Flexible copper foil substrate

Abstract

To provide a flexible copper foil substrate suitable for high frequency transmission.SOLUTION: A flexible copper foil substrate includes a polyimide base material, a nickel copper alloy layer and a copper layer. The nickel copper alloy layer including nickel, copper and phosphorus is formed on at least one surface of the polyimide base material by electroless plating; the nickel copper alloy layer has more than 1.3 and less than 2.3 of the weight ratio of the copper to the nickel; and the content of the phosphorus is larger than 2.1 wt.% and less than 3.0 wt.% of the nickel copper alloy layer; the copper layer formed on the side opposite to the polyimide base material in the nickel copper alloy layer bonds with the nickel copper alloy layer to form a metal conductive layer; and the flexible copper foil substrate capable of reducing an insertion loss in a frequency range of 1-4 GHz is applied to the manufacturing of a flexible circuit board suitable for high frequency transmission.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a flexible copper foil substrate, and more particularly to a flexible copper foil substrate for high frequency transmission. [Background technology]

[0002] Adhesiveless flexible copper clad laminates (2L-FCCL) are primarily made by bonding a polyimide substrate and copper foil using methods such as coating, sputtering, or lamination, and are characterized by their superior heat resistance and dimensional stability. In recent years, a wet metallization method has been developed to manufacture flexible copper clad substrates, in which a nickel layer is formed on the surface of a polyimide substrate and then a copper layer is formed on top of the nickel layer by electroplating.

[0003] By forming a nickel layer between the copper layer and the polyimide substrate, the peel strength between the metal conductive layer consisting of the nickel layer and the copper layer and the polyimide substrate can be improved, and thus the structural strength of the flexible copper foil substrate can be improved. Summary of the Invention [Problem to be solved by the invention]

[0004] However, due to the magnetic properties and low electrical conductivity of nickel metal, it can affect the transmission of electronic signals, and when transmitting at high frequencies, the circuit conductors may experience additional insertion loss due to the skin effect.

[0005] Furthermore, excessive insertion loss can cause circuit conductors to imperfectly transmit signals when transmitting at high frequencies.

[0006] Therefore, in the technical field of the present invention, there is still room for improvement in flexible copper foil substrates for high frequency transmission.

[0007] The inventors have discovered that the flexible copper foil substrate of the present invention is useful for producing flexible printed circuits (FPCs) for high-frequency transmission because it does not cause resonance absorption in the frequency range of 1 to 4 GHz. In other words, the flexible copper foil substrate of the present invention can reduce insertion loss in the frequency range of 1 to 4 GHz by utilizing an electroless plating method and a specific nickel-copper alloy layer composition, making it possible to obtain a flexible copper foil substrate suitable for high-frequency transmission. [Means for solving the problem]

[0008] In order to solve the above problems, a flexible copper foil substrate according to one embodiment of the present invention comprises: The substrate has a polyimide substrate, a nickel-copper alloy layer, and a copper layer, the nickel-copper alloy layer contains nickel, copper, and phosphorus, and is formed on at least one surface of the polyimide substrate by electroless plating, the weight ratio of the copper to the nickel in the nickel-copper alloy layer is greater than 1.3 and less than 2.3, and the phosphorus content is greater than 2.1 wt % and less than 3.0 wt % of the nickel-copper alloy layer, The copper layer is formed on the nickel-copper alloy layer on the opposite side to the polyimide substrate, and is bonded to the nickel-copper alloy layer to form a metal conductive layer.

[0009] In one embodiment of the present invention, the nickel-copper alloy layer is a single plating layer.

[0010] In one embodiment of the present invention, the nickel-copper alloy layer has a thickness of a single layer greater than 60 nm and less than 90 nm.

[0011] In one embodiment of the present invention, the nickel-copper alloy layer has a relative permeability of less than 1 at a frequency of 100 MHz.

[0012] In one embodiment of the present invention, the nickel-copper alloy layer is formed at a plating rate of more than 0.8 nm / sec under the conditions of a metal salt concentration of 4.8 g / L, a reducing agent concentration of 20 g / L, and a plating bath temperature of 38°C in the electroless plating.

[0013] In one embodiment of the present invention, the nickel-copper alloy layer has a sheet resistance of 10 Ω / dm 2 is less than.

