Flexible printed circuit board with embedded optical waveguide structure

Abstract

A flexible printed circuit board with an embedded optical waveguide structure comprises a photoelectric transmission unit which comprises a flexible insulating layer, a first photoelectric unit and a second photoelectric unit embedded in the photoelectric transmission unit, at least one redistribution layer, a first optical waveguide layer and a second optical waveguide layer, the flexible insulating layer is provided with at least one conductive structure which is laminated with the flexible insulating layer and is electrically connected with the first photoelectric unit and the second photoelectric unit, and the optical waveguide structure is laminated with the flexible insulating layer; a first metal bump and a second metal bump adjoin the optical waveguide structure and are optically aligned with the first optoelectronic unit and the second optoelectronic unit, respectively, to provide a reflective surface for optical signals, where the first metal bump and the second metal bump are solid structures made of the same material as the redistribution layer.

Description

Technical Field

[0001] The present invention generally relates to a flexible printed circuit board, and more particularly, to a flexible printed circuit board with an embedded optical waveguide structure. Background Art

[0002] Most existing flexible printed circuit boards are made of copper wires and dielectric materials, which transmit data or information in the form of electrical signals. As the bandwidth requirements of signals increase, the high loss of copper wires at high frequencies will reduce the quality of signal transmission, thereby shortening the distance that the signal can be transmitted. In contrast, the signal transmission method that transmits optical signals through optical waveguide materials has the characteristics of high bandwidth and low loss. It can reduce signal transmission loss and improve the quality of signals at high bandwidth transmission. It is suitable for board-to-board, chip-to-chip, between mobile phone modules and motherboards, chip interconnection in servers in data centers, or high-speed transmission of chips to modules.

[0003] In the existing optical waveguide flexible printed circuit board technology, the optoelectronic unit, the related reflective component, and / or the optical waveguide structure are all installed on a flexible layer structure or a flexible substrate. Such a design will substantially increase the thickness of the entire flexible printed circuit board, and once the die is adhered, the layer-adding process cannot be performed, thereby reducing the number of waveguide layers or waveguide units that can be stacked. Therefore, those skilled in the art still need to improve the structure of the optical waveguide flexible printed circuit board to increase the density of the waveguide circuit and reduce the thickness of the overall structure. Summary of the invention

[0004] In view of the current status of the aforementioned prior art, the present invention proposes a flexible printed circuit board with an embedded optical waveguide structure, wherein the optical waveguide structure is embedded in the flexible layer structure through a layer-building process to reduce the thickness, and the required reflective structure can be formed simultaneously in the process of forming the circuit pattern for the electrical signal, thereby greatly reducing the complexity and steps of the process to improve the alignment between the reflective surface and the optoelectronic element.

[0005] One aspect of the present invention is to provide a flexible printed circuit board with an embedded optical waveguide structure, the structure of which includes: at least one optoelectronic transmission unit, wherein each of the optoelectronic transmission units includes a flexible insulating layer, a first optoelectronic unit embedded in the optoelectronic transmission unit and configured to convert an electrical signal into an optical signal, a second optoelectronic unit embedded in the optoelectronic transmission unit and configured to convert an optical signal into an electrical signal, at least one redistribution layer having at least one conductive structure stacked with the flexible insulating layer and electrically connected to the first optoelectronic unit and the second optoelectronic unit, an optical waveguide structure stacked with the flexible insulating layer for transmitting an optical signal, a first metal bump adjacent to the optical waveguide structure and optically aligned with the first optoelectronic unit to provide a reflection surface for the optical signal, a second metal bump adjacent to the optical waveguide structure and optically aligned with the second optoelectronic unit to provide a reflection surface for the optical signal, and a first flexible covering layer covering the first metal bump, the second metal bump and the optical waveguide structure, wherein the first metal bump and the second metal bump are solid structures made of the same material as the redistribution layer.

[0006] Another aspect of the present invention is to provide a method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure, the steps of which include: embedding a first optoelectronic unit and a second optoelectronic unit into a flexible insulating layer, forming at least one redistribution layer, the redistribution layer having at least one conductive structure stacked with the first optoelectronic unit, the second optoelectronic unit and the flexible insulating layer, forming a first metal bump, a second metal bump and an optical waveguide structure on a first flexible covering layer, and bonding the first flexible covering layer to one side of the flexible insulating layer, the first optoelectronic unit and the second optoelectronic unit, wherein the optical waveguide structure is optically aligned with the first metal bump and the second metal bump on the first flexible covering layer, respectively, wherein the first metal bump and the second metal bump are solid structures made of the same material as the conductive structure of the redistribution layer.

