Method for producing multilayer substrate

Abstract

The present invention provides a multilayer substrate production technology that enables the production of a multilayer substrate at a low cost.　First, a cover sheet 2 is superimposed on a base material sheet 1 that is identical in shape and size. Wiring 11 is provided to one surface of the base material sheet 1. A plurality of holes 21x, 21y, 21z are opened at positions where electronic components are attached to the base material sheet 1 after the cover sheet 2. A prefitted electronic component is attached to the base material sheet through the hole 21y. The base material sheet 1 and the cover sheet 2 that have been superimposed are then folded such that the hole 21y is exposed to the outside. The prefitted electronic component is accommodated in the hole 21x. A retrofitted electronic component is attached to the base material sheet 1 through the hole 21z.

Description

Manufacturing method for multilayer substrates

[0001] This invention relates to a method for manufacturing a multilayer substrate.

[0002] Various electronic devices contain circuit boards with wiring that forms the circuit, on which various electronic components are mounted. Among these circuit boards, there are those called multilayer circuit boards. Multilayer circuit boards are constructed by stacking multiple circuit boards. Each circuit board has a plate-like or sheet-like base material, and wiring is provided on at least one surface of the base material. Although multilayer circuit boards, which have a structure in which multiple circuit boards, each with its own wiring, are stacked, are naturally thicker, they can contain more circuits and electronic components in the same area than single-layer circuit boards, and are therefore widely used because they can achieve high integration.

[0003] In general, the circuits within each substrate in a multilayer substrate need to be electrically connected to each other. Therefore, in multilayer substrates, holes called through-holes are commonly drilled that penetrate through all the substrates in the stacked multilayer substrate. Typically, a metal layer is provided on the inner surface of the through-hole, for example, by plating with a conductive metal, and the wiring in each substrate is made electrically connected to the metal layer, thereby enabling the wiring within each substrate in the multilayer substrate to be electrically connected to each other via the metal layer in the through-hole. For example, wiring provided on both sides of a single substrate can be electrically connected to each other via the metal layer in the through-hole. Alternatively, techniques that do not use through-holes are also employed. For example, it is possible to connect wiring provided on both sides of a single substrate, or on any side of multiple substrates, with conductive external connection wires, thereby enabling electrical connection between wiring located at a distance from each other, and such techniques are also in practical use.

[0004] Although the multilayer substrate described above is as described, there remains room for improvement. Conventional multilayer substrates are provided with through holes and connection lines outside the substrate as described above. When through holes are present, it is necessary to perform a step of drilling holes in the substrate to form the through holes and a step of forming a metal layer on the inner peripheral surface of the through holes, which increases the manufacturing cost of the multilayer substrate. Furthermore, as described above, since through holes are electrically connected to the wirings of each individual substrate included in the multilayer substrate, it is necessary to accurately position the wirings with respect to the base material in each substrate, and to accurately position the plurality of stacked substrates relative to each other when laminating the plurality of substrates. Of course, the two types of positioning accuracy required for the aforementioned reasons are technically achievable. However, achieving them requires labor and tends to cause an increase in cost. Furthermore, the conventional manufacturing process for multilayer substrates involves individually manufacturing a plurality of substrates to be stacked, and then laminating the plurality of individually manufactured substrates together. Alternatively, in some cases, a cover sheet and a copper foil are laminated on the upper and lower sides of a substrate to form circuits on the upper and lower surfaces of the substrate, and this procedure is repeated to sequentially increase the number of layers. Here, when manufacturing each substrate, common steps such as, for example, circuit formation on a copper foil attached to the substrate and gold plating are generally performed for each individual substrate. Therefore, an increase in cost tends to occur due to the repeated execution of common steps for each substrate. The same applies to the case where cover sheets and copper foils are laminated on the upper and lower sides of a substrate. Even when connection lines are provided outside the substrate, performing this step increases the manufacturing cost of the multilayer substrate. In addition, the situation that an increase in cost tends to occur due to the repeated execution of common steps for each substrate remains unchanged.

[0005] Japanese Patent No. 7558604

[0006] In light of these points, the inventor of the present invention has developed a method for manufacturing multilayer substrates that differs from conventional methods and has already obtained a patent. This method for manufacturing multilayer substrates uses a thin, rectangular base sheet made of thermoplastic resin with circuits formed on its front surface, and a cover sheet also made of thermoplastic resin, which is thin and the same size and shape as the base sheet. The cover sheet has holes, usually multiple, at appropriate positions, which are later used to attach electronic components to the base sheet. In this manufacturing method, first, the cover sheet is placed on top of the front surface of the base sheet. In this state, the front surface and circuits of the base sheet are visible through the holes in the cover sheet. Next, the superimposed base sheet and cover sheet are folded while still superimposed. 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 appropriate pressure. When heated, a portion of the cover sheet melts, and then it cools and hardens. The melted cover sheet acts like an adhesive, fixing adjacent base sheet sheets and cover sheets, and adjacent cover sheets to each other. In some cases, adjacent base sheet sheets are also fixed to each other. Finally, electronic components are inserted through holes in the cover sheet and attached to the base sheet while making electrical connections to the circuits provided on the base sheet that are removed from the holes. This completes the multilayer substrate. In multilayer substrates manufactured by this method, when electronic components are attached to the base sheet, the electrical circuit is formed on the front surface of the base sheet in combination with the electronic components, so through holes and external connection wires are unnecessary. Therefore, the process for creating them can be omitted, which reduces the cost of manufacturing multilayer substrates. Furthermore, in the above-described method for manufacturing multilayer substrates, various structures of multilayer substrates can be obtained by changing the way the overlapping base sheet sheets and cover sheets are folded in the second step.

[0007] While this method of manufacturing multilayer substrates is advantageous in that it allows for the low-cost production of multilayer substrates, it tends to impose limitations on the thickness of electronic components that can be attached to the substrate sheet. For example, in order to prevent the electronic components from protruding from the cover sheet, it is preferable that the thickness of the electronic components be less than or equal to the thickness of the cover sheet covering the substrate sheet.

[0008] The object of the present invention is to provide a method for manufacturing a multilayer substrate that enables the low-cost production of multilayer substrates to which electronic components with a greater thickness than conventional substrates are attached.

[0009] The present invention, which solves the above-mentioned problems, is as follows. The present invention is a method for manufacturing a multilayer substrate, which manufactures a multilayer substrate on which multiple electronic components are attached by performing the first, second, third, and fourth steps in that order. Hereinafter, the method for manufacturing a multilayer substrate may be simply referred to as the "manufacturing method". The first to fourth steps in this method for manufacturing a multilayer substrate are as follows. The first step involves preparing a base sheet which is an insulating rectangular sheet made of thermoplastic resin with conductive wiring arranged appropriately on its front surface, and which has holes provided as needed at positions that interfere with pre-attached electronic components which are part of a plurality of electronic components when the second step is performed; a cover sheet which is an insulating sheet made of thermoplastic resin and has substantially the same shape and size as the base sheet, with at least one hole provided at the position where the pre-attached electronic components are attached, and at least one hole provided at the position where the after-attached electronic components which are electronic components other than the pre-attached electronic components are attached, and which has at least one hole provided at the position that interferes with the pre-attached electronic components when the second step is performed, and which is superimposed on the front surface of the base sheet in such a state that its outline matches the outline of the base sheet; and the pre-attached electronic components which are fixed to the base sheet and are visible through the holes, and are inserted into the holes provided on the cover sheet where the pre-attached electronic components are attached, and are electrically connected to the wiring on the front surface of the base sheet visible through the holes. The second step involves folding the base sheet and the cover sheet, which were superimposed in the first step, so that the holes provided in the cover sheet where the retrofitted electronic components are 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 cause problems with the performance of the pre-installed electronic component to melt at least a portion of the cover sheet, and then cooling and hardening it to fix the adjacent base sheet and the cover sheet together. The fourth step involves inserting the aftermarket electronic component into the hole provided in the cover sheet at the position where the aftermarket electronic component is to be attached, and fixing the aftermarket electronic component to the base sheet visible through the hole, while ensuring electrical conductivity with the wiring on the front surface of the base sheet visible through the hole.

