JP2006175570A - Entry board for small bore boring work and small bore boring work method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an entry board for small bore boring work with a lubricating layer excellent in adhesion to a base plate of the lubricating layer, hard to cause peeling and breakage of the lubricating layer and capable of providing favorable boring workability. <P>SOLUTION: This entry board for small bore boring work with the lubricating layer is constituted by forming the lubricating layer 4 through an undercoating layer 3 containing a saponified polyvinyl alcohol at least on a single surface of the aluminum made base plate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an entry board used as a backing plate for forming a small-diameter hole in a workpiece, such as a through hole in a printed wiring board, and a small-diameter drilling method using the entry board.

  In this specification, the term “aluminum” includes aluminum alloys in addition to pure aluminum. Further, the term “plate” includes a foil.

  In recent years, with rapid performance enhancement of communication equipment, OA equipment, home appliances, etc., in the printed wiring board responsible for the control center of these equipment, miniaturization, high density, and multilayering of wiring patterns have progressed, There is a demand for smaller through holes and improved positional accuracy.

  Conventionally, in the drilling of through holes in printed wiring boards, in order to increase the processing efficiency, a plurality of printed wiring boards are stacked on the top board, and the aluminum entry is placed on the topmost board. In this state, a method of forming through holes at once in all stacked substrates by drilling through the entry board from above with a drill and drilling the substrate for printed wiring in this state is adopted. ing. Thus, the entry board is used as a backing plate for the purpose of improving the bite of the drill and preventing the damage on the surface of the base plate and the generation of burrs and burrs around the hole periphery.

  However, the diameter of the drill hole is reduced by reducing the diameter of the through hole. However, when drilling with a small diameter drill with a diameter of 0.3 mm or less, especially when an aluminum plate is used for the entry board, the drill is on the surface of the entry board. As a result, the position accuracy of drilling deteriorates, the drill breaks frequently, and the inner peripheral surface of the hole is roughened. There was a problem that the number of stacked sheets could not be increased and the processing efficiency could not be sufficiently increased.

  For this reason, recently, as the entry board, a board in which a lubricating layer mainly composed of polyethylene glycol is provided on at least one surface of an aluminum substrate has been used. This is because when the drilling process is performed, the lubrication layer of the drilled part is melted by frictional heat, and the cutting resistance in the drilling direction is reduced by the lubrication action, thereby preventing side-slip of the drill and improving the hole position accuracy. In addition, the breakage of the drill is suppressed, the inner peripheral surface of the hole is prevented from being roughened, and the processing efficiency is increased by increasing the number of stacked base plates.

Thus, when providing a lubrication layer in an entry board, the adhesiveness of this lubrication layer and an aluminum substrate becomes a new subject. That is, if this adhesion is poor, peeling or coating film cracking occurs in the lubricating layer, and the original lubricating function of the lubricating layer cannot be fully exhibited, leading to breakage of the drill and a decrease in hole position accuracy. Therefore, an undercoat layer containing a resin component selected from a partially saponified polyvinyl acetate, a vinyl acetate-vinyl chloride copolymer, a vinyl acetate-vinyl chloride-maleic acid copolymer, etc., between the lubricating layer and the aluminum substrate. It has been proposed to improve the adhesion of the lubricating layer to the substrate by interposing a metal (Patent Documents 1 and 2).
JP 2004-9193 A However, in the entry board provided with the lubricating layer through the undercoat layer, the adhesion between the lubricating layer and the aluminum substrate can be satisfied for the time being, but the main component is polyethylene glycol as the lubricating layer. When this component is used, the melting point of this polyethylene glycol is as low as 60 ° C. or less, and the viscosity of the lubricating component melted by the frictional heat accompanying perforation is significantly reduced. It will be dissipated from the part and will not be able to exert a sufficient lubrication effect, causing frequent damage to the drill and deterioration of the positional accuracy of the hole, and the printed wiring board will be soiled due to the dripping of the lubricating component deposited on the drill There was a problem.

  In view of the above circumstances, the present invention provides a small-diameter hole drilling entry board provided with a lubricating layer, and a small-diameter hole drilling method using the same, in the adhesion of the lubricating layer to the substrate in the entry board. Excellent, reliably prevents drill breakage and decrease in hole position accuracy due to peeling of the lubricating layer and cracking of the coating, and in the small-diameter drilling method, the lubricating component is not interrupted at the cutting site by the drill all the time Providing a sufficient lubrication effect, reducing drill breakage and improving hole position accuracy, avoiding contamination of the workpiece due to dripping of the lubrication component, and water-soluble lubrication component An object of the present invention is to provide an undercoat layer that can be easily removed by washing with polyethylene glycol.

