Manufacturing method of printing circuit board and conductive ground layer formation method

Abstract

To form a conductive film on an inner peripheral surface (inner wall) of a hole penetrating to a rear surface from a front surface of an insulating board by an ink jet method without troubles.SOLUTION: When forming at least a conductive ground layer on an inner wall of a hole 13 on an insulating board 11 having the hole 13 penetrating to a rear surface from a front surface, a method executes: a charging step of charging ink 15 into the hole 13 by discharging conductive ink droplets into the hole 13 by an ink jet method in such a state that the hole 13 is blocked by a closing member 17 from the lower side of the board 11; a discharging step of discharging the ink charged into the hole 13 to the outside; and a drying step of drying at least an inner wall ink layer remaining on the inner wall of the hole 13 after the discharging step.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a printed circuit board having via holes and a method for forming a conductive underlayer. [Background technology]

[0002] Conventionally, a known technique involves providing layers of circuit patterns (conductive patterns) on at least two surfaces, the front and back, of an insulating substrate, drilling holes through the substrate to establish electrical continuity between the two layers, and filling the holes with a conductor or forming a conductive film on the inner walls of the holes. Generally, holes intended to establish continuity between such layers are called via holes (or simply vias or via holes) or through holes.

[0003] For example, Patent Document 1 discloses a technique in which a desired circuit pattern is formed on a base member (substrate) with required through-holes by screen printing while one side of the base member is held at low pressure, and at the same time, the conductive ink is sucked into the inner surface of the through-holes by vacuum suction to effect a through-hole conduction process, and then the base member is turned over to form a similar circuit pattern and effect through-hole conduction. It also discloses a technique in which thick plating is applied to the circuit pattern by means of electrolytic plating or electroless chemical plating.

[0004] Patent Document 2 discloses a technique for forming a conductive film on the inner periphery of through holes in an insulating sheet (e.g., a green sheet), in which the insulating sheet is placed on an insulating sheet suction plate with suction holes formed all over the surface, regardless of the through hole pattern of the insulating sheet, with a breathable sheet such as paper interposed between them, and then screen printing with conductive ink is performed from above a mask placed over the insulating sheet, and the inner periphery of the through holes is vacuum-sucked by suction from the insulating sheet suction plate to form through holes. The use of the breathable sheet prevents ink from entering the suction holes in the insulating sheet suction plate or the negative pressure chamber formed by the housing, eliminating the need to clean the suction plate and housing, or reducing the frequency of cleaning.

[0005] Patent Document 3 discloses a technology in which a ceramic green sheet constituting a ceramic multilayer wiring substrate has via holes drilled therein, and a back tape is attached to the sheet so as to cover one of the openings. Then, a mask such as a metal mask or a screen mask is set, and conductive ink (conductive paste) is filled into the via holes by screen printing or the like to fill the holes, and after the ink has hardened and solidified, the back tape is peeled off, thereby preventing the vias (ink, etc.) from falling off the sheet during the via formation process.

[0006] Patent document 4 discloses a technology in which a silver line is formed on a polyimide film using a droplet ejection method, and when a through hole that connects one side of the polyimide film to the other side is formed, droplets of a metal particle dispersion liquid are ejected targeting four locations around the hole, thereby coating the entire side surface and surrounding area of ​​the hole with the metal particle dispersion liquid.

[0007] Patent document 5 discloses a technology for forming an ink film by applying multiple droplets of ink containing metal microparticles to the inner walls of micropores in a substrate in order to electrically connect the front and back electrodes of the substrate.

[0008] Patent Document 6 discloses a technique for forming buried wiring on an interlayer insulating film by discharging metal droplets into via holes in a semiconductor substrate using an inkjet method. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 55-6832 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-111236 [Patent Document 3] Japanese Patent Application Laid-Open No. 2001-111221 [Patent Document 4] Japanese Patent Application Laid-Open No. 2003-318542 [Patent Document 5] Japanese Patent Application Laid-Open No. 2012-129251 [Patent Document 6] Japanese Patent Application Laid-Open No. 2003-243327 Summary of the Invention [Problem to be solved by the invention]

[0010] In the conventional techniques described in Patent Documents 1 and 2, a conductive film is formed on the inner surface of a hole (through hole) in a substrate by vacuum suction from the back side of conductive ink applied to the front side of the substrate by screen printing. Furthermore, in the conventional technique described in Patent Document 3, conductive ink (conductive paste) is applied to the front side of the substrate by screen printing through a mask, and the holes (via holes) are filled with the conductive ink. In both of these conventional techniques, a screen mask must be created when applying the conductive ink in screen printing, which requires initial costs and the effort of storing and maintaining the screen mask.

[0011] In contrast, the conventional techniques described in Patent Documents 4 to 6 use an inkjet method (droplet ejection method) to eject conductive ink containing metal particles as droplets, thereby forming a patterned conductive layer without using a mask. Therefore, inkjet printing can reduce the number of steps required and also requires less material (screen printing, due to its manufacturing process, generates a relatively large amount of discarded material, but inkjet printing ejects only the necessary parts, resulting in almost zero waste).

[0012] From this viewpoint, it can be said that forming a conductive film on the inner wall of a via hole by inkjet printing is preferable.

[0013] Inkjet printing and screen printing not only differ in printing method, but also generally differ in the viscosity of the conductive ink used. Screen printing uses high viscosities ranging from 1,000 cps to 200,000 cps, while inkjet printing typically uses low viscosities ranging from 1 cps to 40 cps. Therefore, the behavior of screen printing inks and inkjet printing inks is completely different. Therefore, conventional screen printing techniques such as those described in Patent Documents 1 to 3 cannot be directly replaced with inkjet printing. This is because inkjet printing using the same method for through-holes results in ink passing through the via holes, resulting in an insufficient ink layer adhering to the inner walls of the via holes. This ink passing through is a phenomenon unique to inkjet printing and does not occur in screen printing, which uses high-viscosity ink to press the ink onto the surface.

[0014] Furthermore, even if the ink droplet size is increased to form a surface film and then sucked from the back, the ink may still "wrap around to the back" and cause problems with the back wiring pattern. This is also a phenomenon that cannot occur with high-viscosity screen printing.

