Method for manufacturing a printed circuit board and printed circuit board

Abstract

A conductive layer with strong adhesion is formed on a substrate made of polyethylene terephthalate using an inkjet printer. [Solution] The method for manufacturing the printed circuit board 1 comprises a primer printing step in which a primer liquid is used as ink to form a pattern of the primer layer 3 on a substrate 2 using an inkjet printer, a copper printing step in which a pattern of the copper fine particle layer 4 is formed on the pattern of the primer layer 3 using an inkjet printer, and a plating step in which the copper fine particle layer 4 is plated to form a pattern of the conductive layer 5. The substrate 2 is made of polyethylene terephthalate. The primer liquid contains a polyolefin resin and has wettability to the substrate 2. The softening point of the primer layer 3 is higher than the temperature range of the primer layer 3 when the printed circuit board 1 is in use, and lower than or equal to the heat resistance temperature of the substrate 2. In the copper printing step, the primer layer 3 is heated to a temperature above the softening point and below the heat resistance temperature of the substrate 2.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a printed circuit board and a printed circuit board manufactured by the manufacturing method.

Background Art

[0002] A printed circuit board having wiring on a substrate and provided with a plurality of light-emitting diodes (LEDs) is known for use in a backlight unit of a liquid crystal display device or a surface light source such as a lighting device. The wiring on the substrate is a pattern of a conductor layer, and a current for lighting the LED flows through it.

[0003] In a printed circuit board for a surface light source, if polyethylene terephthalate having a high reflectance is used as a substrate, the luminous efficiency of the surface light source is improved. And if wiring can be formed on polyethylene terephthalate using an inkjet printer, a flexible printed circuit board can be manufactured. This is because the inkjet printer can flexibly change the pattern to be printed.

[0004] Conventionally, there is a method for manufacturing a conductive circuit in which a pattern of a conductor layer is formed on a substrate made of transparent polyethylene terephthalate (PET) by a printing method (see Patent Document 1). Since the conductive circuit is a transparent conductive circuit for a touch sensor (see paragraph 0002 of Patent Document 1), only a minute current flows through the conductor layer. However, in the conductor layer of a surface light source such as a backlight unit, a current for lighting the LED flows, and temperature rise due to Joule heat and heat cycle occur, so strong adhesion to the substrate is required. Therefore, in order to form the conductor layer of the surface light source on a substrate of polyethylene terephthalate, an inkjet printer cannot be used, and for example, it is necessary to form grooves in the substrate (see paragraph 0028 of Patent Document 1), which is inefficient.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] The present invention aims to solve the above problems and to form a conductive layer with strong adhesion on a substrate made of polyethylene terephthalate using an inkjet printer. [Means for solving the problem]

[0007] The present invention relates to a method for manufacturing a printed circuit board having a substrate and a pattern of a conductive layer, comprising: a primer printing step of forming a pattern of a primer layer on the substrate using an inkjet printer with a primer liquid as an ink; a copper printing step of forming a pattern of a copper nanoparticle layer on the pattern of the primer layer using an inkjet printer with a copper nano ink; and a plating step of forming a pattern of a conductive layer by plating the copper nanoparticle layer, wherein the substrate is made of polyethylene terephthalate, the copper nano ink contains copper nanoparticles dispersed in a dispersion medium, the primer liquid contains a thermoplastic resin and has wettability to the substrate, the thermoplastic resin is a polyolefin resin and has adhesion to the substrate, the softening point of the primer layer is higher than the temperature range of the primer layer when the printed circuit board is in use and lower than the heat resistance temperature of the substrate, and in the copper printing step, the primer layer is heated to a temperature above the softening point and below the heat resistance temperature of the substrate.

[0008] In this method for manufacturing a printed circuit board, the polyethylene terephthalate of the substrate is preferably white PET.

[0009] In this method for manufacturing printed circuit boards, the softening point of the thermoplastic resin is preferably 75°C or higher and 105°C or lower.

[0010] This printed circuit board comprises a substrate made of polyethylene terephthalate, a pattern of a primer layer adhered to the substrate, and a pattern of a conductor layer on the primer layer. The conductor layer has a copper microparticle layer on the primer layer and a plating layer on the copper microparticle layer. The primer layer is made of a polyolefin resin, which is a thermoplastic resin. The softening point of the primer layer is higher than the temperature range of the primer layer during use of the printed circuit board and lower than the heat resistance temperature of the substrate. The copper microparticle layer is characterized in that, at the interface with the primer layer, the copper nanoparticles of the copper microparticle layer are embedded in the primer layer.

[0011] In this printed circuit board, the polyethylene terephthalate of the substrate is preferably white PET.