  In order to achieve the above object, the small-diameter hole drilling entry board according to claim 1 of the present invention has a lubricating layer on at least one surface of an aluminum substrate with an undercoat layer containing a saponified polyvinyl alcohol. It is characterized by being formed.

  Further, in the entry board for small-diameter drilling according to claim 1, the saponification degree of the saponified polyvinyl alcohol in the undercoat layer is 50 to 90 mol%, and the thickness of the undercoat layer is 0. Each of the configurations according to claim 3 is preferably 1 to 5 μm.

  Thus, the lubricating layer in the entry board for small-diameter drilling according to claims 1 to 3 comprises polyethylene glycol having a number average molecular weight of 100 or more and less than 1000, polyethylene glycol having a number average molecular weight of 10,000 or more, and trimethylolpropane. The structure of Claim 4 to contain is suitable.

  On the other hand, in the small-diameter drilling method according to claim 5 of the present invention, the entry board according to any one of claims 1 to 4 is arranged on a plurality of stacked substrates for printed wiring. In this state, a hole having a diameter of 0.3 mm or less is formed in the entry board and the printed wiring board from above using a drill.

  According to the entry board for small-diameter drilling processing according to the invention of claim 1, a saponified product of polyvinyl alcohol is contained in the undercoat layer interposed between the aluminum substrate and the lubricating layer, and the undercoat layer and the substrate Since it has a high affinity for both lubrication layers and has an appropriate elasticity, the lubrication layer adheres strongly to the substrate and does not cause peeling or cracking of the coating film. It is possible to reliably prevent deterioration of the hole position accuracy and roughening of the hole inner peripheral surface. In addition, the saponified polyvinyl alcohol used in the undercoat layer is mixed in the lubricating component during drilling, suppresses the decrease in the viscosity of the lubricating component due to frictional heat of cutting, and exhibits a better lubricating action. Lubrication of the lubricating component from the drill is less likely to occur. Furthermore, since the saponified product of polyvinyl alcohol is water-soluble, even if a part of the undercoat layer cut by drilling adheres to the workpiece together with the lubricating component, there is an advantage that it can be easily removed by washing with water. .

  According to the invention of claim 2, since the saponification product of the polyvinyl alcohol of the undercoat layer has a saponification degree within a specific range, it exhibits good solubility in water, and a part cut by drilling is a lubricating component. In addition, even if it adheres to the workpiece, it can be easily removed by washing with water, and there is an advantage that the lubricating action by the lubricating component is not hindered.

  According to the invention of claim 3, since the undercoat layer has a specific thickness, the effect of preventing the peeling of the lubricating layer and the cracking of the coating film by the undercoat layer is more reliably exhibited.

  According to the invention of claim 4, since the lubricating layer of the entry board contains two types of polyethylene glycol and trimethylol propane having different number average molecular weights, the polyethylene glycol can exhibit a good lubricating action at the time of drilling. And the saponification product of the polyvinyl alcohol of the undercoat layer are good, and the dripping of the lubricating component from the drill can be suppressed by the viscosity lowering action by the compatibility, and the workpiece can be prevented from being soiled by the dripping.

  According to the small-diameter drilling method according to the invention of claim 5, when forming a small-diameter hole having a diameter of 0.3 mm or less at once for a plurality of stacked printed wiring base plates, breakage of the drill is prevented. It is hard to occur, and the hole position accuracy can be set high.

  FIG. 1 shows an embodiment of an entry board for small-diameter drilling according to the present invention. The entry board 1 has a lubricating layer 4 formed on one surface of an aluminum substrate 2 with an undercoat layer 3 interposed therebetween.

  As the aluminum substrate 2 of the entry board 1, for example, a thickness 0.1 made of JIS A1100-H18 material, JIS A1N30-H18 material, JIS A1050-H18 material, JIS A3003-H18 material, JIS A3004-H18 material, etc. A single aluminum plate of about 0.25 mm or a soft aluminum plate having a Vickers hardness (Hv) of 20 to 50 and a thickness of 5 to 100 μm and a Vickers hardness (Hv) of 50 to 140 and a thickness of 30 to 200 μm What stuck together the aluminum plate is used suitably. In the case of using the latter laminated with the soft and hard aluminum plate, the lubricating layer 4 is formed on the surface of the soft aluminum plate via the undercoat layer 3.

