unit  47  is an interface between the personal computer not shown and the circuit pattern forming device. The I/F unit  47  receives a command and circuit pattern drawing data (circuit pattern forming data) from a host device such as the personal computer. The memory control unit  50  transfers the command from the I/F unit  47  to the main control unit  44  and, under the control of the main control unit  44 , generates an address signal and a draw timing to put the circuit pattern drawing data into a buffer memory  45 .  
         [0062]     Further, the main control unit  44  analyzes the command received from the I/F unit  47  and, according to the result of the analysis, sets drawing conditions, such as a drawing speed and a drawing resolution. Then, based on the drawing conditions, the main control unit  44  controls the mechanical unit  46  and the drawing position signal generation unit  41  to execute the drawing operation under the predetermined conditions.  
         [0063]     Further, the circuit pattern drawing data received from the personal computer not shown is stored in the buffer memory  45  or a temporary memory and then transferred to the head control unit  42  by the control of the memory control unit  50  that has received the command from the main control unit  44 .  
         [0064]     In synchronism with the drawing position signal output from the drawing position signal generation unit  41 , the head control unit  42  drives individual nozzles of the liquid ejection head to draw a circuit pattern according to the circuit pattern drawing data transferred from the buffer memory  45 .  
       2. Description of Circuit Pattern Forming Solution  
     2-1. Substrate  
       [0065]     The substrate  1  used in this embodiment basically is shaped like a film, sheet or plate with a planar surface. When forming a circuit pattern layer, the pattern fixing is facilitated by evaporating the solvent. So, it is particularly preferable that the substrate have a good heat resistance. Other than the planar shape, the substrate may have a curved shape as long as a circuit pattern can be formed by the liquid ejection method. The substrate may use the following materials: thermoplastic resin films, such as polyester film, aromatic polyamide film and polyimide film; cloths and nonwoven fabric of glass fibers, polyester fibers and aromatic polyamide fibers impregnated with thermoplastic resin and epoxy resin and then hardened and shaped like a sheet; and a glass epoxy laminated plate used for ordinary printed circuit boards.  
       2-2. Pattern Forming Solution  
       [0066]     For conductivity, the conductive pattern forming solution generally includes metal particles such as Al, Ag and SnO 2 . The metal particles preferably have diameters in a range of several tens to several hundreds of nm in terms of uniformity and stability of circuit patterns. The solution includes water and a water-soluble organic solvent and other components, such as viscosity adjusting agent, pH adjusting agent, antiseptic, surfactant and antioxidant, as necessary.  
         [0067]     The insulating pattern forming solution preferably includes insulating particles of silica, alumina, calcium carbonate and magnesium carbonate. Other materials can also be used if they exhibit an insulating capability. A liquid medium includes water. It is also possible to mix a water-soluble organic solvent and other additives, such as viscosity adjusting agent, pH adjusting agent, antiseptic, various kinds of surfactant, antioxidant and evaporation accelerator, as necessary.  
         [0068]     After a circuit pattern has been drawn with the solution, it is dried to evaporate the solvent. Further, all the patterns after being drawn and dried are sintered to form a highly conductive printed circuit board with metal particles forming metal connections.  
       3. Description of Circuit Pattern Forming Method  
     First Embodiment  
       [0069]     Next, a first embodiment of the circuit pattern forming method according to this invention will be explained by referring to  FIG. 3  and  FIGS. 4A-4F .  FIG. 3  is a flow chart showing a pattern forming process in the first embodiment.  FIGS. 4A-4F  are schematic diagrams showing a process of drawing conductive and insulating patterns,  FIGS. 4A-4E  representing states in which a drawn pattern is dried and a solvent evaporated,  FIG. 4F  representing a state in which the circuit pattern, after being sintered, is completed.  
         [0070]     In this embodiment, a circuit pattern of one integral layer as shown in  FIG. 4F  is constructed of four divided layers, each of which is formed in a separate pattern drawing process, as shown in  FIG. 4E . That is, one pattern drawing operation, which is identical to one scan of the head, forms one divided layer among the four to be formed. Thus, to form all the four divided layers, equivalent to one integral layer of the circuit pattern, requires four drawing operations.  
         [0071]     In step S 1  of  FIG. 3 , the number of pattern drawing operations required to form a pattern of a desired thickness by repetitively applying the pattern forming solution to the same position is determined. The number of pattern drawing operations of repetitively applying the conductive pattern forming solution is equal to the number of pattern drawing operations of repetitively applying the insulating pattern forming solution.  