[0010] In this multilayer substrate manufacturing method, as described above, the first, second, third, and fourth steps are carried out in that order. In the first step, the following is prepared using a base sheet, a cover sheet, and electronic components. Simply put, in the first step, a substrate sheet with pre-attached electronic components, which are part of the electronic components, is manufactured by overlapping the base sheet and cover sheet. The base sheet is an insulating rectangular sheet made of thermoplastic resin. Wiring is provided on the front surface of the base sheet. Wiring is not provided on the cover sheet, but only on the base sheet. Wiring is also provided on only one side of the base sheet. Of the two sides of the base sheet, the side on which the wiring is provided is the front surface. The wiring is conductive and is generally made of a conductive metal. There are no restrictions on the method of providing the wiring on the base sheet; the wiring can be provided on the front surface of the base sheet by a known or well-known method. For example, techniques for attaching wiring to a base sheet can be used, such as the subtractive method, in which copper foil is attached to the entire surface of the front side of the base sheet with adhesive and the unnecessary parts are removed by chemical etching while leaving the wiring portion, or the additive method, in which wiring is plated (in most cases electroless plating and electrolytic plating) onto the front side of the base sheet. The cover sheet is a rectangular sheet that is virtually the same shape and size as the base sheet. The cover sheet is made of thermoplastic resin and has insulating properties. Multiple electronic components are attached to the base sheet, as will be described later. The base sheet may have at least one hole. The hole provided in the base sheet is provided as needed at a position where it may interfere with a pre-attached electronic component, which is part of the multiple electronic components, when the second step is performed. By providing this hole, it is possible to prevent interference between the base sheet and the pre-attached electronic component that may occur when the second step is performed using a pre-attached electronic component that has a thickness greater than the thickness of the cover sheet. If the interference between the base sheet and the pre-attached electronic component described above does not occur even without the hole, then this hole is not necessarily required.As can be understood from the description of the present invention, the manufacturing method of the present invention involves the attachment of multiple electronic components to the base sheet. Some of these electronic components are attached to the base sheet in the first step, and the remaining components are attached 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 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 hole provided in the cover sheet is one that, when the cover sheet is superimposed on the base sheet, is located at a position corresponding to the position on the base sheet where the pre-attached electronic components are attached. Since at least one of these holes exists, the pre-attached electronic components can be attached to the base sheet without being obstructed by the cover sheet. The second type of hole provided in the cover sheet is a hole provided at a position corresponding to the position on the base sheet where the add-on electronic component is attached, when the cover sheet is superimposed on the base sheet. Since at least one of these holes is present, the add-on electronic component can be attached to the base sheet without being obstructed by the cover sheet. The third type of hole provided in the cover sheet is a hole provided at a position that interferes with the pre-attached electronic component when the second step is performed. By providing this hole, it is possible to prevent interference between the cover sheet and the pre-attached electronic component that may occur when the second step is performed using a pre-attached electronic component that has a thickness greater than the thickness of the cover sheet. The first step of the present invention is to prepare a state in which the cover sheet is superimposed on the base sheet and the pre-attached electronic component is attached to the base sheet (a combination of the base sheet, the cover sheet, and the pre-attached electronic component). Once the first step is completed, the pre-attached electronic component is inserted into a hole provided in the cover sheet where the pre-attached electronic component is to be mounted, and is fixed to the base sheet visible through the hole, with electrical conductivity connected to the wiring on the front surface of the base sheet visible through the hole.

[0011] When performing the first step, the cover sheet may be placed on top of the base sheet, and then the pre-attached electronic components may be attached to the base sheet through the holes punched in the cover sheet. Alternatively, the pre-attached electronic components may be attached to the base sheet first, and then the cover sheet may be placed on top of 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 adopted, it is necessary to adjust the cover sheet so that its contour matches that of the base sheet while adjusting it so that the pre-attached electronic components attached to the base sheet are inserted into the holes, which tends to increase the difficulty of the work. In particular, it is expected that the difficulty of the work will increase when there are multiple pre-attached electronic components attached to the base sheet. Therefore, when performing the first step, the former of the two methods described above is generally preferred. In other words, in the first step, it is preferable to first overlap the cover sheet onto the front surface of the base sheet so that its contour matches the contour of the base sheet, and then fix the pre-attached electronic component, which has been 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 visible through the hole, while ensuring electrical conductivity with the wiring on the front surface of the base sheet visible through the hole. Alternatively, in the first step, the cover sheet may be temporarily fixed to the base sheet after overlapping the cover sheet onto the front surface of the base sheet. Temporary fixing means fixing the relative positional relationship between the base sheet and the cover sheet before the third step, which will be described later, is performed. Temporary fixing can be done, for example, by bonding a part of the contact area between the base sheet and the cover sheet (for example, the four corners) with adhesive, but there are no restrictions on the method of temporary fixing. Temporary fixing may be performed using the pin lamination method, which is commonly used in multilayer substrate manufacturing, in which perforated substrates are stacked and each substrate is positioned by passing pins through the holes provided in each substrate, or temporary fixing may be performed using a temporary fixing jig (such as a clip) that holds the base sheet and cover sheet together.

[0012] In the second step, which is performed next, the base sheet and cover sheet, which were superimposed in the first step, are folded together. The folding is performed at least once and may 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 where the aftermarket 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 will be at least double-layered with the rest of the base sheet, although the cover sheet may be in between. As a result, the wiring provided on the front surface of the base sheet will also be at least double-layered. When performing the second step, ensure that the pre-installed electronic components do not interfere with the base sheet and cover sheet. In other words, when the base sheet and cover sheet are folded, pre-installed electronic components that would interfere with the base sheet if there were no holes are positioned within the holes provided in the base sheet. In this way, the pre-installed electronic components can avoid interference with the base sheet. Similarly, when the base sheet and cover sheet are folded, if there were no holes in the cover sheet that would interfere with the pre-installed electronic components, the pre-installed electronic components that would interfere with the cover sheet are positioned within the aforementioned holes in the cover sheet. In this way, the pre-installed electronic components will not interfere with the cover sheet. When the base sheet and cover sheet stacked in the second step are folded, depending on the folding method, a slight misalignment may occur at the edges of both. If such a misalignment is acceptable, the shape and size of the base sheet and cover sheet may be completely identical. Alternatively, if the misalignment of the edges of the base sheet and cover sheet when folded is to be reduced or eliminated, the shape and size of the base sheet and cover sheet can be made almost identical but slightly different. In other words, the cover sheet can be made to be "effectively" the same shape and size as the base sheet. In the third step, which is then performed, the folded, adjacent base sheet and cover sheet are fixed together.In this bonding process, the folded base sheet and cover sheet are heated to melt at least a portion of the cover sheet, and then cooled and hardened. The molten cover sheet is then used as an adhesive to bond adjacent base sheets and cover sheets. In this application, "cooling" does not necessarily require an active cooling process; it 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 permissible to melt not only the cover sheet but also a portion of the base sheet and then cool and harden it. Furthermore, in the third step, adjacent cover sheets among the overlapping base sheets and cover sheets may also be bonded together. 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 performed in the third step must be at a temperature that melts at least a portion of the cover sheet. However, the temperature must be within a range that does not cause problems with the performance of the pre-installed electronic components attached to the base sheet. Then, the fourth step of attaching the electronic components to the base sheet is performed. As described above, after the second step is completed, the holes provided in the cover sheet are exposed to the outside. Holes are provided in the cover sheet at positions on the base sheet corresponding to the positions where the retrofitted electronic components will be attached. The retrofitted electronic components are inserted through these holes and fixed to the base sheet visible through these holes, while being electrically connected to the wiring on the front surface of the base sheet visible through these holes. Needless to say, the wiring provided in the base sheet is designed so that when at least one pre-installed electronic component and at least one retrofitted electronic component are attached to the base sheet, an electrical circuit is formed by the combination of the electronic components. In this way, a multilayer substrate is manufactured.The electronic components used in manufacturing multilayer circuit boards include integrated circuit (IC) chips, ball grid arrays (BGAs), chip size packages (CSPs), resistors, capacitors, etc., which are no different from the electronic components mounted on general circuit boards. Among these, those that tend to be thicker and can withstand the heating in the third process can be candidates for pre-mounted electronic components. For example, resistors and capacitors can be selected as pre-mounted electronic components.

[0013] The multilayer substrate manufactured by the multilayer substrate manufacturing method of the present application, as described above, has an electrical circuit formed by the combination of electronic components when at least one pre-attached electronic component and at least one post-attached electronic component are mounted on the base sheet. Therefore, through-holes and external connection wires are unnecessary. Consequently, the process for manufacturing them can be omitted, thereby reducing the cost of manufacturing the multilayer substrate. Furthermore, in the above-described multilayer substrate manufacturing method, various structures of multilayer substrates can be obtained by changing the way the overlapping base sheet and cover sheet are folded in the second step. In addition, in the multilayer substrate manufactured by the above-described multilayer substrate manufacturing method, the front surface of the base sheet is covered by the cover sheet, except for the perforated portion. As a result, the wiring is protected by the cover sheet, making it less likely for the wiring to break. These are effects that can also be obtained by the patented invention related to the patent previously obtained by the applicant of the present application. In addition, the present invention also has the effect of being able to manufacture multilayer substrates with thicker electronic components attached than conventionally. The pre-attached electronic components in the present application can be thicker electronic components. In the second step, when the superimposed base sheet and cover sheet are folded, the pre-attached electronic component will be positioned within holes in the base sheet that would interfere with the pre-attached electronic component during the second step, and within holes in the cover sheet that would interfere with the pre-attached electronic component during the second step, thus avoiding interference with the base sheet and cover sheet. This makes it possible to increase the thickness of the pre-attached electronic component, for example, to make it thicker than 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 be the same or different. A material that can be used as the thermoplastic resin constituting the base sheet and the cover sheet is, for example, liquid crystal polymer (LCP). Other materials that can be used include polyphenylene sulfide (PPE), polyether ether ketone (PEEK), fluororesin (FR), etc., but other thermoplastic resins can also be used. Liquid crystal polymer is a well-known material and is known as a material with low loss and excellent high-frequency characteristics, making it suitable for use as the material for the base sheet and the cover sheet. The thickness of the base sheet and the cover sheet needs to be thin enough so that the second step can be carried out with both stacked. On the other hand, it is preferable that the thickness of the cover sheet exceeds the thickness of the wiring formed on the base sheet. As mentioned above, the cover sheet protects the wiring after the third step, but if the thickness of the cover sheet is thinner than the thickness of the wiring, the protection of the wiring may be insufficient, and the wiring may not be covered by the cover sheet, leaving the top exposed. Also, even if the wiring is protected by covering it with the cover sheet, the thickness of the wiring may appear as a protrusion on the cover sheet, which may prevent the flatness of the multilayer substrate from being maintained. If this occurs, there is a risk that defects such as poor bonding may occur when mounting electronic components on the surface with inferior 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 overlapping base sheet and cover sheet in the second step is reduced, improving flexibility and also leading to a reduction in processing and material costs.