  The undercoat layer 3 is made of polyvinyl alcohol having a saponification degree of 50 to 90 mol% and a polymerization degree of about 100 to 800, and has a thickness of about 0.1 to 5.0 μm.

  The lubricating layer 4 is composed of a mixed composition containing polyethylene glycol having a number average molecular weight of 100 or more and less than 1000, polyethylene glycol having a number average molecular weight of 10,000 or more, and trimethylolpropane, and the thickness thereof is about 10 to 100 μm. Is set.

  In such an entry board 1, the saponified polyvinyl alcohol forming the undercoat layer 3 has a good affinity for both the aluminum substrate 1 and the lubricating layer 4, so that the lubricating layer 4 is attached to the substrate 1. Since the undercoat layer 3 absorbs strain stress and external force with appropriate elasticity, the coating layer cracking of the lubricating layer 4 is prevented.

  When this entry board 1 is used to drill through holes in a printed wiring board, for example, a plurality of printed wiring boards are first stacked on the top board, and the entry board 1 is a lubricating layer on the top. In this state, drilling is performed from above with a drill from the entry board 1 to the steel plate, so that all the printed wiring boards stacked in a single drilling operation can be made at once. Form a through hole.

  At this time, the lubricating layer 4 of the entry board 1 is melted by the frictional heat accompanying the rotation of the drill, and has a function of reducing the cutting resistance by spreading the lubricating component to the cutting site. In this way, it adheres firmly and uniformly to the aluminum substrate 1 and is subjected to drilling without peeling or cracking of the coating film, so that the original excellent lubricating action can always be exhibited reliably. Therefore, by using this entry board 1, even when drilling with a small diameter of 0.3 mm or less, the drill is prevented from slipping, the frequency of breakage of the drill is significantly reduced, and the positional accuracy of the hole is increased, and Roughness of the inner peripheral surface of the hole is also prevented. Further, since the drill is hardly broken as described above, it is possible to increase the productivity of the printed wiring board by increasing the number of base plates to be stacked at the time of drilling once.

  Moreover, in this entry board 1, since the components of the undercoat layer 3 and the lubricating layer 4 both have good solubility in water, the components adhering to the printed wiring board are easily removed by washing with water. It becomes possible to do. Thus, the saponified polyvinyl alcohol constituting the undercoat layer 3 is partially mixed in the lubricating component at the time of drilling, whereas the melting point of polyethylene glycol as the lubricating component is 60 ° C. or less. Since it has a high melting point of 180 to 190 ° C., it is compatible with the lubricating component and suppresses a decrease in the viscosity of the lubricating component due to frictional heat of cutting, thereby preventing deviation from the cutting site due to excessive fluidity of the lubricating component. This contributes to the maintenance of a good lubricating action and makes it difficult for the lubricating component to sag from the drill, and also contributes to the prevention of contamination of the base plate due to this sag.

  Here, the saponification product of polyvinyl alcohol used for the undercoat layer 3 preferably has a saponification degree in the range of 50 to 90 mol% as described above. If the degree of saponification is less than 50 mol%, the coating of the undercoat layer 3 becomes brittle and the solubility in water becomes insufficient. Therefore, the components of the undercoat layer adhering to the base plate for printed wiring after perforation are formed. It becomes difficult to remove by washing with water. On the contrary, when the degree of saponification exceeds 90 mol%, the lubricating action by the lubricating layer 3 is hindered due to the high viscosity at the time of melting. If the degree of polymerization of the polyvinyl alcohol is too low, the effect of enhancing the adhesion of the lubricating layer 4 cannot be sufficiently exerted. Conversely, if the degree of polymerization is too high, the lubricating effect of the lubricating layer 3 is high due to the high viscosity at the time of melting. While inhibiting, the compatibility with the polyethylene glycol component of the lubricating layer 4 is reduced, so that the range of about 100 to 800 is preferable as described above.

  Furthermore, if the thickness of the undercoat layer 3 is too thin, the adhesion of the lubricating layer 4 to the aluminum substrate becomes insufficient. Conversely, if the thickness is too thick, the manufacturing cost of the entry board 1 is high. It is good to make it in the range of 1-5 μm.

