Multilayer board manufacturing method

Abstract

To provide a multilayer substrate manufacturing technology that enables multilayer substrates to be manufactured at low cost. First, a cover sheet 2 is placed on a base sheet 1 of the same shape and size. Wiring 11 is provided on one side of the base sheet 1. A plurality of holes 21x, 21y, and 21z are opened at positions where electronic components will be attached to the base sheet 1 after the cover sheet 2. The pre-attached electronic components are attached to the base sheet through hole 21y. Then, the superposed base sheet 1 and cover sheet 2 are folded so that hole 21y is exposed to the outside. The pre-attached electronic components fit into hole 21x. The post-attached electronic components are attached to the base sheet 1 through hole 21z.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a multilayer substrate. [Background technology]

[0002] Various electronic devices contain circuit boards with various electronic components attached to them, and among these boards there are those called multilayer boards. A multilayer board is made by stacking multiple substrates. Each substrate has a plate- or sheet-shaped base material, and wiring is provided on at least one surface of the base material. Multilayer boards, which have a structure in which multiple substrates, each with its own wiring, are stacked, are naturally thicker, but can contain more circuits and electronic components in the same area than single-layer boards, allowing for high integration, and are therefore widely used.

[0003] Generally, the circuits in each board included in a multi-layer board must be electrically connected to each other. For this reason, multilayer boards typically have holes called through holes that penetrate all of the stacked boards. Typically, a metal layer is formed on the inner surface of the through hole, for example, by plating with a conductive metal. The wiring on each board is then connected to the metal layer, allowing the wiring on each board to be electrically connected to each other via the metal layer inside the through hole. For example, the wiring on each side of a board is connected to each other via the metal layer inside the through hole. Alternatively, techniques that do not use through holes are also being used. For example, by connecting wires provided on both sides of a single substrate or on any surfaces of multiple substrates with conductive connecting wires outside the substrate, it is possible to electrically connect wires located at distant locations, and such techniques are also being put to practical use.

[0004] Although the multilayer substrate is as described above, there is room for improvement. As mentioned above, conventional multilayer boards have through holes and connecting wires outside the board. When through holes are present, the manufacturing costs of multilayer boards are increased because it is necessary to perform a process of drilling holes in the board to form the through holes and a process of forming a metal layer on the inner surface of the through holes. Furthermore, as described above, because the through holes are electrically connected to the wiring of each board included in the multilayer board, it is necessary to accurately position the wiring relative to the base material of each board, and to accurately position the multiple boards relative to each other when stacking multiple boards. Of course, the two types of positioning accuracy required for the reasons described above are technically achievable. However, achieving these accuracy requirements is time-consuming and likely to increase costs. In addition, conventional manufacturing processes for multilayer boards involve separately manufacturing the multiple boards to be laminated, and then stacking the separately manufactured multiple boards. Alternatively, cover sheets and copper foils are laminated on the top and bottom of the boards to form circuits on the top and bottom surfaces of the boards, and this process is repeated to gradually increase the number of layers. Here, when manufacturing each board, common processes, such as cutting circuits in the copper foil attached to the board and plating with gold, are typically performed for each board. Therefore, costs tend to increase due to the repeated execution of common processes for each board. The same applies when laminating cover sheets and copper foils on the top and bottom of the boards. Even when connecting wires are provided outside the board, the process increases the manufacturing cost of the multilayer board, and the cost is likely to increase due to the repeated execution of common processes on each board. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 7558604 Summary of the Invention [Problem to be solved by the invention]

[0006] In view of this, the inventors of the present invention have developed a method for manufacturing a multilayer substrate that is different from conventional methods and have already obtained a patent for it. The manufacturing method for this multilayer board uses a thin, rectangular base sheet made of thermoplastic resin with a circuit formed on its front surface, and a thin cover sheet made of thermoplastic resin with the same size and shape as the base sheet. Holes (usually multiple) are drilled at appropriate positions on the cover sheet, which will later be used to attach electronic components to the base sheet. In this manufacturing method, a cover sheet is first placed on the front surface of a base sheet, with the front surface of the base sheet and the circuit visible through holes in the cover sheet. Next, the overlapped base sheet and cover sheet are folded while still overlapping. The folding method is basically free, but the holes in the base sheet should be exposed to the outside. The folding of the base sheet and cover sheet can be done, for example, like origami, a traditional Japanese art. Next, the folded base sheet and cover sheet are heated. Preferably, heating is performed while pressing with an appropriate pressure. The heat melts a portion of the cover sheet, which is then cooled and hardened. The melted cover sheet acts like an adhesive, fixing adjacent base sheets and cover sheets, and adjacent cover sheets to each other. In some cases, adjacent base sheets are fixed to each other. Finally, the electronic component is inserted through a hole provided in the cover sheet and attached to the base sheet while being electrically connected to a circuit provided in the base sheet that is removed from the hole. This completes the multilayer board. The multilayer board manufactured by this manufacturing method does not require through holes or external connection wires because when electronic components are attached to the base sheet, an electrical circuit is formed in combination with the electronic components on the front side of the base sheet, which eliminates the need for processes for fabricating them and reduces the cost of manufacturing the multilayer board. Furthermore, in the above-described method for manufacturing a multilayer substrate, it is possible to obtain multilayer substrates with various structures by changing the way in which the superposed base sheet and cover sheet are folded in the second step.

[0007] This method of manufacturing a multilayer board is advantageous in that it allows for the production of multilayer boards at low cost, but it is prone to limiting the thickness of electronic components that can be attached to the base sheet. For example, the thickness of the electronic components is preferably equal to or less than the thickness of the cover sheet that covers the base sheet, so that the electronic components do not protrude from the cover sheet.

[0008] An object of the present invention is to provide a method for manufacturing a multilayer board that enables a multilayer board having electronic components mounted thereon that are thicker than conventional boards to be manufactured at low cost. [Means for solving the problem]

[0009] The present invention for solving the above-mentioned problems is as follows. The present invention is a method for manufacturing a multilayer board, which manufactures a multilayer board having a plurality of electronic components attached thereto by carrying out steps 1, 2, 3, and 4 in that order. Hereinafter, the method for manufacturing a multilayer board may be simply referred to as the "manufacturing method." The first to fourth steps in the method for manufacturing this multilayer substrate are as follows. The first step involves preparing a base sheet that is an insulating rectangular sheet made of thermoplastic resin and has conductive wiring arranged in an appropriate position on its front surface, and has holes formed as necessary in positions that will interfere with pre-attached electronic components that are part of the plurality of electronic components when the second step is carried out; a cover sheet that is made of insulating thermoplastic resin and has substantially the same shape and size as the base sheet, and that has at least one hole formed in a position where the pre-attached electronic component is to be attached and at least one hole formed in a position where a post-attached electronic component that is an electronic component other than the pre-attached electronic component is to be attached, and that has at least one hole formed in a position that will interfere with the pre-attached electronic component when the second step is carried out, and that is overlapped on the front surface of the base sheet such that its outline matches the outline of the base sheet; and the pre-attached electronic component that is inserted into the hole formed in the cover sheet at the position where the pre-attached electronic component is to be attached, and that is fixed to the base sheet exposed through the hole in a state where it is electrically connected to the wiring on the front surface of the base sheet exposed through the hole. The second step involves folding the base sheet and the cover sheet that were superimposed in the first step so that the holes provided in the cover sheet at the positions where the post-installed electronic components are to be attached are exposed to the outside, and so that the pre-installed electronic components do not interfere with the base sheet and the cover sheet. The third step involves heating the folded base sheet and cover sheet at a temperature within a range that does not affect the performance of the pre-attached electronic components to melt at least a portion of the cover sheet, and then cooling and hardening the cover sheet, thereby bonding adjacent base sheets and cover sheets together. The fourth step involves inserting the retrofit electronic component into the hole provided in the cover sheet at the position where the retrofit electronic component is to be attached, and fixing the retrofit electronic component to the base sheet visible through the hole while electrically connecting it to the wiring on the front surface of the base sheet visible through the hole.

[0010] In this method for manufacturing a multilayer substrate, the first step, the second step, the third step, and the fourth step are carried out in this order, as described above. In the first step, the following is prepared using a base sheet, a cover sheet, and an electronic component: Simply put, in the first step, a product is produced in which a pre-attached electronic component, which is part of the electronic component, is attached to the base sheet of the overlapping base sheet and cover sheet. The base sheet is a rectangular sheet made of insulating thermoplastic resin. Wiring is provided on the front surface of the base sheet. Wiring is provided only on the base sheet, not on the cover sheet. Wiring is also provided only on one side of the base sheet. The surface of the base sheet on which wiring is provided is the front surface. Wiring is conductive and is generally made of conductive metal. There are no limitations on the method for providing wiring on the base sheet, and wiring can be provided on the front surface of the base sheet using known or well-known methods. For example, techniques for providing wiring on the base sheet include the subtractive method, in which copper foil is attached to the entire front surface of the base sheet with an adhesive and the unnecessary portion is removed by chemical etching, leaving the wiring, and the additive method, in which wiring is plated (usually by electroless plating or electrolytic plating) on ​​the front surface of the base sheet. The cover sheet is a rectangular sheet with virtually the same shape and size as the base sheet. The cover sheet is made of thermoplastic resin and is insulating. As described below, multiple electronic components are attached to the base sheet. The base sheet may have at least one hole. The hole is provided in the base sheet at a position where it will interfere with a pre-attached electronic component, which is part of the multiple electronic components, when the second step is performed. Providing this hole prevents interference between the base sheet and the pre-attached electronic component, which may occur when the second step is performed using a pre-attached electronic component that is thicker than the cover sheet. If the above-described interference between the base sheet and the pre-attached electronic component does not occur even without the hole, this hole is not necessarily required. As can be understood from the description of the present invention, multiple electronic components are attached to the base sheet in the manufacturing method of the present invention, and some of the multiple electronic components are attached to the base sheet in the first step, while the remaining multiple electronic components are attached to the base sheet in the fourth step. The electronic components attached to the base sheet in the first step are pre-attached electronic components, and the electronic components attached to the base sheet in the fourth step are post-attached electronic components. Post-attached electronic components can also be understood as electronic components other than the pre-attached electronic components. The cover sheet has multiple holes. There are three types of holes provided in the cover sheet.The first type of holes provided in the cover sheet are holes provided at positions corresponding to the positions where pre-attached electronic components will be attached on the base sheet when the cover sheet is overlaid on the base sheet. The presence of at least one such hole allows the pre-attached electronic components to be attached to the base sheet without interference from the cover sheet. The second type of holes provided in the cover sheet are holes provided at positions corresponding to the positions where post-attached electronic components will be attached on the base sheet when the cover sheet is overlaid on the base sheet. The presence of at least one such hole allows the post-attached electronic components to be attached to the base sheet without interference from the cover sheet. The third type of holes provided in the cover sheet are holes provided at positions that will interfere with the pre-attached electronic components when the second step is performed. Providing these holes makes it possible to prevent interference between the cover sheet and the pre-attached electronic components, which may occur when the second step is performed using pre-attached electronic components that are thicker than the cover sheet. The first step of the present invention is to prepare a state in which the cover sheet described above is superimposed on a base sheet and a pre-attached electronic component is attached to the base sheet (a combination of the base sheet, cover sheet, and pre-attached electronic component). When the first step is completed, the pre-attached electronic component is inserted into a hole provided in the cover sheet at the position where the pre-attached electronic component will be attached, and is fixed to the base sheet visible through the hole in a state in which it is electrically connected to the wiring on the front side of the base sheet visible through the hole.

