TWI419634B - Drilling cover - Google Patents

Description

Drilling cover

The present invention relates to an entry sheet for drilling a hole in a copper clad laminate or a multilayer board in the step of manufacturing a printed wiring board.

As a method of drilling a copper-clad laminate or a multi-layered board for a printed wiring board material, a method is generally employed in which a simple aluminum foil for drilling is obtained by forming a resin composition layer on the surface of an aluminum foil before drilling. Or a cover plate as an auxiliary plate for placing on a copper-clad laminate or a multi-layer board or a stacked two or more copper-clad laminates or a top surface of two or more multi-layer sheets. In order to meet the requirements of reliability growth and the development of high densification, printed wiring board materials require high quality drilling. For example, there is a need to improve hole positioning accuracy or reduce hole wall roughness. In order to meet the above requirements for high-quality drilling, it has been proposed and practiced to adopt a hole making method using a plate such as a polyethylene glycol water-soluble resin (for example, JP-A-4-92494) by A water-soluble resin layer is formed on the metal foil to obtain a lubricating plate for pore making (for example, JP-A-6-344297), and a film for forming a thermosetting resin composition on the aluminum foil and further forming a water-soluble resin layer to obtain a pore-forming hole Cover plate (for example, JP-A-2003-136485). With the further development of the densification of the height of the printed wiring board and the reduction of the diameter of the drill bit associated with this development, the influence of winding the cover resin around the drill bit becomes large, so that due to the run-out of a drill, The reduction in chip positioning accuracy and the drill breakage are more likely to occur due to the difference in chip discharge, resin peeling, and the like. Therefore, there is a strong need for a drilling cover plate which is almost free to wrap the cover resin around the drill bit and has excellent hole positioning accuracy.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a cover for drilling which solves the above-mentioned problems associated with drilling a copper-clad laminate or a multi-layered board in which the cover sheet hardly wraps around the drill bit ( It reduces the hole positioning accuracy or causes the drill bit to break, and thus has excellent hole positioning accuracy.

The inventors have studied in detail the improvement of the winding resin around the drill bit and the accuracy of the hole positioning, and as a result, it has been found that the resin layer for winding the cover sheet and the poly ring used in the water-soluble resin composition layer for the cover plate for drilling The relationship between the polydispersity of oxyethane Mw/Mn. Based on the above findings, the inventors completed the present invention.

That is, the present invention relates to a cover plate for drilling, comprising a metal foil and a water-soluble resin composition layer formed on at least one surface of the metal foil, wherein the water-soluble resin composition layer contains polydispersity Mw/ Polyethylene oxide having a Mn of 2.5 or less. In the above-mentioned drilling cover sheet, the polyethylene oxide preferably has a weight average molecular weight Mw of 50,000 to 150,000, and further, the content of polyethylene oxide in the water-soluble resin composition is preferably 20% by weight to 80% by weight.

(effect of the invention)

According to the use of the drilling cover plate provided by the present invention, the winding of the cover resin around the drill bit is reduced at the time of drilling to improve the hole positioning accuracy. Accordingly, excellent drilling quality and productivity are achieved in drilling operations using small diameter drill bits.

The present invention relates to a cover plate for drilling, comprising a metal foil and a water-soluble resin composition layer formed on at least one surface of the metal foil, wherein the water-soluble resin composition layer contains polydispersity Mw/Mn Polyethylene oxide of 2.5 or less. The resin composition layer formed on the metal foil is usually a single layer, but the resin composition layer may be a composite layer composed of at least two resin composition layers.

Generally, the molecular weight of a polymer refers to the average of a plurality of molecular weights of the polymer. The calculation method of the average molecular weight is usually the number average molecular weight Mn calculated by the average molecular weight/molecular weight, or the weight average molecular weight Mw calculated by weight ratio. Generally, the GPC method is suitable for determining the number average molecular weight Mn, and the weight average molecular weight Mw. The number average molecular weight of the water-soluble resin used in the present invention can be measured based on the analysis conditions of the water-soluble GPC. Polyethylene glycol is generally used as a standard substance to calculate the number average molecular weight and the weight average molecular weight of the water-soluble polymer compound.