[0012] In this printed circuit board, the softening point of the thermoplastic resin is preferably 75°C or higher and 105°C or lower. [Effects of the Invention]

[0013] According to the method for manufacturing printed circuit boards of the present invention, the primer layer pattern is formed by an inkjet printer, eliminating waste by not forming the primer layer in unnecessary areas, resulting in lower costs. Furthermore, since the primer layer pattern and the copper nanoparticle layer pattern are formed by an inkjet printer, these patterns can be easily modified. The softening point of the primer layer is higher than the temperature range of the primer layer during use of the printed circuit board, and lower than the heat resistance temperature of the substrate. Therefore, the primer layer can be softened by heating during the copper printing process and is solidified when the printed circuit board is used. During the copper printing process, the primer layer is heated to a temperature above its softening point, so copper nanoparticles penetrate the primer layer at the interface between the copper nanoparticle layer and the primer layer. This strengthens the adhesion of the conductive layer to the substrate via the primer layer. Printed circuit boards manufactured by this method have a conductive layer with strong adhesion. [Brief explanation of the drawing]

[0014] [Figure 1]Figures 1(a) to 1(f) are cross-sectional diagrams showing a method for manufacturing a printed circuit board according to one embodiment of the present invention, in chronological order. [Figure 2] Figure 2 is a cross-sectional view of the same printed circuit board. [Modes for carrying out the invention]

[0015] A method for manufacturing a printed circuit board according to one embodiment of the present invention will be described with reference to Figures 1(a) to 1(f). As shown in Figures 1(e) and 1(f), this method is a method for manufacturing a printed circuit board 1. The printed circuit board 1 has a base material 2 and a pattern of a conductive layer 5. Figures 1(e) and 1(f) show the cross-sectional configuration of the printed circuit board 1, and in the printed circuit board 1, the pattern of the conductive layer 5 is a circuit pattern in plan view.

[0016] As shown in Figure 1(a), the base material 2 is a material molded into a plate, film, or the like. The base material 2 is made of polyethylene terephthalate. In this embodiment, the polyethylene terephthalate (PET) of the base material 2 is white PET, also called white PET, and it is desirable that it be white PET with high reflectivity.

[0017] This printed circuit board manufacturing method comprises a primer printing process, a copper printing process, and a plating process in that order.

[0018] First, as shown in Figure 1(b), in the primer printing process, a pattern for the primer layer 3 is formed on the substrate 2. The pattern for the primer layer 3 is formed on the substrate 2 using an inkjet printer with the primer liquid as the ink.

[0019] The primer liquid contains a thermoplastic resin and has wettability to the base material 2. That is, the primer liquid is a solution in which the thermoplastic resin is dissolved in a solvent. The primer liquid has the wettability for the droplets discharged from the inkjet printer to form a film on the base material 2. The specific numerical value of the contact angle of the droplets, which is an index of the wettability of the primer liquid to the base material 2, is a design matter. A surface conditioner or the like for adjusting the wettability may be added to the primer liquid. The thermoplastic resin is a polyolefin resin and has adhesiveness to the base material 2 (polyethylene terephthalate). The polyolefin resin may be a polyolefin resin having a substituent, that is, a modified polyolefin resin.

[0020] In this embodiment, the primer liquid is a non-reactive type obtained by simply diluting a thermoplastic resin with a solvent, and the primer layer 3 is formed by simply evaporating the solvent after printing on the base material 2. Such a primer liquid is industrially desirable because the formation of the primer layer 3 is fast.

[0021] It is desirable that the polyolefin resin of the primer layer 3 is composed of only the elements of carbon (C), oxygen (O), and hydrogen (H). Such a polyolefin resin is environmentally friendly because only carbon dioxide and water are generated when burned.

[0022] Since the primer liquid has wettability to the base material 2, the primer liquid can be printed on the base material 2 with an inkjet printer.

[0023] The solvent of the primer liquid printed on the base material 2 by the inkjet printer evaporates, and the thermoplastic resin remains on the base material 2, forming the pattern of the primer layer 3. Since the thermoplastic resin of the primer layer 3 has adhesiveness to the base material 2, the primer layer 3 adheres to the base material 2.

[0024] By forming the pattern of the primer layer 3 with an inkjet printer, it is possible to eliminate the waste of forming the primer layer 3 in unnecessary portions. Note that, if necessary, the pattern of the primer layer 3 may cover the entire surface on the base material 2.

[0025] The softening point of the primer layer 3 is higher than the temperature range of the primer layer 3 when the printed circuit board 1 is in use, and lower than the heat resistance temperature of the substrate 2. A softening point of the primer layer 3 being higher than the temperature range of the primer layer 3 when the printed circuit board 1 is in use means that the primer layer 3 does not soften when the printed circuit board 1 is in use. Because the copper nanoparticle layer 4 and the conductor layer 5 have good thermal conductivity, the temperature of the primer layer 3 is substantially the same as the temperature of the conductor layer 5 and is measurable. Generally, the heat resistance temperature is the maximum temperature that can be withstood in a relatively short time. Therefore, the primer layer 3 softens by heating to a temperature above its softening point, and the temperature at which it is heated can be set to a temperature that the substrate 2 can withstand. When the printed circuit board 1 is in use, the primer layer 3 is below its softening point and is solidified. Note that the higher end of the temperature range of the primer layer 3 when the printed circuit board 1 has light-emitting elements such as LEDs is higher than room temperature due to the heat generated by the light-emitting elements and the Joule heating of the conductor layer 5. This temperature range does not include overheating due to a malfunction of the printed circuit board 1.

[0026] The softening point of the thermoplastic resin in the primer solution is, for example, between 75°C and 105°C. A primer solution containing a thermoplastic resin (polyolefin resin) having such a softening point is selected. Generally, the more crosslinking a thermoplastic resin has, the higher its softening point. The softening point of primer layer 3 is approximately the same as that of the thermoplastic resin layer. However, due to additives other than the polyolefin resin contained in the primer solution, the softening point of primer layer 3 may differ slightly from that of the polyolefin resin.

