Optical module and flexible printed circuit board

Abstract

To provide an optical module and a flexible printed circuit board, capable of reducing degradation of a high-speed signal in an electrical connection.SOLUTION: An optical module according to one embodiment includes: a package including a substrate that includes first signal wiring and first ground wiring formed on a second wiring layer located below a first wiring layer; and a flexible printed circuit board having an upper surface wiring layer, a lower surface wiring layer, and a base layer arranged between the upper surface wiring layer and the lower surface wiring layer, and including second signal wiring formed on the upper surface wiring layer and second ground wiring formed on the lower surface wiring layer. The second signal wiring includes a lead portion protruding from the base layer in a first direction. The second ground wiring has a ground terminal located further inside the flexible printed circuit board in the first direction than the lead portion. The lead portion is connected to the first signal wiring, and the ground terminal is connected to the first ground wiring.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present disclosure relates to an optical module and a flexible printed circuit board. [Background technology]

[0002] Patent Document 1 describes an optical transmitter. The optical transmitter includes a driver IC, a light-emitting element, a flexible substrate, and a housing. The housing has a plurality of electrical input units and an optical output unit. The flexible substrate has a signal line that outputs a drive signal output by the driver IC to the light-emitting element, and a plurality of DC lines. The intermediate portions of each of the plurality of DC lines have a flying lead structure in which the base material is removed leaving the electrodes.

[0003] Patent Document 2 describes a modulator module having a modulator, a metal package that houses the modulator, and an intermediate substrate. The modulator has a signal electrode and a ground electrode, and the signal electrode and the ground electrode are led to the outside via the intermediate substrate. A signal electrode is formed on the upper surface of the intermediate substrate. A lead pin is connected to the signal electrode by solder. A flexible substrate is connected to the lead pin. The flexible substrate has a signal electrode and a ground electrode, and a part of the ground electrode is a flying lead that protrudes from an end of the flexible substrate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-210962 A [Patent Document 2] JP 2012-48121 A Summary of the Invention [Problem to be solved by the invention]

[0005] In an optical module, a flexible printed circuit board is connected to a package. The flexible printed circuit board has a plurality of electrical wirings electrically connected to the package, and transmits high-speed signals to the package via the electrical wirings, for example. The optical module converts the high-speed signals transmitted by the flexible printed circuit board into optical signals and outputs them to the outside. Signal transmission by this flexible printed circuit board may require low propagation loss and precise impedance matching over a wide frequency range up to high frequencies. In this case, it is required to reduce degradation of high-speed signal transmission in the electrical connection with the flexible printed circuit board.

[0006] An object of the present disclosure is to provide an optical module and a flexible printed circuit board capable of reducing degradation of high-speed signals in electrical connections. [Means for solving the problem]

[0007] The optical module according to the present disclosure includes a package including a substrate having a first signal wiring formed in a first wiring layer and a first ground wiring formed in a second wiring layer located below the first wiring layer, and a flexible printed circuit board having an upper wiring layer, a lower wiring layer, and a base layer disposed between the upper wiring layer and the lower wiring layer, the flexible printed circuit board including a second signal wiring formed in the upper wiring layer and a second ground wiring formed in the lower wiring layer. The second signal wiring has a lead portion protruding from the base layer in a first direction. The second ground wiring has a ground terminal located on the inside of the flexible printed circuit board in the first direction with respect to the lead portion. The lead portion is connected to the first signal wiring, and the ground terminal is connected to the first ground wiring. Effect of the Invention

[0008] According to the present disclosure, degradation of high-speed signals in electrical connections can be reduced. [Brief description of the drawings]

[0009] [Figure 1]FIG. 1 is a plan view showing a flexible printed circuit board according to an embodiment. [Diagram 2] FIG. 2 is a bottom view showing the flexible printed circuit board of FIG. [Diagram 3] FIG. 3 is a perspective view showing a package according to an embodiment. [Figure 4] FIG. 4 is a diagram illustrating a cross section of a flexible printed circuit board and a package according to an embodiment. [Figure 5A] FIG. 5A is a plan view showing a flexible printed circuit board and a package. [Figure 5B] FIG. 5B is a cross-sectional view taken along line AA in FIG. 5A. [Figure 5C] FIG. 5C is a cross-sectional view taken along line BB in FIG. 5A. [Figure 6] FIG. 6 is a cross-sectional view taken along line CC in FIG. 5A. [Figure 7] FIG. 7 is a diagram showing a schematic diagram of the distribution of the electric field in the optical module according to the reference example and the optical module according to the embodiment. [Figure 8A] FIG. 8A is a plan view showing a flexible printed circuit board and a package according to a first modified example. [Figure 8B] FIG. 8B is a cross-sectional view taken along line AA in FIG. 8A. [Figure 8C] FIG. 8C is a cross-sectional view taken along line BB in FIG. 8A. [Figure 9A] FIG. 9A is a plan view showing a flexible printed circuit board and a package according to a second modified example. [Figure 9B] FIG. 9B is a cross-sectional view taken along line AA in FIG. 9A. [Figure 9C] FIG. 9C is a cross-sectional view taken along line BB in FIG. 9A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [Description of the embodiments of the present disclosure] First, embodiments of an optical module and a flexible printed circuit board according to the present disclosure will be described. (1) An optical module according to an embodiment includes a package including a substrate having a first signal wiring formed in a first wiring layer and a first ground wiring formed in a second wiring layer located below the first wiring layer, and a flexible printed circuit board including an upper wiring layer, a lower wiring layer, and a base layer disposed between the upper wiring layer and the lower wiring layer, the flexible printed circuit board including a second signal wiring formed in the upper wiring layer and a second ground wiring formed in the lower wiring layer. The second signal wiring has a lead portion protruding from the base layer in a first direction. The second ground wiring has a ground terminal located on the inside of the flexible printed circuit board in the first direction with respect to the lead portion. The lead portion is connected to the first signal wiring, and the ground terminal is connected to the first ground wiring.