[0015] In the method described in Patent Document 4, in which a conductive film is formed on the inner surface of a hole by utilizing ink leaking and spreading from an opening, the thickness of the conductive film tends to be uneven, and the film thickness uniformity is insufficient.

[0016] The technology of Patent Document 5 requires highly accurate droplet discharge control in order to apply multiple droplets of ink to the inner wall of a microhole in the substrate at different positions in the depth direction. Also, because this technology is configured to apply droplets to the inner wall in the direction of the inkjet head scan, it is not easy to form a conductive film evenly over the entire inner surface of the hole in the substrate.

[0017] The technology in Patent Document 6 involves filling holes with conductive ink used in inkjet printing and solidifying it, but it is not possible to use the ink layer as a base layer to form plating (which is preferable because it has a lower resistance than ink) on the inner walls of via holes. Furthermore, conductive ink generally needs to be thin to be suitable, as cracks will occur due to shrinkage during firing, which limits the configurations and applications.

[0018] The present invention has been made against this background, and an object of the present invention is to provide a method for manufacturing a printed circuit board and a method for forming a conductive base layer that can form a conductive film on the inner surface (inner wall) of a hole that penetrates from the front surface to the back surface of an insulating substrate using an inkjet method without any problems. [Means for solving the problem]

[0019] A method for manufacturing a printed circuit board according to the present invention includes a surface printing step of printing a circuit pattern by ejecting conductive ink droplets by an inkjet method onto the surface of an insulating substrate having a hole penetrating from the front surface to the back surface; a filling step of filling the hole with ink by ejecting conductive ink droplets into the hole by an inkjet method while the hole is blocked from the underside of the substrate during the surface printing step; a discharging step of discharging the ink that has filled the hole to the outside; and a drying step of drying an inner wall ink layer remaining on the inner wall of the hole and an on-substrate ink layer in the form of a circuit pattern formed on the substrate and connected to this inner wall ink layer after the discharging step.

[0020] With the hole blocked from below the substrate, ink droplets are ejected into the hole using an inkjet method, filling the hole with ink and ensuring that the ink adheres to the entire inner wall of the hole, and then the excess ink is expelled to the outside, resulting in an ink layer of a specified thickness being applied almost evenly and without unevenness to the inner wall of the hole.

[0021] In the filling step, ink droplets can be ejected into the hole after the hole is blocked with a closing member having a water-absorbing function. By using such a closing member, temporary filling of the hole with ink and subsequent discharge of the ink can be achieved in chronological order.

[0022] The closing member may be, for example, a porous sheet member, and the discharging step may include a step of waiting until most of the ink filled in the hole is discharged and a portion of it remains as an ink layer on the wall surface of the hole, and then a step of removing the closing member.

[0023] The method may further include a backside printing step of turning the substrate over after the drying step and printing a circuit pattern on the backside, and after the backside printing, the filling step, the discharging step, and the drying step are performed again, thereby ensuring electrical continuity between the circuit pattern on the backside and the circuit pattern on the frontside via the conductor layer formed in the holes.

[0024] The filling step may include a step of drilling holes in the substrate, or a substrate with pre-drilled holes may be used.

[0025] The method may further include a plating step of applying metal plating to the inner wall ink layer and the conductor layer on the substrate obtained by the drying step.

[0026] The viscosity of the ink is, for example, within the range of approximately 1 cps to 500 cps.

[0027] The diameter of the ink droplets is, for example, approximately within the range of 1 μm to 100 μm.

[0028] The conductive ink may contain, for example, fine particles of at least one of Ag, Au, Cu, Pd, and Ni, or these metals dissolved in an ionic state, or fine particles of a non-metallic conductive material.

[0029] The ink filling rate, which is the ratio of the amount of ink ejected into the hole to the internal volume of the hole in the filling step, is about 100% to about 200% or less. However, if the filling step, discharging step, and drying step are performed again after reverse printing, the ink filling rate may be about 55% to about 200% or less.

[0030] Another method for manufacturing a printed circuit board according to the present invention includes a surface printing step of printing a circuit pattern on the surface of an insulating substrate; a hole drilling step of drilling holes penetrating the substrate on which the circuit pattern has been formed, the holes being formed from the front to the back; a filling step of filling the holes with ink by discharging conductive ink droplets into the holes by an inkjet method while the substrate is placed with the front side facing down and the holes are blocked from below the circuit pattern; a discharging step of discharging the ink filled in the holes to the outside; a drying step of drying an inner wall ink layer remaining on the inner walls of the holes after the discharging step and an on-substrate ink layer formed on the back surface of the substrate and connected to this inner wall ink layer; and a plating step of applying metal plating to the inner wall ink layer and the conductor layer on the substrate obtained by the drying step.

[0031] This method of manufacturing a printed circuit board differs from the above-mentioned method in that holes are drilled after surface printing, but it is the same in that the holes are blocked from below the board and ink droplets are ejected into the holes using an inkjet method, filling the holes with ink and ensuring that the ink adheres to the entire inner wall of the hole, and then the ink is expelled to the outside to remove excess ink from the hole, resulting in an ink layer of a predetermined thickness that is applied almost evenly and without unevenness to the inner wall of the hole.

[0032] During the hole drilling step, a laser ablation process may be performed on the circuit pattern, which allows a portion of the conductor layer to be removed at the same time as the hole drilling is performed with a laser, thereby enabling efficient processing.

[0033] The conductive base layer forming method according to the present invention is a method for forming a conductive base layer on at least the inner wall of a hole in an insulating substrate having a hole penetrating from the front surface to the back surface, and includes the following steps: a filling step in which, with the hole blocked from below, conductive ink droplets are ejected into the hole by an inkjet method to fill the hole with ink; a discharging step in which the ink filled in the hole is discharged to the outside; and a drying step in which, after the discharging step, the inner wall ink layer remaining on at least the inner wall of the hole is dried. In this conductive base layer forming method, the "insulating substrate" includes not only substrates made of a single material, but also substrates such as copper-clad laminates and multilayer substrates in which copper and an insulating base material are laminated.

