Multilayer wiring board

Abstract

This multilayer wiring board, in which a plurality of layers are laminated and differential wiring included in a part of the layers transmits a differential signal, comprises: a first transmission path that transmits the differential signal; a second transmission path that has a longer wiring length than the first transmission path and transmits the differential signal; a first insulation layer laminated between the first transmission path and a first ground wire; and a second insulation layer laminated between the second transmission path and a second ground wire. The differential wiring is composed of the first transmission path and the second transmission path. At least a part of the first transmission path is disposed facing the second transmission path in a lamination direction in which the first insulation layer, the first transmission path, the second transmission path, and the second insulation layer are laminated in this order. The relative permittivity of the first insulation layer is higher than the relative permittivity of the second insulation layer.

Description

multilayer wiring board

[0001] The present invention relates to a multilayer wiring board.

[0002] Conventionally, various products (hereinafter referred to as "AD / ADAS products") such as in-vehicle ECUs (Electronic Control Units) for Advanced Driver Assistance Systems (ADAS) or Autonomous Driving (AD) have been provided. In AD / ADAS products, signal transmission paths used for high-speed communication are wired on a circuit board. Furthermore, as AD / ADAS products have become more sophisticated in recent years, the wiring density of circuit boards has increased, and the number of layers in the circuit boards has tended to increase. Conventionally, to adjust signal delays in high-speed communication wiring (referred to as "skew adjustment"), a method of wiring the transmission paths in a meandering shape (see the lower left diagram in Figure 11, described later) has been adopted.

[0003] Patent Document 1 describes a wiring board having a core layer including an insulating substrate having a glass cloth woven with a plurality of glass warp threads and a plurality of glass weft threads, and differential wiring, and has a filler in a position that reduces the difference in relative dielectric constant between a region that affects the transmission characteristics of a first wiring of the differential wiring and a region that affects the transmission characteristics of a second wiring of the differential wiring.

[0004] JP 2009-259879 A

[0005] In order to reduce the effects of skew that occurs in two transmission paths that transmit differential signals, it has been thought that aligning the wiring lengths of the two transmission paths is effective. Therefore, meander-shaped wiring has been used for the transmission path with the longer wiring length. However, meander-shaped wiring reduces the space available for wiring the transmission path on the board. For this reason, wiring the transmission path in a meander shape increases the number of board layers to provide the required number of transmission paths. Furthermore, since the transmission paths are routed on the inner layers of multiple stacked boards, the manufacturing costs of the board also increase. Therefore, there has been a demand for a space-saving method for wiring the transmission paths without increasing the number of board layers formed on the board.

[0006] The technology disclosed in Patent Document 1 aims to suppress the variation in dielectric constant due to the glass cloth. However, the technology disclosed in Patent Document 1 suppresses the variation in dielectric constant, thereby suppressing the influence of localized relative dielectric constant and keeping it constant, and a meandering shape is required to compensate for the difference in wiring length that occurs depending on the wiring method. For this reason, the above-mentioned space-saving of wiring could not be achieved.

[0007] The present invention has been made in view of the above circumstances, and has as its object to reduce the space required for wiring of transmission paths that transmit differential signals and to enable skew adjustment.

[0008] A multilayer wiring board according to the present invention includes a plurality of layers stacked one on top of the other, and differential wiring included in some of the layers transmits differential signals. The multilayer wiring board includes a first transmission line that transmits differential signals, a second transmission line that is longer than the first transmission line and transmits differential signals, a first insulating layer stacked between the first transmission line and a first ground wiring, and a second insulating layer stacked between the second transmission line and a second ground wiring. The differential wiring is composed of the first transmission line and the second transmission line, and at least a portion of the first transmission line is positioned opposite the second transmission line in the stacking direction in which the first insulating layer, first transmission line, second transmission line, and second insulating layer are stacked in this order, and the dielectric constant of the first insulating layer is higher than the dielectric constant of the second insulating layer.

[0009] According to the present invention, by providing insulating layers with different dielectric constants for differential wiring composed of first and second transmission lines with different wiring lengths, skew adjustment is possible without using a meander shape, and it is also possible to save space for the wiring of the transmission lines. Problems, configurations, and effects other than those described above will become clear from the description of the following embodiments.