[0011] In this optical module, the package has a first signal wiring and a first ground wiring. The first wiring layer has the first signal wiring, and the second wiring layer has the first ground wiring. The flexible printed circuit board has an upper wiring layer, a lower wiring layer, and a base layer, and the base layer is formed between the upper wiring layer and the lower wiring layer. The upper wiring layer has a second signal wiring, and the lower wiring layer has a second ground wiring. The second signal wiring of the flexible printed circuit board has a lead portion protruding from the base layer, and the lead portion is connected to the first signal wiring of the substrate of the package. By making the lead portion exposed to the outside and connectable to the first signal wiring, it is possible to eliminate the need to form a via for solder heat conduction in the connection portion of the flexible printed circuit board to the package. Since no via is formed in the connection portion, it is possible to suppress impedance mismatch in high-speed signal transmission and suppress signal reflection in the signal wiring. Therefore, it is possible to reduce propagation loss in a wide frequency range, and to reduce deterioration of high-speed signal transmission in the electrical connection with the flexible printed circuit board.

[0012] (2) In the above (1), the first ground wiring may extend further out of the package than the first signal wiring in the first direction. In this case, the first ground wiring protrudes further out of the package than the first signal wiring, which facilitates connection of the flexible printed circuit board to the package. This makes it easy to assemble the optical module.

[0013] (3) In the above (1) or (2), the substrate may have a first dielectric layer located between the first wiring layer and the second wiring layer, and a second dielectric layer located on the opposite side of the first dielectric layer from the second wiring layer. An end of the first signal wiring in the direction opposite to the first direction may be spaced apart from the base layer of the flexible printed circuit board, and an end of the second ground wiring in the first direction may be spaced apart from the first dielectric layer. In this case, even if the flexible printed circuit board is thin, contact of the first signal wiring with the second ground wiring can be suppressed. Furthermore, a decrease in impedance caused by the first signal wiring being close to the second ground wiring can be suppressed.

[0014] (4) In any of the above (1) to (3), the upper wiring layer may be located above the base layer, and the lower wiring layer may be located below the base layer.

[0015] (5) A flexible printed circuit board according to one embodiment is a flexible printed circuit board including an upper wiring layer, a lower wiring layer, and a base layer disposed between the upper wiring layer and the lower wiring layer. A second signal wiring is formed on the upper wiring layer, and a second ground wiring is formed on the lower wiring layer. The second signal wiring has a lead portion protruding from the base layer in a first direction, and the second ground wiring has a ground terminal located on the inside of the flexible printed circuit board in the first direction relative to the lead portion. The lead portion is connected to a first signal wiring of a substrate provided in a package of an optical module, and the ground terminal is connected to the first ground wiring of the substrate.

[0016] This flexible printed circuit board has an upper wiring layer, a lower wiring layer, and a base layer, and the base layer is formed between the upper wiring layer and the lower wiring layer. A second signal wiring is formed on the upper wiring layer, and a second ground wiring is formed on the lower wiring layer. The second signal wiring of the flexible printed circuit board has a lead portion protruding from the base layer, and the lead portion is connected to the first signal wiring of the substrate of the package. By making the lead portion exposed to the outside and connectable to the first signal wiring of the package, it is possible to eliminate the need to form a via for solder heat conduction in the connection portion of the flexible printed circuit board to the package. Thus, in this flexible printed circuit board, as in the above-mentioned optical module, the reflection of the signal in the signal wiring can be suppressed by suppressing the impedance mismatch in high-speed signal transmission, and the propagation loss can be reduced in a wide frequency range. Therefore, the deterioration of the transmission of high-speed signals in the electrical connection can be reduced.

[0017] [Details of the embodiment of the present disclosure] Specific examples of the optical module and flexible printed circuit board according to the embodiment will be described below with reference to the drawings. Note that the present invention is not limited to the following examples, but is intended to include all modifications within the scope of the claims and equivalents thereto. In the description of the drawings, the same or corresponding elements are given the same reference numerals, and duplicated descriptions are omitted as appropriate. The drawings may be partially simplified or exaggerated for ease of understanding, and the dimensional ratios and the like are not limited to those shown in the drawings.

[0018] The optical module 1 according to this embodiment has a flexible printed circuit board 10 and a package 20 (see FIG. 4). FIG. 1 is a plan view showing the flexible printed circuit board 10 according to this embodiment. FIG. 2 is a bottom view showing the flexible printed circuit board 10. As shown in FIGS. 1 and 2, the flexible printed circuit board 10 extends in both a first direction D1 and a second direction D2 intersecting the first direction D1. The first direction D1 is the longitudinal direction of the flexible printed circuit board 10, and the second direction D2 is the lateral direction of the flexible printed circuit board 10.

[0019] For example, the flexible printed circuit board 10 has a rectangular shape in a plan view. The flexible printed circuit board 10 has a connection portion 11 at an end portion in the first direction D1. The connection portion 11 is a portion for electrically connecting the flexible printed circuit board 10 to the package 20. The connection portion 11 is provided, for example, in a region extending from a first end 12b to a second end 12c in the second direction D2 at an end portion in the first direction D1 of the flexible printed circuit board 10. Note that although FIGS. 1 and 2 show an end portion in the first direction D1 of the flexible printed circuit board 10, a connection portion similar to the connection portion 11 may be provided at an end portion of the flexible printed circuit board 10 opposite to the first direction D1.

[0020] The flexible printed circuit board 10 has a second signal wiring 13, and the second signal wiring 13 has a lead portion 13b protruding in a first direction D1 from the flexible printed circuit board 10. The second signal wiring 13 extends along the first direction D1. The first direction D1 is the direction in which the second signal wiring 13 extends. The flexible printed circuit board 10 has a plurality of lead portions 13b. The plurality of lead portions 13b each extend along the first direction D1. The plurality of lead portions 13b are arranged along the second direction D2. For example, the plurality of lead portions 13b include a first lead portion 13c and a second lead portion 13d. The flexible printed circuit board 10 has a plurality of sets 13f each consisting of the first lead portion 13c and the second lead portion 13d, and the plurality of sets 13f are arranged along the second direction D2. For example, a distance K1 from the first lead portion 13c to the second lead portion 13d is shorter than a distance K2 from the set 13f to the set 13f adjacent in the second direction D2. The flexible printed circuit board 10 has a second ground wiring 17, and the second ground wiring 17 is formed on the inside of the lead portion 13b in the first direction D1 of the flexible printed circuit board 10.