[0007] These and other objects of the present invention will become more apparent to the reader after reading the following detailed description of the preferred embodiment which is described in various figures and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] This specification contains drawings and constitutes a part of this specification, so that readers can further understand the embodiments of the present invention. These drawings depict some embodiments of the present invention and illustrate their principles together with the description herein. In these drawings:

[0009] Figure 1 is a schematic cross-sectional view of a photoelectric transmission unit with an embedded optical waveguide structure according to a preferred embodiment of the present invention;

[0010] Figure 2is a schematic cross-sectional view of a flexible printed circuit board with an embedded optical waveguide structure according to a preferred embodiment of the present invention;

[0011] Figures 3 to 8 A schematic cross-sectional view showing a manufacturing process of an optoelectronic transmission unit with an embedded optical waveguide structure according to a preferred embodiment of the present invention;

[0012] Figures 9 to 12 A schematic cross-sectional view showing a process of manufacturing a reflective metal bump on a flexible cover layer according to a preferred embodiment of the present invention;

[0013] Fig.13 is an enlarged schematic cross-sectional view of a reflective metal bump according to an embodiment of the present invention;

[0014] Fig.14 is a schematic cross-sectional view of a photoelectric transmission unit with an embedded optical waveguide structure according to a preferred embodiment of the present invention;

[0015] Fig.15 A cross-sectional schematic diagram of a flexible printed circuit board of the present invention connected between two printed circuit boards; and

[0016] Fig.16 FIG. 4 is a schematic cross-sectional view of a photoelectric transmission unit with an embedded optical waveguide structure according to another embodiment of the present invention.

[0017] Please note that all illustrations in this manual are of a legend nature. For the sake of clarity and convenience of illustration, the components in the illustrations may be exaggerated or reduced in size and proportion. Generally speaking, the same reference symbols in the figures will be used to indicate corresponding or similar component features after modification or in different embodiments.

[0018] Description of reference numerals:

[0019] 10 Flexible Printed Circuit Board

[0020] 100 Photoelectric Transmission Unit

[0021] 102 Flexible insulation layer

[0022] 104a Photovoltaic unit (VCSEL diode)

[0023] 104b Photovoltaic Unit (VCSEL Driver)

[0024] 106a Photoelectric unit (light sensor)

[0025] 106b Photoelectric unit (transimpedance amplifier)

[0026] 107 Bottom filler

[0027] 108 Conductive structure

[0028] 108L Heavy Duty Layer

[0029] 110 Dielectric Film

[0030] 112 Adhesive layer

[0031] 114 Optical waveguide structure

[0032] 114a Optical waveguide core

[0033] 114b Optical waveguide cladding structure

[0034] 116 (First) Metal Bump

[0035] 116a Reflective surface

[0036] 116b Base

[0037] 118 (Second) Metal Bump

[0038] 118a Reflective surface

[0039] 119 Luminous position

[0040] 120 Flexible Cover

[0041] 120a Insulation layer

[0042] 121 Light receiving position

[0043] 122 Adhesive layer

[0044] 123 Coating

[0045] 124 Flexible Cover

[0046] 125 Photoresist

[0047] 126 Circuit Pattern

[0048] 127 Opening pattern

[0049] 128 Interconnection Structure

[0050] 130 Adhesive layer

[0051] 132 Temporary Base

[0052] 134 Intermediary Layer

[0053] 136 Opening

[0054] 138 Contact

[0055] 140 System Circuit Board

[0056] 142 Connectors DETAILED DESCRIPTION

[0057] The present invention will be described in detail below with reference to the accompanying illustrations, which form part of the present invention and are presented in the form of drawings and specific embodiments according to which the present invention can be implemented. These embodiments will describe sufficient details to enable a person of ordinary skill in the art to implement the present invention. For the sake of simplicity and convenience, the scale and proportion of certain parts in the illustrations may be deliberately reduced or expressed in an exaggerated manner. Other embodiments or structural, logical and electrical changes may be adopted in the invention without departing from the scope of the present invention. Therefore, the detailed description below should not be viewed in a limiting manner, and the scope of the present invention will be defined by the appended claims.

[0058] The reader should be able to easily understand that the meaning of "on", "over" and "above" in the present disclosure should be interpreted in a broad manner, so that "on" not only means "directly on" something but also includes the meaning of being "on" something with an intervening feature or layer, and "on" or "above" not only means "on" or "above" something but also includes the meaning of being "on" or "above" something with no intervening features or layers (i.e., directly on something). In addition, spatially related terms such as "under", "below", "lower", "above", "upper", etc. may be used herein for convenience of description to describe the relationship of one element or feature to another or more elements or features, as shown in the accompanying drawings.

[0059] As used herein, the term "layer" refers to a material portion including an area with a thickness. A layer may extend over the entirety of a lower or upper structure, or may have a range that is less than the range of a lower or upper structure. In addition, a layer may be a region of a homogeneous or inhomogeneous continuous structure whose thickness is less than the thickness of a continuous structure. For example, a layer may be located between the top and bottom surfaces of a continuous structure or between any horizontal faces at the top and bottom surfaces. A layer may extend horizontally, vertically and / or along an inclined surface. A substrate may be a layer, which may include one or more layers, and / or may have one or more layers thereon, above and / or below. A layer may include multiple layers.

[0060] The reader can generally understand the term at least in part from the usage in the context. For example, depending at least in part on the context, the term "one or more" as used herein can be used to describe any feature, structure, or characteristic in a singular sense, or can be used to describe a combination of features, structures, or characteristics in a plural sense. Similarly, depending at least in part on the context, terms such as "a," "an," "the," or "the" can also be understood to convey a singular usage or to convey a plural usage. In addition, the term "based on" can be understood to not necessarily be intended to convey an exclusive set of factors, but can allow for the presence of additional factors that are not necessarily explicitly described, which also depends at least in part on the context.

[0061] The reader can further understand that when the words "include" and / or "contain" are used in this specification, they specify the existence of the stated features, regions, wholes, steps, operations, elements and / or parts, but do not exclude the possibility of the existence or addition of one or more other features, regions, wholes, steps, operations, elements, parts and / or their combinations.

