Optical transceiver, and method and apparatus for manufacturing same

Abstract

Provided are a configuration for realizing an optical transceiver capable of ultra-high-speed operation, a method for manufacturing the same, and a manufacturing apparatus. An optical transceiver according to the present disclosure includes at least one optical module mounted on a PCB, a DSP, and an FPC that connects the DSP and the optical module. The FPC furthermore includes: a first connection PAD on a first surface, the first connection PAD being connected by soldering to a PAD on a terrace surface of the optical module; a second connection PAD on a second surface, the second connection PAD being connected by soldering to a PAD on an upper surface of the DSP substrate; and a first connection PAD on both side surfaces. The FPC and the first connection PAD, and the FPC and the second connection PAD, are each connected by one or more through-holes or an embedded VIA, the through-holes having a diameter of 100 μm or greater.

Description

Optical transceiver, and manufacturing method and device thereof

[0001] The present disclosure relates to an optical transceiver used in optical communications, and a method and apparatus for manufacturing the same.

[0002] In order to meet the increasing demand for communication traffic, high-speed optical modulators and optical receivers compatible with advanced optical modulation methods are required. In the optical modulators and optical receivers used in early 100G digital coherent communication systems, each component was packaged and mounted on a printed circuit board (PCB). For example, in the case of an optical modulator, the driver IC and optical modulator chip were individually packaged, and the packaged ICs and chips were mounted on the PCB. In the case of an optical receiver, the transimpedance amplifier (TIA) and optical receiver chip were individually packaged and mounted on the PCB as separate components.

[0003] In digital coherent communication systems exceeding 400 Gbps, which are experiencing ever-increasing speeds, optical modulators and optical receivers are also required to have wider bandwidths to handle signals of 40 GHz and above, necessitating reduced high-frequency loss and miniaturization. In the case of optical modulators, the driver IC and optical modulator chip are now integrated into a single package as an optical module. In the case of optical receivers, the TIA and optical receiver chip are now integrated into a single optical module. Regarding the signal format for inputting and outputting baseband signals, a shift from single-ended to differential designs has become common as one means of achieving higher speeds, smaller size, and lower power consumption. Currently, device development is underway to achieve 800 Gbps and 1 Tbps (128 GBd operation).

[0004] The Optical Internetworking Forum (OIF) has standardized an optical transmitter in which a driver IC and an optical modulator are mounted in an integrated package under the name High-Bandwidth Coherent Driver Modulator (HB-CDM). Non-Patent Document 1 specifies various types of modules, including their physical configurations and interface specifications. On the receiving side, a TIA and an optical receiver are mounted in an integrated package, also known as an HB-ICR (High-Bandwidth Intradyne Coherent Receiver). An optical transceiver (optical transmitting and receiving device) includes a transmitting (Tx) optical module and a receiving (Rx) optical module.

[0005] Initially, the optical modules described above used surface-mount (SMT) packages, which offer excellent mountability. Optical modules using SMT packages are mounted on the PCB of an optical transceiver, so a via (VIA) structure is essential for passing high-frequency electrical signals inside the package. Because degradation of high-frequency characteristics is unavoidable with VIAs, the VIA structure is unsuitable for further broadening the bandwidth. Furthermore, with SMT packages, electromagnetic field mode mismatch and impedance mismatch at the connection between the lead pins and the ceramic package cause degradation in the high-frequency transmission characteristics of electrical signals (Non-Patent Document 2). Therefore, a new package configuration using a flexible printed circuit (FPC) has been standardized (Non-Patent Document 1).

[0006] In addition to increasing the speed of individual optical modules, optimizing the design of the digital signal processor (DSP) is crucial to achieving overall high-speed optical transceivers. In early optical transceivers, the DSP, transmitter module, and receiver module were mounted in separate packages, each located at a distance on the PCB. This limited the speed of optical transceivers due to propagation losses within each package and on the PCB. One solution being explored is a "co-package structure" in which the optical modulator chip, optical receiver chip, and DSP are closely spaced within a single package. While this structure is ideal for significantly reducing high-frequency electrical signal loss, it places the DSP, the largest heat source, in close proximity to the optical modulator and receiver. When using optical modulators that require temperature control, such as InP modulators, which offer excellent speed performance, the optical modulator chip must be mounted on a Peltier element. Peltier elements have the potential for increased power consumption and thermal runaway, making it difficult to mount the Peltier element in close proximity to the DSP, which is a source of heat inflow, within a single package. The co-package structure has various problems in terms of the mounting process and realization, so another optical transceiver configuration has been proposed (Patent Document 1).

[0007] FIG. 1 is a side cross-sectional view showing the configuration of a conventional optical transceiver 800 adapted for high-speed operation. The cross-sectional view (z-x plane) of the optical transceiver 800 is shown, cut along a line passing through a transmitter module 807 mounted on the board surface (x-y plane) of a PCB 801. The optical transceiver 800 includes a DSP 802, a transmitter module 807, and a receiver module (not shown). The DSP 802 includes a DSP chip 805 mounted on a DSP board 804 using a ball grid array (BGA). The entire DSP 802 is further mounted on the PCB 801 using a BGA 803. The transmitter module 807 includes a driver IC, an optical modulator chip, and other components (not shown) in a terraced package, and is equipped with an optical fiber 808.

[0008] In the optical transceiver 800, the radio frequency (RF) signal connection between the DSP 802 and the optical module 807 is performed via an FPC 806. Using the FPC 806 as an RF interface allows direct connection between the RF signal line formed on the top surface of the DSP board 804 and the RF signal line on the terrace of the optical module. In early optical transceivers including optical modules in SMT packages, the signal path was configured via the VIA on the DSP board 804, the RF signal line pattern on the PCB 801, and the VIA on the package of the optical module 807. The optical transceiver 800 in Figure 1 minimizes high-frequency loss compared to early optical transceivers using SMT packages. Furthermore, the optical modulator options are not limited as in optical transceivers with a co-package structure, making it more feasible to implement. High-frequency loss can also be significantly reduced compared to early optical transceivers using SMT packages.

[0009] However, in the prior art optical transceiver 800 shown in Figure 1, the specific connection configuration between the FPC 806 and the DSP board 804, the structure of the FPC 806, and the connection configuration between the FPC 806 and the optical module 807 are unclear. The configuration of the DSP and DSP board is also unclear. Furthermore, in the transmitter module 807 shown in Figure 1, heat-generating components such as the driver IC inside the package are thermally connected to the PCB side, which has a terrace, and the heat dissipation direction is downward (in the -z direction) from the optical transceiver 800. On the other hand, in the DSP 802, the DSP chip 805 is located on the upper side, so the heat dissipation direction is upward (in the +z direction) from the optical transceiver. Heat dissipation is split into two directions, which is undesirable for simplifying the heat dissipation structure.

[0010] International Publication Number WO2021 / 171599

[0011] OIF, Implementation Agreement for the High Bandwidth Coherent Driver Modulator (HB-CDM), [online], July 15, 2021, [Retrieved January 20, 2023], Internet <URL: https: / / www.oiforum.com / wp-content / uploads / OIF-HB-CDM-02.0.pdf> J. Ozaki et al., "Over-85-GHz-Bandwidth InP-Based Coherent Driver Modulator Capable of 1-Tb / s / λ-Class Operation," in Journal of Lightwave Technology, vol. 41, no. 11, pp. 3290-3296, June 1, 2023, doi: 10.1109 / JLT.2023.3236962.