[0027] Then, as shown in Figures 1(c) to 1(d), in the copper printing process, a pattern of copper nanoparticle layer 4 is formed on top of the pattern of primer layer 3. The pattern of copper nanoparticle layer 4 is formed on top of the pattern of primer layer 3 using an inkjet printer with copper nano ink 40.

[0028] The copper nano ink 40 consists of copper nanoparticles 41 dispersed in a dispersion medium 42.

[0029] In this embodiment, the copper nanoparticles 41 are copper nanoparticles, i.e., copper fine particles with a median diameter (D50) of 1 nm or more and less than 1 μm, and preferably copper fine particles with a median diameter of 1 nm or more and less than 100 nm. When copper nanoparticles 41 of such particle size are used, high adhesion can be obtained by the anchoring effect by heating for a short time near the softening point of the thermoplastic resin in the copper printing process. If the particle size of the copper nanoparticles is too large, it becomes difficult or impossible to eject the copper nano ink as droplets from the inkjet printer. The copper nanoparticles 41 may be a single particle with the same median diameter, or a mixture of two or more particles with different median diameters. The particle size of the copper nanoparticles 41 is extracted from an image obtained by a scanning electron microscope (SEM image), and the median diameter is calculated from the particle size distribution. The copper nanoparticles 41 need to be dispersed in the dispersion medium 42, and if the median diameter exceeds 100 nm, the weight of the particles increases, resulting in poor dispersion stability. A dispersant that disperses the copper nanoparticles 41 in the dispersion medium 42 may be added to the copper nano ink 40.

[0030] The concentration of copper nanoparticles 41 is preferably between 1% by weight and 80% by weight relative to the copper nanoink 40. If the concentration of copper nanoparticles 41 is less than 1% by weight, a sufficient amount of copper cannot be obtained to form the copper nanoparticle layer 4, and if it exceeds 80% by weight, there is too much copper nanoparticle 41, resulting in poor dispersion stability.

[0031] In this embodiment, the dispersion medium is a polar dispersion medium. It is desirable that the dispersion medium has a boiling point that allows the copper nanoink 40 pattern to be printed with an inkjet printer and that allows the printed copper nanoink 40 pattern to dry quickly. The polar dispersion medium is protic, or if aprotic, has a relative permittivity of 30 or higher. Protic dispersion mediums have a high boiling point due to hydrogen bonding between dispersion medium molecules, and have a low viscosity despite the high boiling point, making them suitable for dispensing copper nanoink as droplets. Aprotic polar dispersion mediums with a relative permittivity of 30 or higher have a high relative permittivity, so their boiling point is high due to electrostatic interactions, and they have a low viscosity despite the high boiling point, making them suitable for dispensing copper nanoink as droplets.

[0032] This protic dispersion medium is, for example, a linear or branched alkyl or alkenyl compound having 5 to 30 carbon atoms and having one hydroxyl group. This protic dispersion medium may have 1 to 10 ether bonds and 1 to 5 carbonyl groups.

[0033] Examples of such protic dispersion media include 3-methoxy-3-methylbutanol, triethylene glycol monomethyl ether, diethylene glycol monobutyl ether, diethylene glycol monomethyl ether, propylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol mono-tert-butyl ether, and 2-octanol.

[0034] The protic dispersion medium may be a linear or branched alkyl or alkenyl compound having 2 to 6 hydroxyl groups and 2 to 30 carbon atoms. This protic dispersion medium may have 1 to 10 ether bonds and 1 to 5 carbonyl groups.

[0035] Examples of such protic dispersion media include 2-methylpentane-2,4-diol, ethylene glycol, propylene glycol, 1,5-pentanediol, diethylene glycol, triethylene glycol, glycerin, and sorbitol.

[0036] Examples of aprotic polar dispersion media with a relative permittivity of 30 or higher include propylene carbonate, 1,3-dimethyl-2-imidazolidinone, hexamethylphosphoramide, N-methylpyrrolidone, N-ethylpyrrolidone, nitrobenzene, N,N-diethylformamide, N,N-dimethylacetamide, furfural, γ-butyrolactone, ethylene sulfite, sulfolane, dimethyl sulfoxide, succinonitrile, and ethylene carbonate.

[0037] These polar dispersion media may be used individually or in mixtures of two or more types.

[0038] The dispersant is, for example, a compound or salt thereof with a molecular weight of 200 to 100,000 having at least one acidic functional group. The acidic functional group of the dispersant is an acidic, i.e., a proton-donating functional group, such as a phosphate group, phosphonic acid group, sulfonic acid group, sulfate group, and carboxyl group.

[0039] When using these dispersants, one type may be used alone, or two or more types may be mixed as appropriate. The concentration of the dispersant should preferably be between 0.5% by weight and 50% by weight relative to the copper nano ink. If the concentration of the dispersant is less than 0.5% by weight, a sufficient dispersion effect will not be obtained, and if it exceeds 50% by weight, it may have an adverse effect on the printing properties of the copper nano ink.