[0021] FIG. 3 is a perspective view showing the package 20. The package 20 extends in both the first direction D1 and the second direction D2, and has a thickness in a third direction D3 intersecting both the first direction D1 and the second direction D2. The package 20 accommodates, for example, a light receiving element and a light emitting element provided in the optical module 1, and an electronic circuit connected to the light receiving element and the light emitting element. The package 20 has a rectangular parallelepiped shape. The package 20 includes a bottom plate 21, a rectangular frame-shaped side wall 22 provided on the bottom plate 21, and a top plate 23 provided on the side wall 22. Hereinafter, the direction in which the top plate 23 is viewed from the bottom plate 21 may be referred to as top, upper side, or upward, and the direction in which the bottom plate 21 is viewed from the top plate 23 may be referred to as bottom, lower side, or downward. However, these directions are for convenience of explanation and do not limit the arrangement position and direction of the components.

[0022] The bottom plate 21, the side walls 22, and the top plate 23 define an internal space of the package 20. The side walls 22 have a front surface 22b and a rear surface 22c that face each other in a first direction D1, and a first side surface 22d and a second side surface 22f that face each other in a second direction D2. The front surface 22b and the rear surface 22c each extend in both the second direction D2 and the third direction D3, and the first side surface 22d and the second side surface 22f each extend in both the first direction D1 and the third direction D3. Hereinafter, the direction in which the front surface 22b is viewed from the rear surface 22c may be referred to as the front, front side, or front, and the direction in which the rear surface 22c is viewed from the front surface 22b may be referred to as the rear, rear side, or rear. However, these directions are for convenience of explanation and do not limit the arrangement position and direction of the components. The first side surface 22d extends from the front surface 22b to the rear surface 22c at an end of the package 20 in the second direction D2. The second side surface 22f extends from the front surface 22b to the rear surface 22c at an end of the package 20 opposite the second direction D2.

[0023] The package 20 further includes a substrate 24. The substrate 24 is also referred to as a feedthrough. The substrate 24 is provided at the front end (front end) of the package 20. The substrate 24 is disposed so as to cover an opening 22g formed in the sidewall 22, and a rear end (rear end) of the substrate 24 faces the internal space of the package 20. The front end of the substrate 24 is exposed to the outside of the package 20. Therefore, at the opening 22g, the substrate 24 penetrates the sidewall 22 along the first direction D1.

[0024] For example, the substrate 24 includes a dielectric layer 25 and a wiring layer 26. The dielectric layer 25 is made of a dielectric material. For example, the dielectric layer 25 is made of alumina. The substrate 24 includes a plurality of dielectric layers 25 and a plurality of wiring layers 26. The plurality of dielectric layers 25 include, for example, a first dielectric layer 25b and a second dielectric layer 25c located below the first dielectric layer 25b. The second dielectric layer 25c and the first dielectric layer 25b are arranged in this order from bottom to top along the third direction D3. The plurality of wiring layers 26 include, for example, a first wiring layer 26b and a second wiring layer 26c located below the first wiring layer 26b. The second wiring layer 26c and the first wiring layer 26b are arranged in this order from bottom to top along the third direction D3. The thickness (length in the third direction D3) of the first wiring layer 26b and the second wiring layer 26c is, for example, 10 μm. The thickness of the first wiring layer 26b and the second wiring layer 26c may be, for example, not less than 5 μm and not more than 20 μm.

[0025] When viewed along the third direction D3, the second dielectric layer 25c protrudes outward in the first direction D1 more than the first dielectric layer 25b, and the second wiring layer 26c extends outward in the first direction D1 more than the first wiring layer 26b. For example, in the first direction D1, the length of the second dielectric layer 25c is greater than the length of the first dielectric layer 25b. Also, in the first direction D1, the length of the second wiring layer 26c is greater than the length of the first wiring layer 26b. The substrate 24 has a first signal wiring 27 formed in the first wiring layer 26b and a first ground wiring 28 formed in the second wiring layer 26c located below the first wiring layer 26b. A plurality of first signal wirings 27 may be formed in the first wiring layer 26b. For example, each of the plurality of first signal wirings 27 extends along the first direction D1 and is arranged along the second direction D2. The first signal wirings 27 are electrically separated from one another and can transmit different signals. The distance between two adjacent first signal wirings 27 in the second direction D2 is, for example, equal to or greater than the shortest distance K3.

[0026] 4 shows a cross-sectional view of the flexible printed circuit board 10, the side wall 22, and the substrate 24 cut along a plane extending in both the first direction D1 and the third direction D3. As shown in FIGS. 3 and 4, the substrate 24 has a first upper surface 24b extending in both the first direction D1 and the second direction D2, a first end surface 24c extending downward from the front end (front end) of the first upper surface 24b, a second upper surface 24d extending forward from the lower end of the first end surface 24c, and a second end surface 24f extending downward from the front end (front end) of the second upper surface 24d. The substrate 24 has a stepped structure including the first upper surface 24b, the first end surface 24c, the second upper surface 24d, and the second end surface 24f. This stepped structure includes the first end surface 24c as a step by setting the length of the second dielectric layer 25c to be greater than the length of the first dielectric layer 25b in the first direction D1. The second upper surface 24d extends in both the first direction D1 and the second direction D2. As shown in FIG. 4, the flexible printed circuit board 10 is located in front of the substrate 24 in the first direction D1, and the substrate 24 is located behind the flexible printed circuit board 10.

