Optical module package and optical module

Abstract

To provide an optical module package capable of reducing crosstalk between adjacent signal wires.SOLUTION: An optical module package comprises a substrate having a first wiring layer, a second wiring layer, and a third wiring layer. The first wiring layer includes a first signal terminal, a second signal terminal, and a first ground terminal provided between the first signal terminal and the second signal terminal and extending in a first direction. The second wiring layer includes a first ground pattern connected with the first ground terminal via a plurality of first vias arranged at a first interval. The third wiring layer includes a second ground pattern extending in the first direction and connected with the first ground pattern via a plurality of second vias arranged in the first direction at the first interval and a plurality of third vias arranged in the first direction at the first interval.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present disclosure relates to an optical module package and an optical module. [Background technology]

[0002] Patent Document 1 discloses a flexible wiring board having a signal line wiring conductor and a ground line wiring conductor on each of the upper surfaces of both main surfaces. Patent Document 1 discloses providing the signal line wiring conductor and the ground line wiring conductor in positions that are symmetrical above and below, respectively. Patent Document 1 also discloses forming via conductors at equal pitch intervals in the longitudinal direction to connect the signal line wiring conductors and the ground line wiring conductor to each other on the flexible wiring board.

[0003] Patent Document 2 discloses a semiconductor device in which a semiconductor element is mounted on the upper surface of a wiring board. Patent Document 2 discloses that a ground interlayer connection via is disposed between interlayer connection vias connected to signal terminals in the second row and signal terminals in the fourth row of the wiring board. Patent Document 2 also discloses that the interlayer connection vias act as a shield to suppress crosstalk between different signals of a differential pair. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-015513 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-192767 Summary of the Invention [Problem to be solved by the invention]

[0005] In optical modules, when high-speed electrical signals are converted into optical signals for transmission and reception, crosstalk between adjacent signal lines increases in the frequency range of 30 GHz or higher, which can limit the transmission and reception characteristics of optical signals.

[0006] The present disclosure provides a package for an optical module that reduces crosstalk between adjacent signal lines. [Means for solving the problem]

[0007] The present disclosure provides a substrate having a first wiring layer, a second wiring layer including a first insulating layer between it and the first wiring layer, and a third wiring layer including a second insulating layer between it and the second wiring layer, wherein the first wiring layer includes a first signal terminal extending along a first direction, a second signal terminal arranged along a second direction intersecting the first direction with respect to the first signal terminal and extending along the first direction, and a first ground terminal provided between the first signal terminal and the second signal terminal and extending along the first direction, and the second wiring layer includes a first ground terminal extending in the first direction and connected to the first ground terminal via a plurality of first vias arranged at a first interval along the first direction. the third wiring layer includes a second ground pattern connected to the first ground pattern via a plurality of second vias arranged at the first interval along the first direction and a plurality of third vias arranged at the first interval along the first direction, the plurality of first vias being arranged between the plurality of second vias and the plurality of third vias in the second direction when viewed in a plan view from a direction perpendicular to the substrate, and the plurality of second vias and the plurality of third vias being shifted from each other by a distance of half the first interval in the first direction when viewed in a plan view from a direction perpendicular to the substrate. [Effects of the Invention]

[0008] According to the optical module package of the present disclosure, crosstalk between adjacent signal wirings can be reduced. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing an outline of the configuration of an optical transceiver according to this embodiment. [Figure 2] FIG. 2 is a perspective view of the optical module assembly according to this embodiment. [Figure 3] FIG. 3 is a perspective view of the optical module according to this embodiment. [Figure 4] FIG. 4 is a top view of the flexible printed circuit board according to this embodiment. [Figure 5] FIG. 5 is a bottom view of the flexible printed circuit board according to this embodiment. [Figure 6] FIG. 6 is a top view of the terminal portion of the optical module according to this embodiment. [Figure 7] FIG. 7 is a perspective view of the substrate of the optical module according to this embodiment. [Figure 8] FIG. 8 is a diagram illustrating a terminal portion of the optical module according to this embodiment. [Figure 9] FIG. 9 is a diagram illustrating a terminal portion of the optical module according to this embodiment. [Figure 10] FIG. 10 is a diagram illustrating a terminal portion of the optical module according to this embodiment. [Figure 11] FIG. 11 is a diagram showing the evaluation results of the optical module according to this embodiment. [Figure 12] FIG. 12 is a diagram showing the evaluation results of the optical module of the comparative example. [Figure 13A] FIG. 13A is a diagram illustrating the arrangement of vias that connect the ground pattern of the second wiring layer and the ground pattern of the third wiring layer. [Figure 13B] FIG. 13B is a diagram illustrating the arrangement of vias that connect the ground pattern of the second wiring layer and the ground pattern of the third wiring layer. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described.

[0011] (1) The optical module package of the present disclosure includes a substrate having a first wiring layer, a second wiring layer including a first insulating layer between the second wiring layer and the first wiring layer, and a third wiring layer including a second insulating layer between the second wiring layer and the second wiring layer. The first wiring layer includes a first signal terminal extending in a first direction, a second signal terminal arranged in a second direction intersecting the first direction with respect to the first signal terminal and extending in the first direction, and a first ground terminal provided between the first signal terminal and the second signal terminal and extending in the first direction. The second wiring layer includes a first ground pattern extending in the first direction and connected to the first ground terminal through a plurality of first vias arranged at a first interval in the first direction. The third wiring layer includes a second ground pattern extending in the first direction and connected to the first ground pattern through a plurality of second vias arranged at the first interval in the first direction and a plurality of third vias arranged at the first interval in the first direction. The first vias are arranged between the second vias and the third vias in the second direction when viewed from a plane perpendicular to the substrate, and the second vias and the third vias are arranged to be shifted from each other by half the first interval in the first direction when viewed from a plane perpendicular to the substrate.

[0012] According to the optical module package of the present disclosure, crosstalk between adjacent signal wirings can be reduced.

[0013] (2) In the above (1), the distance in the second direction between the plurality of first vias and the plurality of second vias is equal to the distance in the second direction between the plurality of first vias and the plurality of third vias.

[0014] According to the optical module package of the present disclosure, crosstalk between adjacent signal wirings can be reduced.

[0015] (3) In the above (1) or (2), the width of the first ground terminal in the second direction is smaller than the width of the first ground pattern in the second direction.

[0016] According to the optical module package of the present disclosure, crosstalk between adjacent signal wirings can be reduced without changing the spacing between the ground terminal and the signal terminal.

[0017] (4) In any one of the above (1) to (3), the centers of the plurality of first vias are arranged on a center line of the first ground terminal that extends in the first direction.

[0018] According to the optical module package of the present disclosure, crosstalk between adjacent signal wirings can be reduced.

[0019] (5) In the above (4), the distance in the second direction between the first signal terminal and the center line is equal to the distance in the second direction between the second signal terminal and the center line.

[0020] According to the optical module package of the present disclosure, crosstalk between adjacent signal wirings can be reduced.

[0021] (6) An optical module according to the present disclosure includes the optical module package according to any one of (1) to (5) above.

[0022] According to the optical module of the present disclosure, crosstalk between adjacent signal lines can be reduced.

[0023] [Details of the embodiments of the present disclosure] Specific examples of the optical module of the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0024] In the description of the specification and drawings of each embodiment, components having substantially the same or corresponding functions may be designated by the same reference numerals to avoid redundant explanation. In addition, the scale of each part in the drawings may differ from the actual scale to facilitate understanding.