[0012] The object of the present disclosure is to present a configuration for a smooth high-frequency connection between a DSP and an optical module in an optical transceiver, and to provide a configuration for realizing an optical transceiver capable of ultra-high speed operation, as well as a manufacturing method and apparatus for the same.

[0013] Based on the above-mentioned object, the present disclosure provides an optical transceiver including at least one optical module mounted on a printed circuit board (PCB), a digital signal processor (DSP) mounted on the PCB, and a flexible printed circuit (FPC) connecting the DSP and the optical module, the DSP including a DSP chip mounted on a DSP substrate formed of a multilayer wiring board having a core layer for adjusting thickness, the FPC including a first connection pad on a first surface that is solder-connected to a pad on a terrace surface of the optical module, and a pad on an upper surface of the DSP substrate that is solder-connected to a pad on the terrace surface of the optical module. and a second connecting pad on the second surface, the FPC and the first connecting pad, and the FPC and the second connecting pad are connected by one or more through holes or embedded vias, the FPC is further provided with lands connected to the through holes or embedded vias and formed on the surface of the FPC, and notches having a diameter of 200 μm or more that can be stably held on both side surfaces of the FPC and at positions 500 μm or more away from the inner ends of the first connecting pad and the second connecting pad, and the through holes have a diameter of φ100 μm or more.

[0014] The present disclosure also provides a manufacturing apparatus for manufacturing the above-mentioned optical transceiver, comprising: a hot bar that contacts the FPC and applies heat and load to the solder formed between the first connection pad and the pad on the DSP board, and between the second connection pad and the pad on the terrace; a mounting base that holds the PCB from below; and a holding mechanism that fits into a notch to hold the FPC from the side and below.

[0015] The present disclosure further provides a manufacturing method for the optical transceiver described above, comprising the steps of: mounting a DSP on a PCB; connecting an optical module and an FPC by soldering using a hot bar; placing the optical module connected to the FPC on the PCB; connecting the FPC and the DSP by soldering using a hot bar; and connecting a DC interface between the PCB and the optical module, wherein the FPC and the first connection pad and the FPC and the second connection pad are through-holes. The manufacturing method according to claim 10 further comprises the steps of: performing a pre-soldering process in which solder is formed on at least a land or a heating pad that is directly heated by the hot bar in the soldering using the hot bar; and heating and pressurizing the solder formed by the pre-soldering process with the hot bar; and the solder melted by the heating and pressurizing fills the through-holes. Alternatively, the FPC and the first connecting pad, and the FPC and the second connecting pad are connected by an embedded via, and in soldering using a hot bar, a pre-soldering process is performed to form solder between the first connecting pad and the terrace and between the second connecting pad and the DSP board, and the hot bar is used to apply heat and pressure to the land or heating pad, and the solder formed by the pre-soldering process melts due to heat transfer from the heated and pressurized land or heating pad, thereby forming solder between the first connecting pad and the terrace and between the second connecting pad and the DSP board.

[0016] FIG. 1 is a side cross-sectional view showing a conventional optical transceiver configuration adapted for higher speeds; FIG. 2 is a top view and a cross-sectional view showing an overview of the configuration of an optical transceiver of the present disclosure; FIG. 3 is an enlarged cross-sectional view of an optical module including an FPC connection portion; FIG. 4 is a view showing an FPC wiring layout in an optical transceiver of the present disclosure; FIG. 5 is a flow chart showing a procedure for a manufacturing method 600 of an optical transceiver 100 according to the present disclosure; FIG. 6 is a side cross-sectional view showing the configuration of a mounting apparatus 700 for an optical transceiver 100 according to the present disclosure; FIG. 7 is a diagram showing in detail the structure of a retention mechanism 704 when the optical transceiver 100 according to the present disclosure is mounted, where (a) is a top view of an FPC 500-1 with the retention mechanism 704 and the notch 512 engaged, (b) is a cross-sectional view taken along line VIIbc-VIIbc, and (c) is a cross-sectional view taken along line VIIbc-VIIbc when the retention mechanism 704 has a different configuration. 1A and 1B are diagrams showing the detailed structure of the DSP board 201 of the optical transceiver 100 according to the present disclosure, where (a) is a top view and (b) is a cross-sectional view taken along the Xb-Xb cross-sectional line. The diagram also shows the cross-sectional structure within the Tx module of the optical transceiver according to the present disclosure.

[0017] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. Numerical values ​​are for illustrative purposes and are not intended to limit the technical scope of the present disclosure. Furthermore, the following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of one embodiment of the present disclosure.

[0018] The optical transceiver disclosed herein provides a specific configuration for achieving a smooth high-frequency connection between a DSP and an optical module. The optical module may include an optical receiver module and an optical modulator module. The optical transceiver may include two separate optical modules, or a single optical module integrating the functions of both the optical receiver and the optical modulator. A flexible printed circuit (FPC) is used for the high-frequency connection. Both sides of the FPC electrically connect the pads on the top surface of the DSP substrate to the pads on the optical module package. The connection pads on one side of the FPC are connected to the DSP substrate, and the connection pads on the other side are connected to the optical module. The DSP may have a DSP chip mounted on the DSP substrate, and the entire DSP may be covered by a lid except for the area of ​​the DSP substrate that is connected to the FPC.

[0019] The present invention discloses detailed structures of the connection pad on the DSP board, the shape of the high-frequency wiring, the structure of the multilayer wiring board, and the FPC structure, in order to avoid degradation of the transmission characteristics of high-frequency signals due to impedance mismatch, high-frequency crosstalk, etc. The detailed configuration of the optical transceiver will be described below with reference to the drawings.

[0020] For simplicity, the following description refers to the transmitting optical module containing an optical modulator and its driver IC as the "Tx module," and the receiving optical module containing a photodetector (PD) and TIA as the "Rx module." A single optical module integrating the functions of both a Tx module and an Rx module is referred to as a TRx module. The following description uses an optical transceiver containing an Rx module and a Tx module as an example, but these two modules can also be replaced with a single TRX module. While the following description focuses on the cross section of a Tx module, the connection between the DSP and optical module via FPC remains the same for an Rx module, which has a roughly symmetrical shape to the Tx module. Having similar pad shapes for the FPC between the DSP and Tx module and between the DSP and Rx module is efficient in terms of high-frequency design. It also reduces variations in solder mounting quality and connection strength throughout the optical module.

[0021] [Basic Configuration of Optical Transceiver] Figure 2 shows a top view and a cross-sectional view illustrating the configuration of the optical transceiver of the present disclosure. Figure 2(a) is a top view of the PCB 101, looking at the board surface (x-y plane), and Figure 2(b) is a cross-sectional side view (x-z plane) taken along line IIb-IIb passing through the Tx module 400. The optical transceiver 100 has a DSP 200, an Rx module 300, and a Tx module 400 mounted on the PCB 101.

[0022] Various other electrical and mechanical components, such as chip capacitors and resistors, will be mounted on the top and bottom surfaces of the PCB 101, but these are omitted from Fig. 1. In the optical transceiver 100, the heat dissipation surfaces of the DSP 200, Rx module 300, and Tx module 400 are aligned in one direction, taking into consideration manufacturability when mounting each component on the PCB 101 and simplification of the optical transceiver's heat dissipation structure. Considering that the DSP chip 202 generates the most heat in the optical transceiver, it is desirable to align the heat dissipation direction upward (+z direction), which is appropriate for heat dissipation from the DSP chip 202.

