OPTICAL MODULE AND MANUFACTURING METHOD OF OPTICAL MODULE

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2022-088998, filed on May 31, 2022, the entire subject matter of which is incorporated herein by reference

TECHNICAL FIELD

The present disclosure relates to an optical module and a method of making an optical module.

BACKGROUND

Japanese Unexamined Patent Application Publication No. 2016-18862 discloses an optical module and a method for manufacturing the optical module. The optical module includes an optical semiconductor element, a stein including a signal pin, a ground layer, and a substrate. The signal pin transmits an electrical signal to the optical semiconductor element and/or transmits an electrical signal output from the optical semiconductor element. The substrate has a first opening through which the signal pin passes and a junction part for joining the stein and the ground layer to each other. The junction part is formed at an end of the substrate or on a top surface of the substrate which the stein is disposed on.

Japanese Unexamined Patent Application Publication No. 2018-82117 discloses an optical module. The optical module includes an optical subassembly having a coaxial housing and a plurality of signal pins, a circuit board, and a flexible printed circuit (FPC) that connects the optical subassembly and the circuit board to each other. The circuit board provides a circuit for transmitting and receiving electrical signals to and from the optical subassembly on a main surface thereof. The FPC includes a ground pattern provided on the bottom surface and a signal line provided on the top surface.

SUMMARY

An optical module according to the present disclosure includes an optical semiconductor element; a stein including a signal pin extending in a first direction and a ground pin, the signal pin being electrically connected to the optical semiconductor element for transmitting an electrical signal, the ground pin being configured to provide a reference potential of the electrical signal; and a circuit board extending in a second direction crossing the first direction, the circuit board including a signal through-hole, a ground through-hole, a signal line extending in the second direction, a ground layer, an assistance through-hole provided within the stein in a planar view of the circuit board from the first direction, and a junction part, the signal through-hole being configured to be pierced by the signal pin, the ground through-hole being configured to be pierced by the ground pin, the signal line being configured to be electrically connected with the signal pin, the ground layer being configured to be electrically connected with the ground pin, the junction part being configured to connect the assistance through-hole and the ground layer around the assistance through-hole with the stein. The circuit board has a first distance between the assistance through-hole and the signal through-hole, the first distance being smaller than a second distance between the ground through-hole and the signal through-hole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a schematic configuration of an optical module according to a first embodiment.

FIG. 2 is a plan view illustrating a bottom surface of an FPC of the optical module according to the first embodiment.

FIG. 3 is a cross-sectional view of III-III line of FIG. 2.

FIG. 4 illustrates an example of an assistance through-hole.

FIG. 5 illustrates an example of an assistance through-hole different from FIG. 4.

FIG. 6 is a perspective view showing a metal stein of an optical module according to a second embodiment.

FIG. 7 is a cross-sectional view showing a flat head of a protrusion of the metal stein of FIG. 6.

FIG. 8 illustrates a step of a method of manufacturing an optical module according to an embodiment.

FIG. 9 illustrates a step of a method of manufacturing the optical module according to the embodiment.

FIG. 10 illustrates a step of a method of manufacturing the optical module according to the embodiment.

DETAILED DESCRIPTION
Details of Embodiments of Present Disclosure

Specific examples of optical modules according to embodiments of the present disclosure are described with reference to the drawings. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and redundant description is omitted as appropriate. The drawings may be partially simplified or exaggerated for ease of understanding, and dimensional ratios and the like are not limited to those illustrated in the drawings.

First Embodiment

FIG. 1 is a cross-sectional view showing a schematic configuration of an optical module 1 according to a first embodiment. As shown in FIG. 1, the optical module 1 includes an optical semiconductor element 2, a metal stein 3 including a signal pin (lead terminal) 3b, and a flexible printed circuit (FPC) 4 which is a circuit board having a signal through-hole 4b through which the signal pin 3b passes. The optical module 1 is, for example, an optical transmission module in which the optical semiconductor element 2 outputs an optical signal. The optical transmitter module includes, for example, a laser diode. The metal stein 3 has a disc-shaped main body 3a, and the main body 3a is made of metal.

