Circuit board and manufacturing method thereof

Abstract

To provide a circuit board that can be properly created by a non-sub subtractive method using metal particle ink even on rigid base materials, and a method for manufacturing the same.SOLUTION: A circuit board 10 is composed of a substrate 11 made of glass epoxy, a resin layer 12 formed on the substrate 11 and containing epoxy resin, an electrical conductive film 13 consisting of a photo-sintered layer of metal particles formed on the resin layer 12, and a plating layer 14 formed on the electrical conductive film 13. In one aspect, the resin comprising the resin layer has a curing temperature in the range from room temperature to 190°C and has an endothermic peak within the region of more than 360°C and less than 450°C in a TG-DTA analysis of the resin.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to a circuit board that can be appropriately produced by a non-subtractive method using an ink containing metal fine particles even on a rigid substrate, and a method for producing the same. [Background technology]

[0002] Conventionally, circuit boards have been manufactured using a subtractive method, in which a metal layer is formed on an insulating substrate (base material) such as resin, and then unnecessary parts of this metal layer are removed by etching to form a wiring pattern. This method requires the use of large amounts of water and excess metal that is discarded during etching, and requires many steps.

[0003] In response to this issue, the present applicant has proposed a method in which a conductive ink containing metal nanoparticles (metal fine particles) is applied only to the required areas on an insulating substrate made of a thermoplastic resin such as polyimide film using an inkjet method or other method, and then the metal layer is thickened by plating to further reduce resistance (Patent Document 1). This method (non-subtractive method), which differs from conventional subtractive methods, enables a significant simplification of the circuit board manufacturing process, particularly by significantly reducing the amount of water used and carbon dioxide emissions. This non-subtractive method can be said to be an environmentally friendly method for manufacturing circuit boards with a small number of steps. Furthermore, the inkjet method is a reliable method for producing small quantities of circuit boards on demand with minimal time and cost.

[0004] Patent Document 2 discloses a method for forming circuits by applying a metal nanoparticle ink onto a polyether ether ketone substrate, forming a photosintered film, and then forming a plating layer on top of that. This method involves applying a resin layer (underlayer) called a primer to the substrate to improve the circuit formation. Photosintering involves sintering (firing) metal nanoparticles using a heat source such as a xenon lamp, which allows selective heating of the metal nanoparticle ink while suppressing temperature rise in the substrate. This minimizes thermal effects and enables sintering in a short time, even for substrates with low heat resistance. The reason for using a resin layer as the underlayer in Patent Document 2 is that when metal nanoparticle ink is directly applied to the substrate surface and photosintered (photosintering: PS), the plating layer does not adhere well to the substrate. Patent Document 3 also describes an example of improving sinterability by providing a resin underlayer with low thermal conductivity on the substrate. While the use of polyimide as this underlayer is described, polyimide has the disadvantage of poor alkali resistance, and other materials are desirable. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6300213 [Patent Document 2] Japanese Patent Publication No. 2020-188074 [Patent Document 3] Special Publication No. 2012-522383 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the case of rigid boards (RPCB) such as so-called glass epoxy boards that have been used for a long time, it has been difficult to manufacture circuit boards in exactly the same manner using the resin layer as a primer of Patent Document 2.

[0007] One reason for this is that when a resin layer is applied as a primer, as in conventional technology, the resin layer (and the metal nanoparticle layer) often blows away during light sintering, making it difficult to create circuit boards.

[0008] Figure 1 is a micrograph of the substrate surface after photosintering, showing that the resin layer and the metal nanoparticle layer applied thereon have been completely blown away during photosintering.

[0009] Figure 2 is a micrograph (×20) of the substrate surface after photosintering, showing that the resin layer and metal nanoparticle layer have been partially blown away.

[0010] Furthermore, even when "blow-off" did not occur, the adhesion strength of the photosintered layer and plating layer formed after photosintering to the substrate was very low, approaching nearly zero N / mm.

[0011] Another reason is that in rigid substrates, which are generally manufactured using a glass epoxy substrate, the weave of the glass fibers creates unevenness on the substrate surface, and these unevenness makes it difficult to perform photosintering of metal nanoparticles in the same way as with smooth polyimide films. Such unevenness on the substrate surface appears on the substrate surface after photosintering as a pattern as shown in the optical microscope photograph (x100) in Figure 3.

[0012] Further observation of the pattern on the substrate surface using an electron microscope (×100) revealed that it was caused by differences in the degree of photosintering. That is, the light-colored surface area 41 shown on the left side of Figure 4 is well sintered, while the dark-colored surface area 42 on the right side of the figure is insufficiently sintered. It was thus discovered that structural unevenness in sintering occurs in rigid substrates.