  In addition to using saponified polyvinyl alcohol alone as the undercoat layer 3, other water-soluble resin components having good affinity for the substrate 2 and the lubricating layer 4 are used as subcomponents. You can also Examples of such subcomponents include the partially saponified polyvinyl acetate described above, which has been used in the undercoat layer of conventional entry boards.

  On the other hand, polyethylene glycol having a number average molecular weight of 100 or more and less than 1000 used for the lubricating layer 4 improves lubricity and water solubility of the lubricating layer, and is compatible with polyethylene glycol having a number average molecular weight of 10,000 or more and trimethylolpropane. Fulfills the function of improving. The blending amount of the low molecular weight polyethylene glycol is preferably in a range that occupies 5 to 70% by mass of the base composition, with the entire component excluding trimethylolpropane in the lubricating layer 4 as the base composition, and 5% by mass. If the ratio is less than 70%, the function becomes insufficient. If it exceeds 70% by mass, the lubricating layer 4 becomes sticky.

  Similarly, the polyethylene glycol having a number average molecular weight of 10,000 or more used for the lubricating layer 4 has a function of increasing the film strength of the lubricating layer 4, but if the blending amount is less than 25% by mass of the base composition, the function is sufficiently achieved. If it exceeds 90% by mass, the lubricating layer 4 becomes hard and brittle. Therefore, the blending amount of the high molecular weight polyethylene glycol is preferably in a range occupying 25 to 90% by mass of the base composition. The upper limit of the number average molecular weight of the high molecular weight polyethylene glycol is about 100,000.

  The trimethylolpropane used in the lubricating layer 4 increases the solubility of the lubricating layer 4 in water, so that the post-drilling process can be washed with water. Therefore, the blending amount of the trimethylolpropane is preferably in the range of 0.5 to 20 parts by mass with respect to 100 parts by mass of the base composition which is the entire other lubricant layer forming component excluding the trimethylolpropane. If the amount is too small, the solubility cannot be sufficiently increased. On the other hand, if the amount is too large, the lubricating layer 4 becomes sticky.

  Furthermore, in addition to the two types of polyethylene glycol and trimethylolpropane having different number average molecular weights, various modifying components may be added to the lubricating layer 4 as necessary. As such a modifying component, for example, a saponified product of polyvinyl alcohol, a propylene glycol alginate ester, a chelating agent and the like can be mentioned as suitable ones.

  Among the above modified components, the saponified product of polyvinyl alcohol is also a constituent component of the undercoat layer 3, so that the blending increases the affinity between the lubrication layer 4 and the undercoat layer 3, and the lubrication layer 4 with respect to the substrate. In addition to further improving the adhesion, the melting point is as high as 180 to 190 ° C., so the heat resistance of the entire lubricating layer 4 is improved, the viscosity of the lubricating component melted by the frictional heat at the time of drilling is suppressed, and the flow of the lubricating component It prevents the escape from the cutting site due to excessive nature, maintains a stable and good lubricating action, and makes it difficult for the lubricating component deposited on the drill to sag.

  Such a saponified product of polyvinyl alcohol as a modifying component of the lubricating layer 4 preferably has a saponification degree of 15 to 90 mol%. If the degree of saponification is too low, the water-solubility is inferior, so that the lubricating component adhering after drilling cannot be easily removed by washing with water. On the other hand, if the degree of saponification is too high, the lubricity is lowered and the compatibility with polyethylene glycol is deteriorated, so that it is difficult to form a uniform lubricating layer. Further, the blending amount of the saponified product of polyvinyl alcohol is preferably in the range occupying 5 to 50% by mass of the base composition, and if it is too small, the effect of improving the heat resistance is not sufficiently exhibited. After that, the compatibility with polyethylene glycol is not obtained, the quality of the lubricating layer 4 is lowered, and the coatability is also lowered, so that the production efficiency of the entry board is lowered.

  Among the above modified components, propylene glycol alginate has a thickening action, so in the lubricating layer containing this, the viscosity of the lubricating component melted by the frictional heat at the time of drilling does not decrease so much. As in the case of blending the saponified product, it prevents the lubrication component from escaping from the cutting site due to excessive fluidity of the lubrication component, maintains a stable and good lubricating action, and prevents the lubrication component deposited on the drill from dripping. Make it difficult to occur.