The signal pin 3b has a columnar shape extending in a direction, the signal pin 3b passing through the main body 3a in the direction. The signal pin 3b transmits, for example, an electrical signal input to or output from the optical semiconductor element 2 between one surface and the other surface of the main body 3a. The signal pin 3b passes through the main body 3a to enable transmission of the electrical signal between one surface (inner surface) 3A and the other surface (outer surface) 3B. For example, the optical semiconductor element 2 is mounted on the one surface 3A and the FPC 4 is connected to the other surface 3B. The signal pin 3b is made of, for example, a conductive material. The signal pin 3b is made of, for example, metal. The signal pin 3b is inserted into a penetration hole 3d bored in the metal stein 3. The signal pin 3b is fixed to the main body 3a and insulated from the main body 3a by, for example, a glass material or the like provided between the signal pin 3b and the main body 3a in the penetration hole 3d. The signal through-hole 4b is provided so that the signal pin 3b protruding from the main body 3a can pass through the FPC 4. The signal through-hole 4b is also referred to as a through-hole.

As an example, the optical module 1 includes a block 5 fixed to the one surface (inner surface) 3A of the main body 3a and a substrate 6 mounted on the block 5, and the optical semiconductor element 2 is mounted on the substrate 6. The substrate 6 has, for example, high thermal conductivity. The substrate 6 is made of, for example, an insulating material having a linear expansion coefficient substantially equal to that of the optical semiconductor element 2. The substrate 6 is made of, for example, ceramic. The substrate 6 is also referred to as a carrier.

For example, the optical semiconductor element 2 is electrically connected to the signal pin 3b via the substrate 6 and a wire W1. More specifically, the substrate 6 provides a conductive wiring pattern, and the wire W1 is electrically connected between one end of the signal pin 3b to one end of the wiring pattern of the substrate 6. The optical semiconductor element 2 is electrically connected to the other end of the wiring pattern of the substrate 6 via, for example, a wire W2 different from the wire W1. Therefore, the signal pin 3b is electrically connected to the optical semiconductor element 2 via the wire W1, the wiring pattern, and the wire W2. As an example, a lens R is provided on the side opposite to the main body 3a when viewed from the optical semiconductor element 2, and the optical signal output by the optical semiconductor element 2 penetrates the lens R and is output to the outside of the optical module 1.

For example, the optical module 1 includes a cap 7. For example, the lens R is fixed to an opening of the cap 7. By fixing the cap 7 to the main body 3a, the position of the lens R with respect to the optical semiconductor element 2 is fixed. The metal stein 3 includes a signal pin 3b and a ground pin 3c. The signal pin 3b and the ground pin 3c pierce the FPC 4 in a first direction D1 which is a thickness direction of the FPC 4. The ground pin 3c is electrically connected to the main body 3a. Electrical connection of the ground pin 3c to a grounded wiring allows the main body 3a to be grounded. The ground pin 3c is preferably disposed near the signal pin 3b in order to electrically reinforce the ground related to transmission of a high-speed electrical signal to be described later. However, for example, the ground pin 3c may be arranged slightly away from the signal pin 3b due to the isolation from a different signal pin for another electrical signal or the configuration of the metal stein 3. The ground pin 3c is provided at a position offset from a centerline L1 shown in FIG. 2 toward an outer edge 3g of the metal stein 3 in the third direction D3. For example, the ground pin 3c may be provided between the outer edge 3g and the centerline L1 in the third direction D3. The ground pin 3c may be provided immediately above the centerline L1. The optical semiconductor element 2 may be hermetically sealed by connecting the cap 7 to the metal stein 3 for example by welding.

