Metal foil provided with electrically resistive layer, and board for printed circuit using said metal foil

Abstract

Metal foil provided with an electrically resistive layer, characterized in that an alloy (in particular, a NiCrAlSi alloy) resistive layer containing 1 to 6 mass % of Si is formed on the metal foil controlled to have a ten-point average roughness Rz, which was measured by an optical method, of 4.0 to 6.0 m, and the variation in the resistance value of the electrically resistive layer is within 10%. Provided is a copper foil that allows embedding of a resistive material in a board by further forming an electrically resistive layer on the copper foil, and further allows improving the adhesiveness and suppressing the variation in resistance value within a certain range. As needed, metal foil provided in advance with a copper-zinc alloy layer formed on the surface thereof and a stabilizing layer composed of at least one component selected from zinc oxide, chromium oxide, and nickel oxide formed on the copper-zinc alloy layer is used.

Claims

1. Metal foil provided with an electrically resistive layer, characterized in that an alloy resistive layer containing 1 to 6 mass % of Si is formed on the metal foil controlled to have a ten-point average roughness Rz, which was measured by an optical method, of 4.0 to less than 6.0 m, and the variation in the resistance value of the electrically resistive layer is within 10%, wherein when a resin substrate is laminated to the metal foil provided with an electrically resistive layer, peel strength as received between the electrically resistive layer and the resin substrate is 0.60 kN/m or more.

2. The metal foil provided with an electrically resistive layer according to claim 1, characterized in that the alloy resistive layer containing 1 to 6 mass % of Si is a NiCrAlSi alloy resistive layer.

3. The metal foil provided with an electrically resistive layer according to claim 2, characterized in that the metal foil is provided with a copper-zinc alloy layer formed on the metal foil surface, a stabilizing layer composed of at least one component selected from zinc oxide, chromium oxide, and nickel oxide formed on the copper zinc alloy layer, and the alloy resistive layer formed on the stabilizing layer.

4. The metal foil provided with an electrically resistive layer according to claim 3, characterized in that the copper-zinc alloy layer has a zinc content per unit area of 1000 to 9000 g/dm.sup.2, and the stabilizing layer has a thickness within a range of 0.5 to 10 nm.

5. The metal foil provided with an electrically resistive layer according to claim 4, characterized in that the metal foil is a copper or copper alloy foil having a thickness of 5 to 35 m.

6. The metal foil provided with an electrically resistive layer according to claim 5, characterized in that the electrically resistive layer is formed on a rolled copper foil or a glossy surface of an electrolytic copper foil, controlled by being subjected to roughening treatment to have a ten-point average roughness Rz, which was measured by an optical method, of 4.0 to less than 6.0 m.

7. A printed circuit board in which the metal foil according to claim 6 is bonded to a resin substrate, characterized in that the peel strength between the electrically resistive layer and the resin substrate is 0.60 kN/m or more.

8. The metal foil provided with an electrically resistive layer according to claim 1, characterized in that the metal foil is provided with a copper-zinc alloy layer formed on the metal foil surface, a stabilizing layer composed of at least one component selected from zinc oxide, chromium oxide, and nickel oxide formed on the copper zinc alloy layer, and the alloy resistive layer formed on the stabilizing layer.

9. The metal foil provided with an electrically resistive layer according to claim 8, characterized in that the copper-zinc alloy layer has a zinc content per unit area of 1000 to 9000 g/dm.sup.2, and the stabilizing layer has a thickness within a range of 0.5 to 10 nm.

10. The metal foil provided with an electrically resistive layer according to claim 1, characterized in that the metal foil is a copper or copper alloy foil having a thickness of 5 to 35 m.

11. The metal foil provided with an electrically resistive layer according to claim 1, characterized in that the electrically resistive layer is formed on a surface of the copper or copper-alloy foil, which is a surface of a rolled copper foil or a glossy surface of an electrolytic copper foil, controlled by being subjected to roughening treatment to have a ten-point average roughness Rz, which was measured by an optical method, of 4.0 to less than 6.0 m.