[0023] 2B, in contrast to the configuration of the prior art, the optical transceiver 100 is mounted on the PCB 101 upside down, with the terrace side facing upward in the height direction (z-axis) of the optical module. The optical module is mounted with the terrace side facing the upper surface of the PCB, and in the case of the Tx module 400, heat is transferred from the driver IC and Peltier element toward the terrace side of the package (in the +z direction).

[0024] FIG. 9 is a diagram showing a cross-sectional structure within a Tx module in an optical transceiver according to the present disclosure. The Tx module 400 includes a Peltier element 406, a substrate 407, and an optical modulator chip 409 arranged in this order within a housing 417. Lenses 410 and 411 are provided on the substrate, which are optically coupled to an optical fiber 402. Within the module, a driver IC 408 is formed on the surface of a base 415, and wiring 412 is formed on a package base 416. The optical modulator chip 409 is connected to the wiring 412 via wires 413 and 414. The housing 417, package base 416, and package sidewall 405 separate the interior and exterior of the module. A portion of the package base 416 is exposed from the housing, forming a terrace surface of the optical module. An FPC 500-1 is connected to the terrace surface.

[0025] A Peltier element 406 for temperature control and a base 415 thermally connected to the driver IC, which is a heat-generating component, are fixed to the top surface of the inside of the housing 417. Heat is dissipated from inside the module in the +z direction, with heat moving from the driver IC and Peltier element to the outside. The Rx module also has a TIA instead of a driver IC, and heat from inside the module moves to the upper side of the module (+z direction).

[0026] The optical module is mounted with heat-generating components inside the package so that heat is dissipated toward the bottom surface, opposite the surface mounted on the PCB. Therefore, a heat dissipation mechanism (housing) with a single common heat dissipation surface (not shown) can be used on the upper side of the optical transceiver 100 to dissipate heat from the DSP 200 and the Tx module 400. By arranging the optical module and DSP in close proximity and aligning the top surfaces of the DSP and optical module, a single heat dissipation mechanism can easily dissipate heat from the DSP and optical module. When using a common heat dissipation mechanism, careful consideration must be given to preventing heat from leaking from the DSP to the optical module. Furthermore, to prevent heat from leaking through the heat dissipation mechanism, separate heat dissipation mechanisms on the same surface can be used for the DSP and optical module.

[0027] The DSP 200 is composed of a DSP substrate 201, which is made up of a multilayer wiring substrate (described later), a DSP chip 202, and a lid 203. The DSP chip 202 and the DSP substrate 201 are connected by a BGA 204. Similarly, the DSP substrate 201 and the PCB 101 are connected by a BGA 205.

[0028] The lid 203 is made of a single piece of metal and covers at least the entire top surface of the DSP chip 202 and the top surface of the DSP substrate excluding the areas where the FPCs 500-1 and 500-2 are connected and fixed. The lid 203 may cover the sides of the DSP chip 202 or may extend beyond the periphery of the DSP substrate 201. The lid 203 prevents unexpected direct damage to the DSP chip and the BGAs 204, 204, etc., which are the connection points, due to mishandling during installation of the DSP 200 in an optical transceiver or during the manufacturing process. By carefully designing the shape of the lid 203, the surface of the DSP substrate 201 can be left open from the RF connection pad for connecting to the FPC to the edge of the substrate. In this way, the DSP 200 can be prepared as a DSP module by mounting the DSP chip 202 and lid 203 on the DSP substrate 201. The manufacturing process of the optical transceiver will be described later, but if it is prepared in the form of a DSP module with a lid, it can be safely mounted on PCB 101 without being damaged during the manufacturing process, just like the Rx module and Tx module.

[0029] As another form of DSP 200, it is also possible to prepare the DSP 200 without a lid, with only the DSP chip 202 mounted on the DSP substrate. The lid-less DSP 200 can be connected to the optical modules 300 and 400 with FPCs 500-1 and 500-2, respectively, and then the lid 203 can be attached. When mounting the lid, there is a risk of applying inadvertent force to the FPC or of the lid 203 shorting the FPC wiring. Therefore, it is necessary to thoroughly optimize the position of the pads in the area including the connection pads on the top surface of the DSP substrate 201 and the shape of the lid. Because the DSP chip and DSP substrate are exposed until just before the lid is mounted, extreme care must be taken to avoid damaging the DSP chip and other components.

[0030] When the DSP 200 includes a lid 203, the footprint of the lid 203 on the substrate surface (xy plane) is necessarily smaller than the footprint of the DSP substrate 201. On the other hand, the footprint of the DSP chip 202 is included within the footprint of the lid. On the substrate surface of the DSP substrate 201, the lid 203 is absent only in the area where at least the RF connection pad is located.

[0031] If damage to the DSP chip and BGAs 204 and 205 can be avoided during the manufacturing process of the optical transceiver, the lid can be omitted. In order to effectively dissipate heat from the DSP chip outside the optical transceiver, a lid may be provided only on the top surface of the DSP chip. The shape of the lid 203 can be modified in various ways, and specific variations will be described later.

[0032] The Rx module 300 and the Tx module 400 are each housed in a package. The two optical modules 300, 400 are directly connected to the DSP 200 via FPCs 500-1, 500-2, which serve as an RF interface for electrical signals. Here, "directly connected" means that the connection pads on the terrace of the optical modules are connected to the connection pads on the DSP board only via the FPCs 500-1, 500-2. Therefore, in the optical transceiver 100, there are no high-frequency paths that pass through the vias connecting the optical modules to the PCB, the RF signal lines within the PCB, the BGAs connecting the PCB to the DSP board, or the vias within the DSP board. In contrast to optical transceivers that include conventional SMT-type packaged optical modules, the electrical signals in the optical transceiver 100 do not pass through the vias or BGAs, which can degrade high-frequency characteristics. The DSP and optical module can be connected via the shortest high-frequency path, significantly reducing high-frequency loss. Furthermore, because the number of VIA and BGA connections is reduced, the optical transceiver configuration shown in Figure 2 is also extremely effective in reducing high-frequency loss due to impedance mismatching that occurs at discontinuous parts of the high-frequency path.

[0033] The direct current (DC) interface of the optical modules 300 and 400 can be, for example, DC lead pins 301 and 401 as shown in FIG. 2A. The DC interface is not limited to a lead pin structure, and an FPC may be used, similar to the RF interface with the DSP 200. The DC lead pins 301 and 401 are soldered to pads (not shown) on the PCB 101 so that power can be supplied from outside the optical transceiver.

[0034] Referring again to the top view of the optical transceiver 100 in FIG. 2A, the Rx module 300, the Tx module 400, and the DSP 200 are optimally positioned to minimize loss of the RF signal, a high-frequency electrical signal. The two modules and the DSP 200 are symmetrically positioned about the centerline along the longitudinal direction of the PCB 101. To achieve a high-speed optical transceiver, it is important to minimize high-frequency loss of the RF signal from the DSP 200 to the Tx module and from the Rx module to the DSP 200. In the optical transceiver 100 of FIG. 2, the lengths of the FPCs 500-1 and 500-2, which function as RF interfaces, must be as short as possible. To transmit high-speed signals exceeding 128 GBd, the distance (gap) between the two optical modules in the y-axis direction is preferably 1 mm or less.

[0035] To place two optical modules close to each other, the DC lead pins must be grouped on one side of each module and oriented in opposite directions. As shown in Figure 2(a), the DC lead pins 401 of the Tx module 400 are oriented in the -y-axis direction (downward in the figure) toward the periphery of the PCB 101, while the DC lead pins 301 of the Rx module 300 are oriented in the +y-axis direction (upward in the figure) toward the periphery of the PCB 101. This configuration conforms to the lead pin orientation specified in the HB-CDM standardized by the Optical Fiber Forum (OIF) as shown in Non-Patent Document 1.

