Method for manufacturing multilayer substrate

Abstract

The present invention provides multilayer substrate manufacturing technology that makes it possible to manufacture a multilayer substrate at low cost.　First, a cover sheet (2) is superimposed on 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 hole (21) is opened at a position as which an electronic component will be attached to the base material sheet (1) after the cover sheet (2). Thereafter, the base material sheet (1) and the cover sheet (2) that have been superimposed are folded such that the hole (21) is exposed to the outside. The electronic component is inserted into the hole (21) and is attached in a state of electrical connection with the wiring (11) of the base material sheet (1).

Description

Multilayer board manufacturing method

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

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

[0003] The circuits in each board included in a multilayer board generally need to be electrically connected to each other. Therefore, holes called through holes are typically drilled in multilayer boards, penetrating each board included in the stacked multilayer board. Typically, a metal layer is formed on the inner surface of the through hole, for example, by plating with a conductive metal, and the wiring in each board is connected to the metal layer, thereby enabling electrical connection between the wiring in each board included in the multilayer board via the metal layer in the through hole. For example, wiring on both sides of a single board is electrically connected to each other via the metal layer in the through hole. Alternatively, wiring on both sides of a single board, or on any side of multiple boards, can be connected to each other with a conductive connecting wire outside the board, thereby enabling electrical connection between wiring located at distant locations, and such a technique is also in practical use.

[0004] While multilayer boards are as described above, there is room for improvement. Conventional multilayer boards include through holes and external connection lines, as described above. The presence of through holes requires the processes of drilling holes in the board to form the through holes and forming metal layers on the inner surfaces of the through holes, which increases the manufacturing cost of the multilayer board. Furthermore, as described above, the through holes are electrically connected to the wiring of each board included in the multilayer board. Therefore, it is necessary to accurately position the wiring relative to the base material in each board, and to accurately position the multiple boards relative to each other when stacking multiple boards. While the two required accuracy levels for the above reasons are technically feasible, achieving them is time-consuming and likely to increase costs. Furthermore, conventional multilayer board manufacturing processes involve separately manufacturing the multiple boards to be stacked, and then stacking the separately manufactured boards. Alternatively, cover sheets and copper foils may be laminated on the top and bottom of each board to form circuits on the top and bottom surfaces of the board, and this process may be repeated to gradually increase the number of layers. Here, when manufacturing each board, common processes, such as cutting circuits in copper foil attached to a base material and plating with gold, are typically performed for each board, which can easily result in increased costs due to the repeated execution of common processes for each board. The same is true when cover sheets and copper foil are laminated on the top and bottom of a board. Even when providing connection lines outside the board, performing this process increases the manufacturing cost of the multilayer board. Additionally, the fact that repeated execution of common processes for each board can easily result in increased costs remains unchanged.

[0005] A main object of the present invention is to provide a multilayer board manufacturing technique that allows for the manufacture of multilayer boards at low cost.

[0006] The present invention for solving the above-mentioned problems is as follows. The present invention is a method for manufacturing a multilayer board, including: a layering step of layering a base sheet, which is a rectangular insulating sheet made of a thermoplastic resin and has conductive wiring arranged in an appropriate position on its front surface, with a cover sheet, which is an insulating thermoplastic resin sheet of substantially the same shape and size as the base sheet, on the front surface of the base sheet, with at least one hole formed in a position where an electronic component will later be placed, so that the contours of the base sheet and the cover sheet match; a folding step of folding the layered base sheet and cover sheet so that the hole is exposed to the outside; a fixing step of heating the folded base sheet and cover sheet to melt at least a portion of the cover sheet, and then cooling and hardening the cover sheet, thereby fixing the adjacent base sheet and cover sheet together; and an electronic component attachment step of fixing an electronic component inserted in the hole to the base sheet exposed through the hole, with the electronic component being electrically connected to the wiring on the front surface of the base sheet exposed through the hole.

[0007] In this method for manufacturing a multilayer board, the lamination process, folding process, bonding process, and electronic component attachment process are performed in this order, as described above. In the lamination process, a cover sheet is placed on the front surface of a base sheet. The base sheet is an insulating rectangular sheet made of a 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 only on one side of the base sheet. The surface of the base sheet on which wiring is provided is the front surface. The wiring is conductive and is generally made of a conductive metal. There are no limitations on the method for providing wiring on the base sheet, and wiring can be provided on the front surface of the base sheet using any known or well-known method. For example, techniques for providing wiring on a base sheet include a subtractive method in which copper foil is attached to the entire front surface of the base sheet with an adhesive, and unnecessary portions are removed by chemical etching while leaving the wiring, and an additive method in which wiring is plated (usually by electroless plating or electrolytic plating) on ​​the front surface of the base sheet. The cover sheet is a rectangular sheet having substantially the same shape and size as the base sheet. The cover sheet is made of a thermoplastic resin and has insulating properties. At least one hole is formed in the cover sheet. The hole is located at a position corresponding to the position where an electronic component will later be placed on the base sheet. The cover sheet is overlaid on the base sheet so that its contour matches the base sheet. In the subsequent folding step, the overlaid base sheet and cover sheet are folded together. The folding is performed at least once, and may be a mountain fold, a valley fold, or a combination of mountain and valley folds. After the folding step is performed, the hole in the cover sheet is exposed to the outside. By folding together with the cover sheet, at least a portion of the base sheet is overlapped with the remainder of the base sheet at least twice, although the cover sheet may be interposed therebetween. As a result, the wiring on the front surface of the base sheet is also overlapped at least twice.In the subsequent bonding process, the adjacent base sheet and cover sheet are bonded together in a folded state. To bond the adjacent base sheet and cover sheet, the folded base sheet and cover sheet are heated to melt at least a portion of the cover sheet, then cooled and hardened. The molten cover sheet is used like an adhesive to bond the adjacent base sheet and cover sheet together. The term "cooling" as used herein does not necessarily require an active cooling process, but also includes leaving the heated base sheet and cover sheet in an atmosphere at a temperature lower than the melting point of the thermoplastic resin constituting the cover sheet (e.g., room temperature). It is also acceptable to melt a portion of the base sheet and then cool and harden it. Furthermore, the bonding process may also bond adjacent cover sheets to each other among the overlapping base sheet and cover sheet. In either case, the bonding process integrates the folded base sheet and cover sheet, essentially fixing their relative positions relative to each other as a whole. Then, the electronic component attachment process is performed to attach electronic components to the base sheet. As described above, after the folding process is completed, the holes in the cover sheet are exposed to the outside. The holes in the cover sheet correspond to the positions where electronic components are attached. The electronic components are inserted through the holes and fixed to the base sheet exposed through the holes while electrically conducting with the wiring on the front surface of the base sheet exposed through the holes. Needless to say, the wiring on the base sheet is designed to form an electrical circuit in combination with at least one electronic component when the electronic component is attached to the base sheet. In this manner, a multilayer substrate is manufactured. The electronic components used in manufacturing a multilayer substrate include integrated circuit (IC) chips, ball grid arrays (BGAs), chip-size packages (CSPs), resistors, capacitors, etc., and are no different from the electronic components attached to general substrates.