[0062] First, please refer to Figure 1 , which is a cross-sectional schematic diagram of an optoelectronic transmission unit with an embedded optical waveguide structure according to a preferred embodiment of the present invention. Figure 1 As shown, the photoelectric transmission unit 100 of the present invention generally includes an internal flexible insulating layer 102 to support the overall structure. The flexible insulating layer 102 can be a modified polyimide layer (PI) having excellent properties such as low dielectric constant, heat resistance, and chemical resistance. Depending on the needs of the invention, the polyimide layer can be surface-modified by wet etching process and / or adsorption of metal catalyst to provide better adhesion and solderability.

[0063] In a preferred embodiment of the present invention, optoelectronic units 104a, 104b, 106a, 106b for converting optical signals into electrical signals are embedded in the flexible insulating layer 102. At one end of the optoelectronic transmission unit 100, the optoelectronic unit has a vertical cavity surface emitting laser (VCSEL) diode 104a and a VCSEL driver 104b. The VCSEL diode 104a can be composed of gallium nitride (GaN) epitaxy and metal electrodes, which can emit a low-scattering laser beam from a predetermined light-emitting position 119 on the surface of the VCSEL diode 104a. The direction of the emitted laser light is preferably perpendicular to the surface of the flexible insulating layer 102. In an embodiment of the present invention, multiple VCSEL diodes 104a can be arranged in a 2D laser array in the flexible insulating layer 102 to achieve maximum power output. The VCSEL driver 104 b is used to control the light emission of the VCSEL diode 104 a, and may be composed of a balancer, a high-speed current modulator, a modulation current generator, a power-on reset circuit, a control logic clock, and a bias current generator. In other embodiments, the VCSEL diode 104 a and the VCSEL driver 104 b may be selectively formed on the surface of the flexible insulating layer 102 .

[0064] Re-reference Figure 1 . At the other end of the photoelectric transmission unit 100, the photoelectric unit may have a photo sensor 106a and a transimpedance amplifier (TIA) 106b. In an embodiment of the present invention, the photo sensor 106a may be a common photodiode, an avalanche photodiode (APD), a single photon avalanche photodiode (SPAD) or a silicon photomultiplier (SiPM), which is used to convert the light from the optical waveguide structure 114 into a current or voltage that can be used in an electronic circuit later. The photo sensor 106a can receive the laser light emitted from the VCSEL diode 104a at the other end. The introduced laser light will illuminate the light receiving position 121 on the surface of the photo sensor 106a. The incident direction of the laser light is preferably perpendicular to the surface of the flexible insulating layer 102. Similarly, a plurality of photo sensors 106a can be arranged in the flexible insulating layer 102 in the form of a sensor array. Furthermore, for applications requiring high speed and high signal bandwidth, the transimpedance amplifier 106b in this embodiment is used to convert the photocurrent generated by the light sensor 106a into a voltage signal, and then the voltage signal can be amplified and used in the subsequent digital circuit. The transimpedance amplifier 106b can be composed of circuits such as an operational amplifier and a feedback resistor. In other embodiments, the VCSEL diode 104a and the VCSEL driver 104b can also be selectively formed on the surface of the flexible insulating layer 102.

[0065] Re-reference Figure 1 . The gaps between the optoelectronic units 104a, 104b, 106a, 106b or between the optoelectronic units 104a, 104b, 106a, 106b and the flexible insulating layer 102 may be filled with an underfill 107. The material of the underfill 107 may be epoxy resin, which may be dispensed into the gaps and then thermally cured. Preferably, the top and bottom surfaces of the embedded optoelectronic units 104a, 104b, 106a, 106b, the cured underfill 107, and the flexible insulating layer 102 are flush to facilitate the subsequent lamination process. In an embodiment of the present invention, at least one redistribution layer 108L is formed on the flexible insulating layer 102, which includes at least one dielectric film 110 and at least one conductive structure 108 located in / on the dielectric film 110. The conductive structure 108 of the redistribution layer 108L is electrically connected to the optoelectronic units 104a, 104b, 106a, 106b. More specifically, a dielectric film 110, such as a polyimide film, is first formed on the flexible insulating layer 102 and the surfaces of the optoelectronic units 104a, 104b, 106a, 106b, and then the conductive structure 108 of the redistribution layer 108L is formed in / on the dielectric film 110 and electrically connected to the optoelectronic units 104a, 104b, 106a, 106b through the openings of the dielectric film 110. The conductive structure material of the redistribution layer 108L can be a metal such as copper, silver, nickel, aluminum, gold, or an alloy thereof. The optoelectronic units 104a, 104b, 106a, 106b can be electrically connected to other interconnect structures in a flexible printed circuit board (FPC) or an external printed circuit board (PCBs) through the conductive structure 108 of the redistribution layer 108L. The openings of the dielectric film 110 can also be correspondingly formed at the light emitting position 119 of the VCSEL diode 104a and the light receiving position 121 of the light sensor 106a to provide a light transmission path.