[0025] Directions such as parallel, right-angled, orthogonal, horizontal, and vertical are permitted to deviate to the extent that the effects of the embodiments are not impaired. The shape of the corners is not limited to right angles and may be rounded like an arch. Parallel, right-angled, orthogonal, horizontal, and vertical may include substantially parallel, substantially right-angled, substantially orthogonal, substantially horizontal, and substantially vertical. For example, substantially parallel means that even if two lines or two surfaces are not completely parallel to each other, they can be treated as being parallel to each other within a manufacturing tolerance. Similar to substantially parallel, the other terms substantially right-angled, substantially orthogonal, substantially horizontal, and substantially vertical are also intended to be applicable as long as the relative positional relationship between the two lines or two surfaces is within a manufacturing tolerance.

[0026] An optical transceiver 1 according to this embodiment will be described below. Fig. 1 is a diagram showing an outline of the configuration of the optical transceiver 1 according to this embodiment.

[0027] The optical transceiver 1 includes optical modules 10T and 10R, flexible printed circuit boards 20T and 20R, and a circuit board 30. The optical modules 10T and 10R are examples of the optical module 10 according to an embodiment of the present disclosure.

[0028] [Optical module] The optical modules 10T and 10R convert electrical signals to optical signals and vice versa. The optical module 10T is, for example, a Transmitter Optical Subassembly (TOSA). The optical module 10R is, for example, a Receiver Optical Subassembly (ROSA).

[0029] The optical module 10T includes, for example, a driver 11 and a transmitting optical element 12. The transmitting optical element 12 is, for example, a light emitting element or an optical modulator.

[0030] The driver 11 (drive circuit) outputs a drive signal Td for driving the transmitting optical element 12 to the transmitting optical element 12 based on a transmission signal Tx2 from a DSP (Digital Signal Processor) 31 provided on the circuit board 30, for example.

[0031] The transmitting optical element 12 outputs a transmission optical signal Lt in response to a drive signal Td. The transmission optical signal Lt output from the transmitting optical element 12 is transmitted to another optical transceiver via an optical fiber. The transmitting optical element 12 is, for example, a laser diode or a Mach-Zehnder modulator.

[0032] The optical module 10R includes, for example, a transimpedance amplifier 13 and a receiving optical element 14. The transimpedance amplifier 13 converts a receiving signal Ri, which is a current signal from the receiving optical element 14, into a receiving signal Rx2, which is a voltage signal, and outputs the voltage signal to a DSP 31 provided on the circuit board 30.

[0033] The receiving optical element 14 is, for example, a photodiode, and converts a received optical signal Lr received from another optical transceiver via an optical fiber into a received signal Ri.

[0034] The optical transceiver 1 may further include a light source 15 (not shown). The light source 15 is, for example, a tunable laser. The light source 15 generates continuous wave light (CW light) Lb having a predetermined peak wavelength and outputs it to the transmitting optical element 12. For example, if the transmitting optical element 12 is an optical modulator, it modulates the CW light Lb supplied from the light source 15 in accordance with a drive signal Td to generate a transmitting optical signal Lt. The receiving optical element 14 may also include a 90° optical hybrid. The receiving optical element 14 may generate a receiving signal Ri from an optical signal generated by interfering the receiving optical signal Lr with the CW light Lb.

[0035] [Flexible printed circuit board] The flexible printed circuit boards 20T and 20R electrically connect the optical modules 10T and 10R to the circuit board 30, respectively. The flexible printed circuit boards 20T and 20R have a plurality of signal wirings. The signal wirings of the flexible printed circuit boards 20T and 20R form, for example, transmission lines. For example, the flexible printed circuit boards 20T and 20R include differential signal wirings consisting of a pair of signal wirings to transmit one differential signal. The differential signal wirings form, for example, differential transmission lines to transmit high-speed differential signals with good waveform quality.

[0036] [Circuit board] The circuit board 30 includes, for example, a control circuit that controls the inside of the optical transceiver 1. For example, the control circuit includes a detection circuit, a microcontroller, and the like for controlling the operation of the optical modules 10T and 10R. The circuit board 30 also communicates with a host (transmission device) to which the optical transceiver 1 is connected for the purpose of monitoring and controlling the optical transceiver 1. Furthermore, the circuit board 30 controls the optical modules 10T and 10R, for example, based on communication with the host. In response to a query from the host, the circuit board 30 transmits information regarding the operating status of the optical modules 10T and 10R to the host.

[0037] The circuit board 30 includes, for example, a DSP 31. The DSP 31 converts a transmission signal Tx1 transmitted from the host into a transmission signal Tx2 to the driver 11. For example, if the DSP 31 includes a CDR (Clock Data Recovery) circuit, it generates a waveform-shaped signal from the transmission signal Tx1 and outputs the waveform-shaped signal as the transmission signal Tx2. The DSP 31 also converts a reception signal Rx2 from the transimpedance amplifier 13 into a reception signal Rx1 to be transmitted to the host. For example, if the DSP 31 includes a CDR circuit, it generates a waveform-shaped signal from the reception signal Rx2 and outputs the waveform-shaped signal as the reception signal Rx1. The DSP 31 transmits and receives a control signal Ctl1 for monitoring control to and from the host via, for example, a dedicated signal wiring provided separately from the signal wiring that transmits the transmission signal Tx1 and the reception signal Rx1.

[0038] Next, the configuration used in the optical transceiver 1 will be described in detail. Note that, for ease of explanation, an XYZ Cartesian coordinate system may be shown in the figures. For example, for a coordinate axis perpendicular to the plane of the drawing, a cross in a circle indicates that the direction toward the back of the plane of the drawing is the positive region of the coordinate axis. Also, for a coordinate axis perpendicular to the plane of the drawing, a black circle in a circle indicates that the direction toward the front of the plane of the drawing is the positive region of the coordinate axis. However, this coordinate system primarily indicates directions for the sake of explanation, and does not limit the coordinates and attitude of the optical module, etc., disclosed herein.

[0039] In this disclosure, unless otherwise specified, the X axis is the direction in which the terminals of the optical modules 10T and 10R are arranged, the Y axis is the thickness direction of the terminals of the optical modules 10T and 10R, and the Z axis is the direction in which the terminals of the optical modules 10T and 10R extend. For example, the transmit signal Tx2 and the receive signal Rx2 are transmitted between the circuit board 30 and the optical modules 10T and 10R along the Z axis. Furthermore, unless otherwise specified, coordinate axes with the same name shown in each figure represent the same thing. For example, the X axis in FIG. 2 represents the same thing as the X axis in FIG. 3.

[0040] [Optical module assembly] The optical module assembly 2 includes an optical module 10T or 10R and a flexible printed circuit board 20T or 20R corresponding to the optical module 10T or 10R. The following describes a case where the optical module assembly 2 is configured from the optical module 10T and the flexible printed circuit board 20T. FIG. 2 is a perspective view of the optical module assembly 2 according to this embodiment. The optical module 10T and the flexible printed circuit board 20T included in the optical module assembly 2 are electrically connected by soldering or the like.

[0041] [Optical module] The optical module 10T includes a driver 11, a transmitting optical element 12, and a package 16. Fig. 3 is a perspective view of the optical module 10T according to this embodiment. The package 16 contains, for example, the driver 11 and the transmitting optical element 12.

