Optical transceiver and manufacturing method and manufacturing equipment thereof

Abstract

A configuration is provided for implementing an optical transceiver capable of operating at ultra-high speed, and a manufacturing method and a manufacturing apparatus for the optical transceiver. The optical transceiver according to the present disclosure includes at least one optical module mounted on a PCB, a DSP, and an FPC connecting the DSP and the optical module, where the FPC further includes: a first connection pad on a first surface, welded to a pad on a platform surface of the optical module; a second connection pad on the second surface, the second connection pad being soldered and connected to a pad on the upper surface of the DSP substrate; and two side surfaces and a first connection pad, the FPC and the first connection pad as well as the FPC and the second connection pad being connected by one or more vias or buried VIAs, and the vias having a diameter of 100 [mu] m or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an optical transceiver for optical communication and a manufacturing method and manufacturing apparatus for the optical transceiver. BACKGROUND

[0002] In order to cope with the increasing demand for communication traffic, a high-speed optical modulator and optical receiver compatible with an advanced optical modulation system are required. In the optical modulator and optical receiver in the early 100G digital coherent communication system, each component is packaged and the parts are mounted on a printed circuit board (PCB). For example, in the case of an optical modulator, a driver IC and an optical modulator chip are packaged separately, and the packaged IC and chip are mounted on a PCB. In the case of an optical receiver, a trans-impedance amplifier (TIA) and an optical light-receiving chip are packaged separately and mounted as separate parts on a PCB.

[0003] In a digital coherent communication system of more than 400G in which the speed is further increased, the optical modulator and optical receiver also need to have a wide band in order to be able to handle a signal of 40GHz or more, and need to reduce high-frequency loss and need to reduce the size. In the case of an optical modulator, a driver IC and an optical modulator chip are integrally mounted in one package as an optical module. In addition, in the case of an optical receiver, a TIA and an optical light-receiving chip are integrally mounted in a single optical module. In addition, for the signal form for input and output baseband signals, a design from a single-ended form to a differential form has been popularized as one of means for achieving high speed, small size, and low power consumption. Currently, development of devices for achieving 800Gbps and 1Tbps (128GBd operation) is in progress.

[0004] The Optical Internetworking Forum (OIF) standardized the name of a high-bandwidth coherent drive modulator (HB-CDM) as an optical transmitter in which a driver IC and an optical modulator are mounted in an integrated package. In Non-Patent Literature 1, a physical configuration, an interface specification, and the like are defined as various types of modules. In addition, on the receiving side, a TIA and an optical light receiver are mounted in an integrated package, and such an integrated package is also called a high-bandwidth intra-differential coherent receiver (HB-ICR). An optical transceiver (optical transmission / reception device) includes a transmitting side (Tx) optical module and a receiving side (Rx) optical module.

[0005] Initially, surface mount technology (SMT) packages, which offer excellent mountability, were used in the aforementioned optical modules. In SMT-packaged optical modules, since the optical module is mounted on the PCB of the optical transceiver, vias (VIAs) are essential for transmitting high-frequency electrical signals into the package. However, degradation of high-frequency characteristics is unavoidable in VIAs, making the VIA structure unsuitable for further broadband implementation. Furthermore, in SMT packages, the high-frequency transmission characteristics of electrical signals are degraded at the connection between the lead pins and the ceramic package due to electromagnetic field mode mismatch or impedance mismatch (Non-Patent Document 2). Therefore, a package configuration using flexible printed circuits (FPCs) was re-standardized (Non-Patent Document 1).

[0006] When achieving speed increases for the entire optical transceiver, in addition to increasing the speed of individual optical modules, optimized design of the digital signal processor (DSP) is also crucial. In early optical transceivers, the DSP, transmitter module, and receiver module were housed in separate packages and mounted in different locations on the PCB. Therefore, speed improvements were limited by propagation losses incurred in these separate packages or on the PCB. As one solution, a "co-package structure" has been investigated, in which the optical modulator chip, optical receiver chip, and DSP are mounted close to each other in a single package. This structure is ideal for significantly reducing losses in high-frequency electrical signals; however, the DSP, being the largest heat source, is placed near the periphery of the optical modulator or receiver. When using optical modulators requiring temperature control (e.g., InP modulators that excel in speed), the modulator chip needs to be mounted on a Peltier element. Peltier elements pose risks of increased power consumption and thermal runaway, and it is difficult to mount them close to the periphery of the DSP, which would cause heat inflow, in a single package. Co-package structures present various challenges during installation. Therefore, another configuration of the optical transceiver has been proposed (Patent Document 1).

[0007] Figure 1 This is a side cross-sectional view showing a configuration of an optical transceiver 800 suitable for high-speed operation according to conventional technology. A side cross-section (zx plane) of the optical transceiver 800 is shown, taken along a line passing through a transmitting module 807 mounted on a substrate surface (xy plane) of a PCB 801. The optical transceiver 800 includes a DSP 802, a transmitting module 807, and a receiving module (not shown). In the DSP 802, a DSP chip 805 is mounted on a DSP substrate 804 using a ball grid array (BGA). The entire DSP 802 is further mounted on the PCB 801 via a BGA 803. The transmitting module 807 includes a driver IC, an optical modulator chip, etc. (not shown) in a package having a platform, and includes an optical fiber 808.

[0008] In optical transceiver 800, high-frequency (RF) signals are connected between DSP 802 and optical module 807 via FPC 806. By using FPC 806 as the RF interface, RF signal lines formed on the upper surface of DSP substrate 804 and RF signal lines on the platform of the optical module are directly connected. In early optical transceivers including SMT-packaged optical modules, signal paths were configured through the VIA of DSP substrate 804, the RF signal line pattern of PCB 801, and the VIA of the optical module 807 package. Compared to early optical transceivers utilizing SMT-packaged... Figure 1 The optical transceiver 800 is configured to minimize high-frequency losses. Furthermore, the option of optical modulators is not limited to optical transceivers with a co-package structure, and installation is more feasible. High-frequency losses are also significantly reduced compared to earlier optical transceivers using SMT packages.

[0009] However, in Figure 1 In the conventional optical transceiver 800, the specific connection configuration between the FPC 806 and the DSP substrate 804, the structure of the FPC 806, and the connection configuration between the FPC 806 and the optical module 807 are not yet clear. The configuration of the DSP and the DSP substrate is also unclear. Furthermore, in... Figure 1 In the transmitter module 807, the heat-generating components within the package (e.g., the driver IC) are thermally connected to the platform side and the PCB side, and the heat dissipation direction is downward (-z direction) relative to the optical transceiver 800. On the other hand, since the DSP chip 805 is located above the DSP 802, the heat dissipation direction is upward (+z direction) relative to the optical transceiver. This division of heat dissipation into two directions is not preferable for simplifying the heat dissipation structure.

[0010] Reference List

[0011] Patent documents

[0012] Patent Document 1: WO 2021 / 171599 A

[0013] Non-patent literature

[0014] Non-Patent Document 1: OIF, Implementation Agreement for the High Bandwidth Coherent Driver Modulator (HB-CDM), [Online], July 15, 2021, [Searched on January 20, 2023], Internet<URL:https: / / www.oiforum.com / wp-content / uploads / OIF-HB-CDM-02.0.pdf>

[0015] Non-patent literature 2: J. Ozaki et al., “Over-85-GHz-Bandwidth InP-Based CoherentDriver Modulator Capable of 1-Tb / s / λ-Class Operation”, Journal of Lightwave Technology, vol.41, No.11, pp.3290-3296, June 1, 2023, doi:10.1109 / JLT.2023.3236962. Summary of the Invention

[0016] The objective of this 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 operating at ultra-high speed, as well as a method and apparatus for manufacturing the optical transceiver.

[0017] Based on the above objectives, this disclosure provides an optical transceiver, comprising: 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, wherein the DSP includes a DSP chip mounted on a DSP substrate formed of a multilayer wiring substrate, the multilayer wiring substrate having a core layer for adjusting thickness; the FPC includes: a first connection pad on a first surface, soldered to a platform surface of the optical module; and a second connection pad on a second surface, soldered to a pad on the upper surface of the DSP substrate; the FPC and the first connection pad, and the FPC and the second connection pad, are connected through one or more through-holes or buried VIAs; the FPC further includes: a connection pad, connected to the through-hole or buried VIA, and formed on the front and rear surfaces of the FPC; and a slot having... A diameter of 300 μm or greater is required to stably hold the FPC. The slot is located on both side surfaces of the FPC, at a position 500 μm or greater from the inner end of each of the first and second connection pads, and the via has… 100μm or larger in diameter.