  The blending amount of such propylene glycol alginate is preferably in the range occupying 0.1 to 5% by mass of the base composition. After that, the compatibility with polyethylene glycol is not obtained, the quality of the lubricating layer 4 is lowered, and the coatability is also lowered, so that the production efficiency of the entry board is lowered.

  In the modified component, the chelating agent has a function of preventing adhesion of cutting waste to the drill bit due to drilling. That is, when drilling with a small diameter of 0.3 mm or less is repeated with the same drill, workpieces such as a base plate for printed wiring and metal cutting scraps such as resin and aluminum accompanying perforation of the entry board 1 are generated. The drill bit adheres to the drill bit, which causes the drill to run out of core, leading to reduced hole position accuracy and increased drill breakage frequency. This can be eliminated by adding a chelating agent.

  Examples of such chelating agents include aminocarboxylic acid chelating agents such as ethylenediamine tetraacetate (EDTA) and nitrilotriacetate (NTA), and phosphonic acids such as hydroxyethylidene diphosphonic acid and aminotrialkylenesulfonic acid. At least one selected from chelating agents is preferably used. Accordingly, the amount of the chelating agent is preferably in the range of 0.01 to 5% by mass of the base composition. If the amount is too small, the effect of use cannot be obtained. Hardens to cause cracks and increase costs.

  If the thickness of the lubricating layer 4 is less than 10 μm, sufficient lubricity cannot be obtained, and if it exceeds 100 μm, the coating efficiency decreases and the production efficiency deteriorates and the material cost increases, so the range of 10 to 100 μm. It is good to do.

  The entry board for drilling a small-diameter hole according to the present invention has a structure in which a lubricating layer 4 is provided on at least one surface of an aluminum substrate 2 with an undercoat layer 3 interposed therebetween. The undercoat layer 3 and the lubricating layer 4 are provided on both surfaces. A configuration can also be adopted. In addition, the entry board of the present invention can be applied to small-diameter drilling for various workpieces, but it is most suitable for drilling for a printed wiring board like the small-diameter drilling method of the present invention. is there.

Example 1
On one side of a 150 μm thick aluminum substrate made of JIS A3004-H18 material, 70 parts by mass of polyvinyl alcohol (degree of polymerization of about 300) having a saponification degree of 80 mol% and polyvinyl alcohol having a saponification degree of 73.5 mol% (polymerization) About 500) 30 parts by mass of the mixture was applied to a thickness of 3 μm to form an undercoat layer. Next, on this undercoat layer, 20 parts by mass of polyethylene glycol having a number average molecular weight of 400, 60 parts by mass of polyethylene glycol having a number average molecular weight of 20000, 20 parts by mass of polyvinyl alcohol having a saponification degree of 80 mol%, and 5 parts by mass of trimethylolpropane. Part of the mixed composition was applied to a thickness of 30 μm to form a lubricating layer, and an entry board for small-diameter drilling was prepared.

Example 2
Entry board for drilling small-diameter holes in the same manner as in Example 1 except that an undercoat layer was formed by coating polyvinyl alcohol having a saponification degree of 85 mol% (degree of polymerization of about 400) to a thickness of 2 μm. Was made.

Comparative Example 1
An entry board for small-diameter drilling was prepared in the same manner as in Example 1 except that an undercoat layer having a thickness of 3 μm was formed using polyvinyl acetate having an average molecular weight of 38000 and a saponification degree of 10 mol%.

Comparative Example 2
A small aperture was obtained in the same manner as in Example 1 except that an undercoat layer having a thickness of 2 μm was formed using a vinyl acetate / vinyl chloride copolymer having an average molecular weight of 31000 and comprising 13 parts by weight of vinyl acetate and 87 parts by weight of vinyl chloride. An entry board for drilling was prepared.

Example 3
For forming the lubricating layer, a mixed composition of 27 parts by mass of polyethylene glycol having a number average molecular weight of 400, 70 parts by mass of polyethylene glycol having a number average molecular weight of 20000, 3 parts by mass of propylene glycol alginate, and 5 parts by mass of trimethylolpropane was used. An entry board for small-diameter drilling was prepared in the same manner as in Example 1 except that this was coated to a thickness of 30 μm to form a lubricating layer.