12. A metal foil according to claim 1, further comprising a resin substrate to which the metal foil is bonded, wherein peel strength between the electrically resistive layer and the resin substrate is 0.60 kN/m or more.

Description

DESCRIPTION

(1) The electrolytic copper foil can be produced with a conventional electrolytic apparatus. The apparatus includes a cathode drum disposed in an electrolytic bath containing an electrolytic solution. The cathode drum is rotatable in a state of being partially (appropriately a half) immersed in the electrolytic solution.

(2) An insoluble anode is disposed so as to surround the lower half part of the circumference of the cathode drum and have a certain gap between the cathode drum and the anode. The electrolytic solution flows in the gap.

(3) Usually, the electrolytic solution is supplied from the bottom part and passes through the gap between the cathode drum and the anode and then overflows from the upper edge of the anode. The electrolytic solution is further circulated.

(4) A rectifier maintains a predetermined voltage between the cathode drum and the anode.

(5) The copper electrodeposited from the electrolytic solution increases in thickness with the rotation of the cathode drum. The raw foil (electrolytic copper foil) that has obtained a certain thickness is peeled and is continuously wound.

(6) The thickness of the thus-produced raw foil is controlled by the distance between the cathode drum and the anode, the flow rate of the supplied electrolytic solution, or the quantity of electricity for the electrolysis. In addition, the conditions of the rough surface of the electrolytic copper foil can be controlled by the electrolytic solution composition and the electrolytic conditions.

(7) The copper foil produced by such a copper foil-producing apparatus has a mirror surface (glossy surface) as the surface being in contact with the cathode drum and a rough surface (mat surface) having asperity as the surface on the opposite side. The electrolytic copper foil can have an arbitrary thickness, usually, a thickness of 5 to 35 m.

(8) The thus-produced copper foil is subjected to a cleaning step for removing the oxide film on the surface and then a water washing step. In the cleaning step, an aqueous solution of 10 to 80 g/L of sulfuric acid is usually used.

(9) A process of producing electrolytic copper foil has been described above. Regarding rolled copper foil, an ingot obtained through melting and casting is subjected to annealing, hot-rolling, and then cold-rolling to produce copper foil having an intended thickness. In the rolled copper foil, the surface that has been brought into contact with the rolling roll becomes a glossy surface. Accordingly, roughening treatment is performed as needed. The roughening treatment may be any known roughening treatment.

(10) An example of the roughening treatment is shown below. The following roughening treatment can also be applied to the glossy surface of the electrolytic copper foil. However, excessive roughening treatment should be avoided in every case, and the roughness is required to be strictly and constantly controlled such that the ten-point average roughness Rz measured by an optical method is 4.0 to 6.0 m. The ten-point average roughness can be measured with, for example, an optical interference surface shape measuring apparatus, specifically, a non-contact three-dimensional surface shape roughness measuring system, model NT1100 (WYKO Optical Profiler (resolution: 0.2 m0.2 m or less): manufactured by Veeco Instruments, Inc.).

(11) Cu ion concentration: 10 to 30 g/L

(12) Sulfuric acid concentration: 20 to 100 g/L

(13) Electrolytic solution temperature: 20 C. to 60 C.

(14) Current density: 5 to 80 A/dm.sup.2

(15) Treatment time: 0.5 to 30 seconds

(16) The thus-produced electrolytic copper foil or rolled copper foil is subjected to a zinc-copper alloy plating process as needed to enhance the heat resistance and the adhesiveness between the copper foil and the surface layer. The bath composition and the electroplating conditions for the zinc-copper alloy plating process are as follows:

(17) (Zinc-Copper Alloy Plating Bath Composition and Treatment Conditions)

(18) Bath composition: CuCN: 60 to 120 g/L Zn(CN).sub.2: 1 to 10 g/L NaOH: 40 to 100 g/L Na(CN): 10 to 30 g/L

(19) pH: 10 to 13

(20) Bath temperature: 60 C. to 80 C.

