TW202415167A - Metal foil, carrier with metal layer comprising the same and printed circuit board comprising the same - Google Patents

Abstract

The present invention relates to a metal layer having a rough surface, a carrier-attached metal foil including the metal layer, and a printed circuit board manufactured using the metal layer. The metal layer includes a plurality of flat-topped projections and has a thickness of 1 μm or less. Each of the projections includes a protrusion having a truncated cone or polygonal truncated pyramidal shape and a plateau formed at the upper end of the protrusion. The projections are formed by electroless plating. The protrusion has a plurality of microprojections formed on the lateral surface thereof other than the plateau to enhance the adhesion to an insulating resin substrate due to its increased surface area.

Description

Metal foil, carrier having a metal layer containing the same, and printed circuit board containing the same

The present invention relates to a metal layer with a rough surface, a carrier-attached metal foil comprising the metal layer, and a printed circuit board manufactured using the metal layer.

Printed circuit boards are typically manufactured by bonding a metal foil to an insulating resin substrate and performing etching of the metal foil for circuit wiring. High adhesion strength is required between the metal foil and the insulating resin substrate to prevent the metal foil from peeling off during circuit wiring.

Many proposals have been made to improve the adhesion between a metal foil and an insulating resin substrate. For example, by roughening the surface of a metal foil to form irregularities thereon, placing an insulating resin substrate on the irregular surface of the metal foil, and pressing the insulating resin substrate to bond the insulating resin substrate to the metal foil, the adhesion strength between the metal foil and the insulating resin substrate is increased. Specifically, Patent Document 1 (Korean Patent Publication No. 2018-0019190) discloses a method for improving the adhesion between a copper foil and a resin layer by electrolyzing, sandblasting, or oxidation-reduction of a copper foil surface at a side of a resin layer to form particle protrusions.

However, this method has a problem in that additionally roughening the metal foil reduces the manufacturing efficiency of the printed circuit board. Furthermore, the formation of irregularities on the surface of the metal foil may reduce the transmission efficiency of high-frequency signals. With the trend toward higher performance of portable electronic devices, in order to quickly process a large amount of information, it is necessary to minimize the loss of high-frequency signal transmission. However, these irregularities make the surface of the metal foil highly rough and act as an obstacle to the transmission of high-frequency signals, resulting in poor high-frequency signal transmission efficiency.

Knowledge and technology documents Patent documents (0001) Korean Patent Publication No. 10-2018-0019190 (0002) Korean Patent Publication No. 10-2021-0016322

Technical Problem The present invention aims to provide a metal layer having high adhesion strength to an insulating resin substrate and capable of avoiding reduction in high-frequency signal transmission efficiency. The present invention also aims to provide a carrier-attached metal foil comprising a metal layer. The present invention also aims to provide a printed circuit board comprising the metal layer.

Technical means One aspect of the present invention provides a metal layer, which includes a plurality of protrusions with flat tops, each of the protrusions includes a projection and a plateau, the protrusion has a shape of a truncated cone or a polygonal truncated pyramid, the plateau is formed at the upper end of the protrusion, the protrusions are formed by electroless plating, and the metal layer has a thickness of 1 μm or less, wherein the protrusion has a plurality of micro-protrusions, the micro-protrusions are formed on the side surface other than the plateau of the protrusion, so as to improve the adhesion to an insulating resin substrate due to its increased surface area.

In one embodiment, there may be 80 to 2500 protrusions per unit area (μm²).

In one embodiment, the metal layer may have a surface roughness (Rz) of 0.20 to 0.80 μm.

In one embodiment, the metal layer may have a developed surface area ratio (SDR) of 110 to 165%.

In one embodiment, the side surface of the protrusions excluding the plateau may have a roughness (Ra) of 0.05 to 0.3 μm.

In one embodiment, the protrusion may form a plurality of micro-protrusions on its surface.

In one embodiment, the ratio of the height (b) of the protrusion to the bottom length (a) of the protrusion may be 0.4:1 to 1.5:1 (b:a).

In one embodiment, the ratio of the length of the plateau (c) to the length of the bottom of the protrusion (a) may be 0.1:1 to 0.7:1 (c/a).

In one embodiment, the shape of the polygonal truncated pyramid can be selected from the group consisting of a pentagonal truncated pyramid, a hexagonal truncated pyramid, a heptagonal truncated pyramid, and an octagonal truncated pyramid.

In one embodiment, the metal layer can be formed by direct electroplating on a substrate or by transfer forming a metal foil on a carrier.

The present invention also provides a carrier-attached metal foil, which includes a carrier, a release layer formed on the carrier, and a metal layer formed on the release layer.

The present invention also provides a substrate to which the surface morphology of the metal layer can be transferred.