[0011] When performing the first step, the cover sheet may be placed on the base sheet, and then the pre-attached electronic components may be attached to the base sheet through holes drilled in the cover sheet, or the pre-attached electronic components may be attached to the base sheet, and then the cover sheet may be placed on the base sheet while adjusting its position so that the pre-attached electronic components attached to the base sheet are inserted into the holes provided in the cover sheet. However, if the latter method is used, it is necessary to place the cover sheet on the base sheet while adjusting the pre-attached electronic components previously attached to the base sheet so that they are inserted into the holes, and aligning the contour of the cover sheet with the base sheet, which tends to make the work more difficult, and it is expected that the work will often be more difficult when there are multiple pre-attached electronic components attached to the base sheet. Therefore, it is considered that the former of the two above-mentioned work orders is often preferable when performing the first step. That is, it is considered preferable in the first step to overlap the cover sheet on the front surface of the base sheet in a state where its outline matches the outline of the base sheet, and then to fix the pre-attached electronic component inserted into the hole provided in the cover sheet at the position where the pre-attached electronic component is to be attached to the base sheet exposed through the hole in a state where the pre-attached electronic component is electrically connected to the wiring on the front surface of the base sheet exposed through the hole. Furthermore, in the first step, the cover sheet may be superimposed on the front surface of the base sheet, and then the cover sheet may be temporarily fixed to the base sheet. Temporary fixing means fixing the relative positional relationship between the base sheet and the cover sheet before the third step described below is performed. Temporary fixing can be performed, for example, by bonding a portion (e.g., the four corners) of the contacting portion of the base sheet and the cover sheet with an adhesive, but there are no limitations on the method of temporary fixing. Temporary fixing may be performed using a pin lamination method, which is used in general multilayer board manufacturing methods, in which boards with holes are stacked and pins are inserted through the holes in each board to position them, or using a temporary fixing jig (such as a clip) that clamps the base sheet and the cover sheet together.

[0012] In the second step, the base sheet and cover sheet that were stacked in the first step are folded together. The folding is performed at least once, and can be either a mountain fold or a valley fold, or a combination of mountain and valley folds. After the second step is performed, the holes provided in the cover sheet at the positions where retrofit electronic components are to be attached are exposed to the outside. By folding together with the cover sheet, at least a portion of the base sheet is at least doubly overlapped with the remainder of the base sheet, although the cover sheet may be interposed between them. As a result, the wiring provided on the front surface of the base sheet is also at least doubly stacked. When the second step is performed, the pre-attached electronic components are prevented from interfering with the base sheet and the cover sheet. That is, when the base sheet and the cover sheet are folded, the pre-attached electronic components that would interfere with the base sheet if there were no holes are positioned inside the holes in the base sheet. This prevents the pre-attached electronic components from interfering with the base sheet. Similarly, when the base sheet and the cover sheet are folded, the pre-attached electronic components that would interfere with the cover sheet if there were no holes positioned to interfere with the pre-attached electronic components are positioned inside the holes in the cover sheet. This prevents the pre-attached electronic components from interfering with the cover sheet either. When the overlapping base sheet and cover sheet are folded in the second step, slight misalignment may occur at the edges of the two, depending on the folding method. If such misalignment is acceptable, the base sheet and cover sheet may be completely identical in shape and size. Alternatively, if it is desired to reduce or eliminate the misalignment at the edges of the base sheet and cover sheet when folded, the base sheet and cover sheet may be almost identical in shape and size, but may differ slightly. In other words, the cover sheet may be a sheet of "virtually" the same shape and size as the base sheet. In the third step, the adjacent base sheet and cover sheet are bonded together in a folded state. To bond them together, the folded base sheet and cover sheet are heated to melt at least a portion of the cover sheet, which is then cooled and hardened. The molten cover sheet is then used like an adhesive to bond the adjacent base sheet and cover sheet together. The term "cooling" used herein does not necessarily require an active cooling process, but also includes leaving the heated base sheet and cover sheet in an atmosphere at a temperature lower than the melting point of the thermoplastic resin constituting the cover sheet (e.g., room temperature). It is also acceptable to melt a portion of the base sheet and then cool and harden it. Furthermore, in the third step, adjacent cover sheets may also be bonded together among the overlapping base sheet and cover sheet. In any case, by performing the third step, the folded base sheet and cover sheet are integrated, and their relative positions as a whole are basically fixed. As described above, the heating in the third step must be performed at a temperature at which at least a portion of the cover sheet melts, but the temperature must be within a range that does not affect the performance of the pre-attached electronic components attached to the base sheet. Then, a fourth step is performed to attach the electronic component to the base sheet. As described above, after the second step is completed, the hole provided in the cover sheet is exposed to the outside. The cover sheet has a hole provided in the base sheet at a position corresponding to the position where the retrofit electronic component will be attached. The retrofit electronic component is inserted through this hole and fixed to the base sheet visible through this hole in a state where it is electrically connected to the wiring on the front side of the base sheet visible through this hole. Needless to say, the wiring provided on the base sheet is designed so that when at least one pre-installed electronic component and at least one post-installed electronic component are attached to the base sheet, an electrical circuit is formed in combination with the electronic components. In this way, a multi-layer substrate is manufactured. The electronic components used in manufacturing multilayer boards include IC (integrated circuit) chips, BGA (ball grid array), CSP (chip size package / chip scale package), resistors, capacitors, etc., which are the same as the electronic components mounted on general boards. Among these, some tend to be thick and can withstand the heat in the third process, making them candidates for pre-attached electronic components. For example, resistors and capacitors can be selected as pre-attached electronic components.

[0013] The multilayer board manufactured by the multilayer board manufacturing method of the present application described above does not require through holes or connecting wires outside the board because an electric circuit is formed by combining at least one pre-installed electronic component and at least one post-installed electronic component that are planned to be attached to the base sheet. Therefore, the process for manufacturing them can be omitted, thereby reducing the cost of manufacturing the multilayer board. Furthermore, in the above-described method for manufacturing a multilayer substrate, it is possible to obtain multilayer substrates with various structures by changing the way in which the superposed base sheet and cover sheet are folded in the second step. Furthermore, in the multilayer board manufactured by the above-described manufacturing method for a multilayer board, the front surface of the base sheet is covered with a cover sheet except for the areas where holes are formed, so that the wiring is protected by the cover sheet and is less likely to break. These effects are also achieved by the patented invention previously granted by the applicant. The present invention also achieves the effect of enabling the manufacture of multilayer boards with thicker electronic components than conventional ones. The pre-attached electronic components in this application can be thicker electronic components. When the second step, i.e., when the superposed base sheet and cover sheet are folded, the pre-attached electronic components are inserted into the holes in the base sheet that will interfere with the pre-attached electronic components when the second step is performed, and the holes in the cover sheet that will interfere with the pre-attached electronic components when the second step is performed, thereby avoiding interference with the base sheet and the cover sheet. This allows the thickness of the pre-attached electronic components to be increased, for example, to be thicker than the thickness of the cover sheet.

[0014] As described above, both the base sheet and the cover sheet used in the present invention are made of thermoplastic resin. The thermoplastic resins constituting the base sheet and the cover sheet may or may not be the same. The thermoplastic resin that constitutes the base sheet and the cover sheet can be, for example, liquid crystal polymer (LCP). Other materials that can be used include polyphenylene sulfide (PPE), polyether ether ketone (PEEK), and fluororesin (FR), but other thermoplastic resins can also be used. Liquid crystal polymers are well known as materials, and are known to have low loss and excellent high frequency characteristics, making them suitable for use as materials for the base sheet and cover sheet. The thicknesses of the base sheet and cover sheet must be thin enough that the second step can be performed with both sheets stacked together. On the other hand, the thickness of the cover sheet is preferably greater than the thickness of the wiring formed on the base sheet. As mentioned above, the cover sheet protects the wiring after the third step. However, if the cover sheet is thinner than the wiring, the wiring may not be sufficiently protected, resulting in the wiring being exposed at the top without being covered by the cover sheet. Even if the wiring can be protected by covering it with the cover sheet, the thickness of the wiring may appear as a convex portion on the cover sheet, potentially preventing the flatness of the multilayer substrate. If this occurs, problems such as poor bonding may occur when electronic components are ultimately mounted on a surface with poor flatness. Furthermore, the thickness of the cover sheet may be thinner than the thickness of the base sheet. By making the cover sheet thinner than the base sheet, the rigidity of the stacked base sheet and cover sheet during the second step is reduced, improving foldability and also reducing processing and material costs.