[0008] The multilayer board manufactured by the multilayer board manufacturing method of the present application described above has an electrical circuit formed in combination with at least one electronic component attached to the base sheet, eliminating the need for through holes or external connection wires. Therefore, the process for fabricating these components can be omitted, thereby reducing the cost of manufacturing the multilayer board. Furthermore, the above-described multilayer board manufacturing method allows for the production of multilayer boards with various structures depending on how the superimposed base sheet and cover sheet are folded during the folding process. Furthermore, in the multilayer board manufactured by the above-described multilayer board manufacturing method, the front surface of the base sheet is covered with a cover sheet except for the portion where holes are drilled. This protects the wiring by the cover sheet, making the wiring less susceptible to breakage.

[0009] As described above, both the base sheet and the cover sheet used in the present invention are made of thermoplastic resin. The thermoplastic resins constituting the base sheet and the cover sheet may or may not be the same. Examples of materials that can be used as the thermoplastic resin constituting the base sheet and the cover sheet include liquid crystal polymer (LCP). Other materials that can be used include polyphenylene sulfide (PPE), polyether ether ketone (PEEK), and fluororesin (FR), but other thermoplastic resins can also be used. Liquid crystal polymer is a well-known material known for its low loss and excellent high-frequency characteristics, making it suitable for use as the material for the base sheet and the cover sheet. The thicknesses of the base sheet and the cover sheet must be thin enough to allow the folding process to be performed while they are stacked. On the other hand, the thickness of the cover sheet is preferably greater than the thickness of the wiring formed on the base sheet. As mentioned above, the cover sheet protects the wiring after the bonding process. However, if the cover sheet is thinner than the wiring, the wiring may not be sufficiently protected, resulting in the wiring being exposed at the top. Even if the wiring is protected by covering it with a cover sheet, the thickness of the wiring may appear as a convex portion on the cover sheet, making it impossible to maintain the flatness of the multilayer substrate. If this occurs, problems such as poor bonding may occur when electronic components are ultimately mounted on a surface with poor flatness. Furthermore, the thickness of the cover sheet may be thinner than the thickness of the base sheet. By making the cover sheet thinner than the base sheet, the rigidity during the bending process is reduced, improving bendability and also reducing processing and material costs.

[0010] As described above, the base sheet used in the present invention is rectangular, and the cover sheet is rectangular with the same shape and size as the base sheet. The base sheet may also be square. As described above, the base sheet and cover sheet are folded by folding them while overlapping each other. However, if the base sheet and cover sheet are square, various folding methods used in origami, a traditional Japanese art, can be applied. For example, a multilayer board having a regular shape, such as a rectangle or a triangle, can be obtained in which the number of base sheets and cover sheets present at every portion in the thickness direction is the same. For example, after the electronic component mounting process is completed, the base sheet and cover sheet can be shaped into a square or a right-angled isosceles triangle. Such a regular shape is convenient when incorporating the multilayer board into a final product. Two right-angled isosceles triangular multilayer boards can also be combined to form a square.

[0011] In the present invention, the folding step can be performed so that after the electronic component mounting step is completed, all portions of the base sheet and the cover sheet are essentially parallel except for the folded portions. The multilayer substrate manufactured in this manner has a plate-like shape, with the base sheet and the cover sheet stacked in a substantially parallel state except for the folded portions. Plate-like multilayer substrates have shapes similar to many conventional multilayer or single-layer substrates and are expected to have a wide range of applications. On the other hand, after the electronic component mounting step is completed, portions of the base sheet and the cover sheet may be non-parallel relative to other portions of the base sheet and the cover sheet. In this case, the multilayer substrate may have a three-dimensional shape rather than a plate-like shape. Depending on the final product to which the substrate will be mounted, a three-dimensional multilayer substrate may be convenient. The ability to obtain such various shapes of multilayer substrates using a similar manufacturing method is one of the advantages of the multilayer substrate manufacturing method of the present invention. The folding process can also be performed so that, after the electronic component attachment process is completed, the number of overlapping base sheets and cover sheets in a certain portion other than the folded portion is different from the number of overlapping base sheets and cover sheets in other portions. For example, suppose a plate-shaped multilayer substrate is manufactured using the multilayer substrate manufacturing method. Then, suppose that in one region of the completed multilayer substrate, four base sheets and four cover sheets are overlapped, while in another region, eight base sheets and eight cover sheets are overlapped. Naturally, the thickness of the other region of the multilayer substrate will be greater than that of the certain region. One advantage of the multilayer substrate manufacturing method of the present invention is that it is possible to obtain multilayer substrates with different thicknesses in different regions using the same manufacturing method. Since it is possible to manufacture substrates with different thicknesses in different regions, less base material is used in the thinner substrate portions, which contributes to reduced material costs and process costs.