[0036] The optical module package is made of ceramic. Considering the layout design of the high-frequency signal lines and DC lines within the optical module, the height from the top surface of the PCB 101 to the terrace surface of the optical module in the cross-sectional view of Figure 2(b) is preferably about 1 to 2 mm. The thickness of the DSP board 201 is also preferably set in the range of about 1 to 2 mm, from the viewpoint of matching the height with the terrace surface of the optical module. By matching the height of the top surface of the DSP board with the height of the terrace surface, bending in the thickness direction of the FCP is unnecessary, as will be described later, and the FCP can be made flat.

[0037] If a DSP board is approximately 1 to 2 mm thick, it will have more layers available than are required for DC and RF lines. Since minimizing the number of layers in a multilayer wiring board is typically desirable from a cost perspective, a core layer can be used internally to adjust the overall thickness of the board. Multilayer wiring boards that include such core layers are sometimes called build-up boards. In addition to adjusting the overall board thickness, the core layer also functions as a separation layer separating the RF wiring layer from the DC wiring layer. The provision of a core layer allows for a significant separation between the DC and RF wiring layers, thereby reducing mutual interference and noise between the wiring layers. The detailed configuration of the DSP board will be described later, along with the detailed configuration of the PAD and signal lines.

[0038] Considering the need for faster DSP operation as described above, it is desirable to house the DSP 200 on a small DSP board 201 to reduce high-frequency signal loss and costs. The two FPCs should be configured to converge on the centerline of the DSP 200 as they move from the optical module side toward the DSP side. As described below, the FPCs shown in Figure 2 are pre-shaped and curved within the plane of the base material (x-y plane). While it is possible to make the FPCs straight without bending, this would require the pads to be arranged in a fan-shaped pattern on the surface of the DSP board 201. This is undesirable because it increases high-frequency loss on the DSP board and increases the size of the DSP board.

[0039] [Connection between DSP and Optical Module in Optical Transceiver] The optical transceiver 100 disclosed herein has one feature in the FPC connection between the DSP and optical module. In the conventional optical transceiver 800 shown in FIG. 1, the FPC 806 is located on the same surface as the board surface of the DSP board 804 and the terrace surface of the optical module 807, which faces the same direction, and is connected only on one side of the FPC 802. In contrast, in the optical transceiver 100 shown in FIG. 2(a), the FPC 500-1 is located on the same surface as the board surface of the DSP board 201 and the terrace surface of the Tx module 400, which faces the opposite direction, and is connected on a different side of the FPC 500-1. The optical module 400 is mounted on the PCB 101 upside down, as compared to the conventional technique, so that the terrace surface with the connection pad for the FPC faces the PCB 101.

[0040] FIG. 3 is an enlarged cross-sectional view of an optical module including an FPC connection. This is an enlarged side cross-section (x-z plane) of the FPC 500-1 and its vicinity in FIG. 2(b), with the height direction (z-axis direction) enlarged. Please note that the relative sizes of the various components are not accurately depicted to facilitate understanding of the configuration near the FPC. FIGS. 3 and 2(b) are cross-sectional views taken perpendicular to the PCB surface through line IIb-IIb in FIG. 2(a). Although the cross-section crosses the signal lines of the bent FPC 500-1, the FPC is actually shown along the signal lines. The optical transceiver 100 disclosed herein employs a configuration that minimizes FPC bending to reduce high-frequency loss caused by bending the FPC in the thickness direction (z-axis direction).

[0041] FIG. 3 shows an FPC 500-1 connecting a connection pad 210 at the end of the DSP board 201 to a connection pad 403 on the terrace surface of the Tx module 400. The FPC 500-1 has metal layers formed on both sides of a base material 501, with a signal line 502 on the upper side of the figure and a ground (GND) surface 503 on the opposite side. A PAD 504 and a PAD 507 for connecting to the optical module are formed on both ends of the signal line 502. Also on the GND surface are a PAD 505 for connecting to the DSP, connected to the PAD 504 via a VIA or through-hole 508, and a PAD 506, connected to the connection pad 507 via a VIA or through-hole 509. As will be described later, by heating the PAD 504, the connection pad 505 and the connection pad 210 are connected by solder 206. Similarly, by heating the PAD 506 , the connection PAD 507 and the connection PAD 403 are connected by the solder 206 .

[0042] In the optical transceiver 100 according to the present disclosure, the height difference ΔH between the terrace surface of the Tx module and the top surface of the DSP substrate 201 is set to 500 μm or less to minimize bending of the FPC in the thickness direction. A height difference of approximately 500 μm eliminates the need to bend the FPC significantly in the thickness direction, allowing for a nearly flat connection. This reduces the high-frequency loss and risk of cracking the metal wiring on the FPC due to significant bending of the FPC, enabling a good high-frequency connection.

[0043] Generally, the thinner the FPC base material 501, the easier it is to handle, allowing for flexible adjustment of its height during the mounting process. On the other hand, a thicker base material is preferable to minimize loss in terms of high-frequency characteristics. Taking these factors into consideration, the thickness of the base material 501 is preferably 50 μm or greater. Specifically, if the base material 501 is approximately 50 to 100 μm thick, even if the height difference ΔH between the terrace surface and the DSP substrate surface is approximately 500 μm, mounting is easy by slightly adjusting the FPC's thickness. Setting ΔH to approximately 500 μm also minimizes damage to the solder joints between the FPC and the optical module during the mounting process. If the solder 206, 404 at both ends of the FPC becomes too thick, the size of the conductor portion of the transmission line may change, potentially reducing the characteristic impedance of the signal line. The solder thickness between the pads is preferably 50 μm or less.

[0044] As mentioned above, the shorter the length of the FPC 500-1, the better its high-frequency characteristics. In practice, the influence of heat from the DSP chip 202, the largest heat source in an optical transceiver, must be considered. Placing the optical modules 300 and 400 and the DSP chip 202 too close together can result in the optical module becoming extremely hot. If the optical module incorporates a Peltier element, there is a risk that the Peltier element's power consumption will increase significantly or that the Peltier element will become inoperable, resulting in thermal runaway. Furthermore, if the FPC is too short, problems may arise during the process of connecting the FPC to the DSP board, such as the lack of a support for the FPC or the inability to secure a sufficient length for bending the FPC. From the perspectives of both thermal design and ease of mounting, it is preferable that the FPC be at least 3 mm long.

[0045] [Outline of Optical Transceiver Assembly Procedure] The following is an outline of the assembly procedure for the optical transceiver 100. The Rx module 300 and Tx module 400 are pre-soldered onto their respective terrace surfaces. First, the DSP 200 is mounted on the PCB 101 by reflow soldering, and then the optical modules 300 and 400 are mounted on the PCB 101. The optical module and DSP are mounted in separate processes because it is difficult to put the optical module through a normal reflow soldering process due to heat resistance limitations and other factors.

[0046] The optical modules 300 and 400 are first assembled by soldering their respective FPCs to the connection pads 210 of the DSP 200. Then, the DC lead pins 301 and 401 of the optical modules are fixed to the PCB 101. Because the FPC of the optical module is an RF interface, the size of the connection pad 505 is limited for impedance matching. Because high-frequency transmission characteristics and connection strength are affected by misalignment between the opposing pads via solder, mounting of the optical module with an FPC requires extremely high precision. Meanwhile, since the lead pins only need to be reliably connected in terms of DC current, the DC pad on the PCB side can be made larger, allowing for a relatively larger tolerance for misalignment. Therefore, the optical module is first connected from the FPC side, and then the lead pins are connected.