[0015] As described above, the base sheet used in the present invention is rectangular, and the cover sheet is a rectangle of the same shape and size as the base sheet. The base sheet may also be square. As described above, the base sheet and cover sheet are folded by folding them while overlapping each other. If the base sheet and cover sheet are square, various folding methods used in origami, a traditional Japanese art form, can be applied. For example, a multilayer substrate with a regular shape, such as a rectangle or triangle, can be obtained in which the total number of base sheets and cover sheets present in all parts 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 the final product. A multilayer substrate with a right-angled isosceles triangle shape can also be made into a square by combining two of them.

[0016] In the present invention, the second step can be performed such that, after the fourth step is completed, all parts of the base sheet and the cover sheet are substantially parallel, except for the folded parts. The multilayer substrate manufactured in this way has a state in which the base sheet and the cover sheet are stacked substantially parallel to each other, except for the folded parts, and the whole is in the shape of a plate. The plate-shaped multilayer substrate has the same shape as many conventional multilayer substrates or single-layer substrates and is expected to have many applications. On the other hand, after the fourth step is completed, a part of the base sheet and the cover sheet may not be parallel to the other parts of the base sheet and the cover sheet. In this case, the multilayer substrate may have a three-dimensional shape rather than a plate shape. Depending on the final product to which the substrate will be attached, a multilayer substrate with a three-dimensional shape may be convenient. The fact that such various shapes of multilayer substrates can be obtained using the same manufacturing method is one of the advantages of the manufacturing method of a multilayer substrate according to the present invention. It is also possible to perform the second step such that, after the fourth step is completed, the number of base sheets and cover sheets overlapped in a certain portion other than the folded portion is different from the number of base sheets and cover sheets overlapped in other portions. For example, suppose a plate-shaped multilayer substrate is manufactured by a multilayer substrate manufacturing method. Suppose that in a certain area of ​​the finished multilayer substrate, four base sheets and four cover sheets are overlapped, and in another area, eight base sheets and eight cover sheets are overlapped. In this case, naturally, the thickness of the other area of ​​the multilayer substrate will be greater than that of the aforementioned area. The ability to obtain a multilayer substrate with different thicknesses in different parts using the same manufacturing method is one of the advantages of the multilayer substrate manufacturing method according to the present invention. Since it is possible to manufacture a substrate with partially different thicknesses, the thinner parts of the substrate use less base material, which has the advantage of contributing to cost reduction in material costs and process costs.

[0017] As described above, the third step involves heating the stacked base sheet and cover sheet. In the third step, the stacked base sheet and cover sheet may be heated while pressurizing. This ensures that the base sheet and cover sheet, or the cover sheet and cover sheet, are evenly and firmly bonded together. The third step may be performed by heating (or heating and pressurizing) the entire base sheet and cover sheet at once after the second step is completed, or by heating (or heating and pressurizing) parts of the base sheet and cover sheet and other parts in multiple stages after the second step is completed. When the resulting multilayer substrate has a three-dimensional shape, the third step is often performed in the latter manner.

[0018] In the method for manufacturing a multilayer substrate according to the present invention, a wiring step may be performed before the first step to provide the wiring on the front surface of the base sheet. As previously stated, there are no particular restrictions on the techniques that can be used to provide the wiring on the front surface of the base sheet in the wiring step, and publicly known or well-known techniques can be used. The wiring may be such that the width of the wiring at the portion where the base sheet is bent is wider than the width of the portions on both sides in the longitudinal direction. In the method for manufacturing a multilayer substrate according to the present invention, the wiring is bent together with the base sheet. As a result, there is some risk of the wiring breaking. If the width of the wiring at the portion where the base sheet is bent is wider than the width of the portions on both sides in the longitudinal direction, it helps to prevent the wiring from breaking when it is bent together with the base sheet. When the wiring step is performed before the first step, it is also possible to manufacture the wiring such that the width of the wiring at the portion where the base sheet is bent is wider than the width of the portions on both sides in the longitudinal direction.

[0019] A diagram showing a base sheet used in one embodiment of the present invention, viewed from the front. A diagram showing a base sheet with wiring provided, viewed from the front. A magnified view of the area near reference numeral X in Figure 2. A diagram showing a cover sheet viewed from the front. A diagram showing the state in which the cover sheet is superimposed on the base sheet in the first step of one embodiment, viewed from the front. A diagram showing the state in which pre-attached electronic components are attached to the base sheet of the base sheet and cover sheet shown in Figure 5(A), viewed from the front. A diagram showing the object shown in Figure 5(B) viewed from the same direction as in Figure 5(B), and is substantially the same as Figure 5(B). A diagram showing the state in Figure 6(A) as viewed from the front, with the portion to the right of the vertical fold line of the base sheet and cover sheet shown in Figure 6(A) folded to the back at the fold line. A diagram showing the state in Figure 6(A) as viewed from the back, with the portion to the right of the vertical fold line of the base sheet and cover sheet shown in Figure 6(A) folded to the back at the fold line. A diagram that is virtually identical to Figure 6(B), showing the object shown in Figure 6(B) viewed from the same direction as in Figure 6(B). A diagram that shows the state as seen from the front of Figure 6(D), where the portion above the upper of the three horizontal fold lines of the base sheet and cover sheet shown in Figure 6(D) is folded to the back at the fold line, and the portion below the lower of the three horizontal fold lines of the base sheet and cover sheet is folded to the back at the fold line. A diagram that shows the state as seen from the back of Figure 6(D), where the portion above the upper of the three horizontal fold lines of the base sheet and cover sheet shown in Figure 6(D) is folded to the back at the fold line, and the portion below the lower of the three horizontal fold lines of the base sheet and cover sheet is folded to the back at the fold line. A diagram that is virtually identical to Figure 6(E), showing the object shown in Figure 6(E) viewed from the same direction as in Figure 6(E). Figure 6(G) shows the base sheet and cover sheet as viewed from the front, with the portion above the horizontal fold line folded back to the front along that fold line.Figure 6(G) shows the base sheet and cover sheet as seen from the back of Figure 6(G), with the portion above the horizontal fold line and folded back to the front at the fold line. Cross-sectional view of the base sheet, cover sheet, and pre-attached electronic component near the pre-attached electronic component when the second step of the embodiment is completed, according to one example. Cross-sectional view of the base sheet, cover sheet, and pre-attached electronic component near the pre-attached electronic component when the second step of the embodiment is completed, according to another example. Cross-sectional view of the base sheet, cover sheet, and pre-attached electronic component near the pre-attached electronic component when the second step of the embodiment is completed, according to yet another example. Figure 6(H) shows the state in which a post-attached electronic component is attached to the base sheet of the base sheet and cover sheet shown. Figure 6(I) shows the state in which a post-attached electronic component is attached to the base sheet of the base sheet and cover sheet shown. Figure 6(I) shows the state in which a post-attached electronic component is attached to the base sheet of the base sheet and cover sheet shown. A view from the front of the base sheet after the cover sheet has been placed on top of the base sheet in the first step of Modification 1, and the pre-attached electronic components have been attached to the base sheet. A view of the object shown in Figure 11 from the same direction as in Figure 11, and is virtually the same as Figure 11. A view from the front of Figure 12(A) showing the portion to the right of the vertical fold line of the base sheet and cover sheet shown in Figure 12(A), folded to the back along that fold line. A view from the front of Figure 12(B) showing the portion below the horizontal fold line of the base sheet and cover sheet shown in Figure 12(B), folded to the back along that fold line.(A) is a view of the base sheet and cover sheet shown in Figure 12(C) from the same direction as Figure 12(B), and is virtually the same as Figure 12(C); (B) is a view of the base sheet and cover sheet shown in Figure 13(A) from the back side of Figure 13(A); (C) is a view of the base sheet of the base sheet and cover sheet shown in Figure 13(A) with the retrofit electronic components attached, from the same direction as Figure 13(A); (D) is a view of the base sheet of the base sheet and cover sheet shown in Figure 13(B) with the retrofit electronic components attached, from the same direction as Figure 13(B); and (E) is a side view of the base sheet and cover sheet shown in Figure 13(D). Cross-sectional view of the base sheet, cover sheet, and retrofit electronic components near the retrofit electronic components when the second step of Modification 1 is completed, according to one example. Cross-sectional view of the base sheet, cover sheet, and pre-attached electronic component near the pre-attached electronic component when the second step of Modification 1 is completed, according to another example. Diagram showing the base sheet with wiring used in Modification 2 viewed from the front. Diagram showing the cover sheet used in Modification 2 viewed from the front. Diagram showing the state in which the cover sheet is superimposed on the base sheet in the first step of Modification 2 viewed from the front. (A) is a view from the front of the state in which the pre-attached electronic component is attached to the base sheet in the base sheet and cover sheet shown in Figure 11, (B) is a view from the front of the state in which the base sheet and cover sheet shown in Figure 18(A) are being folded in the mountain fold with a dashed line and the valley fold with a dashed line, and (C) is a view of the side of the base sheet and cover sheet when the second step of Modification 2 is completed, as seen from the side of the base sheet and cover sheet. Diagram showing the base sheet and cover sheet shown in Figure 18(C) viewed from one side. Figure 18(C) shows the base sheet and cover sheet as viewed from the other side. Cross-sectional view of the base sheet, cover sheet, and pre-attached electronic component near the pre-attached electronic component after the second step of Modification 2 is completed.