The FPC 4 has a top surface 4A and a bottom surface 4B facing away from the top surface 4A. The top surface 4A contacts the metal stein 3 and is fixed to the metal stein 3. More specifically, when the FPC 4 is attached to the metal stein 3, the top surface 4A is in contact with the outer surface 3B of the main body 3a opposite to the inner surface 3A. The signal pin 3b passes through the metal stein 3 along the first direction D1 from the inner surface 3A of the metal stein 3 toward the outer surface 3B of the metal stein 3. For example, one end of the signal pin 3b protrudes from the inner surface 3A of the metal stein 3 and is connected to a wire W1. The other end of the signal pin 3b protrudes from the outer surface 3B of the metal stein 3 to the outside and passes through the signal through-hole 4b in the FPC 4 along the first direction D1. FIG. 2 is a plan view showing the bottom surface 4B of FPC 4. As shown in FIGS. 1 and 2, the FPC 4 has a flat plate shape extending in a second direction D2 intersecting the first direction D1. The bottom surface 4B extends in the second direction D2 and the third direction D3. The third direction D3 intersects the first direction D1 and the second direction D2.

An end portion 4d of the FPC 4 in the second direction D2 is fixed to the metal stein 3. The end portion 4d includes a region where the top surface 4A contacts the outer surface 3B of the main body 3a. For example, when the FPC 4 is viewed from the first direction D1 (which is also referred to as a plan view of the FPC 4), the end portion 4d has a semicircular portion, and when the end portion 4d contacts and is fixed to the outer surface 3B of the main body 3a, the outer edge of the semicircular portion overlaps with the outer edge of the main body 3a. For example, the curvature of the outer edge of the semi-circular portion is the same as the curvature of the outer edge of the main body 3a. The shape of the main body 3a of the metal stein 3 may be a shape having a notch in a part of a circle. The semi-circular portion of the end portion 4d may have a notch which position corresponds to the position of the notch of the main body 3a.

As shown in FIG. 2, the FPC 4 has a signal line 4h provided on the bottom surface 4B. The signal line 4h extends along the second direction D2. For example, the signal line 4h is formed so as to be electrically connectable to the signal pin 3b, and transmits the electrical signal along the second direction D2. The signal line 4h is made of, for example, metal. The signal line 4h transmits, for example, a high-frequency signal such as 100 Gbps. The signal line 4h is formed as a transmission line such as a microstrip line with respect to a ground layer 4t described later. The signal line 4h is formed at a position spaced apart from an outer edge 4j of the FPC 4 in the third direction D3 which is the widthwise direction of the FPC 4. The third direction D3 is a direction crossing both the first direction D1 and the second direction D2. For example, the first direction D1, the second direction D2, and the third direction D3 are orthogonal to each other.

In the third direction D3, the distance from the centerline L1 to the signal line 4h is shorter than the distance from the centerline L1 to the outer edge 4j of the FPC 4. The centerline L1 is, for example, a straight line passing through the center of the FPC 4 in the third direction D3 and extending along the second direction D2. In FIG. 2, the centerline L1 also passes through the center of the metal stein 3 in the third direction D3, and is a centerline common to the metal stein 3 and the FPC 4. The signal line 4h may extend along the centerline L1 of the FPC 4. When the FPC 4 is viewed from the first direction D1, the signal line 4h may have overlapping with the centerline L1 of the FPC 4. The signal through-hole 4b through which the signal pin 3b can pass is provided at one end of the signal line 4h. The signal through-hole 4b may be provided immediately above the centerline L1. The FPC 4 has a ground through-hole 4c through which the ground pin 3c can pass. For example, when the FPC 4 is viewed from the first direction D1 (in a plan view of the bottom surface 4B of the FPC 4 from the first direction D1), the ground through-hole 4c is provided at a position shifted to the outer edge 4j side in the third direction D3 from the centerline L1. For example, the ground through-hole 4c may be provided between the outer edge 4j and the centerline L1 in the third direction D3.