         [0072]     In step S 2 , the liquid ejection head scanns once over the entire conductive pattern forming area on the substrate while ejecting the conductive pattern forming solution onto the substrate  10 , to form a conductive pattern  11  which is then dried, as shown in  FIG. 4A . Since the pattern drawing operation is executed as the head is scanned toward the right in  FIG. 4A , a satellite  12  lands to the right side of the conductive pattern  11 .  
         [0073]     Next, in step S 3 , the insulating pattern forming solution is ejected to where the conductive pattern  11  is not drawn, to form an insulating pattern  13  which is then dried. The insulating pattern  13  drawn in this manner covers the satellite  12  of the conductive pattern forming solution that has previously landed. During the insulating pattern drawing operation a satellite of the insulating pattern forming solution lands, as with the conductive pattern drawing operation. This satellite, however, does not cause any problem in the circuit operation and is therefore not shown.  
         [0074]     Next, in step S 4 , a check is made to see if the number of pattern drawing operations has reached a predetermined number. If the predetermined number is not yet reached, the process returns to step S 2  and repeats the processing of step S 2  and S 3 . At the current stage, since only the first of the four drawing operations is finished, the processing of the subsequent operations will be explained.  
         [0075]     After returning to step S 2 , the conductive pattern is drawn a second time and dried. This state is shown in  FIG. 4C . The second conductive pattern is drawn over the first conductive pattern  11  and a satellite  14  formed during the second operation lands not on the satellite  12  that was formed during the first operation, but on the insulating pattern  13  drawn at step S 3 . Then, as shown in  FIG. 4D , step S 3  is again executed to draw and dry the second insulating pattern as with the first insulating pattern.  
         [0076]     Then, as shown in  FIG. 4E , when the number of drawing operations in step S 2  and S 3  has reached a predetermined total number of four, step S 4  checks if all of the patterns are completely drawn. If so, the pattern drawing process is ended.  
         [0077]     As a final step, the printed substrate is baked in a separate baking device to cause fine metal particles used as the conductive fine particles in the conductive pattern forming solution to make solid metal connections. As a result, a printed circuit board is formed as shown in  FIG. 4F . In the insulating pattern  16 , conductive satellites  17  are present. However, since the drawing of the conductive pattern and the drawing of the insulating pattern are alternated, the conductive satellites are covered with the insulating pattern forming solution. Therefore, the satellites are kept out of touch with one another, thus forming a good circuit pattern which prevents the conductive patterns  15  from undesirably getting short-circuited.  
         [0078]     It is desired that the conductive pattern  15  and the insulating pattern  16  on the completed printed circuit board be flat. To this end, the pattern forming solution must be chosen so that the conductive pattern and the insulating pattern after being drawn once and burned have equal thicknesses. In practice, since a priority is given to the conductive pattern having a desired thickness in terms of conductivity and allowable current, a shrinkage factor of the insulating particles used in the insulating pattern forming solution is preferably almost equal to that of the metal particles used to form the conductive pattern.  
       Second Embodiment  
       [0079]     Next, the second embodiment of this invention will be described. The first embodiment has taken up an example case in which the drawing of the insulating pattern and the drawing of the conductive pattern are alternated every scan. In the second embodiment, a case will be described in which the drawing of conductive pattern is performed continuously in the direction of lamination to the extent that the satellites do not pose any short-circuit problem.  
         [0080]      FIG. 5  is a flow chart showing a drawing procedure in the circuit pattern forming method of the second embodiment.  FIGS. 6A-6F  are schematic diagrams showing a process of drawing conductive and insulating patterns on a substrate.  FIGS. 6A-6E  illustrate a drawn pattern that is dried to evaporate a solvent.  FIG. 6F  shows a completed circuit pattern after being baked.  
         [0081]     Step S 11  determines the number of pattern drawing operations required to form a pattern to a desired thickness, the pattern drawing operations involving repetitively drawing the conductive patterns over the previously drawn one by scanning the head. The desired pattern thickness is set to an appropriate one by considering the conductivity and allowable current. An example case of this embodiment that follows assumes that the conductive pattern of a desired thickness comprises four divided layers which are drawn by four drawing operations in four head scans.  