[0014] In one embodiment of the present invention, the copper layer is formed on the nickel-copper alloy layer by electroplating, and has a thickness of 0.2 to 20 μm. [Effects of the Invention]

[0015] The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a flexible copper foil substrate suitable for high frequency transmission. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a cross-sectional view showing a flexible copper foil substrate according to an embodiment of the present invention. [Figure 2] 3 is a flowchart illustrating a manufacturing process of a flexible copper foil substrate according to an embodiment of the present invention. [Figure 3] FIG. 2 is a comparison diagram showing insertion losses in Example 1 of the present invention and Comparative Examples 1 and 2. [Figure 4] (a) is a photograph of a metallized through hole of Comparative Example 7 of the present invention, (b) is a photograph of a metallized through hole of Comparative Example 6 of the present invention, and (c) is a photograph of a metallized through hole of Example 1 of the present invention. [Figure 5] 1(a) is a photograph of Comparative Example 3 of the present invention after line etching, and FIG. 1(b) is a photograph of Example 1 of the present invention after line etching. DETAILED DESCRIPTION OF THE INVENTION

[0017] The following describes the implementation of the present invention through specific examples, and those skilled in the art can understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can be implemented or applied through different examples, and the details of this specification can be variously modified and changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0018] Unless otherwise specified in the text, the term "A to B" used in the specification and claims includes the meaning of "A or more and B or less." For example, the term "10 to 40% by weight" includes the meaning of "10% by weight or more and 40% by weight or less."

[0019] <Flexible copper foil substrate> Please refer to Figure 1 first. Figure 1 is a cross-sectional view showing a flexible copper foil substrate 100 according to one embodiment of the present invention. As shown in Figure 1, the flexible copper foil substrate 100 according to one embodiment of the present invention has a polyimide substrate 1, a nickel-copper alloy layer 2, and a copper layer 3. The polyimide substrate 1 has a first surface 11 and a second surface 12, and the nickel-copper alloy layer 2 and the copper layer 3 can be bonded to form a metal conductive layer.

[0020] Next, as shown in Fig. 1, a nickel-copper alloy layer 2 is formed on a first surface 11 of a polyimide substrate 1, and a copper layer 3 is formed on the nickel-copper alloy layer 2 on the side opposite to the polyimide substrate 1. That is, a flexible copper foil substrate 100 is configured so that the polyimide substrate 1, the nickel-copper alloy layer 2, and the copper layer 3 are arranged in this order. The nickel-copper alloy layer 2 can also be formed simultaneously on the first surface 11 and the second surface 12 of the polyimide substrate 1. Next, the flexible copper foil substrate 100 according to the present invention will be described in detail.

[0021] <<Polyimide substrate>> The polyimide substrate is a sheet / film-like substrate made of polyimide (PI), and is not particularly limited as long as it has a thickness of about 5 to 150 μm. The polyimide substrate can also be made from transparent polyimide, for example, polyimide with a light transmittance of more than 87%. The polyimide substrate can be a commercially available product, for example, a polyimide film (model number TX6-025) purchased from TAIMIDE Technology Co., Ltd.

[0022] <<Nickel-copper alloy layer>> The nickel-copper alloy layer contains nickel, copper, and phosphorus. Specifically, by adding sodium hypophosphite as a reducing agent to the electroless plating solution, phosphorus is also codeposited as one of the alloy components during the reduction and deposition of nickel ions. In the present invention, the phosphorus content of the nickel-copper alloy layer is greater than 2.1 wt % and less than 3.0 wt %, and can be controlled by the composition and operating conditions of the electroless plating solution.

[0023] Furthermore, if the phosphorus content of the nickel-copper alloy layer is 2.1 wt% or less, the plating speed in electroless plating will be slow (for example, the plating speed will be 0.8 nm / sec or less), which may result in plating leakage. On the other hand, if the phosphorus content of the nickel-copper alloy layer is 3.0 wt% or more, etching with an H2O2 / H2SO4 solution will clearly leave metal residue, which may result in excessive line width when the line is subsequently fabricated, which may lead to problems such as short circuits.