[0047] To reduce the number of steps required for mounting an FPC to a DSP 200 already mounted on a PCB, it is desirable to heat and solder the Tx module and Rx module together using a hot bar. Mounting using a hot bar requires pre-soldering of at least one of the FPC's connection pad 505 or the connection pad 210 on the DSP board. To melt the pre-solder between the connection pads using a hot bar and establish a solder connection, at least one via (VIA) or through-hole 508 is required, as shown in Figure 3. By pressing the hot bar against the heating pad 504 from above in Figure 3, the through-hole 508 transfers heat from the heating pad 504 to the solder 206 between the connection pads 505 and 210. Alternatively, the VIA 508 can be replaced with a through-hole. Here, a through-hole refers to a hole that penetrates a board and has all of its inner surfaces metallized. In the case of through holes, solder can be flowed through the holes, so it is sufficient that at least one of the FPC connection pad 505, heating pad 504, or connection pad 210 on the DSP board is pre-soldered.

[0048] The DC lead pins 301 and 401 of the optical module are DC interfaces, and as long as they are electrically connected, some misalignment is acceptable. The pad size on the PCB can also be set wide enough relative to the lead pins, and high-precision alignment and mounting techniques like those required on the FPC are not required. While lead pins cannot be bent after being fixed, FPCs can be bent slightly. Therefore, after the FPC is fixed, it is possible to fine-tune the FPC to connect and fix the DC lead pins.

[0049] [FPC Wiring Layout] This section describes the specific configuration of the FPC in the optical transceiver 100 of the present disclosure. As shown in FIG. 3 , the FPC has a base material 501 with metal layers on both sides. One side has a signal line 502 for the high-frequency transmission path, and the other side has a GND surface 503 for the signal line. The FPC is preferably configured so that the side with the signal line 502 faces the terrace surface of the Tx module 400. Conversely, if the FPC is configured so that the signal line 502 faces the substrate surface of the PCB 101 below, there is a possibility that the metal wiring pattern on the PCB and the signal line 502 on the FPC will interfere with each other. This interference could result in degradation of the high-frequency transmission characteristics of electrical signals input / output between the DSP 200 and the optical module, or in mutual noise contamination between the electrical signals and the DC signals on the PCB. If the GND surface 503 of the FPC 500-1 faces the PCB 101, as shown in FIG. 3 , the above-mentioned interference problem would not occur. However, if there are no electrical wiring or other components on PCB 101 or if there is a sufficient distance between the PCB and FPC, the signal line surface and GND surface of the FPC may be upside down compared to FIG.

[0050] FIG. 4 illustrates the FPC wiring layout of an optical transceiver disclosed herein. (a) of FIG. 4 illustrates the first surface (x-y plane) on which the signal lines are located, and (b) of FIG. 4 illustrates the second surface (x-y plane) opposite the first surface, which includes the GND surface. In FIG. 4, the FPC coverlay and resist are omitted to make the layout easier to see. Two pairs of GSSG differential signal lines 502a and 502b are shown on the first surface of the FPC 500a in FIG. 4(a) as an example. Here, G represents ground and S represents signal. The number of signal line pairs may vary depending on the type of optical module. Furthermore, the area of ​​the GND side pad is larger than that of the signal line pad. While a smaller pad size is desirable for the signal line pad from the perspective of high-frequency characteristics, a sufficient solder connection area is required to ensure the solder connection strength. Therefore, the GND side pad is made sufficiently wider than the signal line pad to ensure the solder connection strength.

[0051] The maximum area of ​​the GND pad between each channel is uniquely determined by the inter-channel pitch of the signal line pads on the FPC and DSP board. If the inter-channel pitch is narrow and sufficient area for the GND pad cannot be secured, it is effective to expand the size of the GND pad at both ends of the FPC's width direction (y direction). To prevent short circuits if solder flows outside the soldered PDA area, areas of the FPC where soldering is not required are covered with resist. Note that the resist defines the soldered PAD area, thereby defining the GND pad on the GND surface as well. Therefore, multiple PADs are placed at the end of the FPC in a GSSG differential format. Furthermore, because applying resist to the signal lines increases high-frequency loss, care must be taken to minimize the area of ​​the resist.

[0052] 4B, a GND surface 503 for signal lines 502a and 502b is formed on the second surface side of the lower surface of the FPC 500a. As shown in the cross-sectional view of the FPC connection portion in FIG. 3, the first surface of the FPC, on which the signal line 502 is located, is connected to a connection pad on the terrace surface of the optical module 400, and the second surface of the FPC, on which the GND surface 503 is located, is connected to a connection pad on the upper surface of the DSP board 201. The first and second surfaces of the FPC are each used for RF interfaces. Therefore, the VIA 508 included in the path of the signal line 502 not only serves the function of fixing by soldering, but also plays an important role as part of the signal path for transmitting high-frequency signals.

[0053] To transmit high-frequency signals without loss, if vias are not located at the edges of the pads, a portion of the pad away from the vias will form an open stub, adding capacitive impedance to the signal line. To prevent this, as shown in Figure 4, half-through holes 510 are provided at the very end of the FPC 500a for signal lines 502a and 502b. If the pad length is sufficiently short and impedance changes due to stub formation are not a problem within the operating frequency band, half-through holes are not necessary. Similarly, half-through holes 511 are provided at the very end of the FPC 500a to stabilize the high-frequency potential on the GND surface. Half-through holes 510 and 511 not only improve high-frequency characteristics but also strengthen solder connection strength by forming solder fillets on the connecting pads. Regarding half-through holes 510 for signal line pads, if the diameter of the half-through holes is too large and the surrounding land area is large, the capacitive nature of the pads increases, leading to lower impedance and degraded high-frequency characteristics of the connecting pads. In order to prevent deterioration of the high frequency transmission characteristics of the signal line, it is desirable to set the diameter φ of the half through hole 510 to 150 μm or less.

[0054] The configuration of the connection pads and other components of the DSP board 101 will be described later. From the perspective of forming fillets at the FPC end, it is desirable to mount the end of the signal line pad on the FPC and the end of the signal line pad on the DSP board with a displacement of approximately 100 μm. This corresponds to the displacement of the end of the connection pad 210 on the DSP board in the -y direction compared to the end of the connection pad 505 in Figure 3. By setting the size and positional relationship of the connection pads 505 and 210 in this manner, a good fillet is formed in the solder 206 at the connection point, improving the strength quality of the solder. Specifically, the connection pads 505 and 210 can be mounted with the same length, and the FPC position can be adjusted so that the edge positions of the two pads are offset in the longitudinal direction (x direction). 3, one edge of the connection pads 505 and 210 can be aligned, and the length of the connection pad 505 on the FPC side can be made about 100 μm shorter than the connection pad 210 on the DSP board 201. The half-through hole 511 at the end of the GND surface does not have a significant effect on the high-frequency characteristics, so it can be a half-through hole with a larger diameter depending on the required connection strength.

[0055] As shown in FIG. 4 , the FPCs 500-1 and 500-2 according to the present disclosure further include notches 512 formed on both sides of the base material 501. These notches 512 have a shape similar to the half-through holes 510 and 511 described above, but their function is different. They function as a mating portion for a retention mechanism that aligns the FPCs 500-1 and 500-2 during mounting and holds the FPCs 500-1 and 500-2 when heated and pressurized by a hot bar. To prevent heat transfer when mated with the retention mechanism, the notches of the FPCs 500-1 and 500-2 are preferably separated from the metal that forms the GND and are free of metal. From a similar perspective, it is preferable that the retention mechanism itself be made of a material that does not transfer heat, such as resin, rather than metal. This prevents unnecessary heat absorption in areas other than the pads, allowing heat to be concentrated at the pad junctions. The details of the holding mechanism will be described later with reference to FIGS.