[0012] As described above, in the bonding process, the superposed base sheet and cover sheet are heated. In the bonding process, the superposed base sheet and cover sheet may be heated while applying pressure. This allows the base sheet and cover sheet, or the cover sheets, to be bonded evenly and firmly. The bonding process may be performed by heating (or heating and pressurizing) the entire base sheet and cover sheet after the folding process at once, or by heating (or heating and pressurizing) some parts of the base sheet and cover sheet and other parts after the folding process in multiple batches. When the finished multilayer substrate has a three-dimensional shape, the bonding process is often performed in the latter manner.

[0013] In the manufacturing method of the multilayer substrate of the present invention, a wiring step of providing the wiring on the front surface of the base sheet may be performed before the overlapping step. There are no particular limitations on the technology that can be used to provide the wiring on the front surface of the base sheet in the wiring step, and as already mentioned, known or well-known technologies can be used. The width of the wiring at the portion where the base sheet is folded may be wider than the width of both ends in the longitudinal direction. In the manufacturing method of the multilayer substrate of the present application, the wiring is folded together with the base sheet. This may result in some risk of the wiring being broken. Making the width of the wiring at the portion where the base sheet is folded wider than the width of both ends in the longitudinal direction helps prevent the wiring being folded together with the base sheet from being broken. When the wiring step is performed before the overlapping step, it is also possible to fabricate the wiring so that the width of the wiring at the portion where the base sheet is folded is wider than the width of both ends in the longitudinal direction.

[0014] 2 is an enlarged view of the vicinity of the symbol X in FIG. 2; FIG. 3 is a view of the base sheet used in an embodiment of the present invention, as viewed from the front side; FIG. 4 is a view of the base sheet provided with wiring, as viewed from the front side; FIG. 5 is an enlarged view of the vicinity of the symbol X in FIG. 2; FIG. 6 is a view of the cover sheet, as viewed from the front side; FIG. 7 is a view of the base sheet and cover sheet, as viewed from the front side, when the overlapping process is completed in an embodiment; FIG. 8 is a view showing a method of performing a folding process in an embodiment; FIG. 9 is a view of the completed multilayer board after the electronic component mounting process is completed in an embodiment; (A) to (D) are views showing a method of performing the folding process in Modification 1, and (E) is a side view of the base sheet and cover sheet shown in (D); FIG. 10 is a view of the base sheet and cover sheet, as viewed from the front side, when the overlapping process is completed in Modification 2; FIG. 11 is a view showing a method of performing the folding process in Modification 2; FIG. 12 is a view of the completed multilayer board after the electronic component mounting process is completed in Modification 2; FIG. 13 is a view of the base sheet and cover sheet, as viewed from the front side, when the overlapping process is completed in Modification 3; FIG. 14 is a view showing a method of performing the folding process in Modification 3; FIG. 15 is a view of the completed multilayer board after the electronic component mounting process is completed in Modification 3. 10 is a diagram showing the base sheet and the cover sheet when the overlapping step is completed in Modification 4. FIG. 11 is a diagram showing a method of performing the folding step in Modification 4. FIG. 12 is a perspective view showing the completed multilayer board after the electronic component attachment step is completed in Modification 4.

[0015] Hereinafter, an embodiment of the present invention and a modification thereof will be described with reference to the drawings.

[0016] In this embodiment, a method for manufacturing a multilayer substrate will be described. The multilayer substrate is manufactured by performing the steps described below. The multilayer substrate is manufactured using a base sheet, a cover sheet, and electronic components as materials.

[0017] First, the base sheet will be described. FIG. 1 shows the base sheet 1 as viewed from the front side. The wiring described below 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 described below, but in the example shown in FIG. 1 , it is not square. 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 thereto, 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 as long as it allows the base sheet 1 to be folded together with the cover sheet as described below. The thickness can be, for example, 20 μm to 200 μm, preferably 25 μm to 100 μm.

[0018] In this embodiment, wiring 11 is provided on such a base sheet 1 ( FIG. 2 ). Note that the two-dot chain line marked with the symbol 12 in FIG. 2 is a fold line indicating the location where the base sheet 1 will later be folded. The wiring 11 is provided only on the front surface of the base sheet 1. The surface of the base sheet 1 on which the wiring 11 is provided is the front surface of the base sheet 1. The wiring 11 is 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. There are no particular limitations on the technology that can be used to provide the wiring 11 on the base sheet 1, and publicly known or well-known technologies can be used. For example, the wiring 11 can be provided on the front surface of the base sheet 1 by a subtractive method in which copper foil is attached to the entire front surface of the base sheet with an adhesive, and the wiring portion is covered with a mask while the unnecessary portion (the portion of the copper foil not covered by the mask) is removed by chemical etching, and finally the mask is removed, leaving the copper foil covered by the mask on the front surface of the base sheet 1 as the wiring 11. Alternatively, the wiring 11 can be provided on the front surface of the base sheet 1 by a 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 the mask is not present, and then the mask is removed to obtain the wiring 11 on the base sheet 1. The wiring 11 can be provided all at once by performing a process for providing wiring on the base sheet 1, such as a subtractive method or an additive method, in a single process, although this is not limited to this method. The position surrounded by the dashed line indicated by the symbol 13 in FIG. 2 is the planned position where the electronic component will later be attached. The size of the planned positions Y is not uniform because the planned positions 13 are adjusted to the size of the electronic components to be mounted there. Also, the wiring 11 does not necessarily have to be provided on the base sheet 1 shown in FIG.For example, it is possible to provide 10 x 10 = 100 wires 11 on a larger base sheet than the base sheet 1 shown in Figure 1 (e.g., a base sheet 10 times longer and wider than the base sheet 1 shown in Figure 1), and then cut the base sheet 1 into 10 pieces vertically and horizontally to obtain 100 identical base sheets 1 with wires 11 shown in Figure 2 at once. Such a method for producing wires 11 enables efficient mass production of multilayer substrates. Even in this case, the 10 x 10 = 100 wires to be provided on the large base sheet can be provided all at once by performing the process for providing wires on the large base sheet once, thereby reducing the cost of manufacturing the base sheet 1.