[0040] In this copper printing process, copper nano ink 40 is printed onto the pattern of the primer layer 3 using an inkjet printer (see Figure 1(c)), the dispersion medium 42 of the printed copper nano ink 40 evaporates, and copper nanoparticles 41 remain on the primer layer 3, forming a pattern of copper nanoparticle layer 4 (see Figure 1(d)).

[0041] In this copper printing process, the primer layer 3 is heated to a temperature above its softening point and below the heat resistance temperature of the substrate 2 (see Figure 1(c)). For example, the substrate 2 is placed on a hot plate, and the primer layer 3 is heated together with the substrate 2.

[0042] During the copper printing process, the primer layer 3 softens due to heating, allowing the copper nanoparticles 41 to penetrate the primer layer 3 at the interface between the copper nanoparticle layer 4 and the primer layer 3 (see Figure 1(d)). This penetration of the copper nanoparticles 41 into the primer layer 3 strengthens the adhesion of the conductive layer 5 to the substrate 2 via the primer layer 3. There is no need to sinter the copper nanoparticle layer 4.

[0043] If the softening point of the primer layer 3 is too low, the adhesion of the conductor layer 5 to the substrate 2 will decrease. This is because the primer layer 3 softens when the printed circuit board 1 is used. In addition, during the copper printing process, copper nanoparticles 41 penetrate into the interior of the overly softened primer layer 3, reducing the amount of copper nanoparticles 41 that function as anchors for the conductor layer 5.

[0044] In the copper printing process, since the primer layer 3 is heated, the dispersion medium 42 of the copper nano-ink 40 printed by the inkjet printer easily evaporates.

[0045] Furthermore, when forming fine wiring on the order of approximately 100 μm in width using an inkjet printer, it is desirable that the copper nano ink 40 be slightly repelled on the primer layer 3.

[0046] Furthermore, in order to print fine lines with a width of approximately 100 μm sharply, it is necessary to use a dispersion medium with a relatively high boiling point (boiling point of 200°C or higher), eject droplets of copper nano ink of 10 pL or less from the inkjet printer, and quickly dry them almost immediately upon impact to prevent wetting and spreading on the primer layer 3. Therefore, it is desirable to heat the primer layer 3 to 80-100°C.

[0047] Then, as shown in Figure 1(e), in the plating process, the copper nanoparticle layer 4 is plated to form the pattern of the conductive layer 5. Since the copper nanoparticle layer 4 has a pattern, the conductive layer 5 also has the same pattern.

[0048] In this embodiment, the copper nanoparticle layer 4 is used as a seed layer and electroless plating is applied to form the pattern of the electroless plating layer 51. This electroless plating is electroless copper plating. Since the electroless plating layer 51 integrates with the copper nanoparticle layer 4, at this stage the conductor layer 5 consists of the copper nanoparticle layer 4 and the electroless plating layer 51. The electroless plating layer 51 makes the conductor layer 5 thicker and increases its current capacity.

[0049] Then, as shown in Figure 1(f), in this plating process, electroplating is applied to the electroless plating layer 51 to form an electroplated layer 52. In electroplating, the electroless plating layer 51 is immersed in the plating solution and becomes the cathode. In this embodiment, this electroplating is electroplated copper. The conductive layer 5 consists of a copper fine particle layer 4, an electroless plating layer 51, and an electroplated layer 52. The electroplated layer 52 makes the conductive layer 5 even thicker, and its current capacity increases further. When the current capacity of the conductive layer 5 is large, the Joule heat generated in the conductive layer 5 decreases.

[0050] As a variation of this embodiment, if the current capacity required for the conductor layer 5 is small, the electroplating layer 52 may be omitted.

[0051] As described above, according to the method for manufacturing the printed circuit board 1 according to this embodiment, the pattern of the primer layer 3 is formed by an inkjet printer, eliminating the waste of forming the primer layer 3 in unnecessary areas and resulting in low costs. Furthermore, since the patterns of the primer layer 3 and the copper nanoparticle layer 4 are formed by an inkjet printer, these patterns can be easily changed. The primer liquid has wettability to the substrate 2, so droplets ejected from the inkjet printer form a film on the substrate 2, forming the pattern of the primer layer 3. The softening point of the primer layer 3 is higher than the temperature range of the primer layer 3 when the printed circuit board 1 is in use, and lower than the heat resistance temperature of the substrate 2. Therefore, the primer layer 3 can be softened by heating in the copper printing process and is solidified when the printed circuit board 1 is in use. In the copper printing process, the primer layer 3 is heated to a temperature above its softening point, so the copper nanoparticles 41 penetrate the primer layer 3 at the interface between the copper nanoparticle layer 4 and the primer layer 3. This strengthens the adhesion of the conductor layer 5 to the substrate 2 via the primer layer 3. In the copper printing process, the primer layer 3 is heated to a temperature below the heat resistance temperature of the substrate 2, so the substrate 2 can withstand that heating temperature.

[0052] Since the substrate 2 is made of polyethylene terephthalate, if the polyethylene terephthalate is made of white PET, the reflectivity will increase. When such a printed circuit board 1 is used as a surface light source, the luminous efficiency of the surface light source will increase.

[0053] If the median diameter (D50) of the copper nanoparticles 41 is between 1 nm and 100 nm, the thickness of the primer layer 3 should be approximately 0.5 μm to 1 μm. Since polyolefin resin is colorless and transparent at a thickness of approximately 1 μm, it does not impair the optical properties of the substrate 2, such as haze and total light transmittance.