[0013] The reason for this uneven sintering is probably because the thermal conductivity of glass fiber and epoxy resin differs by an order of magnitude, causing heat to be transferred differently depending on the location during light sintering. In other words, this uneven sintering reflects the internal structure of the glass fiber fabric. Incidentally, the thermal conductivity of epoxy resin is 0.17-0.21 W / mK, and that of glass is 1.03 W / mK.

[0014] In fact, as shown in Figure 5, in the case where a thin resin layer was applied as a base layer and sintering unevenness became evident, an electron microscope photograph of the cross section of the substrate after light sintering revealed that metal nanoparticles had periodically accumulated in depressions 51. This is also thought to indicate that light sintering was insufficient at the bottom of the periodic depressions created by the weave of the glass fiber.

[0015] Figure 6 shows a schematic diagram of uneven sintering on the surface of a glass epoxy substrate due to light sintering. Figure 6(a) shows a case where there is no resin layer (primer layer) and uneven sintering occurs. A glass epoxy substrate 61 contains glass fibers 62, and the unevenness of the substrate surface formed by the woven fabric results in unsintered areas 64 in the metal nanoparticle layer (ink layer) 63. It is believed that this structure is a factor that reduces adhesion strength.

[0016] Figure 6(b) shows a case where uneven sintering does not occur thanks to the presence of a resin layer (not shown) that smooths out the irregularities on the substrate surface. However, if we assume that the problem of uneven sintering is due to the irregularities on the substrate surface and simply smooth it out by applying a resin or other coating, the resin layer will be blown away during light sintering, as described above, and it was found that this simple method cannot solve the problem.

[0017] The above problems can be briefly summarized as follows. Specifically, when applying metal nanoparticles (metal fine particles) in ink and then performing photosintering, if a resin layer is provided that is thought to contribute to adhesion between the photosintered layer and the substrate, the resin layer may be blown off depending on the resin used. In addition, the above-mentioned uneven sintering always occurs. Systems with uneven sintering have very low adhesion strength. Furthermore, while the three-dimensional unevenness can be eliminated by thickening the base resin layer, the "blow off" phenomenon occurs when attempting to perform photosintering with a thick base resin layer.

[0018] The present invention has been made against this background, and its object is to provide a circuit board that can be properly produced using a non-subtractive method using an ink containing metal fine particles, even on rigid substrates, and a method for producing the same.

[0019] Another object of the present invention is to provide a circuit board and a method for manufacturing the same that can prevent the resin layer serving as the base layer from being blown away during photosintering.

[0020] It is still another object of the present invention to provide a circuit board and a manufacturing method thereof that can increase the adhesive strength between the conductive film formed by the photosintering layer and the plating layer thereon and the substrate. Note that it is not necessary to achieve all of these multiple objects. [Means for solving the problem]

[0021] To solve the above problem, the inventors adopted a materials-based approach. Specifically, they realized that the same resin, but with a polyimide substrate, was less likely to blow off during photosintering. This led them to analyze the properties of the resin used in the resin layer. Rigid substrates are generally made of a composite of glass fiber and epoxy resin, and are therefore called glass-epoxy substrates. It is easy to predict that an epoxy resin adhesive layer would adhere well to a substrate containing epoxy resin. However, experimental results revealed that even with the same epoxy resin, differences in its composition and components can affect whether or not "blow off" occurs.

[0022] Therefore, the behavior of the epoxy resin from application to curing and the subsequent temperature rise process was analyzed using TG-DTA (Thermogravimetry-Differential Thermal Analysis).

[0023] Figure 7 shows a typical thermogravimetric and differential thermal analysis (TG-DTA) chart for an epoxy resin prone to "blowout" during photosintering. The TG curve (curve B) in this chart represents the weight change (%) of the sample being analyzed as a function of increasing temperature (horizontal axis). Negative values ​​represent weight loss. The DTA curve (curve A) represents the temperature difference between the sample and a reference material as a function of increasing temperature. This temperature difference is expressed as the electromotive force of the differential thermocouple in μV. The DTG curve (curve C) represents the time rate of change of the sample's weight (μg / min) as a function of increasing temperature. (The arrows extending left or right from each curve in the chart indicate the vertical axis that applies to that curve.)