Advantageous Effects According to the present invention, the top flat protrusion is formed on the surface of the metal layer, so that the metal layer has high adhesion strength to an insulating resin substrate and can minimize the loss of high-frequency signal transmission. The plurality of protrusions are naturally formed during electroless plating, avoiding the need for additional roughening for forming irregularities on the metal layer, which is different from the conventional technology. Therefore, the metal layer according to the present invention can enable the manufacture of printed circuit boards to be efficient.

The preferred embodiments of the present invention will now be described in detail. In the description of the present invention, when it is considered that the relevant technologies may unnecessarily obscure the essence of the present invention, their detailed explanation will be omitted. As used herein, the singular forms of "a", "an" and "the" are intended to also include plural forms, unless the context clearly indicates otherwise. In the present invention, it should be understood that terms such as "including" or "having" are intended to indicate the presence of features, numbers, steps, operations, actions, components, parts or combinations thereof disclosed in the specification, and are not intended to exclude the possibility that one or more other features, numbers, steps, operations, actions, components, parts or combinations thereof may exist or be added, or may be added. The individual steps of the method described herein may be performed in an order different from the order explicitly described. In other words, the individual steps may be performed in the same order as described, substantially simultaneously, or in reverse order.

The present invention is not limited to the embodiments shown and may be implemented in a variety of different forms. On the contrary, the disclosed embodiments are provided to make the disclosure thorough and complete and to fully convey the scope of the present invention to those of ordinary skill in the art to which the present invention belongs. In the drawings, the dimensions of the elements, such as width and thickness, may be exaggerated for clarity. The drawings are interpreted from the perspective of the observer. It will be understood that when an element is referred to as "on" another element, it can be directly on the other element, or there may be one or more intervening elements between them. Those of ordinary skill in the art to which the present invention belongs will understand that the present invention may be implemented in a variety of different forms without departing from the spirit and scope of the invention as defined by the attached claims. The same reference numerals represent substantially the same elements throughout the drawings.

As used herein, "and/or" covers the combination of multiple related items disclosed and any items in the multiple related items disclosed. In this specification, the description of "A or B" means "A", "B" or "A and B".

1 , the metal layer 100 of the present invention includes a plurality of protrusions 10 with flat tops. The protrusions 10 may be metal crystal particles protruding vertically upward from the surface of the metal layer 100. Specifically, each protrusion 10 may include a protrusion 11 and a plateau 12.

The protrusion 11 of the protrusion 10 is a portion protruding from the surface of the metal layer 100, and may have a truncated cone or a polygonal truncated pyramid shape. Specifically, the protrusion 11 has a truncated cone with a flat plane (side surface) or a polygonal truncated pyramid shape with an angled surface, as shown in FIG2. Such a shape can enhance the anchoring force of the metal layer and the insulating resin substrate, so that the metal layer 100 can be bonded to the insulating resin substrate with high adhesive strength. More specifically, the protrusion 11 may have at least one polygonal truncated pyramid shape, and the shape of the polygonal truncated pyramid is selected from the group consisting of a pentagonal truncated pyramid, a hexagonal truncated pyramid, a heptagonal truncated pyramid and an octagonal truncated pyramid.

Each of the protrusions 11 may have a plurality of micro-protrusions 11a to improve adhesion to the insulating resin substrate due to its increased surface area. The formation of the micro-protrusions 11a allows the protrusions 11 to have a surface roughness (Ra) of 0.05 to 0.3 μm, specifically 0.08 to 0.2 μm. Here, the surface roughness (Ra) of the protrusion 11 is defined as the roughness of the side surface of the protrusion 11 excluding the plateau 12.

Meanwhile, the ratio of the height (b) of each protrusion 11 to the bottom length (a) of the protrusion 11 can be in the range of 0.4:1 to 1.5:1 (b:a), specifically 0.6:1 to 1.2:1 (b:a). When the ratio (b:a) is within the above-defined range, the adhesion between the metal layer 100 and the insulating resin substrate can be improved, and the loss of high-frequency signal transmission can be minimized.

The plateau 12 of the protrusion 10 is a flat surface at the upper end of the protrusion 11. The plateau 12 may be the upper surface of the protrusion 11 having a truncated cone or a polygonal truncated pyramid shape. According to the known technology, particles protrude sharply or smoothly from the surface of the metal layer to form irregularities that make the surface of the metal layer highly rough. The formation of irregularities can enhance adhesion with the insulating resin substrate, but may result in loss of high-frequency signal transmission. In contrast, the plateau 12 forming the top surface (top) of the protrusion 10 enables the metal layer 100 of the present invention to have a relatively low surface roughness due to its flatness. The relatively low surface roughness minimizes the loss of high-frequency signal transmission. Specifically, the plateau 12 may have a circular, elliptical or polygonal shape. Fine irregularities may be densely formed to provide a flat surface, which can also be considered to be encompassed within the range of the plateau 12.