[0020] An embodiment of the present invention and its modified examples will be described below with reference to the drawings.

[0021] This embodiment describes a method for manufacturing a multilayer substrate. The multilayer substrate is manufactured by performing the following steps. The multilayer substrate is manufactured using a base sheet, a cover sheet, and multiple electronic components as materials.

[0022] First, the base sheet will be described. Figure 1 shows the base sheet 1 as viewed from the front side. The wiring, which will be described later, has not yet been provided on this base sheet 1. The base sheet 1 is rectangular. The base sheet 1 may also be square, as will be described later, but it is not square in the example shown in Figure 1. The base sheet 1 is made of a thermoplastic resin and has insulating properties. Examples of thermoplastic resins that make up 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 makes up the base sheet 1. The thickness of the base sheet 1 can be appropriately selected within a range in which the base sheet 1 can be folded together with the cover sheet, as will be described later. The thickness can be, for example, 20 μm to 200 μm, preferably 25 μm to 100 μm. Holes 1x are punched in the base sheet 1. These holes 1x are not necessarily essential, as will be described later, but they are present in the base sheet 1 of this embodiment. The holes 1x are designed to prevent interference between pre-attached electronic components (electronic components and pre-attached electronic components will be explained later), which are some of the multiple electronic components that make up the multilayer substrate, and the base sheet 1. The holes 1x are shaped and sized in such a way that the pre-attached electronic components can 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 when the second step, which will be described later, is completed, the pre-attached electronic components will be inside the holes 1x. In this embodiment, there are four holes 1x, but this number is not limited to four. There may be one hole 1x or multiple holes 1x. When 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. Depending on the method of folding, as well as differences in the position and number of pre-attached electronic components, it is determined whether the same pre-attached electronic component is located in multiple holes 1x, or whether different pre-attached electronic components are located in multiple holes 1x.

[0023] In this embodiment, wiring 11 is provided on such a base sheet 1 (Figure 2). In Figure 2, the dashed line labeled 12 is a fold line indicating 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 side 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 conductive and is generally made of a conductive metal. Although not limited to this, in this embodiment the wiring 11 is made of a conductive metal. When all the electronic components are attached to the base sheet 1, the wiring 11, in combination with the electronic components, forms an electrical circuit. There are no particular restrictions on the techniques that can be used to provide the wiring 11 on the base sheet 1, and known or well-known techniques can be used. For example, 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, the wiring portion is covered with a mask, unnecessary portions (parts of the copper foil not covered by the mask) are removed by chemical etching, and finally the mask is removed, leaving the copper foil that was covered by the mask as wiring 11 on the front surface of the base sheet 1; or by an additive method, in which a mask is provided on the front surface of the base sheet 1 so that only the portion corresponding to the wiring 11 is exposed, and then plating (e.g., electroless plating and electrolytic plating) is performed to provide a metal plating layer in the areas where there is no mask, and then the mask is removed to obtain wiring 11 on the base sheet 1. The wiring 11 can be provided all at once by performing the wiring process on the base sheet 1, such as the subtractive method or the additive method, in a single step, although this is not limited to this, it is done in this embodiment. Note that the positions enclosed by dashed lines labeled with reference numeral 13 in Figure 2 are the positions where electronic components will be attached later. Such positions will be referred to as planned positions 13 below. There are multiple planned positions 13. The reason why the size of the planned positions 13 is not uniform is that the planned positions 13 are sized to match the size of the electronic components that are to be mounted there. As described above, electronic components are mounted at the planned positions 13. Some electronic components are mounted on the base sheet 1 in the first step, which will be described later, and others are mounted on the base sheet 1 in the fourth step, which will be described later.The former of the electronic components are called pre-attached electronic components, and the latter are called post-attached electronic components. In this embodiment, as will be described later, pre-attached electronic components are attached only to the planned position 13A, which is shaded in Figure 2, and post-attached electronic components are attached to the other planned positions 13. The electronic components are IC chips, BGAs, CSPs, resistors, capacitors, etc., and are appropriately selected according to the performance required for the multilayer substrate. After the pre-attached electronic components are attached to the base sheet 1, they are heated in the third step, which will be described later. In this embodiment, however, the third step involves not only heating but also pressurization. Therefore, the pre-attached electronic components must be able to withstand at least the heating described later (so as not causing problems with the performance of the electronic components), and in this embodiment, they must be able to withstand both heating and pressurization described later. Examples of electronic components that often satisfy such requirements include resistors and capacitors. Also, the wiring 11 does not necessarily have to be provided on the base sheet 1, which has been shaped as shown in Figure 1. For example, it is certainly possible to use a method in which 10 x 10 = 100 wirings 11 are provided on a larger base sheet than the base sheet 1 shown in Figure 1 (for example, a base sheet with 10 times the length and width of the base sheet 1 shown in Figure 1), and then the base sheet 1 is cut into 10 pieces lengthwise and 10 pieces widthwise to obtain 100 identical base sheets 1 with wirings 11 at once, as shown in Figure 2. Such a method for producing wirings 11 (a method for producing base sheets 1 with a large number of pieces) enables efficient mass production of multilayer substrates. In this case as well, the 10 x 10 = 100 wirings provided on the large base sheet can be provided all at once by performing the wiring process on the large base sheet in a single step, thereby reducing the cost of manufacturing the base sheet 1. Note that in the base sheet 1, it does not matter whether the wirings 11 or the holes 1x are made first.

[0024] The wiring 11 may or may not have the same width throughout. It is also possible to give each of the linear wirings 11 connecting the electronic components attached to the base sheet 1 an appropriate width, as will be described later. For example, the width of at least one of the linear wirings 11 may differ from the width of the other linear wirings 11. Furthermore, the width of a single linear wiring 11 does not need to be constant along its entire length. As can be seen from Figure 2, the linear wiring 11 may cross the bend line 12. In such cases, the width of the wiring 11 at the point where it crosses the bend line 12 may be wider than the width on both sides of the wiring 11 in the longitudinal direction. An example of this is shown in Figure 3. Figure 3 is an enlarged view of the portion indicated by the dashed line X in Figure 2. In this way, the wiring 11 is wider at the point where it crosses the bend line 12 than on both sides or front and back in the longitudinal direction. While not limited to this, in this embodiment, all wiring 11 that straddles the bend line 12 have a width at the portion straddling the bend line 12 that is wider than both sides or front and back in the longitudinal direction. In this embodiment, when the wiring 11 is provided on the base sheet 1, the width of the portion straddling the bend line 12 of the wiring 11 is made wider than both sides or front and back in the longitudinal direction. Of course, Figure 3 is an example. In Figure 3, the width of the wiring 11 at the portion straddling the bend line 12 is linearly wider than both sides or front and back in the longitudinal direction, but the wiring 11 may, for example, have a width at the portion straddling the bend line 12 that is curvedly wider than both sides or front and back, or it may widen in a crank shape from a certain position.