[0024] In the chart in Figure 7, a convex peak indicating heat generation is observed in the DTA curve (curve A) below 200°C. This represents the heat generated during thermal polymerization (curving) of the epoxy monomer. The TG curve (curve B), which shows weight change during this time, is flat, indicating no material is entering or leaving the sample. On the other hand, in the range above 360°C and below 450°C, a large weight loss is observed on the TG curve, and the DTA curve indicates that heat is entering or leaving in this region in the direction of heat generation. This generally indicates that combustion occurred in this temperature range. Since heat generation is observed even after this temperature increase above 450°C, it is reasonable to assume that combustion continued. This, as shown by the large, steep peak in the DTG curve (curve C), suggests that combustion occurred during rapid heating by photosintering, and if this explosive combustion occurred, it caused the "blowout" phenomenon.

[0025] Figure 8(a) shows the case where the peak in the temperature range of 360°C to 450°C on the DTA curve (curve A) in Figure 7 is in the exothermic (positive) direction from the baseline. It is best to draw the baseline from the low-temperature side. Since heat generation is observed even after that, even at temperatures above 450°C, it is reasonable to assume that combustion continues to occur. In this case, if the exothermic peak is large, the endothermic peak shown earlier may be canceled out or the baseline may be difficult to draw, so judgment is made based only on the baseline from the low-temperature side. Figure 8(b) shows the case where the peak in the same temperature range is in the endothermic (negative) direction from the baseline.

[0026] On the other hand, Figure 9 shows a typical TG-DTA chart for an epoxy resin that is less susceptible to blow-off during photo-sintering, even when irradiated with the same amount of light. In this example, a large negative peak is observed in the DTA curve (curve A) between 360°C and 450°C. This indicates endothermic heat generation, which is thought to be due to the evaporation of decomposition products, such as monomers, rather than immediate combustion during decomposition. This suggests that rather than sudden heat generation, the decomposition is relatively gradual, resulting in less blow-off during photo-sintering. Of course, combustion must also be occurring in this region, and the overlapping of the positive and negative peaks of both processes is likely the result of the curve shown in the figure. Thus, it can be seen that the behavior of the DTA curve within the temperature range from 360°C to 450°C is deeply related to the occurrence of blow-off.

[0027] Furthermore, in the case of Figure 9, when we look at the weight change on curve B (TG curve), we can see that most of the components have disappeared at around 400°C, at the right end. In this case, there was no "blow-off," but it is thought that the resin layer that should support adhesion has disappeared. Conversely, it was found that a certain amount of resin layer remaining on the right end of the TG curve is a condition for ensuring adhesion. According to experimental results, the average thickness is preferably between 5 μm and 15 μm.

[0028] In the example shown in Figure 9, the resin solution contains 34% by weight of solvent, and a large change in weight (see curves B and C) is observed due to evaporation of the solvent before the first heat generation peak (see curve A) in the region below 200°C. However, the presence of this solvent does not directly affect the occurrence of "blow-off" or adhesion.

[0029] From the above analysis, it is possible to predict and select a resin layer suitable for a glass epoxy substrate from the TGDTA analysis. That is, the circuit board and the manufacturing method thereof according to the present disclosure have the following aspects.

[0030] In a first aspect, the circuit board of the present disclosure includes a glass epoxy substrate, a resin layer containing an epoxy resin formed on the substrate, a conductive film consisting of a photosintered layer of metal fine particles formed on the resin layer, and a plating layer formed on the conductive film. With this configuration, a circuit board can be appropriately produced using a metal fine particle ink by a non-subtractive method even on a rigid substrate.

[0031] In a second aspect of this circuit board, the resin constituting the resin layer has a curing temperature within a range of room temperature to 190° C., and in a TG-DTA analysis of the resin, has an endothermic peak within a range of 360° C. to 450° C. By using such a resin as a base layer for the photosintering layer, the phenomenon of the resin being blown away during photosintering is suppressed.

[0032] In a third aspect of the circuit board, the resin layer has an average thickness of 5 μm to 15 μm. Using such a resin as a base layer for the photosintered layer improves the adhesion strength between the photosintered layer and the substrate. If the thickness is less than 5 μm, the resin layer is susceptible to heat absorption by the base glass during photosintering. If the thickness is more than 15 μm, the epoxy resin is likely to be poorly cured under the manufacturing conditions.

[0033] In a first aspect, the method for manufacturing a circuit board of the present disclosure includes the steps of forming a resin layer containing an epoxy resin on a glass epoxy substrate, forming a conductive film consisting of a photosintered layer on the resin layer by photosintering metal fine particles, and forming a plating layer on the conductive film.