In each of the protrusions 10, the ratio of the length (c) of the plateau 12 to the base length (a) of the protrusion 11 can be in the range of 0.1:1 to 0.7:1, specifically 0.2:1 to 0.6:1. When the ratio (c:a) is within the above-defined range, the adhesion between the metal foil 100 and the insulating resin substrate can be improved, and the loss of high-frequency signal transmission can be minimized. The length (c) of the plateau 12 refers to the maximum length on the plane of the plateau 12.

The number of protrusions 10 per unit area (1 μm²) of the metal layer 100 may be 80 to 2500, taking into account the adhesion between the metal layer 100 and the insulating resin substrate, the transmission efficiency of high-frequency signals, and the circuit line resolution of the metal layer 100. If the number of protrusions per unit area is less than 80, the protrusions will increase in size and tend to fuse, with the result that the surface area of the metal layer may decrease, resulting in a decrease in adhesion strength. At the same time, if the number of protrusions per unit area exceeds 2500, the protrusions will decrease in size, with the result that the surface area of the metal layer may also decrease, resulting in a decrease in adhesion strength.

The protrusions 10 can be formed by electroless plating. Specifically, the protrusions 10 can be formed on the surface of the metal layer 100 by forming a metal seed layer by electroless plating, and then continuously growing crystalline grains on the metal seed layer. According to the known technology, irregularities are formed by additionally roughening the metal layer. In contrast, in the process of forming the metal layer 100 of the present invention, the plurality of protrusions 10 naturally form a rough surface. Thus, the need for additional roughening is avoided, enabling the formation of the metal layer 100 and/or the manufacture of the printed circuit board to be efficient. In addition, electroless plating makes the metal layer 100 thinner and more porous than electroplating.

The composition of the electroless plating solution used to form the metal layer 100 is not particularly limited and may include a metal ion source and a nitrogen-containing compound.

The metal ion source can be specifically a copper ion source selected from copper sulfate, copper chloride, copper nitrate, copper hydroxide, copper sulfamate and mixtures thereof. The metal ion source can be present in a concentration of 0.5 to 10 g/L, specifically 1 to 8 g/L.

The nitrogen-containing compound diffuses metal ions to form a plurality of protrusions 10 on the surface of the metal seed layer formed by the metal ion source. Specifically, the nitrogen-containing compound can be selected from the group consisting of purine, adenine, guanine, hypoxanthine, xanthine, pyridazine, methyl piperidine, 1,2-di-(2-pyridyl)ethylene, 1,2-di-(pyridyl)ethylene, 2,2'-dipyridylamine, 2,2'-bipyridyl, 2,2'-bipyrimidine, 6,6'-dimethyl-2,2'-bipyridyl, di-2-furanone, N,N,N',N'-tetraethylenediamine, 1,8-naphthyridine, 1,6-naphthyridine, tripyridine and mixtures thereof. The nitrogen-containing compound may be present in a concentration of 0.01 to 10 g/L, specifically 0.05 to 1 g/L.

The electroless plating solution may further include one or more additives selected from the group consisting of a chelating agent, a pH adjuster and a reducing agent.

Specifically, the chelating agent can be selected from the group consisting of tartaric acid, citric acid, acetic acid, apple acid, malonic acid, ascorbic acid, oxalic acid, lactic acid, succinic acid, potassium sodium tartrate, dipotassium tartrate, 2,4-imidazolidinone, 1-methyl 2,4-imidazolidinone, 1,3-dimethyl 2,4-imidazolidinone, 5,5-dimethyl 2,4-imidazolidinone, nitrilotriacetic acid, triethanolamine, ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, N-hydroxyethylenediaminetriacetic acid, pentahydroxypropyldiethylenetriamine and mixtures thereof. The chelating agent can be present in a concentration of 0.5 to 150 g/L, specifically 20 to 100 g/L.

Specifically, the pH adjuster can be selected from the group consisting of sodium hydroxide, potassium hydroxide, lithium hydroxide and mixtures thereof. The pH adjuster can adjust the pH value of the electroless plating solution to 8 or higher, specifically 10 to 14, more specifically 11 to 13.5.

Specifically, the reducing agent can be selected from the group consisting of formaldehyde, sodium hypophosphite, sodium hydroxymethanesulfinate, glyoxylic acid, borohydride, dimethylamine borane and mixtures thereof. The reducing agent can be present in a concentration of 1 to 20 g/L, specifically 5 to 20 g/L.

The conditions for forming the metal layer 100 by electroless plating can be appropriately adjusted according to the thickness of the metal layer 100. Specifically, the electroless plating temperature can be 20 to 60° C., specifically 30 to 40° C., and the electroless plating time can be 2 to 30 minutes, specifically 5 to 20 minutes.

The thickness of the metal layer 100 formed by electroless plating can be 5 μm or less, specifically 0.1 to 1 μm. The composition of the metal layer 100 is not particularly limited, and can be any known metal that can form a circuit layer of a printed circuit board. Specifically, the metal can be selected from the group consisting of copper, silver, gold, nickel, aluminum, and mixtures thereof.