[0025] Next, the cover sheet will be described. Figure 4 shows a view of the cover sheet 2 from the front. 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 superimposed 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 you want to align the edges of the base sheet 1 and the cover sheet 2 after carrying out the second step described later, you can adjust the size and shape of the base sheet 1 and the cover sheet 2, taking into consideration the shape, size, and thickness of the base sheet 1 and the cover sheet 2, as well as the way 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 perfectly identical. No wiring is provided on the cover sheet 2. The cover sheet 2 is made of thermoplastic resin and has insulating properties. Examples of thermoplastic resins that make up the cover sheet 2 include liquid crystal polymer (LCP), polyphenylene sulfide (PPE), polyether ether ketone (PEEK), and fluororesin (FR). While not limited to this, in this embodiment, liquid crystal polymer (LCP) is selected as the thermoplastic resin constituting the cover sheet 2. The thermoplastic resin constituting the cover sheet 2 and the thermoplastic resin constituting 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 material 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 temperature, heating time, and pressure applied as needed during the third step, which will be described later. 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 later. For example, the thickness can be 20 μm to 200 μm, 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 needs to be thin enough so that the second step can be carried out with both sheets stacked on top of each other.On the other hand, it is preferable that the thickness of the cover sheet 2 exceeds 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 adequately cover the wiring 11 after the third step, which will be described later, is carried out. 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 that covers the wiring 11 (the surface that does not come into 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, it may cause problems such as poor bonding when mounting electronic components on a surface with poor flatness, but such problems are less likely to occur if the thickness of the cover sheet 2 is thicker than the thickness of the wiring 11. Furthermore, 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 the second process described later is performed, improving their bendability and also leading to cost reductions in processing and material costs. The cover sheet 2 is provided with a plurality of holes 21. The holes 21 can be classified into three types. These will be called holes 21x, holes 21y, and holes 21z, respectively. There is at least one of each type of hole 21x, hole 21y, and hole 21z in the cover sheet 2. The first type of hole, 21x, is for preventing interference between the pre-attached electronic component and the cover sheet 2. The hole 21x is shaped and sized so that the pre-attached electronic component can be placed inside it. Furthermore, the hole 21x is positioned so that when the second process described later is completed, the pre-attached electronic component will be inside the hole 21x. In this embodiment, there are four holes 21x, but there may be cases where there are more than four holes 21x. If there are multiple holes 21x, each hole 21x will be associated with a different pre-installed electronic component in a one-to-one correspondence. After the second step is completed, the same pre-installed electronic component may be located in multiple holes 21x, or different pre-installed 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 method of folding, the position and number of pre-attached electronic components, etc., it is determined whether the same pre-attached electronic component is located in one of the multiple holes 21x, or whether different pre-attached electronic components are located in each of the 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 in a position and shape 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, which will be described later. The second type of hole 21y provided in the cover sheet 2 is a hole provided in a position corresponding to the position where the pre-attached electronic component is attached to the base sheet 1, that is, 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. In this embodiment, although not limited to this, only one hole 21y is provided in the lower right direction of Figure 4. The third type of hole 21z provided in the cover sheet 2 is a hole provided in a position corresponding to the position where the retrofit electronic component will be attached to the base sheet 1, i.e., the planned position 13, when the cover sheet 2 is superimposed on the base sheet 1. In the fourth step, the retrofit electronic component is attached to the base sheet 1 through this hole 21z. In this embodiment, although not limited to this, a total of seven holes 21z are provided in the upper and lower left sides of Figure 4. The second type of hole 21y and the third type of hole 21z are provided in positions where the electronic component will later be placed on the base sheet 1, i.e., the planned position 13 mentioned above. The reason why the sizes of the holes 21y and 21z are not uniform is that the holes 21y and 21z are sized to match the size of the electronic component that will be attached to the base sheet 1 through them. In Figure 4, the dashed line labeled 22 indicates the fold line 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 will be located at corresponding positions because the superimposed base sheet 1 and cover sheet 2 will be folded together.

[0026] Once the base sheet 1 and cover sheet 2 described above are prepared, the first step is performed. In the first step, the cover sheet 2 is placed on top of the front surface of the base sheet 1 so that the contours of both sheets match (Figure 5(A)). When this is done, the four holes 21x in the cover sheet 2 will perfectly overlap with the four holes 1x in the base sheet 1. At this time, the planned position 13 on the base sheet 1 located on the back side of the cover sheet 2 will be visible through the holes 21y and 21z in the cover sheet 2. Also at this time, the ends of the wiring 11 will be slightly visible near 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. In this embodiment, however, there is one pre-attached electronic component 3y. In this embodiment, however, the pre-attached electronic component 3y 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 connected to the wiring 11 in a conductive state by, for example, soldering them to the wiring 11. Since it is necessary to insert the electronic component into the hole 21y in order to fix the pre-attached electronic component 3y to the wiring 11, the hole 21y needs to be slightly larger than the pre-attached electronic component 3y that will 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 when inserting the pre-attached electronic component 3y into the hole 21y, potentially damaging the pre-attached electronic component 3y, or it may become difficult to move the pre-attached electronic component 3y horizontally relative to the front surface of the base sheet 1. Furthermore, in order to enable the connection of the pre-attached electronic component 3y with electrical conductivity to the wiring 11, the hole 21y needs to be sized and shaped such that the end of the wiring 11 is visible from its peripheral portion. The hole 21y in this embodiment is such. The first step is completed by attaching the pre-attached electronic component 3y to the base sheet 1 (Figure 5(B)). Alternatively, the cover sheet 2 may be placed on the base sheet 1 after attaching the pre-attached electronic component 3y to the base sheet 1. In that case as well, the cover sheet 2 should be placed on the base sheet 1 so that the pre-attached electronic component 3y is positioned inside the hole 21y in the cover sheet 2 that is placed on top of the base sheet 1.

[0028] Furthermore, a temporary fixing step may be performed at an appropriate timing during the first step or after the first step is completed but before the second step is executed, in order to prevent the contours of the overlapping base sheet 1 and cover sheet 2 from shifting. This is not necessarily required, but it is done in this embodiment. Temporary fixing can be done by bonding a portion of the contact area (for example, the four corners) of the base sheet 1 and cover sheet 2 with adhesive. However, there are no restrictions on the method of temporary fixing. A pin lamination method, which is used in general multilayer substrate manufacturing methods, may be used, in which perforated substrates are stacked and each substrate is positioned by passing pins through the holes provided in each substrate. Alternatively, temporary fixing may be performed using a temporary fixing jig (clip, etc.) that holds the base sheet 1 and cover sheet 2 together. In this embodiment, although not limited to this, the cover sheet 2 is temporarily fixed to the base sheet 1 after the cover sheet 2 is placed on top of the base sheet 1 and before attaching the pre-installed electronic components 3y to the base sheet 1.

[0029] Next, the second step is performed. In the second step, the superimposed base sheet 1 and cover sheet 2 are folded together. The folding is performed along the overlapping folding lines 12 of the base sheet 1 and 22 of the cover sheet 2. The folding is performed at least once. The folding may be a mountain fold or a valley fold, or a combination of mountain and valley folds. However, after the second step is performed, the holes 21z provided in the cover sheet 2 superimposed on the base sheet 1, corresponding to the positions on the base sheet 1 where the add-on electronic components are attached, are exposed to the outside. When the holes 21z are exposed to the outside, it means that the holes 21z are not covered by the base sheet 1 or the cover sheet 2.