[0034] This method for forming a conductive underlayer can be applied to a wide range of methods for forming circuit patterns on the front and back of a substrate. [Effects of the Invention]

[0035] According to the present invention, it is possible to provide a method for manufacturing a printed circuit board and a method for forming a conductive base layer, which are capable of forming a conductive film on the inner surface (inner wall) of a hole that penetrates from the front surface to the back surface of an insulating substrate using an inkjet method without any problems. [Brief explanation of the drawings]

[0036] [Figure 1] 1A to 1C are diagrams illustrating steps in a method for manufacturing a printed circuit board according to a first embodiment of the present invention. [Figure 2] 5A to 5C are diagrams illustrating steps in a method for manufacturing a printed circuit board according to a first modified example of the first embodiment. [Figure 3] 1 is a graph showing the relationship between inkjet printing resolution (dpi) and ink filling rate. [Figure 4] 5A to 5C are diagrams illustrating steps in a method for manufacturing a printed circuit board according to a second embodiment of the present invention. [Figure 5] 5A to 5C are diagrams illustrating steps in a method for manufacturing a printed circuit board according to a first modified example of the second embodiment shown in FIG. [Figure 6]6 is a diagram showing an example of fine circuit patterns obtained by LA processing in FIG. 5. FIG. [Figure 7] 5A to 5C are diagrams illustrating steps in a method for manufacturing a printed circuit board according to a second modification of the second embodiment shown in FIG. [Figure 8] 5A to 5C are diagrams illustrating steps in a method for manufacturing a printed circuit board according to a third modified example of the second embodiment shown in FIG. [Figure 9] FIG. 1 is a diagram showing the evaluation results of the printed circuit boards obtained in Examples 1 to 9. DETAILED DESCRIPTION OF THE INVENTION

[0037] Hereinafter, embodiments of the present invention will be described in detail. In this embodiment of the method for manufacturing a printed circuit board, inkjet printing is used to eject droplets (microdroplets) of conductive ink onto the inner walls (inner peripheral surfaces) of holes (via holes) that penetrate the front and back of an insulating substrate. To prevent the ink droplets from passing through, a water-absorbing closing member (backing plate) is placed below the substrate. The ink is temporarily stored in the via hole, and then the ink inside the via hole is discharged to the outside. As a result, an ink layer of a predetermined thickness is applied uniformly and evenly to the inner walls of the via hole. This ink layer formed on the inner walls of the via hole is dried to form a conductor layer. This conductor layer is then subjected to a metal plating process as a base layer, resulting in a thicker conductor layer. This embodiment can be broadly divided into two embodiments based on the timing of drilling holes in the insulating substrate, as described in detail below.

[0038] First Embodiment 1 is a diagram showing steps in a method for manufacturing a printed circuit board according to a first embodiment of the present invention. For convenience, this diagram shows only a portion of the printed circuit board, i.e., the area around one via hole, but in reality, there may be multiple via holes.

[0039] In this first embodiment, first, a drilling step is carried out to provide via holes (hereinafter simply referred to as holes) 13 of a predetermined diameter that penetrate from the front surface to the back surface at desired positions in an insulating substrate (or base material) 11 (FIG. 1(a)). Any existing drilling means can be used, such as a drill, a laser, or punching.

[0040] The insulating substrate 11 has a film or plate shape and can be made of, for example, an insulating film such as polyimide, polytetrafluoroethylene (PTFE), or liquid crystal polymer (LCP), or an insulating plate such as glass epoxy (FR-4) or glass composite (CEM3).

[0041] The thickness of the substrate 11 is not particularly limited, but the range that can be adopted is approximately 5 μm to 3 mm, and more preferably 25 μm to 50 μm.

[0042] The diameter of the holes 13 in the substrate 11 can be in the range of approximately 50 μm to 1000 μm, more preferably 100 μm to 300 μm, and typically 200 μm. The depth of the holes 13 is equal to the thickness of the substrate 11, which will be described later.

[0043] After the drilling process, a surface printing process is performed on the insulating substrate 11 with holes. This process involves ejecting tiny droplets of conductive ink 15 onto the surface using an inkjet method to form a circuit pattern (wiring pattern) (Figure 1(b)). The printing method is not a masking technique such as photolithography or etching (e.g., subtractive, semi-additive, modified semi-additive, etc.), but rather a pure additive (registered trademark) method, in which conductive ink is printed using an inkjet method only on the desired wiring areas without masking. This surface printing process includes a filling process in which, while the bottom of the substrate 11 is blocking the holes 13, ink is ejected into the holes 13 using an inkjet method (Figure 1(b1)). Specifically, by using a slowly absorbing material as the blocking member 17 (described in detail below), ink is temporarily filled into the holes 13 whose bottom ends are blocked by the blocking member 17. In this specification, "filling" basically means filling the hole with ink (forming an ink puddle) so that the ink adheres to the entire inner wall of the hole. The formation of the ink puddle 15a in the hole 13 can be performed simultaneously with printing the circuit pattern (including the lands around the hole) on the surface of the substrate 11.

[0044] Conductive ink is an ink containing one or more types of metal particles such as Ag (silver), Au (gold), Cu (copper), Pd (palladium), Ni (nickel), or the like, or these metals dissolved in an ionic state, or non-metallic conductive particles such as C (carbon). The diameter of the ink droplets can be in the range of approximately 1 μm to 100 μm, more preferably 10 μm to 30 μm, and typically 20 μm. The viscosity of the ink can be in the range of approximately 1 cps to 500 cps, more preferably 2 cps to 30 cps, and typically 4 cps.

[0045] As a prerequisite for forming ink reservoir 15a in hole 13, a water-absorbent closing member 17 is placed below substrate 11. Closing member 17 is a film-like or plate-like member made of a porous material, typically a breathable sheet member (or porous sheet member) that has breathability, such as paper, a porous resin plate, cloth, or nonwoven fabric. Paper is preferable from a cost perspective. The pore size of the breathable sheet member can be in the range of approximately 1 μm to 50 μm, more preferably 5 μm to 10 μm, and typically 10 μm. Note that the holes in the closing member are not necessarily circular, and in such cases, the pore size corresponds to the gap width.

[0046] In this embodiment, the basic relationship between the hole diameter of the substrate, the ink droplet diameter, and the hole diameter of the closing member is preferably as follows: hole diameter of substrate>ink droplet diameter>hole diameter of the closing member.