[0027] The first upper surface 24b corresponds to the upper surface of the first dielectric layer 25b. The first wiring layer 26b is formed on the first upper surface 24b. A plurality of wirings spaced apart from each other may be formed on the first wiring layer 26b. For example, the first wiring layer 26b includes a plurality of first signal wirings 27. The second upper surface 24d corresponds to the upper surface of the second dielectric layer 25c. The second wiring layer 26c is formed on the second upper surface 24d. A plurality of wirings spaced apart from each other may be formed on the second wiring layer 26c. For example, the second wiring layer 26c includes a first ground wiring 28. The first ground wiring 28 may be configured as a planar pattern extending in the first direction D1 and the second direction D2 as a so-called solid wiring. For example, the first ground wiring 28 may be configured to cover the entire upper surface of the second dielectric layer 25c. When the first ground wiring 28 is formed on a portion of the second upper surface 24d of the second dielectric layer 25c, the portion of the second upper surface 24d on which the first ground wiring 28 is not formed contacts the lower surface of the first dielectric layer 25b.

[0028] The flexible printed circuit board 10 has a first end surface 10b extending in both the second direction D2 and the third direction D3, a base layer lower surface 10c extending forward from the lower end of the first end surface 10b, and a second end surface 10d extending downward from a front end (front end) of a ground terminal 17b (described later) of the base layer lower surface 10c. The flexible printed circuit board 10 is connected to the substrate 24 in a state in which the first end surface 10b faces the first end surface 24c of the substrate 24, the base layer lower surface 10c is placed on the second upper surface 24d of the substrate 24, and the second end surface 10d faces the second end surface 24f of the substrate 24. For example, the flexible printed circuit board 10 is connected to the substrate 24 via solder. Note that, if a lower surface protective layer 19 (described later) is not provided, the second end surface 10d is omitted.

[0029] A height T1 (length in the third direction D3) from the upper surface of the first ground wiring 28 to the upper surface of the first wiring layer 26b corresponds to the height of the step of the substrate 24. When the first signal wiring 27 is formed on the first wiring layer 26b, the height T1 corresponds to the sum of the thickness (length in the third direction D3) of the first signal wiring 27 and the thickness of the first dielectric layer 25b. The height T1 corresponds to the length of the first end surface 24c in the third direction D3. In the connection portion 11 of the flexible printed circuit board 10, a height T2 from the lower surface of the lower wiring layer 15 to the lower surface of the upper wiring layer 14 corresponds to the sum of the thickness of the lower wiring layer 15 and the thickness of the base layer 16. For example, when the second ground wiring 17 is formed on the lower wiring layer 15, the height T2 corresponds to the sum of the thickness of the second ground wiring 17 and the thickness of the base layer 16.

[0030] In the optical module 1 according to this embodiment, the height T2 is approximately the same as the height T1. "Approximately the same" includes the case where they are the same, and the case where they are not completely the same but have the same effect as when they are the same. When the height T2 is approximately the same as the height T1, for example, solder (or adhesive) of approximately the same thickness can be applied between the second signal wiring 13 and the first signal wiring 27, and between the second ground wiring 17 and the first ground wiring 28, respectively, to ensure the respective connections.

[0031] The flexible printed circuit board 10 has an upper wiring layer 14, a lower wiring layer 15, and a base layer 16 disposed between the upper wiring layer 14 and the lower wiring layer 15. The upper wiring layer 14 is located on the base layer 16, and the lower wiring layer 15 is located below the base layer 16. The base layer 16 is also called a base film, and is made of, for example, polyimide. The thickness (length in the third direction D3) of the base layer 16 may be within a range of, for example, 25 μm to 200 μm. The above-mentioned base layer lower surface 10c corresponds to the lower surface of the base layer 16. The lower wiring layer 15 is formed on the base layer lower surface 10c. The lower wiring layer 15 is made of, for example, copper (Cu). A plurality of wirings separated from each other may be formed on the lower wiring layer 15. For example, the lower wiring layer 15 includes a second ground wiring 17. The second ground wiring 17 may be configured as a planar pattern extending in the first direction D1 and the second direction D2 as a so-called solid wiring. For example, the second ground wiring 17 may be configured to cover the entire lower surface of the base layer 16. The upper surface wiring layer 14 is formed on the upper surface (base layer upper surface) 10e of the base layer 16. The upper surface wiring layer 14 is made of, for example, copper (Cu). A plurality of wirings separated from each other may be formed in the upper surface wiring layer 14. For example, the upper surface wiring layer 14 includes the second signal wiring 13. That is, the second signal wiring 13 is formed in the upper surface wiring layer 14. The second ground wiring 17 is formed in the lower surface wiring layer 15. The flexible printed circuit board 10 has a plurality of lead portions 13b, and the plurality of lead portions 13b are arranged along the second direction D2. The thickness (length in the third direction D3) of the upper surface wiring layer 14 and the lower surface wiring layer 15 is, for example, 20 μm. The thickness of the upper wiring layer 14 and the lower wiring layer 15 may be, for example, not less than 5 μm and not more than 50 μm.

[0032] The flexible printed circuit board 10 may further have an upper surface protective layer 18 and a lower surface protective layer 19. The upper surface protective layer 18 is formed on the upper surface wiring layer 14 so as to cover the upper surface wiring layer 14. The upper surface protective layer 18 protects the upper surface wiring layer 14 from the outside. In this case, in the third direction D3, the upper surface wiring layer 14 is sandwiched between the base layer 16 and the upper surface protective layer 18. The lower surface protective layer 19 is formed below the lower surface wiring layer 15 so as to cover the lower surface wiring layer 15. The lower surface protective layer 19 protects the lower surface wiring layer 15 from the outside. In this case, in the third direction D3, the lower surface wiring layer 15 is sandwiched between the base layer 16 and the lower surface protective layer 19. The upper surface protective layer 18 and the lower surface protective layer 19 are made of, for example, a solder resist. Also, the upper surface protective layer 18 and the lower surface protective layer 19 may be made of, for example, a coverlay. The upper protective layer 18 and the lower protective layer 19 each have a thickness of, for example, 25 μm.