[0030] An example of how to fold the base sheet 1 and cover sheet 2 in the second step will be explained using Figure 6. Figure 6(A) is a view of the same object as in Figure 5, but from the same direction. Although not limited to this, in the second step of this embodiment, first, the portion of the base sheet 1 and cover sheet 2 (Figures 5(B) and 6(A)) that has been superimposed as a result of the first step, to the right of the vertically running fold lines 12 and 22, is folded back towards the back side of Figure 6(A) along the vertically running fold lines 12 and 22. As a result, the base sheet 1 and cover sheet 2 are in the state shown in Figures 6(B) and (C). Figure 6(B) shows the state of the base sheet 1 and cover sheet 2 as viewed from the front when the portion to the right of the vertically running fold lines 12 and 22 is folded back towards the back side of Figure 6(A) along the vertically running fold lines 12 and 22, and Figure 6(C) shows the same object as viewed from the back side. After this first fold, the two holes 21x (and the hole 1x overlapping the holes 21x) located relatively to the right in Figure 6(A) overlap the two holes 21x (and the hole 1x overlapping the holes 21x) located relatively to the left. Next, the portion of the base sheet 1 and cover sheet 2 (for explanatory purposes, the same drawing as in Figure 6(B) and (C) shown above the upper of the three horizontal fold lines 12 and 22 is folded back to the back side of Figure 6(D) at the fold lines 12 and 22, and the portion of the base sheet 1 and cover sheet 2 below the lower of the three horizontal fold lines 12 and 22 is folded back to the back side of Figure 6(D) at the fold lines 12 and 22. As a result, the folded base sheet 1 and cover sheet 2 are in the state shown in Figures 6(E) and (F). Figure 6(E) shows the folded base sheet 1 and cover sheet 2 as viewed from the front, and Figure 6(F) shows the same as viewed from the back. After the second and third folding, the pre-attached electronic component 3y attached to the base sheet 1 will be located in the hole 21x (and the hole 1x overlapping the hole 21x) which is relatively lower in Figure 6(D).The pre-attached electronic component 3y is visible from the front side of the base sheet 1 and cover sheet 2 through the hole 21x (and the hole 1x overlapping the hole 21x) (see Figure 6(E)). Next, the portions of the base sheet 1 and cover sheet 2 (for explanatory purposes, the same drawing as in Figure 6(E) is shown again in Figure 6(G)) that are in the state shown in Figures 6(E) and (F) are folded back to the front side (front side) of Figure 6(G) at the fold 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) shows the folded base sheet 1 and cover sheet 2 as seen from the front side, and Figure 6(I) shows the same as seen from the back side. In this state, all four holes 21x (and holes 1x overlapping the holes 21x) shown in Figure 6(A) are overlapping at the same position. Figure 7(A) shows a cross-section of the folded base sheet 1 and cover sheet 2 near the overlapping holes 21x and holes 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. The pre-attached electronic component 3y attached to the base sheet 1 is located within this large space S1. The thickness of the pre-attached electronic component 3y (length in the vertical direction in Figure 7(A)) is greater than the thickness of the cover sheet 2, and in this embodiment, it is also greater than the thickness of the base sheet 1. Of course, it is not limited to this, but in this embodiment, the height of the above-mentioned space S1 is the height obtained 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 upper surface of the pre-attached electronic component 3y) can be increased as long as it reaches the height of the space S1 determined as described above. In other words, according to this embodiment, the thickness of the pre-attached electronic component 3y can be greater than the thickness of the cover sheet 2. This concludes the second step in this embodiment.At the end of the second process, four holes 21z in the cover sheet 2 are exposed on the surface shown in Figure 6(H), and three are exposed on the surface shown in Figure 6(I). In other words, all seven holes 21z in the cover sheet 2 that originally existed are exposed to the outside at the end of the second process. Also, in the example explained using Figures 6(A) to 6(I), all parts of the base sheet 1 and the cover sheet 2 are virtually parallel except for the folded parts. In other words, the folded base sheet 1 and cover sheet 2 are stacked from bottom to top, as shown in the cross-sectional view of Figure 7(A), from the surface shown in Figure 6(I) to the surface shown in Figure 6(H). All wiring 11 that straddles the folding line 12 have a width at the part that straddles the folding line 12 that is wider than both sides or front and back in the length direction, so even if the wiring 11 is folded 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 end of the second step, the space S1 created by overlapping the holes 1x in the base sheet 1 and the holes 21x in the cover sheet 2 is blocked above by at least one of the base sheet 1 and the cover sheet 2 (in this embodiment, two laminated base sheets 1 and one cover sheet 2). However, the space S1 can be made into an open space S2, for example, as shown in Figure 7(B), which is a cross-sectional view equivalent to that of Figure 7(A). It is obvious that to create such a space S2, holes 1x and 21x should be provided in the base sheet 1 and cover sheet 2 in Figure 7(A) at locations above space S1. In other words, to create a space S2 as shown in Figure 7(B), two more holes 1x are needed in the base sheet 1 and one more hole 21x is needed in the cover sheet 2 than when creating a space S1 as shown in Figure 7(A). Furthermore, the space S1 described above may not have the same 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 that of Figure 7(A). The space S3 shown in Figure 7(C) narrows as it moves upward in the figure. Such a space S3 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, there was one space S1 provided in the folded base sheet 1 and cover sheet 2, but this is of course not limited to this, and there may be spaces other than space S1 in the folded base sheet 1 and cover sheet 2. To have multiple spaces, additional holes 1x can be provided in the base sheet 1 and additional holes 21x can be provided in the cover sheet 2 as needed.

[0032] Next, three steps are performed. In the third step, the base sheet 1 and cover sheet 2, which have been fixed after the second step, are fixed together. This third step is performed by heating the folded base sheet 1 and cover sheet 2. When performing the third step, it is preferable not only to heat the base sheet 1 and cover sheet 2, but also to pressurize the folded base sheet 1 and cover sheet 2 by sandwiching them from both sides, the side shown in Figure 6(H) and the side shown in Figure 6(I). This is done in this embodiment. The heating and pressing conditions for the base sheet 1 and cover sheet 2 can be adjusted depending on the thickness and melting point of the material. The heating and pressing conditions described below are an example where the thermoplastic resins constituting the base sheet 1 and 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 reason for selecting a thermoplastic resin for the cover sheet 2 that has a lower melting point than the liquid crystal polymer used as the thermoplastic resin for the base sheet 1 is to allow the cover sheet 2 to be softened preferentially over the base sheet 1 in the third step, thereby enabling the liquid crystal polymer constituting the cover sheet 2, which has softened and become fluid, to effectively fill the gaps between adjacent base sheet 1 and cover sheet 2 (where there may be irregularities due to the wiring 11), or the gaps between adjacent cover sheets 2. Thus, in this invention, the thermoplastic resins constituting the base sheet 1 and the cover sheet 2 can be selected such that the melting point of the thermoplastic resin constituting the cover sheet 2 is lower than the melting point of the thermoplastic resin constituting the base sheet 1. For example, heating of the base sheet 1 and the cover sheet 2 can be done using a general press device. (1) First, the temperature of the base sheet 1 and the cover sheet 2 is raised from room temperature to a temperature at which the thermoplastic resins constituting them (especially the thermoplastic resin constituting the cover sheet 2) soften slightly, for example, 150°C, and this temperature is maintained for several minutes.This step is performed with the aim of filling to some extent the gaps between adjacent base sheets 1 and cover sheets 2, or between adjacent cover sheets 2, by softening at least the cover sheet 2 of the base sheet 1 and cover sheet 2, so that the softened and fluidized thermoplastic resin fills the gaps between adjacent base sheets 1 and cover sheet 2, or between adjacent cover sheets 2. (2) Next, the temperature of the base sheet 1 and cover sheet 2 is raised to a temperature close to the melting point of the thermoplastic resin that constitutes them (especially the thermoplastic resin that constitutes cover sheet 2), for example, 290°C, and the temperature is maintained for about 30 minutes. This step is performed with the aim of further softening at least the cover sheet 2 of the base sheet 1 and cover sheet 2, so that the softened and fluidized thermoplastic resin fills the gaps between adjacent base sheets 1 and cover sheet 2, or between adjacent cover sheets 2. (3) Finally, the temperature of the base sheet 1 and cover sheet 2 is cooled to, for example, room temperature. Through this process, the thermoplastic resin that has filled the gaps between adjacent base sheets 1 and cover sheet 2, or between adjacent cover sheets 2, is cured. As a result, the gaps between adjacent base sheet 1 and cover sheet 2, or the gaps between adjacent cover sheet 2, are effectively filled with thermoplastic resin, and the adjacent base sheet 1 and cover sheet 2, or the adjacent cover sheet 2, are fixed to each other. When carrying out steps (1) to (3) described above, the base sheet 1 and cover sheet 2 are pressurized in a press device. In this embodiment, for example, pressurization is not performed for a while from the start of step (1) described above, and from about the middle of the time period in the same step when the temperature of the base sheet 1 and cover sheet 2 is maintained at 150°C until the end, the base sheet 1 and cover sheet 2 are pressurized at, for example, 3 MPa while maintaining a constant pressure. This is to ensure that the gaps between adjacent base sheet 1 and cover sheet 2, or the gaps between adjacent cover sheet 2, are well filled with fluidized thermoplastic resin. Then, from the start of step (2) described above until the end of step (3), the base sheet 1 and cover sheet 2 are pressurized at a constant pressure of 1 MPa.The reason why the pressure applied at this time is less than the pressure applied during the time when the temperature of the base sheet 1 and cover sheet 2 in step (1) is maintained at 150 degrees (as mentioned above in this example, this pressure is 3 MPa) is that although a certain amount of pressure is necessary to fill the gaps between adjacent base sheet 1 and cover sheet 2, or between adjacent cover sheet 2, with the fluidized thermoplastic resin, if the pressure is too strong, the fluidized thermoplastic resin may leak out from the gaps between adjacent base sheet 1 and cover sheet 2, or between adjacent cover sheet 2.