[0024] Furthermore, in the nickel-copper alloy layer of the present invention, the weight ratio of copper to nickel (i.e., copper weight / nickel weight) is greater than 1.3 and less than 2.3. Here, if the weight ratio of copper to nickel is 1.3 or less, there is a risk that metal will be clearly left behind when etching with a H2O2 / H2SO4 solution. On the other hand, if the weight ratio of copper to nickel is 2.3 or more, there is a risk that plating leakage will occur. Note that the copper content is preferably in the range of 50 to 65 wt% of the nickel-copper alloy layer.

[0025] On the other hand, in addition to nickel, copper, and phosphorus, any metal that can be plated together with nickel can be added to the nickel-copper alloy layer according to the required properties, and there are no particular limitations. Specifically, the nickel-copper alloy layer of the present invention may further contain at least one selected from the group consisting of molybdenum, tungsten, tin, chromium, and zinc. However, the nickel-copper alloy layer of the present invention does not contain magnetic iron or cobalt.

[0026] In addition, the nickel-copper alloy layer of the present invention can be a single plating layer, i.e., without combining with other layers, it can achieve the effect of no resonance absorption in the frequency range of 1 to 4 GHz, thereby reducing manufacturing costs. Furthermore, the thickness of the nickel-copper alloy layer metallized on one side is preferably greater than 60 nm and less than 90 nm, and the total thickness of the nickel-copper alloy layer metallized on both sides can be greater than 120 nm and less than 180 nm.

[0027] Next, in the present invention, a nickel-copper alloy layer is formed on at least one side of the polyimide substrate by electroless plating. For electroless plating, a roll-shaped polyimide substrate (purchased from TAIMIDE Technology, model number TX6-025) is first subjected to continuous hydrophilization treatment using a corona treatment machine (purchased from WEDGE Co., Ltd., Japan) under the operating conditions of a power of 3 kW and a speed of 3 m / min.

[0028] Furthermore, the electroless plating may be conventional electroless plating, and is not particularly limited, for example, reference may be made to the applicant's prior application (Taiwan Application No. TW112142862, the contents of which are incorporated herein by reference).

[0029] The hydrophilic polyimide substrate is then cut into 20 cm x 20 cm pieces and immersed in a 2 wt% KOH solution at 40°C for 150 seconds. A catalyst is then applied using the SLP metallization process (SLP process) developed by Okuno Pharmaceutical Co., Ltd. of Japan, to obtain a polyimide substrate with a palladium catalyst on one or both sides. The palladium catalyst used here is SLP-400, part of the SLP series of electroless nickel plating reagents.

[0030] Thereafter, the polyimide substrate having the palladium catalyst is subjected to the above-mentioned electroless plating treatment, thereby forming a nickel-copper alloy layer.

[0031] <<Copper layer>> The copper layer of the present invention is not particularly limited as long as it is a copper layer that can be used to form an etching line later. In one embodiment of the present invention, the copper layer is preferably formed on a nickel-copper alloy layer by electroplating. The electroplating solution that can be used for the copper layer may be a commercially available product, such as a copper sulfate electroplating solution (purchased from ALL-IN-LINE-CHEMICALS ENTERPRISE, Inc.). The thickness of the copper layer is preferably 0.2 to 20 μm.

[0032] Specifically, the electroplated copper layer can be formed by conventional methods, for example, see the applicant's prior application (Taiwan Application No. TW112142862, the contents of which are incorporated herein by reference), and an electroplated copper layer having a thickness of about 1 μm can be obtained on one or both sides.

[0033] Here, the thickness of the copper layer can be measured using a copper thickness measuring device (purchased from SHIN SHEN Co., Ltd.) Specifically, a 10 cm × 10 cm FCCL sample is placed on a measuring table, and a four-point probe is evenly brought into contact with the copper surface of the FCCL to measure the thickness of the copper layer.

[0034] <How to make a flexible copper foil board> First, please refer to Figure 2. Figure 2 is a flowchart of a manufacturing method for a flexible copper foil substrate 100 according to one embodiment of the present invention. As shown in Figure 2, the manufacturing method for a flexible copper foil substrate according to one embodiment of the present invention includes the steps of preparing a polyimide substrate 1, forming a nickel-copper alloy layer 2 on a first surface 11 of the polyimide substrate 1 by electroless plating, and forming a copper layer 3 on the nickel-copper alloy layer 2 on the opposite side of the polyimide substrate 1 by electroplating.