[0010] 1 is a top view of glass cloth; FIG. 2 is a cross-sectional view of a multilayer wiring board including glass cloth; FIG. 3 is a simplified cross-sectional view of a simplified multilayer wiring board according to one embodiment of the present invention; FIG. 4 is a cross-sectional view of a multilayer wiring board according to one embodiment of the present invention; FIG. 5 is a line graph showing the relationship between the relative dielectric constant of wiring and the distance between fibers of glass cloth according to one embodiment of the present invention; FIG. 6 is a line graph showing the relationship between the relative dielectric constant of wiring and the thickness of glass cloth according to one embodiment of the present invention; FIG. 7 is a diagram showing an example of a conventional horizontally arranged first transmission line and a second transmission line; FIG. 8 is a diagram showing an example of a vertically arranged first transmission line and a second transmission line according to one embodiment of the present invention; FIG. 9 is a top view of a multilayer wiring board in which the first transmission line and the second transmission line are wired on a surface layer and an inner layer according to one embodiment of the present invention; FIG. 10 is a side view of a multilayer wiring board B in which the first transmission line and the second transmission line are wired in a BGA arrangement according to one embodiment of the present invention; FIG. 11 is a diagram showing differences in the configuration of the first transmission line and the second transmission line according to one embodiment of the present invention; FIG. 12 is a diagram showing an example of measurement results of skew adjustment according to one embodiment of the present invention; FIG. 13 is a top view of a configuration example of a multilayer wiring board according to one embodiment of the present invention; FIG. 14 is a cross-sectional view showing an example of wiring when the IC is a footprint according to one embodiment of the present invention; FIG. 15 is a cross-sectional view showing an example of wiring when the IC is a BGA according to one embodiment of the present invention. 1 is a diagram showing an example of an inner layer wiring for explaining a characteristic impedance value of a transmission line according to an embodiment of the present invention, and is a diagram showing a configuration example of a multilayer wiring board configured with inner layer wiring having different widths according to an embodiment of the present invention.

[0011] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functions or configurations are designated by the same reference numerals, and redundant description will be omitted. The present invention is applicable, for example, to a multilayer wiring board mounted on a vehicle control computing device with which an on-board ECU, which is an AD / ADAS product, can communicate.

[0012] [One embodiment] Fig. 1 is a top view of a glass cloth 1. For example, a plurality of crosses 1x are arranged in the x-axis direction of the glass cloth 1, and a plurality of crosses 1y are arranged in the y-axis direction of the glass cloth 1. The glass cloth 1 is formed by interweaving crosses 1x in the x-axis direction and crosses 1y in the y-axis direction. Here, the distance between two crosses 1x arranged adjacent to each other in the x-direction is called the inter-fiber distance.

[0013] 2 is a cross-sectional view of a multilayer wiring board including the glass cloth 1. Here, a cross-sectional view of the glass cloth 1 shown in FIG. 1 taken along the AA direction is shown.

[0014] The glass cloth 1 is configured such that cloth 1x is placed in the recess of cloth 1y, which has a wavy shape within the substrate. Many layers each consisting of a pair of cloths 1x and 1y are formed in the stacking direction indicated by the downward arrow in the figure. Three wirings 2 are provided on top of the glass cloth 1, and copper foil 3 is also provided. The copper foil 3 is, for example, a ground wiring. A first insulating layer 4, such as a prepreg, is provided to fill the gaps between the cloths 1x and 1y. The first insulating layer 4 is an example of an intermediate material in which fibers other than the cloths 1x and 1y are impregnated with resin.

[0015] FIG. 3 is a cross-sectional view of a simplified multilayer wiring board. FIG. 3 shows one layer of glass cloth 1 (cloths 1x and 1y) provided as a first insulating layer 4 on a copper-clad laminate 6. Copper foil 3 is formed on top of the glass cloth 1. Note that FIG. 3 shows an example in which one layer of glass cloth 1 is provided, but in this embodiment, multiple layers of glass cloth 1 are provided. The fiber-to-fiber distance between adjacent cloths 1x is indicated by d1. The thickness of the cloth 1 in the stacking direction is indicated by thickness th1.

[0016] 4 is a cross-sectional view of a multilayer wiring board 10 according to one embodiment. The multilayer wiring board 10 is composed of multiple substrates stacked in the order of 1st layer, 2nd layer, 3rd layer, and 4th layer from top to bottom. Ground wiring or normal wiring is formed, for example, from copper foil, on layers numbered 1 and 2. An insulating layer is provided between each layer.