(21) Current density: 100 to 10000 A/dm.sup.2

(22) Treatment time: 2 to 60 seconds

(23) As a result, a copper-zinc alloy layer having a zinc content per unit area of 1000 to 9000 g/dm.sup.2 can be formed. The electroplating is suitable for zinc-copper alloy plating. The conditions are not limited to the above as long as a copper-zinc alloy layer having a zinc content per unit area of 1000 to 9000 g/dm.sup.2 can be formed.

(24) Accordingly, a copper-zinc alloy layer may be formed by zinc-plating on copper and then heat diffusion of the plating product. In general, as long as zinc plating is formed, a copper-zinc alloy layer is formed by heat diffusion due to the heat during pressing, and such heat diffusion can be utilized. A suitable example of the zinc plating is shown below.

(25) (Zinc-Plating Bath Composition and Plating Conditions)

(26) Bath composition ZnSO.sub.4.7H.sub.2O: 50 to 350 g/L

(27) pH: 2.5 to 4.5

(28) Bath temperature: 40 C. to 60 C.

(29) Current density: 5 to 40 A/m.sup.2

(30) Treatment time: 1 to 30 seconds

(31) Subsequently, a stabilizing layer composed of at least one component selected from zinc oxide, chromium oxide, and nickel oxide and having a thickness within a range of 0.5 to 10 nm is formed as needed on the copper foil or the zinc-copper alloy layer to provide functions of preventing rust, i.e., preventing the oxidation corrosion of the copper foil, preventing the dielectric base material from being decomposed by copper, and maintaining stable peel strength.

(32) In an embodiment, a coating layer can be formed using an electrolytic solution containing zinc ions and chromium ions. Examples of the source of zinc ions in the electrolytic solution include ZnSO.sub.4, ZnCO.sub.3, and ZnCrO.sub.4. Examples of the source of chromium ions in the electrolytic solution include hexavalent chromium salts or compounds such as ZnCrO.sub.4 and CrO.sub.3.

(33) The concentration of the zinc ions in the electrolytic solution is within a range of 0.1 to 2 g/L, preferably 0.3 to 0.6 g/L, and more preferably 0.45 to 0.55 g/L. The concentration of the chromium ions in the electrolytic solution is within a range of 0.5 to 5 g/L, preferably 0.5 to 3 g/L, and more preferably 1.0 to 3 g/L. These conditions are those for efficient plating, and the coating layer may be formed under conditions out of the conditions mentioned above, as needed.

(34) In another embodiment, the stabilizing layer can be formed by coating with nickel oxide and nickel metal; or zinc oxide; or chromium oxide; or a combination thereof. Examples of the source of nickel ions of the electrolytic solution include Ni.sub.2SO.sub.4, NiCO.sub.3, and a combination thereof.

(35) The concentration of the nickel ions in the electrolytic solution is preferably 0.2 to 1.2 g/L. Furthermore, a stabilizing layer containing phosphorus as described in U.S. Pat. No. 5,908,544 can also be used. These conditions are those for efficiently forming a stabilizing layer composed of at least one component selected from zinc oxide, chromium oxide, and nickel oxide and having a thickness within a range of 0.5 to 10 nm, and the stabilizing layer may be formed under conditions out of the conditions mentioned above, as needed.

(36) The electrolytic solution can contain other conventional additives such as Na.sub.2SO.sub.4 within a range of 1 to 50 g/L, preferably 10 to 20 g/L, and more preferably 12 to 18 g/L. The electrolytic solution generally has a pH of 3 to 6 and preferably 4 to 5.

(37) The electrolytic solution preferably has a temperature of 20 C. to 100 C. and preferably 25 C. to 45 C.

(38) In the formation of the stabilizing layer, for example, in order to apply a current density to the copper foil, an anode is arranged adjacent to each side of the copper foil. By application of a voltage to the anode, a stabilizing layer, for example, consisting of zinc oxide and chromium oxide, is deposited on the exposing surface of the copper foil.

(39) The current density is within a range of 1 to 100 A/ft.sup.2 (about 10.8 to about 1080 A/m.sup.2), preferably 5 to 25 A/ft.sup.2 (about 55 to about 270 A/m.sup.2), and more preferably 7 to 15 A/ft.sup.2 (about 85 to about 160 A/m.sup.2). In a case of arranging a plurality of anodes, the current densities of the anodes can be different from each other.