[0054] By setting the softening point of the thermoplastic resin in the primer liquid to 75°C to 105°C, the softening point of the primer layer 3 can be set higher than the temperature range of the primer layer 3 when the printed circuit board 1 is in use, and lower than the heat resistance temperature of the substrate 2 made of polyethylene terephthalate. For example, when white PET is used for the substrate 2, the heat resistance temperature of the substrate 2 is approximately 130°C.

[0055] The printed circuit board 1 manufactured as described above will be explained with reference to Figure 2. This printed circuit board 1 comprises a substrate 2, a pattern for a primer layer 3, and a pattern for a conductor layer 5. The substrate 2 is made of polyethylene terephthalate (PET). The primer layer 3 is adhered to the substrate 2. The conductor layer 5 has a copper nanoparticle layer 4 on the primer layer 3 and plating layers (51, 52) on the copper nanoparticle layer 4. The primer layer 3 is made of polyolefin resin. This polyolefin resin is a thermoplastic resin. This polyolefin resin is a resin that can be dissolved in a solvent and used as ink for an inkjet printer. The softening point of the primer layer 3 is higher than the temperature range of the primer layer 3 when the printed circuit board 1 is in use, and lower than or equal to the heat resistance temperature of the substrate 2. At the interface with the primer layer 3, the copper nanoparticles of the copper nanoparticle layer 4 are embedded in the primer layer 3.

[0056] In this embodiment, the polyethylene terephthalate of the substrate 2 is white PET.

[0057] The softening point of the thermoplastic resin in the primer liquid is, for example, between 75°C and 105°C.

[0058] Preferably, the printed circuit board 1 has at least an electroless copper plating layer 51 as a plating layer on the copper fine particle layer 4, and an electroplating layer 52 on the electroless plating layer 51.

[0059] With the printed circuit board 1 configured as described above, the polyolefin resin can be dissolved in a solvent and used as ink for an inkjet printer, and a primer layer 3 made of polyolefin resin can be formed by an inkjet printer. The softening point of the primer layer 3 is higher than the temperature range of the primer layer 3 when the printed circuit board 1 is in use, and lower than the heat resistance temperature of the substrate 2, so the primer layer 3 can be softened by heating and is solidified when the printed circuit board 1 is in use. At the interface with the primer layer 3, the copper nanoparticles of the copper nanoparticle layer 4 are embedded in the primer layer 3, so the conductive layer 5 having the copper nanoparticle layer 4 has high adhesion to the primer layer 3. Since the primer layer 3 is adhered to the substrate 2, the conductive layer 5 has high adhesion to the substrate 2 via the primer layer 3.

[0060] Since the substrate 2 is made of polyethylene terephthalate, if the polyethylene terephthalate is made of white PET, the reflectivity will increase. When such a printed circuit board 1 is used as a surface light source, the luminous efficiency of the surface light source will increase.

[0061] By setting the softening point of the thermoplastic resin in the primer solution to 75°C to 105°C, the softening point of the primer layer 3 can be set higher than the temperature range of the primer layer 3 when the printed circuit board 1 is in use, and lower than the heat resistance temperature of the substrate 2 made of polyethylene terephthalate. For example, when white PET is used for the substrate 2, its heat resistance temperature is approximately 130°C.

[0062] As an example, a printed circuit board 1 was manufactured using the printed circuit board manufacturing method of the present invention, and a test was conducted to evaluate the printed circuit board 1.

[0063] First, I will explain the points common to all examples. Substrate 2 was a high-reflectance type white PET (polyethylene terephthalate) film. The heat resistance temperature of this white PET is 130°C. The size of substrate 2 was 50 mm square and 78 μm thick.

[0064] A non-reactive primer solution was prepared by dissolving a primer (solid component for the primer) in an organic solvent. In this primer printing process, substrate 2 was heated on a hot plate at 130°C. The primer solution was then printed onto substrate 2 using an inkjet printer so that the primer layer 3 after drying would be 0.5 to 1 μm thick. The printed pattern covered the entire surface of substrate 2. The primer solution ejected from the inkjet printer dried rapidly on substrate 2 almost immediately after impact.

[0065] For the copper nano-ink, we used a copper nano-ink (manufactured by Ishihara Chemical Co., Ltd., model number "IJ-02") in which copper nanoparticles are dispersed in butyl carbitol (dispersion medium). The median diameter of the copper nanoparticles in this copper nano-ink is approximately 40 nm, and the concentration of copper nanoparticles is 40% by weight. In this copper printing process, the substrate 2 was placed on a hot plate at 100°C, and the primer layer 3 was heated together with the substrate 2. The copper nano-ink was then printed onto the primer layer 3 using an inkjet printer so that the copper nanoparticle layer 4 after drying was 0.5 μm thick. The printed pattern covered the entire surface of the primer layer 3. The liquid of the copper nano-ink ejected from the inkjet printer dried rapidly on the primer layer 3 almost immediately after impact.

[0066] A copper nanoparticle layer 4 formed on the primer layer 3 was used as a seed layer for electroless copper plating for 10 minutes, and then electroplated copper was performed using the electroless plated layer 51 as the cathode. The goal was to form a layer 18 μm thick, and the electroplating current was 4 A / dm². 2 The time limit was set at 20 minutes.