[0017] The first and fourth layers are respectively formed with a first ground wiring (denoted as "GND" in the figure) and a second ground wiring. A first insulating layer (first insulating layer 4) is laminated between the first and second layers, between the first transmission path (first transmission path 11) and the first ground wiring. The first insulating layer (first insulating layer 4) includes glass cloth 1A (cloth 1x_1, 1y_1) as an example of a first glass cloth. Furthermore, cloth 1x of glass cloth 1A has a thickness th1. Note that transmission paths capable of transmitting signals may also be formed in the first and fourth layers.

[0018] In both the second and third layers, copper foil is formed and then etched to form differential wiring arranged vertically in the stacking direction. The differential wiring is composed of a first transmission line (first transmission line 11) and a second transmission line (second transmission line 12). The second layer is provided with the first transmission line (first transmission line 11) that transmits differential signals. The third layer is provided with a second transmission line (second transmission line 12) that transmits differential signals and has a wiring length longer than the first transmission line (first transmission line 11). The first transmission line 11 and the second transmission line 12 have different wiring lengths, as shown in FIG. 9 (described later). A second insulating layer (second insulating layer 5) is laminated between the third and fourth layers, between the second transmission line (second transmission line 12) and the second ground wiring. The second insulating layer (second insulating layer 5) includes glass cloth 1B (cloths 1x_2 and 1y_2) as an example of the second glass cloth. Although not shown in the figure to avoid complicating the drawing, an insulating layer is also provided between the second layer and the third layer.

[0019] The multilayer wiring board 10 according to this embodiment is configured by stacking multiple layers, with differential wiring included in some of the layers transmitting differential signals, and the insulating layers and differential wiring are configured in multiple layers. Furthermore, in the stacking direction in which the first insulating layer (first insulating layer 4), the first transmission path (first transmission path 11), the second transmission path (second transmission path 12), and the second insulating layer (second insulating layer 5) are stacked in this order, at least a portion of the first transmission path (first transmission path 11) is arranged opposite the second transmission path (second transmission path 12).

[0020] For the first transmission line 11, which has a shorter wiring length than the second transmission line 12, the glass cloth 1x_1 is densely arranged between the first and second layers. On the other hand, for the second transmission line 12, which has a longer wiring length than the first transmission line 11, the glass cloth 1x_2 is sparsely arranged between the third and fourth layers. The density of the glass cloth is expressed by the weaving density of the glass cloth. The weaving density is defined, for example, by the length of the inter-fiber distance d1 shown in FIG. 1. For example, if the inter-fiber distance d1 is short, the weaving density is high, and if the inter-fiber distance d1 is long, the weaving density is low.

[0021] In this embodiment, the signal transmission speed due to the difference in wiring length between the first transmission line 11 and the second transmission line 12 is adjusted not by the physical wiring length such as a meandering shape, but by the relative dielectric constant εr of the glass cloths 1A and 1B, as shown in the following equation (1).

[0022]

[0023] In equation (1), v represents the transmission speed of the differential signal, c represents the speed of light, and εr represents the relative dielectric constant. Equation (1) shows that as the relative dielectric constant increases, the signal transmission speed v decreases, and as the relative dielectric constant εr decreases, the signal transmission speed v increases.

[0024] The skew adjustment of the differential wiring due to the difference in wiring length between the first transmission line 11 and the second transmission line 12 is achieved by changing the signal speed by changing the relative dielectric constant rather than the physical length. Therefore, the relative dielectric constant of the first insulating layer (first insulating layer 4) is configured to be higher than the relative dielectric constant of the second insulating layer (second insulating layer 5). As a result, it is not necessary to configure the first transmission line 11 in a meandering shape, and the wiring space for the first transmission line 11 is reduced.

[0025] Here, the relationship between the inter-fiber distance and thickness of the glass cloth and the relative dielectric constant of the wiring will be described with reference to Fig. 5 and Fig. 6. Fig. 5 is a line graph (hereinafter abbreviated as "graph") showing the relationship between the inter-fiber distance of the glass cloth and the relative dielectric constant of the wiring. The horizontal axis of Fig. 5 represents the inter-fiber distance of the glass cloth [µm], and the vertical axis represents the relative dielectric constant of the wiring [εr].