The polydispersity Mw/Mn is obtained by dividing the weight average molecular weight Mw of the polymer compound having a molecular weight distribution by the number average molecular weight Mn thereof. As the polydispersity value increases, the molecular weight distribution becomes wider. As the polydispersity value decreases, the molecular weight distribution becomes narrow. Generally, natural polymers such as cellulose or starch have a narrow molecular weight distribution, and synthetic polymers such as polyethylene or polystyrene have a broad molecular weight distribution.

The water-soluble resin to be used in the water-soluble resin composition layer of the cover sheet for drilling according to the present invention is not particularly limited as long as it can be dissolved in 100 g of water in an amount of 1 g or more at normal temperature and pressure. The polymer compound is sufficient. Examples of preferred water-soluble resins include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, sodium polyacrylate, polyacrylamide, polyvinylpyrrolidone, carboxymethylcellulose, polytetramethylene glycol, polyether. Ester, polyvinyl alcohol, polyethylene glycol, polypropylene glycol; monoether of polyoxyethylene, such as polyoxyethylene oleyl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, polyoxyethylene laurel Ether, polyoxyethylene nonylphenyl ether and polyoxyethylene octyl phenyl ether; polyoxyethylene monostearate, polyoxyethylene sorbitan monostearate; polyglycerol monostearate Esters such as hexaglycerol monostearate and hexadecaglycerol monostearate; and polyoxyethylene propylene copolymers. A water-soluble resin may be used alone or a mixture of at least two water-soluble resins may be used as needed.

The polydispersity Mw/Mn of the polyethylene oxide contained in the water-soluble resin composition layer of the present invention is preferably 2.5 or less, more preferably 2.0 or less. When the polydispersity of polyethylene oxide exceeds 2.5, the molecular weight distribution is broadened so that the amount of resin of the wound drill bit is increased.

The molecular weight of the polyethylene oxide having a polydispersity Mw/Mn of 2.5 or less in the water-soluble resin composition layer of the present invention is preferably 50,000 to 150,000 in terms of the weight average molecular weight Mw. According to the use of polyethylene oxide having a weight average molecular weight Mw of 50,000 to 150,000, the lubricating effect at the time of drilling is excellent, and in addition, the amount of resin wound around the drill bit is small, making it possible to complete the drilling with high hole positioning accuracy. hole.

The content of the polyethylene oxide having a polydispersity Mw/Mn of 2.5 or less in the water-soluble resin composition of the present invention is preferably from 20% by weight to 80% by weight, more preferably from 30% by weight to 60% by weight. %. When it is less than 20% by weight, the amount of the resin wound around the drill is large. When it exceeds 80% by weight, the strength of the resin layer is lowered.

The thickness of the water-soluble resin composition layer formed on at least one surface of the metal foil of the drilling cover plate provided by the present invention, the diameter of the drill bit used for drilling or the structure of the copper-clad laminate or the multi-layer board to be drilled And change. Generally, the thickness of the water-soluble resin composition layer is preferably from 5 to 250 μm, more preferably from 10 to 200 μm. When the thickness of the water-soluble resin composition layer is less than 5 μm, a sufficient lubricating effect cannot be obtained. When the thickness of the water-soluble resin composition layer exceeds 250 μm, the amount of the cover resin of the wound drill bit increases. Further, the water-soluble resin composition layer may further contain various additives. For example, organic or inorganic fillers, dyes, coloring pigments, and the like can be used depending on the intended purpose.

The metal foil used in the cover plate for drilling provided by the present invention is preferably an aluminum foil having a thickness of 50 to 300 μm. When the thickness of the aluminum foil is less than 50 μm, the burrs of the laminate are apt to occur during drilling. When the thickness of the aluminum foil exceeds 300 μm, it is difficult to discharge the chips generated at the time of drilling. For aluminum foil materials, aluminum having a purity of at least 95% is preferred. Specific examples thereof include 5052, 3004, 3003, 1N30, 1050, 1070, and 1085, each of which is defined in JIS-H4160. The use of high-purity aluminum foil as a metal foil mitigates the vibration of the drill bit and improves the biting performance of the drill bit. The above-described use of the high-purity aluminum foil and the lubricating effect of the water-soluble resin composition layer on the drill bit improve the hole positioning accuracy of the drilled hole. In view of the adhesiveness of the water-soluble resin composition, it is preferred to use an aluminum foil having a film thickness of 0.1 μm to 10 μm formed thereon. Examples of the adhesive for the mucosa include a urethane adhesive, a vinyl acetate adhesive, a vinyl chloride adhesive, a polyester adhesive, a copolymer adhesive of the compounds, an epoxy adhesive, and a cyanate adhesive. Agent.