[0035] In one embodiment, the nickel-copper alloy layer 2 can be formed simultaneously on the first surface 11 and the second surface 12. In this case, the copper layer 3 is formed on the opposite side of each of the nickel-copper alloy layers 2 relative to the polyimide substrate 1.

[0036] Next, the electroless plating method and the electroplating method are not particularly limited and may be any known electroless plating method or electroplating method. Specifically, the electroless plating method or the electroplating method described above may be used, and detailed description thereof will be omitted here. [Example]

[0037] EXAMPLES The present invention will be described in detail below through examples and comparative examples, but the present invention is not limited to these examples and comparative examples.

[0038] (Measurement of elemental composition of nickel-copper alloy layer) Using a scanning electron microscope (SEM / EDS) from JIE DONG Co., Ltd., the sample of the polyimide substrate plated with a nickel-copper alloy layer without gold plating was placed directly in the SEM, and after vacuuming, the elemental composition of the nickel-copper alloy layer was analyzed within an area of ​​200 μm × 150 μm using EDS.

[0039] (Remaining plating layer (etchability)) Using a Taiwan Keyence microscope (VK-X3000), the etched flexible circuit board sample was placed directly on the analysis stage, and a 50x optical lens was used to observe the thinnest line zone (line width / line spacing = 25 / 25 μm) to check for any metal residues on its shape and periphery.

[0040] In the present invention, the case where there is no remaining metal is designated as ○, the case where there is a small amount of remaining metal is designated as △, and the case where there is clearly remaining metal (not cleanly etched) is designated as ×. Here, "no remaining metal" means that there are no traces of metal on the edge of the track, "a small amount of remaining metal" means that the width of the metal trace on the edge of the track is less than 2 μm, and "clearly remaining metal" means that the width of the metal trace on the edge of the track is greater than 2 μm.

[0041] (Circuit board insertion loss) First, a 25μm thick PI substrate is electrolessly plated with a nickel-copper alloy as a seed layer, and then a 12μm thick copper layer is electroplated on top of that to obtain a double-sided flexible copper foil substrate (FCCL).Next, this FCCL is used to fabricate a differential microstrip circuit board for loss testing, with a line width of 40-50μm, line height of 20-22μm, and resistance controlled to 100Ω±10%.

[0042] The differential microstrip used for the loss test has two types of signal lines, 2 inches and 10 inches in length, with a cover layer attached to the line and electroless nickel immersion gold (ENIG) surface treatment on the contacts. Before the official test, a network analyzer (Keysight Technologies, N5224B) must be used to check whether the resistance of the signal line is within 100±5Ω, and the transmission loss is measured in the frequency range of 10MHz to 43.5GHz using signal lines that pass the test.

[0043] Finally, connect the measurement head to the 2-inch and 10-inch signal lines under test, measure and record the signal loss of the 2-inch and 10-inch signal lines, respectively, and subtract the signal loss of the 2-inch signal line from the signal loss of the 10-inch signal line to determine the actual signal loss in the 8-inch signal line, excluding connector and other losses.

[0044] (Absorbance value) First, a polyimide (PI) substrate plated with a nickel-copper alloy layer of a specific thickness on both sides was prepared as a sample. It was immersed in a fast etching solution of H2O2 / H2SO4 at room temperature for 20 seconds, then removed, rinsed with water, and dried. Next, the remaining plating layer on the PI substrate was analyzed using a UV-Vis spectrometer (SUNWAY SCIENTIFIC CORPORATION, JASCO / V-750) in absorbance mode. Specifically, the absorbance value of the etched sample at a wavelength of 500 nm was measured using the unplated PI substrate as the reference. Furthermore, the higher the absorbance value, the more remaining plating layer, i.e., the more difficult it was to etch the nickel-copper alloy layer.

[0045] (sheet resistance) The PI substrate plated with the nickel-copper alloy layer was cut into 10cm x 10cm samples and placed on a measuring table. The sheet resistance was measured by uniformly contacting the nickel-copper alloy layer with a four-point probe using a low resistance analyzer (Loresta / MCP-T370, South & North International). The test data was the average value of five points.