(40) The plating time is desirably 1 to 30 seconds and preferably 5 to 20 seconds.

(41) In a preferred example, the molar ratio of the chromium ions to the zinc ions in the electrolytic solution is preferably 0.2 to 10, more preferably 1 to 5, and most preferably about 1.4. In the present invention, the stabilizing layer formed on the copper foil desirably has a thickness of 0.5 to 10 nm, preferably 2 to 5 nm.

(42) In the embodiments described above, the stabilizing layer is composed of chromium oxide and zinc oxide, but it may be composed of chromium oxide only. In such a case, the stabilizing layer is formed under conditions different from those described above.

(43) The bath for forming a stabilizing layer of chromium oxide is, for example, as follows:

(44) CrO.sub.3: 1 to 10 g/L of aqueous solution

(45) pH: 2

(46) Bath temperature: 25 C.

(47) Current density: 10 to 30 A/ft.sup.2 (108 to 320 A/m.sup.2)

(48) Treatment time: 5 to 10 seconds

(49) After the process of forming the stabilizing layer, washing is performed. In the washing step, for example, water is sprayed to the surfaces of copper foil (having a stabilizing layer) with spray apparatuses disposed at the upper and lower sides of the copper foil to rinse the surfaces to remove the remaining electrolytic solution to give clean surfaces. The wastewater can be collected in a container disposed under the spray nozzle.

(50) Copper foil having a stabilizing layer on the upper face is further dried. As shown in the embodiments, forced-air dryers disposed at the upper and lower sides of the copper foil eject air to dry the surfaces of the copper foil.

(51) An electrically resistive layer composed of an alloy containing 1 to 6 mass % of Si, typically a NiCrAlSi alloy (an alloy composed of Cr: 5 to 40 mass %, Al: 1 to 3 mass %, Si: 1 to 6 mass %, and the remainder being Ni), is formed on the copper foil subjected with surface treatment described above as needed. This film is used as an electrically resistive element of a circuit board.

EXAMPLES

(52) Examples will now be described. These Examples are intended to facilitate understanding of the present invention and do not limit the invention.

(53) That is, modifications, embodiments, and other examples based on the technical idea of the present invention are included in the present invention.

Example 1 and Comparative Example 1

(54) In Example 1, an electrodeposited copper foil having a thickness of 18 m and a rough surface having a ten-point average roughness Rz of 5.2 m measured by an optical method (with WYKO Optical Profiler, manufactured by Veeco Instruments, Inc.) was used.

(55) In Example 1, a copper-zinc alloy layer was formed on the rough surface of the electrolytic copper foil without performing roughening treatment, whereas in Comparative Example 1, a copper-zinc alloy layer was formed on the rough surface of the electrolytic copper foil subjected to roughening treatment to have an Rz of 7.2 m.

(56) The copper-zinc alloy layer was formed under conditions shown below so as to have a zinc content per unit area of about 3500 g/dm.sup.2 (rounded to the nearest hundred). The coating amount was adjusted by controlling the treatment time.

(57) (Copper-Zinc Alloy Plating Bath Composition and Plating Conditions)

(58) Bath composition CuCN: 90 g/L Zn(CN).sub.2: 5 g/L NaOH: 70 g/L Na(CN): 20 g/L

(59) Bath temperature: 70 C.

(60) Current density: 500 A/dm.sup.2

(61) Treatment time: 5 to 20 seconds

(62) Subsequently, a stabilizing layer composed of zinc oxide-chromium oxide and having a thickness of about 5 nm was formed on the copper-zinc alloy layer under the following treatment conditions:

(63) (Stabilizing Treatment Bath Composition and Treatment Conditions)

(64) Bath composition: ZnSO.sub.4: 0.53 g/L in terms of zinc CrO.sub.3: 0.6 g/L in terms of chromium Na.sub.2SO.sub.4: 11 g/L

(65) Bath pH: 5.0

(66) Bath temperature: 42 C.