         [0082]     In step S 12 , of the number of drawing operations (scans) determined by step S 11 , the number of pattern drawing operations that are performed continuously in drawing the conductive pattern and the insulating pattern is determined. The method of determining the number of drawing operations in a continuous drawing layers for conductive pattern involves checking beforehand the amount of satellites that is actually produced in one drawing operation of the conductive pattern and determining the maximum number of drawing operations in the continuous drawing session that does not cause any pattern short-circuit by the satellites produced in these drawing operations getting connected together. The amount of satellites produced changes depending on various conditions as described above, so it is important that the number of drawing operations be determined so as to prevent short-circuits from occurring even if a somewhat greater amount of satellites than expected is produced. In this embodiment, the following description assumes that the conductive pattern is drawn by performing two drawing operations continuously.  
         [0083]     Next, how many of the divided insulating pattern layers is to be drawn continuously or the number of continuous drawing operations is determined. It is desired that the finally formed conductive pattern and insulating pattern are uniform in height. To this end, it is necessary to draw the insulating pattern so that the thickness of the insulating pattern after being burned is almost equal to the thickness of the conductive pattern that was drawn two times in a row and burned. When, for example, fine insulating particles with their shrinkage factor almost equal to that of fine metal particles are used, let us consider a case where an amount of insulating particles contained in one droplet of an insulating pattern forming solution is two times that of metal particles. In this case, the thickness of the insulating pattern formed by one insulating pattern drawing operation is equal to the thickness of the conductive pattern obtained by performing two drawing operations continuously. Conversely, when the amount of insulating fine particles contained in one droplet is smaller than the amount of metal fine particles, two or more drawing operations are required. This embodiment uses the conductive and insulating pattern forming solutions which have almost the same coagulation factors and almost the same amount of particles contained in one droplet. It is assumed that two insulating pattern drawing operations are performed continuously, the same number as that of conductive pattern drawing operations performed in a row. This invention, however, is not limited to these conditions.  
         [0084]     In step S 13 , the conductive pattern is drawn the number of times that was determined in step S 12 .  FIG. 6A  shows a first divided layer (formed by the first drawing operation) of the conductive pattern that is to be completed by two successive drawing operations. A conductive pattern  21  is drawn on a substrate  20 . Since the scan direction of the head is toward right, a satellite  22  lands on the right side of the conductive pattern  21 .  
         [0085]     Next, the second divided layer (formed by the second drawing operation) which is drawn and dried is shown in  FIG. 6B . A second conductive pattern is formed over the first conductive pattern  21 . A satellite  23  formed during the second drawing operation lands on the substrate  20  but often does not overlap the satellite  22  that has landed during the first drawing operation.  
         [0086]     Next, in step S 14 , at locations where the conductive pattern  21  is not drawn, two divided layers of the insulating pattern are similarly drawn by two successive drawing operations.  FIG. 6C  shows two divided layers of insulating pattern drawn in two drawing operations and dried. The satellites  22 ,  23  that have landed in previous operations are now completely covered with the insulating pattern  24 .  
         [0087]     Next, in step S 15  a check is made to see if the pattern drawing operation has been executed a predetermined number of times. If that number is not yet reached, the process returns to step S 13  to repeat the processing of step S 13  and S 14 . At the current stage, only two of the four conductive pattern drawing operations have been performed, so the subsequent processing will be explained below.  
         [0088]     Returning from step S 15  to step S 13 , the third and fourth divided layer (formed by the third and fourth drawing operation) of the conductive pattern are drawn in two successive drawing operations and dried. This is shown in  FIG. 6D . These layers are formed over the already drawn conductive pattern  21 . Satellites  25  formed in these drawing operations land on the insulating pattern  24  drawn in step S 14 . Then, as shown in  FIG. 6E , the insulating pattern is drawn in two successive operations and dried in the same way as described above.  
         [0089]     Then, when the number of conductive pattern drawing operations reaches a predetermined number (a total of four drawing operations), step S 15  decides that all pattern drawing operations are finished and ends the pattern drawing process.  
         [0090]     As a final step, the substrate drawn with circuit patterns is baked to form a printed circuit board as shown in  FIG. 6F . As shown in the figure, in the insulating pattern  27  to the right of the conductive pattern  26 , a plurality of satellites  28  formed during the two successive drawing operations are situated close to each other at two locations. However, they are not close enough to short-circuit the conductive patterns  26  and thus good circuit patterns can be formed. Further, in this embodiment, since two scans are performed successively and then dried, the printed circuit board can be completed in a shorter time than when the layers are dried after each drawing operation as in the first embodiment.  
       Third Embodiment  
       [0091]      FIG. 7  is a flow chart showing a pattern drawing procedure in the circuit pattern forming method in a third embodiment of this invention.  FIGS. 8A-8H  are schematic diagrams showing a process of drawing conductive and insulating patterns.  FIGS. 8A-8G  illustrate a circuit pattern drawn and dried to evaporate a solvent, and  FIG. 8F  shows a completed circuit pattern after being burned.  