As a method of forming a water-soluble resin composition layer on at least one surface of a metal foil, for example, a method of laminating a plate of a water-soluble resin composition prepared in advance on a metal foil, and water-soluble by a coating method or the like A method in which a molten material of a resin composition or a solution of a water-soluble resin composition is directly applied to a metal foil and then dried to form a water-soluble resin composition layer on the metal foil.

When drilling a hole in a printed wiring board material such as a copper-clad laminate or a multi-layer board, the cover sheet of the present invention is placed in a copper-clad laminate, a multi-layer board or a plurality of laminated copper-clad laminates or a plurality of layers when drilling. The at least one top surface of the plate is such that the cover plate is in contact with the printed wiring board material, and the side of the water-soluble resin composition layer of the cover plate faces upward, and the surface of the water-soluble resin composition layer of the drilling cover plate is drilled hole.

[Examples]

The invention will be specifically explained below with reference to examples and comparative examples.

(Example 1)

50 parts by weight of polyethylene oxide having a polydispersity Mw/Mn of 1.5 and a weight average molecular weight Mw of 110,000 (ALKOX L11, by Meisei Chemical Works, Ltd.) was kneaded in a kneader at a temperature of 130 ° C under a nitrogen atmosphere. Provided) and 50 parts by weight of polyethylene glycol (PEG 20000, supplied by Sanyo Chemical Industries, Ltd.) having a weight average molecular weight Mw of 20,000, and then passing the kneaded mixture through a heating roll at 100 ° C to obtain a thickness of 50 μm A water-soluble resin composition plate. The water-soluble resin composition sheet was laminated on an aluminum foil having a thickness of 100 μm (provided by Mitsubishi Aluminum Co., Ltd., material 3004) via a laminator to obtain a cover sheet A for drilling. The drilling cover A was placed on the top surfaces of the stacked three copper-clad laminates each having a thickness of 0.8 mm (CCL-HL832, copper foil 12 μm on both sides, supplied by Mitsubishi Gas Chemical Company, Inc. The side of the water-soluble resin composition layer of the cover A is made to face upward. A backing plate (phenolic resin sheet) was placed on the lower side of the stacked copper-clad laminate. Drilling was carried out under conditions of a drill diameter of 0.2 mm, a rotational frequency of 250,000 rpm, and a feed rate of 15 μm/rev. Each bit has a number of hits of 2,000 and is drilled with 15 drill bits. The degree of winding of the resin around the drill bit and the accuracy of the hole positioning were evaluated. Table 1 shows the results.

(Example 2)

50 parts by weight of polyethylene oxide having a polydispersity Mw/Mn of 1.5 and a weight average molecular weight Mw of 80,000 (ALKOX L8, by Meisei Chemical Works, Ltd.) was kneaded in a kneader at a temperature of 140 ° C under a nitrogen gas. Provided) and 50 parts by weight of polyoxyethylene monostearate (NONION S-40, supplied by NOF CORPORATION), and then extruding the kneaded mixture with an extruder to obtain a water-soluble resin composition having a thickness of 100 μm board. The water-soluble resin composition sheet was placed on one surface of an aluminum foil having a thickness of 100 μm (supplied by Nippon Foil Mfg Co., Ltd., material 1N30) as a polyester adhesive for the mucosa (R820, by CEMEDINE Co.). , Ltd.) has been coated to a thickness of 2 microns. The water-soluble resin composition plate was bonded to the aluminum foil with a heating roll at 100 ° C to obtain a cover plate B for drilling. The drilled cover plate B was placed on the top surfaces of the stacked three copper-clad laminates each having a thickness of 0.4 mm (CCL-HL832, copper foil on both sides 12 micrometers, supplied by Mitsubishi Gas Chemical Company, Inc. The side of the water-soluble resin composition layer of the cover B is made to face upward. A backing plate (phenolic resin sheet) was placed on the lower side of the stacked copper-clad laminate. Drilling was carried out under the conditions of a drill diameter of 0.15 mm, a rotational frequency of 300,000 rpm, and a feed rate of 12 μm/rev. Each drill has a drill number of 2,000 and is drilled with 15 drill bits. The degree of winding of the resin around the drill bit and the accuracy of the hole positioning were evaluated. Table 1 shows the results.