[0019] The wiring 11 may or may not have the same width throughout. It is also possible to provide an appropriate width for each of the linear wirings 11 connecting electronic components attached to the base sheet 1, as described below. For example, the width of at least one of the linear wirings 11 may be different from the width of the other linear wirings 11. Furthermore, the width of a single linear wiring 11 does not need to be constant over its entire length. As can be seen from FIG. 2 , the linear wiring 11 may straddle a fold line 12. In such a case, the width of the wiring 11 at the position straddling the fold line 12 may be wider than the widths on both sides of the wiring 11 in the longitudinal direction. An example of this case is shown in FIG. 3 . FIG. 3 is an enlarged view of the portion indicated by the dashed line X in FIG. 2 . In this way, the wiring 11 is wider at the portion straddling the fold line 12 than on either side or before and after the fold line 12 in the longitudinal direction. Although not limited thereto, in this embodiment, all of the wirings 11 that straddle the fold line 12 have a width that is wider at the portion that straddles the fold line 12 than at either side or at the front and back in the length direction. In this embodiment, when wirings 11 are provided on the base sheet 1, the width of the wirings 11 at the portion that straddles the fold line 12 is wider than at either side or at the front and back in the length direction. Of course, Figure 3 is an example. In Figure 3, the width of the wirings 11 at the portion that straddles the fold line 12 is linearly wider than at either side or at the front and back in the length direction, but the width of the wirings 11 at the portion that straddles the fold line 12 may be curved wider than at either side or at the front and back, or may be wider in a crank-like manner from a certain position.

[0020] Next, the cover sheet will be described. Figure 4 shows a view of the cover sheet 2 from the front side. The front side of the cover sheet 2 is the side that will 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 with substantially the same shape and size as the base sheet 1. No wiring is provided on the cover sheet 2. The cover sheet 2 is made of a thermoplastic resin and has insulating properties. Examples of thermoplastic resins that make up the cover sheet 2 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 cover sheet 2. Note that the thermoplastic resin that makes up the cover sheet 2 and the thermoplastic resin that makes up the base sheet 1 may or may not be the same. Whether the base sheet 1 and the cover sheet 2 are made of the same or different materials, the necessary processing of the base sheet 1 and the cover sheet 2 can be performed by adjusting the processing conditions, such as the heating temperature and heating time, and the pressure applied as needed, during the bonding process described below. The thickness of the cover sheet 2 can be appropriately selected within a range that allows the cover sheet 2 to be folded together with the base sheet 1 as described below. The thickness can be, for example, 20 μm to 200 μm, 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 thicknesses of the base sheet 1 and the cover sheet 2 need to be thin enough to allow the folding process to be performed with the two sheets overlapped. On the other hand, the thickness of the cover sheet 2 is preferably greater than the thickness of the wiring 11 formed on the base sheet 1. If the thickness of the cover sheet 2 is thinner than the thickness of the wiring 11, the cover sheet 2 may not adequately cover the wiring 11 after the bonding process described below is performed.Furthermore, if the cover sheet 2 is thinner than the wiring 11, even if the cover sheet 2 covers the wiring 11 after the fixing process, the thickness of the wiring 11 may appear as a convex portion on the surface of the cover sheet 2 covering the wiring 11 (the surface not in contact with the base sheet 1). If the thickness of the wiring 11 appears as a convex portion on the surface of the cover sheet 2, problems such as poor bonding may occur when mounting electronic components on a surface with poor flatness. However, such problems are less likely to occur if the cover sheet 2 is thicker than 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 during the folding process described below is reduced, improving foldability and also leading to cost savings in processing and material costs. The cover sheet 2 has at least one hole 21. The hole 21 is provided at a position where the electronic component will later be placed on the base sheet 1, i.e., at a position corresponding to the intended position 13. The holes 21 are not uniform in size because they are adjusted to the size of the electronic components to be attached thereto. Although not limited to this, seven holes 21 are provided in the cover sheet 2 in this embodiment. Note that the two-dot chain line labeled 22 in Figure 4 is a fold line indicating the location where the cover sheet 2 will later be folded together with the base sheet 1. The fold line 12 of the base sheet 1 and the fold line 22 of the cover sheet 2 are located at corresponding positions because the superimposed base sheet 1 and cover sheet 2 are folded together.

[0021] After the base sheet 1 and cover sheet 2 are prepared as described above, the overlapping process is first carried out. In the overlapping process, the cover sheet 2 is overlapped on the front surface of the base sheet 1 so that the outlines of the two sheets match (FIG. 5). As a result, the holes 21 in the cover sheet 2 are aligned with the planned positions 13 on the base sheet 1. The wiring 11 is exposed near the periphery of each hole 21.

[0022] Next, a folding process is performed. In the folding process, the overlapping base sheet 1 and cover sheet 2 are folded together. The folding is performed along the overlapping fold line 12 of the base sheet 1 and the fold line 22 of the cover sheet 2. The folding is performed at least once. The folding may be either a mountain fold or a valley fold, or a combination of mountain and valley folds. However, after the folding process is performed, the above-mentioned holes 21 provided in the cover sheet 2 are exposed to the outside. The hole 21 being exposed to the outside means that the hole 21 is not covered by the base sheet 1 or the cover sheet 2. Note that after the overlapping process and before the folding process, a temporary fixing process may be performed to temporarily fix the base sheet 1 and the cover sheet 2 to prevent the contours of the overlapping base sheet 1 and the cover sheet 2 from shifting during the folding process. This is not necessarily required, but is performed in this embodiment. The temporary joining can be performed by bonding a portion (for example, the four corners) of the contacting portion of the base sheet 1 and the cover sheet 2 with an adhesive. However, there are no limitations on the method of temporary joining. A pin lamination method, which is used in a general multilayer board manufacturing method, in which boards with holes are stacked and each board is positioned by inserting a pin through the hole provided in each board may be used, or the temporary joining may be performed using a temporary fixing jig (such as a clip) that clamps the base sheet 1 and the cover sheet 2 together.