[0015] As described above, the base sheet used in the present invention is rectangular, and the cover sheet is rectangular with the same shape and size as the base sheet. The base sheet may be square. As described above, the base sheet and the cover sheet are folded by being folded while overlapping each other, but if the base sheet and the cover sheet are square, various folding methods used in origami, a traditional Japanese art, can be applied, and for example, a multilayer board having a regular shape such as a rectangle or a triangle can be obtained in which the number of sheets, including the base sheet and the cover sheet, present at every portion in the thickness direction is the same. For example, the shape of the base sheet and cover sheet after the fourth step can be a square or a right-angled isosceles triangle. Such a regular shape is convenient when incorporating the multilayer substrate into a final product. Two right-angled isosceles triangle multilayer substrates can also be combined to form a square.

[0016] In the present invention, the second step can be carried out so that after the fourth step is completed, all parts of the base sheet and the cover sheet are essentially parallel except for the folded parts. The multilayer substrate thus manufactured has the base sheet and cover sheet stacked in parallel except for the folded portions, forming a plate-like overall structure. The plate-like multilayer substrate has the same shape as many conventional multilayer or single-layer substrates, and is expected to find many applications. On the other hand, after the fourth step is completed, the base sheet and a portion of the cover sheet may be arranged not to be parallel to the other portions of the base sheet and the cover sheet. In this case, the multilayer board may have a three-dimensional shape rather than a plate shape. Depending on the final product to which the board will be attached, a multilayer board having a three-dimensional shape may be convenient. The fact that multilayer substrates of such various shapes can be obtained using the same manufacturing method is one of the advantages of the manufacturing method of the multilayer substrate according to the present invention. It is also possible to carry out the second step so that, after the fourth step is completed, the number of the base sheets and the cover sheets stacked in a certain portion other than the folded portion is different from the number of the base sheets and the cover sheets stacked in other portions. For example, suppose a plate-shaped multilayer substrate is manufactured using a multilayer substrate manufacturing method. In one area of ​​the completed multilayer substrate, four base sheets and four cover sheets are stacked, while in another area, eight base sheets and eight cover sheets are stacked. Naturally, the thickness of the other area of ​​the multilayer substrate will be greater than that of the one area. One advantage of the multilayer substrate manufacturing method of the present invention is that it is possible to obtain multilayer substrates with different thicknesses in different areas using the same manufacturing method. Because it is possible to manufacture substrates with different thicknesses in different areas, less base material is used in the thinner areas, which has the advantage of contributing to reduced material costs and process costs.

[0017] As described above, in the third step, the superposed base sheet and cover sheet are heated. In the third step, the superposed base sheet and cover sheet may be heated while being pressed. This allows the base sheet and cover sheet, or the cover sheets, to be firmly and evenly bonded to each other. The third step may be performed by heating (or heating and pressurizing) the entire base sheet and cover sheet after the second step is completed all at once, or by heating (or heating and pressurizing) a portion of the base sheet and cover sheet and another portion of the cover sheet after the second step is completed in multiple batches. When the completed multilayer substrate is to have a three-dimensional shape, the third step is often performed in the latter manner.

[0018] In the method for manufacturing a multilayer substrate of the present invention, a wiring step of providing the wiring on the front surface of the base sheet may be carried out before the first step. There are no particular restrictions on the technology that can be used to provide the wiring on the front surface of the base sheet in the wiring step, and as already mentioned, any known or well-known technology can be used. The width of the wiring at the portion where the base sheet is folded may be wider than the width of the portions on both sides in the length direction. In the manufacturing method of a multilayer board of the present application, the wiring is folded together with the base sheet. This may result in some risk of the wiring being broken. Making the width of the wiring at the portion where the base sheet is folded wider than the width of the portions on both sides in the length direction helps prevent the wiring from being broken when folded together with the base sheet. When the wiring step is carried out before the first step, it is also possible to prepare the wiring so that the width of the wiring at the portion where the base sheet is folded is wider than the width of the portions on both sides of the length. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 2 is a view showing a substrate sheet used in an embodiment of the present invention as viewed from the front side. [Figure 2] FIG. 2 is a diagram showing a substrate sheet provided with wiring as viewed from the front side. [Figure 3] An enlarged view of the area around symbol X in Figure 2. [Figure 4] FIG. 4 is a diagram showing the cover sheet as seen from the front side. [Figure 5(A)] FIG. 10 is a view from the front side showing the state in which the cover sheet is superimposed on the base sheet in the first step in one embodiment. [Figure 5(B)] 5(B) is a front view of the base sheet and the cover sheet shown in FIG. 5(A) with pre-attached electronic components attached to the base sheet. FIG. [Figure 6(A)] 5(B) is a view substantially the same as FIG. 5(B), showing the object shown in FIG. 5(B) from the same direction as FIG. 5(B). [Figure 6(B)] This figure shows the state of the base sheet and cover sheet shown in Figure 6(A) when folded back to the right of the vertical fold line at the fold line, as viewed from the front side of Figure 6(A). [Figure 6(C)]A figure showing the state of the base sheet and cover sheet shown in Figure 6(A) when folded back to the right of the vertical fold line at the fold line, as viewed from the back of Figure 6(A). [Figure 6(D)] 6(B) is a view substantially the same as FIG. 6(B), showing the object shown in FIG. 6(B) from the same direction as FIG. 6(B). [Figure 6(E)] A figure showing the state of the base sheet and cover sheet, viewed from the front side of Figure 6(D), with the upper part of the base sheet and cover sheet shown in Figure 6(D) above the upper of the three fold lines running horizontally folded back to the back at the fold line, and the lower part of the base sheet and cover sheet shown in Figure 6(D) below the lower of the three fold lines running horizontally folded back to the back at the fold line. [Figure 6(F)] A figure showing the state of the base sheet and cover sheet, viewed from the back of Figure 6(D), with the portion above the upper of the three fold lines running horizontally through the base sheet and cover sheet shown in Figure 6(D) folded back to the back at the fold line, and the portion below the lower of the three fold lines running horizontally through the base sheet and cover sheet folded back to the back at the fold line. [Figure 6(G)] 6(E) is a view substantially the same as FIG. 6(E), showing the object shown in FIG. 6(E) from the same direction as FIG. 6(E). [Figure 6(H)] A figure showing the state of the base sheet and cover sheet shown in Figure 6(G) when the portion above the horizontal fold line is folded back to the front side at the fold line, as seen from the front side of Figure 6(G). [Figure 6(I)] A figure showing the state of the base sheet and cover sheet shown in Figure 6(G) when the portion above the horizontal fold line is folded back to the front at the fold line, as seen from the back of Figure 6(G). [Figure 7(A)] FIG. 10 is a cross-sectional view of the base sheet, the cover sheet, and the pre-attached electronic component in the vicinity of the pre-attached electronic component when the second step of the embodiment is completed, according to an example. [Figure 7(B)]FIG. 10 is a cross-sectional view of the base sheet, the cover sheet, and the pre-attached electronic component in the vicinity of the pre-attached electronic component when the second step of the embodiment is completed, according to another example. [Figure 7(C)] FIG. 10 is a cross-sectional view of the base sheet, the cover sheet, and the pre-attached electronic component in the vicinity of the pre-attached electronic component when the second step of the embodiment is completed, according to yet another example. [Figure 8(A)] 7A is a diagram showing a state in which an add-on electronic component is attached to the base sheet of the base sheet and cover sheet shown in FIG. 6(H). FIG. [Figure 8(B)] 7 is a diagram showing a state in which an after-installation electronic component is attached to the base sheet of the base sheet and cover sheet shown in FIG. 6(I). FIG. [Figure 9] FIG. 10 is a diagram showing a base sheet provided with wiring used in Modification 1, viewed from the front side. [Figure 10] FIG. 10 is a diagram showing the cover sheet used in the first modification example as viewed from the front side. [Figure 11] FIG. 10 is a view from the front side showing the state in which the cover sheet is superimposed on the base sheet in the first step of Modification 1, and pre-attached electronic components are further attached to the base sheet. [Figure 12(A)] 12 is a view substantially the same as FIG. 11, showing the object shown in FIG. 11 viewed from the same direction as FIG. 11; [Figure 12(B)] A figure showing the state of the base sheet and cover sheet shown in Figure 12(A) when viewed from the front side, with the portion to the right of the vertical fold line folded back to the back at the fold line. [Figure 12(C)] A figure showing the state of the base sheet and cover sheet shown in Figure 12(B) folded back to the back at the fold line, viewed from the front side of Figure 12(B). [Figure 13](A) is a view essentially identical to FIG. 12(C), showing the base sheet and cover sheet shown in FIG. 12(C) viewed from the same direction as FIG. 12(B); (B) is a view showing the base sheet and cover sheet shown in FIG. 13(A) viewed from the back of FIG. 13(A); (C) is a view showing the base sheet of the base sheet and cover sheet shown in FIG. 13(A) with retrofit electronic components attached, viewed from the same direction as FIG. 13(A); (D) is a view showing the base sheet of the base sheet and cover sheet shown in FIG. 13(B) with retrofit electronic components attached, viewed from the same direction as FIG. 13(B); and (E) is a side view of the base sheet and cover sheet shown in FIG. 13(D). [Figure 14(A)] FIG. 10 is a cross-sectional view of the base sheet, the cover sheet, and the pre-attached electronic component in the vicinity of the pre-attached electronic component when the second step of the first modified example is completed, according to an example. [Figure 14(B)] FIG. 10 is a cross-sectional view of the base sheet, the cover sheet, and the pre-attached electronic component in the vicinity of the pre-attached electronic component when the second step of the first modified example is completed, according to another example. [Figure 15] FIG. 11 is a diagram showing a base sheet provided with wiring used in Modification 2, viewed from the front side. [Figure 16] FIG. 11 is a view showing the cover sheet used in the second modification example as seen from the front side. [Figure 17] FIG. 10 is a view from the front side showing the state in which the cover sheet is superimposed on the base sheet in the first step of Modification 2. [Figure 18] (A) is a view from the front side of the base sheet and cover sheet shown in Figure 11 with pre-attached electronic components attached to the base sheet, (B) is a view from the front side of the base sheet and cover sheet shown in Figure 18(A) in the middle of being mountain-folded along the dashed lines and valley-folded along the dotted lines, and (C) is a view from the side of the base sheet and cover sheet when the second step of variant example 2 is completed. [Figure 19(A)] FIG. 18(B) is a view of the base sheet and cover sheet shown in FIG. 18(C) as viewed from one side. [Figure 19(B)]18(C) is a view of the base sheet and the cover sheet shown in FIG. 18(C) when viewed from the other side. [Figure 20] 10 is a cross-sectional view of the base sheet, the cover sheet, and the pre-attached electronic component in the vicinity of the pre-attached electronic component when the second step of the second modification example is completed. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, an embodiment of the present invention and a modification thereof will be described with reference to the drawings.

