num="0058">The electro-conductive ink is an ink including one kind or plural kinds of metal fine particles of at least one of Ag (silver), Au (gold), Cu (copper), Pd (palladium), and Ni (nickel), a solution in which the metal is dissolved in an ionic state, or non-metallic conductor fine particles, or non-metallic conductor fine particles such as C (carbon). A range of the diameter of the ink droplet that can be adopted is about 1 μm to 100 μm, more preferably 10 μm to 30 μm, and typically 20 μm. A range of the viscosity of the ink that can be adopted is about 1 cps to 500 cps, more preferably about 2 cps to 30 cps, and typically 4 cps.

Assuming that the ink puddle 15a is formed in the hole 13, the closing member 17 having water absorption properties is disposed on the lower side of the substrate 11. The closing member 17 is a film-shaped or plate-shaped member formed of a porous material, and is typically an air-permeable sheet member (or a porous sheet member) having air permeability. For example, paper, a porous resin plate, fabric, or non-woven fabric may be used. From the viewpoint of cost, paper is suitable. A range of a pore diameter of the air-permeable sheet member that can be adopted is about 1 μm to 50 μm, more preferably 5 μm to 10 μm, and typically 10 μm. The pore shape of the closing member is not limited to a circular shape. Therefore, in this case, the pore diameter corresponds to a gap width.

In the present embodiment, it is preferable that a basic relationship between the hole diameter of the substrate, the ink droplet diameter, and the pore diameter of the closing member satisfies the hole diameter of the substrate>the ink droplet diameter>the pore diameter of the closing member.

Regarding the filling of the ink in the hole 13, when the water absorption ability of the closing member 17 is not sufficient, the water absorption capacity of the closing member 17 is substantially full while the ink is filling in the hole 13, and there may be a situation in which the discharge does not progress well even after a certain period of time from the filling. In addition, when the water absorption ability of the closing member 17 is excessive, there may be a situation in which an enormous amount of time is required until the ink is completely filled in the hole 13. From this viewpoint, it is desirable that an appropriate relationship is selected as a relationship (condition) between the water absorption ability of the closing member 17 and the amount of the ink ejected per unit time and unit area.

More specifically, it is considered that the water absorption ability of the closing member 17 relates to a water absorption speed and the water absorption capacity of the closing member 17. When the pore diameter of the closing member 17 changes, the most affected factor is the water absorption speed. When the water absorption speed changes, the following situation is expected. That is, when the water absorption speed is sufficiently fast, there is a possibility that the ink filling may not occur. One case where this situation occurs is that a relationship of the ink droplet diameter<the pore diameter of the closing member is satisfied. Conversely, when the water absorption speed is slow, for example, when a plate that is not porous is used as the closing member, it is considered that an excess amount of ink remains in the hole for a long period of time such that the fixing of the ink to the wall surface is inhibited by the surface tension of the ink remaining in the bottom portion.

The front surface printing step also includes a discharge step of discharging the ink filled in the hole 13 to the outside from the hole 13 (FIG. 1(b2)).

Next, as the time elapses, the closing member 17 absorbs the ink, and most of the ink in the hole 13 is discharged from the hole 13. However, the ink layer attached to the inner wall of the hole 13 remains without being discharged. It should be noted that, in FIG. 1(b2), the shape or size of a region of an infiltration portion 18 formed by the closing member 17 absorbing the ink is schematically illustrated simply for convenience of description. Actually, the shape or size of the region of the infiltration portion 18 may change depending on the capacity of the ink puddle 15a in the hole 13 and the material, composition, thickness, and the like of the closing member 17.

The front surface printing step also includes a drying step of drying an ink layer (inner wall ink layer) that remains on the inner wall of the hole 13 and an on-substrate ink layer having a circuit pattern shape that is connected to the inner wall ink layer and is formed on the substrate after discharging the ink from the hole 13 (FIG. 1(b3). The film thickness of the electro-conductive layer (electro-conductive film) obtained in the drying step is 100 nm to 20 μm and more preferably 500 nm to 2 μm.

That is, after waiting for a predetermined period of time from the ink-jet printing on the front surface of the substrate 11, the closing member 17 is removed, and the drying step is executed on the circuit pattern and the land on the front surface of the substrate 11 and the ink layer of the inner wall of the hole 13 connected to the land. As a result, a conductor layer 16 is formed on the front surface and in the via hole. The drying step includes one or a plurality of steps among a blowing process and a heating process.