[0047] With regard to filling ink into hole 13, if the water absorption capacity of closing member 17 is insufficient, the water absorption capacity of closing member 17 will be nearly full while filling hole 13 with ink, and the ink will not be discharged very quickly even after filling. On the other hand, if the water absorption capacity of closing member 17 is too great, it will take an excessively long time for ink to fill hole 13. From this perspective, it is desirable to select an appropriate relationship (condition) between the water absorption capacity of closing member 17 and the amount of ink discharged per unit time per unit area.

[0048] More specifically, the water absorption capacity of the closing member 17 is thought to be related to the water absorption speed and water absorption capacity of the closing member 17. When the pore size of the closing member 17 changes, the water absorption speed is most affected. When the water absorption speed changes, the following situation is expected. That is, if the water absorption speed becomes sufficiently fast, ink filling may not occur. This situation can occur when the relationship between the ink droplet diameter and the pore size of the closing member is met. Conversely, if the water absorption speed slows down, for example, if a non-porous plate is used as the closing member, excess ink will remain in the holes for a long time, and the surface tension of the ink accumulated at the bottom will likely hinder ink adhesion to the wall surface.

[0049] The front surface printing step also includes a discharging step of discharging the ink filled in the holes 13 from the holes 13 to the outside (FIG. 1(b2)).

[0050] Thereafter, the closing member 17 absorbs the ink over time, and most of the ink inside the hole 13 is discharged from the hole 13. However, the ink layer adhering to the inner wall of the hole 13 remains without being discharged. The size and shape of the area of ​​the wetting portion 18 that is formed when the closing member 17 absorbs the ink in Figure 1 (b2) are shown simply as a schematic for the purpose of explanation, and in reality, they may vary depending on the capacity of the ink reservoir 15a inside the hole 13 and the material, composition, thickness, etc. of the closing member 17.

[0051] The surface printing process also includes a drying process (FIG. 1(b3)) in which the ink layer remaining on the inner wall of the hole 13 (inner wall ink layer) after the ink is discharged from the hole 13 and the ink layer on the substrate in the form of a circuit pattern formed on the substrate and connected to this inner wall ink layer are dried. The thickness of the conductive layer (conductive film) obtained by this drying process is 100 nm to 20 μm, and more preferably 500 nm to 2 μm.

[0052] That is, after inkjet printing is performed on the surface of the substrate 11, the closing member 17 is removed after waiting for a predetermined time, and a drying process is performed on the circuit pattern and lands on the surface of the substrate 11 and the ink layer on the inner walls of the holes 13 connected to the lands, thereby forming a conductor layer 16 on the surface and in the via holes. The drying process may include one or more of a blowing process and a heating process.

[0053] Following the front surface printing process, a back surface printing process is carried out (FIG. 1(c)).

[0054] In the backside printing process, the substrate 11 obtained in Figure 1(b3), on which the conductor layer 16 on the front side and in the via holes has been formed, is inverted (turned upside down), and then the same printing, ink discharge, and drying processes as described above are carried out.

[0055] That is, in the backside printing process, first, tiny droplets of conductive ink 15 are ejected onto the backside using an inkjet method to print the circuit pattern on the backside, and form lands around the holes and ink pools 15a inside the holes (Fig. 1(c1)). Note that the "backside" here refers to the side opposite to the front side of substrate 11 in Fig. 1(b), and is located on the top side in Fig. 1(c) after substrate 11 has been turned over.

[0056] As a prerequisite for forming ink reservoir 15a in hole 13, a water-absorbent closing member 17 is placed on the underside of substrate 11 (Fig. 1(c1)). Next, ink is filled into hole 13, the lower end of which is blocked by closing member 17, using an inkjet method. Over time, the ink is absorbed by closing member 17, and most of the ink in hole 13 is discharged from hole 13 (Fig. 1(c2)). After waiting a predetermined time after inkjet printing on the rear surface of substrate 11, closing member 17 is removed, and a drying process is performed on the circuit pattern and lands on the rear surface of substrate 11 and the ink layer on the inner walls of hole 13 connected to these lands, thereby forming conductor layer 16 on the rear surface and in the via hole (Fig. 1(c3)).

[0057] In principle, the conductor layer 16 has already been formed on the inner wall of the hole 13 in the front printing process, so the main purpose of the back printing process is to connect the conductor layer 16 in the via hole with the land on the back surface. However, there is no problem even if an ink layer from the back surface printing remains on top of the conductor layer 16 on the inner wall of the hole 13 formed in the front printing process; rather, this makes the electrical connection between the back surface land and the conductor layer 16 more reliable.

[0058] Following the backside printing process, a metal plating process is carried out (FIG. 1(d)). Generally, plating methods include electrolytic plating and electroless plating, but electroless plating is used in this embodiment. However, the plating process is not essential for all applications of the present invention. While Cu, Ni, Ag, Au, etc. can be used as the plating metal, Cu is used in this embodiment. The thickness of the plating layer is approximately 0.03 μm to 100 μm, and more preferably 3 μm to 18 μm. Furthermore, its resistance value is 0.001 Ω to 10 Ω, and more preferably 0.001 Ω to 1 Ω.

[0059] In this way, the circuit patterns and lands on the front and back surfaces of the substrate 11 formed in the front and back printing processes, as well as the conductor layer 16 on the inner walls of the via holes, are thickened by metal plating as an underlayer, thereby increasing the mechanical strength of the circuit patterns and reducing the electrical resistance of the conductor layer.

[0060] As described above, according to this embodiment, the circuit patterns (including lands) on the front and back of the substrate 11 and the inner walls of the holes 13 can be appropriately made conductive with a small number of steps by inkjet printing.

[0061] 1(b2) and 1(c2), a means for creating a negative pressure on the closing member 17 side of the substrate 11 of the vacuum suction means may be used to forcibly suck the ink out of the holes 13 through the closing member 17. This suction is an essential element when using highly viscous ink such as in screen printing, as in the prior art described in Patent Documents 1 and 2 above. However, when using low-viscosity ink such as in inkjet printing, as in this embodiment, it is not an essential element and can be used simply to accelerate the speed at which the ink is discharged.

[0062] 2 shows steps in a method for manufacturing a printed circuit board according to Modification 1 of the first embodiment. Elements similar to those shown in FIG. 1 are given the same reference numerals, and duplicated explanations will be omitted.