[0026] Graph G1 in FIG. 5 represents the relative dielectric constant versus inter-fiber distance for a glass cloth having a thickness of 20 μm. Graph G2 represents the relative dielectric constant versus inter-fiber distance for a glass cloth having a thickness of 40 μm. Graph G3 represents the relative dielectric constant versus inter-fiber distance for a glass cloth having a thickness of 60 μm. Graphs G1 to G3 show that, regardless of the thickness of the glass cloth, the narrower the inter-fiber distance, the higher the relative dielectric constant. Therefore, if the glass cloths have the same thickness, the inter-fiber distance of glass cloth 1A (an example of a first glass cloth) is configured to be narrower than the inter-fiber distance of glass cloth 1B (an example of a second glass cloth).

[0027] 6 is a line graph showing the relationship between the thickness of the glass cloth and the relative dielectric constant of the wiring, where the horizontal axis represents the thickness of the glass cloth [μm] and the vertical axis represents the relative dielectric constant [εr] of the wiring.

[0028] Graph G11 shown in FIG. 6 represents the dielectric constant versus thickness of a glass cloth with an inter-fiber distance of 200 μm. Graph G12 represents the dielectric constant versus thickness of a glass cloth with an inter-fiber distance of 150 μm. Graph G13 represents the dielectric constant versus thickness of a glass cloth with an inter-fiber distance of 100 μm. Graphs G11 to G13 show that, regardless of the inter-fiber distance of the glass cloth, the greater the thickness, the higher the dielectric constant. Therefore, with the same inter-fiber distance, the thickness of glass cloth 1A (an example of a first glass cloth) is configured to be thicker than the thickness of glass cloth 1B (an example of a second glass cloth).

[0029] The graphs in Figures 5 and 6 show that the dielectric constant increases when the inter-fiber distance in the glass cloth arrangement is narrow, and increases as the thickness of the glass cloth increases. The dielectric constant can be adjusted by utilizing this tendency of glass cloths. For example, the inter-fiber distance of the first glass cloth (glass cloth 1A) can be made narrower than the inter-fiber distance of the second glass cloth (glass cloth 1B), thereby increasing the dielectric constant of the first glass cloth (glass cloth 1A). Furthermore, the thickness of the first glass cloth (glass cloth 1A) can be made thicker than the thickness of the second glass cloth (glass cloth 1B), thereby increasing the dielectric constant of the first glass cloth (glass cloth 1A).

[0030] Next, examples of wiring layouts of the first and second transmission lines will be described with reference to Figures 7 and 8. Here, the first and second transmission lines formed on the substrate will be described as simplified schematic diagrams.

[0031] FIG. 7 is a diagram showing an example of a conventional horizontally arranged first transmission line 101 and second transmission line 102. In FIG.

[0032] In a conventional multilayer wiring board 100, a first transmission line 101 and a second transmission line 102 are arranged side by side in the horizontal direction on the second layer. The first transmission line 101 and the second transmission line 102 are separated by a distance d11. Electric field lines 103 are generated in the horizontal direction between the first transmission line 101 and the second transmission line 102. Electric field lines 104 are also generated in the vertical direction between the first transmission line 101 and the second transmission line 102. The electric field lines 104 are generated from the first transmission line 101 and the second transmission line 102 toward the upper and lower ground wirings, and current leaks to the ground wirings on the first and third layers.

[0033] 8 is a diagram showing an example of a first transmission line 11 and a second transmission line 12 arranged vertically according to this embodiment. The multilayer wiring board 10 shown in FIG. 8 is the same as the multilayer wiring board 10 shown in FIG. 4 except that the glass cloths 1A and 1B are omitted.

[0034] In the multilayer wiring board 10 according to this embodiment, a first transmission line 11 and a second transmission line 12 are arranged side by side in a vertical direction perpendicular to the horizontal direction, i.e., in the stacking direction. The first transmission line 101 and the second transmission line 102 are separated by a distance d12. Electric field lines 13 are generated in the vertical direction between the first transmission line 11 and the second transmission line 12. Electric field lines 14 are also generated in the vertical direction toward the ground wiring of each of the first transmission line 11 and the second transmission line 12, causing current to leak into the ground wiring.

[0035] 7 has four electric field lines 104, whereas the multilayer wiring board 10 according to this embodiment has two electric field lines 14. Therefore, the amount of current leaking from the first transmission line 11 and the second transmission line 12 of the multilayer wiring board 10 to the ground wiring is smaller than that of the conventional multilayer wiring board 100. In this way, in the multilayer wiring board 10 according to this embodiment, the amount of coupling of the electric field lines 13 (also referred to as "coupling between differential lines") generated between the first transmission line 11 and the second transmission line 12 (also referred to as "between differential lines") is greater than in the conventional horizontal arrangement, making it less likely that current will leak to the ground wiring.