[0067] After drying the fabricated printed circuit board 1 in an air-drying oven at 100°C for 30 minutes, the printed circuit board 1 (sample) was subjected to evaluation tests. For all examples, the adhesion of the conductive layer 5 was evaluated using a 90° peel test (JIS K6854-1:1999). In addition, for some examples, constant temperature and humidity tests and thermal shock tests were performed to evaluate the effect of high temperature, high humidity, and heat cycles on adhesion, i.e., the reliability of the printed circuit board 1.

[0068] The 90° peel test is described below. A sample dried at 100°C for 30 minutes was cut into 5 x 50 mm pieces to prepare peel test specimens. Approximately 5 mm of the edge of each specimen was peeled off, and the base material side (2) was attached to a board material with double-sided tape. The conductive layer side (5) was then fixed to the measuring jig of the peel strength tester. The peeling speed was set to 50 mm / min, and the peeling distance was set to 20 mm or more, and the peel strength was measured. Two peel test specimens were measured for each sample, and the average value of these two trials was taken as the peel strength.

[0069] The constant temperature and humidity test is described below. A 50 mm square sample was placed in a constant temperature and humidity chamber (manufactured by ESPEC) maintained at a temperature of 85°C and a relative humidity (RH) of 85%. After 1000 hours, the sample was removed from the constant temperature and humidity chamber, and the 90° peel test described above was performed.

[0070] The thermal shock test is described below. A 50mm square sample was placed in a thermal shock testing apparatus (manufactured by ESPEC) and moved between -40°C and 100°C chambers for 15 minutes each, with each cycle lasting up to 1000 cycles. The sample was then cut out, and the 90° peel test described above was performed. [Examples]

[0071] A primer solution was prepared using a primer (manufactured by Unitika Ltd., product name "Arrowbase DA-1010") containing a modified polyolefin resin (thermoplastic resin) with a softening point of 75°C. The softening point of this resin, 75°C, is lower than the heating temperature of the hot plate in the copper printing process (100°C) and lower than the heat resistance temperature of the substrate 2 (white PET) (130°C). The softening point of the primer layer 3 made of this resin is also considered to be approximately 75°C. In the primer printing process, this primer solution was used as an ink, and the primer layer 3 was formed (film-formed) on the substrate 2 (white PET) using an inkjet printer. In the copper printing process, a copper fine particle layer 4 was formed (film-formed) on the primer layer 3 using an inkjet printer. In the plating process, a conductive layer 5 was formed (film-added) by plating. In other words, a printed circuit board 1 was fabricated.

[0072] In the 90° peel test, strong adhesion of 5 N / cm or more was confirmed. It is inferred that the primer layer 3 softened during the copper printing process. More specifically, the white PET substrate 2 broke when the adhesion exceeded approximately 5 N / cm during the test, so it was not possible to measure the adhesion beyond that point. In other words, the conductive layer 5 of the sample showed adhesion exceeding the breaking strength of the white PET. [Examples]

[0073] A primer solution was prepared using a primer (manufactured by Unitika Ltd., product name "Arrowbase SB-1200") containing a modified polyolefin resin with a softening point of 80°C. The softening point of this resin (80°C) is lower than the heating temperature of the hot plate in the copper printing process (100°C) and lower than the heat resistance temperature of the substrate 2 (130°C). All other conditions were the same as in Example 1. A printed circuit board 1 was successfully fabricated, similar to Example 1. The results of the 90° peel test were the same as in Example 1, confirming strong adhesion to the conductive layer 5. [Examples]

[0074] A primer solution was prepared using a primer (manufactured by Unitika Ltd., product name "Arrowbase SE-1200") containing a modified polyolefin resin with a softening point of 95°C. The softening point of this resin (95°C) is lower than the heating temperature of the hot plate in the copper printing process (100°C) and lower than the heat resistance temperature of the substrate 2 (130°C). All other conditions were the same as in Example 1. A printed circuit board 1 was successfully fabricated in the same manner as in Example 1. The results of the 90° peel test were the same as in Example 1, confirming strong adhesion to the conductive layer 5. [Examples]

[0075] A primer solution was prepared using a primer (manufactured by Unitika Ltd., product name "Arrowbase SD-1200") containing a modified polyolefin resin with a softening point of 105°C. The softening point of this resin (105°C) is higher than the heating temperature of the hot plate in the copper printing process (100°C) and lower than the heat resistance temperature of the substrate 2 (130°C). All other conditions were the same as in Example 1. A printed circuit board 1 was successfully fabricated, similar to Example 1. The results of the 90° peel test were the same as in Example 1, confirming strong adhesion to the conductive layer 5.