(Example 3)

60 parts by weight of polyethylene oxide (SE-8, supplied by Nihon Waters KK) having a polydispersity Mw/Mn of 1.1, a weight average molecular weight Mw of 85,000, and 40 parts by weight of polyethylene glycol in a stirred tank (Trade name: PEG 20000, supplied by Sanyo Chemical Industries, Ltd.) was dissolved in 250 parts by weight of water to prepare a resin solution. The resin solution was applied to one surface (material: 1085) of an aluminum foil having a thickness of 100 μm by a die coater, to which a polyester adhesive (R820) having a thickness of 5 μm was applied, so that application was performed. The thickness of the resin solution was 140 microns. By passing the obtained aluminum foil at a speed of 0.1 m/sec through a roll-supported drying oven having 3 chambers each having a length of 3 m (predetermined temperature: 60-100-140 ° C), and allowing hot air The applied resin solution was dried by blowing the surface of the applied resin solution of the aluminum foil at a wind speed of 20 m/min, thereby obtaining a drilling cover plate C having a total thickness of 140 μm. The drilling cover C was placed on the top surfaces of the stacked three copper-clad laminates each having a thickness of 0.8 mm (CCL-HL832, copper foil 12 μm on both sides, supplied by Mitsubishi Gas Chemical Company, Inc. The side of the water-soluble resin composition layer of the cover C is made to face upward. A backing plate (phenolic resin sheet) was placed on the lower side of the stacked copper-clad laminate. Drilling was carried out under conditions of a drill diameter of 0.2 mm, a rotational frequency of 250,000 rpm, and a feed rate of 15 μm/rev. Each drill has a drill number of 2,000 and is drilled with 15 drill bits. The degree of winding of the resin around the drill bit and the accuracy of the hole positioning were evaluated. Table 1 shows the results.

(Example 4)

20 parts by weight of polyethylene oxide (ALKOX L15, supplied by Meisei Chemical Works, Ltd.) having a polydispersity Mw/Mn of 1.5 and a weight average molecular weight Mw of 150,000 in a stirring tank, 50 parts by weight of polyethylene Glycol. Dimethyl terephthalate polycondensate (Paogen PP-15, supplied by Daiichi-Kogyo Seiyaku Co;, Ltd.) and 30 parts by weight of polyoxyethylene monostearate (NONION S-40) dissolved in 250 weight A portion of the water was used to prepare a resin solution. The resin solution was applied to one surface (material: 1070) of an aluminum foil having a thickness of 70 μm by a die coater so that the thickness of the applied resin solution was 140 μm. The obtained aluminum foil was passed through a roll-supported drying oven having 3 chambers each having a length of 3 m at a rate of 0.07 m/sec (predetermined temperature: 60-90-120 ° C), and hot air was 15 m / min. The wind speed was blown over the surface of the applied resin solution of the aluminum foil, thereby obtaining a drilling cover D having a total thickness of 110 μm. The drilling cover plate D was placed on the top surfaces of the stacked three copper-clad laminates each having a thickness of 0.4 mm (CCL-HL832, copper foil on both sides 12 micrometers, supplied by Mitsubishi Gas Chemical Company, Inc. The side of the water-soluble resin composition layer of the cover D is made to face upward. A backing plate (phenolic resin sheet) was placed on the lower side of the stacked copper-clad laminate. Drilling was carried out under the conditions of a drill diameter of 0.15 mm, a rotational frequency of 300,000 rpm, and a feed rate of 12 μm/rev. Each drill has a drill number of 2,000 and is drilled with 15 drill bits. The degree of winding of the resin around the drill bit and the accuracy of the hole positioning were evaluated. Table 1 shows the results.