[0021] In this embodiment, a method for manufacturing a multilayer substrate will be described. The multilayer substrate is manufactured by carrying out each of the steps described below. The multilayer substrate is manufactured using a base sheet, a cover sheet, and a plurality of electronic components as materials.

[0022] First, the base sheet will be described. The front side of the base sheet 1 is shown in Fig. 1. This base sheet 1 has not yet been provided with wiring, which will be described later. The base sheet 1 is rectangular. As will be described later, the base sheet 1 may also be square, but in the example shown in FIG. The base sheet 1 is made of a thermoplastic resin and has insulating properties. Examples of the thermoplastic resin that constitutes the base sheet 1 include liquid crystal polymer (LCP), polyphenylene sulfide (PPE), polyether ether ketone (PEEK), and fluororesin (FR). Although not limited to these, in this embodiment, liquid crystal polymer (LCP) is selected as the thermoplastic resin that constitutes the base sheet 1. The thickness of the base sheet 1 can be appropriately selected within a range that allows the base sheet 1 to be folded together with the cover sheet as described below. The thickness can be, for example, 20 μm to 200 μm, preferably 25 μm to 100 μm. Holes 1x are formed in the base sheet 1. As will be described later, these holes 1x are not necessarily required, but are present in the base sheet 1 of this embodiment. The holes 1x are intended to prevent interference between the base sheet 1 and pre-attached electronic components (electronic components; pre-attached electronic components will be described later), which are some of the multiple electronic components that make up the multilayer board. The holes 1x have a shape and size that allows the pre-attached electronic components to be positioned inside them, thereby preventing interference between the pre-attached electronic components and the base sheet 1. Furthermore, the holes 1x are positioned so that the pre-attached electronic components will be inserted inside the holes 1x when the second step, which will be described later, is completed. In this embodiment, there are four holes 1x, but this number is not limited to four. There may be one hole 1x, or there may be multiple holes 1x. After the second step is completed, the same pre-attached electronic component may be located in multiple holes 1x, or different pre-attached electronic components may be located in multiple holes 1x. In the second step, as will be described later, the base sheet 1 and the cover sheet 2 are folded, and whether the same pre-attached electronic component is located in multiple holes 1x or different pre-attached electronic components are located in multiple holes 1x will be determined depending on the folding method, the position and number of pre-attached electronic components, etc.

[0023] In this embodiment, wiring 11 is provided on such a base sheet 1 (FIG. 2). In FIG. 2, the two-dot chain line labeled 12 is a folding line indicating the location where the base sheet 1 will be folded later. The wiring 11 is provided only on the front surface of the base sheet 1. The surface of the base sheet 1 on which the wiring 11 is provided is the front surface of the base sheet 1. The wiring 11 is electrically conductive and is generally made of an electrically conductive metal. Although not limited to this, in this embodiment the wiring 11 is made of an electrically conductive metal. When all electronic components are attached to the base sheet 1, the wiring 11 forms an electric circuit in combination with the electronic components. There are no particular limitations on the technology that can be used to provide the wiring 11 on the base sheet 1, and any known or well-known technology can be used. For example, the wiring 11 can be provided on the front surface of the base sheet 1 by a subtractive method in which copper foil is attached to the entire front surface of the base sheet 1 with an adhesive, and while the wiring portion is covered with a mask, unnecessary portions (portions of the copper foil not covered by the mask) are removed by chemical etching, and finally the mask is removed, leaving the copper foil covered by the mask on the front surface of the base sheet 1 as the wiring 11; or by an additive method in which a mask is provided on the front surface of the base sheet 1 so that only the portions corresponding to the wiring 11 are exposed, and then plating (e.g., electroless plating and electrolytic plating) is performed to provide a metal plating layer in the areas where the mask is not present, and then the mask is removed to obtain the wiring 11 on the base sheet 1. The wiring 11 can be provided all at once on the base sheet 1 by performing a process for providing wiring on the base sheet 1, such as a subtractive method or an additive method, in a single step, although this is not limited to this. In Fig. 2, the positions enclosed by dashed lines and marked with the reference numeral 13 are positions where electronic components will later be attached. Such positions will be referred to as planned positions 13 hereinafter. There are multiple planned positions 13. The planned positions 13 are not all the same size because they are adjusted to fit the size of the electronic components that will be attached there. As described above, electronic components are attached to the planned positions 13. There are two types of electronic components: those that are attached to the base sheet 1 in a first step described below, and those that are attached to the base sheet 1 in a fourth step described below. The former are called pre-attached electronic components, and the latter are called post-attached electronic components. In this embodiment, as described below, pre-attached electronic components are attached only to planned positions 13A, which are planned positions 13 shaded in FIG. 2, and post-attached electronic components are attached to the other planned positions 13. The electronic components may be IC chips, BGAs, CSPs, resistors, capacitors, etc., and are selected appropriately according to the performance required of the multilayer substrate. After being attached to the base sheet 1, the pre-attached electronic components are heated in the third step described below. In this embodiment, not only heating but also pressure is applied in the third step, although this is not limited to this. Therefore, the pre-attached electronic components must be able to withstand at least the heating described below (so that the performance of the electronic components is not affected), and in this embodiment, they must be able to withstand the heating and pressure described below. Examples of electronic components that often meet these requirements include resistors and capacitors. Furthermore, the wiring 11 does not necessarily have to be provided on a base sheet 1 with a uniform outline as shown in FIG. 1 . For example, it is also possible to provide 10 × 10 = 100 wirings 11 on a base sheet larger than the base sheet 1 shown in FIG. 1 (e.g., a base sheet 1 with dimensions 10 times larger than the base sheet 1 shown in FIG. 1 ), and then cut the base sheet 1 into 10 pieces vertically and horizontally to obtain 100 identical base sheets 1 with wiring 11 shown in FIG. 2 at once. Such a method for producing wiring 11 (a method for producing a base sheet 1 with a large number of pieces) enables efficient mass production of multilayer substrates. Even in this case, the 10 × 10 = 100 wirings to be provided on the large base sheet can be provided all at once by performing the process for providing wiring on the large base sheet once, thereby reducing the cost of producing the base sheet 1. In the base sheet 1, it does not matter whether the wiring 11 or the holes 1x are formed first.

[0024] The wiring 11 may or may not have the same width throughout. As will be described later, it is also possible to provide an appropriate width for each of the linear wirings 11 connecting electronic components attached to the base sheet 1. For example, the width of at least one of the linear wirings 11 may be different from the width of the other linear wirings 11. Furthermore, the width of a single linear wiring 11 does not need to be constant over its entire length. As can be seen from FIG. 2, the linear wiring 11 may cross a folding line 12. In such a case, the width of the wiring 11 at the position where it crosses the folding line 12 may be wider than the widths on both sides of the wiring 11 in the length direction. An example of this case is shown in FIG. 3. FIG. 3 is an enlarged view of the portion surrounded by the dashed line X in FIG. 2. As such, the wiring 11 is wider at the portion where it crosses the folding line 12 than on both sides or at the front and back in the length direction. Although not limited to this, in this embodiment, all wirings 11 that cross the folding line 12 have a wider width at the portion where it crosses the folding line 12 than on both sides or at the front and back in the length direction. In this embodiment, when wirings 11 are provided on the base sheet 1, the width of the wiring 11 at the portion where it crosses the folding line 12 is wider than on both sides or at the front and back in the length direction. Of course, FIG. 3 is an example. In Figure 3, the width of the wiring 11 at the portion spanning the bending line 12 is linearly wider than on either side or in front and behind in the longitudinal direction, but the width of the wiring 11 at the portion spanning the bending line 12 may be curvedly wider than on either side or in front and behind, or may be crank-shaped wider from a certain position, for example.