[0066] Re-reference Figure 1. An adhesive layer 112 is formed on the dielectric film 110 and the redistribution layer 108L conductive structure 108 to provide adhesion for bonding to external stacked layers or structures. The material of the adhesive layer 112 can be epoxy resin or polyimide, and the adhesive layer 112 can be filled in the openings on the light-emitting position 119 and the light-receiving position 121 without blocking the transmission of optical signals. The outermost flexible cover layer 120 will cover and bond to the adhesive layer 112, wherein a first metal bump 116, an optical waveguide structure 114 and a second metal bump 118 are arranged between the adhesive layer 112 and the flexible cover layer 120 to form a light transmission path. In a preferred embodiment of the present invention, the first metal bump 116 and the second metal bump 118 are arranged to be optically aligned with the optoelectronic unit 104a (such as a VCSEL diode) and the optoelectronic unit 106a (such as a light sensor), respectively, to provide reflection surfaces 116a, 118a for the optical signal. More specifically, the reflective surface 116a of the first metal bump 116 is optically aligned with the light emitting position 119 of the photoelectric unit 104a, and the reflective surface 118a of the second metal bump 118 is optically aligned with the light receiving position 121 of the photoelectric unit 106a, so that the emitted light signal can be reflected by the reflective surfaces 116a, 118a. In addition, a circuit pattern for electrical signals can be formed on the outer surface of the flexible cover layer 120 as required.

[0067] Furthermore, if Figure 1 As shown, the optical waveguide structure 114 is formed between the first metal bump 116 and the second metal bump 118. In a preferred embodiment of the present invention, the optical waveguide structure 114 is designed to be adjacent to the reflective surfaces 116a, 118a of the first metal bump 116 and the second metal bump 118, so that the optical signal reflected by the reflective surfaces 116a, 118a can be transmitted in the optical waveguide structure 114 without being dispersed. In operation, the optical signal emitted from the light emitting position 119 of the VCSEL diode 104a at one end is reflected by the reflective surface 116a of the first metal bump 116 and introduced into the optical waveguide structure 114. The optical signal is then transmitted to the other end of the optoelectronic transmission unit 100 by total reflection in the optical waveguide structure 114, and the optical signal is further reflected by the reflective surface 118a of the second metal bump 118 to the light receiving position 121 of the optical sensor 106a, and the optical signal can be converted into an electrical signal, such as a voltage signal, as a digital signal of the optical sensor 106a.

[0068] In the embodiment of the present invention, the material of the flexible cover layer 120 can be modified polyimide like the flexible insulating layer 102 to provide properties such as low dielectric constant, heat resistance, and chemical resistance. The material of the optical waveguide structure 114 can be polyimide or cyclo-olefin polymer (COP), which has excellent optical properties, including high light transmittance, low birefringence, low water absorption, and good heat resistance. It is worth mentioning that in the embodiment of the present invention, the first metal bump 116 and the second metal bump 118 are completely made of metal materials such as copper, silver, nickel, aluminum, gold, or alloys thereof, including their outer reflective surfaces and internal structures. This feature allows them to be simply formed on the flexible cover layer 120 through the same machine or process, such as the same electroplating process as the conductive structure 108 of the redistribution layer 108L, which will be described in detail in the subsequent embodiments.

[0069] Re-reference Figure 1 The aforementioned redistribution layer 108L, metal bumps 116, 118, and optical waveguide structure 114 are all formed on one side of the flexible insulating layer 102. In the embodiment of the present invention, the other side of the flexible insulating layer 102 can also be bonded to a flexible cover layer 124 via an adhesive layer 122, and the circuit pattern 126 can also be formed on the outer surface of the flexible cover layer 124 as required.

[0070] In the aforementioned embodiment, the redistribution layer 108L and the optical waveguide structure 114 are stacked on the same surface of the flexible insulating layer 102. In other embodiments, such as Fig.16 As shown, the redistribution layer 108L and the optical waveguide structure 114 can be stacked on two opposite surfaces of the flexible insulating layer 102. In this embodiment, since the light transmission path will not be blocked by the dielectric film 110 of the redistribution layer 108L, it is not necessary to form an opening for the light transmission path in the dielectric film 110. In another embodiment of the present invention, the redistribution layer 108L and the optical waveguide structure 114 can be stacked on both sides of the flexible insulating layer 102.

[0071] Now please refer to Figure 2 In the embodiment of the present invention, a plurality of the aforementioned photoelectric transmission units 100 can be laminated to form a flexible printed circuit board (FPC) 10. Figure 2As shown, three exemplary optoelectronic transmission units 100 are laminated and bonded by an adhesive layer 130 (such as an epoxy resin or polyimide layer) to form a multi-layer optical transmission path, wherein the optoelectronic transmission units 100 can be electrically connected to each other or to an external circuit pattern 126 through the conductive structure 108 of the redistribution layer 108L and the interconnection structure 128 that passes through the optoelectronic transmission unit 100 stack, and they can be further electrically connected to an external circuit together, such as being connected to a system circuit board through contacts such as golden fingers, solder bumps, or connectors.

[0072] After describing the structure of the photoelectric transmission unit 100 and the flexible printed circuit board 10 of the present invention, the following will now refer to Figures 3 to 8 The manufacturing process of the optoelectronic transmission unit with an embedded optical waveguide structure according to the preferred embodiment of the present invention is described.