[0023] Although not limited thereto, in the folding process in this embodiment, first, the portions of the base sheet 1 and cover sheet 2 (FIGS. 5 and 6(A)) that are overlapped as a result of the overlapping process, above the horizontally running fold lines 12 and 22, are folded back toward the back side of FIG. 6 at the horizontally running fold lines 12 and 22. This results in the base sheet 1 and cover sheet 2 being in the state shown in FIG. 6(B). Next, the portions of the base sheet 1 and cover sheet 2 in the state shown in FIG. 6(B) that are to the right of the vertically running fold lines 12 and 22 in FIG. 6(B) are folded back toward the back side of FIG. 6 at the vertically running fold lines 12 and 22. This results in the folded base sheet 1 and cover sheet 2 being in the state shown in FIG. 6(C). When viewed from the back side of FIG. 6(C), the folded base sheet 1 and cover sheet 2 are in the state shown in FIG. 6(D). This completes the folding process in this embodiment. After the folding process is completed, four holes 21 are exposed on the surface shown in FIG. 6(C) and three on the surface shown in FIG. 6(D). In other words, all seven holes 21 in the cover sheet 2 that were originally present are exposed to the outside after the folding process is completed. In addition, in the example described using FIG. 6, 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 in the following order from the surface shown in FIG. 6(C) to the surface shown in FIG. 6(D): cover sheet 2, base sheet 1, base sheet 1, cover sheet 2, cover sheet 2, base sheet 1, base sheet 1, cover sheet 2. All wiring 11 that straddles the fold line 12 have a width wider at the part straddling the fold line 12 than on either side or before and after in the longitudinal direction. Therefore, even if the wiring 11 is bent together with the base sheet 1, there is little risk of the wiring 11 being damaged, such as being broken.

[0024] Next, a fixing process is performed. In the fixing process, the base sheet 1 and the cover sheet 2 are fixed together after the folding process. This fixing process is performed by heating the folded base sheet 1 and cover sheet 2. When performing the fixing process, it is preferable to not only heat the base sheet 1 and the cover sheet 2, but also to pressurize the folded base sheet 1 and cover sheet 2 so that they are sandwiched from both the side shown in FIG. 6(C) and the side shown in FIG. 6(D). This is done in this embodiment. The heating and pressing conditions for the base sheet 1 and the cover sheet 2 can be adjusted depending on the thickness and melting point of the material. The heating and pressing conditions described below are an example in which the thermoplastic resins constituting the base sheet 1 and the cover sheet 2 are both liquid crystal polymers, more specifically, the base sheet 1 is a liquid crystal polymer with a melting point of 335°C, and the cover sheet 2 is a liquid crystal polymer with a melting point of 280°C. The thermoplastic resin constituting the cover sheet 2 is selected to have a melting point lower than that of the liquid crystal polymer constituting the base sheet 1. This is because the cover sheet 2 is preferentially softened over the base sheet 1 during the bonding process, allowing the softened and fluidized liquid crystal polymer constituting the cover sheet 2 to effectively fill gaps between adjacent base sheets 1 and 2 (where irregularities due to wiring 11 may exist) or gaps between adjacent cover sheets 2. Thus, in the present application, the thermoplastic resins constituting the base sheet 1 and the cover sheet 2 can be selected so that the melting point of the thermoplastic resin constituting the cover sheet 2 is lower than that of the thermoplastic resin constituting the base sheet 1. For example, the base sheet 1 and the cover sheet 2 can be heated using a general press. (1) First, the temperatures of the base sheet 1 and the cover sheet 2 are raised from room temperature to a temperature at which the thermoplastic resins constituting them (particularly the thermoplastic resin constituting the cover sheet) slightly soften, for example, 150°C, and maintained at that temperature for several minutes.This process is carried out with the aim of softening at least the cover sheet 2 of the base sheet 1 and the cover sheet 2, thereby filling to some extent the gap between adjacent base sheets 1 and cover sheets 2, or the gap between adjacent cover sheets 2, with the softened and fluidized thermoplastic resin. (2) Next, the temperatures of the base sheet 1 and cover sheet 2 are raised to a temperature close to the melting point of the thermoplastic resin (particularly the thermoplastic resin constituting the cover sheet), for example, 290°C, and maintained at that temperature for approximately 30 minutes. This process is carried out with the aim of further softening at least the cover sheet 2 of the base sheet 1 and cover sheet 2, thereby virtually completely filling the gap between adjacent base sheets 1 and cover sheets 2, or the gap between adjacent cover sheets 2, with the softened and fluidized thermoplastic resin. (3) Finally, the temperatures of the base sheet 1 and cover sheet 2 are cooled, for example, to room temperature. This process hardens the thermoplastic resin that has virtually completely filled the gap between adjacent base sheets 1 and cover sheets 2, or the gap between adjacent cover sheets 2. As a result, the gap between adjacent base sheet 1 and cover sheet 2, or the gap between adjacent cover sheets 2, is essentially filled with the thermoplastic resin, and adjacent base sheet 1 and cover sheet 2, or adjacent cover sheets 2, are fixed to each other. When performing the above-described steps (1) to (3), the base sheet 1 and cover sheet 2 are pressurized in a press. In this embodiment, for example, pressure is not applied for a while after the start of the above-described step (1), and then, from the middle to the end of the period during which the temperatures of the base sheet 1 and cover sheet 2 are maintained at 150°C in the same step, the base sheet 1 and cover sheet 2 are pressed while maintaining a constant pressure of, for example, 3 MPa. This is to allow the fluidized thermoplastic resin to better fill the gap between adjacent base sheet 1 and cover sheet 2, or the gap between adjacent cover sheets 2. Thereafter, the base sheet 1 and cover sheet 2 are pressed at a constant pressure of 1 MPa from the start of the above-described step (2) to the end of the step (3).The pressure applied at this time is smaller than the pressure applied during the period in step (1) when the temperature of the base sheet 1 and cover sheet 2 is maintained at 150 degrees (as described above, in this example, this pressure is 3 MPa).The reason for this is that, although a certain amount of pressure is required to better fill the gaps between adjacent base sheets 1 and cover sheets 2, or the gaps between adjacent cover sheets 2, with the fluidized thermoplastic resin, if the pressure is applied too strongly, there is a risk that the fluidized thermoplastic resin will leak out from the gaps between adjacent base sheets 1 and cover sheets 2, or the gaps between adjacent cover sheets 2.