[0034] In a second aspect of the method for manufacturing a circuit board, the resin constituting the resin layer is cured at a temperature within the range of room temperature to 190°C.

[0035] In a third aspect of the method for producing a circuit board, the resin layer is characterized in that it has an endothermic peak in the range of 360° C. or higher and 450° C. or lower in TG-DTA analysis of the cured resin. [Effects of the Invention]

[0036] According to one aspect of the present invention, it is possible to provide a circuit board that can be appropriately produced by a non-subtractive method using an ink containing metal fine particles even on a rigid substrate, and a method for producing the same.

[0037] According to another aspect of the present invention, it is possible to provide a circuit board and a method for manufacturing the same that can prevent the phenomenon in which a resin layer serving as a base layer is blown away during photosintering.

[0038] According to yet another aspect of the present invention, a circuit board and a manufacturing method thereof can be provided that can increase the adhesive strength between the conductive film formed by the photosintered layer and the plating layer thereon and the substrate. [Brief explanation of the drawings]

[0039] [Figure 1] This is a micrograph of the substrate surface after photosintering (in the case of complete blow-off). [Figure 2] 1 is a micrograph of the substrate surface after photosintering (in the case of partial blow-off). [Figure 3] FIG. 10 is a diagram showing an optical microscope photograph of a case where a pattern is formed by photosintering. [Figure 4] Electron microscope photographs taken from above of the dark and light colored areas after photosintering. [Figure 5] 1 is an electron microscope photograph of a cross section of a substrate after photosintering, in which sintering unevenness is evident. [Figure 6] 1 is a schematic diagram showing uneven sintering caused by light sintering in a glass epoxy substrate. FIG. [Figure 7] This is a typical TG-DTA chart for epoxy resin, which is prone to "blowout" during photosintering. [Figure 8] FIG. 1 is an explanatory diagram of a DTA curve (curve A) when (a) an exothermic peak appears relative to the baseline, and (b) when an endothermic peak appears relative to the baseline. [Figure 9]This is a typical TG-DTA chart for an epoxy resin that is less likely to "blow away" during photosintering. [Figure 10] 1 is a cross-sectional view schematically illustrating a basic configuration of a circuit board according to an embodiment of the present disclosure. [Figure 11] FIG. 1 is a diagram showing the profile (TG-DTA data) of the epoxy resin solution used in Example 1. [Figure 12] 1 is a photograph of the surface of a substrate after photosintering in Example 1. [Figure 13] 1 is an optical microscope photograph of the surface of a substrate after photosintering in Example 1. [Figure 14] FIG. 1 is a diagram showing the results of a peel test in Example 1. [Figure 15] 1 is an optical microscope photograph after photosintering in the case of a thinly applied resin layer in Comparative Example 1. [Figure 16] FIG. 1 is a diagram showing the profile (TG-DTA data) of the epoxy resin solution used in Comparative Example 2. [Figure 17] 10 is an optical microscope photograph of the resin layer used in Comparative Example 2 after photosintering. [Figure 18] FIG. 1 shows the profile (TG-DTA data) of the epoxy resin solution of Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0040] The present invention will be described in detail below with reference to an embodiment thereof. This embodiment employs an approach in which an underlying resin layer that meets specific conditions is used when applying ink containing metal nanoparticles as metal fine particles onto a glass epoxy substrate.

[0041] <Circuit board configuration> Figure 10 shows a cross-sectional view that schematically illustrates the basic configuration of the circuit board according to this embodiment, where Figure 10(a) shows a general configuration, and Figure 10(b) shows a specific configuration example.

[0042] 10(a), the circuit board 10 is basically configured to include a substrate 11 made of glass epoxy as an insulating base material, a resin layer 12 applied on the substrate 11, a photosintered layer 13 (conductive film) formed of a metal nanoparticle layer (ink layer) applied on the resin layer 12, and a plating layer 14 formed on the photosintered layer 13. The plating layer 14 forms a conductive layer for the circuit.

[0043] Figure 10(b) shows a cross-sectional view of a circuit board in a partially printed wiring state. In this figure, the photosintered layer 13a is formed by photosintering an ink layer 13a of a metal nanoparticle layer formed in a pattern. A plating layer 14a is formed on this photosintered layer 13a. This plating layer 14a is formed in a similar pattern to the photosintered layer 13a.

[0044] The specific configuration of the components of the circuit board 10 is as follows.