[0018] Additionally, this disclosure provides a manufacturing apparatus for manufacturing an optical transceiver, the apparatus comprising: a heat rod that contacts the FPC and applies heat and load to solder formed between a first connecting pad and a pad on a DSP substrate and between a second connecting pad and a pad on a platform; a mounting substrate that holds the PCB from below; and a holding mechanism that holds the FPC from a side surface and a lower surface by fitting with a slot.

[0019] Furthermore, this disclosure provides a manufacturing method for an optical transceiver, the method comprising: mounting a DSP on a PCB; connecting an optical module and an FPC by soldering using a hot rod; mounting the optical module connected to the FPC on the PCB, and connecting the FPC and the DSP by soldering using a hot rod; and connecting the PCB to the DC interface of the optical module, wherein the FPC and a first connection pad and the FPC and a second connection pad are connected via through-holes. The manufacturing method further comprises: performing a pre-soldering process to form solder at least on a connection pad or heated pad directly heated by the hot rod during soldering; and heating and pressurizing the solder formed by the pre-soldering process using a hot rod, wherein the solder melted by heating and pressurizing reaches the first connection pad through the through-hole. Solder is formed between a first connecting pad and a platform, and between a second connecting pad and a DSP substrate, and between the first connecting pad and the platform, 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 a buried VIA. The manufacturing method further includes: performing a pre-soldering process to form solder between the first connecting pad and the platform, and between the second connecting pad and the DSP substrate when soldering with a hot rod; and heating and pressurizing the connecting pad or heated pad with a hot rod, wherein the solder formed by the pre-soldering process is melted by heat transfer from the heated and pressurized connecting pad or heated pad, and solder is formed between the first connecting pad and the platform, and between the second connecting pad and the DSP substrate. Attached Figure Description

[0020] Figure 1 This is a side cross-sectional view showing an optical transceiver configuration suitable for high-speed operation based on conventional technology.

[0021] Figure 2 These are top views and cross-sectional views illustrating an overview of the configuration of the optical transceiver of this disclosure.

[0022] Figure 3 This is an enlarged cross-sectional view of the optical module, including the connection part of the FPC.

[0023] Figure 4 This is a diagram illustrating the FPC cabling layout in the optical transceiver of this disclosure.

[0024] Figure 5 This is a flowchart illustrating the process of a manufacturing method 600 for an optical transceiver 100 according to the present disclosure.

[0025] Figure 6 This is a side cross-sectional view showing the configuration of the mounting device 700 for the optical transceiver 100 according to the present disclosure.

[0026] Figure 7The diagram shows in detail the structure of the holding mechanism 704 when the optical transceiver 100 according to this disclosure is installed. Figure 7 (a) shows a top view of the FPC 500-1 with the retaining mechanism 704 and the slot 512 assembled together. Figure 7 (b) shows a cross-sectional view taken along line VIIbc-VIIbc, and Figure 7 (c) shows a cross-sectional view taken along line VIIbc-VIIbc with the retaining mechanism 704 having a different shape.

[0027] Figure 8 This is a diagram showing the detailed structure of the DSP substrate 201 of the optical transceiver 100 according to the present disclosure. Figure 8 (a) shows a top view, and Figure 8 (b) shows a cross-sectional view taken along line Xb-Xb.

[0028] Figure 9 This is a diagram showing the cross-sectional structure of the Tx module in the optical transceiver of this disclosure. Detailed Implementation

[0029] In the following, various embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The same or similar reference numerals denote the same or similar components, and repeated descriptions may be omitted. Numerical values ​​are for illustrative purposes only and are not intended to limit the scope of the present disclosure. Furthermore, the following description is merely illustrative, and some configurations may be omitted, modified, or implemented with additional configurations without departing from the spirit of the embodiments of the present disclosure.

[0030] The optical transceiver disclosed herein presents 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 may include a single optical module integrating the functions of both an optical receiver and an optical modulator. For the high-frequency connection, a flexible substrate (hereinafter referred to as FPC) is used. The FPC uses two surfaces to electrically connect pads on the upper surface of the DSP substrate to pads on the optical module's package. Connection pads on one surface of the FPC are connected to the DSP substrate side, and connection pads on the other surface are connected to the optical module side. The DSP chip can be mounted on the DSP substrate within the DSP, and the entire DSP substrate, except for the area of ​​the DSP substrate connected to the FPC, can be covered with a cover.

[0031] The detailed structures of the connection pads on the DSP substrate, the shape of the high-frequency wiring, the structure of the multilayer wiring substrate, and the FPC structure are disclosed. These structures are used 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 accompanying drawings.

[0032] In the following description, for simplicity, the transmitter-side optical module including the optical modulator and its driver IC will be referred to as a "Tx module," and the receiver-side optical module including the photodetector (PD) and TIA will be referred to as an "Rx module." A single optical module integrating the functions of both Tx and Rx modules will be referred to as a TRx module. In the following description, an optical transceiver including both Rx and Tx modules is described as an example, but both modules can be replaced with TRx modules. Furthermore, the following description will be based on the cross-section of the Tx module, but for the Rx module, whose shape is substantially symmetrical to the Tx module, the connection method using the FPC for the DSP and optical module is the same. Similar configurations of the FPC pad shapes between the DSP and Tx modules, and between the DSP and Rx modules, are also effective in high-frequency design. This also helps to suppress differences in solder mounting quality and connection strength throughout the optical module.

[0033] [Basic Configuration of Optical Transceiver]

[0034] Figure 2 These are top views and cross-sectional views illustrating an overview of the configuration of the optical transceiver of this disclosure. Figure 2 (a) is a top view of the substrate surface (xy plane) of PCB101, and Figure 2 (b) is a diagram of the side section (xz plane) taken along IIb-IIb passing through Tx module 400. In optical transceiver 100, DSP 200, Rx module 300 and Tx module 400 are mounted on PCB 101.

[0035] In addition, various electronic and mechanical components, such as chip capacitors and resistors, are mounted on the upper and lower surfaces of PCB 101, but... Figure 1 The text is omitted. In the optical transceiver 100, considering manufacturability when each component is mounted on the PCB 101 and simplification of the optical transceiver's heat dissipation structure, the heat dissipation surfaces of the DSP 200, Rx module 300, and Tx module 400 are aligned in one direction. Considering that the heat dissipation from the DSP chip 202 is the greatest in the optical transceiver, it is desirable that the heat dissipation direction be aligned upwards (+z direction), which is suitable for the heat dissipation of the DSP chip 202.

[0036] refer to Figure 2In the cross-sectional view in (b), compared to the configuration of conventional technology, the optical transceiver 100 is configured such that the upper and lower surfaces are reversed when mounted on the PCB 101, and the side where the platform is located is the upper side of the optical module in the height direction (z-axis). The optical module is mounted such that the platform surface faces the upper surface of the PCB, and in the case of the Tx module 400, heat from the driver IC and Peltier elements is configured to move towards the side where the platform surface of the package is located (along the +z direction).

[0037] Figure 9 This diagram illustrates the cross-sectional structure of the Tx module in the optical transceiver of this disclosure. In the Tx module 400, a Peltier element 406, a substrate 407, and an optical modulator chip 409 are arranged in this order within a housing 417. Lenses 410 and 411 are disposed on the substrate and are optically coupled to an optical fiber 402. In this module, a driver IC 408 is formed on the surface of a substrate 415, and wiring 412 is formed on a package substrate 416. The optical modulator chip 409 is connected to wiring 412 via wirings 413 and 414. The interior and exterior of the module are divided by the housing 417, the package substrate 416, and the package sidewall 405. The platform surface of the optical module is configured by exposing a portion of the package substrate 416 from the housing. An FPC 500-1 is attached to the platform surface.

[0038] A Peltier element 406 for temperature control and a base 415 thermally connected to the driver IC, which serves as a heat-generating component, are fixed to the upper surface inside the housing 417. Heat dissipation from within the module is configured such that heat is moved from the driver IC or Peltier element to the outside along the +z direction. Additionally, in the Rx module, a TIA is provided instead of a driver IC, and heat from within the module is moved to the upper side of the module (+z direction).