[0025] As described above, after the folding process is completed, the folded base sheet 1 and cover sheet 2 are laminated with the base sheet 1 and cover sheet 2 in the following order from the surface shown in FIG. 6(C) to the surface shown in FIG. 6(D): cover sheet 2, base sheet 1, base sheet 1, cover sheet 2, cover sheet 2, base sheet 1, base sheet 1, cover sheet 2. At least a portion of the cover sheet 2 melts upon heating and hardens after heating is completed; in this embodiment, a portion of the base sheet 1 also melts and hardens after heating is completed. Therefore, of the above-mentioned layers contained in the folded base sheet 1 and cover sheet 2, adjacent base sheets 1 and 1, and adjacent cover sheets 2 and 2 are welded to each other. As a result, after the fixing process is completed, the folded base sheet 1 and cover sheet 2 are fixed into a plate-like shape.

[0026] After the bonding process is completed, the electronic component attachment process is carried out. The electronic components may be IC chips, BGAs, CSPs, resistors, capacitors, etc., and are selected appropriately according to the performance required of the multilayer substrate. The electronic components 3 are electrically connected to the wiring 11 provided on the front surface of the base sheet 1, which is a plate-like substrate after the bonding process, and the cover sheet 2 (FIGS. 7A and 7B), thereby being fixed to the base sheet 1. The electronic components 3 are inserted into holes 21 drilled in the cover sheet 2, and terminals protruding from the electronic components 3 are electrically connected to the wiring 11, for example, by soldering. Since the electronic components 3 must be inserted into the holes 21 to be fixed to the wiring 11, the holes 21 must be slightly larger than the electronic components 3. Otherwise, when attaching the electronic components 3 to the base sheet 1 through the holes 21, pressure may be applied when inserting the electronic components 3 into the holes 21, which may damage the electronic components 3, or it may be difficult to move the electronic components 3 horizontally relative to the front surface of the base sheet 1. Furthermore, in order to enable the electronic components 3 to be electrically connected to the wiring 11, the holes 21 must be sized and shaped so that the ends of the wiring 11 can be seen from their periphery. The holes 21 in this embodiment are of this size and shape. Closely adjacent holes 21 may also be combined into a single hole 21. For example, the two holes 21 located horizontally in the upper left corner of FIG. 7A that are smaller than the other holes 21 may be combined into a single hole 21. In this case, two electronic components 3 are attached to a single hole 21. The multilayer board is completed by fixing all of the intended electronic components 3 to the base sheet 1 and forming an electrical circuit using the wiring 11 and the electronic components 3. In the multilayer substrate manufactured in this embodiment, the base sheet 1 and the cover sheet 2 are substantially parallel except for the folded portions, and the number of superimposed base sheets 1 and cover sheets 2 (total number) is the same throughout the multilayer substrate (eight in this embodiment). A multilayer substrate having such a structure is in the form of a plate.In a multilayer board manufactured by this manufacturing method for a multilayer board, the front surface of the base sheet 1 is covered with the cover sheet 2 except for the portion where the hole 21 is formed. As a result, the wiring 11 is protected by the cover sheet 2, making the wiring 11 less likely to break.

[0027] <Variation 1> The manufacturing method of the multilayer board of Variation 1 is almost the same as the manufacturing method of the multilayer board described in the embodiment. The multilayer board is manufactured using a base sheet 1, a cover sheet 2, and electronic components 3 as materials. The configurations of the base sheet 1 and the cover sheet 2 are basically the same between Variation 1 and the embodiment described above. However, in Variation 1, the hole 21 at the bottom right of the cover sheet 2 shown in FIG. 4 is slightly smaller, and the position of the hole 21 is slightly closer to the bottom left corner of the cover sheet 2. Furthermore, in Variation 1, the configuration of the wiring 11 provided on the front surface of the base sheet 1 is appropriately modified in accordance with the change in the position and size of the hole 21.

[0028] In Modification 1, the same overlapping step as in the above-described embodiment is performed. Also, in Modification 1, the same folding step as in the above-described embodiment is performed. In the above-described embodiment, the overlapped base sheet 1 and cover sheet 2 are folded twice. In Modification 1, the base sheet 1 and cover sheet 2 are folded twice in the same manner. As a result, the base sheet 1 and cover sheet 2 are in the state shown in FIGS. 8(A) and 8(B). FIGS. 8(A) and 8(B) are views similar to FIGS. 6(C) and 6(D). In the folding step of Modification 1, the overlapped base sheet 1 and cover sheet 2 are folded once more from this state. This third folding is performed along the fold lines 12 and 22 shown in FIGS. 8(A) and 8(B). The third fold involves folding the portions of the base sheet 1 and cover sheet 2, which are superimposed in FIG. 8(A), below the horizontal fold lines 12 and 22 toward the back side of FIG. 8(A) at the horizontal fold lines 12 and 22. This results in the base sheet 1 and cover sheet 2 being in the state shown in FIGS. 8(C) and 8(D). FIGS. 8(C) and 8(D) are views of the base sheet 1 and cover sheet 2, respectively, from the same direction as FIGS. 8(A) and 8(B). This completes the folding process of Variation 1. In this case, too, after the folding process is complete, all seven holes 21 in the cover sheet 2 are exposed to the outside. Furthermore, in this case, too, after the folding process is complete, all portions of the base sheet 1 and cover sheet 2 are virtually parallel except for the folded portions. Furthermore, the base sheet 1 and cover sheet 2 folded in this manner have an upper portion in which eight sheets are stacked together, including the base sheet 1 and cover sheet 2, and a lower portion in which 16 sheets are stacked together, including the base sheet 1 and cover sheet 2. As a result, in Modification 1, the base sheet 1 and cover sheet 2 after the folding process have steps as shown in Figure 8(E), in other words, have a stepped cross section.