[0025] Next, the cover sheet will be described. 4 shows the cover sheet 2 as viewed from the front side. The front side of the cover sheet 2 is the side that does not face the base sheet 1 when the cover sheet 2 is later laminated on the base sheet 1. The cover sheet 2 is a rectangular sheet that is substantially the same shape and size as the base sheet 1. The meaning of "substantially" has already been explained. If it is desired to align the edges of the base sheet 1 and the cover sheet 2 after carrying out the second step described below, the size and shape of the base sheet 1 and the cover sheet 2 can be adjusted taking into consideration the shape, size, and thickness of the base sheet 1 and the cover sheet 2, as well as the way in which the base sheet 1 and the cover sheet 2 are folded. The shape and size of the base sheet 1 and the cover sheet 2 may be completely the same. No wiring is provided on the cover sheet 2. The cover sheet 2 is made of a thermoplastic resin and has insulating properties. Examples of the thermoplastic resin that constitutes the cover sheet 2 include liquid crystal polymer (LCP), polyphenylene sulfide (PPE), polyether ether ketone (PEEK), and fluororesin (FR). Although not limited thereto, in this embodiment, liquid crystal polymer (LCP) is selected as the thermoplastic resin that constitutes the cover sheet 2. The thermoplastic resin that constitutes the cover sheet 2 and the thermoplastic resin that constitutes the base sheet 1 may or may not be the same. Whether the base sheet 1 and the cover sheet 2 are made of the same or different materials, the necessary processing can be performed on the base sheet 1 and the cover sheet 2 by adjusting the processing conditions, such as the heating temperature and heating time in the third step described below, and the pressure applied as needed. The thickness of the cover sheet 2 can be appropriately selected within a range that allows the cover sheet 2 to be folded together with the base sheet 1 as described below. The thickness can be, for example, 20 μm to 200 μm, and preferably 25 μm to 100 μm. The thickness of the base sheet 1 and the cover sheet 2 may or may not be the same. As already mentioned, the thickness of the base sheet 1 and the cover sheet 2 must be thin enough that the second step can be carried out with the two sheets stacked together. On the other hand, the thickness of the cover sheet 2 is preferably greater than the thickness of the wiring 11 formed on the base sheet 1. If the thickness of the cover sheet 2 is thinner than the thickness of the wiring 11, the cover sheet 2 may not cover the wiring 11 well after the third step described below is performed. Also, if the cover sheet 2 is thinner than the thickness of the wiring 11, even if the cover sheet 2 covers the wiring 11 after the third step, the thickness of the wiring 11 may appear as a protrusion on the surface of the cover sheet 2 covering the wiring 11 (the surface not in contact with the base sheet 1). If the thickness of the wiring 11 appears as a protrusion on the surface of the cover sheet 2, problems such as poor bonding may occur when mounting electronic components on a surface with poor flatness. However, such problems are less likely to occur if the thickness of the cover sheet 2 is thicker than the thickness of the wiring 11. The thickness of the cover sheet 2 may be thinner than the thickness of the base sheet 1. By making the cover sheet 2 thinner than the base sheet 1, the rigidity of the base sheet 1 and the cover sheet 2 is reduced when performing the second step described below, improving their foldability and also leading to cost reductions in processing costs and material costs. The cover sheet 2 has a plurality of holes 21. The holes 21 can be classified into three types. These will be referred to as holes 21x, holes 21y, and holes 21z, respectively. At least one of each of holes 21x, holes 21y, and holes 21z is provided in the cover sheet 2. The first type of holes 21x are intended to prevent interference between the pre-attached electronic components and the cover sheet 2. The holes 21x are shaped and sized so that the pre-attached electronic components can be placed inside them. The holes 21x are also positioned so that the pre-attached electronic components will be inside the holes 21x when the second step, which will be described later, is completed. In this embodiment, there are four holes 21x, but the number of holes 21x may be different. When there are multiple holes 21x, each hole 21x corresponds one-to-one with a different pre-attached electronic component. After the second step is completed, the same pre-attached electronic component may be located in multiple holes 21x, or different pre-attached electronic components may be located in multiple holes 21x. In the second step, as will be described later, the base sheet 1 and the cover sheet 2 are folded. Depending on the folding method, the positions and number of the pre-attached electronic components, etc., it is determined whether the same pre-attached electronic component is located in multiple holes 21x, or different pre-attached electronic components are located in multiple holes 21x. Although not limited to this, in this embodiment, each of the four holes 21x provided in the cover sheet 2 is provided at a position that exactly matches each of the four holes 1x provided in the base sheet 1 when the cover sheet 2 is superimposed on the base sheet 1 in the first step described below, and with a shape and size that exactly matches each of the four holes 1x provided in the base sheet 1. The second type of hole 21y provided in the cover sheet 2 is a hole provided at a position on the base sheet 1 where a pre-attached electronic component is to be attached, that is, at a position corresponding to the planned position 13A, when the cover sheet 2 is superimposed on the base sheet 1. In the first step, the pre-attached electronic component is attached to the base sheet 1 through this hole 21y. Although not limited to this, in this embodiment, only one hole 21y is provided in the lower right direction in FIG. The third type of holes 21z provided in the cover sheet 2 are holes provided at positions on the base sheet 1 where retrofit electronic components are to be attached, i.e., at positions corresponding to the planned positions 13, when the cover sheet 2 is superimposed on the base sheet 1. In the fourth step, the retrofit electronic components are attached to the base sheet 1 through these holes 21z. Although not limited to this, in this embodiment, a total of seven holes 21z are provided on the upper and lower left side of FIG. The second type of holes 21y and the third type of holes 21z are provided at positions where electronic components will be disposed later on the base sheet 1, that is, at positions corresponding to the planned positions 13 described above. The holes 21y and 21z are not uniform in size because the holes 21y and 21z are adjusted to the sizes of the electronic components that are to be attached to the base sheet 1 through them. 4 is a fold line indicating the location where the cover sheet 2 will later be folded together with the base sheet 1. The fold line 12 of the base sheet 1 and the fold line 22 of the cover sheet 2 are located at corresponding positions because the superimposed base sheet 1 and cover sheet 2 are folded together.

[0026] After the base sheet 1 and cover sheet 2 described above are prepared, the first step is carried out. In the first step, the cover sheet 2 is superimposed on the front surface of the base sheet 1 so that the contours of the two sheets match (FIG. 5(A)). In this case, the four holes 21x provided in the cover sheet 2 will be aligned exactly with the four holes 1x provided in the base sheet 1. At this time, the planned positions 13 on the base sheet 1 located on the back side of the cover sheet 2 can be seen through the holes 21y and 21z provided in the cover sheet 2. Also, at this time, the ends of the wiring 11 can be seen slightly around the edges of the holes 21y and 21z.

[0027] In the first step, pre-attached electronic components are attached to the base sheet 1. As described above, pre-attached electronic components refer to electronic components that are attached to the base sheet 1 in the first step. Although not limited thereto, in this embodiment, the number of pre-attached electronic components 3y is one. Although not limited thereto, the pre-attached electronic component 3y in this embodiment is a resistor or a capacitor. The pre-attached electronic component 3y is inserted into a hole 21y drilled in the cover sheet 2, and the terminals protruding from the pre-attached electronic component 3y are electrically connected to the wiring 11 by, for example, soldering. Because an electronic component must be inserted into the hole 21y to secure the pre-attached electronic component 3y to the wiring 11, the hole 21y must be slightly larger than the pre-attached electronic component 3y to be inserted into the hole 21y. Otherwise, when attaching the pre-attached electronic component 3y to the base sheet 1 through the hole 21y, pressure may be applied during insertion of the pre-attached electronic component 3y into the hole 21y, which could damage the pre-attached electronic component 3y or make it difficult to move the pre-attached electronic component 3y horizontally relative to the front surface of the base sheet 1. Furthermore, to enable electrically connected pre-attached electronic component 3y to the wiring 11, the hole 21y must be sized and shaped so that the end of the wiring 11 can be seen through its periphery. The hole 21y in this embodiment is such. The first step is completed by attaching the pre-attached electronic components 3y to the base sheet 1 (FIG. 5(B)). It is also possible to attach the pre-attached electronic components 3y to the base sheet 1 and then overlay the cover sheet 2 on the base sheet 1. In this case, too, the cover sheet 2 is overlaid on the base sheet 1 so that the pre-attached electronic components 3y are positioned inside the holes 21y in the cover sheet 2 overlaid on the base sheet 1.

[0028] In addition, a temporary fixing process may be carried out to temporarily fix the base sheet 1 and the cover sheet 2 together to prevent the contours of the overlapping base sheet 1 and cover sheet 2 from shifting during the first process or at an appropriate time after the first process is completed and before the second process is carried out.This is not necessarily the case, but is done in this embodiment. The temporary joining can be achieved by using an adhesive to bond a portion of the abutting portion (for example, the four corners) of the base sheet 1 and the cover sheet 2. However, there are no limitations on the method of temporary joining. A pin lamination method, which is used in general multilayer board manufacturing methods, can be used, in which boards with holes are stacked and each board is positioned by inserting a pin through the hole in each board, or the temporary joining can be achieved using a temporary fixing jig (such as a clip) that clamps the base sheet 1 and the cover sheet 2 together. Although not limited to this, in this embodiment, after the cover sheet 2 is placed on the base sheet 1, the cover sheet 2 is temporarily attached to the base sheet 1 before the pre-attached electronic component 3y is attached to the base sheet 1.

[0029] Next, the second step is carried out. In the second step, the overlapping base sheet 1 and cover sheet 2 are folded together. The folding is performed along the overlapping fold line 12 of the base sheet 1 and the fold line 22 of the cover sheet 2. The folding is performed at least once. The folding may be either a mountain fold or a valley fold, or a combination of mountain and valley folds. However, after the second step is performed, the above-mentioned hole 21z, which is provided on the cover sheet 2 overlapping the base sheet 1 at a position corresponding to the position on the base sheet 1 where the retrofit electronic component will be attached, is exposed to the outside. "The hole 21z being exposed to the outside" means that the hole 21z is not covered by the base sheet 1 or the cover sheet 2.