[0073] Please refer to Figure 3 At the beginning of the process, a temporary substrate 132 is provided as a supporting base for the components and layer structures to be formed or attached thereto during the process. The temporary substrate 132 may be a substrate with a low thermal expansion coefficient, such as a glass substrate. An interposer 134 is formed on the temporary substrate 132 as a release film or release paper, such as a polyester fiber (PET) film or a polyimide film coated with silicone, which can provide good heat resistance, glue resistance, filling and separation properties. After the temporary substrate 132 and the interposer 134 are prepared, the photovoltaic units 104a, 104b, 106a, 106b are attached to the interposer 134. The interposer 134 can provide moderate adhesion to fix the photovoltaic units 104a, 104b, 106a, 106b on the temporary substrate 132. After the photovoltaic units 104a, 104b, 106a, 106b are attached, a flexible insulating layer 102 is laminated on the interposer 134. The flexible insulating layer 102 may be a modified polyimide layer, which has excellent properties such as low dielectric constant, heat resistance and chemical resistance. In the embodiment of the present invention, the flexible insulating layer 102 has a predetermined opening pattern, so that the photoelectric units 104a, 104b, 106a, 106b fixed on the intermediate layer 134 will be located in the opening pattern after the flexible insulating layer 102 is pressed.

[0074] Please refer to Figure 4. After the flexible insulating layer 102 is pressed together, gaps may be formed between the optoelectronic units 104a, 104b, 106a, 106b or between the optoelectronic units 104a, 104b, 106a, 106b and the flexible insulating layer 102. In an embodiment of the present invention, these gaps are filled with bottom filler 107, such as epoxy resin, which can be filled into the gaps by dispensing and then thermally cured. The bottom filler 107 can enhance the mechanical strength and reliability of the entire flexible printed circuit board, and avoid problems such as component displacement or solder joint breakage and peeling during the temperature cycle of the FPC product. Preferably, the top and bottom surfaces of the embedded optoelectronic units 104a, 104b, 106a, 106b, the cured bottom filler 107, and the flexible insulating layer 102 are flush to facilitate the subsequent lamination process. In the present invention, since the optoelectronic units 104a, 104b, 106a, 106b are embedded in the flexible insulating layer 102, the subsequent FPC layer-building process will not be hindered by the components or require additional steps, which can increase the transmission density. In addition, since the bottom filler 107 is filled in the gap on the side of the optoelectronic unit, it will not block the light transmission path, which can reduce signal loss and can use an opaque bottom filler 107.

[0075] Please refer to Figure 5 After filling the bottom filler 107, a redistribution layer 108L including at least one conductive structure 108 and at least one dielectric film 110 is then formed on the flexible insulating layer 102 to electrically connect the optoelectronic units 104a, 104b, 106a, 106b. More specifically, the dielectric film 110, such as a modified polyimide film, is first formed on the surface of the flexible insulating layer 102 and the optoelectronic units 104a, 104b, 106a, 106b, and then an opening pattern or a fabricated conductive pattern is formed in the dielectric film 110 by a photolithography process or an electroplating process to interconnect the conductive structure 108 of the redistribution layer 108L with the optoelectronic units 104a, 104b, 106a, 106b. The conductive structure 108 of the redistribution layer 108L can be formed in / on the dielectric film 110 by first depositing a metal layer on the dielectric film 110 by an electroplating or chemical plating process and then patterning the metal layer into a redistribution layer circuit by a photolithography process. The material of the conductive structure 108 of the redistribution layer 108L can be a metal such as copper, silver, nickel, aluminum, gold, or an alloy thereof. The metal layer formed in the opening pattern serves as a contact connecting the conductive structure 108 of the redistribution layer 108L and the optoelectronic units 104a, 104b, 106a, 106b. In an embodiment of the present invention, some openings 136 are designed to remain on the optoelectronic unit (i.e., VCSEL diode) 104a and the optoelectronic unit (i.e., optical sensor) 106a to provide a light transmission path.

[0076] Please refer to Figure 6After the redistribution layer 108L is formed, an adhesive layer 112 is laminated on the conductive structure 108 of the redistribution layer 108L and the dielectric film 110 to provide adhesion for bonding the outer stack or structure. In an embodiment of the present invention, the material of the adhesive layer 112 may be highly transparent polyimide, polyester fiber, acrylic or modified epoxy resin, which can be directly pressed onto the layer structure, and the adhesive layer 112 will fill the opening of the dielectric film 110 and will not block the transmission of the optical signal.

[0077] After the adhesive layer 112 is formed, an outer flexible cover layer (also referred to as a cover layer) 120 is laminated on the adhesive layer 112. In the embodiment of the present invention, one surface of the flexible cover layer 120 has two metal bumps 116, 118 and an optical waveguide structure 114, wherein the first metal bump 116, the optical waveguide structure 114 and the second metal bump 118 are disposed between the adhesive layer 112 and the flexible cover layer 120 to form a light transmission path. More specifically, the optical waveguide structure 114 is designed to be adjacent to the reflection surfaces 116a, 118a of the first metal bump 116 and the second metal bump 118, so that the optical signal reflected by the reflection surfaces 116a, 118a can be transmitted in the optical waveguide structure 114 without being dispersed. The material of the metal bumps 116, 118 can be the same metal material as the conductive structure 108 of the redistribution layer 108L, such as copper, silver, nickel, aluminum, gold or alloys thereof, which can be formed on the flexible cover layer 120 by electroplating. The material of the optical waveguide structure 114 can be polyimide or cycloolefin polymer, which has excellent optical properties, including high light transmittance, low birefringence, low water absorption and good heat resistance, and can be formed on the flexible cover layer 120 by a doctor blade coating process.