[0033] The lead portion 13b protrudes from the first end surface 10b of the flexible printed circuit board 10 in the first direction D1. The lead portion 13b is integral with the second signal wiring 13 and protrudes from the base layer 16 in the first direction D1. The lead portion 13b also protrudes from the upper protective layer 18 and the lower protective layer 19. For example, when a plurality of second signal wirings 13 are formed on the flexible printed circuit board 10, each of the second signal wirings 13 may have a lead portion 13b. The lead portions 13b of the plurality of second signal wirings 13 are electrically connected to the plurality of first signal wirings 27 as terminals. The second ground wiring 17 is located on the inside of the flexible printed circuit board 10 relative to the lead portion 13b in the first direction D1. That is, the front end of the second ground wiring 17 is located forward of the front end of the lead portion 13b. Alternatively, the lead portion 13b is located between the second ground wiring 17 and the front surface 22b of the package 20 in the first direction D1.

[0034] The lead portion 13b does not have the base layer 16, the upper surface protective layer 18, and the lower surface protective layer 19, and is a terminal composed only of electrical wiring, and is called a flying lead. A structure having the terminal is called a flying lead structure. A flexible printed circuit board having a flying lead structure is called a flying lead FPC (Flexible Printed Circuit). The flexible printed circuit board 10 according to the present embodiment has a flying lead structure formed at an end in the first direction D1. In the lead portion 13b, for example, when the electrical wiring is composed of copper (Cu), the surface of the electrical wiring may be plated with gold (Au). Furthermore, a base metal layer may be formed between the copper constituting the electrical wiring and the gold plated on the surface of the electrical wiring. The base metal layer is composed of, for example, nickel (Ni) and palladium (Pd).

[0035] The second ground wiring 17 has a ground terminal 17b, and the ground terminal 17b is located inside the flexible printed circuit board 10 with respect to the lead portion 13b in the first direction D1. The second ground wiring 17 extends in both the first direction D1 and the second direction D2 on the base layer lower surface 10c of the flexible printed circuit board 10. For example, the second ground wiring 17 may be formed in the lower wiring layer 15 so as to cover the entire base layer lower surface 10c. For example, the second ground wiring 17 may be formed as a so-called solid wiring (solid pattern). The lead portion 13b is connected to the first signal wiring 27 in a state where it is placed on the first signal wiring 27 of the substrate 24. For example, the lead portion 13b is joined to the corresponding first signal wiring 27 by soldering. The ground terminal 17b is connected to the first ground wiring 28 in a state where it is placed on the first ground wiring 28 of the substrate 24. For example, the ground terminal 17b is joined to the first ground wiring 28 by soldering. The ground terminal 17b corresponds to a portion of the second ground wiring 17 connected to the first ground wiring 28. For example, when the flexible printed circuit board 10 has a bottom protective layer 19, the ground terminal 17b is exposed from the bottom protective layer 19. The first ground wiring 28 extends outside the package 20 further than the first signal wiring 27 in the first direction D1. That is, in the first direction D1, the first signal wiring 27 is located between the first ground wiring 28 and the front surface 22b. The first signal wiring 27 is formed on the first upper surface 24b of the substrate 24, and the first ground wiring 28 is formed on the second upper surface 24d of the substrate 24.

[0036] FIG. 5A is a plan view showing a connection portion 11 of the flexible printed circuit board 10. FIG. 5B is a cross-sectional view taken along line AA in FIG. 5A. FIG. 5C is a cross-sectional view taken along line BB in FIG. 5A. FIG. 6 is a cross-sectional view taken along line CC in FIG. 5A. As shown in FIGS. 5A, 5B, 5C, and 6, in a plan view taken along the third direction D3, one first ground wiring 28, two first signal wirings 27, and one first ground wiring 28 are arranged in this order along the second direction D2. That is, the optical module 1 has a GSSG (ground-signal-signal-ground) configuration. The two first signal wirings 27 transmit, for example, a pair of complementary signals. The pair of complementary signals may be, for example, a differential signal having a positive phase signal and a negative phase signal.

[0037] For example, on the first upper surface 24b, the thickness (length in the second direction D2) of the first ground wiring 28 is larger than the thickness of the first signal wiring 27. The lead portion 13b is disposed so as to overlap the first signal wiring 27. For example, the thickness W1 of the lead portion 13b is approximately the same as the thickness of the first signal wiring 27. As an example, the thickness W1 of the lead portion 13b is 100 μm. However, the thickness W1 of the lead portion 13b does not have to be approximately the same as the thickness of the first signal wiring 27 and can be changed as appropriate. For example, the thickness of the first signal wiring 27, the distance (spacing) between the two first signal wirings 27, and the distance (spacing) between the first signal wiring 27 and the first ground wiring 28 may be set according to the characteristic impedance of the transmission line configured by the GSSG configuration. In addition, the thickness of a pair of second signal wirings 13 connected to a pair of first signal wirings 27 and the distance (spacing) between the two second signal wirings may be set taking into consideration mutual impedance matching between the pair of first signal wirings 27 and the pair of second signal wirings 13.

[0038] In this embodiment, the second signal wiring 13 (particularly the lead portion 13b) does not have a via for connecting to the first signal wiring 27. Since the second signal wiring 13 does not have a via, it is possible to set the thickness of the second signal wiring 13 arbitrarily. For example, since the second signal wiring 13 does not have a via, it is possible to make the thickness of the second signal wiring 13 smaller than the lower limit of the via diameter in manufacturing. More specifically, when the second signal wiring 13 has a via, the thickness of the second signal wiring 13 is about 200 μm, whereas when the second signal wiring 13 does not have a via, the thickness W1 of the second signal wiring 13 can be reduced to about 100 μm. In this embodiment, the second signal wiring 13 can be made thinner to widen the interval W2 between the second signal wirings 13, so that the impedance can be set to an appropriate value in a wider range. The characteristic impedance of the differential line according to electromagnetic field analysis is 63Ω when the second signal wiring 13 has a via, whereas in this embodiment, the second signal wiring 13 does not have a via, it can be increased to 100Ω.