(67) Current density: 85 to 160 A/m.sup.2

(68) Plating time: 3 to 4 seconds

(69) Subsequently, in each of Example 1 and Comparative Example 1, a film having sheet resistance shown in Table 1 was formed on the stabilizing layer of the chromium-zinc oxide by sputtering an alloy consisting of 56% nickel (Ni), 38% chromium (Cr), and 2% aluminum (Al) and 4% silicon (Si) as dopants with a 14-inch sputtering apparatus under the following conditions:

(70) Power: 0.85 to 2.3 kW

(71) Linear velocity: 0.49 ft/min (0.15 m/min)

(72) Regarding the thus-produced copper foil laminated on a resin substrate, the peel strength as received, the peel strength after soldering (heat resistance), and the peel strength after hydrochloric acid treatment (hydrochloric acid resistance) were investigated.

(73) The peel strength after soldering is the value measured after immersion in a molten solder bath at 260 C. for 20 seconds (i.e., in a state where it was subjected to heat treatment), that is, the peel strength after soldering indicates the peeling strength after the treatment (after receiving thermal influence). The measurement was performed for evaluating heat resistance.

(74) The peel strength after hydrochloric acid treatment is the value measured after immersion in an 18 mass % hydrochloric acid solution at room temperature for 1 hour. The measurement was performed for evaluating hydrochloric acid resistance. The same applies to the following.

(75) The results were that the peel strength as received was 1.05 kN/m and that the peel strength after soldering (heat resistance) was 0.96 kN/m. Thus, the deterioration was low even after soldering to maintain satisfactory properties. The average sheet resistance was 24 /sq (), and the variation thereof was 7.2%.

(76) In contrast, in the sample of Comparative Example 1, the peel strength as received was 1.42 kN/m, and the peel strength after soldering (heat resistance) was 1.30 kN/m. Thus, the deterioration was low even after soldering to maintain satisfactory properties, but the sheet resistance was 40 /sq (), and the variation thereof was high at 16.0%.

Comparative Examples 2 and 3

(77) As Comparative Examples 2 and 3, films having sheet resistance shown in Table 1 were formed as electrically resistive layers on the respective copper foil treated as in Example 1 and Comparative Example 1 by sputtering a nickel 80%-chromium 20% alloy with the 14-inch sputtering apparatus under the following conditions:

(78) Power: 3 kW

(79) Linear velocity: 0.4 m/min

(80) In these cases, when the surface roughness was low, the variation in sheet resistance was low, at 7.5%, but the peel strength as received was an insufficient value, at 0.57 kN/m. When the surface roughness was high, the peel strength as received was sufficient, at 0.92 kN/m, but the variation in sheet resistance was an unsuitable value, at 18.0%. The results are shown in Table 1.

(81) The results above demonstrate that a NiCrAlSi alloy shows sufficiently high strength of adhesiveness to a substrate and a resin and is effective as an electrically resistive layer in the process of producing an electrically resistive element in a circuit board.

(82) TABLE-US-00001 TABLE 1 Variation Peel Solder in strength bath Roughness Sheet sheet as peel Rz resistance resistance received strength (m) (/) (%) (kN/m) (kN/m) Example 1 5.2 24 7.2 1.05 0.96 Comparative 7.2 40 16.0 1.42 1.30 Example 1 Comparative 5.2 23 7.5 0.57 0.53 Example 2 Comparative 7.2 38 18.0 0.92 0.85 Example 3 Roughness Rz: ten-point average roughness Rz measured by an optical method Solder bath peel strength: peel strength after immersion in molten solder bath at 260 C.
[Influence by Ten-Point Average Roughness Rz]

Examples 2 and 3 and Comparative Examples 4 and 5

(83) Subsequently, investigation was made on the characteristics when the ten-point average roughness Rz measured by an optical method was changed based on the conditions of Example 1 which showed satisfactory characteristics. The Rz values were adjusted to 4.0 m, 6.0 m, 6.7 m, and 3.5 m in Examples 2 and 3 and Comparative Examples 4 and 5, respectively.

(84) Other conditions were the same as those in Example 1. The results are shown in Table 2.