The metal layer may have a surface roughness (Rz) in the range of 0.20 to 0.80 μm. The formation of the protrusions enables the metal layer to have an appropriate surface roughness (Rz) in the above-defined range, thereby ensuring significantly improved adhesion to the insulating resin substrate. The Rz of the metal layer below 0.2 μm may reduce the adhesion strength of the metal layer. At the same time, the Rz of the metal layer exceeding 0.8 μm, that is, excessively large protrusions, may also lead to reduced adhesion strength of the metal layer.

The metal layer may have a developed surface area ratio (SDR) of 110% to 165%. The developed surface area ratio is defined as the ratio of the actual surface area viewed from above to the unit area, and the developed surface area ratio is a numerical value representing the increase in surface area per unit area. The increased surface area results in an increase in the area of the metal layer to which the substrate will bond, which generally improves the adhesion of the metal layer to the substrate. In an attempt to increase the surface area of the metal layer above a predetermined level, cavities are formed in the metal layer or protrusions of uneven sizes are formed on the metal layer. However, this attempt also results in a decrease in the adhesion strength of the metal layer. If the SDR is less than 110%, a not-significantly increased surface area of the metal layer may not result in a significant improvement in adhesion strength. At the same time, if the SDR exceeds 165%, the adhesion strength may be reduced due to the above reasons.

The present invention also provides a carrier-attached metal foil including a metal layer. Specifically, the carrier-attached metal foil includes a carrier, a release layer and a metal layer, wherein the metal layer is the same as the above-mentioned metal layer. The carrier-attached metal foil will be described in detail below.

The carrier of the carrier-attached metal foil according to the present invention is used to avoid deformation of the metal layer during transportation or use of the carrier-attached metal foil. The carrier can be made of metals such as copper or aluminum. Alternatively, the carrier can be made of polymers such as polyethylene terephthalate (PET), polyphenylene sulfide (PPS) or Teflon. The thickness of the carrier can be specifically 10 to 50 μm.

The release layer design of the carrier-attached metal foil is used to easily remove the carrier from the carrier-attached metal foil bonded to the insulating resin substrate. The release layer can have a single-layer or multi-layer structure. Specifically, the release layer can have a single-layer structure, which contains a nitrogen-containing cyclic compound as an organic material and contains a metal selected from the group consisting of nickel, molybdenum, cobalt, phosphorus, manganese and iron. That is, the single layer is an organic/inorganic composite layer. Alternatively, the release layer can have a multi-layer structure, in which an organic layer is bonded to an alloy layer, the organic layer is composed of a nitrogen-containing cyclic compound, and the alloy layer includes one or more metals selected from the group consisting of nickel, molybdenum, cobalt, phosphorus, manganese and iron. The release layer may have a thickness of 30 nm to 1 μm.

The metal layer of the carrier-attached metal foil is the same as the above-mentioned metal layer, so the detailed description is omitted.

The carrier-attached metal foil of the present invention may further include an electrolytic metal layer formed on the metal layer to enhance the mechanical strength and electrical conductivity of the metal layer. The electrolytic metal layer may be composed of the same or different components as the metal layer.

The carrier-attached metal foil of the present invention may further include a rust-proof layer formed on the metal layer to protect the metal layer from rusting. For example, the rust-proof layer may include zinc or chromium.

The carrier-attached metal foil of the present invention may further include a diffusion barrier layer formed between the carrier and the alloy layer of the release layer to improve its performance. For example, the diffusion barrier layer may include nickel or phosphorus.

The carrier-attached metal foil of the present invention may further include an oxidation barrier layer formed between the organic layer of the release layer and the metal layer. For example, the oxidation barrier layer may include nickel or phosphorus.

The present invention also provides a printed circuit board manufactured using a metal layer. Specifically, the printed circuit board of the present invention includes a metal circuit layer and an insulating resin layer. The metal circuit layer is formed by directly electroplating the metal layer on a substrate or transferring a metal foil to a substrate to form a metal layer, which will be described below.

The metal circuit layer of the printed circuit board is a layer that forms a circuit line. The metal circuit layer can be formed by electroless plating of the metal layer directly on the substrate, as described above. The metal layer and the metal circuit using the metal layer can be formed by a suitable method known in the art.

The metal circuit layer is obtained by forming a circuit line on a metal foil. The metal foil ensures miniaturization and high resolution of the printed circuit board. Specifically, the printed circuit board of the present invention is manufactured by forming a laminate by combining an insulating resin substrate and a metal foil, and etching the laminate to form a circuit line on the metal foil. The metal layer for the metal foil is combined with an insulating resin substrate having a high adhesion strength, and the metal layer has a relatively small thickness, enabling a fine and high-resolution circuit line to be formed thereon. In addition, the circuit line formed on the metal foil has a high adhesion strength to the insulating resin substrate.