[0042] The package 16 has, for example, a rectangular parallelepiped shape. More specifically, the package 16 includes a substrate 16a, a mounting substrate 16b, a sidewall 16c, and a lid 16d.

[0043] The substrate 16a, for example, introduces a signal from outside the optical module 10T into the inside of the optical module 10T. Note that the substrate 16a may also output a signal from inside the optical module 10T to the outside of the optical module 10T. The substrate 16a has signal wiring for transmitting signals between the inside and outside of the package 16. Such signal wiring is also called a feedthrough. Furthermore, the substrate 16a having such signal wiring is sometimes called a feedthrough.

[0044] The substrate 16a is, for example, a ceramic substrate. The substrate 16a is a multilayer wiring substrate having wiring layers on its surface and inside. The substrate 16a has, for example, an insulating layer made of ceramic between the wiring layers. The optical module 10T has a terminal section 10a on the substrate 16a. The wiring layer is provided with signal wiring for transmitting a transmission signal Tx2 and a reception signal Rx2, a ground line, a power supply line for supplying power to the interior, and the like. The terminal section 10a is, for example, provided on a wiring layer (first wiring layer) on the surface of the substrate 16a.

[0045] The mounting substrate 16b has a rectangular, flat plate-like outer shape. The mounting substrate 16b has, for example, an upper surface parallel to the XZ plane. For example, the driver 11 and the transmitting optical element 12 provided in the optical module 10T are mounted on the upper surface of the mounting substrate 16b. The mounting substrate 16b closes the lower side (-Y side) of the side wall portion 16c. For example, the mounting substrate 16b is joined to the lower side of the side wall portion 16c or is formed integrally with the side wall portion 16c.

[0046] The side wall portion 16c forms the side wall of the package 16. The side wall portion 16c has a pair of side walls (hereinafter referred to as lateral walls) parallel to the YZ plane and a pair of side walls (hereinafter referred to as front and rear walls) parallel to the XY plane.

[0047] A substrate 16a is provided on the front wall. The rear wall is located on the opposite side of the front wall in the Z-axis direction. For example, an optical component (not shown) for outputting the transmission optical signal Lt to the outside is provided on the rear wall. The front wall and the rear wall are each connected to a pair of side walls. The pair of side walls, the front wall, and the rear wall form a space inside (internal space). The internal space is surrounded by the pair of side walls, the front wall, and the rear wall in the XZ plane. The internal space accommodates a driver 11 and a transmitting optical element 12 provided in the optical module 10T.

[0048] The lid 16d closes the upper side (+Y side) of the side wall 16c. For example, the lid 16d is joined to the upper side of the side wall 16c. The lid 16d is, for example, a metal lid, and is joined to the upper part of the side wall 16c by seam welding. An internal space is defined by the substrate 16a, the mounting substrate 16b, the side wall 16c, and the lid 16d. For example, the package 16 is hermetically sealed so that the internal space is isolated from the outside of the package 16. The optical module 10R has a package similar to the package 16 of the optical module 10T, and houses the transimpedance amplifier 13 and the receiving optical element 14 in the internal space of the package. A description of the package of the optical module 10R will be omitted.

[0049] [Flexible printed circuit board] The configuration of the flexible printed circuit board 20T will be described in detail. FIG. 4 is a top view of the flexible printed circuit board 20T according to this embodiment. Specifically, FIG. 4 is a top view of the flexible printed circuit board 20T as viewed from the positive region of the Y axis toward the negative region of the Y axis. FIG. 5 is a bottom view of the flexible printed circuit board 20T according to this embodiment. Specifically, FIG. 5 is a top view of the flexible printed circuit board 20T as viewed from the negative region of the Y axis toward the positive region of the Y axis. The flexible printed circuit board 20R has a configuration similar to that of the flexible printed circuit board 20T. A detailed description of the flexible printed circuit board 20R will be omitted.

[0050] The flexible printed circuit board 20T extends along the Z-axis direction. For example, the flexible printed circuit board 20T has the same shape at both ends in the direction of extension (Z-axis direction). The shapes at both ends may be different. 4 and 5 show the end of the flexible printed circuit board 20T on the side that connects to the optical module 10T (negative side of the Z-axis), and the end of the flexible printed circuit board 20T on the opposite side from the side that connects to the optical module 10T (positive side of the Z-axis) will not be described.

[0051] The flexible printed circuit board 20T has a pair of signal wirings WaS1p and WaS1n on the upper surface LmS1 of the insulating layer Lm. The pair of signal wirings WaS1p and WaS1n are formed as part of the upper surface wiring layer. The signal wirings WaS1p and WaS1n each extend along the Z-axis direction. Differential signals are transmitted through the signal wirings WaS1p and WaS1n. The signal wirings WaS1p and WaS1n are formed as transmission lines, for example.

[0052] The flexible printed circuit board 20T has a signal terminal FaS1p connected to the signal wiring WaS1p at an end of the upper surface LmS1 of the insulating layer Lm that is connected to the optical module 10T. The flexible printed circuit board 20T has a signal terminal FaS1n connected to the signal wiring WaS1n at an end of the upper surface LmS1 that is connected to the optical module 10T. The signal terminals FaS1p and FaS1n may be collectively referred to as differential signal terminals FaS1.

[0053] Similarly, the flexible printed circuit board 20T has a pair of signal wirings WaS2p and WaS2n on the upper surface LmS1 of the insulating layer Lm. The pair of signal wirings WaS2p and WaS2n are formed as part of the upper surface wiring layer. The signal wirings WaS2p and WaS2n each extend along the Z-axis direction. Differential signals different from the differential signals transmitted by the signal wirings WaS1p and WaS1n are transmitted through the signal wirings WaS2p and WaS2n. The signal wirings WaS2p and WaS2n are formed as, for example, transmission lines. The flexible printed circuit board 20T has a signal terminal FaS2p connected to the signal wiring WaS2p and a signal terminal FaS2n connected to the signal wiring WaS2n at an end of the upper surface LmS1 on the side connected to the optical module 10T. The signal terminals FaS2p and FaS2n may be collectively referred to as differential signal terminals FaS2.

[0054] The flexible printed circuit board 20T also has a pair of signal wirings WaS3p and WaS3n and a pair of signal wirings WaS4p and WaS4n on its upper surface LmS1. The pair of signal wirings WaS3p and WaS3n has the same configuration as the pair of signal wirings WaS1p and WaS1n. The pair of signal wirings WaS4p and WaS4n has the same configuration as the pair of signal wirings WaS1p and WaS1n. Detailed descriptions of the pair of signal wirings WaS3p and WaS3n and the pair of signal wirings WaS4p and WaS4n will be omitted.

[0055] Furthermore, the flexible printed circuit board 20T has a signal terminal FaS3p connected to the signal wiring WaS3p, a signal terminal FaS3n connected to the signal wiring WaS3n, a signal terminal FaS4p connected to the signal wiring WaS4p, and a signal terminal FaS4n connected to the signal wiring WaS4n. Detailed descriptions of the signal terminals FaS3p and FaS3n and the signal terminals FaS4p and FaS4n will be omitted.