After the front surface printing step, a back surface printing step is executed (FIG. 1(c)).

In the back surface printing step, the front and back of the substrate 11 obtained in FIG. 1(b3) where the conductor layer 16 is formed on the front surface and in the via hole is reversed (vertically flips), and each of the printing step, the ink discharge step, and the drying step is executed as described above.

That is, in the back surface printing step, fine droplets of the electro-conductive ink 15 are ejected to the back surface using an ink-jet method, the circuit pattern is printed on the back surface, and the ink puddle 15a is formed in the land around the hole and in the hole (FIG. 1(c1)). It should be noted that “back surface” described herein refers to a surface opposite to the front surface of the substrate 11 of FIG. 1(b) and is positioned on the upper surface side in FIG. 1(c) after reversing the front and back of the substrate 11.

Assuming that the ink puddle 15a is formed in the hole 13, the closing member 17 having water absorption properties is disposed on the lower side of the substrate 11 (FIG. 1(c1). Incidentally, the ink is filled in the hole 13 of which the lower end is blocked by the closing member 17 using an ink-jet method. Next, as the time elapses, the closing member 17 absorbs the ink, and most of the ink in the hole 13 is discharged from the hole 13 (FIG. 1(c2). After waiting for a predetermined period of time from the ink-jet printing on the back surface of the substrate 11, the closing member 17 is removed, and the drying step is executed on the circuit pattern and the land on the back surface of the substrate 11 and the ink layer of the inner wall of the hole 13 connected to the land. As a result, a conductor layer 16 is formed on the back surface and in the via hole (FIG. 1(c3)).

In principle, the conductor layer 16 is already formed on the inner wall of the hole 13 in the front surface printing step. Therefore, in the back surface printing step, the connection between the conductor layer 16 in the via hole and the land on the back surface is the main object. It should be noted that there is no problem even when the ink layer remains in the back surface printing to overlap the conductor layer 16 on the inner wall of the hole 13 formed in the front surface printing step and the electrical connection between the back surface land the conductor layer 16 will be more assured.

After the back surface printing step, a metal plating step is executed (FIG. 1(d)). In general, as the plating method, electrolytic plating and electroless plating can be executed. In the present embodiment, electroless plating is executed. It should be noted that, in the present invention, the plating step is not essential for various uses. As the plating metal, Cu, Ni, Ag, Au, or the like can be adopted. In the present embodiment, Cu is adopted. The film thickness of the plating layer is about 0.03 μm to 100 μm and more preferably 3 μm to 18 μm. In addition, the resistance value is 0.001Ω to 10Ω and more preferably 0.001Ω to 1Ω.

This way, by executing the metal plating on the conductor layer 16, as an underlayer, on the circuit patterns and the lands on the front and back surfaces of the substrate 11 formed in the front surface printing step and the back surface printing step and on the inner wall of the via hole, the thickness of the conductor layer 16 increases. As a result, an increase in the mechanical strength of the circuit pattern and the like and a decrease in the electrical resistance of the conductor layer are realized.

This way, in the present embodiment, the appropriate formation of the conductor on the circuit patterns (including the lands) on the front and back surfaces of the substrate 11 and on the inner wall of the hole 13 by ink-jet printing can be realized with little man-hours.

In the ink discharge step of (b2) and (c2) of FIG. 1, the ink in the hole 13 may be forcibly sucked through the closing member 17 by using vacuum suction means for adjusting the pressure of the closing member 17 side of the substrate 11 to a negative pressure. This suction is an essential element for a high-viscosity ink as in the screen printing in the techniques of the related art described in PTLs 1 and 2. However, when a low-viscosity ink is used in the ink-jet printing as in the present embodiment, the suction is not an essential element and can be used simply for accelerating the speed of the ink discharge.

FIG. 2 is a diagram illustrating steps of a method of producing a printed circuit board according to a first modification example of the first embodiment. The same elements as those illustrated in FIG. 1 will be represented by the same reference numerals, and the repeated description thereof will not be repeated.