The method of forming the circuit line is not particularly limited. For example, the circuit line can be formed by a subtractive process, an additive process, a full additive process, a semi-additive process, or a modified semi-additive process.

The insulating resin layer of the printed circuit board is an insulating layer formed on the metal circuit layer. The insulating resin layer can be any suitable insulating resin substrate known in the art. Specifically, the insulating resin layer can be a substrate having a structure in which a known resin is impregnated in an inorganic or organic fiber. For example, the resin substrate can be a prepreg.

The printed circuit board of the present invention can be manufactured by using an insulating resin substrate or by a coreless process without using an insulating resin substrate.

A metal layer attached to an insulating resin substrate by transfer or electroplating can have an adhesion strength of 250 to 800 _gf /cm. Conventional metal layers include uneven protrusions or have inappropriate physical properties, which explains their low adhesion strength. Specifically, conventional metal layers are known to have an adhesion strength of up to 250 gf/cm. In contrast, the metal layer of the present invention has a higher adhesion strength than conventional metal layers because it has suitable physical properties and includes protrusions of uniform size.

The present invention also provides a substrate to which the surface morphology of the metal layer can be transferred.

Before use, the metal layer can be transferred to an insulating substrate. The insulating substrate is the same as described above. In this case, the metal layer can be removed after the shape of the protrusion is transferred to the substrate. Since the protrusion forms a depression of the same shape on the substrate, the shape transferred to the substrate has a roughness (Rz) and SDR value similar to that of the metal layer. The metal layer can be simply peeled off after transfer. Alternatively, since the presence of the protrusion gives the metal layer a high adhesion strength, the metal layer can be removed by etching after transfer.

After the surface morphology of the conventional metal layer is transferred, a large amount of etching is required because the protrusions formed on the metal layer are not uniform in shape. Thus, there is a limit to increasing the adhesion strength of the substrate to which the surface morphology of the metal layer is transferred. In contrast, since the protrusions of the metal layer according to the present invention are uniform in size, the metal layer can be removed from the substrate surface by a minimum amount of etching, thereby making it possible to make the substrate have high adhesion strength.

In detail, if the protrusions formed on the metal layer are uneven in size, etching should be performed until the largest protrusion in the reference is removed. However, in this case, since the smaller protrusions are etched before the largest protrusion is removed, etching is continued for a longer time than necessary on the portion of the metal layer from which the smaller protrusions have been removed, resulting in low surface roughness. That is, if the protrusions are uneven in size, it may be difficult to obtain a surface roughness higher than a predetermined level even after the surface morphology transfer on the substrate is completed, resulting in a decrease in adhesion strength.

In contrast, since the protrusions of the metal layer of the present invention are of constant size, as shown in FIG3 , they can be etched away almost uniformly. Therefore, the protrusions can be prevented from being overetched, thereby minimizing the reduction in surface roughness. Therefore, the substrate of the present invention has a higher adhesion strength than the substrate of the previous invention.

Invention Mode The present invention will be explained more specifically with reference to the following examples. However, these examples are provided for illustrative purposes only and are not intended to limit the scope of the present invention. A person having ordinary knowledge in the technical field to which the present invention belongs will understand that various modifications and changes can be made without departing from the scope and spirit of the present invention.

[Example 1-27] A copper (Cu) foil carrier is bonded to a release layer (alloy layer composed of nickel and molybdenum + organic layer composed of sodium nitrobenzotriazole) to prepare a laminate. The laminate is electrolessly plated in an electroless plating bath to form a metal layer (copper layer) on the release layer. The electroless plating solution used for electroless plating contains CuSO ₄ ·5H ₂ O as a metal ion source (A), guanosine as a nitrogen-containing compound (B), potassium sodium tartrate as a chelating agent (C), NaOH as a pH adjuster (D), and formaldehyde as a reducing agent (E). The concentrations of the metal ion source, nitrogen-containing compound, chelating agent, pH adjuster and reducing agent and the electroless plating conditions were adjusted as shown in Table 1 to form the metal foil. The size and number of protrusions in the metal foil were measured. In Table 1, all concentrations are expressed in g/L.