[0039] As shown in FIG. 6, the first ground wiring 28 of the substrate 24 includes, for example, an upper wiring 28b formed as the first wiring layer 26b on the first upper surface 24b, and a lower wiring 28c formed as the second wiring layer 26c located inside the substrate 24. The first ground wiring 28 of the substrate 24 may further include an intermediate wiring 28d located between the upper wiring 28b and the lower wiring 28c. The substrate 24 has a via 29 extending in the third direction D3. The via 29 is filled with, for example, a metal inside. The via 29 extends from the upper wiring 28b through the intermediate wiring 28d to the lower wiring 28c. As an example, the via 29 has a cylindrical shape extending along the third direction D3. The substrate 24 has a plurality of vias 29, and the plurality of vias 29 are arranged along the first direction D1. The first ground wiring 28 includes an upper wiring 28b and a lower wiring 28c electrically connected to each other by the via 29. The first ground wiring 28 may include a wiring formed in the intermediate wiring 28d shown in Fig. 6. The intermediate wiring 28d is omitted in Fig. 5C.

[0040] For example, in each upper wiring 28b, the vias 29 are arranged at equal intervals along the first direction D1. For example, in the multiple upper wirings 28b, the positions of the vias 29 in the first direction D1 of each upper wiring 28b are the same as the positions of the vias 29 in the first direction of the other upper wirings 28b. Therefore, in a plan view along the third direction D3, the multiple vias 29 are arranged along the second direction D2. The second ground wiring 17 is connected to the first ground wiring 28, and the lead portion 13b is connected to the first signal wiring 27 via solder. For example, by heating a heater tool P extending along the second direction D2 and applying it to the vias 29 and the lead portion 13b from above, the heat of the heater tool P is transferred to the first ground wiring 28, the vias 29, the second ground wiring 17, the lead portion 13b, and the first signal wiring 27. This allows the second ground wiring 17 to be soldered to the first ground wiring 28, and the lead portion 13b to the first signal wiring 27 to be soldered to one time.

[0041] Next, the effects obtained from the optical module 1 and the flexible printed circuit board 10 according to the present embodiment will be described. In the optical module 1, the package 20 has a first signal wiring 27 and a first ground wiring 28. The first signal wiring 27 is formed in the first wiring layer 26b, and the first ground wiring 28 is formed in the second wiring layer 26c. The flexible printed circuit board 10 has an upper wiring layer 14, a lower wiring layer 15, and a base layer 16, and the base layer 16 is formed between the upper wiring layer 14 and the lower wiring layer 15. The upper wiring layer 14 is formed on the upper surface (base layer upper surface) 10e of the base layer, and the lower wiring layer 15 is formed on the lower surface (base layer lower surface) 10c of the base layer. The second signal wiring 13 is formed in the upper wiring layer 14, and the second ground wiring 17 is formed in the lower wiring layer 15. The second signal wiring 13 of the flexible printed circuit board 10 has a lead portion 13b protruding from the base layer 16 as a terminal, and the lead portion 13b is electrically connected to the first signal wiring 27 of the substrate 24 of the package 20. Since the lead portion 13b is exposed to the outside and connected to the first signal wiring 27, it is possible to eliminate the need to form a via for solder heat conduction in the connection portion 11 of the flexible printed circuit board 10 to the package 20. Since no via is formed in the connection portion 11, it is possible to adjust the characteristic impedance in a wider range, and it is possible to suppress impedance mismatch between the signal wirings and suppress signal reflection in the signal wiring. Therefore, it is possible to reduce propagation loss in a wide frequency range, and it is possible to reduce deterioration of high-speed signal transmission in the electrical connection with the flexible printed circuit board 10.

[0042] As described above, the front end (front end) of the first ground wiring 28 may be located further forward in the package 20 than the first signal wiring 27 in the first direction D1. In this case, the first ground wiring 28 protrudes further outward from the package 20 than the first signal wiring 27, which facilitates connection of the flexible printed circuit board 10 to the package 20. Therefore, the optical module 1 can be easily assembled.

[0043] 7 is a schematic diagram showing the directions of electric fields of differential signals transmitted from the flexible printed circuit boards 10 and 110 to the package 20 in the optical module 1 according to the embodiment and the optical module 100 according to the reference example. In FIG. 7, the differential signals propagate along a direction perpendicular to the paper surface. In the optical module 1, the flexible printed circuit board 10 is connected to the package 20 in a state in which the first signal wiring 27 of the substrate 24 is located above the first ground wiring 28 and the second signal wiring 13 of the flexible printed circuit board 10 is located above the second ground wiring 17.

[0044] Therefore, in a state where the flexible printed circuit board 10 is connected to the package 20, the height (position in the third direction D3) of the second ground wiring 17 formed on the lower wiring layer 15 is approximately the same as the height of the first ground wiring 28 formed on the second wiring layer 26c, and the height of the second signal wiring 13 formed on the upper wiring layer 14 is approximately the same as the height of the first signal wiring 27 formed on the first upper surface. In contrast, the optical module 100 according to the reference example does not have a lead portion, so that a terminal is provided on the inside of the flexible printed circuit board 110. Therefore, the second signal wiring 113 exposed downward of the flexible printed circuit board 110 is located below the second ground wiring 117. The flexible printed circuit board 110 is connected to the package 20 in a state where the second signal wiring 113 and the second ground wiring 117 are upside down compared to the flexible printed circuit board 10.