(85) TABLE-US-00002 TABLE 2 Variation Peel Solder in strength bath Roughness Sheet sheet as peel Rz resistance resistance received strength (m) (/) (%) (kN/m) (kN/m) Example 2 4.0 21 7.0 0.98 0.90 Example 3 6.0 28 8.0 1.15 1.07 Comparative 6.7 36 10.0 1.30 1.21 Example 4 Comparative 3.5 17 5.7 0.80 0.72 Example 5 Roughness Rz: ten-point average roughness Rz measured by an optical method Solder bath peel strength: peel strength after immersion in molten solder bath at 260 C.

Examples 4 to 8

NiCrAlSi Alloy Resistive Layers Having Different Compositions

(86) All of the resistive layers had a thickness of about 170 nm, and the compositions of the NiCrAlSi alloys were five types shown in Table 3. The compositions of the resistive films shown in Table 3 are indicated by mass % values of Ni/Cr/Al/Si. That is, the compositions were Ni: 65 mass %, Cr: 29 mass %, Al: 4 mass %, and Si: 2 mass % in Example 4; Ni: 54 mass %, Cr: 37 mass %, Al: 6 mass %, and Si: 3 mass % in Example 5; Ni: 89 mass %, Cr: 5 mass %, Al: 4 mass %, and Si: 2 mass % in Example 6; Ni: 54 mass %, Cr: 40 mass %, Al: 4 mass %, and Si: 2 mass % in Example 7; and Ni: 57 mass %, Cr: 40 mass %, Al: 2 mass %, and Si: 1 mass % in Example 8. The copper foil had a surface roughness Rz of 5.2 m.

(87) Resistive layers were formed on the stabilizing layers by sputtering the respective NiCrAlSi alloys under the same conditions as those in Example 1 with a 14-inch sputtering apparatus under the following conditions:

(88) Power: 5 to 8 kW

(89) Linear velocity: 1.8 to 2.8 ft/min (0.55 to 0.85 m/min)

(90) As shown in Table 3, Examples 4 to 8, respectively, have sheet resistance of 14 to 30/, variation in the sheet resistance of 6.9 to 7.5%, peel strength as received of 1.02 to 1.06 kN/m, and solder bath peel strength of 0.96 to 0.98 kN/m. All samples showed satisfactory properties.

(91) TABLE-US-00003 TABLE 3 Variation in Peel Solder Composition Sheet sheet strength bath peel of resistive resistance resistance as received strength film (/) (%) (kN/m) (kN/m) Example 4 65/29/4/2 21 7.3 1.04 0.98 Example 5 54/37/6/3 30 7.5 1.06 0.97 Example 6 89/5/4/2 14 6.9 1.02 0.96 Example 7 54/40/4/2 27 7.4 1.04 0.97 Example 8 57/40/2/1 23 7.4 1.03 0.96 Solder bath peel strength: peel strength after immersion in molten solder bath at 260 C.
[Rolled Copper Foil]

Examples 9 and 10 and Comparative Examples 6 and 7

(92) In these Examples, rolled copper foil having a thickness of 18 m was used. This rolled copper foil was subjected to roughening treatment under the conditions shown below. The surfaces after roughening treatment were adjusted so as to have a ten-point average roughness Rz of 4.1 m (Example 9), 5.9 m (Example 10), 3.5 m (Comparative Example 6), or 6.8 m (Comparative Example 7) when measured by an optical method.

(93) Cu ion concentration: 20 g/L

(94) Sulfuric acid concentration: 60 g/L

(95) Electrolytic solution temperature: 40 C.

(96) Current density: 30 A/dm.sup.2

(97) Treatment time: 5 seconds

(98) Subsequently, a Zn-plated layer was formed on each roughened rolled copper foil at 3500 g/dm.sup.2 under the following conditions. The thickness of the plated zinc was controlled by the treatment time.

(99) Zinc plating bath composition: ZnSO.sub.4.7H.sub.2O: 50 to 350 g/L

(100) pH: 3

(101) Bath temperature: 50 C.