In the first modification example, in the back surface printing step (FIG. 2(c1)), the passing of ink droplets 15b ejected into the hole 13 through the lower side of the substrate 11 is allowed during the back surface printing without using the closing member 17 illustrated in FIG. 1. At this time, in order to reduce the amount of the ink droplets passing through the lower side of the substrate 11, the ejection of the ink droplets to a center region of the hole 13 may be skipped. In order to ensure the connection between the conductor of the inner wall of the hole 13 and the peripheral portion (the land portion of the back surface) of the hole 13 of the back surface of the substrate 11, the ejection of the ink droplets to at least the outer peripheral portion of the hole 13 is executed. The other steps of FIG. 2 are as described above with reference to FIG. 1.

FIG. 3 is a graph illustrating a relationship between a resolution (dpi) and an ink filling rate of ink-jet printing. This data is acquired for Examples where the amount of ink per dot is about 6 pL, the diameter of the hole is 0.5 mm, and the depth of the hole is 0.025 mm. The ink filling rate represents a ratio of the amount of ink ejected into the hole to the inner volume of the hole (via volume). As can be seen from the drawing, a correlation (substantially proportional) between the resolution and the ink filling rate is seen.

In the present embodiment, in principle, the amount of ink ejected into the hole 13 needs to be set to be sufficient with respect to the via volume. The parameters at this time are the amount of ink and the print resolution. When the amount of ink ejected is not sufficient with respect to the via volume, there is a possibility that the fixing of the ink to the inner wall may not occur. Conversely, when the amount of ink is excessively large, error may occur. As an example of the error occurring when the amount of ink is large, an ink puddle may be formed in the hole inner wall to cause cracking during drying. As a result, the plating shape becomes also irregular, and a defect may also occur in that the front and back surfaces are not electrically connected. In the present embodiment, in principle, the ink filling rate is desirably about 100% to about 200%. A desirable range of the print resolution is about 1700 dpi to about 2300 dpi. It should be noted that, however, when the printing step, the filling step, and the drying step are executed on each of the front and back surfaces of the substrate as in the embodiment illustrated in FIG. 1, the ink filling rate may be lower than 100%. The reason for this is that, when the ink is filled in more than half of the lower side in the hole in the filling step from the front side in a thickness direction of the substrate, the substrate is reversed, and the ink is filled in more than new half of the lower side (more than half of the original front side) in the filling step, the steps of the front and back surfaces complement each other such that the conductor layer that is continuous over the entire inner wall of the hole 13 can be obtained. In an actual experiment by the present inventors, it was verified that, even when the thickness of the substrate is relatively large at 125 μm, the ink filling rate of about 55% to about 60% is sufficient in the present embodiment.

Second Embodiment

FIG. 4 is a diagram illustrating steps of a method of producing a printed circuit board according to a second embodiment of the present invention. In this embodiment, the hole forming step of the substrate 11 is executed after the front surface printing.

First, a circuit pattern 21 is formed on the front surface of the substrate 11 (FIG. 4(a)). In this case, the circuit pattern 21 does not need to be formed by ink-jet printing. However, in the present embodiment, the circuit pattern 21 is formed by ink-jet printing, and the ink layer is dried to form a front surface conductor layer.

Next, a hole forming process of forming the hole 13 at a desired via position is executed on the substrate 11 obtained in FIG. 4(a) (FIG. 4(b)).

Next, a back surface printing step is executed (FIG. 4(c)).

In the back surface printing step, first, the front and back of the substrate 11 obtained in FIG. 4(b) where the circuit pattern 21 is formed on the front surface is reversed, and each of the back surface printing step (including the ink filling in the hole 13), the ink discharge step, and the drying step is executed. It should be noted that “back surface” described herein refers to a surface opposite to the front surface of the substrate 11 of FIG. 4(b) and is positioned on the upper surface side in FIG. 4(c) after reversing the front and back of the substrate 11.

Assuming that the ink puddle 15a is formed in the hole 13, the closing member 17 having water absorption properties is disposed on the lower side of the substrate 11 (FIG. 4(c1). Thereafter, the ink is filled in the hole 13 of which the lower end is blocked by the closing member 17 using an ink-jet method. Next, as the time elapses, the closing member 17 absorbs the ink, and most of the ink in the hole 13 is discharged from the hole 13 (FIG. 4(c2). After waiting for a predetermined period of time from the ink-jet printing on the back surface of the substrate 11, the closing member 17 is removed, and the drying step is executed on the circuit pattern and the land on the back surface of the substrate 11 and the ink layer of the inner wall of the hole 13 connected to the land. As a result, a conductor layer 16 is formed on the back surface and in the via hole (FIG. 4(c3)).