[0029] Thereafter, in Modification 1, the multilayer board is completed by carrying out the fixing step and the electronic component mounting step in the same manner as in the above-described embodiment. The completed multilayer board will be as shown in Figures 8(C), (D), and (E) except for the electronic components 3 being mounted. As a result, the multilayer board manufactured by the manufacturing method of Modification 1 will have a step as shown in Figure 8(E), in other words, a stepped cross section.

[0030] <Variation 2> The manufacturing method of the multilayer board of Variation 2 is almost the same as the manufacturing method of the multilayer board described in the embodiment. The multilayer board is manufactured using a base sheet 1, a cover sheet 2, and electronic components 3 as materials. The configurations of the base sheet 1 and the cover sheet 2 are basically the same between Variation 2 and the above-mentioned embodiment. However, in Variation 2, both the base sheet 1 and the cover sheet 2 are square. Although not shown in its entirety, wiring 11 similar to that in the above-mentioned embodiment is provided on the front surface of the base sheet 1 of Variation 2.

[0031] In Modification 2, the overlapping step is performed in the same manner as in the above-described embodiment ( FIG. 9 ). Holes 21 are formed in the cover sheet 2, as in the above-described embodiment. The number of holes 21 in Modification 2 is eight, although this is not limited thereto. Also, in Modification 2, the folding step is performed after the overlapping step, as in the above-described embodiment. The dashed lines 22A in FIG. 9 indicate mountain fold lines along which the base sheet 1 and cover sheet 2 are mountain-folded, and the two-dot chain lines 22B indicate valley fold lines along which the base sheet 1 and cover sheet 2 are valley-folded. In this embodiment, the base sheet 1 and cover sheet 2 are simultaneously mountain-folded along the mountain fold lines 22A and valley-folded along the valley fold lines 22B, thereby folding the base sheet 1 and cover sheet 2 from the state shown in FIG. 10(A) through the state shown in FIG. 10(B) to the plate-like state shown in FIG. 10(C). This folding method is common in origami. In Fig. 10(B), the centers of the base sheet 1 and the cover sheet 2 are located on the front side of the page, and the edges of the base sheet 1 and the cover sheet 2 are located on the back side of the page. This completes the folding process of variant 2. In this case, too, after the folding process is completed, all eight holes 21 in the cover sheet 2 are exposed to the outside (Fig. 11). Fig. 11(A) is a view of one side of the base sheet 1 and the cover sheet 2 of variant 2 that have been formed into a plate shape after the folding process is completed, and Fig. 11(B) is a view of the other side. Also in this case, after the folding process is completed, all parts of the base sheet 1 and the cover sheet 2 are virtually parallel except for the folded parts.

[0032] Thereafter, in Modification 2, the multilayer board is completed by carrying out the fixing step and the electronic component mounting step in the same manner as in the above-described embodiment. The completed multilayer board is as shown in Figures 10(C) and 11 except for the electronic components 3 mounted thereon. As a result, the multilayer board manufactured by the manufacturing method of Modification 2 has the shape of an isosceles right triangle with the electronic components 3 mounted on both the front and back sides.

[0033] <Variation 3> The manufacturing method of the multilayer board of Variation 3 differs from that of Variation 2 in the way the base sheet 1 and the cover sheet 2 are folded in the folding process, but is otherwise the same as Variation 2. The multilayer board is manufactured using a base sheet 1, a cover sheet 2, and electronic components 3 as materials. The configurations of the base sheet 1 and the cover sheet 2 are basically the same between Variation 3 and Variation 2. In Variation 3, both the base sheet 1 and the cover sheet 2 are square. Although not shown in its entirety, wiring 11 similar to that in the above-described embodiment is provided on the front surface of the base sheet 1 of Variation 3.

[0034] In Modification 3, the overlapping step is performed in the same manner as in Modification 2 ( FIG. 12 ). Holes 21 are formed in the cover sheet 2, as in the above-described embodiment. The number of holes 21 in Modification 3 is eight, although not limited thereto. Also, in Modification 3, the folding step is performed after the overlapping step, as in the above-described embodiment. The dashed lines 22A in FIG. 12 indicate mountain fold lines along which the base sheet 1 and cover sheet 2 are mountain-folded, and the two-dot chain lines 22B indicate valley fold lines along which the base sheet 1 and cover sheet 2 are valley-folded. In this embodiment, the base sheet 1 and cover sheet 2 are simultaneously mountain-folded along the mountain fold lines 22A and valley-folded along the valley fold lines 22B, thereby folding the base sheet 1 and cover sheet 2 from the state shown in FIG. 13(A) through the state shown in FIG. 13(B) to the plate-like state shown in FIG. 13(C). This folding method is common in origami. In Figure 13(B), the centers of the base sheet 1 and the cover sheet 2 are located on the front side of the page, and the edges of the base sheet 1 and the cover sheet 2 are located on the back side of the page. This completes the folding process of variant 3. In this case, too, after the folding process is completed, all eight holes 21 in the cover sheet 2 are exposed to the outside (Figure 14). Note that Figure 14(A) is a view of one side of the base sheet 1 and the cover sheet 2 of variant 3 that have been formed into a plate shape after the folding process is completed, and Figure 14(B) is a view of the other side. Also in this case, after the folding process is completed, all parts of the base sheet 1 and the cover sheet 2 are virtually parallel except for the folded parts.

[0035] Thereafter, in Modification 3, the multilayer board is completed by carrying out the fixing step and the electronic component mounting step in the same manner as in the above-described embodiment. The completed multilayer board will be as shown in Figures 13(C) and 14, except for the electronic components 3 mounted on it. As a result, the multilayer board manufactured by the manufacturing method of Modification 3 will be a square plate with electronic components 3 mounted on both the front and back sides.

[0036] <Variation 4> The manufacturing method of a multilayer board in Variation 4 differs from Variation 2 in that the base sheet 1 and the cover sheet 2 are folded differently in the folding process and the method for performing the fixing process is different, but otherwise it is the same as Variation 2. The multilayer board is manufactured using a base sheet 1, a cover sheet 2, and electronic components 3 as materials. The configurations of the base sheet 1 and the cover sheet 2 are basically the same between Variation 4 and Variation 2. In Variation 4, both the base sheet 1 and the cover sheet 2 are square. Although not shown in its entirety, wiring 11 similar to that in the above-mentioned embodiment is provided on the front surface of the base sheet 1 in Variation 4.