[0056] The signal wiring WaS1p, signal wiring WaS1n, signal wiring WaS2p, signal wiring WaS2n, signal wiring WaS3p, signal wiring WaS3n, signal wiring WaS4p, and signal wiring WaS4n are arranged side by side on the upper surface LmS1 in order from the negative side of the X axis. The flexible printed circuit board 20T has an insulating coating Lga on the upper surface LmS1 to cover the signal wiring WaS1p, signal wiring WaS1n, and other signal wirings. The insulating coating Lga is formed of an insulating material and, for example, prevents the signal wirings from electrically contacting each other or from being damaged by contact with the outside. The insulating coating Lga may be, for example, a coverlay or a solder resist.

[0057] The flexible printed circuit board 20T has a ground pattern FaG1 at the end on the negative side of the X-axis that is connected to the optical module 10T. The flexible printed circuit board 20T also has a ground pattern FaG2 between the signal terminals FaS1n and FaS2p, i.e., between the differential signal terminals FaS1 and FaS2. Similarly, the flexible printed circuit board 20T has a ground pattern FaG3 between the signal terminals FaS2n and FaS3p, i.e., between the differential signal terminals FaS2 and FaS3. The flexible printed circuit board 20T also has a ground pattern FaG4 between the signal terminals FaS3n and FaS4p, i.e., between the differential signal terminals FaS3 and FaS4. The flexible printed circuit board 20T has a ground pattern FaG5 at the end on the positive side of the X-axis that is connected to the optical module 10T.

[0058] The flexible printed circuit board 20T has a ground pattern WbG on the lower surface LmS2 of the insulating layer Lm. The ground pattern WbG is a planar pattern extending in the X-axis direction and the Z-axis direction and is formed, for example, as a so-called solid pattern. The ground pattern WbG is formed as part of the wiring layer on the lower surface LmS2. The flexible printed circuit board 20T also has, in order from the negative side of the X-axis, ground terminals FbG1, FbG2, FbG3, FbG4, and FbG5 at an end of the lower surface LmS2 of the insulating layer Lm that is connected to the optical module 10T. The ground terminals FbG1, FbG2, FbG3, FbG4, and FbG5 each extend along the Z-axis direction and are connected to the ground pattern WbG. The configuration of the ground terminal FbG1, signal terminal FbS1p, signal terminal FbS1n, and ground terminal FbG2 arranged along the X-axis direction is also called a GSSG configuration, where S stands for signal wiring (signal terminal), and G stands for ground wiring (ground terminal).

[0059] The ground pattern FaG1 is electrically connected to the ground terminal FbG1 via a through-hole via (hereinafter simply referred to as a via). The circles shown in FIGS. 4 and 5 represent vias. The vias penetrate the insulating layer Lm along the Y-axis direction and are internally plated to electrically connect the wiring layer on the upper surface LmS1 and the wiring layer on the lower surface LmS2. Similarly, the ground pattern FaG2 is electrically connected to the ground terminal FbG2 via a via. Similarly, the ground patterns FaG3, FaG4, and FaG5 are electrically connected to the ground terminals FbG3, FbG4, and FbG5, respectively, via vias.

[0060] The flexible printed circuit board 20T has a signal terminal FbS1p and a signal terminal FbS1n between the ground terminals FbG1 and FbG2. The signal terminal FbS1p is electrically connected to the signal terminal FaS1p through a via. The signal terminal FbS1n is electrically connected to the signal terminal FaS1n through a via.

[0061] Similarly, the flexible printed circuit board 20T has signal terminals FbS2p and FbS2n between the ground terminals FbG2 and FbG3. The signal terminal FbS2p is electrically connected to the signal terminal FaS2p through a via. The signal terminal FbS2n is electrically connected to the signal terminal FaS2n through a via.

[0062] The flexible printed circuit board 20T also has a signal terminal FbS3p and a signal terminal FbS3n between the ground terminals FbG3 and FbG4. The signal terminal FbS3p is electrically connected to the signal terminal FaS3p through a via. The signal terminal FbS3n is electrically connected to the signal terminal FaS3n through a via.

[0063] Furthermore, the flexible printed circuit board 20T has a signal terminal FbS4p and a signal terminal FbS4n between the ground terminals FbG4 and FbG5. The signal terminal FbS4p is electrically connected to the signal terminal FaS4p through a via. The signal terminal FbS4n is electrically connected to the signal terminal FaS4n through a via.

[0064] The flexible printed circuit board 20T has an insulating coating Lgb on its lower surface LmS2 to cover the ground pattern WbG. The insulating coating Lgb is made of an insulating material and, for example, prevents the ground pattern from coming into electrical contact with other wiring or from being damaged by external contact. The insulating coating Lgb may be, for example, a coverlay or a solder resist. Note that the insulating coating Lgb may cover areas other than those that need to expose the wiring layer, such as the signal terminals FbS1p and FbS1n.

[0065] In the flexible printed circuit board 20T, the signal wirings WaS1p and WaS1n and the ground pattern WbG form transmission lines. Similarly, the signal wirings WaS2p and WaS2n, the signal wirings WaS3p and WaS3n, and the signal wirings WaS4p and WaS4n form transmission lines with the ground pattern WbG. These transmission lines are configured as differential transmission lines particularly suited to transmitting differential signals. The transmission lines may be, for example, microstrip lines or grounded coplanar lines.

[0066] A plurality of ground terminals and a plurality of signal terminals formed on the lower surface LmS2 of the flexible printed circuit board 20T are connected by solder to a plurality of ground terminals and a plurality of signal terminals formed on the terminal section 10a of the optical module 10T. In this case, the ground terminals of the flexible printed circuit board 20T are connected one-to-one to the ground terminals of the optical module 10T, and the plurality of signal terminals of the flexible printed circuit board 20T are connected one-to-one to the corresponding plurality of signal terminals of the optical module 10T. When connecting the flexible printed circuit board 20T and the optical module 10T, the flexible printed circuit board 20T and the optical module 10T are connected so that the impedances at the connection portion are matched between them in order to reduce signal reflection at the connection portion. Note that, when the flexible printed circuit board 20T is turned upside down, the plurality of ground terminals and a plurality of signal terminals formed on the upper surface LmS1 of the flexible printed circuit board 20T may be connected by solder to the corresponding ground terminals and a plurality of signal terminals formed on the terminal section 10a of the optical module 10T. When the flexible printed circuit board 20T is turned upside down, the order of the differential signal terminals FaS1, FaS2, FaS3, and FaS4 in the X-axis direction will be reversed from before the flexible printed circuit board 20T was turned upside down. However, the order of the differential signals can be appropriately changed so that the multiple signal terminals of the flexible printed circuit board 20T and the multiple signal terminals of the optical module 10T correspond correctly to each other.

[0067] <Details of the substrate 16a> The substrate 16a constituting the terminal section 10a of the optical module 10T will now be described in detail. Fig. 6 is a top view of the terminal section 10a of the optical module 10T according to this embodiment. Specifically, Fig. 6 is a top view of the optical module 10T as viewed from the positive region of the Y axis toward the negative region of the Y axis (or as viewed from the Y axis direction).

[0068] The optical module 10T has a ground terminal TaG1, a signal terminal TaS1p, a signal terminal TaS1n, and a ground terminal TaG2 on the terminal unit 10a, more specifically, on the top surface 16aS of the substrate 16a, in this order from the negative end of the X-axis toward the positive end of the X-axis. The ground terminal TaG1, the signal terminal TaS1p, the signal terminal TaS1n, and the ground terminal TaG2 are formed as part of the wiring layer on the top surface 16aS. The signal terminals TaS1p and TaS1n each extend along the Z-axis direction. The signal terminals TaS1p and TaS1n are sometimes collectively referred to as differential signal terminals TaS1. Note that the configuration of the ground terminals TaG1, TaS1p, TaS1n, and ground terminal TaG2 arranged along the X-axis direction is an example of a GSSG configuration.