[0045] (Base material 11) The insulating substrate (insulating substrate) 11 in this embodiment is a glass fiber fabric impregnated with epoxy resin, and is typically plain woven, but is not necessarily limited to this. The substrate 11 can be of various types depending on the degree of modification of the epoxy resin and additives, but is not particularly limited to this. The thickness is typically about 0.2 mm to 1.6 mm, but is not necessarily limited to this.

[0046] (Resin layer 12 (primer, undercoat layer)) The resin solution for the resin layer 12 as a primer in this embodiment is composed of an epoxy resin monomer to which 1 to 5% by weight of an amine-based curing agent is added, diluted to 50% by weight with ethylene glycol monoethyl ether.

[0047] Modified epoxy resin monomers can be used, and oligomers or monomers with different odd functionalities can be blended. The curing agent may be one other than an amine-based one. Note that polymers (molecular weight of 10,000 or more) are not included in the embodiments of the present invention.

[0048] (Metal nanoparticle layer 13: ink layer: layer containing metal nanoparticles: photosintered layer: conductive film) In this embodiment, the thickness of the metal nanoparticle layer 13 is preferably 100 nm to 20 μm, more preferably 200 nm to 5 μm, and most preferably 500 nm to 2 μm. If this layer is too thin, the mechanical strength may be reduced. Conversely, if the ink coating layer is too thick, the manufacturing cost may increase because metal nanoparticles are generally more expensive than regular metals.

[0049] Materials used for metal nanoparticles (metal fine particles) include gold (Au), silver (Ag), copper (Cu), palladium (Pd), and nickel (Ni), and may contain one or more metals, but gold, silver, and copper are preferred from the perspective of conductivity. Silver is preferred because it is less susceptible to oxidation than copper and is cheaper than gold, but if copper is available as it is even cheaper, that would be even better.

[0050] The average particle size of the metal nanoparticles is preferably 1 to 200 nm, more preferably 10 to 100 nm. If the particle size is too small, the reactivity of the particles may be high, which may adversely affect the storage stability and durability of the ink. If the particle size is too large, it may be difficult to form a uniform thin film and the ink particles may be more likely to settle.

[0051] (Plating layer 14) The plating layer 14 as a conductive layer is formed by plating (electrolytic plating or electroless plating) on ​​the metal nanoparticle layer 13. For electroless plating, a standard copper sulfate solution with formaldehyde as a reducing agent and a pH of 10 or higher can be used.

[0052] As the plating metal, copper, nickel, tin, silver, gold, etc. can be used, but copper is most preferably used from the viewpoint of economy and conductivity.

[0053] The thickness of the plating layer 14 is preferably 3 μm to 100 μm, and more preferably 3 μm to 35 μm. If the plating layer 14 is too thin, the mechanical strength may be insufficient and the conductivity may not be sufficient for practical use. Conversely, if the plating layer 14 is too thick, the time required for the plating process may be longer, which may increase manufacturing costs. Generally, electrolytic plating requires less time than electroless plating, so electrolytic plating can accommodate thicker plating layers at a more realistic cost. However, electroless plating has the advantage that it can plate not only connected electrode lines but also raised areas such as islands.

[0054] <Circuit board manufacturing method> (Primer application process) A resin solution for the resin layer 12 serving as a primer is applied to the surface of a glass epoxy substrate. While the method for applying this primer is not particularly limited, sheet-fed printing is often used because rigid substrates are hard. For example, screen printing is considered suitable. Alternatively, gravure printing or offset printing can also be used. The applied resin layer serving as a primer is cured at a temperature ranging from room temperature to 190°C. While room temperature to 200°C is the appropriate temperature range for curing epoxy resin serving as a primer, glass epoxy substrates melt or burn if heated above 200°C, so the upper limit of the curing temperature is set to 190°C. It is preferable to use a resin composition used as a primer that will cure within the range of room temperature to 190°C.

[0055] (Ink application process) In this process, ink containing metal nanoparticles is applied to the surface of the substrate, which has been coated and cured with a primer. This application can be done either over the entire surface of the substrate or in a pattern. When applying in a pattern, a printing method can be used, and the inkjet method is typically used. However, this is not necessarily limited to the inkjet method, and other application methods can also be used. In the experiments described below, application was also performed using a bar coater.

[0056] After applying the ink containing metal nanoparticles to a substrate, a drying process is performed to remove the solvent, if any. This process is similar to the drying process for known metal nanoparticle inks. Methods for drying the ink containing metal nanoparticles include heating in an oven or hot air drying.