[0039] Within the optical module's package, heat-generating components are mounted to dissipate heat to the bottom surface opposite the mounting surface on the PCB. Therefore, a heat dissipation mechanism (housing) with a common heat dissipation surface (not shown) can be used, relative to heat dissipation from the DSP 200 and from the Tx module 400, on the upper side of the optical transceiver 100. The optical module and DSP are positioned close to each other, and both have the same upper surface height, allowing for easy heat dissipation of both the DSP and optical module using a single heat dissipation mechanism. In the case of a common heat dissipation mechanism, it is necessary to carefully consider preventing heat transfer from the DSP to the optical module. Alternatively, to suppress heat movement via the heat dissipation mechanism, the DSP and optical module can use independent heat dissipation mechanisms on the same plane.

[0040] DSP 200 includes a DSP substrate 201, a DSP chip 202, and a cover 203. The DSP substrate 201 includes a multilayer wiring substrate, which will be described below. The DSP chip 202 and the DSP substrate 201 are connected via a BGA 204. Similarly, the DSP substrate 201 and the PCB 101 are also connected via a BGA 205.

[0041] The cover 203 is made of a single piece of metal to cover at least the entire area of ​​the upper surface of the DSP chip 202 and the area of ​​the upper surface of the DSP substrate except for the area where FPCs 500-1 and 500-2 are connected / fixed. The cover 203 may cover the side surface of the DSP chip 202 or protrude from the periphery of the DSP substrate 201. The cover 203 prevents accidental damage to the DSP chip, BGAs 204 and 205 (which serve as connection parts), etc., when the DSP 200 is mounted on the optical transceiver or during manufacturing due to processing errors. Through the finely designed shape of the cover 203, the surface of the DSP substrate 201 can be kept open from the RF connection pads for connection with the FPC to the substrate end. As described above, the DSP 200 can be fabricated as a DSP module in which the DSP chip 202 and the cover 203 are mounted on the DSP substrate 201. Although the manufacturing process of the optical transceiver will be described below, when the DSP is fabricated as a covered DSP module, the DSP can be safely mounted on PCB 101 without being damaged during the manufacturing process, similar to the Rx and Tx modules.

[0042] As another form of DSP 200, DSP 200 can be fabricated with only the DSP chip 202 mounted on the DSP substrate without a cover. Alternatively, the coverless DSP 200 and optical modules 300 and 400 can be connected via FPCs 500-1 and 500-2 respectively, and then the cover 203 is finally installed. When installing the cover, there is a risk of accidental force acting on the FPC or the cover 203 causing a short circuit in the FPC's wiring. Therefore, the pad positions on the upper surface of the DSP substrate 201 and the shape of the cover need to be carefully optimized. Since the DSP chip and DSP substrate are exposed until the cover is installed, extra care is required to avoid damaging the DSP chip, etc.

[0043] When the DSP 200 includes a cover 203, the area occupied by the cover 203 on the substrate surface (xy plane) is always smaller than the area occupied by the DSP substrate 201. On the other hand, the area occupied by the DSP chip 202 is included within the area occupied by the cover. On the substrate surface of the DSP substrate 201, the cover 203 is not provided only in areas where at least RF connection pads are provided.

[0044] In the manufacturing process of the optical transceiver, the cover can be omitted if damage to the DSP chip and BGAs 204 and 205 can be avoided. To effectively dissipate heat from the DSP chip to the outside of the optical transceiver, a cover can be placed only on the upper surface of the DSP chip. The shape of the cover 203 can be modified in various ways, and specific variations will be described below.

[0045] Both the Rx module 300 and the Tx module 400 are housed within a package. The two optical modules 300 and 400, along with the DSP 200, are directly connected via FPCs 500-1 and 500-2 as RF interfaces for electrical signals. Here, "direct" connection means that the connection pads on the optical module platform and the connection pads on the DSP substrate are connected only via FPCs 500-1 and 500-2. Therefore, in the optical transceiver 100, there is no high-frequency path via the VIA connecting the optical module to the PCB, the RF signal lines in the PCB, the BGA connecting the PCB and the DSP substrate, and the VIA in the DSP substrate. In the optical transceiver 100, compared to optical transceivers including SMT-type packages using conventional technology, electrical signals do not pass through the VIA or BGA, thus avoiding degradation of high-frequency characteristics. The DSP and optical module can be connected via the shortest high-frequency path, significantly reducing high-frequency losses. Furthermore, due to the reduction in the number of connection parts such as VIAs and BGAs, Figure 2 The configuration of the optical transceiver is also very effective in reducing high-frequency losses caused by impedance mismatch at discontinuous parts of the high-frequency path.

[0046] The DC interface of optical modules 300 and 400 can be, for example, Figure 2 DC lead pins 301 and 401 in (a). The DC interface is not limited to the lead pin structure and is similar to the RF interface with DSP 200, which can use an FPC. DC lead pins 301 and 401 are soldered onto pads (not shown) on PCB 101 so that power can be supplied from outside the optical transceiver.

[0047] Refer again Figure 2 The top view of the optical transceiver 100 in (a) shows the optimal arrangement of the Rx module 300, Tx module 400, and DSP 200 to minimize the loss of the RF signal, which is a high-frequency electrical signal. These two modules and the DSP 200 are arranged symmetrically with respect to the centerline along the longitudinal direction of the PCB 101. To achieve a high-speed optical transceiver, it is important to suppress high-frequency losses in the RF signal from the DSP 200 to the Tx module and from the Rx module to the DSP 200. Figure 2In the optical transceiver 100 (where the lengths of the FPCs 500-1 and 500-2, which serve as RF interfaces, need to be as short as possible), the distance (gap) between the two optical modules in the y-axis direction is preferably close to 1 mm or less in order to transmit high-speed signals exceeding 128 GBd.

[0048] To position the two optical modules close to each other, the DC lead pins need to be converged on one side of each module, with the DC lead pins facing opposite directions. For example... Figure 2 As shown in (a), the DC lead pin 401 of the Tx module 400 points towards the outer periphery of the PCB 101 along the -y-axis direction (downward in the figure), and the DC lead pin 301 of the Rx module 300 points towards the outer periphery of the PCB 101 along the +y-axis direction (upward in the figure). This configuration follows the lead pin orientation defined in the OIF-standardized HB-CDM, as described in Non-Patent Document 1.

[0049] The optical module's package is made of ceramic. Considering the layout design of the high-frequency signal lines and DC lines within the optical module, in... Figure 2 In the cross-sectional view of (b), the height from the upper surface of PCB 101 to the platform surface of the optical module is expected to be approximately 1 to 2 mm. Similarly, the thickness of the DSP substrate 201, from the angle of matching this height with the platform surface of the optical module, is expected to be set in the range of approximately 1 to 2 mm. By matching the height of the upper surface of the DSP substrate with the height of the platform surface, as described later, bending of the FPC in the thickness direction is not required, and planarization can be performed.

[0050] When the DSP substrate has a thickness of approximately 1 to 2 mm, more layers than are required for DC and RF lines can be used. Generally, in multilayer wiring substrates, to reduce costs, it is desirable to minimize the number of layers; therefore, a core layer can be used internally to adjust the overall thickness of the substrate. Multilayer wiring substrates including such a core layer can be called assembly substrates. The core layer acts as a separator layer, separating the RF wiring layer from the DC wiring layer in addition to adjusting the overall substrate thickness. By providing a core layer, the DC and RF wiring layers can be largely separated from each other, thus suppressing mutual interference and noise effects between wiring layers. The detailed configuration of the DSP substrate, as well as the detailed configuration of the pads and signal lines, will be described below.

[0051] Considering the high-speed DSP operation described above, it is desirable to house the DSP 200 within a smaller DSP substrate 201 to suppress high-frequency signal loss and reduce cost. The two FPCs should be configured to converge along the centerline of the DSP 200 from the optical module side toward the DSP side. As will be described below, Figure 2The FPCs shown all have a curved shape within the plane (xy plane) of the substrate, which has been previously formed into a predetermined shape. Although FPCs can also be formed into straight shapes without bending, it is necessary to spread out in a fan shape and arrange the pads on the substrate surface of the DSP substrate 201. This increases the high-frequency loss on the DSP substrate and also increases the size of the DSP substrate, which is not desirable.