[0063] In this variant 1, in the back surface printing step (FIG. 2(c1)), ink droplets 15b ejected into holes 13 are allowed to pass through to the underside of substrate 11 during back surface printing without using closing member 17 as shown in FIG. 1. In this case, ejection of ink droplets into the central region of hole 13 may be omitted to reduce the amount of ink droplets that pass through, but ink droplets are ejected at least onto the outer periphery of hole 13 to ensure connection between the conductor on the inner wall of hole 13 and the peripheral portion of hole 13 on the back surface of substrate 11 (land portion on the back surface). The other steps in FIG. 2 are as shown in FIG. 1.

[0064] Figure 3 shows the relationship between inkjet printing resolution (dpi) and ink filling rate. This data was obtained for an example where the ink volume per ink dot was approximately 6 pL, the hole diameter was 0.5 mm, and the depth was 0.025 mm. The ink filling rate represents the ratio of the amount of ink ejected into the hole to the internal volume of the hole (via volume). As can be seen from this figure, there is a correlation (almost proportional) between the resolution and the ink filling rate.

[0065] In this embodiment, the amount of ink ejected into the hole 13 must be set to a volume sufficient to fill the via volume. The parameters for this are the ink volume and the printing resolution. If the ink volume is not sufficient to fill the via volume, there is a high possibility that the ink will not adhere to the inner wall. Conversely, if too much ink is used, an error may occur. An example of an error caused by too much ink is that ink may accumulate on the inner wall of the hole, causing cracks during drying. This may result in an uneven plating shape and a lack of electrical continuity between the front and back surfaces. In this embodiment, the ink filling rate is preferably approximately 100% to approximately 200%. The desirable range of printing resolution is approximately 1700 dpi to approximately 2300 dpi. However, if printing, filling, and drying processes are performed on both the front and back surfaces of the substrate, as in the embodiment shown in Figure 1, the ink filling rate may be lower than 100%. The reason for this is that more than the lower half of the hole is filled with ink in the filling process from the front side in the thickness direction of the substrate, and then more than the new lower half (more than the original front half) is filled in the filling process after the substrate is turned over, so that the front and back processes complement each other to obtain a continuous conductor layer over the entire inner wall of hole 13. In fact, experiments by the inventors and others have confirmed that in this embodiment, even with a relatively large substrate thickness of 125 μm, an ink filling rate of about 55 to 60% is sufficient.

[0066] <Second embodiment> 4 shows the steps of a method for manufacturing a printed circuit board according to a second embodiment of the present invention, in which the drilling process of the board 11 is carried out after the surface printing.

[0067] First, a circuit pattern 21 is formed on the surface of the substrate 11 (FIG. 4(a)). In this case, the circuit pattern 21 does not necessarily have to be formed by inkjet printing, but in this embodiment, it is printed by inkjet printing, and the ink layer is dried to form a surface conductor layer.

[0068] Next, the substrate 11 obtained in FIG. 4(a) is subjected to a drilling process to open holes 13 at desired via positions (FIG. 4(b)).

[0069] After that, the backside printing process is carried out (FIG. 4(c)).

[0070] In this backside printing process, first, the substrate 11 with the front side circuit pattern 21 formed thereon, obtained in Figure 4(b), is turned over, and then the processes of printing the back side (including filling the holes 13 with ink), discharging the ink, and drying are carried out. Note that the "backside" here refers to the side opposite to the front side of the substrate 11 in Figure 4(b), and is located on the upper side in Figure 4(c) after the substrate 11 has been turned over.

[0071] That is, in the backside printing process, tiny droplets of conductive ink 15 are first ejected onto the backside using an inkjet method to print the circuit pattern on the backside and form lands around the holes and ink pools 15a inside the holes (Figure 4(c1)).

[0072] As a prerequisite for forming ink reservoir 15a in hole 13, a water-absorbent closing member 17 is placed on the underside of substrate 11 (Fig. 4(c1)). Next, ink is filled into hole 13, the lower end of which is blocked by closing member 17, using an inkjet method. Over time, the ink is absorbed by closing member 17, and most of the ink in hole 13 is discharged from hole 13 (Fig. 4(c2)). After waiting a predetermined time after inkjet printing on the rear surface of substrate 11, closing member 17 is removed, and a drying process is performed on the circuit pattern and lands on the rear surface of substrate 11 and the ink layer on the inner walls of hole 13 connected to these lands, thereby forming conductor layer 16 on the rear surface and in the via hole (Fig. 4(c3)).

[0073] Following the rear printing step, a metal plating step is carried out (FIG. 4(d)). In this embodiment, electroless copper plating is used as the metal plating. This results in a thickened conductor layer (electroless plated layer) 19 obtained by electroless plating using the conductor layers on the circuit patterns, lands, and inner walls of holes on the front and back sides of the substrate 11 formed in the front and rear printing steps as a base layer. This increases the mechanical strength of the circuit patterns and reduces the electrical resistance of the conductor layer.

[0074] As described above, according to the second embodiment, it is possible to realize the circuit patterns (including lands) on the front and back of the substrate 11 and the inner walls of the holes 13 as appropriate conductors with a small number of steps by inkjet printing. In particular, it is possible to simplify the procedure of the front surface printing process. The relationship between the inkjet printing resolution (dpi) and the ink filling rate and the preferred range are the same as those in the first embodiment.

[0075] Furthermore, according to the second embodiment, alignment of the surface printing is not required, facilitating roll-to-roll processing. "Roll-to-roll processing" refers to a transport method in which a film prepared in a roll is wound up on a roll after processing when processing film or paper-based materials. This method is adopted to improve mass productivity compared to batch processing. In this embodiment, printing and drying can be processed continuously at once. If hole drilling is performed first, the land drawing at the inkjet drawing location must be aligned with the processed hole, requiring precision of several tens of microns, and precise transport control is required. On the other hand, if hole drilling is performed after this process, the printing precision on the substrate only needs to fit within the film width, so the minimum precision is on the order of a few millimeters, significantly reducing the precision required for the equipment.

[0076] 4(c2), a means for creating a negative pressure on the closing member 17 side of the substrate 11 of the vacuum suction means may be used to forcibly suck the ink out of the holes 13 through the closing member 17. When using low-viscosity ink for inkjet printing as in this embodiment, this suction is not an essential element and can be used simply to accelerate the speed of ink discharge.