(102) Current density: 20 A/m.sup.2

(103) Treatment time: 2 to 3 seconds

(104) The copper foil having the treatment layer was heated at 300 C. to form a copper-zinc alloy layer. The thus-formed copper-zinc alloy layer had a zinc content per unit area of about 3500 g/dm.sup.2 (rounded to the nearest hundred).

(105) Subsequently, a stabilizing layer composed of zinc oxide-chromium oxide and having a thickness of about 50 A was formed on the copper-zinc alloy layer under the following treatment conditions:

(106) Bath composition: ZnSO.sub.4: 0.53 g/L in terms of zinc CrO.sub.3: 0.6 g/L in terms of chromium Na.sub.2SO.sub.4: 11 g/L

(107) pH of bath: 5.0

(108) Bath temperature: 42 C.

(109) Current density: 85 to 160 A/m.sup.2

(110) Plating time: 3 to 4 seconds

(111) Subsequently, a film was deposited on the stabilizing layer by sputtering an electrically resistive alloy material consisting of 56% nickel (Ni), 38% chromium (Cr), and 2% aluminum (Al) and 4% silicon (Si) as dopants with a 14-inch sputtering apparatus under the following conditions:

(112) Power: 0.85 to 2.3 kW

(113) Linear velocity: 0.49 ft/min (0.15 m/min)

(114) Thickness of NiCrAlSi alloy resistive layer: about 10 nm

(115) The sheet resistance of the resistive material, which slightly varied depending on the roughness, was about 160 /sq ().

(116) Regarding the coating layer for the above copper foil, the peel strength as received, the heat resistance (peel strength after soldering), and the hydrochloric acid resistance (peel strength after hydrochloric acid treatment) were investigated.

(117) The results are shown in Table 4. As shown in Table 4, the samples of Examples 9 and 10 and Comparative Example 7 having Rz higher than the lower limit, respectively, showed satisfactory properties such that peel strength as received was 0.97 to 1.28 kg/cm, and the peel strength after solder immersion was 0.89 to 1.19 kg/cm. In contrast, the sample of Comparative Example 6 having Rz lower than the lower limit of the roughness did not have sufficient strengths.

(118) The variations in the resistance value of the thus-formed NiCrAlSi alloy electrically resistive layers were 6.9 to 8.2% in Examples 9 and 10, which are satisfactory results.

(119) In contrast, as in electrolytic copper foil, in the sample of Comparative Example 7 having a surface roughness Rz larger than the range, the variation was high, at 10.3%.

(120) Examples using rolled copper foil have been described above. It will be easily understood that the similar results are also obtained in the glossy surfaces (S surfaces) of electrolytic copper foil, because the roughened surface of each copper foil is the common issue. These Examples include cases of the glossy surfaces.

(121) TABLE-US-00004 TABLE 4 Variation in Peel strength Solder bath Sheet sheet as peel resistance resistance received strength (/) (%) (kN/m) (kN/m) Example 9 20 6.9 0.97 0.89 Example 10 28 8.2 1.12 1.05 Comparative 18 5.8 0.78 0.69 Example 6 Comparative 35 10.3 1.28 1.19 Example 7 Solder bath peel strength: peel strength after immersion in molten solder bath at 260 C.

(122) In the present invention, the use of copper foil comprising an electrically resistive layer does not require separate formation of an electrically resistive element in designing a circuit and allows formation of a resistive element merely by exposing the electrically resistive layer formed on the copper foil with an etching solution such as a cupric chloride solution. Accordingly, soldering is unnecessary or is highly reduced to give an effect of significantly simplifying mounting steps. In addition, the present invention particularly has a considerable effect of suppressing the variation in the resistance value of the electrically resistive layer within a certain range.

(123) The present invention has effects of considerably lessening the circuit-designing and producing processes and improving the signal characteristics in a high frequency region by comprising a resistive element in the copper foil. In addition, the present invention can suppress the decrease in adhesive force, which is a disadvantage associated with the copper foil comprising an electrically resistive layer, and therefore has an excellent effect of maintaining the satisfactory heat resistance and acid resistance of copper foil to be effective as a printed circuit board.