(Comparative Example 1)

50 parts by weight of polyethylene oxide having a polydispersity Mw/Mn of 3.5 and a weight average molecular weight Mw of 150,000 (ALKOX R-150, by Meisei Chemical Works, kneaded in a kneader at a temperature of 130 ° C under a nitrogen atmosphere) Ltd. provided) and 50 parts by weight of polyethylene glycol (PEG 20000, supplied by Sanyo Chemical Industries, Ltd.) having a weight average molecular weight Mw of 20,000, and then subjecting the kneaded mixture to a heating roll at 100 ° C to obtain a thickness of 50 micron resin composition plate. The resin composition plate was laminated on an aluminum foil having a thickness of 100 μm (provided by Mitsubishi Aluminum Co., Ltd., material 3004) via a laminator to obtain a cover plate E for drilling. The drilling cover E was placed on the top surfaces of the stacked three copper-clad laminates each having a thickness of 0.8 mm (CCL-HL832, copper foil 12 μm on both sides, supplied by Mitsubishi Gas Chemical Company, Inc. The side of the water-soluble resin composition layer of the cover E is made to face upward. A backing plate (phenolic resin sheet) was placed on the lower side of the stacked copper-clad laminate. Drilling was carried out under conditions of a drill diameter of 0.2 mm, a rotational frequency of 250,000 rpm, and a feed rate of 15 μm/rev. Each drill has a drill number of 2,000 and is drilled with 15 drill bits. The degree of winding of the resin around the drill bit and the accuracy of the hole positioning were evaluated. Table 1 shows the results.

(Comparative Example 2)

50 parts by weight of polyethylene oxide (ALKOX R-150, having a weight average molecular weight Mw of 150,000) having a polydispersity Mw/Mn of 3.5 and a weight average molecular weight of 150,000 was kneaded in a kneader at a temperature of 140 ° C under a nitrogen gas (by Meisei Chemical Works, Ltd. supplied) and 50 parts by weight of polyoxyethylene monostearate (NONION S-40, supplied by NOF CORPORATION), and then the kneaded mixture was extruded by an extruder to obtain a plate having a thickness of 100 μm. The plate was placed on one surface of an aluminum foil having a thickness of 100 μm (supplied by Nippon Foil Mfg Co., Ltd., material 1N30), and a polyester adhesive (R820, supplied by CEMEDINE Co., Ltd.) as a mucous film thereon. ) has been coated to a thickness of 2 microns. The plate was bonded to an aluminum foil with a heating roll at 100 ° C to obtain a cover plate F for drilling. The drilling cover F was placed on the top surfaces of the stacked three copper-clad laminates each having a thickness of 0.4 mm (CCL-HL832, copper foil 12 μm on both sides, supplied by Mitsubishi Gas Chemical Company, Inc. The side of the water-soluble resin composition layer of the cover F is made to face upward. A backing plate (phenolic resin sheet) was placed on the lower side of the stacked copper-clad laminate. Drilling was carried out under the conditions of a drill diameter of 0.15 mm, a rotational frequency of 300,000 rpm, and a feed rate of 12 μm/rev. The number of drills per drill was 2,000 and was drilled with 15 drill bits. The degree of winding of the resin around the drill bit and the accuracy of the hole positioning were evaluated. Table 1 shows the results.

(Comparative Example 3)

60 parts by weight of polyethylene oxide (ALKOX R-150, supplied by Meisei Chemical Works, Ltd.) having a polydispersity Mw/Mn of 3.5 and a weight average molecular weight Mw of 150,000 and 40 parts by weight in a stirring tank Polyethylene glycol (trade name: PEG 20000, supplied by Sanyo Chemical Industries, Ltd.) was dissolved in 250 parts by weight of water to prepare a resin solution. The resin solution was applied to a surface (material: 1085) of an aluminum foil having a thickness of 100 μm by a die coater to which a polyester adhesive (R820) having a thickness of 5 μm was applied so that the applied resin solution The thickness is 140 microns. By passing the obtained aluminum foil at a speed of 0.1 m/sec through a roll-supported drying oven having 3 chambers each having a length of 3 m (predetermined temperature: 60-100-140 ° C) and allowing hot air to be 20 m The wind speed of /min was blown through the surface of the applied resin solution of the aluminum foil to dry the applied resin solution, thereby obtaining a drilling cover plate G having a total thickness of 140 μm. The drilling cover G was placed on the top surfaces of the stacked three copper-clad laminates each having a thickness of 0.8 mm (CCL-HL832, copper foil on both sides 12 micrometers, supplied by Mitsubishi Gas Chemical Company, Inc. The side of the water-soluble resin composition layer of the cover G is made to face upward. A backing plate (phenolic resin sheet) was placed on the lower side of the stacked copper-clad laminate. Drilling was carried out under conditions of a drill diameter of 0.2 mm, a rotational frequency of 250,000 rpm, and a feed rate of 15 μm/rev. The number of drills per drill was 2,000 and was drilled with 15 drill bits. The degree of winding of the resin around the drill bit and the accuracy of the hole positioning were evaluated. Table 1 shows the results.