[0078] Please refer to Figure 7. During the bonding process, the first metal bump 116 and the second metal bump 118 are configured to be optically aligned with the optoelectronic unit 104a (such as a VCSEL diode) and the optoelectronic unit 106a (such as a photo sensor), respectively, to provide reflection surfaces 116a, 118a for the optical signal. More specifically, the reflection surface 116a of the first metal bump 116 is optically aligned with the light emitting position 119 of the optoelectronic unit 104a through the opening of the dielectric film 110, and the reflection surface 118a of the second metal bump 118 is optically aligned with the light receiving position 121 of the optoelectronic unit 106a through the opening of the dielectric film 110, so that the emitted optical signal can be reflected by the reflection surfaces 116a, 118a. In the present invention, since the optical waveguide structure 114 is formed next to the metal bumps 116, 118, the optoelectronic units 104a, 106a can be accurately aligned with the optical waveguide structure 114 through the metal bumps 116, 118, effectively reducing the alignment error. In addition, a circuit pattern (not shown) can be formed on the other side of the flexible cover layer 120 as required. After the flexible cover layer 120 is laminated, the temporary substrate 132 and the intermediate layer 134 can be easily peeled off from the flexible insulating layer 102 under a suitable release environment, such as a suitable release temperature or treatment.

[0079] Please refer to Figure 8 After the temporary substrate 132 and the intermediate layer 134 are removed, the entire substrate is turned over with the back side facing upward. Similarly, a flexible cover layer 124 is pressed onto the back side of the flexible insulating layer 102 through the adhesive layer 122. The material of the flexible cover layer 124 can be modified polyimide, and the material of the adhesive layer 122 can be epoxy resin or polyimide. Similarly, a circuit pattern (not shown) can be formed on the other side of the flexible cover layer 124 as required.

[0080] After the photoelectric transmission unit 100 is prepared, Figure 2 As shown, a plurality of photoelectric transmission units 100 can be pressed together via an adhesive layer 130 to form a flexible printed circuit board 10, and an interconnection structure 128 can be formed in the stack of the flexible printed circuit board 10 to electrically connect the conductive structure 108 of the redistribution layer 108L on the photoelectric units 104a, 104b, 106a, 106b in the photoelectric transmission unit 100 to the external circuit pattern 126, and these circuits can be further electrically connected to an external circuit together, such as being connected to a system circuit board via contact members such as slots, gold fingers, solder bumps, or connectors.

[0081] Please refer to Figures 9 to 12, which shows a cross-sectional schematic diagram of a process for making a reflective metal bump on a flexible cover layer according to a preferred embodiment of the present invention. In these figures, a metal bump 116 and a circuit pattern 126 are shown as examples to illustrate the method of the present invention. Please note that a plurality of metal bumps and circuit patterns may be formed on the flexible cover layer, and they may be formed on the same surface or different surfaces of the flexible cover layer depending on product requirements.

[0082] As described in the previous embodiment, the metal bumps 116, 118 may be pre-formed on the flexible cover layer 120 of the present invention before being attached to the substrate. Fig. 9 The original flexible cover layer 120 before the process may be a flexible metal clad laminate, such as a flexible copper clad laminate (FCCL), which is composed of an insulating layer 120a (such as a polyimide layer) located in the middle and two coating layers 123 (such as copper foil) located on both sides of the insulating layer 120a. Photoresists 125 are formed on the two coating layers 123, respectively, wherein the photoresists 125 are formed with opening patterns 127 by a photolithography process to define the shape of the metal bumps. The shape of the metal bumps expected by the present invention can be formed and adjusted by adjusting the photolithography process parameters.

[0083] Please refer to Fig.10 . After the photoresist 125 is formed, an electroplating process is performed to form metal bumps 116 and circuit patterns 126 on the coating layer 123. Please note that in the present invention, the first metal bump 116 and the second metal bump 118 are solid structures, which are made of the same material as the conductive structure 108 of the redistribution layer 108L, the external metal pattern 126, or other interconnect structures in the flexible circuit board such as contacts. The material of the metal bump 116 and the circuit pattern 126 can be a metal such as copper, silver, nickel, aluminum, gold, or an alloy thereof. The copper (metal) coating layer 123 can be used as a seed layer in the electroplating process to grow the metal bump 116 and the circuit pattern 126, and the height of the metal bump 116 or the circuit pattern 126 can be controlled by the duration of the electroplating process. In the present invention, since the metal bumps 116 and the circuit patterns 126 are integrated and formed simultaneously in the same process, the complexity and steps of the process can be greatly reduced, the alignment between the reflective surface and the optoelectronic element can be improved, and the overall performance of signal transmission can be enhanced.

[0084] Please refer to Fig.11After the metal bumps 116 and the circuit patterns 126 are formed, the photoresist 125 is removed from the cover layer 123 , and then an etching process is performed to remove the exposed cover layer 123 from the insulating layer 120 a , thereby separating the different metal bumps 116 and circuit patterns 126 on the insulating layer 120 a .