FIG. 3 is a cross-sectional view of III-III line of FIG. 2. As shown in FIGS. 2 and 3, the FPC 4 has a junction part 4f for joining the ground layer 4t to the metal stein 3. For example, the junction part 4f has a circular shape when viewed along the first direction D1 (i.e., in a plan view from the first direction D1). The junction part 4f consists of at least an assistance through-hole 4g passing through the FPC 4 along the first direction D1, and a solder 4k applied to the assistance through-hole 4g. The junction part 4f may further include the ground layer 4t exposed by an opening 4x and the solder 4k applied to the ground layer 4t. The opening 4x is provided so as to include the assistance through-hole 4g in the plan view from the first direction D1. The assistance through-hole 4g is a hole (through-hole) passing through the FPC 4 from the top surface 4A to the bottom surface 4B along the first direction D1. A metal layer is formed on an inner surface of the assistance through-hole 4g. For example, the inner surface of the assistance through-hole 4g may be plated with copper. Thus, the inside of the assistance through-hole 4g is electrically connected to the ground layer 4t at the top surface 4A. The ground layer 4t is provided around the assistance through-hole 4g. In other words, in the plan view from the first direction D1, the assistance through-hole 4g is provided inside the ground layer 4t visible via the opening 4x. The solder 4k is also filled inside the assistance through-hole 4g. The ground layer 4t is joined to the metal stein 3 via the solder 4k, and the inside of the assistance through-hole 4g is also joined to the metal stein 3 via the solder 4k. Since the solder 4k is coated on the ground layer 4t exposed by the opening 4x and is also filled in the assistance through-hole 4g, the junction part 4f more firmly bonds the FPC 4 to the metal stein 3. The area of each junction part 4f when viewed from the first direction D1 is 0.1 mm 2 or more and 1.0 mm 2 or less. The area of the junction part 4f depends on the size of the assistance through-hole and the size of the opening 4x included in the junction part 4f. For example, the diameter of the assistance through-hole 4g may be greater than or equal to 50% and less than or equal to 300% of the diameter of the ground through-hole 4c. When the size of the opening 4x is larger than the size of the assistance through-hole 4g, the area of the junction part 4f gets larger as much as the size of the opening 4x gets larger than that of the assistance through-hole 4g.

The junction part 4f is provided inside the metal stein 3 in a plan view of the FPC 4 from the first direction D1. That is, the junction part 4f is provided inside a virtual circle C whose center corresponds with the center 3f of the signal pin 3b when viewed from the first direction D1 and whose outer periphery is in contact with the ground through-hole 4c. When viewed from the first direction D1, the distance (first distance) from the junction part 4f to the center 3f of the signal pin 3b is shorter than the distance (the radius R of the virtual circle C) from the ground through-hole 4c to the center 3f of the signal pin 3b. That is, the first distance between the junction part 4f and the signal through-hole 4b is smaller than a distance between the ground through-hole 4c and the signal through-hole 4b. The assistance through-hole 4g is included in the junction part 4f in a plan view from the first direction D1. Therefore, the assistance through-hole 4g is provided inside the virtual circle C. Also, the first distance between the assistance through-hole 4g and the signal through-hole 4b may be smaller than a distance between the ground through-hole 4c and the signal through-hole 4b.

For example, the optical module 1 includes a plurality of junction parts 4f. At least one of the junction parts 4f is formed at a position closer to the outer edge 3g of the metal stein 3 than the signal through-hole 4b. For example, the distance between the junction part 4f and the outer edge 3g may be smaller than the distance between the signal through-hole 4b and the outer edge 3g. The plurality of junction parts 4f include a first junction part 4p and a second junction part 4q, and the first junction part 4p and the second junction part 4q are formed at positions sandwiching the signal line 4h in a plan view from the first direction D1. As an example, the first junction part 4p and the second junction part 4q are arranged at positions line-symmetric to each other with respect to a virtual line extending along the signal line 4h. The virtual line may be the centerline L1. For example, the distance between the signal line 4h and the first junction part 4p extending along the second direction D2 is approximately equal to the distance between the signal line 4h and the second junction part 4q. Here, ‘approximately equal’ includes a situation in which two values are different from each other within a range of an allowable manufacturing variation. For example, if a difference between the two distances is within 5% as a relative error of one of the distances, the two distances may be considered to be equal to each other. Each of the first junction part 4p and the second junction part 4q is formed at a position closer to the outer edge 3g of the metal stein 3 than the signal through-hole 4b. For example, when viewed along the first direction D1, the distance from the center of the first junction part 4p to the outer edge 3g of the metal stein 3 may be shorter than the distance from the center of the first junction part 4p to the center of the signal through-hole 4b. The same applies to the second junction part 4q. For example, the plurality of junction parts 4f further includes a third junction part 4r. The third junction part 4r is provided on the side opposite to the signal line 4h when viewed from the signal through-hole 4b. For example, when viewed along the first direction D1, the third junction part 4r is provided between a center 3j of the metal stein 3 and the signal pin 3b in the second direction D2. The FPC 4 has a plurality of assistance through-holes 4g corresponding to the plurality of junction parts 4f, respectively. For example, the plurality of assistance through-holes 4g may include a first assistance through-hole 4P, a second assistance through-hole 4Q, and a third assistance through-hole 4R. In a plan view from the first direction D1, the first junction part 4p includes the first assistance through-hole 4P therein, the second junction part 4q includes the second assistance through-hole 4Q therein, and the third junction part 4r includes the third assistance through-hole 4R therein.