[0030] An example of how to fold the base sheet 1 and the cover sheet 2 in the second step will be described with reference to Fig. 6. Fig. 6(A) is a view of the same thing as Fig. 5, seen from the same direction. Although not limited to this, in the second step in this embodiment, first, the portions of the base sheet 1 and cover sheet 2 (FIGS. 5(B) and 6(A)) that have been stacked as a result of the first step, to the right of the vertical fold lines 12 and 22, are folded back toward the back side of FIG. 6(A) at the vertical fold lines 12 and 22. This results in the base sheet 1 and cover sheet 2 being in the state shown in FIGS. 6(B) and 6(C). FIG. 6(B) is a diagram showing the base sheet 1 and cover sheet 2 as viewed from the front side when the portions to the right of the vertical fold lines 12 and 22 are folded back toward the back side of FIG. 6(A) at the vertical fold lines 12 and 22, and FIG. 6(C) is a diagram showing the same as viewed from the back side. After this first folding, the two holes 21x (and the holes 1x overlapping with the holes 21x) located relatively to the right in Figure 6(A) overlap with the two holes 21x (and the holes 1x overlapping with the holes 21x) located relatively to the left. Next, the base sheet 1 and cover sheet 2 in the state shown in Figures 6(B) and (C) (for convenience of explanation, the same drawing as Figure 6(B) is shown again in Figure 6(D)) are folded back to the back side of Figure 6(D) at the folding lines 12, 22 in the upper part of the three horizontally running folding lines 12, 22, and the base sheet 1 and cover sheet 2 in the lower part of the three horizontally running folding lines 12, 22 are folded back to the back side of Figure 6(D) at the folding lines 12, 22. The folded base sheet 1 and cover sheet 2 are then folded back to the state shown in Figures 6(E) and (F). Figure 6(E) shows the folded base sheet 1 and cover sheet 2 as seen from the front side, and Figure 6(F) shows the same as seen from the back side. After the second and third folding, the pre-attached electronic component 3y attached to the base sheet 1 is positioned in the hole 21x (and the hole 1x overlapping the hole 21x) located relatively lower in Fig. 6(D). The pre-attached electronic component 3y is visible from the front side of the base sheet 1 and the cover sheet 2 through the hole 21x (and the hole 1x overlapping the hole 21x) (see Fig. 6(E)). Next, the base sheet 1 and cover sheet 2 in the state shown in Figures 6(E) and (F) (for convenience of explanation, the same drawing as Figure 6(E) is shown again in Figure 6(G)) above the horizontal folding lines 12 and 22 are folded back to the front side (near side) of Figure 6(G) at the folding lines 12 and 22. As a result, the folded base sheet 1 and cover sheet 2 are in the state shown in Figures 6(H) and (I). Figure 6(H) is a diagram showing the folded base sheet 1 and cover sheet 2 as seen from the front side, and Figure 6(I) is a diagram showing the same as seen from the back side. In this state, all four holes 21x (and holes 1x overlapping holes 21x) shown in Figure 6(A) are overlapping at the same position. FIG. 7A shows a cross section of the folded base sheet 1 and cover sheet 2 near the overlapping holes 21x and 1x. The holes 1x in the base sheet 1 and the holes 21x in the cover sheet 2 overlap inside the overlapping base sheet 1 and cover sheet 2, forming a large space S1. A pre-attached electronic component 3y attached to the base sheet 1 is located in this large space S1. The thickness (vertical length in FIG. 7A) of the pre-attached electronic component 3y is thicker than the thickness of the cover sheet 2, and in this embodiment, is thicker than the thickness of the base sheet 1. While not limited to this, in this embodiment, the height of the space S1 is calculated by adding the thickness of the four base sheets 1 and the thickness of the four cover sheets 2. Therefore, in this embodiment, the thickness of the pre-attached electronic component 3y (more precisely, the height from the base sheet 1 to which the pre-attached electronic component 3y is attached to the top surface of the pre-attached electronic component 3y) can be increased up to the height of the space S1 calculated as described above. That is, according to this embodiment, the thickness of the pre-attached electronic component 3y can be made larger than the thickness of the cover sheet 2. This completes the second step in this embodiment. After the second step, four holes 21z in the cover sheet 2 are exposed on the surface shown in Figure 6(H) and three on the surface shown in Figure 6(I). In other words, all of the seven holes 21z originally present in the cover sheet 2 are exposed to the outside after the second step is completed. 6(A) to 6(I), all portions of the base sheet 1 and the cover sheet 2 are substantially parallel except for the folded portions. In other words, the folded base sheet 1 and cover sheet 2 are stacked from the bottom up, as shown in the cross-sectional view of FIG. 7(A), from the surface shown in FIG. 6(I) to the surface shown in FIG. 6(H). All of the wiring 11 that crosses the folding line 12 has a width at the crossing portion that is wider than the width on either side or in front and behind in the longitudinal direction, so that even if the wiring 11 is bent together with the base sheet 1, there is little risk of the wiring 11 being damaged, such as being broken.

[0031] In this embodiment, at the time when the second step is completed, the above-mentioned space S1 created by overlapping the hole 1x provided in the base sheet 1 and the hole 21x provided in the cover sheet 2 is blocked by at least one of the base sheet 1 and the cover sheet 2 (in this embodiment, two stacked base sheets 1 and one cover sheet 2). However, the above-mentioned space S1 can be, for example, a space S2 that is open upward, as shown in Figure 7(B), which is a cross-sectional view equivalent to Figure 7(A). It is obvious that creating such a space S2 requires providing holes 1x and holes 21x in the base sheet 1 and cover sheet 2 in Figure 7(A) at locations above the space S1. In other words, creating the space S2 shown in Figure 7(B) requires two more holes 1x in the base sheet 1 and one more hole 21x in the cover sheet 2 than creating the space S1 shown in Figure 7(A). Furthermore, the above-mentioned space S1 may not have the same cross-section shape or area at all locations in the height direction (thickness direction of the base sheet 1 and cover sheet 2), as shown in Figure 7(C), which is a cross-sectional view equivalent to Figure 7(A). The space S3 shown in Figure 7(C) narrows as it moves upward in the figure. A space S3 having such a cross-section can be formed by appropriately changing the shape and size of each hole 1x provided in the base sheet 1 and each hole 21x provided in the cover sheet 2. In this embodiment, at the end of the second step, the number of spaces S1 provided in the folded base sheet 1 and cover sheet 2 is one, but this is of course not limited to this, and spaces other than space S1 may be provided in the folded base sheet 1 and cover sheet 2. To provide multiple spaces, additional holes 1x may be provided in the base sheet 1 as needed, and additional holes 21x may be provided in the cover sheet 2 as needed.

[0032] Next, the third step is carried out. In the third step, the base sheet 1 and the cover sheet 2 after the second step are fixed together. The third step is performed by heating the folded base sheet 1 and cover sheet 2. When performing the third step, it is preferable to not only heat the base sheet 1 and cover sheet 2, but also to pressurize the folded base sheet 1 and cover sheet 2 so that they are sandwiched from both the surface shown in Figure 6(H) and the surface shown in Figure 6(I). This is done in this embodiment. The heating and pressurizing conditions for the base sheet 1 and the cover sheet 2 can be adjusted depending on the thickness and melting point of the material. The heating and pressurizing conditions described below are an example in which the thermoplastic resins constituting the base sheet 1 and the cover sheet 2 are both liquid crystal polymers, more precisely, the base sheet 1 is a liquid crystal polymer with a melting point of 335°C, and the cover sheet 2 is a liquid crystal polymer with a melting point of 280°C. The thermoplastic resin constituting the cover sheet 2 is selected to have a melting point lower than that of the liquid crystal polymer constituting the thermoplastic resin constituting the base sheet 1. This allows the cover sheet 2 to be softened preferentially over the base sheet 1 in the third step, so that the liquid crystal polymer constituting the softened and fluidized cover sheet 2 can effectively fill gaps between adjacent base sheets 1 and 2 (where irregularities due to wiring 11 may exist) or gaps between adjacent cover sheets 2. Thus, in the present application, the thermoplastic resins constituting the base sheet 1 and the cover sheet 2 can be selected so that the melting point of the thermoplastic resin constituting the cover sheet 2 is lower than that of the thermoplastic resin constituting the base sheet 1. For example, the base sheet 1 and the cover sheet 2 can be heated using a general press device. (1) First, the temperatures of the base sheet 1 and the cover sheet 2 are raised from room temperature to a temperature at which the thermoplastic resins that make them up (particularly the thermoplastic resin that makes up the cover sheet 2) soften slightly, for example, 150°C, and this temperature is maintained for several minutes. This process is carried out for the purpose of softening at least the cover sheet 2 out of the base sheet 1 and the cover sheet 2, so that the softened and fluidized thermoplastic resin fills to some extent the gaps between adjacent base sheets 1 and cover sheets 2, or the gaps between adjacent cover sheets 2. (2) Next, the temperatures of the base sheet 1 and the cover sheet 2 are raised to a temperature close to the melting point of the thermoplastic resin that constitutes them (particularly the thermoplastic resin that constitutes the cover sheet 2), for example, 290°C, and maintained at that temperature for about 30 minutes. This step is carried out for the purpose of further softening at least the cover sheet 2 of the base sheet 1 and the cover sheet 2, so that the softened and fluidized thermoplastic resin virtually completely fills the gaps between adjacent base sheets 1 and cover sheets 2, or the gaps between adjacent cover sheets 2. (3) Finally, the base sheet 1 and the cover sheet 2 are cooled to, for example, room temperature. This process hardens the thermoplastic resin that has virtually completely filled the gap between the adjacent base sheet 1 and cover sheet 2, or the gap between the adjacent cover sheets 2. As a result, the gap between the adjacent base sheet 1 and cover sheet 2, or the gap between the adjacent cover sheets 2, is virtually filled with the thermoplastic resin, and the adjacent base sheet 1 and cover sheet 2, or the adjacent cover sheets 2, are fixed to each other. When carrying out the above steps (1) to (3), the base sheet 1 and the cover sheet 2 are pressed together in a press device. In this embodiment, for example, no pressure is applied for a while after the start of step (1) above, but from the middle to the end of the period during which the temperature of the base sheet 1 and the cover sheet 2 is maintained at 150°C in this step, the base sheet 1 and the cover sheet 2 are pressed together while maintaining a constant pressure of, for example, 3 MPa. This is to allow the fluidized thermoplastic resin to better fill gaps between adjacent base sheets 1 and cover sheets 2, or gaps between adjacent cover sheets 2. Thereafter, from the start of step (2) to the end of step (3) described above, the base sheet 1 and the cover sheet 2 are pressed together at a constant pressure of 1 MPa. The reason why the pressure applied at this time is smaller than the pressure applied during the period in step (1) when the temperatures of the base sheet 1 and the cover sheet 2 are maintained at 150°C (in this example, as described above, this pressure is 3 MPa) is that, although applying a constant pressure is necessary to better fill the gaps between adjacent base sheets 1 and cover sheets 2 or the gaps between adjacent cover sheets 2 with the fluidized thermoplastic resin, applying too much pressure could cause the fluidized thermoplastic resin to leak out from the gaps between adjacent base sheets 1 and cover sheets 2 or the gaps between adjacent cover sheets 2.

[0033] As described above, after the second step is completed, the folded base sheet 1 and cover sheet 2 are laminated as shown in Fig. 7(A) from the surface shown in Fig. 6(H) to the surface shown in Fig. 6(I). At least a portion of the cover sheet 2 melts when heated and hardens after heating is completed; in this embodiment, a portion of the base sheet 1 also melts and hardens after heating is completed. Therefore, of the above-mentioned layers contained in the folded base sheet 1 and cover sheet 2, adjacent base sheets 1 and cover sheets 2, adjacent base sheets 1 and base sheets 1, and adjacent cover sheets 2 and cover sheets 2 are welded to each other. As a result, when the third step is completed, the folded base sheet 1 and cover sheet 2 are fixed in a plate-like shape as a whole.