[0036] The wiring length of the first transmission line 11 is set shorter than the wiring length of the second transmission line 12. For this reason, the glass cloths 1A and 1B are laid so that the relative dielectric constant εr of the first insulating layer 4 and the relative dielectric constant εr of the second insulating layer 5 are also high.

[0037] Generally, the phenomenon in which part of the current flowing between differential lines flows into the ground wiring, converting the signal flowing through the differential lines, is called mode conversion loss. When mode conversion loss occurs, the current flowing between the signals generates noise from the ground wiring, or external noise intrudes into the transmission path. For this reason, it is desirable for multilayer wiring boards to have little effect from mode conversion loss.

[0038] The distance between the first transmission line 101 and the second transmission line 102 due to manufacturing constraints of the conventional board is represented by "a." In this case, the first transmission line 101 and the second transmission line 102 of the conventional multilayer wiring board 100 (see FIG. 7 ) can only be spaced apart by a μm due to manufacturing constraints. On the other hand, in the multilayer wiring board 10 according to this embodiment, the first transmission line 11 and the second transmission line 12 can be spaced apart to a / 2 μm or less, which is the thickness of the insulating layer. Therefore, the distance d12 between the first transmission line 11 and the second transmission line 12 of the multilayer wiring board 10 according to this embodiment is shorter than the distance d11 between the first transmission line 101 and the second transmission line 102 of the conventional multilayer wiring board 100.

[0039] In this way, in the multilayer wiring board 10 according to this embodiment, by shortening the distance d12 between the first transmission line 11 and the second transmission line 12, the amount of coupling between the differential lines is greater than the amount of coupling between the differential lines in the conventional multilayer wiring board 100. As a result, the multilayer wiring board 10 according to this embodiment can reduce the amount of coupling of the electric field lines 14 to the ground wiring of the first transmission line 11 and the second transmission line 12, making it difficult for current to flow through the ground wiring. Furthermore, the multilayer wiring board 10 according to this embodiment has better mode conversion loss characteristics than the conventional multilayer wiring board 100. Furthermore, the noise resistance of the first transmission line 11 and the second transmission line 12 provided in the inner layer is enhanced.

[0040] Next, examples of wiring of the first transmission line 11 and the second transmission line 12, which have a difference in wiring length, in the multilayer wiring board 10 according to this embodiment will be described with reference to FIGS.

[0041] 9 is a top view of a multilayer wiring board 10A in which a first transmission line 21 and a second transmission line 22 are wired on a surface layer and an inner layer. The first transmission line 21 and the second transmission line 22 are denoted by different reference numerals from the first transmission line 11 and the second transmission line 12 shown in FIG. 4 in order to explain how the first transmission line 21 and the second transmission line 22 are wired on the multilayer wiring board 10A.

[0042] Two integrated circuits (ICs) 31 and 32 are attached to the multilayer wiring board 10A. The ICs 31 and 32 are wired via a first transmission path 21 and a second transmission path 22. Of the first transmission path 21, the portion indicated by the solid line is wired on the surface layer of the multilayer wiring board 10A, and the portion indicated by the two-dot chain line is wired on an inner layer of the multilayer wiring board 10A. Similarly, of the second transmission path 22, the portion indicated by the solid line is wired on the surface layer of the multilayer wiring board 10A, and the portion indicated by the dashed line is wired on an inner layer of the multilayer wiring board 10A. The line length of the first transmission path 21 is 100 mm, and the line length of the second transmission path 22 is 123 mm.

[0043] FIG. 10 is a side view of a multilayer wiring board 10B in which a first transmission line 21 and a second transmission line 22 are wired in a BGA (Ball Grid Array) arrangement.

[0044] Two ICs 31 and 32 are also attached to the multilayer wiring board 10B. The first transmission line 21 and the second transmission line 22 are both formed in a BGA arrangement on an inner layer (between the L1 layer and the L2 layer) of the multilayer wiring board 10B. In the multilayer wiring board 10B as well, the line length of the first transmission line 21 is set to 100 mm, and the line length of the second transmission line 22 is set to 123 mm.

[0045] FIG. 11 is a diagram showing the difference in configuration between the first transmission path 21 and the second transmission path 22. In FIG.