[0052] [Connection method between the DSP and optical module of the optical transceiver]

[0053] The optical transceiver 100 disclosed herein is characterized by the FPC connection between the DSP and the optical module. Figure 1 In the conventional optical transceiver 800 shown, the FPC 806 is on a common plane of the substrate surface of the DSP substrate 804 and the platform surface of the optical module 807 in the same direction, and is connected only on one surface of the FPC 806. On the other hand, in Figure 2 In the optical transceiver 100 of (a), the FPC 500-1 is on a common plane of the substrate surface of the DSP substrate 201 and the platform surface of the Tx module 400 in the opposite direction, and is connected to different surfaces of the FPC 500-1. The optical module 400 is mounted upside down on the PCB 101 relative to conventional technology, such that the platform surface with connection pads for the FPC faces the PCB 101 side.

[0054] Figure 3 This is an enlarged cross-sectional view of the optical module, including the connection part of the FPC. Figure 3 By magnification Figure 2 The side section (xz plane) obtained in (b) is located near FPC 500-1 and is enlarged to indicate the height direction (z-axis direction). It should also be noted that, to facilitate understanding of the structure near the FPC, the relative dimensions of each part are not precisely drawn. Figure 3 and Figure 2 (b) is along Figure 2 The cross-sectional view of line IIb-IIb in (a) is taken perpendicular to the PCB surface, and the line passes through the signal line of the bent FPC 500-1, but is shown along the signal line in the FPC. In the optical transceiver 100 of this disclosure, in order to reduce high-frequency losses caused by bending the FPC in the thickness direction (z-axis direction), a structure that minimizes the bending of the FPC is adopted.

[0055] Figure 3An FPC 500-1 is shown, which connects the connection pad 210 at the end of the DSP substrate 201 and the connection pad 403 on the platform surface of the Tx module 400. In the FPC 500-1, metal layers are formed on both surfaces of the substrate 501, and a signal line 502 is provided on the upper side in the figure, while a ground (GND) surface 503 is provided on the opposite side. Pads 504 and connection pads 507 for optical modules are formed at both ends of the signal line 502. On the GND surface side, a connection pad 505 for the DSP is also formed, connected to the pad 504 via a VIA or through-hole 508, and a pad 506 is formed, connected to the connection pad 507 via a VIA or through-hole 509. As described below, the connection pads 505 and 210 are connected by solder 206 by heating the pad 504. Similarly, the connection pads 507 and 403 are connected by solder 404 by heating the pad 506.

[0056] In the optical transceiver 100 according to this disclosure, the height difference ΔH between the platform surface of the Tx module and the upper surface of the DSP substrate 201 is set to 500 μm or less to minimize the bending of the FPC in the thickness direction. When the height difference is at most about 500 μm, there is no need to bend the FPC significantly in the thickness direction, and the FPC can be connected almost flat. This suppresses high-frequency losses caused by significant bending of the FPC and the risk of cracking of metal wiring on the FPC, and achieves good high-frequency connectivity.

[0057] When the substrate 501 of the FPC is thinner, allowing for flexible adjustment of its shape in the height direction during installation, the substrate is generally easier to handle. On the other hand, for high-frequency characteristics, a thicker substrate is preferable to suppress losses. Considering these factors, the thickness of the substrate 501 is desirable, for example, 50 μm or more. Specifically, when the substrate 501 has a thickness of about 50 to 100 μm, even if the height difference ΔH between the aforementioned platform surface and the DSP substrate surface is about 500 μm, installation can be easily performed by slightly adjusting the shape of the FPC in the thickness direction. When ΔH is set to about 500 μm, damage to the solder joints between the FPC and the optical module can also be suppressed during installation. When the solder in the finished state of the solder 206 and 404 at both ends of the FPC becomes thicker, the dimensions of the conductor portion of the transmission line change, and the characteristic impedance of the signal line may decrease. The solder thickness between the pads is desirable, being 50 μm or less.

[0058] As mentioned above, it is advantageous for the FPC 500-1 to be as short as possible in terms of high-frequency characteristics. However, the impact of heat from the DSP chip 202, which is the largest heat source in the optical transceiver, must be considered. If the optical modules 300 and 400 are placed too close to the DSP chip 202, the optical modules may reach very high temperatures. When the optical module includes Peltier elements, there is a risk of a significant increase in the power consumption of the Peltier elements or the Peltier elements failing, leading to thermal runaway. Furthermore, if the FPC is too short, problems arise during the connection of the FPC to the DSP substrate, such as the disappearance of the portion used to hold the FPC or the inability to guarantee the length for bending the FPC. From a thermal design and installability perspective, the length of the FPC is preferably 3 mm or longer.

[0059] [An overview of the process for assembling optical transceivers]

[0060] An overview of the process for mounting the optical transceiver 100 will be described. The Rx module 300 and Tx module 400 are pre-soldered to their respective platform surfaces using an FPC. First, the DSP 200 is mounted on the PCB 101 via reflow, and then the optical modules 300 and 400 are mounted on the PCB 101. The reason for mounting the optical modules and DSP in different processes is that it is difficult to perform a normal reflow process for the optical modules due to heat resistance limitations, etc.

[0061] First, the corresponding FPCs of optical modules 300 and 400 are soldered 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. Since the FPCs of the optical modules are RF interfaces, the size of the connection pads 505 is limited by impedance matching. High-frequency transmission characteristics and connection strength are affected by the positional deviation of the pads facing each other via solder, therefore, the mounting of the optical modules with FPCs needs to be performed with very high precision. On the other hand, since it is sufficient for the lead pins to be reliably connected in DC, the size of the DC pads on the PCB side can be increased, and the allowable positional deviation error can also be relatively large. Therefore, the optical modules are connected from the FPC side, and then the lead pins are connected.

[0062] To reduce the number of processes involved in mounting the FPC onto the DSP 200 already mounted on the PCB, it is desirable to use hot-dip soldering for joint thermal / soldering of the Tx and Rx modules. For hot-dip soldering to perform the mounting, at least one of the connection pads 505 on the FPC and the connection pads 210 on the DSP substrate needs to be pre-soldered. Figure 3 As shown, at least one or more vias (VIAs) or through-holes 508 are required to perform a solder joint by melting the pre-solder between the bonding pads with a hot rod. Figure 3In this configuration, a heat rod is pressed from above onto the heating pad 504, and the through-hole 508 transfers heat from the heating pad 504 to the solder 206 between the connecting pads 505 and 210. Alternatively, the VIA 508 can be a through-hole instead of a VIA. Here, a through-hole refers to a hole formed through the substrate, and the entire inner surface of the hole is metallized. In the case of a through-hole, since solder can flow through the hole, it is sufficient to perform pre-soldering treatment on at least one of the connecting pads 505, the heating pad 504, or the connecting pads 210 on the DSP substrate.

[0063] The DC lead pins 301 and 401 of the optical module are DC interfaces, and only a simple electrical connection is required, thus allowing for slight deviations. The pad size on the PCB can also be set wide enough relative to the lead pins, eliminating the need for the high-precision alignment and mounting methods required on the FPC side. The lead pins cannot be bent after being secured, but the FPC can be bent to a certain extent. Therefore, after initially securing the FPC, fine adjustments can be made to connect and secure the DC lead pins.

[0064] [FPC cabling layout]

[0065] Here, a more specific configuration of the FPC in the optical transceiver 100 of this disclosure will be described. For example... Figure 3 As shown, the FPC has the following structure: metal layers are included on both surfaces of the substrate 501, a signal line 502 for transmission lines is disposed on one surface, and a GND surface 503 for the signal line is disposed on the other surface. The FPC can be preferably configured such that the surface containing the signal line 502 faces the platform surface of the Tx module 400. Conversely, when the FPC is configured such that the signal line 502 faces the substrate surface of the underlying PCB 101, the metal wiring pattern on the PCB and the signal line 502 on the FPC may interfere with each other. Due to this interference, there is a risk of degradation in the high-frequency transmission characteristics of the input and output electrical signals between the DSP 200 and the optical module, and a risk of noise mixing between the electrical signals and the DC signals on the PCB, etc. Figure 3 As shown, the aforementioned interference problem does not occur when the GND surface 503 of FPC 500-1 faces the PCB 101 side. However, in the absence of wires or other components on PCB 101, or when a sufficient distance can be maintained between the PCB and FPC, the surface of the FPC's signal lines and the GND surface can be... Figure 3 The corresponding surfaces in the image are upside down.