[0037] In Modification 4, the overlapping step is performed in the same manner as in Modification 2 ( FIG. 15 ). Holes 21 are formed in the cover sheet 2, as in the above-described embodiment. The number of holes 21 in Modification 4 is ten, although not limited thereto. Also, in Modification 4, the folding step is performed after the overlapping step, as in the above-described embodiment. The dashed lines 22A in FIG. 12 indicate mountain fold lines along which the base sheet 1 and cover sheet 2 are mountain-folded, and the two-dot chain lines 22B indicate valley fold lines along which the base sheet 1 and cover sheet 2 are valley-folded. In this embodiment, the base sheet 1 and cover sheet 2 are simultaneously mountain-folded along the mountain fold lines 22A and valley-folded along the valley fold lines 22B, thereby folding the base sheet 1 and cover sheet 2 from the state shown in FIG. 16(A) through the state shown in FIG. 16(B) to the plate-like state shown in FIG. 16(C). This folding method is common in origami. In FIG. 16(B), the centers of the base sheet 1 and the cover sheet 2 are located on the near side of the page, and the edges of the base sheet 1 and the cover sheet 2 are located on the far side of the page. Also, in FIG. 16(B), the holes 21 in the cover sheet 2 and the wiring 11 on the base sheet 1 visible through the holes 21 are omitted. This completes the folding process of Variation 4. In Variation 4, after the folding process is completed, the base sheet 1 and the cover sheet 2 have a shape in which, when viewed from above, the base sheet 1 and the cover sheet 2 have the same size right-angled isosceles triangular plates extending in four symmetrical directions, connected so that the non-hypotenuse sides of the right-angled isosceles triangles overlap (see FIGS. 16(C) and 17). Even in this case, after the folding process is completed, all ten holes 21 in the cover sheet 2 are exposed to the outside. This is because all ten holes 21 are located on either side of the four plate-shaped portions of the right-angled isosceles triangle described above after the folding process is completed. 17 is a perspective view of the base sheet 1 and cover sheet 2 of Modified Example 4 after the folding process is completed. In Modified Example 4, after the folding process is completed, parts of the base sheet 1 and cover sheet 2 are not parallel to other parts of the base sheet 1 and cover sheet 2. As a result, in Modified Example 4, after the folding process is completed, the base sheet 1 and cover sheet 2 have a three-dimensional shape.

[0038] Thereafter, in the fourth modification, the multilayer substrate is completed by performing the bonding process and the electronic component attachment process in the same manner as in the above-described embodiment. However, in this embodiment, the bonding process is performed four times for each of the plate-shaped portions of the right-angled isosceles triangle. Each plate-shaped portion of the right-angled isosceles triangle is heated while applying pressure. By doing so, the base sheet 1 and the cover sheet 2 that are in contact with each other, and the base sheet 1 and the base sheet 1 that are in contact with each other, are fixed in each of the four plate-shaped portions. The completed multilayer substrate is as shown in Figures 16(C) and 17, except for the electronic components 3 that are attached. As a result, the multilayer substrate manufactured by the manufacturing method of the fourth modification has the three-dimensional structure described above. In addition, the angle between adjacent plate-like portions of each right-angled isosceles triangle (θ in Figure 17) when viewed in a plane is basically 90 degrees, but this angle is variable because the plate-like portions of the right-angled isosceles triangle can rotate around the axis of the non-hypotenuse side that overlaps between the four plate-like portions.

[0039] REFERENCE SIGNS LIST 1 base sheet 2 cover sheet 3 electronic component 11 wiring 12 bending line 21 hole 22 bending line

Claims

1. A method for manufacturing a multilayer board, comprising: a layering step of layering a base sheet, which is a rectangular insulating sheet made of thermoplastic resin and has conductive wiring arranged in an appropriate position on its front surface, with a cover sheet, which is an insulating thermoplastic resin sheet of substantially the same shape and size as the base sheet, with at least one hole formed in a position where an electronic component will later be placed, on the front surface of the base sheet, so that the contours of the base sheet and the cover sheet match; a folding step of folding the layered base sheet and cover sheet so that the hole is exposed to the outside; a fixing step of heating the folded base sheet and cover sheet to melt at least a portion of the cover sheet, and then cooling and hardening the cover sheet, thereby fixing the adjacent base sheet and cover sheet together; and an electronic component attachment step of fixing an electronic component inserted in the hole to the base sheet exposed through the hole, with the electronic component being electrically connected to the wiring on the front surface of the base sheet exposed through the hole.

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

3. The method for manufacturing a multilayer board according to claim 2, wherein the shape of the base sheet and the cover sheet after the electronic component mounting step is completed is a square or a right-angled isosceles triangle.

4. The method for manufacturing a multilayer board according to claim 1, wherein the folding step is carried out so that after the electronic component mounting step is completed, all parts of the base sheet and the cover sheet are substantially parallel except for the folded parts.

5. A method for manufacturing a multilayer board as described in claim 4, wherein the folding step is carried out so that the number of the base sheets and the cover sheets overlapped in a certain portion other than the folded portion after the electronic component attachment step is completed is different from the number of the base sheets and the cover sheets overlapped in other portions.

6. The method for manufacturing a multilayer board according to claim 1, wherein in the fixing step, adjacent cover sheets of the superposed base sheet and cover sheet are also fixed to each other.

7. The method for manufacturing a multilayer substrate according to claim 1 or 5, wherein in the fixing step, the superposed base sheet and cover sheet are heated while being pressurized.

8. The method for manufacturing a multilayer board according to claim 1, wherein a wiring step of providing the wiring on the front surface of the base sheet is carried out before the overlapping step.

9. A method for manufacturing a multilayer board according to claim 1 or 8, wherein the width of the wiring at the portion where the base sheet is folded is wider than the width of the portions on both sides in the length 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.

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