[0077] Fig. 5 is a diagram showing steps in a method for manufacturing a printed circuit board according to Modification 1 of the second embodiment shown in Fig. 4. Elements similar to those shown in Fig. 4 are given the same reference numerals, and duplicated explanations will be omitted.

[0078] In this first modification of the second embodiment, laser ablation (LA) processing 25 is performed on substrate 11 on which circuit pattern 21 has been formed. LA processing removes a portion 23 of the conductor layer on substrate 11 in order to miniaturize the circuit pattern (including lands) on substrate 11. Similar LA processing can also be performed in the steps shown in FIGS. 1 and 2, but in an embodiment in which holes are drilled after the circuit pattern is formed on substrate 11, as shown in FIG. 5, it is possible to remove a portion of the conductor layer at the same time as performing hole drilling with a laser, allowing for efficient processing.

[0079] Figure 6 shows an example of circuit pattern miniaturization using LA processing. Figure 6(a) shows an example of a conductor layer formed on the front (or back) surface of a substrate by inkjet printing. This conductor layer has a series of multiple land portions 41, low-density conductor portions 43 connected to one end of these lands 41, tapered solid portions 45 connected to the other end of the low-density conductor portions 43, and narrow solid portions 47 connected to the end of the tapered solid portions 45. As shown in Figure 6(b), the gray area in the center of the land portion 41 is a hole (through hole) formed by LA processing, and the gray areas of the tapered solid portion 45 and narrow solid portion 47 are the portions of the conductor layer surface removed by LA processing. Figure 6(c) shows the conductor pattern obtained as a result of this LA processing. That is, a high-density conductor portion 57, which is denser than the low-density conductor portion 43, and a connecting conductor portion 55 connecting the two portions are formed. Even if the low-density conductor portion 43 has the density (resolution) limit that can be achieved by the inkjet method, the LA processing can provide a high-density conductor portion 57 with an even higher density.

[0080] FIG. 7 is a diagram showing steps of a method for manufacturing a printed circuit board according to Modification 2 of the second embodiment shown in FIG.

[0081] First, a circuit pattern 21 is formed on the surface of the substrate 11 (FIG. 7(a)), and then the circuit pattern 21 is subjected to electrolytic plating to form a circuit pattern 31, which is a conductor layer with a relatively large thickness (FIG. 7(b)).

[0082] Next, holes 13 are formed at desired via positions in the substrate 11 on which the circuit patterns 21 and 31 have been formed (FIG. 7(c)). At this time, the LA processing 25 described above can be performed as needed. That is, a portion 23 of the conductor layer on the substrate 11 is removed to miniaturize the circuit pattern (including the lands) on the substrate 11. This processing can be performed using the laser used for the drilling.

[0083] After that, the backside printing process is carried out (FIG. 7(d)).

[0084] In this backside printing process, first, the substrate 11 with the front side circuit patterns 21 and 31 formed as shown in Figure 7(c) is turned over, and then the processes of printing the back side (including filling the holes 13 with ink), discharging the ink, and drying are carried out. Note that the "backside" here refers to the side opposite to the front side of the substrate 11 in Figure 7(b), and is located on the upper side in Figure 7(c) after the substrate 11 has been turned over.

[0085] That is, in the backside printing process, first, tiny droplets of conductive ink 15 are ejected onto the backside using an inkjet method to print the circuit pattern on the backside, and also form lands around the holes and ink pools 15a inside the holes (Figure 7(d1)).

[0086] As a prerequisite for forming ink reservoir 15a in hole 13, a water-absorbent closing member 17 is placed on the underside of substrate 11 (Fig. 7(d1)). Next, ink is filled into hole 13, the lower end of which is blocked by closing member 17, using an inkjet method. Over time, closing member 17 absorbs the ink, and most of the ink in hole 13 is discharged from hole 13 (Fig. 7(d2)). After waiting a predetermined time after inkjet printing on the rear surface of substrate 11, closing member 17 is removed, and a drying process is performed on the circuit pattern and lands on the rear surface of substrate 11 and the ink layer on the inner walls of hole 13 connected to these lands, thereby forming conductor layer 16 on the rear surface and in the via hole (Fig. 7(d3)).

[0087] Following the backside printing step, a metal plating step is carried out (FIG. 7(e)). In this example, electroless copper plating is used as the metal plating. As a result, a thickened conductor layer (electroless plated layer) 19 is obtained by electroless plating, using the conductor layer 16 on the circuit patterns, lands, and inner walls of holes on the front and back sides of the substrate 11 formed in the front side printing step and the back side printing step as a base layer. This increases the mechanical strength of the circuit patterns, etc., and reduces the electrical resistance of the conductor layer.

[0088] In this second modification, similarly to the above, alignment of the front surface printing is not required, which facilitates roll-to-roll processing.

[0089] 7(d2), a means for creating a negative pressure on the closing member 17 side of the substrate 11 of the vacuum suction means may be used to forcibly suck the ink out of the holes 13 through the closing member 17. When using low-viscosity ink for inkjet printing as in this embodiment, this suction is not an essential element and can be used simply to accelerate the speed of ink discharge.

[0090] Fig. 8 is a diagram showing steps in a method for manufacturing a printed circuit board according to Modification 3 of the second embodiment shown in Fig. 4. This is similar to Modification 2 shown in Fig. 7, and explanations of steps that are the same as those in Modification 2 will be omitted, and only different steps will be described in detail.

[0091] The steps from Figure 8(a) to Figure 8(d) are the same as the steps from Figure 7(a) to Figure 7(d). In this modification 3, after the step of Figure 8(d), electrolytic plating is performed instead of electroless plating (Figure 8(e)). This results in a conductor layer (electrolytic plated layer) 32 that is thicker than the film thickness obtained by electroless plating. In this case, depending on the conditions, the entire inside of the hole 13 may be filled with the electrolytic plated layer, resulting in a solid state. Such a condition is approximately "via hole diameter < (plated film thickness x 2)".

[0092] Electrolytic plating allows for thicker films to be formed and the thickness to be controlled in a relatively short time compared to electroless plating.

[0093] Specific examples will be described below. Example 1 In Example 1, various parameters in the first embodiment are limited to predetermined values ​​(or objects, methods).