Example 4
For forming a lubricating layer, a mixed composition of 29 parts by mass of polyethylene glycol having a number average molecular weight of 400, 70 parts by mass of polyethylene glycol having a number average molecular weight of 20000, 1 part by mass of ethylenediaminetetraacetate, and 5 parts by mass of trimethylolpropane is used. An entry board for small-diameter drilling was prepared in the same manner as in Example 1 except that the lubricating layer was formed by coating to a thickness of 30 μm.

Example 5
For forming a lubricating layer, 28 parts by mass of polyethylene glycol having a number average molecular weight of 400, 70 parts by mass of polyethylene glycol having a number average molecular weight of 20000, 1 part by mass of propylene glycol alginate, 1 part by mass of nitrilotriacetate, and 5 parts by mass of trimethylolpropane. An entry board for small-diameter drilling was prepared in the same manner as in Example 1 except that a part of the mixed composition was used.

Comparative Example 3
Except that an undercoat layer having a thickness of 2 μm was formed using a vinyl acetate / vinyl chloride / maleic acid copolymer having an average molecular weight of 32,000 consisting of 13 parts by mass of vinyl acetate, 86 parts by mass of vinyl chloride and 1 part by mass of maleic acid. In the same manner as in Example 5, a small-diameter drilling entry board was produced.

Evaluation test 1
Using the entry boards of Examples 1 to 5 and Comparative Examples 1 to 3, the state of the lubricating layer when the surface on which the lubricating layer was formed was bent 180 degrees was observed. The case where no peeling or cracking of the lubricating layer occurred was indicated by “◎”, and the case where the lubricating layer was slightly peeled or cracked was indicated by “◯”. The results are shown in Table 1.

Evaluation test 2
4 layers of printed wiring board made of epoxy resin with a total thickness of 0.1 mm, with 12 μm thick copper plates laminated on both sides of a 1.5 mm thick baking sheet, the topmost base plate The entry board for drilling a small-diameter of any of Examples 1 to 5 and Comparative Examples 1 to 3 is arranged with the lubricating layer side facing upward, and the entry board is drilled from above with a drill having a diameter of 0.1 mm. A 2000-hit drilling process reaching the glass plate was performed. Then, a hole position of 2000 hits is measured with a coordinate measuring machine for each base plate of the drilling process, and the deviation from the set position is checked, and the lubricant component drips on the uppermost base plate after each drilling process. It was investigated whether there was any. The results are shown in Table 1.

  As is clear from the results in the above table, the entry boards of Examples 1 to 5 according to the present invention are excellent in the adhesion of the lubricating layer to the substrate and do not cause any peeling or cracking of the lubricating layer even under severe bending. In addition, even when used for drilling with a very small diameter such as a drill diameter of 0.1 mm, holes can be formed with very high positional accuracy, and contamination due to dripping of lubricating components during processing can be prevented. On the other hand, in the entry boards of Comparative Examples 1 to 3, in addition to causing slight peeling and cracking of the lubricating layer by bending, in addition to the poor position accuracy of the holes formed when used for drilling, It can be seen that contamination due to dripping of the lubricating component during processing is likely to occur.

It is an expanded sectional view of the entry board for small diameter drilling processing concerning one embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Entry board for small diameter drilling process 2 ... Aluminum substrate 3 ... Undercoat layer 4 ... Lubrication layer

Claims (5)

  A small-diameter drilling entry board characterized in that a lubricating layer is formed on at least one surface of an aluminum substrate via an undercoat layer containing a saponified polyvinyl alcohol.   The entry board for small-diameter drilling according to claim 1, wherein the saponification degree of the saponified product of polyvinyl alcohol in the undercoat layer is 50 to 90 mol%.   The entry board for small-diameter drilling according to claim 1 or 2, wherein the undercoat layer has a thickness of 0.1 to 5 µm.   The small-diameter hole according to any one of claims 1 to 3, wherein the lubricating layer contains polyethylene glycol having a number average molecular weight of 100 or more and less than 1000, polyethylene glycol having a number average molecular weight of 10,000 or more, and trimethylolpropane. Entry board for drilling.   The entry board according to any one of claims 1 to 4 is arranged on the plurality of stacked printed wiring boards, and in this state, the entry board and the printed wiring board are used from above using a drill. A small-diameter drilling method, wherein a hole having a diameter of 0.3 mm or less is formed in a plate.

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