[0057] (Photosintering process) After the ink application and drying processes, the photo-sintering process is carried out. For this, a commercially available photo-sintering (PS) device, such as the photo-sintering device (B0320-A) manufactured by Ushio Inc., can be used. The distance between the substrate and the lamp is set, and the voltage and irradiation time are adjusted. Photo-sintering is completed instantly, so the time required to proceed to the next process is short.

[0058] (Plating process) The formed photosintered layer is subjected to a plating treatment. This causes a plating metal to be deposited on the surface and inside of the sintered layer. The plating method is similar to a known plating treatment using a known plating solution, and specifically may include electroless copper plating, electrolytic copper plating, electrolytic nickel plating, etc.

[0059] Next, a specific example will be described in which photosintering and plating treatment are carried out using the resin layer (underlayer) of this embodiment.

[0060] Example 1 In Example 1, a 100 mm x 100 mm glass epoxy resin substrate, ES-33515 (manufactured by Risho Kogyo Co., Ltd.), was used as the glass epoxy resin substrate. A 50 wt. % solution containing monomers, oligomers, and polymers to form the epoxy resin layer (underlayer) was applied to this substrate and cured at 150°C for 1 hour. The substrate was first washed with acetone, and then coated at a speed of 40 mm / sec using a bar coater and a Bevs 1818S Miniautomatic Applicator (manufactured by Bevs) to a thickness of 30 μm before drying. Measurements using a micrometer revealed that the underlayer thickness varied from 11 μm to 15 μm due to unevenness. An electron microscope photograph of the cross section showed a thickness of 8 μm, which differed from the micrometer measurement. In this embodiment, the thickness measurement using a micrometer was calculated as an average of measurements taken at five or more locations. To measure the thickness on an electron microscope photograph, two lines were drawn at the upper and lower boundaries of the layer to be measured, and the distance between the two lines was measured.

[0061] The epoxy resin used as a primer here is a blend of epoxy monomers with a bisphenol A skeleton, bifunctional and trifunctional monomers necessary for three-dimensional hardening, and an amine-based hardener. The solvent is ethylene glycol monoethyl ether, initially 66% by weight, to which the same solvent was added to make it 50% by weight before application.

[0062] The characteristics (profile) of this epoxy resin are shown in the TG-DTA chart in Figure 11.

[0063] This resin solution contains about 30% by weight of solvent, and on the chart it evaporates at temperatures above 100°C, causing a decrease in weight (see curve B), and it can be seen that heat is absorbed (see curve A). On the other hand, an endothermic portion is confirmed above 400°C, followed by the appearance of heat generation (see curve A). The heat generation around this temperature can be interpreted as being due to combustion. The endothermic portion is thought to be the heat absorbed when some of the resin components that hardened by 200°C decompose and evaporate.

[0064] Such epoxy resins are not limited to a specific composition as long as they can be identified by their TG-DTA results. Modified epoxy resins with various additives may also be used as long as they provide equivalent TG-DTA results.

[0065] After the resin layer (undercoat layer) was cured, a 15% by weight copper nanoparticle ink was applied to the substrate with a bar coater to a coating thickness of 10 μm. After confirming that the solvent had dried at room temperature, the coated substrate was dried in an oven at 60°C for 30 minutes. Ethylene glycol monobutyl ether was used as the solvent for this nanoparticle ink.

[0066] After drying, the substrate was placed in a photosintering device B0320-A (manufactured by Ushio Inc.) and photosintered at a voltage between 2700 V and 3000 V. Specifically, the voltage and irradiation time (milliseconds) were adjusted using an Ohir thermal sensor L50(150)A-LP2-35 so that the xenon lamp output was within the range of 1.2 W ± 0.02 W. The distance from the xenon lamp to the sample surface was 5 cm.

[0067] Severe cases of resin layer blow-off were immediately visible to the naked eye, but subtle cases were confirmed by taking optical microscopic photographs using a digital microscope (VHX-8000, manufactured by Keyence Corporation). Resistance values ​​were measured using a four-terminal resistance meter RM3544 (manufactured by Hioki E.E. Corporation).

[0068] As a result, in Example 1, there was no blow-off, and a conductive film made of a photosintered layer (photosintered film) with a resistance value of 0.5Ω or less could be obtained.

[0069] Figure 12 shows a photograph of the substrate surface after light sintering in Example 1. From the reflection of light, even though sintering was performed under good conditions, the texture of the substrate can be seen on the surface. In other words, this photograph shows that large undulations on the substrate surface are still visible. On the other hand, as shown in Figure 13, the optical microscope photograph (x100) shows that the uneven sintering shown in Figure 3 is almost invisible.