[0069] The optical module 10T also has, on the top surface 16aS, signal terminals TaS2p, TaS2n, and ground terminal TaG3, located adjacent to the ground terminal TaG2 and arranged in this order toward the positive side of the X-axis. Each of the signal terminals TaS2p and TaS2n extends along the Z-axis direction. The signal terminals TaS2p and TaS2n are sometimes collectively referred to as differential signal terminals TaS2. The configuration of the ground terminals TaG2, TaS2p, TaS2n, and TaG3 arranged along the X-axis direction is an example of a GSSG configuration. In this way, this GSSG configuration and the above-described GSSG configuration may be configured by sharing the ground terminal (G) at the end. Therefore, for example, the configuration may be expressed as GSSGSSG.

[0070] Furthermore, the optical module 10T has, on the top surface 16aS, signal terminals TaS3p, TaS3n, and ground terminal TaG4, located adjacent to the ground terminal TaG3 and arranged in this order toward the positive side of the X-axis. The signal terminals TaS3p and TaS3n each extend along the Z-axis. The signal terminals TaS3p and TaS3n are sometimes collectively referred to as differential signal terminals TaS3. The configuration of the ground terminals TaG3, TaS3p, TaS3n, and TaG4 arranged along the X-axis is an example of a GSSG configuration. The configuration of the ground terminals TaG3, TaS3p, TaS3n, and TaG4 arranged along the X-axis may have the same shape, spacing, etc. as the configuration of the ground terminals TaG2, TaS2p, TaS2n, and ground terminal TaG3.

[0071] Furthermore, the optical module 10T has, on the top surface 16aS, signal terminals TaS4p, TaS4n, and ground terminal TaG5, located adjacent to the ground terminal TaG4 and in this order toward the positive side of the X axis. The signal terminals TaS4p and TaS4n each extend along the Z axis. The signal terminals TaS4p and TaS4n are sometimes collectively referred to as differential signal terminals TaS4. The configuration of the ground terminal TaG4, signal terminal TaS4p, signal terminal TaS4n, and ground terminal TaG5 arranged along the X axis direction is an example of a GSSG configuration.

[0072] The ground terminals TaG1, TaG2, TaG3, TaG4, and TaG5 each extend along the Z-axis direction on the upper surface 16aS. The length (width) in the X-axis direction, the spacing (pitch) in the X-axis direction, and the length in the Z-axis direction of each ground terminal may be set to the same values. For example, the spacing between the ground terminals TaG1 and TaG2 may be set to the same value as the spacing between the ground terminals TaG2 and TaG3.

[0073] FIG. 7 is a perspective view of the substrate 16a of the optical module 10T according to this embodiment. In FIG. 7, the insulating layer of the substrate 16a is transparent, and the wiring layer and vias are drawn with solid lines. FIG. 8 is an enlarged top view of the terminal portion 10a of the optical module 10T according to this embodiment, showing region A in FIG. 6. Specifically, FIG. 8 is a top view of the terminal portion of the optical module 10T as viewed from the positive region of the Y axis toward the negative region of the Y axis. In FIG. 8, the insulating layer of the substrate 16a is transparent, and the wiring layer is drawn with solid lines, and the vias are drawn with dashed lines.

[0074] Fig. 9 is a cross-sectional view taken along the line II in Fig. 8. Specifically, the cross-sectional view is taken when the terminal portion 10a of the optical module 10T is cut along the line II in Fig. 8 and viewed from the positive region of the X axis toward the negative region of the X axis. In Fig. 9, the insulating layer of the substrate 16a is transparent, and the wiring layer and vias are drawn with solid lines.

[0075] The substrate 16a includes a first wiring layer L1 provided on the upper surface 16aS of the substrate 16a, a second wiring layer L2 including a first insulating layer Li1 between the first wiring layer L1 and the second wiring layer L2, and a third wiring layer L3 including a second insulating layer Li2 between the second wiring layer L2 and the first wiring layer L2. That is, the third wiring layer L3, the second insulating layer Li2, the second wiring layer L2, the first insulating layer Li1, and the first wiring layer L1 are stacked in this order along the Y-axis direction from the negative region of the Y-axis toward the positive region of the Y-axis. The first wiring layer L1, the second wiring layer L2, and the third wiring layer L3 are formed parallel to the XZ plane. The first wiring layer L1, the second wiring layer L2, and the third wiring layer L3 are formed of a conductive metal (e.g., copper foil).

[0076] (1st wiring layer L1) The substrate 16a has ground terminals TaG1, TaG2, TaG3, TaG4, and TaG5 on the first wiring layer L1. The substrate 16a also has signal terminals TaS1p, TaS1n, TaS2p, TaS2n, TaS3p, TaS3n, TaS4p, and TaS4n on the first wiring layer L1. The signal terminals TaS1p and TaS1n are used to transmit one differential signal. FIG. 6 shows an example of an embodiment in which four differential signals are transmitted in parallel. One differential signal or one differential signal wiring is also referred to as a channel. That is, FIG. 6 shows an example of an embodiment in which four channels are handled. When handling only one channel, the first wiring layer L1 is required to have at least the ground terminals TaG1, TaS1p, TaS1n, and TaG2. In this case, the ground terminal TaG1, the signal terminal TaS1p, the signal terminal TaS1n, and the ground terminal TaG2 may be configured to have the above-mentioned GSSG configuration. Furthermore, when handling two channels, the first wiring layer L1 only needs to have at least the ground terminal TaG1, the signal terminal TaS1p, the signal terminal TaS1n, the ground terminal TaG2, the signal terminal TaS2p, the signal terminal TaS2n, and the ground terminal TaG3. In this case, the ground terminal TaG1, the signal terminal TaS1p, the signal terminal TaS1n, the ground terminal TaG2, the signal terminal TaS2p, the signal terminal TaS2n, and the ground terminal TaG3 may be configured to have the above-mentioned GSSGSSG configuration.

[0077] Each of the ground terminal and the signal terminal is formed of a conductive material. Each of the ground terminal and the signal terminal extends along the Z-axis direction. The ground terminal has a width W1 in the X-axis direction (see FIG. 10). The width of the signal terminal, the distance between the pair of signal terminals, and the distance between the signal terminal and the ground terminal are determined so as to form a differential transmission line according to the thickness of the insulating layer in the Y-axis direction, the relative dielectric constant of the insulating layer, and the like. On the other hand, for miniaturization or high-density mounting of the optical module 10T, it is preferable that the length of the terminal portion 10a in the X-axis direction (e.g., equal to the width W of the optical module 10T) be small (see FIG. 6). Therefore, it is preferable that the width W1 of the ground terminal, the width of the signal terminal, the distance between the pair of signal terminals, and the distance between the signal terminal and the ground terminal be small.