The FPC 4 includes, for example, a top surface coverlay 4s, the ground layer 4t, a core (core layer) 4v, the signal line 4h, and a bottom surface coverlay 4w. For example, the top surface coverlay 4s and the bottom surface coverlay 4w are protective films on the FPC 4, and the core 4v is a dielectric layer on the FPC 4. The top surface coverlay 4s, the ground layer 4t, the core 4v, the signal line 4h, and the bottom surface coverlay 4w are stacked in this order from the top surface 4A toward the bottom surface 4B along the first direction D1. The bottom surface coverlay 4w, the signal line 4h, the core 4v, the ground layer 4t, and the top surface coverlay 4w may be alternatively stacked in this order from the bottom surface 4B toward the top surface 4B along the first direction D1. The ground layer 4t is a layer that gives a reference potential of an electrical signal transmitted by the signal line 4h, and is formed between the signal line 4h and the metal stein 3 in the first direction D1. The ground layer 4t is formed as part of the first wiring layer. The first wiring layer is a thin film formed by a conductive metal between the top surface coverlay 4s and the core 4v. The top surface coverlay 4s protects the electrical wiring formed by the first wiring layer. The top surface coverlay 4s covers the electric wiring formed by the first wiring layer, and covers the core 4v in a portion where the electric wiring is not formed. Further, the signal line 4h is formed as a part of the second wiring layer. The second wiring layer is a thin film formed by a conductive metal between the bottom surface coverlay 4w and the core 4v. The bottom surface coverlay 4w protects the electrical wiring formed in the second wiring layer. The bottom surface coverlay 4w covers the electrical wiring formed by the second wiring layer, and covers the core 4v in a portion where the electrical wiring is not formed. The signal line 4h, the core 4v, and the ground layer 4t may configure a transmission line.

When the FPC 4 is viewed along the first direction D1, the ground layer 4t may extend in the second direction D2 and the third direction D3 to have the same shape as the outer shape of the FPC 4. That is, the ground layer 4t may be provided as a so-called solid pattern in a multilayer wiring substrate. A space is provided between the ground layer 4t and the signal pin 3b so that the signal pin 3b is not in contact with the ground layer 4t. The ground layer 4t is, for example, a thin film formed of a conductive metal. The signal pin 3b is insulated from the ground layer 4t by the space.

The assistance through-hole 4g is configured to pierce the FPC 4, namely all of the top surface coverlay 4s, the ground layer 4t, the core 4v, and the bottom surface coverlay 4w in the first direction D1. The assistance through-hole 4g has the opening 4x formed in the top surface coverlay 4s on the top surface 4A and an opening 4y formed in the bottom surface coverlay 4w on the bottom surface 4B. The ground layer 4t around the assistance through-hole 4g is exposed from the top surface coverlay 4s through the opening 4x. The assistance through-hole 4g has a land 4z inside the opening 4y on the bottom surface 4B. The land 4z is a thin film formed of a conductive metal. The land 4z is exposed from the bottom surface coverlay 4w through the opening 4y. The land 4z is formed as a part of the second wiring layer, for example. The land 4z is formed so as to surround the outside of the assistance through-hole 4g in a plan view of the FPC 4. The land 4z is electrically connected to the metal film formed on the inner surface of the assistance through-hole 4g, thereby being electrically connected to the ground layer 4t. The solder 4k is applied to the ground layer 4t exposed by the opening 4x and the land 4z exposed by the opening 4y, and the solder 4k is filled in the assistance through-hole 4g to form the junction part 4f. The solder 4k is melted by heating and bonded to each of the main body 3a of the metallic stein 3, the ground layer 4t, and the metallic film of the assistance through-hole 4g, whereby the main body 3a and the FPC 4 are firmly connected to each other. At this time, the main body 3a is electrically connected to the ground layer 4t through the solder 4k. That is, the FPC 4 electrically connects the ground layer 4t to the metal stein 3 via the junction part 4f. The junction part 4f electrically connects the ground layer 4t and the metal stein 3 to each other. The junction part 4f joins the FPC 4 to the metal stein 3 by the solder 4k filled in the opening 4x and the assistance through-hole 4g.