[0085] Please refer to Fig.12 After the etching process, the optical waveguide structure 114 is formed on the insulating layer 120a by a doctor blade coating process or by laminating a thin optical waveguide layer. The material of the optical waveguide structure 114 can be polyimide or cycloolefin polymer. Preferably, the optical waveguide structure 114 can be directly formed to contact the metal bump 116 and one side of the cladding layer 123 to receive the optical signal reflected by the metal bump 116, and the top surface of the metal bump 116 is flush with the surface of the optical waveguide structure 114.

[0086] Please refer to Fig.13 , which is an enlarged schematic diagram of the cross section of the reflective metal bump. In the present invention, the height of the metal bump 116 is 15-30 μm, the height of the coating layer 123 can be 3-12 μm, and the width of the base 116b of the metal bump 116 can be 30-60 μm. Preferably, the angle between the reflective surface 116a of the metal bump 116 and the vertical normal of the flexible cover layer 120 is 45±5°.

[0087] Please refer to Fig.14, which is a cross-sectional schematic diagram of an optoelectronic transmission unit with an embedded optical waveguide structure according to a preferred embodiment of the present invention. In the embodiment of the present invention, the optical waveguide structure 114 may include an optical waveguide core 114a for optical transmission and an optical waveguide cladding structure 114b around the optical waveguide core 114a for isolating the optical signal. The optical waveguide structure 114 is disposed between the VCSEL diode 104a (directly below the light emitting position of the VCSEL diode 104a) and the flexible cover layer 120, and its width (in a direction orthogonal to the optical transmission path of the optical waveguide structure 114) is greater than or approximately equal to the width of the VCSEL diode 104a. In order to achieve a total reflection effect, the refractive index of the optical waveguide core 114a must be greater than the refractive index of the optical waveguide cladding structure 114b. The material of the optical waveguide structure 114 may be polyimide or cycloolefin polymer, which has excellent optical properties, including high light transmittance, low birefringence, low water absorption, and good heat resistance. The waveguide core 114a and the optical waveguide cladding structure 114b can be formed of the same material, such as polyimide, which is partially modified by using different UV curing processes in the photolithography process so that they have different refractive indexes. During operation, the light emitted by the VCSEL diode 104a is guided into the optical waveguide core 114a at one end of the optoelectronic transmission unit and transmitted to the other end of the optoelectronic transmission unit. Due to the total reflection effect provided by the optical waveguide core and the cladding layer, the transmitted optical signal will not be diverged and weakened.

[0088] Please refer to Fig.15 , which is a cross-sectional schematic diagram of the flexible printed circuit board of the present invention connected between two printed circuit boards. The flexible printed circuit board 10 composed of a plurality of photoelectric transmission units 100 can be used to connect two system circuit boards 140. The connection between the two can be established by using the contact pieces 138 (such as gold fingers) of the circuit pattern 126 formed on the flexible printed circuit board 10 to connect the contact pieces in the connector (or slot) 142 installed on the two system circuit boards 140. In other embodiments, the flexible printed circuit board 10 can be directly connected to the contact pieces formed on the system circuit board 140 by welding (such as solder) instead of using a connector (or slot), and such connection can be a horizontal board-to-board connection or a vertical board-to-board connection (such as a flip-port).

[0089] The above descriptions are only preferred embodiments of the present invention. All equivalent changes and modifications made according to the claims of the present invention should fall within the scope of the present invention.

Claims

1. A flexible printed circuit board with an embedded optical waveguide structure, comprising at least one photoelectric transmission unit, wherein each of the photoelectric transmission units comprises: a flexible insulating layer; a first optoelectronic unit, embedded in the optoelectronic transmission unit and configured to convert the electrical signal into an optical signal; a second optoelectronic unit, embedded in the optoelectronic transmission unit and configured to convert the optical signal into an electrical signal; At least one redistribution layer has at least one conductive structure stacked with the flexible insulating layer and electrically connected to the first photovoltaic unit and the second photovoltaic unit; an optical waveguide structure stacked with the flexible insulating layer, for transmitting optical signals; a first metal bump adjacent to the optical waveguide structure and optically aligned with the first optoelectronic unit to provide a reflection surface for optical signals; a second metal bump adjacent to the optical waveguide structure and optically aligned with the second optoelectronic unit to provide a reflection surface for the optical signal; as well as a first flexible covering layer, covering the first metal bump, the second metal bump and the optical waveguide structure; The first metal bump and the second metal bump are solid structures made of the same material as the redistribution layer.

2. The flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 1, wherein the reflective surface of the first metal bump is optically aligned with a light emitting position of the first optoelectronic unit, and the reflective surface of the second metal bump is optically aligned with a light receiving position of the second optoelectronic unit.

3. The flexible printed circuit board with an embedded optical waveguide structure as described in claim 2, wherein the redistribution layer further includes at least one dielectric film between the flexible insulating layer and the conductive structure, the dielectric film has two openings, and the reflective surfaces of the first metal bump and the second metal bump are respectively aligned with the light-emitting position and the light-receiving position through the two openings. 4 . The flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 1 , wherein a material of the reflective surface of the first metal bump and the second metal bump is the same as a material of an internal structure of the first metal bump and the second metal bump. 5 . The flexible printed circuit board with embedded optical waveguide structure as claimed in claim 1 , wherein the base width of the first metal bump and the second metal bump is 30-60 μm, and the height of the first metal bump and the second metal bump is 15-30 μm. 6 . The flexible printed circuit board with embedded optical waveguide structure as claimed in claim 1 , wherein the first optoelectronic unit comprises a vertical cavity surface emitting laser diode and a vertical cavity surface emitting laser driver. 7 . The flexible printed circuit board with embedded optical waveguide structure as claimed in claim 1 , wherein the second photoelectric unit comprises a photodiode and a transimpedance amplifier. 8 . The flexible printed circuit board with embedded optical waveguide structure as claimed in claim 1 , further comprising a bottom filling glue filled in the gaps between the first optoelectronic unit, the second optoelectronic unit and the flexible insulating layer. 9 . The flexible printed circuit board with embedded optical waveguide structure as claimed in claim 1 , further comprising a second flexible covering layer covering the other side of the flexible insulating layer relative to the first flexible covering layer.