[0076] Since the conductive layer 5 exhibits very strong adhesion, it is inferred that the primer layer 3 softened during the copper printing process, causing the copper nanoparticles 41 to penetrate the primer layer 3. In other words, the softening point of the primer layer 3 is below the heating temperature of the hot plate in the copper printing process, which is 100°C or lower, and at least 5 degrees lower than the softening point of the modified polyolefin resin of the primer layer 3, which is 105°C. It is thought that the additive (undisclosed) contained in the primer (manufactured by Unitika Ltd., product name "Arrowbase SD-1200") caused the softening point of the primer layer 3 to be lower than that of the polyolefin resin. Furthermore, although hypothetical, it is possible that at the interface between the primer layer 3 and the copper nanoparticle layer 4, the interfacial free energy is lower when the copper nanoparticles 41 are embedded in the primer layer 3 than when they are in contact with the primer layer 3, thus further lowering the softening point at the interface. [Examples]

[0077] A primer solution was prepared using a primer containing a modified polyolefin resin with a softening point of 105°C (manufactured by Unitika Ltd., product name "Arrowbase SD-1205J2"). The softening point of the modified polyolefin resin was the same as in Example 4, but the additives were different. All other conditions were the same as in Example 1. Printed circuit board 1 was successfully fabricated in the same manner as in Example 1. The results of the 90° peel test were the same as in Example 1, and strong adhesion to the conductive layer 5 was confirmed.

[0078] A higher softening point of the primer layer 3, while maintaining good adhesion of the conductor layer 5, allows the printed circuit board 1 to withstand high temperatures and improves reliability. Therefore, a constant temperature and humidity test and a thermal shock test were performed on the printed circuit board 1 of Example 5. In the constant temperature and humidity test, the adhesion of the conductor layer 5 was maintained even after 1000 hours. In the thermal shock test, the adhesion of the conductor layer 5 was maintained even after applying thermal shock for 1000 cycles. [Examples]

[0079] A primer solution was prepared using a primer (manufactured by Unitika Ltd., product name "Arrowbase SD-1210J2") containing a modified polyolefin resin with a softening point of 105°C. The softening point of the modified polyolefin resin (105°C) was the same as in Examples 4 and 5, but the additives were different from those in Examples 4 and 5. All other conditions were the same as in Example 1. A printed circuit board 1 was successfully fabricated, similar to Example 1. The results of the 90° peel test were the same as in Example 1, confirming strong adhesion to the conductive layer 5.

[0080] A constant temperature and humidity test and a thermal shock test were performed on printed circuit board 1 of Example 6. The results of these tests were the same as those of Example 5.

[0081] Examples 1-3 showed that if the softening point of the thermoplastic resin in the primer layer 3 is lower than the heating temperature in the copper printing process, the primer layer 3 will soften in the copper printing process, resulting in strong adhesion to the conductor layer 5. The higher the softening point of the primer layer 3 is while maintaining adhesion to the conductor layer 5, the more the printed circuit board 1 can withstand high temperatures and the higher its reliability. Examples 4-6 showed that the softening point of the primer layer 3 may be slightly lower than the softening point of the thermoplastic resin in the primer layer 3. While it is not easy to directly observe whether the primer layer 3 has softened in the copper printing process, it can be easily determined from the adhesion of the conductor layer 5. Therefore, by using a thermoplastic resin with a softening point slightly higher than the heating temperature in the copper printing process and confirming the adhesion of the conductor layer 5 (a difference of 5 degrees in the examples), the softening point of the thermoplastic resin used can be raised to the maximum possible level.

[0082] As a comparative example, tests were conducted by changing the thermoplastic resin contained in the primer solution.

[0083] (Comparative Example 1) A primer solution was prepared using a primer (manufactured by Unitika Ltd., product name "Arrowbase YA-6010") containing a modified polyolefin resin (thermoplastic resin) with a softening point of 145°C. The softening point of this resin (145°C) is higher than the heating temperature of the hot plate (100°C) in the copper printing process and higher than the heat resistance temperature of the substrate (white PET) (130°C). The softening point of the primer layer made of this resin is also approximately 145°C. All other conditions were the same as in Example 1. In the primer printing process, this primer solution was used as ink to form a primer layer on the substrate using an inkjet printer. In the copper printing process, a copper microparticle layer was formed on the primer layer using an inkjet printer. However, the plating film peeled off in the plating process. This was because the primer layer did not soften in the copper printing process, resulting in poor adhesion of the copper microparticle layer.

[0084] (Comparative Example 2) A primer solution was prepared using a primer (manufactured by Unitika Ltd., trade name "Arrowbase DC-1010") containing a modified polyolefin resin (thermoplastic resin) with a softening point of 140°C. The softening point of this resin (140°C) is higher than the heating temperature of the hot plate in the copper printing process (100°C) and higher than the heat resistance temperature of the substrate (white PET) (130°C). The softening point of the primer layer made of this resin is also approximately 140°C. All other conditions were the same as in Comparative Example 1. In the primer printing process, the primer layer was formed on the substrate. In the copper printing process, a layer of copper microparticles was formed on the primer layer. However, the plating film peeled off in the plating process. This was because the primer layer did not soften in the copper printing process, so adhesion of the copper microparticle layer could not be obtained.

[0085] (Comparative Example 3) A primer solution was prepared using a primer (manufactured by Unitika Ltd., trade name "Elitel UE-9900") containing a saturated copolymer polyester resin (thermoplastic resin) with a softening point of 137°C. The softening point of this resin (137°C) is higher than the heating temperature of the hot plate in the copper printing process (100°C) and higher than the heat resistance temperature of the substrate (white PET) (130°C). The softening point of the primer layer made of this resin is also approximately 137°C. All other conditions were the same as in Comparative Example 1. In the primer printing process, the primer layer was formed on the substrate. In the copper printing process, a copper microparticle layer was formed on the primer layer. However, the plating film peeled off in the plating process. This was because the primer layer did not soften in the copper printing process, so adhesion of the copper microparticle layer could not be obtained.