[Table 1] A B C D E Electroplating conditions Temperature(℃) Time (seconds) Example 1 0.1 0.05 70 15 20 30 10 Example 2 0.5 0.05 70 15 20 30 10 Example 3 1 0.05 70 15 20 30 10 Example 4 5 0.05 70 15 20 30 10 Example 5 8 0.05 70 15 20 30 10 Example 6 10 0.05 70 15 20 30 10 Example 7 12 0.05 70 15 20 30 10 Example 8 5 0.001 70 15 20 30 10 Example 9 5 0.01 70 15 20 30 10 Example 10 5 0.05 70 15 20 30 10 Example 11 5 1 70 15 20 30 10 Example 12 5 5 70 15 20 30 10 Example 13 5 10 70 15 20 30 10 Example 14 5 15 70 15 20 30 10 Example 15 5 0.05 70 15 20 10 10 Example 16 5 0.05 70 15 20 20 10 Example 17 5 0.05 70 15 20 30 10 Example 18 5 0.05 70 15 20 40 10 Example 19 5 0.05 70 15 20 60 10 Example 20 5 0.05 70 15 20 80 10 Example 21 5 0.05 70 15 20 30 1 Example 22 5 0.05 70 15 20 30 2 Example 23 5 0.05 70 15 20 30 5 Example 24 5 0.05 70 15 20 30 10 Example 25 5 0.05 70 15 20 30 20 Example 26 5 0.05 70 15 20 30 30 Example 27 5 0.05 70 15 20 30 40

Comparative Example 1 A metal layer (copper layer) was formed in the same manner as in Example 4, except that electroless plating was performed in an electroless plating solution containing 8-15 g/L of CuSO ₄ ·5H ₂ O and NiSO ₄ ·6H ₂ O as metal ion sources, 0.5-0.8 g/L of 2,2-bipyridine as a nitrogen-containing compound, 60-80 g/L of sodium potassium tartrate as a chelating agent, and 28% formaldehyde as a reducing agent, at 34° C. for 20 minutes.

Test Example 1 The surface and cross section of the metal layer formed in Example 1 and Comparative Example 1 were observed to compare the shapes of the protrusions formed in the metal foil.

The surface and cross section of the metal layer were analyzed using a scanning electron microscope (SEM) and an ion beam cross section polisher (CP). The results are shown in Figure 3.

3 , the innovative metal foil of Example 1 has a plurality of surface protrusions with flat tops, while the metal foil of Comparative Example 1 has a plurality of surface protrusions with sharp tops.

It was concluded that the use of a plating solution having the same composition as that used in Example 1 enabled the formation of protrusions having flat tops.

Test Example 2 The size of the protrusions of the metal foil formed in Example 1-27, as well as the Rz, surface roughness, and SDR values were measured as follows.

The surface of each metal foil was observed using a field emission scanning electron microscope (FE-SEM, model SU-8010, Hitachi) at a magnification of 50,000× and the protrusion size per unit area was measured.

The 10-point average surface roughness (Rz) of each metal layer was measured using an atomic force microscope (AFM, model Multimode IVa, Brooke) at a scan size of 10 μm and a scan rate of 0.5 Hz.

The roughness (Ra) of the protrusion side surface was measured using an atomic force microscope (AFM, model Multimode IVa, Brooke) at a scan size of 1 μm and a scan rate of 0.5 Hz.

The surface area of each metal layer was measured using an atomic force microscope (AFM, model Multimode IVa, Brook) at a scan size of 10 μm and a scan rate of 0.5 Hz to determine the SDR of the metal layer.

[Table 2] Protrusions (number/μm ² ) Rz (μm) Ra (μm) SDR (%) Example 1 3158 0.1667 0.013 101.61 Example 2 2416 0.2217 0.059 114.35 Example 3 1108 0.2948 0.071 121.11 Example 4 981 0.3219 0.085 126.24 Example 5 718 0.4997 0.104 121.01 Example 6 168 0.2679 0.226 111.54 Example 7 79 0.1158 0.315 110.15 Example 8 - 0.0115 - 100.78 Example 9 1280 0.1684 0.067 115.11 Example 10 981 0.3219 0.085 126.24 Example 11 894 0.3341 0.095 122.49 Example 12 574 0.4468 0.067 119.05 Example 13 137 0.2208 0.059 115.16 Example 14 118 0.1884 0.046 111.00 Example 15 - 0.0108 - 100.55 Example 16 1649 0.2647 0.068 116.47 Example 17 981 0.3219 0.085 126.24 Example 18 618 0.3349 0.146 114.80 Example 19 463 0.2249 0.667 115.16 Example 20 184 0.1637 0.846 110.00 Example 21 2687 0.1982 0.035 108.15 Example 22 2451 0.2144 0.068 114.14 Example 23 1138 0.2864 0.070 118.60 Example 24 981 0.3219 0.085 126.24 Example 25 155 0.5630 0.137 116.84 Example 26 92 0.7360 0.498 110.11 Example 27 81 0.8355 0.849 108.46

It can be seen from the results in Table 2 that the conditions applied in Examples 4, 10, 17 and 24 were most effective in increasing the surface area of the metal layer. In addition, the number of protrusions of the metal layer formed in Example 4 and the Rz and Ra values of the metal layer were also found to be appropriate.