[0066] Figure 4 This is a diagram illustrating the FPC cabling layout in the optical transceiver of this disclosure. Figure 4(a) shows the first surface (xy plane) on the side where the signal line is located, and (b) shows the second surface side (xy plane) opposite the first surface and having a GND surface. Figure 4 In this design, the FPC's cover layer and solder resist are omitted to allow for easy layout viewing. Figure 4 On the first surface of FPC 500a in (a), two sets of signal lines 502a and 502b in GSSG differential form are shown. Here, G represents ground, and S represents a signal line. The number of signal line sets can vary depending on the type of optical module. Additionally, the pad area on the GND surface is made larger than the pad area of ​​the signal lines. From a high-frequency characteristics perspective, the pad size of the signal lines is expected to be smaller, but sufficient solder joint area is required to ensure solder joint strength. Therefore, solder joint strength can be configured to be guaranteed by making the pads on the GND side much wider than the signal line pads.

[0067] The maximum area of ​​the GND pads between channels is uniquely determined by the channel spacing of the signal line pads on the FPC and DSP substrate. When the channel spacing is narrow and the area for the GND pads cannot be adequately guaranteed, increasing the size of the GND pads at both ends of the FPC in the width direction (y-direction) is effective. Since there is a risk of short circuits when solder flows in areas other than the areas to be soldered, areas in the FPC where solder is not needed are covered with solder resist. It should be noted that the GND pads on the GND surface are also defined by dividing the areas to be soldered using solder resist. Therefore, multiple pads in a differential GSSG configuration are provided at the ends of the FPC. Furthermore, since applying solder resist to the signal lines increases high-frequency losses, the area of ​​the solder resist needs to be as small as possible.

[0068] Figure 4 In (b), a GND surface 503 for signal lines 502a and 502b is formed on the second surface side of the lower surface of FPC 500a. Figure 3 As shown in the cross-sectional view of the FPC connection portion, the first surface of the FPC with signal line 502 is connected to the connection pads on the platform surface of the optical module 400, and the second surface of the FPC with GND surface 503 is connected to the connection pads on the upper surface of the DSP substrate 201. Both the first and second surfaces of the FPC are used for RF interfaces. Therefore, the VIA 508 included in the path of signal line 502 plays an important role, not only serving as a solder fixing function, but also as part of the signal path for transmitting high-frequency signals.

[0069] To transmit high-frequency signals without loss, when no VIA is provided at the edge of the pad portion, the area of ​​the pad far from the VIA forms an open-circuit stub, adding capacitive impedance to the signal line. To prevent this from happening, such as... Figure 4 As shown, a half-via 510 is provided at the very end of FPC 500a relative to signal lines 502a and 502b. A half-via is not necessary when the pad length is sufficiently short and impedance variations caused by stub formation are not a problem within the used frequency band. Similarly, a half-via 511 is provided at the very end of FPC 500a to stabilize the high-frequency potential on the GND surface. Half-vias 510 and 511 not only improve high-frequency characteristics but also contribute to the fillet formation of solder in the connection pads, thereby enhancing the solder joint strength. For the half-via 510 of the signal lines relative to the pads, if the half-via diameter is too large and the surrounding contact area is large, the capacitance at the pad portion increases, leading to a decrease in impedance at the connection pads and a deterioration in high-frequency characteristics. To suppress the deterioration of high-frequency transmission characteristics in the signal lines, the diameter of the half-via 510 is desired to be... It is 150μm or smaller.

[0070] The configuration of the connection pads, etc., on the DSP substrate 201 will be described below. However, from the perspective of the fillet formation at the FPC end, it is expected that the mounting positions of the signal line pads on the FPC and the signal line pads on the DSP substrate should be offset by approximately 100 μm. This corresponds to... Figure 3 The fact that the end of the connection pad 210 on the DSP substrate is offset in the -x direction relative to the end of the connection pad 505. By setting the size and positional relationship of the connection pads 505 and 210 in this way, a favorable fillet is formed in the solder 206 at the connection portion, and the strength quality of the solder can be improved. Specifically, the connection pads 505 and 210 have the same length, and the FPC position can be adjusted and mounted such that the edge positions of the two pads are offset in the length direction (x direction). Additionally, as... Figure 3 As shown in the cross-sectional view, it is sufficient as long as one edge of the connection pads 505 and 210 are aligned and the length of the connection pad 505 on the FPC side is about 100 μm shorter than the length of the connection pad 210 on the DSP substrate 201. Since the semi-through hole 511 at the end of the GND surface does not significantly affect the high-frequency characteristics, the semi-through hole can be a semi-through hole with a large diameter, depending on the required connection strength.

[0071] In addition, such as Figure 4As shown, FPCs 500-1 and 500-2 according to this disclosure also include slots 512 formed on two side surfaces of the substrate 501. The slots 512 have a shape similar to the semi-through holes 510 or 511 described above, but have a different function and act as assembly portions with retaining mechanisms that position FPCs 500-1 and 500-2 during installation and hold them during heating / pressurization using hot rods. It is desirable that the slot portions of FPCs 500-1 and 500-2 be separated from the metal forming the GND and have no metal, so that no heat is transferred when assembled to the retaining mechanism. From the same perspective, it is desirable that the retaining mechanism portion itself be formed of resin or the like rather than metal, so that no heat is transferred. Therefore, it is desirable to suppress wasteful heat absorption by portions other than the pad portions and to concentrate heat applied to the connection of the pads. Reference will be made below. Figure 6 and Figure 7 Describe the details of the retaining mechanism.

[0072] [Details of the process for assembling the optical transceiver]

[0073] Figure 5 This is a flowchart illustrating the process of a manufacturing method 600 for an optical transceiver 100 according to this disclosure. Figure 5 As shown, the manufacturing method 600 includes: mounting components other than optical modules 300 and 400 (e.g., DSP 200, other capacitors not shown, etc.) on PCB 101 (S601); connecting optical modules 300 and 400 to FPCs 500-1 and 500-2 by soldering with hot rods (S602); mounting optical modules 300 and 400 connected to FPCs 500-1 and 500-2 on PCB 101, and connecting FPCs 500-1 and 500-2 to DSP 200 by soldering with hot rods (S603); and connecting PCB 101 to optical modules 300 and 400 via DC lead pins 301 and 401 (S604).

[0074] In S601, components other than optical modules 300 and 400 are typically mounted on the PCB via reflow. As mentioned above, the reason why mounting optical modules 300 and 400 is set as a separate process is that it is difficult to allow the optical modules to undergo a normal reflow process due to heat resistance limitations, etc.

[0075] In S602, FPC 500-1 and Tx module, as well as FPC 500-2 and Rx module 300, are connected by soldering using hot rods. The details of soldering using hot rods are described below using S603 (the connection between FPC 500-1 and 500-2 and DSP substrate 201) as an example.

[0076] When installing FPC 500-1 and 500-2 in S603, in order to reduce the number of installation processes and minimize the thermal impact (heat load) and damage risk to DSP 200, it is desirable to install both FPC 500-1 and 500-2 by co-soldering with heat rods.

[0077] Following the connection on the RF interface side in S603, the DC side interface of the optical module is to be fixed in S604. From an impedance matching perspective, pad sizes with narrow design tolerances and very small widths (e.g., 100 to 200 μm) are required at the high-frequency connection sections. Therefore, even with small positional deviations, good bonding cannot be achieved, or bonding strength cannot be guaranteed. Furthermore, since unintended solder joint shapes can lead to impedance mismatch and high-frequency characteristic degradation, mounting must be performed with very high accuracy and precision.

[0078] [Welding methods using heat pipes]

[0079] In the following text, the connection between FPC 500-1 and 500-2 and DSP substrate 201 (corresponding to...) will be used to explain the connection between the FPC 500-1 and 500-2 and DSP substrate 201. Figure 5 The welding method using a heat rod when installing an optical transceiver, according to this disclosure, will be described in detail using S603 as an example.