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

[0034] Once the third step is completed, the fourth step is carried out. In the fourth step, add-on electronic components 3z, which are electronic components other than the pre-attached electronic components 3y, are attached to the base sheet 1. Add-on electronic components 3z are, for example, IC chips, BGAs, CSPs, resistors, capacitors, etc., and are appropriately selected according to the performance required for the multilayer substrate. The electronic components are connected in a conductive manner to the wiring 11 provided on the front surface of the base sheet 1 included in the cover sheet 2, which is in a plate shape after the third step is completed (Figures 8(A) and (B)), and are thereby fixed to the base sheet 1. Figures 8(A) and (B) show the base sheet 1 and cover sheet 2 as viewed from the front side (one side) and back side (the other side), respectively, after the fourth step is completed. The add-on electronic components 3z are inserted into each of the holes 21z drilled in the cover sheet 2, and the terminals protruding from the add-on electronic components 3z are connected to the wiring 11 in a conductive manner by, for example, soldering them to the wiring 11. In order to fix the add-on electronic component 3z to the wiring 11, it is necessary to insert the add-on electronic component 3z into the hole 21, so the hole 21z needs to be slightly larger than the add-on electronic component 3z. This is for the same reason that the hole 21y needs to be slightly larger than the pre-installed electronic component 3y. Also, in order to enable the add-on electronic component 3z to be connected to the wiring 11 in a conductive state, the hole 21z needs to be sized and shaped so that the end of the wiring 11 is visible from its periphery. The hole 21z in this embodiment is designed in this way. Note that adjacent holes 21 at close range can be combined into a single hole 21. For example, it is possible to combine two holes 21z located side by side on the upper side of Figure 8(A) into one. In such a case, two add-on electronic components 3z will be installed in a single hole 21z. The multilayer substrate is completed by fixing all the planned add-on electronic components 3z to the base sheet 1 and configuring an electrical circuit with the wiring 11 and electronic components.In this embodiment, the multilayer substrate manufactured is such that the base sheet 1 and the cover sheet 2 are substantially parallel except in the bent portion, and the number of superimposed base sheet 1 and cover sheet 2 (the sum of the two) is the same in all parts of the multilayer substrate (16 in this embodiment). A multilayer substrate with such a structure is in the form of a plate. In a multilayer substrate manufactured by this manufacturing method, the front surface of the base sheet 1 is covered by the cover sheet 2, except for the portion where the hole 21y is made (as described above, it is also possible to expose the space S2 to the outside). As a result, the wiring 11 is protected by the cover sheet 2, making it less likely for the wiring 11 to break. In addition, although not limited to this, in this embodiment, the pre-attached electronic component 3y is covered on both sides in the height direction by at least one of the base sheet 1 and the cover sheet 2 so that it is not exposed to the outside. By preventing the pre-attached electronic component 3y from being exposed to the outside, it is possible to protect the pre-attached electronic component 3y by at least one of the base sheet 1 and the cover sheet 2.

[0035] <Modification 1> The manufacturing method of the multilayer substrate in Modification 1 is almost the same as the manufacturing method of the multilayer substrate described in the embodiment. Except for the points mentioned as differences, the manufacturing method of Modification 1 is basically the same as the manufacturing method according to the embodiment described above. In Modification 1 as well, the multilayer substrate is manufactured using a base sheet 1, a cover sheet 2, and electronic components as materials. The configuration of the base sheet 1 and the cover sheet 2 is basically the same in Modification 1 as in the embodiment described above. As shown in Figure 9, the base sheet 1 of Modification 1 also has four holes 1x of the same size and shape as the holes 1x of the base sheet 1, which were four in the embodiment, and is provided in the same positions as in the embodiment described above. Wiring 11 is also provided on the front surface of the base sheet 1 of Modification 1 as in the embodiment described above, but the pattern is somewhat different from the embodiment described above because the number of pre-attached electronic components 3y is different from the embodiment described above. In addition, the base sheet 1 of Modification 1 has two planned positions 13A, unlike the embodiment described above. The base sheet 1 of Modification 1 also has folding lines 12, similar to the embodiment described above, but the number or arrangement of these lines differs from that of the embodiment described above. The cover sheet 2 of Modification 1 is shown in Figure 10. The cover sheet 2 of Modification 1 is almost the same as the cover sheet 2 of the embodiment described above, but the number of holes 21x, which was four in the embodiment described above, has been reduced to three. Specifically, the hole 21x that was shown in the lower left of Figure 4 is absent in Modification 1.

[0036] In Modification 1, the same first step as in the above-described embodiment is performed. When the first step is performed, in Modification 1, the cover sheet 2 is first placed on the front surface of the base sheet 1 and the two are temporarily fixed together. The cover sheet 2 shown in Figure 10 is placed on the base sheet 1 shown in Figure 9 in the same orientation. The two holes 21y provided in the cover sheet 2 align 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 of the base sheet 1. The method of attaching the pre-attached electronic components 3y to the base sheet 1 is the same in Modification 1 and in the above-described embodiment. However, in Modification 1, the pre-attached electronic components 3y are attached to each of the two planned positions 13A. This completes the first step. Figure 11 shows the stacked base sheet 1 and cover sheet 2 at the time the first step is completed.

[0037] In the modified example 1, the second step is performed after the first step is completed. Using Figure 12, we will explain how to fold the base sheet 1 and cover sheet 2 that have been superimposed as a result of the first step. Figure 12(A) is a view of the same base sheet 1 and cover sheet 2 shown in Figure 11, from the same direction. Although not limited to this, in the second step of this embodiment, first, the base sheet 1 and cover sheet 2 that have been superimposed as a result of the first step are folded back towards the back side of Figure 12(A) at the vertical fold lines 12 and 22, as indicated by the arrows in Figure 12(A). Then the base sheet 1 and cover sheet 2 are in the state shown in Figure 12(B). After this first fold, in Figure 12(A), of the two holes 21x (and the hole 1x overlapping hole 21x) that are side by side, the hole 21x (and the hole 1x overlapping hole 21x) located on the right side will overlap the hole 21x (and the hole 1x overlapping hole 21x) located on the left side. In other words, in this section, the multiple holes 21x and holes 1x that exist at the same position become continuous, forming a series of holes that penetrate the base sheet 1 and the cover sheet 2. Next, the portions of the base sheet 1 and cover sheet 2 below the horizontal fold lines 12 and 22, which are in the state shown in Figure 12(B), are folded back to the back side of Figure 12(B) at the fold lines 12 and 22. As a result, the folded base sheet 1 and cover sheet 2 are in the state shown in Figure 12(C). The same diagram as shown in Figure 12(C) is shown in Figure 13(A). Figure 13(B) shows the base sheet 1 and cover sheet 2 as seen from the back, as shown in Figure 13(A). The second step in Modification 1 is completed when this second fold is finished. After the second fold, one of the pre-attached electronic components 3y attached to the base sheet 1 (the pre-attached electronic component 3y located relatively lower in Figure 12(A)) will be located in a series of holes that penetrate the base sheet 1 and cover sheet 2 as described above, which are formed after the second fold.This pre-attached electronic component 3y is visible from the front side of the base sheet 1 and cover sheet 2 through the series of holes described above (see Figures 12(C) and 13(A)). In other words, one of the pre-attached electronic components 3y is housed within the series of holes described above. Figure 14(A) shows a cross-sectional view of the folded base sheet 1 and cover sheet 2 near this pre-attached electronic component 3y. The space labeled S4 in Figure 14(A) is the series of holes described above. The other pre-attached electronic component 3y (the pre-attached electronic component 3y located relatively higher in Figure 12(A)) is housed within a space S5 formed by the two lower holes of the horizontally aligned holes 1x in the base sheet 1 shown in Figure 9 and the lower hole of the vertically aligned holes 21x in the right half of the cover sheet 2 shown in Figure 10 (Figure 14(B)). Thus, in Modification 1, one of the two pre-attached electronic components 3y is exposed to the outside, while the other is not. However, this is not the only option. Both pre-attached electronic components 3y may be exposed to the outside, or neither may be exposed to the outside. In other words, in Modification 1, any of the pre-attached electronic components 3y may be exposed, or none may be exposed. Whether or not each pre-attached electronic component 3y is exposed can be changed by appropriately changing the holes 1x provided in the base sheet 1 and the holes 21x provided in the cover sheet 2. This concludes the second step in this embodiment. After completing the second step, four holes 21z in the cover sheet 2 are exposed on the side shown in Figure 13(A), and three holes are exposed on the side shown in Figure 13(B). That is, all seven holes 21z in the cover sheet 2 that originally existed are exposed to the outside after completing the second step. Furthermore, in the example explained using Figures 13(A) and (B), all parts of the base sheet 1 and the cover sheet 2 are virtually parallel except for the folded parts.