[0078] (First insulating layer Li1) The substrate 16a has a first insulating layer Li1 between the first wiring layer L1 and the second wiring layer L2. The first insulating layer Li1 is formed, for example, by molding a green sheet containing a ceramic material and sintering it. The first insulating layer Li1 is made of an insulating material and electrically insulates the first wiring layer L1 from the second wiring layer L2. The thickness t1 of the first insulating layer Li1 is, for example, 100 to 300 micrometers. The substrate 16a is provided with vias penetrating the first insulating layer Li1 to electrically connect the wiring or terminals provided on the first wiring layer L1 to the wiring provided on the second wiring layer L2. The vias will be described later.

[0079] (2nd wiring layer L2) The substrate 16a has a ground pattern TbG1 on the second wiring layer L2 below the ground terminal TaG1 on the first wiring layer L1. Similarly, the substrate 16a has a ground pattern TbG2 below the ground terminal TaG2, a ground pattern TbG3 below the ground terminal TaG3, a ground pattern TbG4 below the ground terminal TaG4, and a ground pattern TbG5 below the ground terminal TaG5. The ground patterns have a width W2 in the X-axis direction (see FIG. 10). The width W1 of the ground terminal is smaller than the width W2 of the ground pattern. For example, as shown in FIG. 10, the ground terminal TaG3 is included in the ground pattern TbG3 in a plan view from the Y-axis direction.

[0080] In the second wiring layer L2, no conductive member such as an electrode is provided below each of the signal terminals TaS1p, TaS1n, TaS2p, TaS2n, TaS3p, TaS3n, TaS4p, and TaS4n. In the regions without conductive members such as electrodes, the first insulating layer Li1 and the second insulating layer Li2 are formed in contact with each other.

[0081] (Second insulating layer Li2) The substrate 16a has a second insulating layer Li2 between the second wiring layer L2 and the third wiring layer L3. The second insulating layer Li2 is formed, for example, by molding a green sheet containing a ceramic material and sintering it. The second insulating layer Li2 is made of an insulating material and electrically insulates the second wiring layer L2 from the third wiring layer L3. The thickness t2 of the second insulating layer Li2 is, for example, 100 to 300 micrometers. Note that vias are provided in the second insulating layer Li2 to electrically connect the wiring provided in the second wiring layer L2 and the wiring provided in the third wiring layer L3. The vias will be described later.

[0082] (Third wiring layer L3) The substrate 16a has a ground pattern TcG in the third wiring layer L3, which extends in the X-axis direction from below the ground terminal TaG1 to below the ground terminal TaG5. Therefore, the ground pattern TcG extends along the Y-axis and X-axis directions. The ground pattern TcG may be a so-called solid pattern (solid ground). In a plan view from the Y-axis direction, the ground terminals TaG1, TaG2, TaG3, TaG4, and TaG5 are included in the ground pattern TcG3.

[0083] (Connection structure between first wiring layer L1 and second wiring layer L2) Since the ground terminals TaG1, TaG2, TaG3, TaG4, and TaG5 have the same configuration, the ground terminal TaG3 will be described as an example. The same applies below. The ground terminal TaG3 is provided between the differential signal terminals TaS2 and TaS3 (see FIG. 6).

[0084] FIG. 10 is a top view of FIG. 8, with center lines and dimensions added for illustrative purposes.

[0085] The substrate 16a has vias Va1, Va2, Va3, and Va4 that electrically connect the ground terminal TaG3 provided on the first wiring layer L1 and the ground pattern TbG3 provided on the second wiring layer L2. The vias Va1, Va2, Va3, and Va4 are formed to penetrate the first insulating layer Li1 along the Y-axis direction. The vias Va1, Va2, Va3, and Va4 are holes provided from the first wiring layer L1 to the second wiring layer L2, and the insides are plated to electrically connect the first wiring layer L1 and the second wiring layer L2. The vias Va1, Va2, Va3, and Va4 are, for example, blind vias. The insides of the vias may be filled with a conductive metal. In a plan view from the Y-axis direction, the centers of the vias Va1, Va2, Va3, and Va4 are aligned on a center line CL1 extending in the Z-axis direction of the ground terminal TaG3 and the ground pattern TbG3.

[0086] The vias Va1, Va2, Va3, and Va4 are arranged at equal intervals pa with their centers along a center line CL1. The center line CL1 is a virtual straight line extending along the Z-axis direction in a plan view from the Y-axis direction. The interval pa is, for example, the distance between the center of the circle of the via Va1 and the center of the circle of the via Va2.

[0087] (Connection structure between second wiring layer L2 and third wiring layer L3) The substrate 16a has vias Vb11, Vb12, Vb13, and Vb14 that connect the ground pattern TbG3 provided on the second wiring layer L2 and the ground pattern TcG provided on the third wiring layer L3. The vias Vb11, Vb12, Vb13, and Vb14 are formed to penetrate the second insulating layer Li2 along the Y-axis direction. The vias Vb11, Vb12, Vb13, and Vb14 are holes provided from the second wiring layer L2 to the third wiring layer L3, and are plated on the inside to electrically connect to the second wiring layer L2 and the third wiring layer L3. The vias Vb11, Vb12, Vb13, and Vb14 are, for example, buried vias. The substrate 16a also has vias Vb21, Vb22, Vb23, and Vb24 that connect the ground pattern TbG3 provided on the second wiring layer L2 and the ground pattern TcG provided on the third wiring layer L3. The vias Vb21, Vb22, Vb23, and Vb24 are formed to penetrate the second insulating layer Li2 along the Y-axis direction. The vias Vb21, Vb22, Vb23, and Vb24 are holes provided from the second wiring layer L2 to the third wiring layer L3, and are plated on the inside to electrically connect to the second wiring layer L2 and the third wiring layer L3. The vias Vb21, Vb22, Vb23, and Vb24 are, for example, buried vias.

[0088] The vias Vb11, Vb12, Vb13, and Vb14 are arranged at equal intervals pb1 with their centers along a center line CL21. The center line CL21 is a virtual straight line extending along the Z-axis direction in a plan view from the Y-axis direction. The interval pb1 is, for example, the distance between the center of the circle of the via Vb12 and the center of the circle of the via Vb13. The center line CL21 is spaced apart from the center line CL1 by an interval d1 on the negative side of the X-axis direction in a plan view from the Y-axis direction (when viewed from the positive side to the negative side in the Y-axis direction). Therefore, the center line CL21 is located between the center line CL1 and the differential signal terminal TaS2.

[0089] The vias Vb21, Vb22, Vb23, and Vb24 are arranged at equal intervals pb2 with their centers along a center line CL22. The center line CL22 is a virtual straight line extending along the Z-axis direction in a plan view from the Y-axis direction. The interval pb2 is, for example, the distance between the center of the circle of the via Vb22 and the center of the circle of the via Vb23. The center line CL22 is spaced apart from the center line CL1 by an interval d2 on the positive side of the X-axis direction in a plan view from the Y-axis direction (when viewed from the positive side to the negative side in the Y-axis direction). Therefore, the center line CL22 is located between the center line CL1 and the differential signal terminal TaS3. The interval d2 may be set to the same length as the interval d1. The vias Vb21, Vb22, Vb23, and Vb24 are offset from the vias Vb11, Vb12, Vb13, and Vb14 by half the distance pb1 or half the distance pb2 along the Z-axis. Note that the distance pb2 may be set to the same length as the distance pb1.