[0046] The upper side of Fig. 11 shows a top view of the multilayer wiring board 10A shown in Fig. 9. As described above, in conventional multilayer wiring boards, when there is a difference in wiring length between two transmission lines, a meandering shape is used to make the wiring lengths of the two transmission lines approximately the same in order to prevent signal transmission delays.

[0047] 11 shows a partially enlarged view of a multilayer wiring board 10A having a conventional configuration. In the conventional configuration, the first transmission line 21 is configured in a meander shape to match the wiring length of the second transmission line 12, and the wiring length of the first transmission line 21 is long in the partially enlarged region. However, the first transmission line 21 takes up extra space at a portion 110 where the meander shape is configured.

[0048] A partially enlarged view of a multilayer wiring board 10A configured according to this embodiment is shown in the lower right of Fig. 11. In the configuration according to this embodiment, the glass cloth 1 shown in Fig. 4 is provided in an inner layer of the multilayer wiring board 10A. Therefore, it is not necessary to configure the first transmission path 21 in a meandering shape, and extra space due to the first transmission path 21 is not required.

[0049] 12 is a diagram showing an example of the measurement results of skew adjustment. In each graph of FIG. 12, the horizontal axis represents time [nanoseconds], and the vertical axis represents voltage [V].

[0050] Graph (1) at the top of Figure 12 shows an example of differential signal changes in a multilayer wiring board 10 with a non-meandered wiring and a difference in wiring length. The multilayer wiring board 10 with a difference in wiring length has its relative dielectric constant εr not adjusted. The dashed line below graph (1) represents the differential signal changes in the first transmission line 21 with a line length of 100 mm. The dashed line above graph (1) represents the differential signal changes in the second transmission line 22 with a line length of 123 mm. The first transmission line 21 and the second transmission line 22 both have the same relative dielectric constant εr of "4." Therefore, the arrival timing of the differential signal in the second transmission line 22 is delayed by a skew sk1 from the arrival timing of the differential signal in the first transmission line 21. Skew represents the difference in signal arrival time, i.e., delay, between the differential signals transmitted through two transmission lines.

[0051] Graph (2) at the bottom of Figure 12 shows an example of differential signal changes in a multilayer wiring board 10 configured according to this embodiment. According to this embodiment, glass cloth 1 is laid in both directions of the first transmission line 21, and the relative dielectric constant εr is adjusted to "6." As a result, the differential signal in the first transmission line 21 is delayed, reducing the skew sk1, which is the difference in signal arrival time between the differential signals. By reducing the difference in signal arrival time between the differential signals, the timing at which the differential signal change in the first transmission line 21 arrives and the timing at which the differential signal change in the second transmission line 22 arrives become approximately the same. Adjusting the relative dielectric constant εr in this way makes it possible to adjust the timing of differential signal transmission and the skew sk1. Furthermore, among the transmission lines provided in the multilayer wiring board 10, the transmission line with the shorter wiring length does not need to be configured in a meandering shape, thereby saving space for wiring on the board.

[0052] Next, examples of wiring having a configuration according to this embodiment will be described with reference to Figures 13 to 15. Figure 13 is a top view showing an example of the configuration of a multilayer wiring board 10C according to this embodiment.

[0053] In the multilayer wiring board 10C, ICs 31 and 32 are wired by a first transmission line 21 and a second transmission line 22 via footprints provided on the top surface of the board. The wiring length of the first transmission line 21 is shorter than the wiring length of the second transmission line 22. The solid line portions of the first transmission line 21 and the second transmission line 22 represent the portions that are wired on the surface layer of the multilayer wiring board 10C. The dashed line portions represent the portions that are wired on the inner layer of the multilayer wiring board 10C. The circled portions in the figure represent layer switching vias.

[0054] In the multilayer wiring board 10C, the inner layer wiring portions of the first transmission line 21 and the second transmission line 22, indicated by dashed lines, overlap at the same position. An example of the wiring of the first transmission line 21 is shown in the lower right of Fig. 13, and an example of the wiring of the second transmission line 22 is shown in the lower left of Fig. 13. This figure also shows that the wiring length of the first transmission line 21 is shorter than the wiring length of the second transmission line 22.

[0055] Fig. 14 is a cross-sectional view showing an example of wiring when ICs 31 and 32 are footprints. Fig. 14 is a cross-sectional view of the multilayer wiring board 10C shown in Fig. 13. The layer numbers shown in Fig. 14 correspond to the layer numbers shown in Fig. 4, and layer numbers are not assigned to layers not used in the description.