[0038] Subsequently, in the modified example 1, the third and fourth steps are performed in the same manner as in the embodiment described above, thereby completing the multilayer substrate. The completed multilayer substrate is as shown in Figures 13(A) and (B), except that the after-market electronic component 3z is attached. The base sheet 1 and cover sheet 2 after the after-market electronic component 3z is attached are shown in Figures 13(C) and (D). The base sheet 1 and cover sheet 2 shown in Figure 13(C) are viewed from the same side as in Figure 13(A), and the base sheet 1 and cover sheet 2 shown in Figure 13(D) are viewed 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 in Figure 13(E). This multilayer substrate contains 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] <Modification 2> The manufacturing method of the multilayer substrate in Modification 2 is almost the same as the manufacturing method of the multilayer substrate described in the embodiment. Except for the points mentioned as differences, the manufacturing method of Modification 2 is basically the same as the manufacturing method according to the embodiment described above. In Modification 2 as well, the multilayer substrate is manufactured using a base sheet 1, a cover sheet 2, and electronic components as materials. The configuration of the base sheet 1 and the cover sheet 2 is basically the same in Modification 2 as in the embodiment described above. However, in Modification 2, both the base sheet 1 and the cover sheet 2 are square. Figure 15 shows a view of the base sheet 1 from the front side, and Figure 16 shows a view of the cover sheet 2 superimposed on the base sheet 1 shown in Figure 15 from the front side. Note that the front surface of the base sheet 1 in Modification 2 is provided with wiring 11 similar to that in the embodiment described above, but this is omitted from the illustration. The base sheet 1 is provided with one planned position 13A for the later attachment of pre-attached electronic components and eight planned positions 13 for the later attachment of post-attached electronic components. The base sheet 1 is also provided with holes 1x. The base sheet 1 has fold lines, which consist of dashed lines 12A and dotted lines 12B. The former is a mountain fold line where the base sheet 1 (and cover sheet 2) is folded in a mountain fold, and the latter is a valley fold line where the base sheet 1 (and cover sheet 2) is folded in a valley fold. As shown in Figure 16, the cover sheet 2 is also square, and its size is equal to that of the base sheet 1. In the portion of the cover sheet 2 corresponding to the planned position 13A on the base sheet 1, there is a hole 21y of the shape and size corresponding to the planned position 13A. In the portion of the cover sheet 2 corresponding to the planned position 13 on the base sheet 1, there are eight holes 21z of the shape and size corresponding to the planned position 13. In the portion of the cover sheet 2 corresponding to the planned position 13 on the base sheet 1, there is a hole 21x of the shape and size corresponding to the hole 1x on the cover sheet 2. The cover sheet 2 has fold lines, which consist of dashed lines 22A and dotted lines 22B. The former is the mountain fold line at which the cover sheet 2 (and base sheet 1) is folded in a mountain fold, and the latter is the valley fold line at which the cover sheet 2 (and base sheet 1) is folded in a valley fold.Later, when the cover sheet 2 is placed on top of the base sheet 1, the dashed line 22A will overlap exactly with the dashed line 12A, and the dotted line 22B will overlap exactly with the dotted line 12B.

[0040] In Modification 2, the first step is performed in the same manner as in the above-described embodiment. In the first step, the cover sheet 2 is placed on top of the base sheet 1, and the cover sheet 2 is temporarily fixed to the base sheet 1 as needed. This is the same in Modification 2 and the above-described embodiment. Figure 17 shows the state in which the cover sheet 2 is placed on top of the base sheet 1. The cover sheet 2 is placed on the base sheet 1 shown in Figure 15 as shown in Figure 17. The holes 21y in the cover sheet 2 are aligned with the planned position 13A in the base sheet 1, the holes 21y in the cover sheet 2 are aligned with the planned position 13 in the base sheet 1, and the holes 1x in the base sheet 1 are aligned with the holes 21x in the cover sheet 2. Next, the pre-attached electronic component 3y is attached to the planned 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 Modification 2 and the above-described embodiment. This completes the first step. Figure 18(A) shows the stacked base sheet 1 and cover sheet 2 at the end of the first process.

[0041] In Modification 2, as in the embodiment described above, the second step is performed after the first step. The dashed line 22A shown in Figure 18 is the mountain fold line where the base sheet 1 and cover sheet 2 are mountain folds, and the dashed line 22B is the valley fold line where the base sheet 1 and cover sheet 2 are valley folds. In this embodiment, the base sheet 1 and cover sheet 2 are folded from the state in Figure 18(A) through the state in Figure 18(B) to the plate-like state shown in Figure 18(C) by simultaneously performing mountain folds along the mountain fold line 22A and valley folds along the valley fold line 22B. This type of folding is common in origami. In Figure 18(B), the centers of the base sheet 1 and cover sheet 2 are located on the front side of the paper, and the edges of the base sheet 1 and cover sheet 2 are located on the back side of the paper. This completes the second step of Modification 2. In this case as well, after the second step is completed, all eight holes 21z of the cover sheet 2 are exposed to the outside (Figure 19). Figure 19(A) shows one side of the base sheet 1 and cover sheet 2 of the modified example 2, which have become plate-like after the second step is completed, and Figure 19(B) shows the other side. In this case as well, all parts of the base sheet 1 and cover sheet 2 after the second step is completed are virtually parallel except for the folded parts. When the second step is completed, the pre-attached electronic component 3y is encased in the folded base sheet 1 and cover sheet 2 and is not exposed to the outside. Figure 20 shows a cross-sectional view of the base sheet 1 and cover sheet 2 near the pre-attached electronic component 3y. As shown in Figure 20, the pre-attached electronic component 3y is housed inside the space S6 formed by the holes 1x in the base sheet 1 and the holes 21x in the cover sheet 2.

[0042] Subsequently, in the modified example 2, the third and fourth steps are performed in the same manner as in the embodiment described above, thereby completing the multilayer substrate. The completed multilayer substrate is as shown in Figures 19(A), (B), and 18(C), except that the add-on electronic components 3z are attached. As a result, the multilayer substrate manufactured by the manufacturing method of modified example 2 is a right-angled isosceles triangular plate with the pre-attached electronic components 3y embedded in the space S6 and the add-on electronic components 3z attached to both the front and back surfaces.

[0043] 1 Base sheet 1x hole 2 Cover sheet 3y Pre-attached electronic component 3z Post-attached electronic component 11 Wiring 12 Bent wire 21x hole 21y hole 21z hole 22 Bent wire

Claims

1. A method for manufacturing a multilayer substrate, comprising carrying out the first, second, third, and fourth steps in that order to manufacture a multilayer substrate on which multiple electronic components are attached, The first step involves preparing a base sheet which is an insulating rectangular sheet made of thermoplastic resin with conductive wiring arranged appropriately on its front surface, and which has holes provided as needed at positions that interfere with pre-attached electronic components which are part of a plurality of electronic components when the second step is performed; a cover sheet which is an insulating sheet made of thermoplastic resin and has substantially the same shape and size as the base sheet, with at least one hole provided at the position where the pre-attached electronic components are attached, and at least one hole provided at the position where the after-attached electronic components which are electronic components other than the pre-attached electronic components are attached, and which has at least one hole provided at the position that interferes with the pre-attached electronic components when the second step is performed, and which is superimposed on the front surface of the base sheet in such a state that its outline matches the outline of the base sheet; and the pre-attached electronic components which are fixed to the base sheet and are visible through the holes, and are inserted into the holes provided on the cover sheet where the pre-attached electronic components are attached, and are electrically connected to the wiring on the front surface of the base sheet visible through the holes. The second step involves folding the base sheet and the cover sheet, which were superimposed in the first step, so that the holes provided in the cover sheet where the retrofitted electronic components are 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 cause problems with the performance of the pre-attached electronic components to melt at least a part of the cover sheet, and then cooling and hardening it to fix the adjacent base sheet and the cover sheet together.The fourth step is to insert the add-on electronic component into a hole provided in the cover sheet at a position where the add-on electronic component is to be attached, and fix the add-on electronic component to the base sheet visible through the hole, while making electrical contact with the wiring on the front surface of the base sheet visible through the hole, the method for manufacturing a multilayer substrate.

2. The method for manufacturing a multilayer substrate according to claim 1, wherein in the first step, the cover sheet is superimposed on the front surface of the base sheet such that its contour matches the contour of the base sheet, and 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 is fixed to the base sheet visible through the hole while electrically connected to the wiring on the front surface of the base sheet visible through the hole.

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

4. The method for manufacturing a multilayer substrate according to claim 1, wherein the second step is performed such that all parts of the base sheet and the cover sheet after the fourth step are substantially parallel except for the folded parts.

5. The method for manufacturing a multilayer substrate according to claim 4, wherein the second step is performed such that, after the completion of the fourth step, the number of substrate sheets and cover sheets overlapped in a certain portion other than the folded portion is different from the number of substrate sheets and cover sheets overlapped in other portions.

6. The method for manufacturing a multilayer substrate according to claim 1, wherein in the third step, adjacent cover sheets among the superimposed base sheet and cover sheet are also fixed together.

7. The method for manufacturing a multilayer substrate according to claim 1 or 6, wherein in the third step, the superimposed base sheet and the cover sheet are heated under pressure.

8. A method for manufacturing a multilayer substrate according to claim 1, wherein, prior to the first step, a wiring step is performed to provide the wiring on the front surface of the base sheet.

9. The method for manufacturing a multilayer substrate according to claim 1 or 8, wherein the width of the wiring at the portion where the base sheet is bent is wider than the width of the portions on both sides in the longitudinal direction.

10. The method for manufacturing a multilayer substrate according to claim 1 or 9, wherein the thickness of the cover sheet is thinner than the thickness of the base sheet.

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

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