[0090] In the optical module 10T according to this embodiment, the intervals pb1 and pb2 may be set to the same length as the interval pa. It is preferable to make the intervals pa, pb1, and pb2 as small as possible depending on, for example, the accuracy of via drilling and the thickness and strength of the insulating layer. For example, the intervals pa, pb1, and pb2 are set to 200 to 350 micrometers. In the optical module 10T according to this embodiment, the vias Vb21, Vb22, Vb23, and Vb24 are offset from the vias Vb11, Vb12, Vb13, and Vb14, thereby narrowing the apparent interval between the vias provided between the differential signal terminals TaS2 and TaS3.

[0091] For example, if the intervals pa, pb1, and pb2 are 300 micrometers and the via diameter is 75 micrometers, then the interval between vias Vb11 and Vb21 in FIG. 9 is 75 micrometers. For example, the wavelength of an electromagnetic wave with a frequency of 80 gigahertz that propagates through an insulating layer is approximately 1 to 2 mm. Therefore, the apparent intervals between the vias are sufficiently smaller than a quarter wavelength of the electromagnetic wave that propagates through the insulating layer, and therefore vias Vb21, Vb22, Vb23, and Vb24, and vias Vb11, Vb12, Vb13, and Vb14 act as shields between differential signal terminals TaS2 and TaS3.

[0092] On the other hand, if the vias are not offset, the spacing between them will be 225 micrometers, which may result in the vias not acting as a shield, especially at high frequencies. For example, if the differential signal contains frequency components above 80 GHz, crosstalk may occur between differential signal terminals TaS2 and TaS3, degrading the waveform quality of the differential signal.

[0093] <Effects of crosstalk> FIG. 11 shows the results of an electromagnetic field analysis performed to calculate crosstalk between differential signals when using the optical module 10T according to this embodiment. FIG. 11 shows the electric field distribution when differential signals are input to signal terminals provided in region Rtx. Region Rgnd indicates the region where ground terminals are provided. Region Rct indicates the region where signal terminals are provided. Note that no differential signals are input to the signal terminals in region Rct. Although the differential signals input to the signal terminals provided in region Rtx actually propagate to the right along the Z-axis direction in FIG. 11, FIG. 11 only shows the results related to the terminal portion 10a. More specifically, only the results related to vias Vb12, Vb13, Vb21, Vb22, and Vb23 are shown.

[0094] As shown in Figure 11, the electric field generated in region Rtx is shielded by region Rgnd. Therefore, the shade of color represents the strength of the electric field of the differential signal, but the spread of the electric field in the X-axis direction is suppressed by the ground terminal in region Rgnd, and no electric field is observed in region Rct.

[0095] Here, Fig. 12 shows the results of an electromagnetic field analysis of an optical module according to a comparative example, in which vias connecting the ground pattern of the second wiring layer L2 and the ground pattern of the third wiring layer L3 are arranged in a row in the Z-axis direction in the terminal section 10a. More specifically, the two white circles in Fig. 12 represent a via that connects the second wiring layer and the third wiring layer, located directly below via Va2, and a via that connects the second wiring layer and the third wiring layer, located directly below via Va3. The vias Va2 and Va3 that connect the first wiring layer L1 and the second wiring layer are the same as the two black circles in Fig. 11, and are therefore omitted in Fig. 12.

[0096] As shown in Fig. 12, it can be seen that the electric field generated in region Rtx spreads in the X-axis direction, passes through region Rgnd, and leaks out to region Rct. In other words, the results in Fig. 12 indicate that crosstalk occurs between differential signals transmitted through adjacent signal terminals in the optical module according to the comparative example.

[0097] By arranging the vias Vb21, Vb22, Vb23, and Vb24 arranged along the Z-axis direction and the vias Vb11, Vb12, Vb13, and Vb14 arranged along the Z-axis direction with a distance half the interval pb1 from each other along the Z-axis direction, the vias Vb21, Vb22, and Vb12 are arranged so that their centers form an equilateral triangle in a plan view from the Y-axis direction (see FIG. 13A). By arranging the two via rows with a distance half the interval pb1 from each other along the Z-axis direction, the interval pc between the two via rows can be made smaller than the aforementioned interval pa (= interval pb1 = interval pb2). For example, if the two via rows are arranged in the same position in the Z-axis direction without being shifted by half the interval pa in the Z-axis direction, the interval pc between the two via rows can be made equal to the interval pa, but cannot be made smaller than the interval pa (see FIG. 13B). By making the interval pc smaller than the interval pa, the horizontal width W2 of the ground pattern TbG3 of the second wiring layer L2 can be reduced.

[0098] The package 16 of the optical module 10T according to this embodiment can prevent an electric field generated at one signal terminal by a differential signal from leaking from the via of the ground terminal and being transmitted to the differential signal transmitted through the adjacent signal terminal. In other words, the optical module according to this embodiment can suppress crosstalk between signals transmitted through adjacent signal terminals.

[0099] Crosstalk between signals is suppressed by vias in the ground terminals between adjacent signal terminals. For example, by placing vias at a narrow pitch (spacing), crosstalk can be suppressed up to the higher frequency components contained in the signals (shielding effect of the ground terminals). However, the via pitch is subject to package manufacturing constraints. In mass production, if the via pitch cannot be made narrower than the package manufacturing constraints, the shielding effect may be insufficient for signals with very high signal speeds, and crosstalk may increase.

[0100] In the package 16 of the optical module 10T according to this embodiment, the vias Vb21, Vb22, Vb23, and Vb24 arranged along the Z-axis direction and the vias Vb11, Vb12, Vb13, and Vb14 arranged along the Z-axis direction are offset from one another by half the interval pb1 in the Z-axis direction. By offsetting the vias by half the interval pb1, the apparent via pitch when viewed from the X-axis direction can be made smaller than the constraints imposed by package manufacturing. Therefore, the package 16 of the optical module 10T according to this embodiment can suppress crosstalk between signals transmitted adjacent to one another, even for higher-frequency components included in the signals.

[0101] Furthermore, in the package 16 of the optical module 10T according to this embodiment, the vias Va1, Va2, Va3, and Va4 are arranged in a single row at equal intervals along a centerline CL1 whose centers extend in the Z-axis direction. Therefore, according to the package 16 of the optical module 10T according to this embodiment, the width of the ground terminal of the first wiring layer L1 can be set to the minimum value allowed by manufacturing constraints. As a result, for example, in the GSSG configuration of the terminal, the characteristic impedance of the differential signal SS can be set to a desired value, and the distance (spacing) between the GG terminals at both ends can be set to the minimum value allowed by manufacturing constraints. This is advantageous for miniaturizing the optical module 10T because it minimizes the length of the terminal section 10a in the X-axis direction (corresponding to the width W in FIG. 3).

[0102] For example, the width of the ground terminal can be increased to connect the first wiring layer L1 and the second wiring layer L2 with two rows of vias arranged along the Z-axis direction. However, if the two rows of vias are arranged so that each via is at the same position along the Z-axis, the width of the ground terminal will be wider than the width of a ground terminal including only one row of vias by the spacing between the vias. Increasing the width of the ground terminal is difficult when the length of the package 16 in the X-axis direction (see width W in Figure 6) is limited. Furthermore, the shape and spacing of terminal portions such as signal terminals and ground terminals are designed taking into account the characteristic impedance of the transmission line formed by the signal terminals and ground terminals. However, increasing the width of the ground terminal may result in an inability to obtain the desired characteristic impedance.