[0094] Polyimide was used as the insulating substrate (base material). More specifically, a 25 μm thick polyimide film without any coating was used as the base film.

[0095] Laser processing was used as the hole processing method. That is, the through holes were formed using a laser processing machine with a wavelength of 355 nm and a laser spot diameter of 20 μm. The hole diameter (diameter) of the substrate was 200 μm.

[0096] The conductive ink used was Ag nanoink, which has an average particle diameter of 30 nm, a viscosity of 5 cps, a silver nanoparticle content of 15% by weight, and water as the main solvent.

[0097] The ink ejection medium is plain paper, here with a basis weight of 64.0 g / m 2 The paper used was an uncoated paper mainly composed of pulp of about 100%.

[0098] The inkjet method was used for the surface printing. That is, plain paper was placed on the backside, and ink was ejected onto the substrate at a rate of 21 mL per square meter of coating area using the inkjet method. The ink filling rate was 174%.

[0099] The surface was dried using an infrared (IR) heater for 30 minutes to form a silver nanoparticle layer (first conductive layer) with an average thickness of 1 μm on the substrate.

[0100] For the backside printing method, ink was ejected by the inkjet method, similar to the front side printing.

[0101] The back surface was dried in the same manner as the front surface to form a second conductive layer.

[0102] The plating method used was electroless copper plating. Specifically, the base film with the silver nanoparticle layer was cleaned for one minute at room temperature using a weak alkaline cleaner solution. Then, without going through a process such as adding a palladium catalyst, a pre-dip was performed in electroless copper plating solution, and electroless copper plating was performed for 50 minutes at a solution temperature of 65°C using an electroless copper plating solution whose main components are copper, alkali, and formaldehyde. The film was then immersed in a discoloration prevention agent at room temperature for one minute and then dried.

[0103] <Examples 2 to 6> Examples 2 to 6 each employed the same manufacturing process as Example 1, except that in the manufacturing process of Example 1, the hole diameter of the substrate was changed to five different diameters: 100 μm, 150 μm, 300 μm, 500 μm, and 1000 μm.

[0104] <Examples 7 to 9> Examples 7 to 9 each used the same manufacturing process as Example 1, except that in the manufacturing process of Example 1, the thickness of the polyimide film serving as the base film, which is the insulating substrate (base material), was changed to three types: 50 μm, 75 μm, and 125 μm, and the ink filling rates were changed to 145%, 97%, and 58%, respectively.

[0105] <Results of Examples 1 to 6> Figure 9(a) shows the evaluation results for the printed circuit boards obtained as a result of Examples 1 to 6. In this evaluation, the cross section of the printed circuit board after copper plating was observed at approximately 1000x magnification using an electron microscope. A rating of "good" was given to cases where it was confirmed that the conductive layers were connected at the top and bottom (all of Examples 1 to 6 were given a rating of "good"). The thickness of the copper plating film was measured at three locations at the same time, and the average value was recorded. Furthermore, the resistance between the front and back surfaces was measured at five points, and the average value was recorded, along with the ink filling rate used.

[0106] <Results of Examples 7 to 9> Figure 9(b) shows the evaluation results for the printed circuit boards obtained in Examples 7 to 9. In this evaluation, the cross section of the printed circuit board after copper plating was observed at approximately 1000x magnification using an electron microscope. A rating of "Good" was given to cases where it was confirmed that the conductive layers were connected between the top and bottom (all of Examples 7 to 9 were "Good"). The thickness of the copper plating film was measured at three locations at the same time, and the average value was recorded. Furthermore, the resistance between the front and back was measured and recorded, and the ink filling rate used was also recorded.

[0107] <Other variations> In the above, the method for manufacturing printed circuit boards is not a method that uses a technique that requires masking such as photolithography or etching (e.g., subtractive method, semi-additive method, modified semi-additive method, etc.), but rather a method that prints metal only on the areas where wiring is desired, without masking, to form a conductor layer in a via hole. However, the above-mentioned conductive base layer formation method can also be used as a part of the process of other methods that require masking.

[0108] For example, in the subtractive method, a hole is formed through an insulating substrate with copper foil on both sides, and then a conductive base layer is formed on the inner wall of the hole and the surrounding land by electroless plating or the like. That is, to form a conductive film on the inner wall of the hole, a plating catalyst is adsorbed onto the entire surface, and a copper film is formed by electroless plating using this plating catalyst as a base. This copper film is then used as a conductive base layer to thicken the copper film by electrolytic plating. Instead of such electroless plating or the like, the conductive base layer formation method using inkjet printing described above can be adopted. This can eliminate the need for labor-intensive and costly wet processes such as electroless plating.

[0109] In the semi-additive process, a hole is formed in an insulating substrate, penetrating from the front to the back, and then a conductive base layer is formed, primarily by electroless plating, on the inner wall of the hole and the surrounding land. That is, to form a conductive film on the inner wall of the hole, a plating catalyst is adsorbed onto the entire surface, and a copper film is formed by electroless plating using this plating catalyst as a base. This copper film is then used as a conductive base layer to be thickened by electrolytic plating. Instead of such electroless plating, the conductive base layer formation method using inkjet printing described above can be adopted. This can eliminate the need for labor-intensive, costly wet processes such as electroless plating.

[0110] In the modified semi-additive process, holes are formed through an insulating substrate with copper films on both sides, and then a carbon conductive film, primarily called a black hole, is formed on the inner wall of the hole using a carbon-based conductive material that only adheres to the resin. This is then used as a conductive layer to be thickened with a copper film by electroplating. Instead of using black holes, the aforementioned inkjet printing method for forming a conductive base layer can be used. This eliminates the need for laborious and costly wet processes such as black holes.

[0111] In these methods, electroless plating or electrolytic plating can be carried out as needed following the drying step after the backside printing as described above.