It was observed that the physical properties of the metal layers formed in Examples 1-7 changed as the concentration of the metal ion source changed. When the concentration of the metal ion source was low, the protrusions were not formed smoothly. Meanwhile, when the concentration of the metal ion source exceeded a predetermined level, the metal ion source had an insignificant effect on increasing the surface area of the metal layer. These results appear to be due to the growth and fusion of the metal protrusions, demonstrating that when the concentration of the metal ion source exceeded a predetermined level, the Rz value of the metal layer decreased.

It was observed that the physical properties of the metal layers formed in Examples 8-14 changed as the concentration of the nitrogen-containing compound changed. When the concentration of the nitrogen-containing compound was lower than a predetermined level, the protrusions were not formed smoothly (Example 8). Meanwhile, when the concentration of the nitrogen-containing compound exceeded the predetermined level, the roughness of the side surface of the protrusions was not high enough (Examples 13-14). These results lead to the conclusion that the use of a nitrogen-containing compound at a concentration exceeding the predetermined level results in insufficient protrusion formation and poor physical properties of the metal layer.

The temperature-dependent differences in the physical properties of the metal layers formed in Examples 15-20 were studied. When the temperature was low, the protrusions were not easily formed and grown. Meanwhile, when the reaction was performed at an excessively high temperature, excessive growth and fusion of the protrusions were observed. Specifically, the growth and fusion of lateral protrusions at high temperatures resulted in high Ra values, but the surface area of the metal layer could not be effectively increased.

The reaction time-dependent differences in the physical properties of the metal layers formed in Examples 21-27 were studied. As the reaction time increased, a large number of protrusions grew and fused. However, after the reaction time was increased to exceed a predetermined level, the protrusions tended to fuse, causing their number to decrease. In summary, a reaction time exceeding a predetermined level would reduce its effect on increasing the surface area of the metal layer. As the reaction time increased, the lateral protrusions also fused. In summary, the surface area of the metal layer would be effectively increased only when the reaction time was lower than a predetermined level.

[Example 28-44]

Electroplating was performed to form a metal layer having the number of protrusions and the Rz and Ra values shown in Table 3 to study the effects of the number of protrusions and the Rz and Ra values of the metal layer on the adhesion strength and high-frequency signal transmission performance of the metal layer. The protrusions were formed in the same manner as in Example 4, except that the reaction time and the concentration of the nitrogen-containing compound were changed.

[table 3] Protrusions (number/μm ² ) Rz (μm) Ra (μm) Example 28 76 0.1668 0.064 Example 29 95 0.1749 0.059 Example 30 249 0.1744 0.069 Example 31 939 0.2367 0.084 Example 32 1275 0.1628 0.064 Example 33 2676 0.1649 0.066 Example 34 919 0.1116 0.058 Example 35 946 0.2490 0.069 Example 36 944 0.3598 0.074 Example 37 926 0.6870 0.085 Example 38 918 0.8599 0.081 Example 39 911 0.3468 0.041 Example 40 938 0.3119 0.059 Example 41 967 0.3496 0.084 Example 41 916 0.3331 0.129 Example 43 936 0.3198 0.294 Example 44 918 0.3448 0.448

Test Example 3

The following procedures were performed to determine the physical properties and high frequency signal transmission performance of the metal layers formed in Examples 28-44.

The SUS plate, kraft paper, release film, insulating resin substrate (DS-7409HG), each metal layer (including the release layer) formed in Examples 1 to 7 and Comparative Example 1, kraft paper, and SUS plate were laminated in this order and vacuum compressed at a pressure of 3.5 MPa and a temperature of 200° C. for 100 minutes to prepare a laminate. After the kraft paper and SUS plate in the laminate were removed through the release layer, the peel strength between the metal foil and the insulating resin substrate was evaluated by the IPC-TM-650 test method (BMSP-90P peel tester, test speed: 50 mm/min, test speed: 90°).

Copper foil (18 μm thick), insulating resin substrate (DS-7402, 50 μm), and each metal layer formed in Examples 1-7 and Comparative Example 1 were laminated in this order, and wires (width: 40 μm, length: 10 cm) were formed on the metal foil by mSAP. An insulating resin substrate (DS-7402) and copper foil were laminated on the metal foil forming the wire, and through holes were processed to manufacture a printed circuit board. The high-frequency signal transmission performance of the printed circuit board was measured using PNA N5225A (KEYSIGHT). The S21 parameter (dB/cm) was measured at 10 MHz to 40 GHz. The results are shown in Table 4.

The same test was performed on Comparative Example 1 to determine the effect of the metal layer without the surface plateau.