[0070] The substrate with the resulting photosintered layer (conductive film) containing metal nanoparticles was treated with 10% sulfuric acid for 10 seconds and then rinsed with water. It was then pre-dipped in an electroless copper plating solution, and electroless copper plating was performed at a solution temperature of 65°C for 4 hours using an electroless copper plating solution whose main components were copper, alkali, and formaldehyde. It was then immersed in a discoloration inhibitor at room temperature for 1 minute and then dried.

[0071] As a result, a plating layer was formed on the surface of the substrate after photosintering, ensuring conductivity suitable for use in ordinary devices.

[0072] The substrate thus completed was annealed at 200°C for one hour and subjected to a peel test, resulting in a stable adhesion strength of over 0.4 N / mm, as shown in Figure 14(a). This strength exceeds the US UL standard (0.35 N / mm). For comparison, Figure 14(b) shows the result before annealing, when there was almost no peel strength (0.113 N / mm).

[0073] (Comparative Example 1) Comparative Example 1 describes a case where a 5 wt% solution was applied to a 10 μm coating thickness using a bar coater using the epoxy resin layer of Example 1. Other than that, there are no differences from Example 1. The thickness of the resin layer in Comparative Example 1 was measured with a micrometer and an electron microscope photograph of the cross section, and was found to be less than 2 μm. Figure 15 shows an optical microscope photograph of Comparative Example 1 after photosintering. In addition to uneven sintering, this photograph reveals that some of the copper has been blown away, creating holes.

[0074] In this case, the peel test result after plating was almost 0.0 N / mm. Therefore, it was found that even if the resin is the same, if the resin layer is thin, the effect on adhesion strength is small and no strength can be obtained at all.

[0075] (Comparative Example 2) The experimental method for Comparative Example 2 was the same as that for Example 1, but the TG-DTA profile of the epoxy resin in the resin layer used in the primer was different, as shown in Figure 16. In this example, there was an exothermic peak above 300°C, suggesting that the resin was primarily decomposed by combustion. In this case, it was confirmed that explosive combustion occurred during photosintering, making "blowout" more likely to occur.

[0076] In this case, the applied film thickness was 10 μm using a bar coater, and as can be imagined from the TG-DTA profile, there was a tendency for "blow-off" to occur, so uneven sintering was clearly observed after photosintering. Figure 17 shows an optical microscope image of this resin layer after photosintering.

[0077] The substrate with the ink layer containing metal nanoparticles was cleaned with running water for one minute. It was then pre-dipped in electroless copper plating solution, and electroless copper plating was performed at a solution temperature of 60°C for 180 minutes using an electroless copper plating solution whose main components were copper, alkali, and formaldehyde. The substrate was then immersed in a discoloration inhibitor at room temperature for one minute and then dried.

[0078] In this case, even after annealing at 200°C for one hour after plating, the peel test result was a small value of 0.066 N / mm. Before annealing, it was 0.018 N / mm. Therefore, it was found that the TG-DTA profile was poor and sufficient adhesive strength could not be obtained when the resin layer was thin.

[0079] Example 2 In Example 2, a 100 mm x 100 mm glass epoxy resin substrate (ES-33515, manufactured by Risho Kogyo Co., Ltd.) was used as the base material. A 50 wt. % solution containing monomers, oligomers, and polymers that would form the epoxy resin layer (underlayer) was applied to the substrate, followed by curing at 150°C for 1 hour. The substrate was first washed with acetone, and then coated at a speed of 40 mm / sec using a Bevs 1818S Miniautomatic Applicator (manufactured by Bevs) with a bar coater to coat a thickness of 30 μm before drying. Measurements with a micrometer revealed that the underlayer thickness ranged from 11 μm to 15 μm, with some variation due to unevenness.

[0080] The epoxy resin used as a primer here is composed mainly of an epoxy monomer with a bisphenol A skeleton, as in Example 1, and is blended with bifunctional and trifunctional monomers necessary for three-dimensional curing, and an amine-based curing agent, but the blending is slightly different from that in Example 1. The solvent is also ethylene glycol monoethyl ether, initially at 66% by weight, to which the same solvent was added to make it 50% by weight before application.

[0081] The characteristics of this epoxy resin are shown in the TG-DTA chart in Figure 18.

[0082] According to the DTA curve in this chart, an endothermic portion is recognized above 400°C, followed by the appearance of heat generation (see curve A). The heat generation around this temperature can be interpreted as being due to combustion. The endothermic portion is thought to be the heat absorption occurring when some of the resin components that have hardened up to 200°C decompose and evaporate.