         [0092]     This third embodiment can not only be applied to a case where one layer of a circuit pattern is formed over an insulating substrate but also to a case where a circuit pattern is formed over a conductive substrate. Further, this embodiment is also applicable to a case in which another pattern is formed over an already formed circuit pattern, i.e., a so-called multilayered substrate is formed. Here, an example case will be explained in which a second layer of circuit pattern is formed in the multilayered substrate.  
         [0093]     As shown in  FIG. 8A  to  8 G, the circuit pattern P 2  of a second layer is formed over circuit pattern P 1  of a first layer made up of a conductive pattern  51  and an insulating pattern  52  both formed on a substrate  50 . The circuit pattern P 1  of a first layer shown in  FIG. 8A  to  8 G may be formed in any way, not limited to the method of this invention.  
         [0094]     In this embodiment, an example case is described in which, as shown in  FIG. 8G , the circuit pattern (P 2 ) of the second layer is formed by dividing it into four layers and drawing them in four drawing operations and the layer of the insulating pattern is also formed by dividing it into five layers and drawing them in five drawing operations.  
         [0095]     In this embodiment, the number of times that a pattern forming solution is repetitively applied to the same position (the number of layers into which the second layer is to be divided) to form circuit pattern p 2  of the second layer to a desired thickness, is determined. That is, step S 21  determines the number of layers into which the second layer is to be divided (the number of times that the conductive pattern is drawn). Here, the conductive pattern layer is divided into four layers which are then drawn in four drawing operations to produce a desired thickness of circuit pattern p 2 . The following step S 22  determines the number of layers into which the insulating pattern is to be divided (the number of drawing operations). The number of insulating pattern drawing operations is one more than the number of conductive pattern drawing operations determined by step S 21 . Therefore, the number of insulating pattern drawing operations in this embodiment is five.  
         [0096]     In step S 23 , as shown in  FIG. 8B , one divided layer of an insulating pattern  31  is drawn (one time) over the circuit pattern P 1  of a first layer of  FIG. 8A  and then dried. Next, step S 24  draws one divided layer of conductive pattern  32  (one time) and dries it, as shown in  FIG. 8C . Since the head scan direction is toward the right, satellites  33  land on the right side of the conductive pattern  32 . In the next step S 25 , one divided layer of insulating pattern is again drawn (one time) and dried, as shown in  FIG. 8D . With the insulating pattern  34  drawn, the satellites  33  that have landed in the preceding step are covered with the insulating pattern  34 .  
         [0097]     Next, step S 26  checks if the number of pattern drawing operations has reached a predetermined number. If the predetermined number is not yet reached, the process returns to step S 24  to repeat the processing of step S 24  and S 25 . At the current stage, since only one of the four conductive pattern drawing operations is finished, the process repeats step  524  to step  25 .  
         [0098]      FIG. 8E  shows a second divided layer of conductive pattern drawn and dried in step S 24  again. Satellites  35  produced during this conductive pattern drawing operation land on the previously drawn insulating pattern  34 , not on the satellites  33  that landed during the first drawing operation. Then, in step S 25  again, an insulating pattern is drawn and dried, as shown in  FIG. 8F .  
         [0099]     After this, the above step S 24  and S 25  are similarly repeated. Then, as shown in  FIG. 8G , when the number of times that the conductive pattern has been drawn reaches four and the number of times that the divided insulating pattern has been drawn reaches five, step S 26  decides that all pattern drawing operations are completed, terminating the drawing process.  
         [0100]     As a final step, the substrate with drawn circuit patterns is baked in a baking device to form a printed circuit board as shown in  FIG. 8H . The conductive satellites  38  are present in the insulating pattern  37 . In the pattern of second layer formed in this embodiment, the insulating pattern is drawn prior to the drawing of the conductive pattern, and the drawing of the insulating pattern is alternated with the drawing of the conductive pattern. Therefore, the satellites  38  that land during each conductive pattern drawing operation do not contact each other, preventing the conductive pattern  51  in the first layer and the conductive pattern  36  in the second layer from getting short-circuited undesirably. This makes it possible to form a good circuit pattern.  