10. The flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 1, comprising a plurality of the photoelectric transmission units, wherein the photoelectric transmission units are laminated together, the first photoelectric units are located at one end of the flexible printed circuit board, and the second photoelectric units are located at the other end of the flexible printed circuit board.

11. The flexible printed circuit board with embedded optical waveguide structure as claimed in claim 10, further comprising two connectors respectively located at two ends of the flexible printed circuit board, and the two connectors electrically connect the first optoelectronic units and the second optoelectronic units through the conductive structure of the redistribution layer.

12. The flexible printed circuit board with embedded optical waveguide structure as claimed in claim 1, wherein the optical waveguide structure comprises an optical waveguide core and an optical waveguide cladding structure surrounding each of the optical waveguide cores, and the refractive index of the optical waveguide core is greater than the refractive index of the optical waveguide cladding structure.

13. A method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure, comprising: Embedding a first photovoltaic unit and a second photovoltaic unit into a flexible insulating layer; forming at least one redistribution layer, the redistribution layer having at least one conductive structure stacked with the first photovoltaic unit, the second photovoltaic unit and the flexible insulating layer; forming a first metal bump, a second metal bump and an optical waveguide structure on a first flexible cover layer; and Bonding the first flexible cover layer to one side of the flexible insulating layer, the first optoelectronic unit and the second optoelectronic unit, wherein the optical waveguide structure is optically aligned with the first metal bump and the second metal bump on the first flexible cover layer, respectively; The first metal bump and the second metal bump are solid structures made of the same material as the conductive structure of the redistribution layer.

14. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure according to claim 13, further comprising: Providing the first flexible covering layer, wherein the first flexible covering layer comprises an insulating layer and two metal coating layers respectively located on two sides of the insulating layer; Forming a patterned photoresist on the two metal coating layers, wherein the photoresist has opening patterns of the first metal bump and the second metal bump; Performing an electroplating process to form the first metal bump and the second metal bump on one of the metal coating layers; removing the patterned photoresist; Performing an etching process to remove the metal coating layer exposed from the first metal bump and the second metal bump; as well as The optical waveguide structure is formed on the insulating layer between the first metal bump and the second metal bump.

15. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure according to claim 14, further comprising: forming a patterned photoresist on another of the metal coating layers; and An electroplating process is performed to form a circuit pattern on the other metal coating layer.

16. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 14, wherein the first flexible covering layer is a flexible covering metal base plate, the first metal bump, the second metal bump and the optical waveguide structure are located on the flexible covering metal base plate, the first metal bump and the second metal bump both have a reflective surface, and two ends of the optical waveguide structure contact the reflective surfaces of the first metal bump and the second metal bump respectively.

17. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 13, wherein the first flexible cover layer is bonded to the flexible insulating layer through an adhesive layer at a side thereof having the first metal bump, the second metal bump and the optical waveguide structure.

18. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure according to claim 13, further comprising: providing a temporary substrate; and The first photovoltaic unit, the second photovoltaic unit and the flexible insulating layer are attached to the temporary substrate. 19 . The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 18 , further comprising filling a bottom filling glue in the gaps between the first optoelectronic unit, the second optoelectronic unit and the flexible insulating layer.

20. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 13, further comprising laminating a plurality of the photoelectric transmission units to form the flexible printed circuit board, the first photoelectric unit and the second photoelectric unit are respectively located at two ends of the flexible printed circuit board, and the optical waveguide structure is connected between the first photoelectric unit and the second photoelectric unit.

21. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 13, further comprising bonding a second flexible cover layer to the other surface of the flexible insulating layer.

22. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 21, further comprising forming an interconnection structure extending through the flexible insulating layer and electrically connecting the redistribution layers and the circuit patterns on the first flexible cover layer and the second flexible cover layer.

23. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 13, further comprising connecting two connectors to the first optoelectronic unit and the second optoelectronic unit located at two ends of the flexible printed circuit board respectively through the redistribution layer.

24. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 13, wherein the optical waveguide structure comprises an optical waveguide core and an optical waveguide cladding structure surrounding each of the optical waveguide cores, and the refractive index of the optical waveguide core is greater than the refractive index of the optical waveguide cladding structure.

25. The method for manufacturing a flexible printed circuit board with an embedded optical waveguide structure as claimed in claim 23, wherein the step of forming the optical waveguide core and the optical waveguide cladding structure comprises: forming an optical waveguide material on the first flexible cover layer; and A photolithography process is performed on the optical waveguide material to form a patterned optical waveguide core and an optical waveguide cladding structure with different refractive indices.

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