[0083] Such epoxy resins are not limited to a specific composition as long as they can be identified by their TG-DTA results. Modified epoxy resins with various additives may also be used as long as they provide equivalent TG-DTA results.

[0084] After the resin layer (undercoat layer) was cured, a 15% by weight copper nanoparticle ink was similarly applied to the substrate using a bar coater to a coating thickness of 10 μm. After confirming that the solvent had dried at room temperature, the coated substrate was dried in an oven at 60°C for 30 minutes. Ethylene glycol monobutyl ether was used as the solvent for this nanoparticle ink.

[0085] The dried substrate was set in a photosintering apparatus B0320-A (manufactured by Ushio Inc.) and photosintered at a voltage between 2700 V and 3000 V in the same manner as in Example 1. As a result, a conductive film was obtained that was free of blow-off and consisted of a photosintered layer (photosintered film) with a resistance of 0.5 Ω or less.

[0086] (Resin layer (base layer) thickness) Table 1 shows the experimental results (within a 100 mm x 100 mm substrate sample) for the thickness (film thickness) of the resin layer (underlayer). [Table 1]

[0087] As explained in Example 1 above, when a 50 wt % epoxy resin solution was applied using a 30 μm bar coater, the thickness of the undercoat layer (after drying and curing) was measured with a micrometer to be 11 μm to 15 μm in an experimental example with an adhesion strength of around 0.4 N / mm. Furthermore, the cross-sectional SEM measurement value was 8 μm.

[0088] In contrast, when applying two coats of the primer layer or directly applying the undiluted solution, a 30μ bar coater was also used, but the film thickness was very large.

[0089] In samples with a base layer thickness exceeding 100 μm, blow-off was likely to occur during light sintering, making adjustments difficult. It is thought that light sintering conditions are easier to adjust when the base layer is 20 μm or less. In this situation, we attempted to produce samples with a film thickness of 20 to 100 μm, but this was difficult and reproducible. However, in the case of a film thickness of 25 μm that was obtained by chance, blow-off occurred, so it is thought that thicker film thicknesses are more likely to cause blow-off.

[0090] When the same epoxy resin undercoat was applied at 5% by weight, cross-sectional observations using an electron microscope revealed that the undercoat thickness was 3 to 4 μm. In this case, there was less blow-off, but the effect of improved adhesion strength was not achieved.

[0091] Based on the above experimental results, it is considered appropriate that the average thickness of the resin layer as the underlayer is 5 μm or more and 15 μm or less from the viewpoint of adjusting the light sintering conditions and adhesive strength.

[0092] (Variation) Although the preferred embodiment has been described above, various modifications and changes other than those mentioned above are possible. The materials, lengths, thicknesses, ratios, temperatures, times, etc. used are examples only and are not necessarily limited to these. [Explanation of symbols]

[0093] 10 Circuit Board 11. Base material (insulating base material) 12 Resin layer (primer, base layer) 13 Metal nanoparticle layer (ink layer, photo-sintered layer, conductive film) 14 plating layer 14b Wiring pattern (conductive pattern) 41,42 Surface area 51 hollow 61 Glass epoxy substrate 62 Glass Fiber 63 Metal nanoparticle layer (ink layer, photo-sintered layer, conductive film) 64 Unsintered part 65 plating layer A curve (DTA curve) B curve (TG curve) C-curve (DTG curve)

Claims

1. A glass epoxy substrate, a resin layer formed on the substrate and containing an epoxy resin; a conductive film formed on the resin layer and including a sintered layer of metal fine particles; a plating layer formed on the conductive film, the resin constituting the resin layer has a curing temperature within a range of room temperature to 190°C, and in TG-DTA analysis of the resin, has an endothermic peak within a range of 360°C or higher and 450°C or lower; The average thickness of the resin layer is 5 μm or more and 15 μm or less. Circuit board.

2. forming a resin layer containing an epoxy resin on a glass epoxy substrate; forming a conductive film made of a sintered layer on the resin layer by sintering metal fine particles; forming a plating layer on the conductive film; the resin layer has an endothermic peak in a range of 360°C or higher and 450°C or lower in a TG-DTA analysis of the resin after curing, The average thickness of the resin layer is 5 μm or more and 15 μm or less. A method for manufacturing a circuit board.

3. The method for manufacturing a circuit board according to claim 2 , wherein the resin constituting the resin layer is cured at a temperature within a range of from room temperature to 190° C.

Images