After the back surface printing step, a metal plating step is executed (FIG. 4(d)). As the metal plating, electroless copper plating is executed in the present embodiment. This way, by executing the electroless plating on the conductor layer, as an underlayer, on the circuit patterns and the lands on the front and back surfaces of the substrate 11 formed in the front surface printing step and the back surface printing step and on the inner wall of the hole, a conductor layer (electroless plating layer) 19 of which the thickness increases is obtained. As a result, an increase in the mechanical strength of the circuit pattern and the like and a decrease in the electrical resistance of the conductor layer are realized.

This way, in the second embodiment, the appropriate formation of the conductor on the circuit patterns (including the lands) on the front and back surfaces of the substrate 11 and on the inner wall of the hole 13 by ink-jet printing can be realized with little man-hours. In particular, the procedure of the front: surface printing step can be simplified. A relationship between the resolution (dpi) and the ink filling rate of the ink-jet printing and preferable ranges thereof are the same as those of the first embodiment.

In addition, in the second embodiment, the alignment of the front surface printing is not necessary, and thus a roll-to-roll process is easy. “The roll-to-roll process” refers to a transport configuration in which, when an object on a film or paper is processed, a film prepared from a roll is wound around another roll after being processed. The roll-to-roll process is a process that is adopted to improve the mass productivity as compared to a batch process. In the present embodiment, the printing and the drying can be continuously processed in a batch process. When the hole formation is executed first, land drawing at an ink-jet drawing position needs to be aligned with the formed hole, an accuracy of several tens of micrometers is required, and a transport control needs to be accurately executed. On the other hand, when hole formation is executed after this step, the printing accuracy on the substrate only needs to be less than the film width. Therefore, the accuracy is at least about several millimeters, and the required accuracy of the device is significantly reduced.

In the ink discharge step of FIG. 4(c2), the ink in the hole 13 may be forcibly sucked through the closing member 17 by using vacuum suction means for adjusting the pressure of the closing member 17 side of the substrate 11 to a negative pressure. However, when a low-viscosity ink is used in the ink-jet printing as in the present embodiment, the suction is not an essential element and can be used simply for accelerating the speed of the ink discharge.

FIG. 5 is a diagram illustrating steps of a method of producing a printed circuit board according to a first modification example of the second embodiment illustrated in FIG. 4. The same elements as those illustrated in FIG. 4 will be represented by the same reference numerals, and the repeated description thereof will not be repeated.

In the first modification example of the second embodiment, laser ablation (LA) processing 25 is executed on the substrate 11 where the circuit pattern 21 is formed. For fining the circuit pattern (including the land) on the substrate 11, the LA processing is executed to remove a part 23 of the conductor layer on the substrate 11. Even in the steps illustrated in FIGS. 1 and 2, the same LA processing can be executed. In the embodiment where the hole is formed after forming the circuit pattern on the substrate 11 as illustrated in FIG. 5, when the hole formation is executed with a laser, a part of the conductor layer can be removed at the same time. Thus, the processing can be efficiently executed.

FIG. 6 illustrates an example of the fining process of the circuit pattern by the LA processing. FIG. 6(a) illustrates an example of the conductor layer formed on the front surface (or the back surface) of the substrate by ink-jet printing. This conductor layer includes: a plurality of sequential land portions 41; a low-density conductive pattern portion 43 of which one end is connected to each of the land portions 41; a tapered solid portion 45 that is connected to another end of the low-density conductive pattern portion 43; and a narrow solid portion 47 that is connected to an end portion of the tapered solid portion 45. As illustrated in FIG. 6(b), a gray region of a center portion of the land portion 41 is a hole portion (through-hole) formed by the LA processing, and a gray region in the tapered solid portion 45 and the narrow solid portion 47 is a portion to be removed by the LA processing on the surface of the conductor layer. FIG. 6(c) illustrates a conductor pattern obtained by the LA processing. That is, a high-density conductive pattern portion 57 having a higher density than the low-density conductive pattern portion 43 and a connection conductive pattern portion 55 that connects both of the portions are formed. Even when the low-density conductive pattern portion 43 has a limit density (resolution) obtained by the ink-jet method, the high-density conductive pattern portion 57 having a high density can be provided by the LA processing.