[Table 4] Adhesion strength (g _f /cm) High frequency signal transmission performance 10 GHz 20 GHz 30 GHz 40 GHz Example 28 86.44 0.750 1.334 1.847 2.446 Example 29 186.49 0.764 1.241 1.890 2.449 Example 30 519.46 0.795 1.344 1.900 2.557 Example 31 683.16 0.768 1.348 1.907 2.507 Example 32 448.55 0.817 1.359 1.998 2.648 Example 33 168.11 0.829 1.448 2.007 2.667 Example 34 88.49 0.751 1.310 1.888 2.411 Example 35 384.55 0.749 1.334 1.890 2.448 Example 36 685.49 0.765 1.347 1.900 2.501 Example 37 719.57 0.789 1.447 1.994 2.687 Example 38 721.44 0.866 1.505 2.058 2.994 Example 39 318.66 0.719 1.338 1.894 2.448 Example 40 495.46 0.774 1.345 1.884 2.455 Example 41 684.55 0.745 1.344 1.895 2.469 Example 42 557.48 0.746 1.349 1.886 2.455 Example 43 219.55 0.759 1.359 1.998 2.569 Example 44 201.40 0.807 1.447 2.049 2.778 Comparison Example 1 284.48 0.868 1.547 2.118 2.997

The amount of flat areas on the surface increases as the number of protrusions decreases (Examples 28-33), resulting in improved high-frequency signal transmission performance. However, when the number of protrusions is less than a predetermined level (Examples 28 and 29), the adhesion strength of the metal layer decreases. Meanwhile, when the number of protrusions exceeds a predetermined level, no fusion of the protrusions occurs, resulting in a decrease in the amount of top flat areas, resulting in decreased adhesion strength and signal transmission performance.

For the metal layers of Examples 34-38 with similar protrusion numbers, the adhesion strength decreases as Rz decreases. Increasing Rz results in high adhesion strength but poor signal transmission performance.

When Ra is low (Example 39), the adhesion strength decreases but the signal transmission performance slightly increases. Increasing Ra results in high adhesion strength but poor signal transmission performance. From these results, it is believed that higher Ra leads to the formation of a greater number of lateral protrusions protruding outward from the surface.

The metal layer without surface plateaus (Comparative Example 1) has inferior physical properties compared to the innovative metal layer.

Although the specific contents of the present invention have been described in detail, it is obvious to those with ordinary knowledge in the technical field to which the present invention belongs that these specific contents are only preferred embodiments and are not intended to limit the scope of the present invention. Therefore, the true scope of the present invention is defined by the appended claims and their equivalents.

100: metal layer 10: protrusion 11: bulge 11a: micro protrusion 12: platform a: bottom length b: height c: length

FIG. 1 shows a portion of the surface of a metal layer according to an embodiment of the present invention. FIG. 2 shows a possible shape of a protrusion of a metal layer according to an embodiment of the present invention. FIG. 3 shows the results of scanning electron microscopy (SEM) and ion beam cross-sectional polishing (CP) of the metal layers of Example 1 and Comparative Example 1 in Test Example 1.

without

100:Metal layer

10: Protrusions

11: Bump

11a: Microprotrusions

12: High platform

a: Bottom length

b:Height

c: Length

Claims (12)

A metal layer comprises a plurality of protrusions with flat tops and a plateau, wherein each of the protrusions comprises a projection having a shape of a truncated cone or a polygonal truncated pyramid, the plateau is formed at the upper end of the protrusion, and the protrusions are formed by electroless plating, the metal layer has a thickness of 1 μm or less, wherein the protrusion has a plurality of micro-projections formed on the side surface of the protrusion other than the plateau to improve adhesion to an insulating resin substrate due to its increased surface area. The metal layer as described in claim 1, wherein the number of protrusions is 80 to 2500 per unit area (μm²). The metal layer as described in claim 1, wherein the metal layer has a surface roughness (Rz) of 0.20 to 0.80 μm. The metal layer of claim 1, wherein the metal layer has a developed surface area ratio (SDR) of 110 to 165%. The metal layer as described in claim 1, wherein the side surface of the protrusions except the plateau has a roughness (Ra) of 0.05 to 0.3 μm. A metal layer as claimed in claim 1, wherein the protrusion forms a plurality of micro-protrusions on its surface. The metal layer as described in claim 1, wherein the ratio of the height (b) of the protrusion to the bottom length (a) of the protrusion is 0.4:1 to 1.5:1 (b:a). The metal layer as described in claim 1, wherein the ratio of the length (c) of the plateau to the bottom length (a) of the protrusion is 0.1:1 to 0.7:1 (c/a). The metal layer as described in claim 1, wherein the shape of the polygonal truncated pyramid is selected from the group consisting of a pentagonal truncated pyramid, a hexagonal truncated pyramid, a heptagonal truncated pyramid and an octagonal truncated pyramid. The metal layer as described in claim 1, wherein the metal layer is formed by directly electroplating on a substrate or transferring a metal foil formed on a carrier. A carrier-attached metal foil comprises a carrier, a release layer formed on the carrier, and a metal layer as described in any one of claims 1 to 10 formed on the release layer. A substrate, wherein the surface morphology of the metal layer as described in any one of claims 1 to 10 is transferred to the substrate.