[0056] [Details of Optical Transceiver Assembly Procedure] Figure 5 is a flow diagram showing the steps of a manufacturing method 600 for the optical transceiver 100 according to the present disclosure. As shown in Figure 5, the manufacturing method 600 includes mounting components other than the optical modules 300 and 400 (e.g., the DSP 200 and other capacitors (not shown)) on the PCB 101 (S601), connecting the optical modules 300 and 400 to the FPCs 500-1 and 500-2 by soldering using a hot bar (S602), placing the optical modules 300 and 400 to which the FPCs 500-1 and 500-2 are connected on the PCB 101, and connecting the FPCs 500-1 and 500-2 to the DSP 200 by soldering using a hot bar (S603), and connecting the PCB 101 and the optical modules 300 and 400 via the DC lead pins 301 and 401 (S604).

[0057] In S601, all elements other than the optical modules 300 and 400 are generally mounted on the PCB together by reflow soldering. The reason for mounting the optical modules 300 and 400 in a separate process is that, as mentioned above, it is difficult to put the optical modules through a normal reflow soldering process due to limitations on heat resistance, etc.

[0058] In S602, the FPC 500-1 and the Tx module, and the FPC 500-2 and the Rx module 300 are connected by soldering using a hot bar. Details of soldering using a hot bar will be described later using S603 (connection of the FPCs 500-1 and 500-2 to the DSP board 201) as an example.

[0059] When mounting the FPCs 500-1 and 500-2 in S603, it is desirable to solder both the FPCs 500-1 and 500-2 together using a hot bar in order to reduce the number of mounting steps and minimize the risk of thermal effects (thermal load) on the DSP 200 and damage.

[0060] It is desirable to fix the DC side interface of the optical module in S604 after connecting the RF interface side in S603. This is because, from the perspective of impedance matching, the design margin for the pad size at the high-frequency connection point is narrow, and it is necessary to use a very small width, for example, 100 to 200 μm. Therefore, even a slight misalignment can result in an insufficient connection or the bonding strength cannot be guaranteed. Furthermore, an unexpected solder connection shape can occur, which can lead to impedance mismatch or degradation of high-frequency characteristics, so extremely high-precision and precise mounting is required.

[0061] [Method of soldering using a hot bar] Below, the method of soldering using a hot bar in implementing an optical transceiver according to the present disclosure will be explained in detail, using the example of connecting FPCs 500-1, 500-2 to DSP board 201 (corresponding to S603 in Figure 5).

[0062] In order to mount the FPCs using a hot bar, it is not possible to heat only the connection surfaces between the FPCs 500-1 and 500-2 and the DSP board 201. Therefore, heat must be applied from the top surfaces of the FPCs 500-1 and 500-2 while applying pressure with the hot bar. Therefore, the FPCs 500-1 and 500-2 must be structured so that heat applied from the top surface is transferred to the connection portions with the DSP board 201. Therefore, as will be described in detail later, the FPCs 500-1 and 500-2 of the optical transceiver according to the present disclosure are formed with metal patterns on both sides of the base material, and have a heating pad 504 on the top surface and an RF connection pad 505 on the bottom surface. Furthermore, at least one through-hole or buried via 508 is formed in each pad. The through hole or buried VIA has a land, and soldering is performed between the RF connection PAD 505 and the RF connection PAD 210 by transmitting molten heat or solder (only in the case of a through hole) on the DSP board 201 side via the metal of the land and the through hole or buried VIA 508.

[0063] To effectively transfer heat applied from the top surface of the FPCs 500-1 and 500-2, the more through holes or buried vias there are and the larger the heating pad area, the better. However, increasing the number of through holes or buried vias and the area of ​​the pads increases the capacitance due to the structure, leading to lower impedance and degradation of high-frequency characteristics at the solder joints. For this reason, the heating pads may also serve as lands for forming vias or through holes. This configuration can suppress the increase in capacitance. Furthermore, since the land size increases as the diameter of the through holes or buried vias increases, it is desirable to keep the diameter of the through holes or buried vias as small as possible. However, it should be noted that through holes with a diameter of less than 100 μm significantly reduce solder flow, potentially preventing stable solder mounting. On the other hand, buried vias do not require solder flow, so they can be set to any desired size.

[0064] For mounting using a hot bar, it is necessary to perform pre-soldering in advance on at least one of the heating pads of the FPCs 500-1 and 500-2 and the RF connection pad of the DSP board 201. The method for this pre-soldering differs depending on whether the above-mentioned 508 is a through-hole or a buried via.

[0065] If 508 is a through-hole, the hollow through-hole makes it difficult to efficiently transfer heat applied from the hot bar to the DSP board 201. Therefore, solder is pre-formed on the heating pad 504 using solder paste or the like (pre-soldering), and this pre-formed solder is then heated and pressurized with the hot bar. This melts the pre-formed solder simultaneously with heating, and flows through the through-hole 508 to the connection, forming the solder 206. However, from the perspective of flowing the solder through the through-hole 508, it is desirable for the diameter of the through-hole 508 to be φ100 μm or greater. Alternatively, a method is also useful in which the solder paste is applied, and then the solder portion is heat-treated to fill the through-hole portion in advance during the pre-soldering process. This pre-soldering process may also be additionally performed on the RF connection pad 210 on the DSP board 201. This allows the solder to better settle and ensures stable solder mounting.

[0066] On the other hand, if 508 is an embedded via, pre-soldering on the heating pad 504 would not allow the molten solder to flow. In this case, pre-soldering is performed on the RF connection pad 210 on the DSP board 201 or on the RF connection pad 505 on the FPCs 500-1 and 500-2. When the heating pad 504 is subsequently heated and pressurized with a hot bar, since the via 508 is made of metal, heat is applied to the solder formed by the pre-soldering through the via 508, forming solder 206. Note that pre-soldering only needs to be performed on at least one of the RF connection pad 210 or the RF connection pad 505. In the case of an embedded via, there is no need to allow solder to flow through the hole, so the diameter does not necessarily need to be 100 μm or larger like a through-hole. However, it should be noted that if the diameter is too small, the amount of heat transfer decreases. It is desirable to set an appropriate buried via diameter from the perspective of both impedance and heat. For example, considering the flow of heat, a diameter of 50 μm or more is necessary. Note that due to the nature of the general FPC manufacturing process, it is impossible to combine buried vias and through holes, so only one of them is always formed.

[0067] As mentioned above, whether 508 is a through-hole or a buried via, the formation of solder 206 using the hot bar is performed via the FPCs 500-1 and 500-2. Therefore, the hot bar temperature must be set higher than the melting temperature of the solder being used, taking into account the insulating effect of the FPCs 500-1 and 500-2. For example, if the FPCs 500-1 and 500-2 are 50 μm thick, the hot bar temperature must be set at least 50°C higher than the melting temperature. However, if the hot bar temperature is set too high, excessive heat will be transferred to the DSP chip 202, potentially damaging it. For this reason, it is recommended to use a thermocouple or similar device and monitor the solder temperature with a small thermistor before soldering.