[0080] To perform mounting using a heat pipe, it is not feasible to heat only the surface of the connection portion between FPCs 500-1 and 500-2 and the DSP substrate 201. Therefore, it is practically necessary to apply heat from the upper surface of FPCs 500-1 and 500-2 while applying pressure using a heat pipe. Thus, FPCs 500-1 and 500-2 need to have a structure where heat applied from the upper surface is transferred to the connection portion with the DSP substrate 201. Therefore, although details will be described below, the FPCs 500-1 and 500-2 of the optical transceiver according to this disclosure are formed with metallic patterns on both surfaces of the substrate, and have heating pads 504 on the upper surface and RF connection pads 505 on the lower surface. Additionally, at least one through-hole or buried via 508 is formed for each pad. The via or buried VIA has a connecting pad, and the molten heat or solder (only in the case of via) is transferred to the DSP substrate 201 side via the metal of the connecting pad and the via or buried VIA 508, thereby performing soldering between the RF connection pad 505 and the RF connection pad 210.

[0081] To effectively transfer heat applied from the upper surfaces of FPCs 500-1 and 500-2, the number of vias or buried VIAs is preferably large, and the area of ​​the heated pads is preferably large. However, when the number of vias or buried VIAs increases and the area of ​​the heated pads also increases, the capacitance due to the structure increases, leading to a decrease in impedance and deterioration of high-frequency characteristics at the solder joint. Therefore, the heated pads can be provided by connecting pads used to form VIAs or vias. This configuration can suppress the increase in capacitance. Furthermore, since the size of the connecting pads increases with the diameter of the vias or buried VIAs, it is desirable that the diameter of the vias or buried VIAs be as small as possible. However, it should be noted that when the via size is smaller than... At 100 μm, solder flow is significantly reduced, and there is a possibility that stable soldering installation cannot be performed. On the other hand, since it is not necessary for solder to flow through the buried VIA, the buried VIA can be set to the desired size.

[0082] When using a hot rod for mounting, pre-soldering treatment needs to be performed on at least one of the heated pads of FPC 500-1 and 500-2 and the RF connection pads of DSP substrate 201. The method for pre-soldering treatment differs depending on whether 508 is a through-hole or a buried VIA.

[0083] When 508 is a through-hole, it is difficult to effectively transfer the heat applied by the hot rod to the DSP substrate 201 side because the through-hole portion is hollow. Therefore, solder is pre-formed on the heating pad 504 using solder paste or the like (pre-soldering treatment), and then the pre-formed solder is heated / pressurized using a hot rod. In this way, during heating, the previously formed solder melts and flows out through the through-hole 508 to the connection portion, thereby forming solder 206. However, from the perspective of solder flowing through the through-hole 508, the diameter of the through-hole 508 is expected to be... 100 μm or larger. Alternatively, a method of heat-treating the solder portion after solder paste application and pre-burying the via portion during the pre-soldering process is also useful. Additionally, this pre-soldering process can be performed on the RF connection pads 210 on the DSP substrate 201. By doing so, the solder will spread, and soldering can be performed stably.

[0084] On the other hand, when 508 is a buried VIA, even if pre-soldering is performed on the heated pad 504, the molten solder cannot flow. In this case, pre-soldering is performed on the RF connection pad 210 on the DSP substrate 201 or the RF connection pad 505 on FPCs 500-1 and 500-2. Then, when the heated pad 504 is heated / pressurized by a hot bar, since VIA 508 is metallic, heat is applied through VIA 508 to the solder formed by the pre-soldering process, and solder 206 is formed. Note that it is sufficient to apply pre-soldering to at least one of the RF connection pads 210 or RF connection pad 505. In the case of a buried VIA, since it is not necessary to consider the solder flowing through the hole, a buried VIA does not always need to have the same characteristics as a through-hole. 100μm or larger. However, it should be noted that when the diameter becomes too small, heat transfer will decrease. It is desirable to set the appropriate buried VIA diameter based on both impedance and heat transfer. For example, considering heat flow, a diameter of 50μm or larger is required. Note that because both buried VIAs and vias cannot be provided for ease of use in general FPC manufacturing processes, it is necessary to form only one of the buried VIAs and vias.

[0085] As described above, regardless of whether 508 is a through-hole or buried VIA, since the solder 206 is formed via hot bars through FPCs 500-1 and 500-2, considering the thermal insulation effect of FPCs 500-1 and 500-2, the setting temperature of the hot bars needs to be set higher than the melting temperature of the solder to be used. For example, when the thickness of FPCs 500-1 and 500-2 is 50 μm, the temperature of the hot bars needs to be set at least +50°C or more higher than the melting temperature. However, if the temperature of the hot bars is set too high, excessive heat is transferred to the DSP chip 202 side, which may damage the DSP chip 202. Therefore, it is desirable to perform soldering after monitoring the solder temperature using a thermocouple via a small thermistor or similar device.

[0086] The solder material can be SnAgCu-based lead-free solder commonly used in optical equipment, but from the perspective of suppressing the thermal impact on the DSP chip 202 and the installation tolerance, it can be a low-temperature solder based on Sn or Bi with a melting temperature of 184°C or lower.

[0087] As described above, when manufacturing the optical transceiver according to this disclosure, it is desirable to use a single heat pipe to simultaneously connect FPC500-1 and FPC 500-2. Given this premise, the load applied by the heat pipe needs to be 10 N or greater.

[0088] Additionally, for example, when Tx module 400 is HB-CDM and Rx module 300 is HB-ICR, and the mounting gap between Tx module 400 and receiving module 300 is 1mm, the expected distance from the edge of FPC 500-1 to the edge of FPC 500-2 is approximately 20 to 30mm. Therefore, it is assumed that the appropriate length of the heat rod in the Y direction is 20 to 30mm. Generally, this length needs to be set considering the temperature distribution of the heat rod. For example, in the case of a type where heat is applied from the center of the heat rod, the area near the center is longer, while the area near the sides has lower heat, and in this case, it is desirable to increase the length as much as possible so that heating is performed in the area near the center of the heat rod. However, when the length of the heat rod in the Y direction is too long, interference may occur between the heat rod and other components and members (e.g., cover 203), so the length of the heat rod in the Y direction needs to be set so as not to interfere with other components and members.

[0089] On the other hand, from the perspective of ensuring high frequency, high-frequency signals cannot pass through when the capacitance becomes too large. Therefore, the dimensions of RF connection pad 505 and RF connection pad 210 are expected to be 1 mm or less in length and 0.2 mm or less in width. In addition, the width (length in the X direction) of the heat pipe is expected to be approximately the same as or slightly smaller than the dimensions of RF connection pad 505 and RF connection pad 210 (e.g., about 200 to 300 μm smaller than the pad size), so that the location where the solder 206 is formed can be effectively heated by the heat pipe.

[0090] [Equipment Configuration]

[0091] Figure 6 This is a side cross-sectional view showing the configuration of a mounting device 700 for an optical transceiver according to this disclosure. (See diagram below.) Figure 6 As shown, the mounting apparatus 700 for an optical transceiver according to the present disclosure includes: a heat rod 701 for applying heat and pressure between RF connection pads 505 and RF connection pads 210 to form solder 206; a mounting base 702 for holding PCB 101 during mounting; a lower support structure 703 disposed between the lower surface of PCB 101 and the mounting base 702 and supporting the portion pressurized by the heat rod 701 from below; and a holding mechanism 704 for achieving high-precision alignment of FPCs 500-1 and 500-2 during mounting and for holding FPCs 500-1 and 500-2 when heated / pressurized using the heat rod 701.

[0092] In use Figure 6In the manufacturing method 600 of the illustrated optical transceiver, it is necessary to effectively apply the heat applied by the heat rod 701 to the RF connection pads 505 and RF connection pads 210. More specifically, the mounting device 700 is configured such that the heat applied by the heat rod 701 is concentrated on the RF connection pads 505 and RF connection pads 210 and conduction to other components and release to the outside is suppressed. As an example of this configuration, such as Figure 7 As shown, the space between the mounting base 702 of the mounting device 700 and the PCB 101 can be configured to be hollow. Because the space between the mounting base 702 and the PCB 101 is hollow, heat conduction is interrupted in the hollow portion, and heat is concentrated on the RF connection pads 505 and 210. However, when the space between the mounting base 702 and the PCB 101 becomes hollow, the PCB 101 deforms (warps) when the hot rod 701 applies a load, causing the load applied by the hot rod 701 to be insufficiently applied to the location where the aforementioned pre-soldering process is performed, or to the RF connection pads 505 and 210. Therefore, by configuring the lower support structure 703 to have a structure that can withstand the load applied from the hot rod 701, appropriate loads can be applied to the pre-soldering portion on the heated pads, the RF connection pads 505, and the RF connection pads 210. Note that the lower support structure 703 is expected to be formed of a material with low thermal conductivity (e.g., resin).