[0046] (Thickness of nickel-copper alloy layer) X-ray film thickness measurement meter (TONG YUIN TECHNOLOGY ELECTRIC CO., LTD., FISCHERSCOPE (R) After calibration using XDL210 before measurement, the thickness of the nickel-copper alloy layer can be measured by cutting a 10cm x 10cm sample of a PI substrate plated with a nickel-copper alloy layer on one side and placing it on the measurement area. Here, the test data is the average value of 5 points.

[0047] (Relative permeability at 100MHz) After calibration before measurement using a resistance analyzer (Keysight Technologies, E4991B) and a dielectric material test jig (Keysight Technologies, 16453A), 200 to 300 circular coating samples with an outer diameter of 18 mm and an inner diameter of 5 mm are stacked on the electrodes of the jig, and the relative permeability can be measured in the frequency range of 1 kHz to 1 GHz by entering the sample size.

[0048] (Confirmation of through-hole metallization quality) Using a stereo microscope (URANUS TECHNOLOGY, Motic / SMZ-171TP), a sample with an electroless nickel-copper alloy layer is placed on the measurement stage, and a 5x optical lens is used to observe the metallization condition of the outer edge of the through-hole, which can be used to check whether there is any plating leakage on the outer edge of the through-hole.

[0049] In the present invention, the case where the PI substrate is not exposed at the outer edge of the through-hole is recorded as good, the case where the PI substrate is slightly exposed at the outer edge of the through-hole is recorded as partial plating leakage, and the case where the PI substrate is heavily exposed at the outer edge of the through-hole is recorded as severe plating leakage.

[0050] (comprehensive evaluation) If at least one of the characteristics in Tables 1 and 3 below is marked "X", the overall rating is "X", if at least one is marked "△", the overall rating is "△", and if all characteristics are marked "○" or meet the needs, the overall rating is "○".

[0051] Example 1 (Pretreatment of polyimide substrate) A roll of polyimide substrate (purchased from TAIMIDE Technology, model number TX6-025) was continuously hydrophilized using a corona treatment machine (purchased from WEDGE, Japan). The operating conditions were a power of 3 kW and a speed of 3 m / min. Next, the hydrophilized polyimide substrate was cut into a 20 cm x 20 cm piece and immersed in a 2 wt% KOH solution at 40°C for 150 seconds.

[0052] (Electroless nickel-copper alloy layer) First, as mentioned above, according to the SLP metallization process, the hydrophilic-treated polyimide substrate is subjected to the steps of charge adjustment, pre-soaking, catalyst activation, and acceleration in order to obtain a polyimide substrate with palladium catalysts on both sides.

[0053] Next, an electroless plating solution was poured into a 5-liter beaker (bath volume: 5 L). The electroless plating solution contained 86.0 g of nickel sulfate, 18.9 g of copper sulfate, 100 g of sodium hypophosphite (reducing agent), 300 g of sodium citrate (adjusting agent), and 120 g of boric acid (buffering agent), and was adjusted to pH 8.5 with 50 wt % NaOH solution.

[0054] The polyimide substrate with the palladium catalyst was then immersed in the electroless plating solution and reacted at 38°C for 90 seconds so that the total thickness of the plating layer on both sides was 160±10 μm.Then, it was removed, washed with water, and dried to form a polyimide substrate plated with a nickel-copper alloy layer (a ternary alloy containing nickel / copper / phosphorus) on both sides.

[0055] (Through-hole metallization of pre-drilled PI substrate) The following steps are followed to perform through-hole metallization of the pre-drilled PI substrate.

[0056] (1) Laser drilling: The PI substrate was cut to A4 size and placed on the suction stage of a UV laser processing machine. Holes were drilled to diameters of 30, 50, 100, 200, and 500 μm, with 100 through-holes in a 2 cm x 2 cm area for each diameter. The PI substrate was then immersed in a beaker containing a 5 wt% aqueous solution of isopropyl alcohol (IPA) and subjected to ultrasonic vibration for 5 minutes to clean the substrate surface and through-holes. The PI substrate was then removed and washed with water before proceeding to the next metallization step.

[0057] (2) Pretreatment: The PI substrate subjected to step (1) above is immersed in a 2 wt % KOH solution at 40°C for 150 seconds.

[0058] (3) Catalyst activation: As mentioned above, the catalyst is activated according to the SLP metallization process.