[0034] Once the third step is completed, the fourth step is carried out. In a fourth step, post-attachment electronic components 3z, which are electronic components other than the pre-attachment electronic components 3y, are attached to the base sheet 1. The post-attachment electronic components 3z are, for example, IC chips, BGAs, CSPs, resistors, capacitors, etc., and are appropriately selected according to the performance required of the multilayer board. The electronic components are electrically connected to the wiring 11 provided on the front surface of the base sheet 1, which has become a plate after the third step, and the cover sheet 2 (FIGS. 8(A) and (B)), thereby being fixed to the base sheet 1. FIGS. 8(A) and (B) are diagrams showing the base sheet 1 and the cover sheet 2 after the fourth step, as viewed from the front side (one side) and the back side (the other side), respectively. The retrofit electronic components 3z are inserted into the holes 21z formed in the cover sheet 2, and terminals protruding from the retrofit electronic components 3z are connected to the wiring 11 in a conductive state by, for example, soldering them to the wiring 11. Since the retrofit electronic components 3z must be inserted into the holes 21z to secure the retrofit electronic components 3z to the wiring 11, the holes 21z must be slightly larger than the retrofit electronic components 3z. This is for the same reason that the holes 21y must be slightly larger than the pre-installed electronic components 3y. Furthermore, to enable the retrofit electronic components 3z to be connected to the wiring 11 in a conductive state, the holes 21z must be sized and shaped so that the ends of the wiring 11 can be seen through their peripheries. The holes 21z in this embodiment are of this size and shape. It is also possible to combine adjacent holes 21 into one hole 21. For example, it is possible to combine two holes 21z located side by side on the upper left and right sides of Fig. 8(A) into one hole 21z. In such a case, two add-on electronic components 3z are attached in one hole 21z. The multilayer board is completed by fixing all of the planned electronic components 3z to be attached to the base sheet 1 and forming an electric circuit with the wiring 11 and electronic components. In the multilayer board manufactured in this embodiment, the base sheet 1 and the cover sheet 2 are essentially parallel except for the folded portions, and the number of overlapping base sheets 1 and cover sheets 2 (total number) is the same throughout the multilayer board (16 in this embodiment). A multilayer board with this structure is plate-shaped. In a multilayer board manufactured by this manufacturing method for a multilayer board, the front surface of the base sheet 1 is covered with the cover sheet 2, except for the portion where the hole 21y is opened (as described above, it is also possible to expose the space S2 to the outside). This means that the wiring 11 is protected by the cover sheet 2, making the wiring 11 less likely to break. In addition, although not limited to this, in this embodiment, the pre-attached electronic component 3y is prevented from being exposed to the outside by having both sides in the height direction covered by at least one of the base sheet 1 and the cover sheet 2. By preventing the pre-attached electronic component 3y from being exposed to the outside, it becomes possible to protect the pre-attached electronic component 3y by at least one of the base sheet 1 and the cover sheet 2.

[0035] <Variation 1> The manufacturing method of the multilayer substrate of Modification 1 is almost the same as the manufacturing method of the multilayer substrate described in the embodiment. Except for the points noted as differences, the manufacturing method of Modification 1 is basically the same as the manufacturing method according to the embodiment described above. In the first modification, the multilayer substrate is also manufactured using a base sheet 1, a cover sheet 2, and electronic components as materials. The configurations of the base sheet 1 and the cover sheet 2 are basically the same between the first modified example and the above-described embodiment. 9, the base sheet 1 of Modification 1 also has four holes 1x of the same size and shape as the four holes 1x in the base sheet 1 in the embodiment, at the same positions as in the above-mentioned embodiment. Wiring 11 is also provided on the front surface of the base sheet 1 of Modification 1, as in the above-mentioned embodiment, but the pattern is somewhat different from that of the above-mentioned embodiment because the number of pre-attached electronic components 3y is different from that in the above-mentioned embodiment. Furthermore, unlike the above-described embodiment, the base sheet 1 of Modification 1 has two planned positions 13A. The base sheet 1 of Modification 1 also has folding lines 12 arranged thereon, as in the above-described embodiment, but the number or arrangement of the folding lines 12 differs from that of the above-described embodiment. The cover sheet 2 of Modified Example 1 is shown in Figure 10. The cover sheet 2 of Modified Example 1 is almost the same as the cover sheet 2 in the above-described embodiment, but the number of holes 21x is reduced by one, from four in the above-described embodiment to three. Specifically, the hole 21x depicted in the lower left of Figure 4 does not exist in Modified Example 1.

[0036] In the first modification, the same first step as in the above-described embodiment is carried out. When the first step is performed, even in the first modification, the cover sheet 2 is first placed on the front surface of the base sheet 1 and the two are temporarily fastened together. The cover sheet 2 shown in Fig. 10 is placed on the base sheet 1 shown in Fig. 9 in the same orientation. The two holes 21y provided in the cover sheet 2 overlap with the two planned positions 13A provided in the base sheet 1. Next, the pre-attached electronic components 3y are attached to the planned positions 13A on the base sheet 1. The method of attaching the pre-attached electronic components 3y to the base sheet 1 is the same between the first modification and the above-described embodiment. However, in the first modification, the pre-attached electronic components 3y are attached to each of the two planned positions 13A. This completes step 1. The superposed base sheet 1 and cover sheet 2 at the time step 1 is completed are shown in FIG.

[0037] In the first modification, once the first process is completed, the second process is executed. Using Figure 12, we will explain how to fold the base sheet 1 and cover sheet 2 that are stacked together as a result of carrying out the first step. Figure 12(A) is a view of the same base sheet 1 and cover sheet 2 as shown in Figure 11, viewed from the same direction. Although not limited thereto, in the second step of this embodiment, the base sheet 1 and cover sheet 2, which have been stacked as a result of the first step, are first folded back toward the back side of FIG. 12(A) at the vertically running fold lines 12 and 22, as indicated by the arrows in FIG. 12(A). This results in the base sheet 1 and cover sheet 2 being in the state shown in FIG. 12(B). After this first folding, of the two holes 21x (and the hole 1x overlapping with hole 21x) that are side-by-side in FIG. 12(A), the hole 21x (and the hole 1x overlapping with hole 21x) located on the relatively right side overlaps the hole 21x (and the hole 1x overlapping with hole 21x) located on the relatively left side. In other words, in this portion, the holes 21x and holes 1x present in the same position are connected to form a series of holes penetrating the base sheet 1 and the cover sheet 2. Next, the portions of the base sheet 1 and cover sheet 2 below the lateral folding lines 12 and 22 in the state shown in Figure 12(B) are folded back to the back side of Figure 12(B) at the folding lines 12 and 22. The folded base sheet 1 and cover sheet 2 then assume the state shown in Figure 12(C). Figure 13(A) shows the same view as in Figure 12(C). Figure 13(B) shows the backside of the base sheet 1 and cover sheet 2 shown in Figure 13(A). When this second folding is completed, the second step in the first modification is completed. After the second folding, one of the pre-attached electronic components 3y attached to the base sheet 1 (the pre-attached electronic component 3y located relatively lower in FIG. 12(A)) is positioned in the series of holes that penetrate the base sheet 1 and the cover sheet 2 after the second folding. This pre-attached electronic component 3y is visible from the front side of the base sheet 1 and the cover sheet 2 through the series of holes (see FIGS. 12(C) and 13(A)). In other words, one of the pre-attached electronic components 3y is housed in the series of holes. FIG. 14(A) shows a cross-sectional view of the folded base sheet 1 and cover sheet 2 near the pre-attached electronic component 3y. The space marked S4 in FIG. 14(A) is the series of holes. The other of the pre-attached electronic components 3y (the pre-attached electronic component 3y located relatively higher in Figure 12(A)) is fitted into a space S5 formed by the two lower holes 1x arranged horizontally in the base sheet 1 shown in Figure 9 and the lower hole 21x arranged vertically in the right half of the cover sheet 2 shown in Figure 10 (Figure 14(B)). In this way, in Modification 1, one of the two pre-attached electronic components 3y is exposed to the outside and the other is not exposed to the outside, but this is of course not limited to this. Both of the two pre-attached electronic components 3y may be exposed to the outside, or neither of the two pre-attached electronic components 3y may be exposed to the outside. In other words, in Modification 1, it is not necessary to expose any of the pre-attached electronic components 3y, but whether or not each pre-attached electronic component 3y is exposed can be changed by appropriately changing the holes 1x formed in the base sheet 1 and the holes 21x formed in the cover sheet 2. This completes the second step in this embodiment. After the second step, four holes 21z in the cover sheet 2 are exposed on the surface shown in Figure 13(A) and three holes 21z are exposed on the surface shown in Figure 13(B). In other words, all seven holes 21z originally present in the cover sheet 2 are exposed to the outside after the second step is completed. In the example described with reference to FIGS. 13(A) and 13(B), the entire base sheet 1 and the entire cover sheet 2 are substantially parallel to each other except for the folded portions.

[0038] Thereafter, in the first modification, the third and fourth steps are carried out in the same manner as in the above-described embodiment, thereby completing the multilayer substrate. The completed multilayer board will be as shown in Figures 13(A) and (B) except for the attachment of the post-installation electronic component 3z. Figures 13(C) and (D) show the base sheet 1 and cover sheet 2 after the attachment of the post-installation electronic component 3z. The base sheet 1 and cover sheet 2 shown in Figure 13(C) are seen from the same side as in Figure 13(A), and the base sheet 1 and cover sheet 2 shown in Figure 13(D) are seen from the same side as in Figure 13(B). The completed multilayer substrate has a stepped or stepped cross-sectional shape, as shown in the side cross-sectional view of Figure 13(E). This multilayer substrate includes four base sheets 1 and four cover sheets 2 in the relatively thicker portion, and two base sheets 1 and two cover sheets 2 in the relatively thinner portion.