Next, the effects obtained from the optical module 1 according to the embodiment of the present disclosure will be described. In the optical module 1, the FPC 4 has a junction part 4f for connecting the ground layer 4t to the metal stein 3, and the ground layer 4t and the metal stein 3 are electrically joined to each other via the junction part 4f. In addition, the FPC 4 is mechanically firmly attached to the metal stein 3 by the junction part 4f. When viewed from a first direction D1 in which the signal pin 3b extends, the assistance through-hole 4g is provided inside the metal stein 3, and a first distance between the assistance through-hole 4g and the signal through-hole 4b is smaller than a distance between the ground through-hole 4c and the signal through-hole 4b.

Incidentally, for example, when a high-speed electrical signal is transmitted from the signal line 4h to the signal pin 3b, a return current generated with the transmission of the electrical signal returns from the ground layer 4t to the signal source. For example, when the optical semiconductor element 2 is a laser diode, the electrical signal may be a drive signal for driving the laser diode at high speed, and the signal source may be a drive circuit for outputting the drive signal. In the optical module 1, since the junction part 4f is provided closer to the signal pin 3b than the ground through-hole 4c, the ground is reinforced and the return current flows closer to the signal line 4h. Therefore, the parasitic inductance and parasitic capacitance between the ground layer 4t and the metal stein 3 are reduced by the ground reinforcement, and a resonance phenomenon due to the return current flows and the parasitic capacitance is suppressed, so that the frequency response characteristic of the electrical signal can be improved up to a higher frequency. Such broadening of the frequency response characteristic (bandwidth) of the optical module 1 is preferable for improving the performance of the optical transmission device.

The junction part 4f may be formed at a position closer to the outer edge 3g of the metal stein 3 than the signal through-hole 4b in the FPC 4 plan view. In this case, by providing the junction part 4f joining the metal stein 3 and the ground layer 4t to each other at a position closer to the outer edge 3g of the metal stein 3, for example, the mechanical strength of the optical module 1 against bending along the first direction of the FPC 4 can be increased, so that the possibility of disconnection of the signal line 4h due to bending stresses can be reduced.

The junction part 4f may include the first junction part 4p and the second junction part 4q, and the first junction part 4p and the second junction part 4q may be formed to sandwich the signal line 4h in a plan view of the FPC 4. In this case, since the first junction part 4p and the second junction part 4q are formed so as to sandwich the signal line 4h, the return current flows in the vicinity of the signal line 4h by the first junction part 4p and the second junction part 4q. In addition, since the number of junction parts is increased, the FPC 4 can be mechanically more firmly fixed to the metal stein 3. Thus, the mechanical resistance of the optical module 1 against bending stress can be improved.

By the way, the above-described electrical reinforcement of the ground and strengthening of the mechanical strength can be expected to be more effective by increasing the area of the junction part 4f. However, when the area of the junction part 4f is increased, the required amount of the solder 4k to form the junction part 4f is increased, and a larger amount of heat is required to heat and melt the solder 4k. Since heat is also conducted to the metal stein 3 due to heating of the solder 4k, the amount of the solder 4k is preferably small in order to reduce thermal influence on the metal stein 3 on which the optical semiconductor element 2 is mounted.