[0059] (4) Electroless plating: As mentioned above, a nickel-copper alloy layer is electrolessly plated onto the PI substrate.

[0060] (electroplated copper layer) The through-hole metallized PI substrate was fixed in a stainless steel frame and immersed in a 3 wt% H2SO4 solution for 1 minute to clean the surface. It was then placed in an electroplating bath for copper electroplating. The electroplating area was 15cm x 15cm, and the electroplating solution contained 150g / L H2SO4, 120g / L CuSO4, 50ppm chloride ion concentration, and appropriate amounts of brightener and leveler. The current conditions were 6A, voltage 3V, and time 2 minutes. The PI substrate was removed, washed with water, and dried, resulting in a flexible copper foil substrate with a copper thickness of approximately 1µm on both sides.

[0061] (Train track production) Next, referring to the applicant's prior application (Taiwan Application No.: TW112142862), the flexible copper foil substrate obtained in Example 1 can be sequentially subjected to conventional semi-additive processes such as pretreatment, lamination, exposure, development, copper plating, film removal, and rapid etching to produce a flexible circuit board with a line width / line spacing of 25 / 25 μm.

[0062] Here, rapid etching refers to fixing a semi-finished circuit that has undergone pattern electroplating with a plastic frame and placing it on a rapid etching line (purchased from FUJICHEM PRECISION MACHINERIES) to perform metal etching on the bottom layer. The rapid etching solution contains 5% by weight sulfuric acid, 10% by weight hydrogen peroxide, copper ions with a concentration of less than 30 g / L, and an appropriate amount of etching additive (purchased from JCU TAIWAN CORPORATION). The working speed is 0.9 m / min, and upon completion, a flexible circuit board with fine lines is obtained.

[0063] Finally, the metal concentrations in the electroless plating solution, the composition of the nickel-copper alloy layer, and the properties of the nickel-copper alloy layer in Example 1 are summarized in Tables 1 and 2. For the measurement methods of each property, please refer to the above content and will not be repeated here.

[0064] <Comparative Example 1> An electroless plating solution was poured into a 5-liter beaker (bath volume: 5 L). The electroless plating solution contained 107.5 g of nickel sulfate, 100 g of sodium hypophosphite, 200 g of sodium citrate, and 120 g of boric acid, and was adjusted to pH 8.5 with a 50 wt % NaOH solution. Next, the polyimide substrate with palladium catalysts on both sides was immersed in the electroless plating solution and reacted at 38°C for 95 seconds. After that, the substrate was rinsed with water and dried, resulting in a polyimide substrate plated with a nickel-phosphorus alloy on both sides to a total thickness of 160 ± 10 μm.

[0065] Thereafter, in the same manner as in Example 1, the pre-punched PI substrate is subjected to through-hole metallization, the copper layer is electroplated, and the wiring is fabricated, thereby obtaining the flexible copper foil substrate and flexible circuit board of Comparative Example 1.

[0066] <Comparative Examples 2 to 9 and Examples 2 to 5> Next, similarly to Example 1, flexible copper foil substrates and flexible circuit boards of Comparative Examples 2 to 9 and Examples 2 to 5 can be obtained by changing the copper to nickel weight ratio and phosphorus content in the electroless plating solution based on Table 1 below. The metal concentrations in the electroless plating solution, the composition of the nickel-copper alloy layer, and the properties of the nickel-copper alloy layer in Comparative Examples 2 to 9 and Examples 2 to 5 are also summarized in Tables 1 and 2.

[0067] [Table 1]

[0068] [Table 2]

[0069] First, as can be seen from Table 1, in Examples 1 to 5, the nickel-copper alloy layer is formed without electroplating. In the nickel-copper alloy layer, the weight ratio of copper to nickel is greater than 1.3 and less than 2.3, and the phosphorus content is greater than 2.1 wt % and less than 3.0 wt % of the nickel-copper alloy layer. Therefore, no metal remains in the nickel-copper alloy layer after etching, and the insertion loss of the manufactured FPC at a frequency of 2 GHz can be 0.2 dB / cm or less, that is, the insertion loss is reduced.

[0070] Furthermore, as shown in Table 1, the relative permeability at 100 MHz for the nickel-copper alloy layers of Examples 1 to 5 is less than 1. In other words, since the plating layer is non-magnetic, it can be seen that there is no additional insertion loss when transmitting at high frequencies.