[0039] <Variation 2> The method for manufacturing a multilayer substrate according to Modification 2 is almost the same as the method for manufacturing a multilayer substrate described in the embodiment. Except for the points noted as differences, the manufacturing method according to Modification 2 is basically the same as the manufacturing method according to the embodiment described above. In the second modification, the multilayer substrate is also manufactured using the base sheet 1, the cover sheet 2, and the electronic components as materials. The configurations of the base sheet 1 and the cover sheet 2 are basically the same between Modification 2 and the above-described embodiment. However, in Modification 2, both the base sheet 1 and the cover sheet 2 are square. Fig. 15 shows the base sheet 1 as viewed from the front side, and Fig. 16 shows the cover sheet 2 to be overlaid on the base sheet 1 shown in Fig. 15 as viewed from the front side. Note that wiring 11 similar to that in the above-described embodiment is provided on the front surface of the base sheet 1 of Modification 2, but is not shown. The base sheet 1 has one planned position 13A where a pre-installed electronic component will be attached later, and eight planned positions 13 where post-installed electronic components will be attached later. Holes 1x are also provided in the base sheet 1. The base sheet 1 has folding lines, which consist of dashed lines 12A and two-dot chain lines 12B. The former are mountain fold lines along which the base sheet 1 (and cover sheet 2) are mountain folds, and the latter are valley fold lines along which the base sheet 1 (and cover sheet 2) are valley folds. As shown in FIG. 16, the cover sheet 2 is also square, and its size is equal to that of the base sheet 1. A hole 21y of a shape and size corresponding to the planned position 13A is provided in the portion of the cover sheet 2 corresponding to the planned position 13A on the base sheet 1. Eight holes 21z of a shape and size corresponding to the planned position 13 are provided in the portion of the cover sheet 2 corresponding to the planned position 13 on the base sheet 1. Holes 21x of a shape and size corresponding to the holes 1x are provided in the cover sheet 2 at positions corresponding to the holes 1x on the base sheet 1. The cover sheet 2 has folding lines, which are made up of dashed lines 22A and two-dot chain lines 22B. The former are mountain fold lines along which the cover sheet 2 (and base sheet 1) are mountain folds, and the latter are valley fold lines along which the cover sheet 2 (and base sheet 1) are valley folds. When the cover sheet 2 is later superimposed on the base sheet 1, the dashed line 22A exactly overlaps with the dashed line 12A, and the two-dot chain line 22B exactly overlaps with the two-dot chain line 12B.

[0040] In the second modification, the first step is carried out in the same manner as in the above-described embodiment. In the first step, the cover sheet 2 is superimposed on the base sheet 1, and if necessary, the cover sheet 2 is temporarily attached to the base sheet 1. This is the same in Modification 2 and the above-described embodiment. Figure 17 shows the state in which the cover sheet 2 is superimposed on the base sheet 1. The cover sheet 2 is placed on the base sheet 1 shown in Fig. 15 as shown in Fig. 17. The hole 21y in the cover sheet 2 is aligned with the predetermined position 13A in the base sheet 1, the hole 21y in the cover sheet 2 is aligned with the predetermined position 13 in the base sheet 1, and the hole 1x in the base sheet 1 is aligned with the hole 21x in the cover sheet 2. Next, the pre-attached electronic component 3y is attached to the predetermined position 13A of the base sheet 1. The method of attaching the pre-attached electronic component 3y to the base sheet 1 is the same in the second modified example and the above-described embodiment. This completes step 1. The superposed base sheet 1 and cover sheet 2 at the time when step 1 is completed are shown in Figure 18(A).

[0041] In Modification 2, as in the above-described embodiment, the second step is performed after the first step. The dashed lines 22A shown in Fig. 18 are mountain fold lines along which the base sheet 1 and the cover sheet 2 are mountain-folded, and the two-dot chain lines 22B are valley fold lines along which the base sheet 1 and the cover sheet 2 are valley-folded. In this embodiment, the base sheet 1 and the cover sheet 2 are folded from the state shown in FIG. 18(A) through the state shown in FIG. 18(B) to the plate-like state shown in FIG. 18(C) by simultaneously mountain-folding along the mountain fold line 22A and valley-folding along the valley fold line 22B. This type of folding method is common in origami. In FIG. 18(B), the centers of the base sheet 1 and the cover sheet 2 are located on the front side of the page, and the edges of the base sheet 1 and the cover sheet 2 are located on the back side of the page. This completes the second step of the second modification. In this case, too, after the second step is completed, all eight holes 21z of the cover sheet 2 are exposed to the outside (Fig. 19). Fig. 19(A) is a diagram of one side of the base sheet 1 and cover sheet 2 of Modification Example 2 that have become plate-shaped after the second step is completed, and Fig. 19(B) is a diagram of the other side. Also in this case, after the second step is completed, all portions of the base sheet 1 and the cover sheet 2 are substantially parallel to each other except for the folded portions. When the second step is completed, the pre-attached electronic component 3y is wrapped in the folded base sheet 1 and cover sheet 2 and is not exposed to the outside. A cross-sectional view of the base sheet 1 and cover sheet 2 in the vicinity of the pre-attached electronic component 3y is shown in Figure 20. As shown in FIG. 20, the pre-attached electronic component 3y is housed inside the space S6 defined by the hole 1x provided in the base sheet 1 and the hole 21x provided in the cover sheet 2.

[0042] Thereafter, in the second modification, the third and fourth steps are carried out in the same manner as in the above-described embodiment, thereby completing the multilayer substrate. 19(A), 19(B), and 18(C) except for the attachment of post-attached electronic component 3z. As a result, the multilayer substrate manufactured by the manufacturing method of Modification 2 has a plate shape of an isosceles right triangle with pre-attached electronic component 3y built in space S6 and post-attached electronic components 3z attached to both the front and back sides. [Explanation of symbols]

[0043] 1 Base sheet 1x hole 2 Cover Sheet 3y Pre-installed electronic parts 3z Aftermarket electronic parts 11 Wiring 12 Bend lines 21x holes 21y hole 21z hole 22 Bend line

Claims

1. A method for manufacturing a multilayer substrate, comprising the steps of: manufacturing a multilayer substrate having a plurality of electronic components attached thereto by carrying out a first step, a second step, a third step, and a fourth step in that order; the first step includes preparing a base sheet that is an insulating rectangular sheet made of thermoplastic resin and has conductive wiring provided in an appropriate arrangement on its front surface, and that has holes provided as necessary in positions that will interfere with pre-attached electronic components that are some of the electronic components when the second step is performed; an insulating thermoplastic resin cover sheet that has at least one hole provided in a position where the pre-attached electronic component is to be attached and at least one hole provided in a position where a post-attached electronic component that is an electronic component other than the pre-attached electronic component is to be attached, and that has at least one hole provided in a position that will interfere with the pre-attached electronic component when the second step is performed, and that has substantially the same shape and size as the base sheet, and that is superimposed on the front surface of the base sheet such that its outline matches the outline of the base sheet; and the pre-attached electronic component that is inserted into the hole provided in the cover sheet at the position where the pre-attached electronic component is to be attached, and that is fixed to the base sheet exposed through the hole in a state where it is electrically connected to the wiring on the front surface of the base sheet exposed through the hole. The second step involves folding the base sheet and the cover sheet that have been superimposed in the first step so that the holes provided in the cover sheet at positions where the post-attached electronic components are to be attached are exposed to the outside and so that the pre-attached electronic components do not interfere with the base sheet and the cover sheet, the third step involves heating the folded base sheet and the cover sheet at a temperature within a range that does not affect the performance of the pre-attached electronic components to melt at least a portion of the cover sheet, and then cooling and hardening the cover sheet, thereby fixing the adjacent base sheet and the cover sheet together; The fourth step is to fix the retrofit electronic component inserted into the hole provided in the cover sheet at a position where the retrofit electronic component is to be attached to the base sheet exposed through the hole in a state where the retrofit electronic component is electrically connected to the wiring on the front surface of the base sheet exposed through the hole. A method for manufacturing a multilayer board.

2. In the first step, the cover sheet is superimposed on the front surface of the base sheet in such a manner that its contour coincides with the contour of the base sheet, and then the pre-attached electronic component is inserted into the hole provided in the cover sheet at a position where the pre-attached electronic component is to be attached, and the pre-attached electronic component is fixed to the base sheet exposed through the hole in a state where the pre-attached electronic component is electrically connected to the wiring on the front surface of the base sheet exposed through the hole. The method for manufacturing the multilayer substrate according to claim 1.

3. The base sheet is square. The method for manufacturing the multilayer substrate according to claim 1.

4. The second step is carried out so that all portions of the base sheet and the cover sheet are substantially parallel to each other except for the folded portions after the fourth step is completed. The method for manufacturing the multilayer substrate according to claim 1.

5. the second step is carried out so that the number of the base sheets and the cover sheets superposed in a certain portion other than the folded portion after the fourth step is completed is different from the number of the base sheets and the cover sheets superposed in the other portion. The method for manufacturing a multilayer substrate according to claim 4.

6. In the third step, adjacent cover sheets of the superposed base sheet and cover sheet are also fixed to each other. The method for manufacturing the multilayer substrate according to claim 1.

7. In the third step, the superposed base sheet and cover sheet are heated while being pressed. The method for manufacturing a multilayer substrate according to claim 1 or 6.

8. a wiring step of providing the wiring on the front surface of the base sheet is carried out before the first step; The method for manufacturing the multilayer substrate according to claim 1.

9. The width of the wiring at the portion where the base sheet is folded is wider than the width of both sides of the wiring in the length direction. The method for manufacturing a multilayer substrate according to claim 1 or 8.

10. The thickness of the cover sheet is thinner than the thickness of the base sheet. The method for manufacturing the multilayer substrate according to claim 1.

11. In the first step, the cover sheet is superimposed on the front surface of the base sheet, and then the cover sheet is temporarily fixed to the base sheet. The method for manufacturing the multilayer substrate according to claim 1.

12. The thickness of the cover sheet is thinner than the thickness of the base sheet. The method for manufacturing a multilayer substrate according to claim 9.

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