[0086] (Comparative Example 4) A primer solution was prepared using a primer (manufactured by Unitika Ltd., trade name "Elitel Emulsion KA-3556") containing a saturated copolymer polyester resin (thermoplastic resin) with a softening point of 80°C. The softening point of this resin (80°C) is lower than the heating temperature of the hot plate in the copper printing process (100°C) and lower than the heat resistance temperature of the substrate (white PET) (130°C). The softening point of the primer layer made of this resin is also approximately 80°C. All other conditions were the same as in Comparative Example 1. In the primer printing process, the primer layer could not be formed on the substrate using an inkjet printer. This was because the primer solution did not wet well with the white PET and did not form a film on the substrate.

[0087] (Comparative Example 5) A primer solution was prepared using a primer (manufactured by Unitika Ltd., trade name "Elitel UE-3320") containing a saturated copolymer polyester resin (thermoplastic resin) with a softening point of 40°C. The softening point of this resin (40°C) is lower than the heating temperature of the hot plate (100°C) in the copper printing process. The softening point of the primer layer made of this resin is also approximately 40°C. All other conditions were the same as in Comparative Example 1. In the primer printing process, the primer layer was formed on the substrate. In the copper printing process, a layer of copper nanoparticles was formed on the primer layer. However, the plating film peeled off in the plating process. If the softening point of the primer layer is too low, the primer layer melts in the copper printing process and mixes with the copper nano-ink, resulting in poor plating deposition and poor adhesion of the conductive layer. Also, when the printed circuit board is used as a surface light source, the primer layer softens during use because its softening point of 40°C is too low.

[0088] In Comparative Examples 1-3, the softening point of the thermoplastic resin in the primer layer was higher than the heating temperature in the copper printing process. As a result, the primer layer did not soften during the copper printing process, and adhesion of the conductive layer to the substrate was not achieved, making plating impossible. Comparative Example 4 showed that the primer solution needs to be wettable to the substrate. Comparative Example 5 showed that if the softening point of the primer layer is too low, adhesion of the copper fine particle layer to the substrate is not achieved, making plating impossible.

[0089] It should be noted that the present invention is not limited to the configuration of the above embodiments, and various modifications are possible without changing the essence of the invention. For example, the plating metal of the electroless plating layer 51 and the electroplating layer 52 may be a metal other than copper. [Explanation of symbols]

[0090] 1 Printed circuit board 2 Base material 3. Prima layer 4 Copper fine particle layer 40 Copper Nano Ink 41 Copper nanoparticles 42 Dispersion medium 5 Conductor Layers

Claims

1. A method for manufacturing a printed circuit board having a substrate and a pattern of a conductive layer, A primer printing process in which a primer liquid is used as an ink to form a pattern of the primer layer on a substrate using an inkjet printer, A copper printing process in which a pattern of copper nanoparticle layers is formed on the pattern of the primer layer using an inkjet printer with copper nano ink, The process includes a plating step of applying plating to the copper fine particle layer to form a pattern for the conductive layer, The aforementioned substrate is made of polyethylene terephthalate. The aforementioned copper nanoink has copper nanoparticles dispersed in a dispersion medium. The primer liquid contains a thermoplastic resin and has wettability to the substrate, The thermoplastic resin is a polyolefin resin and has adhesive properties to the substrate. The softening point of the primer layer is higher than the temperature range of the primer layer during use of the printed circuit board, and lower than or equal to the heat resistance temperature of the substrate. A method for manufacturing a printed circuit board, characterized in that, in the copper printing process, the primer layer is heated to a temperature above the softening point and below the heat resistance temperature of the substrate, the copper fine particle layer is not fired, and the copper nanoparticles of the copper fine particle layer penetrate the primer layer at the interface with the primer layer and do not penetrate into the interior of the primer layer.

2. The method for manufacturing a printed circuit board according to claim 1, characterized in that the polyethylene terephthalate of the substrate is white PET.

3. The method for manufacturing a printed circuit board according to claim 2, characterized in that the softening point of the thermoplastic resin is 75°C or higher and 105°C or lower.

4. A substrate made of polyethylene terephthalate, The pattern of the primer layer adhered to the substrate, A printed circuit board comprising a pattern of a conductive layer on the primer layer, The conductor layer has a layer of copper fine particles on the primer layer and a plating layer on the copper fine particle layer. The primer layer is made of a polyolefin resin, which is a thermoplastic resin. The softening point of the primer layer is higher than the temperature range of the primer layer during use of the printed circuit board, and lower than or equal to the heat resistance temperature of the substrate. The printed circuit board is characterized in that the copper nanoparticle layer is not fired, and at the interface with the primer layer, the copper nanoparticles of the copper nanoparticle layer are embedded in the primer layer and do not penetrate into the interior of the primer layer.

5. The printed circuit board according to claim 4, characterized in that the polyethylene terephthalate of the substrate is white PET.

6. The printed circuit board according to claim 5, characterized in that the softening point of the thermoplastic resin is 75°C or higher and 105°C or lower.

Images