[0071] Next, in Comparative Examples 1 to 5 and 8, the weight ratio of nickel to copper in the nickel-copper alloy layer was 1.3 or less, and the phosphorus content was 3.0 wt% or more in the nickel-copper alloy layer, so that in Comparative Examples 1 to 5 and 8, a small amount of metal remained after etching or the etching was not complete, which is considered to not meet the needs.In addition, in Comparative Examples 6 to 7 and 9, the weight ratio of nickel to copper in the nickel-copper alloy layer was 2.3 or more, and the phosphorus content was 2.1 wt% or less in the nickel-copper alloy layer, so that when the through-holes were metallized, partial or severe plating leakage occurred, making it impossible to measure the insertion loss.

[0072] Please refer to Figure 3. Figure 3 is a comparison diagram showing the insertion loss in Example 1 of the present invention and Comparative Examples 1 and 2. As can be seen from Figure 3, Example 1 of the present invention has a lower insertion loss than Comparative Examples 1 and 2. In particular, compared to Comparative Example 1, Example 1 of the present invention can significantly reduce the insertion loss in the frequency range of 1 to 4 GHz.

[0073] Also, please refer to Figure 4. Figure 4(a) is a photograph of a metallized through-hole in Comparative Example 7 of the present invention, Figure 4(b) is a photograph of a metallized through-hole in Comparative Example 6 of the present invention, and Figure 4(c) is a photograph of a metallized through-hole in Example 1 of the present invention. Furthermore, as shown in Figure 4(a), in Comparative Example 7, a large amount of the PI substrate is exposed at the outer edge of the through-hole, resulting in severe plating leakage. As shown in Figure 4(b), in Comparative Example 6, a small amount of the PI substrate is exposed at the outer edge of the through-hole, resulting in partial plating leakage. As shown in Figure 4(c), in Example 1, the PI substrate is not exposed at the outer edge of the through-hole, resulting in good plating.

[0074] Please refer to Figure 5. Figure 5(a) is a photograph of Comparative Example 3 of the present invention after line etching, and Figure 5(b) is a photograph of Example 1 of the present invention after line etching. Furthermore, as shown in Figure 5(a), in Comparative Example 3, the phosphorus content is excessive, so it is thought that there is metal residue of the nickel-copper alloy layer between the lines (see the area surrounded by the dotted line). On the other hand, as shown in Figure 5(b), it is thought that there is no metal residue between the lines in Example 1.

[0075] As shown in Table 1, in the electroless plating, under conditions where the metal salt concentration in the plating bath was 4.8 g / L, the reducing agent (sodium hypophosphite) concentration was 20 g / L, and the plating bath temperature was 38° C., the nickel-copper alloy layers of Examples 1 to 5 were all formed at a plating speed of more than 0.8 nm / sec. On the other hand, in Comparative Examples 6 to 7 and 9, the phosphorus content was too low and the plating speed was slower than 0.8 nm / sec, so a good nickel-copper alloy layer could not be obtained.

[0076] Furthermore, as can be seen from Table 2, the larger the absorbance value, the greater the remaining amount of the plating layer, i.e., the nickel-copper alloy layer is less likely to be etched and remains on the PI substrate. Specifically, in Comparative Examples 1 to 5 and 8, the absorbance value is 0.27% or more, which means that there is a small amount of metal remaining or the layer is not cleanly etched. Furthermore, as can be seen from Table 2, by making the total thickness of the nickel-copper alloy layer on both sides of each Example and Comparative Example greater than 120 nm and less than 180 nm, i.e., by making the thickness of a single layer greater than 60 nm and less than 90 nm, a good sheet resistance (for example, 10 Ω / dm 2 In the following test examples, the thickness of the plating layer is examined.

[0077] (Test example) First, as Test Examples 1 to 6, nickel-copper alloy layers having different plating layer thicknesses were formed based on the plating layer composition of Example 1 (i.e., 34.0 wt % Ni, 63.5 wt % Cu, and 2.5 wt % P) with reference to the following Table 3. In addition, the properties of the nickel-copper alloy layers of Test Examples 1 to 6 were measured according to the above-mentioned measurement methods, and the results are summarized in Table 3.