[0056] As shown in Figure 14, the first transmission line 21 and the second transmission line 22 are both routed via footprints on inner layers of the multilayer wiring board 10C, and have different wiring lengths. A first insulating layer 4 is provided between the first and second layers, and a second insulating layer is provided between the third and fourth layers. The relative dielectric constant εr of the first insulating layer 4 is set to a value higher than the relative dielectric constant εr of the second insulating layer 5. As a result, even if the first transmission line 21 and the second transmission line 22 partially overlap in the stacking direction, skew in the differential wiring can be suppressed.

[0057] Fig. 15 is a cross-sectional view showing an example of wiring when ICs 31 and 32 are BGA. Fig. 15 is a cross-sectional view of multilayer wiring board 10C shown in Fig. 13 when ICs 31 and 32 of multilayer wiring board 10C are installed as BGA. The layer numbers shown in Fig. 15 correspond to the layer numbers shown in Fig. 4, and layer numbers are not assigned to layers not used in the description.

[0058] Even when the ICs 31 and 32 are BGAs, the first transmission path 21 and the second transmission path 22 are wired on inner layers of the multilayer wiring substrate 10C and have different wiring lengths. A first insulating layer 4 is provided between the first and second layers, and a second insulating layer is provided between the third and fourth layers. As a result, even when the first transmission path 21 and the second transmission path 22 partially overlap in the stacking direction, skew in the differential wiring can be suppressed.

[0059] Next, the characteristic impedance of the transmission line using the wiring configured according to this embodiment will be described with reference to Fig. 16 and Fig. 17. Fig. 16 is a diagram showing an example of inner layer wiring for explaining the characteristic impedance value Zo of the transmission line.

[0060] When a designer designs the wiring of the first transmission line 11 and the second transmission line 12, they need to set a characteristic impedance value Zo of, for example, 100 Ω. To ensure signal stability and prevent noise in differential wiring, the characteristic impedance values ​​Zo of the first transmission line 11 and the second transmission line 12 need to be the same. The following formula (2) is a simple formula for calculating the general characteristic impedance for inner layer wiring.

[0061]

[0062] In equation (2), εr represents the relative dielectric constant, b represents the height of the insulating layer in the stacking direction, t represents the height of the inner layer wiring in the stacking direction, and w represents the horizontal line width of the inner layer wiring. As shown in equation (2), when the relative dielectric constant εr changes, the characteristic impedance value Zo also changes. For example, when the relative dielectric constant εr increases, the characteristic impedance value Zo decreases. Conversely, when the relative dielectric constant εr decreases, the characteristic impedance value Zo increases.

[0063] When the relative dielectric constant εr changes in this way, the line width w of the inner layer wiring must be adjusted. For example, if the relative dielectric constant εr is lowered by placing the glass cloth 1 close to the inner layer wiring, the characteristic impedance value Zo increases. Therefore, it is necessary to adjust the line width w to widen it so that the characteristic impedance value Zo does not change. However, even if this adjustment increases the line width w of the inner layer wiring from its original line width, the space that the inner layer wiring increases horizontally is much smaller than with conventional meandering shape adjustments. Conversely, when the relative dielectric constant εr increases, the characteristic impedance value Zo decreases, so the line width w is narrowed to adjust it.

[0064] Fig. 17 is a diagram showing an example of the configuration of a multilayer wiring board 10D configured by changing the width of the inner layer wiring. The multilayer wiring board 10D shown in Fig. 17 has the same configuration as the multilayer wiring board 10 shown in Fig. 4, but the line width w1 of the first transmission line 11 is narrower than the line width w2 of the second transmission line 12.

[0065] The line width w1 of the first transmission line 11 is narrowed in this manner because the relative dielectric constant εr of the glass cloth 1A provided between the first and second layers is higher than the relative dielectric constant εr of the glass cloth 1B provided between the third and fourth layers. According to the above-mentioned formula (2), as the relative dielectric constant εr increases, the characteristic impedance value Zo decreases. Therefore, in order to maintain the characteristic impedance value Zo unchanged, the line width w is reduced.

[0066] In this way, the width of the first transmission path (first transmission path 11) in a direction intersecting the stacking direction is narrower than the width of the second transmission path (second transmission path 12) in a direction intersecting the stacking direction, and the characteristic impedance of the first transmission path (first transmission path 11) and the characteristic impedance of the second transmission path (second transmission path 12) are adjusted. For example, even if the wiring length of the first transmission path 11 is longer than the wiring length of the second transmission path 12, the wiring width w1 of the first transmission path 11 is formed narrower than the wiring width w2 of the second transmission path 12. By forming the wiring widths in this way, the characteristic impedance values ​​Zo of the first transmission path 11 and the second transmission path 12 can be made the same.