[0068] The solder material may be a SnAgCu-based lead-free solder, which is commonly used in optical devices, but from the standpoint of suppressing the thermal impact on the DSP chip 202 and ensuring mounting tolerance, a low-temperature solder with a melting point of 184°C or less, such as a Sn- or Bi-based solder, may also be used.

[0069] As described above, in the manufacture of optical transceivers according to the present disclosure, it is desirable to simultaneously connect FPC 500-1 and FPC 500-2 together using a single hot bar. Based on this premise, the load applied by the hot bar needs to be 10 N or more.

[0070] Furthermore, assuming the Tx module 400 is HB-CDM and the Rx module 300 is HB-ICR, mounted with a 1 mm gap between them, the distance from the edge of FPC 500-1 to the edge of FPC 500-2 is expected to be approximately 20-30 mm. Therefore, a hot bar length of 20-30 mm is considered appropriate. Generally, the length must be set taking into account the temperature distribution of the hot bar. For example, if heat is applied from the center of the hot bar, the temperature near the center will be higher and the temperature near the sides will be lower. In this case, it is desirable to make the hot bar as long as possible so that heating can occur near the center of the hot bar. However, if the hot bar's Y-direction length is excessively long, it may interfere with other elements or components (e.g., the lid 203). Therefore, the Y-direction length of the hot bar must be set to avoid interference with these elements or components.

[0071] On the other hand, from the viewpoint of ensuring high frequency, it is desirable that the size of the RF connection PAD 505 and the RF connection PAD 210 be 1 mm or less in length and 0.2 mm or less in width, because if the capacitance becomes too large, high frequency signals will not pass through. Also, so that the hot bar can efficiently heat the area where solder 206 is to be formed, it is desirable that the width of the hot bar (length in the X direction) be approximately the same as or slightly smaller than the size of the RF connection PAD 505 and the RF connection PAD 210 (for example, approximately 200 to 300 μm smaller than the pad size).

[0072] 6 is a side cross-sectional view showing the configuration of an optical transceiver mounting apparatus 700 according to the present disclosure. As shown in FIG. 6, the optical transceiver mounting apparatus 700 according to the present disclosure includes a hot bar 701 for applying heat and load to form solder 206 between RF connection PAD 505 and RF connection PAD 210, a mounting base 702 for holding PCB 101 during mounting, a lower support structure 703 disposed between the underside of PCB 101 and mounting base 702 for supporting from below the area where pressure is applied by hot bar 701, and a holding mechanism 704 for realizing high-precision alignment of FPCs 500-1 and 500-2 during mounting and for holding FPCs 500-1 and 500-2 when heating and pressurizing FPCs 500-1 and 500-2 using hot bar 701.

[0073] In the optical transceiver manufacturing method 600 shown in FIG. 6 , heat applied by the hot bar 701 needs to be efficiently transferred to the RF connecting PAD 505 and the RF connecting PAD 210. More specifically, the mounting device 700 is configured so that the heat applied by the hot bar 701 is intensively transferred to the RF connecting PAD 505 and the RF connecting PAD 210 and is prevented from being conducted to other elements or dissipated to the outside. As an example of such a configuration, as shown in FIG. 7 , the mounting device 700 may be configured so that a hollow space is formed between the mounting base 702 and the PCB 101. By providing a hollow space between the mounting base 702 and the PCB 101 of the mounting device 700, heat conduction is blocked in the hollow space, and heat is intensively transferred to the RF connecting PAD 505 and the RF connecting PAD 210. However, if the space between the mounting base 702 and the PCB 101 is hollow, deformation (warping) of the PCB 101 occurs when a load is applied by the hot bar 701, and the load applied by the hot bar 701 is not sufficiently applied to the areas where the above-mentioned pre-soldering process has been performed, or to the RF connection pad 505 and the RF connection pad 210. Therefore, by providing the lower support structure 703 and creating a structure that receives the load applied from the hot bar 701, it becomes possible to apply an appropriate load to the pre-soldering process areas on the heating pad, the RF connection pad 505 and the RF connection pad 210. It is desirable that the lower support structure 703 be formed from a material with low thermal conductivity (e.g., resin).

[0074] As another example, the optical transceiver mounting device according to the present disclosure may be configured so that the mounting base 702 holds the PCB 101 while contacting the entire lower surface of the PCB 101. In this case, since there is no hollow portion as described above, the PCB 1010 will not deform when a load is applied. Therefore, the lower support structure 703 is not required. However, in order to efficiently apply heat to the RF connection pad 505 and the RF connection pad 210, the mounting base 702 needs to be made of a highly insulating material (e.g., resin).

[0075] The holding mechanism 704 must be positioned near the contact area of ​​the hot bar 701 to facilitate alignment and prevent movement of the FPCs 500-1 and 500-2 during heating and pressure application. Furthermore, attempting to hold the FPCs 500-1 and 500-2 on their flat surfaces results in unstable holding positions. Therefore, as shown in FIG. 4A, the holding mechanism 704 is preferably a round bar structure that fits into the notches 512 formed on both sides of the base material 501 of the FPCs 500-1 and 500-2. If the diameter of the notches 512 is too small, the diameter of the matching round bar structure becomes too small, resulting in unstable holding and making positioning extremely difficult. Therefore, the notches 512 must have a diameter of at least 300 μm to ensure stable holding. 7(b) and (c), the holding mechanism 704 has a mechanism that supports only the base material 501 of the FPCs 500-1 and 500-2 on the sides and bottom. This configuration enables stable mounting. Based on the same concept as the device base described above, it is desirable that the holding mechanism 704 be formed from a highly insulating material (e.g., resin). However, as shown in FIG. 4(c), if the space between the bottom surface of the base material 501 and the holding mechanism 704 is hollow and the notch 512 is formed solely from a dielectric, the holding mechanism 704 may be formed from a highly conductive material such as metal.

[0076] By fixing the notch 512 using a rod-shaped holding mechanism 704, it becomes possible to hold the FPCs 500-1 and 500-2 without moving even when they are heated and pressurized by the hot bar 701. Also, to prevent the holding mechanism 704 from interfering with the surface of the DSP board 201 or the hot bar 701, it is desirable to position the notch 512 at a position 500 μm or more away from the inner edge of the RF connection pad 505.

[0077] [DSP Configuration for Realizing the Mounting Method] Figure 8 shows the detailed structure of the DSP board 201 of the optical transceiver 100 according to the present disclosure, with (a) showing a top view and (b) showing a cross-sectional view taken along the Xb-Xb line. In the solder mounting using the hot bar 701 described above, to reduce the risk of deformation or damage to the DSP board 201 due to the load applied by the hot bar 701, the DSP board 201 must have a thickness of 1 mm or more (e.g., 1 to 2 mm). To achieve a thickness of 1 mm or more, the DSP board 201 typically requires the use of a core layer. This is because, considering high-frequency characteristics, etc., it is desirable for the vias formed in the DSP board 201 to be φ100 μm or less. In this case, the thickness of the build-up board that can be used is 100 μm or less. To achieve a thickness of 1 mm or more for the DSP board 201, more layers are required than those required for RF wiring and DC wiring, which is disadvantageous in terms of cost and other factors.

[0078] As shown in FIG. 8( b), the DSP board 201 not only has a core layer inserted to increase its thickness to 1 mm or more, but also has second vias 215 disposed in the core layer below the pads. This configuration reinforces the DSP board 201 and reduces deformation and damage to the DSP board 201 when pressure is applied with a hot bar. Suppressing deformation (e.g., warpage) is also effective in ensuring uniform load application (reducing uneven contact) during solder mounting. Because the second vias 215 do not affect high-frequency characteristics, they may be significantly larger than other embedded vias, e.g., φ200 μm or larger. As shown in FIG. 8( b), it is desirable to form at least one second via 215 below each of the signal and ground pads formed on the top surface of the DSP board 201.