[0112] In addition, various modifications and variations can be made without departing from the scope of the claims of the present invention. [Explanation of symbols]

[0113] 11 Insulating substrate 13 holes (via holes) 15 Conductive ink 15a Ink puddle 15b Ink Drops 16 Conductor Layer 17 Closure member 18 Infiltrated area 19 Conductor layer (electroless plating layer) 21 Circuit Pattern 23 part 25 Laser ablation (LA) processing 31 Circuit Pattern 41 Land Department 43 Low density conductor section 45 Tapered solid area 47 Narrow solid area 55 Connecting conductor section 57 High-density conductor section

Claims

1. a surface printing process in which conductive ink droplets are ejected by an inkjet method onto the surface of an insulating substrate having holes penetrating from the front surface to the back surface to print a circuit pattern; a filling step of filling the holes with ink by discharging conductive ink droplets into the holes by an inkjet method in a state in which the holes are blocked from below the substrate in the surface printing step; a discharging step of discharging the ink filled in the hole to the outside; a drying step of drying the inner wall ink layer remaining on the inner wall of the hole and the circuit pattern-shaped on-substrate ink layer connected to the inner wall ink layer formed on the substrate after the discharging step; A method for manufacturing a printed circuit board comprising:

2. 2. The method for manufacturing a printed circuit board according to claim 1, wherein in the filling step, ink droplets are ejected into the holes after the holes are closed with a closing member having a water-absorbing function.

3. the closing member is a porous sheet member, The discharging step includes a step of waiting until most of the ink filled in the hole is discharged and a portion of the ink remains on the wall surface of the hole as an ink layer, and a step of removing the closing member thereafter. The method for manufacturing a printed circuit board according to claim 2 .

4. a backside printing step of turning the substrate over after the drying step and printing a circuit pattern on the backside, After the rear surface printing, the filling step, the discharging step, and the drying step are performed again. The method for manufacturing the printed circuit board according to any one of claims 1 to 3.

5. The method for manufacturing a printed circuit board according to any one of claims 1 to 4, further comprising the step of drilling holes in the board before the filling step.

6. 6. The method for producing a printed circuit board according to claim 1, further comprising a plating step of applying metal plating to the inner wall ink layer and the conductor layer on the substrate obtained by the drying step.

7. 7. The method for manufacturing a printed circuit board according to claim 1, wherein the viscosity of the ink is within a range of approximately 1 cps to 500 cps.

8. 8. The method for manufacturing a printed circuit board according to claim 1, wherein the diameter of the ink droplets is within a range of approximately 1 μm to 100 μm.

9. The method for manufacturing a printed circuit board according to any one of claims 1 to 8, wherein the conductive ink contains at least one metal particle selected from Ag, Au, Cu, Pd, and Ni, or a solution of these metals in an ionic state, or non-metallic conductive particle.

10. 10. The method for manufacturing a printed circuit board according to claim 1, wherein the ink filling rate, which is the ratio of the amount of ink ejected into the hole to the internal volume of the hole in the filling step, is from about 100% to about 200%.

11. 5. The method for manufacturing a printed circuit board according to claim 4, wherein the ink filling rate, which is the ratio of the amount of ink ejected into the hole to the internal volume of the hole in the filling step, is from about 55% to about 200%.

12. a surface printing process of printing a circuit pattern on the surface of an insulating substrate; a drilling step of drilling holes penetrating the substrate on which the circuit pattern is formed from front to back; a filling step of filling the holes with ink by discharging conductive ink droplets into the holes by an inkjet method while the substrate is placed with its front side facing down and the holes are blocked from below the circuit pattern; a discharging step of discharging the ink filled in the hole to the outside; a drying step of drying the inner wall ink layer remaining on the inner wall of the hole after the discharging step and the on-substrate ink layer formed on the back surface of the substrate connected to the inner wall ink layer; A method for manufacturing a printed circuit board comprising:

13. The method for manufacturing a printed circuit board according to claim 12, wherein in the filling step, ink droplets are ejected into the holes after the holes are closed with a closing member having a water-absorbing function.

14. the closing member is a porous sheet member, The discharging step includes a step of waiting until most of the ink filled in the hole is discharged and a portion of the ink remains on the wall surface of the hole as an ink layer, and a step of removing the closing member thereafter. The method for manufacturing a printed circuit board according to claim 13 .

15. The method for manufacturing a printed circuit board according to any one of claims 12 to 14, wherein the circuit pattern is subjected to a laser ablation process during the hole drilling process.

16. The method for producing a printed circuit board according to any one of claims 12 to 14, further comprising a plating step of applying metal plating to the inner wall ink layer and the conductor layer on the substrate obtained by the drying step.

17. The method for manufacturing a printed circuit board according to claim 12, further comprising an electrolytic plating step of performing electrolytic plating on the circuit pattern after the surface printing step and before the hole drilling step.

18. The method for manufacturing a printed circuit board according to claim 17, wherein the circuit pattern and the conductor layer formed by electrolytic plating are subjected to laser ablation during the hole drilling step.

19. a backside printing step of printing a circuit pattern on the backside of the substrate after the drilling step, and the filling step, the discharging step, and the drying step are carried out after the backside printing. The method for manufacturing a printed circuit board according to any one of claims 12 to 18.

20. The method for manufacturing a printed circuit board according to claim 19, further comprising the step of performing electroless plating following the drying step after the rear surface printing.

21. The method for manufacturing a printed circuit board according to claim 19, further comprising the step of performing electrolytic plating following the drying step after the rear surface printing.

22. The method for manufacturing a printed circuit board according to claim 21, wherein the circuit pattern and the conductor layer formed by electrolytic plating on the rear surface of the board are subjected to laser ablation.

23. A method for forming a conductive base layer on at least an inner wall of an insulating substrate having a hole penetrating from a front surface to a back surface, the method comprising: a filling step of filling the holes with ink by discharging conductive ink droplets into the holes by an inkjet method while the holes are blocked from below the substrate; a discharging step of discharging the ink filled in the hole to the outside; a drying step of drying at least the inner wall ink layer remaining on the inner wall of the hole after the discharging step; A method for forming a conductive underlayer comprising:

24. 24. The method for forming a conductive base layer according to claim 23, wherein in the filling step, ink droplets are ejected into the holes after the holes are closed with a closing member having a water absorbing function.

25. the closing member is a porous sheet member, The discharging step includes a step of waiting until most of the ink filled in the hole is discharged and a portion of the ink remains on the wall surface of the hole as an ink layer, and a step of removing the closing member thereafter. The method for forming a conductive underlayer according to claim 24.

26. The method for forming a conductive underlayer according to claim 23, further comprising the step of performing electroless plating following the drying step.

27. The method for forming a conductive underlayer according to claim 23, further comprising the step of performing electrolytic plating following the drying step.

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