[0067] In the multilayer wiring boards 10 to 10D according to the above-described embodiment, when the first transmission line 11 and the second transmission line 12 used in differential wiring have different wiring lengths, a glass cloth 1 with a high relative dielectric constant εr is provided in the layer adjacent to the first transmission line 11, which has a shorter wiring length. This allows the arrival timing of the differential signal transmitted through the first transmission line 11 to be aligned with the arrival timing of the differential signal transmitted through the second transmission line 12, thereby reducing skew sk1. This enables skew adjustment without reducing wiring efficiency. Furthermore, even when the first transmission line 11 and the second transmission line 12, which perform high-speed communication, are wired closely to each other, wiring with enhanced noise resistance suitable for high-speed transmission in AD / ADAS products can be achieved.

[0068] Furthermore, by arranging the first transmission line 11 and the second transmission line 12 in the stacking direction, it is possible to reduce mode conversion loss in the multilayer wiring boards 10 to 10D and improve noise resistance.

[0069] Furthermore, when glass cloth 1 with a high relative dielectric constant εr is laid in the first transmission line 11, the line width w1 of the first transmission line 11 is made narrower than the line width w2 of the second transmission line 12. With this configuration, the characteristic impedance values ​​Zo of the first transmission line 11 and the second transmission line 12 can be made uniform. Therefore, even if there is a difference in wiring length between the first transmission line 11 and the second transmission line 12, the characteristic impedance values ​​Zo can be made uniform by adjusting the line width of the transmission line in which glass cloth 1 with a high relative dielectric constant εr is laid.

[0070] When glass cloth with a wide fiber spacing is embedded in a layer, the glass cloth may be compressed during the lamination process. If the glass cloth is compressed during the lamination process, the transition distance may become narrower, and the initial relative dielectric constant may become higher. The glass cloth may be laminated in anticipation of such a change in the relative dielectric constant.

[0071] The present invention is not limited to the above-described embodiments, and various other applications and modifications are possible without departing from the spirit of the present invention as defined in the claims. For example, the above-described embodiments have described the configuration of a multilayer wiring board in detail and specifically in order to clearly explain the present invention, and are not necessarily limited to those having all of the described configurations. Furthermore, it is possible to add, delete, or replace some of the configurations of the present embodiments with other configurations. Furthermore, the control lines and information lines shown are those considered necessary for explanation, and do not necessarily represent all control lines and information lines in the product. In reality, it can be assumed that almost all of the configurations are interconnected.

[0072] 1, 1A, 1B...glass cloth, 4...first insulating layer, 5...second insulating layer, 10 to 10D...multilayer wiring board, 11, 21...first transmission line, 12, 22...second transmission line, 31, 32...IC

Claims

1. A multilayer wiring board in which a plurality of layers are laminated and differential wirings included in a part of the layers transmit differential signals, the multilayer wiring board including: a first transmission path that transmits the differential signals; a second transmission path that has a longer wiring length than the first transmission path and transmits the differential signals; a first insulating layer laminated between the first transmission path and a first ground wiring; and a second insulating layer laminated between the second transmission path and a second ground wiring, wherein the differential wiring is composed of the first transmission path and the second transmission path, at least a part of the first transmission path is disposed to face the second transmission path in a stacking direction in which the first insulating layer, the first transmission path, the second transmission path, and the second insulating layer are stacked in this order, and a relative dielectric constant of the first insulating layer is higher than a relative dielectric constant of the second insulating layer.

2. The multilayer wiring board according to claim 1, wherein the first insulating layer includes a first glass cloth, the second insulating layer includes a second glass cloth, and an inter-fiber distance of the first glass cloth is narrower than an inter-fiber distance of the second glass cloth.

3. The multilayer wiring board according to claim 1, wherein the first insulating layer includes a first glass cloth, the second insulating layer includes a second glass cloth, and a thickness of the first glass cloth is thicker than a thickness of the second glass cloth.

4. The multilayer wiring board according to claim 2, wherein a width of the first transmission path in a direction intersecting the stacking direction is narrower than a width of the second transmission path in a direction intersecting the stacking direction, and characteristic impedances of the first transmission path and the second transmission path are adjusted.

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