[0078] [Table 3]

[0079] As shown in Table 3, in Test Examples 1 and 2, the total thickness of the nickel-copper alloy layers on both sides was greater than 180 nm, and therefore the sheet resistance of Test Examples 1 and 2 was 10 Ω / dm 2 Although the total thickness of the nickel-copper alloy layers on both sides was less than 120 nm, the absorbance value of the PI substrate after etching was too high, resulting in a situation where etching was not complete or a small amount of metal remained. In addition, in Test Examples 5 and 6, since the total thickness of the nickel-copper alloy layers on both sides was less than 120 nm, although there was no metal remaining on the PI substrate after etching in Test Examples 5 and 6, the sheet resistance was too high (10 Ω / dm 2 (larger), there is an operational risk of using too high a voltage during continuous roll-to-roll electroplating.

[0080] On the other hand, in Test Examples 3 and 4, the total thickness of the nickel-copper alloy layers on both sides was greater than 120 nm and less than 180 nm, and therefore, in Test Examples 3 and 4, the sheet resistance was 10 Ω / dm 2 As can be seen from Tables 2 and 3, the thickness of the single nickel-copper alloy layer is preferably greater than 60 nm and less than 90 nm, and the total thickness of the nickel-copper alloy layers on both sides is preferably greater than 120 nm and less than 180 nm.

[0081] The present invention utilizes an electroless plating method and a specific nickel-copper alloy layer composition to reduce insertion loss in the frequency range of 1 to 4 GHz, thereby providing a flexible copper foil substrate suitable for high-frequency transmission.

[0082] Furthermore, in a preferred embodiment of the present invention, by controlling the thickness of the nickel-copper alloy layer, it is possible to obtain a nickel-copper alloy layer with good sheet resistance while leaving no metal residue on the PI substrate after etching, thereby making it possible to manufacture flexible copper foil substrates and flexible circuit boards suitable for high-frequency transmission.

[0083] In addition, in a preferred embodiment of the present invention, by controlling the plating speed of electroless plating, a nickel-copper alloy layer having well-metallized through-holes can be obtained, and thus flexible copper foil substrates and flexible circuit boards suitable for high-frequency transmission can be manufactured.

[0084] The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included within the technical scope of the present invention. [Explanation of symbols]

[0085] 1 Polyimide substrate 11 1st surface 12 Second surface 2 Nickel-copper alloy layer 3 copper layers 100 Flexible copper foil board

Claims

1. A flexible copper foil substrate having a polyimide substrate, a nickel-copper alloy layer, and a copper layer, the nickel-copper alloy layer contains nickel, copper, and phosphorus, and is formed on at least one surface of the polyimide substrate by electroless plating, the weight ratio of the copper to the nickel in the nickel-copper alloy layer is greater than 1.3 and less than 2.3, and the phosphorus content is greater than 2.1 wt % and less than 3.0 wt % of the nickel-copper alloy layer, The copper layer is formed on the nickel-copper alloy layer on the opposite side from the polyimide substrate, and is bonded to the nickel-copper alloy layer to form a metal conductive layer.

2. The flexible copper foil substrate according to claim 1 , wherein the nickel-copper alloy layer is a single plating layer.

3. The flexible copper foil substrate according to claim 1 , wherein the nickel-copper alloy layer has a single layer thickness of more than 60 nm and less than 90 nm.

4. The flexible copper foil substrate according to claim 1 , wherein the nickel-copper alloy layer has a relative magnetic permeability of less than 1 at a frequency of 100 MHz.

5. 2. The flexible copper foil substrate according to claim 1, wherein the nickel-copper alloy layer is formed at a plating rate of more than 0.8 nm / sec under the conditions of a metal salt concentration of 4.8 g / L, a reducing agent concentration of 20 g / L, and a plating bath temperature of 38°C in the electroless plating.

6. The nickel-copper alloy layer has a sheet resistance of 10 Ω / dm 2 The flexible copper foil substrate of claim 1 , wherein the thickness is less than 1 / 2 mm.

7. The flexible copper foil substrate according to any one of claims 1 to 6, wherein the copper layer is formed on the nickel-copper alloy layer by electroplating and has a thickness of 0.2 to 20 µm.

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