FIG. 7 is a diagram illustrating steps of a method of producing a printed circuit board according to a second modification example of the second embodiment illustrated in FIG. 4.

First, the circuit pattern 21 is formed on the front surface of the substrate 11 (FIG. 7(a)), and by executing the electrolytic plating process on the circuit pattern 21, a circuit pattern 31 that is a conductor layer having a relatively large film thickness is formed (FIG. 7(b)).

Next, by executing the hole forming process on a desired via position of the substrate 11 where the circuit patterns 21 and 31 are formed, the hole 13 is formed (FIG. 17(c)). At this time, optionally, the above-described LA processing 25 can be executed. That is, for fining of the circuit pattern (including the land) on the substrate 11, a part 23 of the conductor layer on the substrate 11 is removed. This process can be executed using a laser used for the hole forming process.

Next, a back surface printing step is executed (FIG. 7(d)).

In the back surface printing step, first, the front and back of the substrate 11 obtained in FIG. 7(c) where the circuit patterns 21 and 31 are formed on the front surface is reversed, and each of the back surface printing step (including the ink filling in the hole 13), the ink discharge step, and the drying step is executed. It should be noted that “back surface” described herein refers to a surface opposite to the front surface of the substrate 11 of FIG. 7(b) and is positioned on the upper surface side in FIG. 7(c) after reversing the front and back of the substrate 11.

Assuming that the ink puddle 15a is formed in the hole 13, the closing member 17 having water absorption properties is disposed on the lower side of the substrate 11 (FIG. 7(d1). Thereafter, the ink is filled in the hole 13 of which the lower end is blocked by the closing member 17 using an ink-jet method. Next, as the time elapses, the closing member 17 absorbs the ink, and most of the ink in the hole 13 is discharged from the hole 13 (FIG. 7(d2). After waiting for a predetermined period of time from the ink-jet printing on the back surface of the substrate 11, the closing member 17 is removed, and the drying step is executed on the circuit pattern and the land on the back surface of the substrate 11 and the ink layer of the inner wall of the hole 13 connected to the land. As a result, a conductor layer 16 is formed on the back surface and in the via hole (FIG. 7(d3)).

After the back surface printing step, a metal plating step is executed (FIG. 7(e)). In this example, as the metal plating, electroless copper plating is executed. This way, by executing the electroless plating on the conductor layer 16, as an underlayer, on the circuit patterns and the lands on the front and back surfaces of the substrate 11 formed in the front surface printing step and the back surface printing step and on the inner wall of the hole, a conductor layer (electroless plating layer) 19 of which the thickness increases is obtained. As a result, an increase in the mechanical strength of the circuit pattern and the like and a decrease in the electrical resistance of the conductor layer are realized.

In the second modification example, as described above, the alignment of the front surface printing is not necessary, and thus a roll-to-roll process is easy.

In the ink discharge step of FIG. 7(d2), the ink in the hole 13 may be forcibly sucked through the closing member 17 by using vacuum suction means for adjusting the pressure of the closing member 17 side of the substrate 11 to a negative pressure. However, when a low-viscosity ink is used in the ink-jet printing as in the present embodiment, the suction is not an essential element and can be used simply for accelerating the speed of the ink discharge.

FIG. 8 is a diagram illustrating steps of a method of producing a printed circuit board according to a third modification example of the second embodiment illustrated in FIG. 4. The third modification example is similar to the second modification example illustrated in FIG. 7, the description of the same steps as those of the second modification example will not be repeated, and only different steps will be described in detail.

The steps of FIG. 8(a) to FIG. 8(d) are the same as the steps of FIG. 7(a) to FIG. 7(d). In the third modification example, electrolytic plating is executed instead of electroless plating after the step of FIG. 8(d) (FIG. 8(e)). As a result, a conductor layer (electrolytic plating layer) 32 having a larger film thickness than that obtained by electroless plating can be obtained. In this case, depending on a condition, the entire inside of the hole 13 is embedded with the electrolytic plating layer to enter a solid state. This condition is substantially “the via hole diameter<(the plating film thickness×2)”.

In the electrolytic plating, an increase in film thickness and a control of the film thickness can be executed in a shorter period of time as compared to the electroless plating.