[0103] According to the package 16 of the optical module 10T of this embodiment, by suppressing an increase in the width W2 of the ground pattern of the second wiring layer, the desired characteristic impedance value can be maintained within a range that minimizes the impact on high-frequency characteristics, and crosstalk between adjacent signals can be suppressed. For example, let Z0 be the differential characteristic impedance when the vias between the first wiring layer L1 and the second wiring layer L2 and the vias between the second wiring layer L2 and the third wiring layer L3 are arranged in a single row (Comparative Example 1). The differential characteristic impedance Z1 of the package 16 of the optical module 10T of this embodiment, in which the vias between the first wiring layer L1 and the second wiring layer L2 are arranged in a single row and the vias between the second wiring layer L2 and the third wiring layer L3 are arranged in two rows, is calculated to be 0.7% smaller than the differential characteristic impedance Z0 (see FIG. 13A for the arrangement of the vias connecting the ground pattern of the second wiring layer L2 and the ground pattern of the third wiring layer). On the other hand, when the vias between the first wiring layer L1 and the second wiring layer L2 and the vias between the second wiring layer L2 and the third wiring layer L3 are arranged in two rows (Comparative Example 2), the differential characteristic impedance Z2 is calculated to be 3.9% smaller than the differential characteristic impedance Z0 (see FIG. 13B for the arrangement of vias connecting the ground pattern of the second wiring layer L2 and the ground pattern of the third wiring layer L3 in this case). In this way, by arranging the two via rows connecting the ground pattern of the second wiring layer L2 and the ground pattern of the third wiring layer L3 so that they are shifted from each other by half the distance pa along the Z-axis direction, the change in the differential characteristic impedance Z2 can be reduced.

[0104] The above results are due to the fact that the width of the ground terminal of the first wiring layer L1 is the same as that of Comparative Example 1, and the width of the ground pattern of the second wiring layer L2 is smaller than that of Comparative Example 2, thereby suppressing an increase in parasitic capacitance between the signal terminal and the ground. Therefore, the package 16 of the optical module 10T according to this embodiment can suppress changes in the value of the characteristic impedance and also suppress crosstalk between adjacent signals being transmitted.

[0105] The Z-axis direction is an example of a first direction, and the X-axis direction is an example of a second direction intersecting the first direction. For example, if the ground terminal TaG3 is taken as the first ground terminal, the differential signal terminal TaS2 is an example of a first signal terminal, and the differential signal terminal TaS3 is an example of a second signal terminal. Furthermore, the ground pattern TbG3 is an example of a first ground pattern, and the ground pattern TcG is an example of a second ground pattern.

[0106] Also, vias Va1, Va2, Va3, and Va4 are examples of first vias, vias Vb11, Vb12, Vb13, and Vb14 are examples of second vias, and vias Vb21, Vb22, Vb23, and Vb24 are examples of third vias. Spacing pa, spacing pb1, and spacing pb2 are examples of first spacings.

[0107] <Modification> Although the optical module 10 according to the present embodiment has been described using the optical module 10T as an example of a TOSA, the optical module 10 is not limited to a TOSA. For example, the optical module 10 may be the optical module 10R as an example of a ROSA. Furthermore, for example, the optical module 10 may be an optical module in which a light source, an optical modulator, and an optical receiver are housed in a single package. In other words, the package 16 can be used as a package for various optical modules that input and output multiple high-speed signals.

[0108] Furthermore, in the optical module 10 according to the present embodiment, the signals transmitted by the signal terminals have been described as differential signals, but the signals transmitted to the signal terminals may be single-ended signals. [Explanation of symbols]

[0109] 1 Optical transceiver 2 Optical Module Assembly 10, 10T, 10R optical modules 10a Terminal part 11 Driver (drive circuit) 12 Transmitting optical element 13 Transimpedance Amplifier 14 Receiving optical element 15 light source 16 packages 16a board 16aS top 16b Mounting board 16c Side wall part 16d Lid 20T, 20R flexible printed circuit board 30 Circuit Board Area A CL1, CL21, CL22 center line d1, d2 interval FaG1, FaG2, FaG3, FaG4, FaG5 ground patterns FaS1, FaS2, FaS3, FaS4 differential signal terminals FaS1n, FaS1p, FaS2n, FaS2p, FaS3n, FaS3p, FaS4n, FaS4p signal terminals FbG1, FbG2, FbG3, FbG4, FbG5 Ground terminals FbS1n, FbS1p, FbS2n, FbS2p, FbS3n, FbS3p, FbS4n, FbS4p signal terminals L1 1st wiring layer L2 2nd wiring layer L3 3rd wiring layer Li1 First insulating layer Li2 second insulating layer LGA insulation coating Lgb insulation coating Lm insulating layer LmS1 top surface LmS2 bottom surface Lb CW light Lr Received optical signal Lt Transmitted optical signal pa, pb1, pb2 interval Rct, Rgnd, Rtx area Ri received signal Rx1 Received signal Rx2 Received signal TaG1, TaG2, TaG3, TaG4, TaG5 Ground terminals TaS1, TaS2, TaS3, TaS4 differential signal terminals TaS1n, TaS1p, TaS2n, TaS2p, TaS3n, TaS3p, TaS4n, TaS4p signal terminals TbG1, TbG2, TbG3, TbG4, TbG5 ground pattern TcG Grand Pattern Td drive signal Tx1, Tx2 transmit signal Va1, Va2, Va3, Va4 vias Vb11, Vb12, Vb13, Vb14 vias Vb21, Vb22, Vb23, Vb24 vias W1, W2 width WaS1n, WaS1p, WaS2n, WaS2p, WaS3n, WaS3p, WaS4n, WaS4p signal wiring WbG ground pattern t1, t2 thickness

Claims

1. a first wiring layer; a second wiring layer including a first insulating layer between the second wiring layer and the first wiring layer; a third wiring layer including a second insulating layer between the third wiring layer and the second wiring layer; a substrate having the first wiring layer includes a first signal terminal extending along a first direction, a second signal terminal disposed along a second direction intersecting the first direction with respect to the first signal terminal and extending along the first direction, and a first ground terminal provided between the first signal terminal and the second signal terminal and extending along the first direction; the second wiring layer includes a first ground pattern that is connected to the first ground terminal through a plurality of first vias that are arranged at a first interval along the first direction and that extends in the first direction; the third wiring layer includes a second ground pattern that is connected to the first ground pattern via a plurality of second vias that are arranged at the first intervals along the first direction and a plurality of third vias that are arranged at the first intervals along the first direction and that extends along the first direction; the plurality of first vias are arranged between the plurality of second vias and the plurality of third vias in the second direction in a plan view from a direction perpendicular to the substrate, the plurality of second vias and the plurality of third vias are arranged to be shifted from each other by a distance of half the first interval in the first direction in a plan view from a direction perpendicular to the substrate; Package for optical module.

2. a distance in the second direction between the plurality of first vias and the plurality of second vias is equal to a distance in the second direction between the plurality of first vias and the plurality of third vias; 2. The optical module package according to claim 1.

3. a width of the first ground terminal in the second direction being smaller than a width of the first ground pattern in the second direction; 2. The optical module package according to claim 1.

4. a center of each of the plurality of first vias is disposed on a center line of the first ground terminal extending in the first direction; 2. The optical module package according to claim 1.

5. a distance in the second direction between the first signal terminal and the center line is equal to a distance in the second direction between the second signal terminal and the center line; 5. The optical module package according to claim 4.

6. An optical module comprising the optical module package according to any one of claims 1 to 5.

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