[0045] In the optical module 100, the direction of the electric field from the first signal wiring 27 to the first ground wiring 28 in the package 20 is reversed from the direction of the electric field from the second signal wiring 113 to the second ground wiring 117 in the flexible printed circuit board 110. Furthermore, the direction of the electric field from the first ground wiring 28 to the first signal wiring 27 in the package 20 is reversed from the direction of the electric field from the second ground wiring 117 to the second signal wiring 113 in the flexible printed circuit board 110.

[0046] In contrast, in the optical module 1, the direction of the electric field from the first signal wiring 27 to the first ground wiring 28 in the package 20 is the same as the direction of the electric field from the second signal wiring 13 to the second ground wiring 17 in the flexible printed circuit board 10. The direction of the electric field from the first ground wiring 28 to the first signal wiring 27 in the package 20 is the same as the direction of the electric field from the second ground wiring 17 to the second signal wiring 13 in the flexible printed circuit board 10.

[0047] The reversal of the electric field direction in the optical module 100 described above is unlikely to be a problem for frequency components of, for example, about 50 GHz or less. However, for frequency components exceeding 50 GHz, the optical module 100 may cause degradation of signal waveform quality due to electromagnetic radiation caused by disturbance in the electric field distribution. In contrast, in the optical module 1, the electric field direction is the same as described above, so degradation of signal waveform quality can be reduced.

[0048] Next, an optical module 1A according to a first modified example will be described with reference to Figures 8A, 8B, and 8C. A portion of the configuration of the optical module 1A is the same as a portion of the configuration of the above-mentioned optical module 1. Therefore, hereinafter, the same components as those in the optical module 1 will be given the same reference numerals, and duplicated descriptions will be omitted as appropriate.

[0049] The optical module 1A has a flexible printed circuit board 10A and a package 20A having shapes different from those of the flexible printed circuit board 10 and the package 20 described above. The first ground wiring 28 of the substrate 24A of the package 20A includes an upper wiring 28b formed on the first upper surface 24b, a lower wiring 28c located inside the substrate 24A, and an intermediate wiring 28f formed on the second upper surface 24d. The upper wiring 28b is formed in the first wiring layer as in the package 20, and the lower wiring 28c is formed in the second wiring layer as in the package 20. The substrate 24A has a first via 29b extending from the upper wiring 28b to the lower wiring 28c, and a second via 29c extending from the intermediate wiring 28f to the lower wiring 28c. Each of the first via 29b and the second via 29c is filled with a metal. Package 20A differs from package 20 in that it has an intermediate wiring layer between the first wiring layer and the second wiring layer, intermediate wiring 28f is formed on the second upper surface 24d by the intermediate wiring layer, and intermediate wiring 28f is connected to the lower wiring 28c via a second via 29c.

[0050] The optical module 1A differs from the optical module 1 described above in that it has a second via 29c extending downward from the intermediate wiring 28f. That is, in the substrate 24A of the optical module 1A, the first ground wiring 28 (intermediate wiring 28f) is formed at a position higher than the lower wiring 28c. This makes it possible to reduce the step between the first upper surface 24b and the second upper surface 24d of the substrate 24A, and to connect a flexible printed circuit board 10A having a thinner base layer 16 than the flexible printed circuit board 10. In this way, by making it possible to set the distances between the signal wiring and the ground wiring in the third direction D3 separately between the substrate 24A and the flexible printed circuit board 10A, it is possible to more freely adjust the characteristic impedance of the transmission lines formed by the respective signal wirings.

[0051] For example, by applying a heater tool P extending along the second direction D2 to the first via 29b from above, heat from the heater tool P is transferred from the first via 29b to the intermediate wiring 28f through the lower wiring 28c and the second via 29c. This allows the second ground wiring 17 to be soldered to the intermediate wiring 28f and the lead portion 13b to the first signal wiring 27 to be soldered at the same time.

[0052] A distance H1 in the third direction D3 between the second signal wiring 13 and the second ground wiring 17 in the flexible printed circuit board 10A is smaller than a distance H2 in the third direction D3 between the first signal wiring 27 and the first ground wiring 28 in the substrate 24A. For example, the distance H1 is 100 μm and the distance H2 is 200 μm. The distance H1 is approximately the same as a distance H3 in the third direction D3 between the first signal wiring 27 and the intermediate wiring 28f in the substrate 24A.

[0053] Further, the distance H4 in the third direction D3 from the upper surface of the first ground wiring 28 to the upper surface of the first signal wiring 27 in the substrate 24A is larger than the distance H5 in the third direction D3 from the lower surface of the second ground wiring 17 to the lower surface of the second signal wiring 13 in the flexible printed circuit board 10A. When the distance in the third direction D3 from the upper surface of the intermediate wiring 28f to the upper surface of the first signal wiring 27 is distance H6, the height from the upper surface of the lower wiring 28c to the upper surface of the intermediate wiring 28f is approximately the same as the difference between the distance H4 and the distance H6. As described above, in the optical module 1A according to the first modification, the flexible printed circuit board 10A having a thinner base material can be connected to the package 20A. In this way, the distances H2 and H1 can be set to different values ​​in the substrate 24A and the flexible printed circuit board 10A, so that the characteristic impedances of the transmission lines formed in each can be adjusted more freely without being restricted to make the distances H2 and H1 the same.

[0054] Next, an optical module 1B according to a second modified example will be described with reference to Figs. 9A, 9B, and 9C. The optical module 1B is the same as the optical module 1A except that it has offset portions 31 and 32, which will be described later. In the optical module 1B, in a state in which the flexible printed circuit board 10B is connected to the substrate 24B of the package 20B, the second ground wiring 17B of the flexible printed circuit board 10B is spaced apart from the first dielectric layer 25b of the substrate 24B along the first direction D1. In a state in which the flexible printed circuit board 10B is connected to the substrate 24B, the first signal wiring 27B of the substrate 24B is spaced apart from the base layer 16 of the flexible printed circuit board 10B along the first direction D1.