[0079] 8A , in order to efficiently apply heat to the RF connecting PAD 505 and the RF connecting PAD 210 and to prevent excessive heat from flowing to the DSP chip 202 and damaging the BGA 204, etc., it is desirable to include a thermal isolation portion 231 in the ground of the RF connecting PAD 210. This thermal isolation portion 231 has a structure in which the surface metal of the RF connecting PAD 210 is separated into a portion where the solder 206 is formed and other portions (only the thermal isolation portion 231 has no surface metal). By providing such a thermal isolation portion 231, it is possible to prevent heat applied from the hot bar 701 from propagating directly to the DSP chip 202, thereby preventing the BGA 204 and the DSP chip 202 from being damaged by heat. From a similar perspective, with regard to the signal of the RF connection PAD 210, only the portion where the solder 206 is formed is provided on the surface layer of the DSP board 201, and for the portion thereafter, a VIA is provided and wiring is provided on the inner layer of the DSP board 201. Also, from the perspective of high-frequency wiring, providing a groove for thermal isolation in the GND metal portion means that if that portion is used as surface wiring, there will be no GND metal in that portion, which will result in impedance mismatch and degradation of high-frequency characteristics. Therefore, from the perspective of ensuring high-frequency characteristics, it is desirable to use a VIA in this groove or closer to the PAD than this groove to provide inner-layer wiring.

[0080] Although the above description uses an example in which the Tx module and Rx module are packaged separately, the optical transceiver configuration according to the present disclosure is also effective for optical transmitter / receiver modules in which the optical modulator and optical receiver are packaged together (such as IC-TROSA) or other package formats.

[0081] The optical transceiver according to the present disclosure can be used for optical communications.

[0082] 100 Optical transceiver 101 DSP board 200 DSP 201 DSP board 202 DSP chip 203 Lid 206 Solder 210 RF connection pad 215 Second VIA 231 Thermal isolation section 300 Rx module (optical module) 301 DC lead pin 400 Tx module (optical module) 401 DC lead pin 402 Optical fiber 403 RF connection pad 404 Solder 405 Package side wall 406 Peltier element 407 Substrate 409 Optical modulator chip 410 Lens 411 Lens 412 Wiring 413 Wire 414 Wire 415 Base 416 Package base 417 Housing 501 Base material 502 Signal line 502a, b Signal line 503 GND surface 504 Heating pad 505 RF connection pad 506 Heating pad 507 RF connection pad 508 VIA or through hole 509 VIA or through hole 510 Half through hole 511 Half through hole 512 Notch 600 Manufacturing method 701 Hot bar 702 Mounting base 703 Lower support structure 704 Holding mechanism 800 Optical transceiver 802 DSP board 804 DSP board 805 DSP chip 807 Transmission module 808 Optical fiber

Claims

1. (delete)

2. (delete)

3. (delete)

4. (delete)

5. (delete)

6. (delete)

7. (delete)

8. (delete)

9. It is an optical transceiver, At least one optical module mounted on a printed circuit board (PCB), A digital signal processor (DSP) mounted on the aforementioned PCB, A flexible printed circuit board (FPC) connects the DSP and the optical module. Equipped with, The DSP comprises a DSP chip mounted on a DSP substrate formed of a multilayer wiring board having a core layer for adjusting the thickness. The FPC includes a first connecting pad on a first surface which is soldered to a PAD on the terrace surface of the optical module, and a second connecting pad on a second surface which is soldered to a PAD on the upper surface of the DSP substrate. The aforementioned FPC is On both sides of the FPC, and at a distance of 500 μm or more from the inner ends of the first and second connecting pads, there are notches having a diameter of φ300 μm or more that can stably hold the FPC. Furthermore, Optical transceiver.

10. The FPC and the first connecting pad, and the FPC and the second connecting pad are connected by one or more through-holes or embedded vias. The FPC further comprises lands formed on the front and back surfaces of the FPC, which are connected to the through-hole or the embedded VIA. The through-hole has a diameter of φ100 μm or more. The optical transceiver according to claim 9.

11. The aforementioned DSP board is Having a thickness of 1-2 mm, The core layer is provided with at least one second VIA at a position corresponding to the lower part of the PAD on the DSP substrate, The optical transceiver according to claim 9.

12. The optical transceiver according to claim 10, wherein the FPC further comprises a heating pad connected to the land and installed on the side opposite to the position where the first connecting pad and the second connecting pad are located.

13. The optical transceiver according to claim 19, wherein the PAD on the DSP substrate is provided with a thermal separation portion that thermally separates the portion on which the solder is formed from the other portion, and a high-frequency line for connecting the FPC and the DSP chip on the DSP substrate is formed in the inner layer wiring of the DSP substrate.

14. A manufacturing apparatus for manufacturing an optical transceiver as described in any one of claims 9 to 13, A hot bar that contacts the FPC and applies heat and load to the solder formed between the first connecting pad and the pad on the DSP substrate, and between the second connecting pad and the pad on the terrace, A mounting base that holds the PCB from below, A retaining mechanism that holds the FPC from the side and bottom by fitting into a notch, A manufacturing apparatus equipped with the following features.

15. The aforementioned mounting base is configured such that there is a hollow space between the PCB and the mounting base. The system further comprises a lower support structure positioned between the PCB and the mounting base, at a position corresponding to the position where the hotbar makes contact, The retaining mechanism is formed of resin or metal if the notch is formed only of dielectric material, and is formed of resin if the notch is metallized. The manufacturing apparatus according to claim 14.

16. The aforementioned hotbar is The FPC, which has a length of 20 to 30 mm and is connected to an optical transmitter module including an optical modulator and driver integrated circuit, and an optical receiver module including a photodetector and transimpedance amplifier, is soldered together as a single unit. The FPC is brought into contact with the solder at a temperature at least 50 degrees higher than the melting temperature of the solder. Apply a load of 10 N or more. The manufacturing apparatus according to claim 14, configured as described above.

17. A manufacturing method for manufacturing an optical transceiver according to any one of claims 10 to 13, Mounting the DSP on the aforementioned PCB, The optical module and the FPC are connected by soldering using a hot bar. The optical module with the FPC connected to it is placed on the PCB, and the FPC and DSP are connected by soldering using a hot bar. The DC interface of the PCB and the optical module are connected, A manufacturing method comprising the FPC and the first connecting pad, and the FPC and the second connecting pad being through holes, In soldering using the hot bar, a pre-soldering process is performed to form solder on the land or heating pad that is directly heated by the hot bar, The solder formed by the pre-soldering process is heated and pressurized by the hot bar, Furthermore, The solder, melted by the heating and pressurizing described above, reaches through the through-holes between the first connecting pad and the terrace, and between the second connecting pad and the DSP substrate, thereby forming solder between the first connecting pad and the terrace, and between the second connecting pad and the DSP substrate, or The FPC and the first connecting pad, and the FPC and the second connecting pad are connected by the embedded VIA. In soldering using the hot bar, a pre-soldering process is performed to form solder between the first connecting pad and the terrace, and between the second connecting pad and the DSP substrate. The hot bar is used to heat and pressurize the land or heating pad, Furthermore, A manufacturing method in which solder formed by the pre-soldering process melts due to heat transfer from the heated and pressurized land or heating pad, thereby forming solder between the first connecting pad and the terrace, and between the second connecting pad and the DSP substrate.