[0093] As another example, the mounting device for an optical transceiver according to this disclosure can be configured to hold PCB 101, wherein the mounting base 702 is in contact with the entire lower surface of PCB 101. In this case, since the aforementioned hollow portion is absent, no deformation of PCB 101 occurs when a load is applied. Therefore, the lower support structure 703 is unnecessary. However, in order to effectively apply heat to RF connection pads 505 and RF connection pads 210, the mounting base 702 needs to be formed of a material with high thermal insulation properties (e.g., resin).

[0094] To facilitate alignment and prevent FPCs 500-1 and 500-2 from moving during heating and pressurization, the retaining mechanism 704 needs to be positioned near the portion in contact with the hot rod 701. Additionally, during retention, the retaining position is unstable when attempting to hold FPCs 500-1 and 500-2 on a flat surface. Therefore, as... Figure 7 As shown in (a), it is desirable that the retaining mechanism 704 has a round rod structure, which is fitted into slots 512 formed on two side surfaces of the substrate 501 of the aforementioned FPCs 500-1 and 500-2. When the diameter of the slot 512 is too small, the diameter of the round rod structure to be fitted is also too small, resulting in unstable retention. Furthermore, maintaining the positioning becomes very difficult, making it necessary for the slot 512 to have at least A diameter of 300 μm or larger is required for stable retention. Additionally, as... Figure 7 As shown in (b) and (c), the retaining mechanism 704 has a mechanism that supports only the substrate 501 of FPC 500-1 and 500-2 on the side and lower surfaces. This configuration achieves stable mounting. Furthermore, from the same angle as the aforementioned device substrate, it is desirable that the retaining mechanism 704 be made of a material with high thermal insulation properties (e.g., resin). However, as... Figure 7 As shown in (c), when the space between the lower surface of the substrate 501 and the holding mechanism 704 is hollow and the slot 512 portion is formed only by a dielectric, the holding mechanism 704 can be formed by a highly conductive material such as metal.

[0095] By fixing the rod-shaped retaining mechanism 704 to the slot 512, FPCs 500-1 and 500-2 can be held even when heated / pressurized by the hot rod 701. Furthermore, to prevent interference between the retaining mechanism 704 and the surface of the DSP substrate 201 and the hot rod 701, the slot 512 is preferably positioned 500 μm or more away from the inner end of the RF connection pad 505.

[0096] [DSP configuration for implementing the installation method]

[0097] Figure 8 This is a diagram showing the detailed structure of the DSP substrate 201 of the optical transceiver 100 according to the present disclosure. Figure 8 (a) shows a top view, and Figure 8 (b) shows a cross-sectional view taken along line Xb-Xb. When performing welding installation using the aforementioned hot rod 701, in order to reduce the risk of deformation or damage to the DSP substrate 201 due to the load applied by the hot rod 701, the DSP substrate 201 needs to have a thickness of 1 mm or greater (e.g., 1 to 2 mm). To achieve a thickness of 1 mm or greater for the DSP substrate 201, a core layer is typically used. This is because: considering high-frequency characteristics, etc., the VIA formed in the DSP substrate 201 is expected to be... 100μm or less, but in this case, the layer thickness of the assembled substrate that can be used is 100μm or less. This is because: in order to set the thickness of the DSP substrate 201 to 1mm or more, the number of layers required is greater than the number of layers required for RF wiring and DC wiring, and there are huge disadvantages in terms of cost, etc.

[0098] In addition, such as Figure 8As shown in (b), in the DSP substrate 201, not only is a core layer with a thickness of 1 mm or greater inserted, but a second VIA 215 is also provided in the core layer below the pads. This configuration strengthens the DSP substrate 201 and suppresses deformation and breakage of the DSP substrate 201 when pressurized by hot rods. Suppressing such deformation (e.g., warping) is also effective from the perspective of achieving uniform load application (suppressing partial contact) during soldering. Since the second VIA 215 does not affect high-frequency characteristics, it can be significantly larger than, for example, [missing information - likely referring to a specific type of VIA 215]. Other buried VIAs of 200 μm or larger. Additionally, such as... Figure 8 As shown in (b), it is desirable to form at least one second VIA 215 below each signal pad and ground pad formed on the upper surface of the DSP substrate 201.

[0099] In addition, in order to effectively apply heat to RF connection pads 505 and RF connection pads 210 and to suppress damage to BGA 204 and the like due to excessive heat flowing through DSP chip 202, it is desirable that the ground portion of RF connection pad 210 includes... Figure 8 The thermal isolation portion 231 is shown in (a). The thermal isolation portion 231 has the following structure: the surface metal of the RF connection pad 210 is separated from the portion where the solder 206 is formed and other portions (only the surface layer metal is not formed in the thermal isolation portion 231). By installing this thermal isolation portion 231, the direct propagation of heat applied from the heat bar 701 to the DSP chip 202 side can be suppressed, and the BGA 204 and DSP chip 202 can be suppressed from being damaged by heat. From the same perspective, regarding 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 substrate 201, the VIA is provided for the subsequent portion, and the wiring is provided in the inner layer of the DSP substrate 201. In addition, from the perspective of high-frequency wiring, providing a groove for thermal isolation in the GND metal portion will lead to impedance mismatch / high-frequency characteristic degradation, because when the portion has surface layer wiring, there is no GND metal in that portion. Therefore, from the perspective of ensuring high-frequency characteristics, it is desirable to use VIA to form inner layer wiring in the groove portion or in the portion closer to the pad than the groove portion.

[0100] In the above description, as an example, the Tx module and Rx module are packaged separately. However, the configuration of the optical transceiver according to this disclosure, in which the optical modulator and optical receiver are packaged in the same package, is also effective in optical transmitter / receiver module form (e.g., IC-TROSA), other package forms, etc.

[0101] Industrial applicability

[0102] The optical transceiver disclosed herein can be used for optical communication.

[0103] List of reference numerals

[0104] 100 optical transceivers

[0105] 101 DSP substrate

[0106] 200 DSP

[0107] 201 DSP substrate

[0108] 202 DSP chip

[0109] 203 Lid

[0110] 206 solder

[0111] 210 RF connection pad

[0112] 215 Second VIA

[0113] 231 Thermal Insulation Section

[0114] 300 Rx Module (Optical Module)

[0115] 301 DC lead pins

[0116] 400 Tx module (optical module)