[0055] The rear end of the second ground wiring 17B is located closer to the inside (front side) in the first direction D1 of the flexible printed circuit board 10B than the rear end of the base layer 16. The front end of the first signal wiring 27B is located closer to the inside (rear side) in the first direction D1 of the substrate 24B than the front end of the first dielectric layer 25b. As a result, an offset portion 31 is formed between the second ground wiring 17B and the first dielectric layer 25b, and an offset portion 32 is formed between the first signal wiring 27B and the base layer 16. For example, the offset portion 31 is a region where the rear end of the second ground wiring 17B is removed in the flexible printed circuit board 10B, and the offset portion 32 is a region where the front end of the first signal wiring 27B is removed in the substrate 24B.

[0056] As an example, the offset portion 31 is a space formed between the second ground wiring 17B and the first dielectric layer 25b, and the offset portion 32 is a space formed between the first signal wiring 27B and the base layer 16. However, the offset portion 31 and the offset portion 32 may not be hollow, and may be filled with, for example, a resin material. For example, the length L1 of the offset portion 31 in the first direction D1 and the length L2 of the offset portion 32 in the first direction D1 are 50 μm or more and 200 μm or less (100 μm, for example).

[0057] As described above, in the optical module 1B according to the second modification, the substrate 24B has the first dielectric layer 25b located between the first wiring layer 26b and the second wiring layer 26c, and the second dielectric layer 25c located on the opposite side of the first dielectric layer 25b as viewed from the second wiring layer 26c. The front end of the first signal wiring 27B is spaced apart from the base layer 16 of the flexible printed circuit board 10B. The rear end of the second ground wiring 17B is spaced apart from the first dielectric layer 25b. In this case, even if the flexible printed circuit board 10B is thin, the contact of the first signal wiring 27B with the second ground wiring 17B can be suppressed. Furthermore, the decrease in impedance of the signal wiring constituting the transmission line due to the proximity of the first signal wiring 27B to the second ground wiring 17B can be suppressed.

[0058] The above describes the embodiment and various modifications of the optical module and flexible printed circuit board according to the present disclosure. However, the present invention is not limited to the above-mentioned embodiment or various modifications, and can be modified as appropriate within the scope of the gist described in the claims. In addition, the optical module and flexible printed circuit board according to the present disclosure may be a combination of multiple examples of the above-mentioned embodiment, first modification, and second modification. For example, the configuration, shape, size, material, number, and arrangement of each part of the optical module and flexible printed circuit board according to the present disclosure are not limited to the above-mentioned embodiment or modification, and can be modified as appropriate. For example, in the above-mentioned embodiment, the flexible printed circuit board 10 having two wiring layers including the upper wiring layer 14 and the lower wiring layer 15 was described. However, the flexible printed circuit board may have three or more wiring layers. The same applies to the substrate of the package. [Explanation of symbols]

[0059] 1,1A,1B...Optical module 10, 10A, 10B...Flexible printed circuit board 10b...first end surface 10c…Base layer bottom 10d…Second end surface 10e…Top surface of base layer 11...Connection part 12b...first end 12c…Second end 13…Second signal line 13b…Lead section 13c…First lead section 13d…Second lead section 13f…Group 14…Top wiring layer 15…Bottom wiring layer 16...Base layer 17, 17B: Second ground wiring 17b...Ground terminal 18…Top protective layer 19…Bottom protective layer 20, 20A, 20B…Package 21…Bottom plate 22…Side wall 22b…Front 22c…back 22d…1st side 22f…Second side 22g…Aperture 23…Tabletop 24, 24A, 24B...Board 24b...first top surface 24c...first end surface 24d…Second top surface 24f…Second end face 25...Dielectric layer 25b...first dielectric layer 25c…Second dielectric layer 26...Wiring layer 26b...1st wiring layer 26c…Second wiring layer 27,27B…1st signal wiring 28…First ground wiring 28b…Upper wiring 28c…Lower wiring 28d, 28f...Intermediate wiring 29…Beer 29b…First via 29c…Second via 31, 32…Offset section 100...Optical module 110...Flexible printed circuit board 113…Second signal line 117…Second ground wiring P…Heater tool

Claims

1. a package including a substrate having a first signal wiring formed in a first wiring layer and a first ground wiring formed in a second wiring layer located below the first wiring layer; a flexible printed circuit board having an upper surface wiring layer, a lower surface wiring layer, and a base layer disposed between the upper surface wiring layer and the lower surface wiring layer, the flexible printed circuit board including a second signal wiring formed on the upper surface wiring layer and a second ground wiring formed on the lower surface wiring layer; Equipped with the second signal wiring has a lead portion protruding from the base layer in a first direction, the second ground wiring has a ground terminal located on an inner side of the flexible printed circuit board relative to the lead portion in the first direction, The lead portion is connected to the first signal wiring, The ground terminal is connected to the first ground wiring. Optical module.

2. the first ground wiring is located on an outer side of the package than the first signal wiring in the first direction; 2. The optical module according to claim 1.

3. the substrate has a first dielectric layer located between the first wiring layer and the second wiring layer, and a second dielectric layer located on the opposite side of the second wiring layer to the first dielectric layer, an end portion of the first signal wiring in a direction opposite to the first direction is spaced apart from the base layer of the flexible printed circuit board; an end portion of the second ground wiring in the first direction is spaced apart from the first dielectric layer; 3. The optical module according to claim 1 or 2.

4. the upper wiring layer is located above the base layer, and the lower wiring layer is located below the base layer.

3. The optical module according to claim 1 or 2.

5. A flexible printed circuit board including an upper wiring layer, a lower wiring layer, and a base layer disposed between the upper wiring layer and the lower wiring layer, A second signal wiring is formed on the upper wiring layer, a second ground wiring is formed on the lower wiring layer, the second signal wiring has a lead portion protruding from the base layer in a first direction, the second ground wiring has a ground terminal located on an inner side of the flexible printed circuit board relative to the lead portion in the first direction, the lead portion is connected to a first signal wiring of a substrate provided in a package of an optical module; The ground terminal is connected to a first ground wiring of the substrate. Flexible printed circuit board.

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