         [0101]     It is preferred that the conductive pattern  36  and the insulating pattern  37  on the completed printed circuit board be flat on the same plane with no height difference. To meet this requirement, it is necessary in this embodiment to select pattern forming solutions in such a manner that a conductive pattern and an insulating pattern, when formed and baked, have the same thicknesses, with the conductive pattern being formed by dividing it into four layers and drawing them in four drawing operations and with the insulating pattern being formed by dividing it into five layers and drawing them in five drawing operations. In practice, since a priority is given to the conductive pattern being set to a desired thickness in terms of conductivity and allowable current, it is preferable to select an insulating pattern forming solution according to the thickness of the conductive pattern. That is, it is preferred that the insulating particles used in the insulating pattern forming solution have a shrinkage factor almost equal to that of metal particles used in the conductive pattern forming solution and that the amount of insulating particles contained in one droplet be set about 20% less than that of metal particles.  
       Fourth Embodiment  
       [0102]     In the first embodiment described above, divided layers of a circuit pattern have been described to be formed by drawing a conductive pattern of one scan followed by the drawing of an insulating pattern of one scan. In this embodiment, as shown in  FIG. 1 , a conductive pattern and an insulating pattern are formed in the same scan by taking advantage of the configuration of the device in which a conductive head and an insulation head are arranged side by side. Descriptions of those portions similar to the above embodiments are omitted.  
         [0103]      FIGS. 9A-9K  are schematic views showing a process of drawing conductive and insulating patterns on a substrate  60 .  FIGS. 9A-9J  show a pattern drawn and dried to evaporate a solvent, and  FIG. 9K  shows a completed circuit pattern after baking.  
         [0104]     In the fourth embodiment, a circuit pattern similar to that of the first embodiment is formed.  
         [0105]     One complete layer of a circuit pattern in its final form shown in  FIG. 9K  is, in the drawing process, divided into four layers which are formed individually, as shown in  FIG. 9J . That is, each of the four layers making up the circuit pattern is formed by one scan of the head or one drawing operation. Thus, to form all of the four layers that combine to form one complete layer of the circuit pattern requires four drawing operations.  
         [0106]     First, as shown in  FIG. 9A , a first scan forms an insulating pattern  61 , corresponding to the first divided layer of the circuit pattern, by ejecting droplets  2   d  of an insulating solution from the insulation head. Then, as shown in  FIG. 9B , as the conduction head  3  scans over a conductive pattern forming area on the substrate  60 , the head  3  ejects droplets  3   d  of a conductive solution to form a conductive pattern  62  on the substrate  60 . Because the head draws the conductive pattern as it scans toward the right in the figure, satellites  3   s  of the conductive liquid land on the right side of the conductive pattern  62 . When an insulating pattern is drawn, satellites of the insulating solution also land as in the case with the conductive pattern drawing operation. However, because these satellite do not pose any problem in the circuit operation, they are not shown in the figure.  
         [0107]     Then, as shown in  FIG. 9C , when the insulation head  2  scans over the insulating pattern forming area on the substrate, the head  2  ejects droplets  2   d  of the insulating solution to form an insulating pattern  63  on the substrate. By drawing the insulating pattern  63  as described above, the satellites  3   s  of the conductive solution that have landed during the previous drawing operation are covered with the insulating pattern  63 . Similarly, as shown in  FIGS. 9D, 9E  and  9 F, a conductive pattern  64  and an insulating pattern  65  are formed. An operation up to this step is performed by one scan of the head and this operation forms one of the four divided layers that combine to form a complete layer of the circuit pattern.  
         [0108]     A fixing process based on drying may be performed by warming the substrate  60  at any desired time or every scan.  
         [0109]     Next, in a process shown in  FIGS. 9G, 9H  and  9 I, a second scan of the head forms a second divided layer of the four divided layers making up the one complete layer of the circuit pattern. In this operation, too, the conductive satellites  3   s  are covered with insulating patterns  68 ,  70 .  
         [0110]     As shown in  FIG. 9J , when the number of times that the head performs the drawing operation reaches a predetermined number of four, it is decided that all patterns have been completely drawn, terminating the pattern drawing operation.  
         [0111]     Then, as shown in  FIG. 9K , the substrate formed with circuit patterns is baked in a separate baking device, causing metal particles contained as conductive particles in the conductive pattern forming solution to form metal connections. Now, the fabrication of circuit patterns is complete.  
         [0112]     In all of the above embodiments, although the drawing operation has been described to be performed in only one of the forward and backward scans of the head, this invention can also be applied to a configuration in which the drawing operation is performed in both directions.  
         [0113]     The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspect, and it is the intention, therefore, in the appended claims to cover all such changes.  
         [0114]     This application claims priority from Japanese Patent Application No. 2005-181626 filed Jun. 22, 2005, which is hereby incorporated by reference herein.