[0117] 401 DC lead pins

[0118] 402 Fiber Optic

[0119] 403 RF connection pad

[0120] 404 solder

[0121] 405 package sidewall

[0122] 406 Peltier element

[0123] 407 substrate

[0124] 409 Optical Modulator Chip

[0125] 410 lens

[0126] 411 Lens

[0127] 412 Wiring

[0128] 413 Wiring

[0129] 414 Wiring

[0130] 415 Base

[0131] 416 package substrate

[0132] 417 Casing

[0133] 501 substrate

[0134] 502 signal line

[0135] 502a, 502b signal lines

[0136] 503 GND surface

[0137] 504 heated pads

[0138] 505 RF connection pads

[0139] 506 heated pad

[0140] 507 RF connection pads

[0141] 508 VIA or through hole

[0142] 509 VIA or through hole

[0143] 510 semi-through hole

[0144] 511 Semi-through hole

[0145] 512 slot

[0146] 600 Manufacturing Method

[0147] 701 Heatsink

[0148] 702 Mounting Base

[0149] 703 Lower Support Structure

[0150] 704 Maintenance Agency

[0151] 800 optical transceiver

[0152] 802 DSP substrate

[0153] 804 DSP substrate

[0154] 805 DSP chip

[0155] 807 Transmission Module

[0156] 808 fiber optic cable. Claims (as amended under Article 19 of the Treaty) 1. [Delete] 2. [Delete] 3. [Delete] 4. [Delete] 5. [Delete] 6. [Delete] 7. [Delete] 8. [Delete] 9. [Additional] An optical transceiver, comprising: At least one optical module is mounted on a printed circuit board (PCB). A digital signal processor (DSP) is mounted on the PCB; and The flexible printed circuit (FPC) connects the DSP and the optical module. in, The DSP includes a DSP chip mounted on a DSP substrate formed of a multilayer wiring substrate, the multilayer wiring substrate having a core layer for adjusting thickness. The FPC includes: a first connection pad on a first surface, soldered to a pad on the platform surface of the optical module; and a second connection pad on a second surface, soldered to a pad on the upper surface of the DSP substrate. The FPC also includes: The slot has A diameter of 300 μm or greater is required to stably hold the FPC, and the notch is located on both side surfaces of the FPC and at a position 500 μm or greater away from the inner end of each of the first and second connection pads. 10. [Additional] The optical transceiver according to claim 9, wherein, The FPC and the first connection pad, as well as the FPC and the second connection pad, are connected via one or more through-holes or buried VIAs. The FPC also includes: A connecting disc, connected to the through-hole or the buried VIA, is formed on the front and rear surfaces of the FPC. The through hole has 100μm or larger in diameter. 11. [Additional] The optical transceiver according to claim 9, wherein, The DSP substrate: It has a thickness of 1 to 2 mm, and At least one second VIA is included in the core layer and at each location corresponding to the lower part of the pads on the DSP substrate. 12. [Additional] The optical transceiver of claim 10, wherein the FPC further comprises a heated pad connected to the connection pad and mounted on a surface opposite to the location where the first connection pad and the second connection pad are provided. 13. [Additional] The optical transceiver of claim 19, wherein the pads on the DSP substrate include thermally isolated portions that thermally isolate the solder-bearing portions from portions other than the solder-bearing portions, and high-frequency lines for connecting the FPC and DSP chip on the DSP substrate are formed in the inner layer wiring of the DSP substrate. 14. [Additional] A manufacturing apparatus for manufacturing an optical transceiver according to any one of claims 9 to 13, said manufacturing apparatus comprising: A hot rod 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 platform. Mounting base, holding the PCB from below; and A retaining mechanism, by fitting with the slot, retains the FPC from the side and bottom surfaces. 15. [Additional] The manufacturing equipment according to claim 14, wherein, The mounting base is configured to form a hollow space between the PCB and the mounting base, and The manufacturing equipment also includes: The lower support structure is disposed at a position corresponding to the contact position of the heat pipe between the PCB and the mounting base, and in, When the slot is formed solely of dielectric material, the retaining mechanism is formed of resin or metal, and when the slot is metallized, the retaining mechanism is made of resin. 16. [Additional] The manufacturing equipment according to claim 14, wherein, The heat pipe is configured as follows: The FPC, with a length of 20 to 30 mm, is soldered and mounted together with the optical transmission module, including the optical modulator and driver integrated circuits, and the optical receiving module, including the photodetector and transimpedance amplifier. The FPC is contacted at a temperature 50 degrees or more higher than the melting temperature of the solder, and Apply a load of 10N or greater. 17. [Additional] A method for manufacturing an optical transceiver according to any one of claims 10 to 13, the method comprising: The DSP is mounted on the PCB; The optical module and the FPC are connected by welding using hot rods; The optical module connected to the FPC is mounted on the PCB, and the FPC and the DSP are connected by soldering using a heat gun; and Connect the PCB to the DC interface of the optical module. in, The FPC and the first connection pad, as well as the FPC and the second connection pad, are connected via through-holes. The manufacturing method further includes: When soldering using the hot rod, a pre-soldering process is performed to form solder on at least the connecting pad or heated solder pad directly heated by the hot rod; and The solder formed through the pre-soldering process is heated and pressurized using the heat rod. in, The solder, melted by heating and pressurization, passes through the through-hole to the space between the first connecting pad and the platform, and between the second connecting pad and the DSP substrate, forming solder between the first connecting pad and the platform, and between the second connecting pad and the DSP substrate. The FPC and the first connection pad, as well as the FPC and the second connection pad, are connected via a buried VIA. The manufacturing method further includes: When soldering using the hot rod, a pre-soldering process is performed to form solder between the first connecting pad and the platform, and between the second connecting pad and the DSP substrate; and The heat pipe is used to heat and pressurize the connecting pad or the heating pad. in, The solder formed by the pre-soldering process is melted by heat transfer from the heated and pressurized connection pad or heated pad, and the solder is formed between the first connection pad and the platform and between the second connection pad and the DSP substrate.

Claims

1. An optical transceiver comprising: 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, wherein the DSP includes a DSP chip mounted on a DSP substrate formed of a multi-layer wiring substrate having a core layer for adjusting a thickness, the FPC includes first connection pads of a first surface solder-connected to pads of a platform surface of the optical module and second connection pads of a second surface solder-connected to pads of an upper surface of the DSP substrate, the FPC and the first connection pads and the FPC and the second connection pads are connected by one or more through holes or buried vias (VIA), the FPC further includes: connection pads connected to the through holes or the buried VIA and formed on a front surface and a rear surface of the FPC; and 2.The optical transceiver of claim 1, wherein A notch having 300μm or more in diameter, capable of stably holding the FPC, the notch being on both side surfaces of the FPC and at a position of 500μm or more from an inner end of each of the first connection pad and the second connection pad, and The through hole has 100 μm or more in diameter. the DSP substrate: has a thickness of 1 to 2 mm, and includes at least one second VIA in the core layer and at each position corresponding to a lower portion of the pads on the DSP substrate. the FPC further includes a heating pad connected to the connection pads and mounted on a surface opposite to a position where the first connection pads and the second connection pads are disposed.

3. The optical transceiver of claim 1, wherein, the pads on the DSP substrate include a thermal isolation portion that thermally isolates a portion formed with solder from a portion other than the portion formed with solder, and a high-frequency line for connecting the FPC and the DSP chip on the DSP substrate is formed in an inner layer wiring of the DSP substrate.

4. The optical transceiver of claim 1, wherein, 5.A manufacturing apparatus for manufacturing the optical transceiver of any one of claims 1 to 4, the manufacturing apparatus comprising: a thermal bar in contact with the FPC and applying heat and a load to solder formed between the first connection pads and the pads on the DSP substrate and between the second connection pads and the pads on the platform; a mounting base holding the PCB from below; and a holding mechanism holding the FPC from a side surface and a lower surface by fitting with a slot. 6.The manufacturing apparatus of claim 5, wherein the mounting base is configured to form a hollow between the PCB and the mounting base, and the manufacturing apparatus further comprises: a lower support structure disposed at a position corresponding to a position in contact with the thermal bar between the PCB and the mounting base, and wherein when the slot is formed of only a dielectric, the holding mechanism is formed of resin or metal, and when the slot is metalized, the holding mechanism is composed of resin. 7.The manufacturing apparatus of claim 5, wherein the thermal bar is configured to: have a length of 20 to 30 mm, and collectively solder-mount an FPC connected with an optical transmission module including an optical modulator and a driver integrated circuit and an optical reception module including a photodetector and a transimpedance amplifier, ​ ​ contacting the FPC at a temperature that is 50 degrees or more higher than the melting temperature of the solder, and applying a load of 10 N or more.

8. A manufacturing method for manufacturing the optical transceiver according to any one of claims 1 to 4, the method comprising: mounting a DSP on the PCB; connecting the optical module and the FPC by soldering using a hot bar; mounting the optical module connected with the FPC on the PCB and connecting the FPC and the DSP by soldering using a hot bar; and connecting the PCB with a DC interface of the optical module, wherein the FPC and the first connection pad and the FPC and the second connection pad are connected through a via, the manufacturing method further comprises: performing a pre-soldering process of forming a solder on at least a connection pad or a heating pad directly heated by the hot bar when soldering using the hot bar; and heating and pressurizing the solder formed through the pre-soldering process using the hot bar, wherein the solder melted through the heating and pressurizing reaches between the first connection pad and the platform and between the second connection pad and the DSP substrate through the via and forms a solder between the first connection pad and the platform and between the second connection pad and the DSP substrate, or the FPC and the first connection pad and the FPC and the second connection pad are connected through a buried VIA, the manufacturing method further comprises: performing a pre-soldering process of forming a solder between the first connection pad and the platform and between the second connection pad and the DSP substrate when soldering using the hot bar; and heating and pressurizing the connection pad or the heating pad using the hot bar, wherein the solder formed through the pre-soldering process is melted through heat transfer from the heated and pressurized connection pad or heating pad and forms the solder between the first connection pad and the platform and between the second connection pad and the DSP substrate. ​

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