(Comparative Example 4)

20 parts by weight of polyethylene oxide (ALKOX R150, supplied by Meisei Chemical Works, Ltd.) having a polydispersity Mw/Mn of 3.5 and a weight average molecular weight Mw of 150,000 in a stirring tank, 50 parts by weight of polyethylene A diol ‧ terephthalate condensate (Paogen PP-15) and 30 parts by weight of polyoxyethylene monostearate (NONION S-40) were dissolved in 250 parts by weight of water to prepare a resin solution. The resin solution was applied to one surface (1070) of an aluminum foil having a thickness of 70 μm by a die coater so that the thickness of the applied resin solution was 140 μm. The obtained aluminum foil was passed through a roll-supported drying oven having 3 chambers each having a length of 3 m at a rate of 0.07 m/sec (predetermined temperature: 60-90-120 ° C), and hot air was 15 m / min. The wind speed was blown over the surface of the applied resin solution of the aluminum foil, thereby obtaining a drilling cover plate H having a total thickness of 110 μm. The drilling cover H was placed on the top surfaces of the stacked three copper-clad laminates each having a thickness of 0.4 mm (CCL-HL832, copper foil 12 μm on both sides, supplied by Mitsubishi Gas Chemical Company, Inc. The side of the water-soluble resin composition layer of the cover H is made to face upward. A backing plate (phenolic resin sheet) was placed on the lower side of the stacked copper-clad laminate. Drilling was carried out under the conditions of a drill diameter of 0.15 mm, a rotational frequency of 300,000 rpm, and a feed rate of 12 μm/rev. The number of drills per drill was 2,000 and was drilled with 15 drill bits. The degree of winding of the resin around the drill bit and the accuracy of the hole positioning were evaluated. Table 1 shows the results.

<Evaluation method>

1) Degree of winding of the resin: After 2,000 holes were drilled, each of the 15 drill bits was observed with a microscope at a magnification of 25 times, thereby obtaining the maximum diameter of the resin wound around the drill bit in terms of the diameter of the drill. The 15 drill bits were evaluated on the following 1 to 3 scales (less than 1.5 times the diameter of the drill: very small, 1.5 to 5 times the diameter of the drill: small, greater than 5 times the diameter of the drill: large). "The number of related drill bits among the 15 drill bits" represents the degree of winding.

2) Hole positioning accuracy: The hole position of each drill bit was measured 2,000 times with respect to the back surface of the lowest copper-clad laminate on the laminated copper-clad laminate using a hole analyzer (supplied by Hitachi Via Mechanics, Ltd.). The displacement of the target coordinates on the top. The mean and standard deviation (σ) are obtained. Thus, "average + 3σ" and "maximum value" are calculated. Table 1 shows the average of "average + 3σ" and "maximum value" in 15 drilling processes.

3) Molecular weight measurement: The molecular weight was measured using a water-soluble GPC in which the columns of Shodex SB-G, Shodex SB-803HQ and Shodex SB-806MHQ (supplied by Showa Denko K. K.) were sequentially arranged. The number average molecular weight and the weight average molecular weight of the polymer compound were measured under the following conditions. Analyze the measurement conditions of the sample:

Carrier: 50 mM-NaCl aqueous solution.

Flow rate: 0.7 ml/min.

Furnace temperature: 35 ° C.

Injection volume of the sample: 20 microliters.

Detection: Differential refractometer (RID-6A, supplied by Shimadzu Corporation).

Reference material: Polyethylene glycol kit (supplied by POLYMER LABORATORIES Ltd.).

Claims (3)

A drilling cover plate comprising a metal foil and a water-soluble resin composition layer formed on at least one surface of the metal foil, wherein the water-soluble resin composition layer contains a polydispersity Mw/Mn of 2.5 or less Polyethylene oxide. The cover sheet of claim 1, wherein the polyethylene oxide has a weight average molecular weight Mw of 50,000 to 150,000. A cover sheet according to claim 1 or 2, wherein the water-soluble resin composition has a polyethylene oxide content of from 20% by weight to 80% by weight.