The junction part 4f may be connected to the metal stein 3 by the solder 4k, and an area of the solder 4k in the junction part 4f when viewed from the first direction D1 may be not less than 0.1 mm²and not more than 1.0 mm²per junction part 4f. As a result, it is possible to suppress the thermal influence of the heating of the solder 4k on the metal stein 3 and to sufficiently reinforce the ground and the mechanical strength. Since the solder 4k is filled in the assistance through-hole 4g, heat can be efficiently transmitted to the solder 4k at a portion in contact with the metal stein 3, for example, by applying a solder iron to the solder 4k from the side of the bottom surface 4B of the FPC 4. As a result, the heating time required for melting the solder 4k can be shortened and the thermal influence on the metal stein 3 can be reduced.

In the first embodiment, an example in which the shape of the assistance through-hole 4g configuring the junction part 4f when viewed along the first direction D1 is a circular shape has been described above. However, the shape of the assistance through-hole is not limited to a circular shape, and may be an elliptical shape, an oval shape, an arc shape, or a polygonal shape, and can be appropriately changed. For example, as shown in FIG. 4 which is a modification, the FPC may have an assistance through-hole 4g having a shape extending in one direction. The assistance through-hole 4g has a shape extending in the second direction D2. For example, the assistance through-hole 4g may have an oval shape and may have a major axis extending along the second direction D2. The length of the assistance through-hole 4g in the second direction D2 may be greater than the length (width) of the assistance through-hole 4g in the third direction D3. Further, the assistance through-hole 4g may have a portion in which the ground layer 4t is not exposed around the assistance through-hole 4g. This portion corresponds to a non-filling portion 14g to which the solder 4k is not applied. For example, the non-filling portion 14g may be provided at one side of the assistance through-hole 4g in the long axis direction.

As described above, in the case of the assistance through-hole 4g according to the modification, the assistance through-hole 4g has a shape extending in one direction, that is, a shape extending in the second direction D2. Such an assistance through-hole 4g allows a worker to easily inspect the finished condition of the solder 4k partially filled in the assistance through-hole 4g. Further, if the assistance through-hole 4g has the non-filling portion 14g, when the solder 4k is applied between the exposed ground layer 4t around the assistance through-hole 4g and the main body 3a of the metal stein 3 and the exposed ground layer 4t is bonded to the main body 3a by melting the solder 4k, the amount of the solder 4k and the finished condition of shape formed between the metal stein 3 and the ground layer 4t in the junction part 4f can be visually inspected with the naked eye or a microscope through the non-filling portion 14g where the solder 4k is not applied from the bottom surface 4B.

In the case of a junction part 14f to be described later, a portion covered by the coverlay 4w corresponds to a non-filling portion to which solder is not applied. The relationship between the non-filling portion and the land 4z will be described later. The same effect as that of the junction part 4f can be obtained from the junction part 14f.

FIG. 5 is a cross-sectional view taken along line V-V of FIG. 4. The assistance through-hole 4g extends along the second direction D2. FIG. 5 shows a state in which the FPC 4 is attached to the metal stein 3 by the junction part 14f formed in the assistance through-hole 4g. In an opening 14x, the top surface coverlay 4s covers the ground layer 4t adjacent to the non-filling portion 14g, and only the ground layer 4t of the other portion (soldered portion) is exposed from the top surface coverlay 4s. The land 4z is not formed in a portion adjacent to the non-filling portion 14g and is formed only in the soldered portion. Although the land 4z is formed to surround the assistance through-hole 4g in the first embodiment shown in FIG. 3, the land 4z is formed to surround only a portion of the assistance through-hole 4g in FIGS. 4 and 5. In addition, in the assistance through-hole 4g, a portion adjacent to the non-filling portion 14g is not plated, and only a soldered portion is plated. Thus, the solder 4k is not wettable to the non-filling portion 14g. The solder 4k is applied to the ground layer 4t exposed by the opening 14x, a part (filling portion) of the assistance through-hole 4g, and the land 4z, and is melted by heating to form the junction part 14f. Accordingly, the junction part 14f is formed only at one end of the assistance through-hole 4g in the second direction D2. The finished condition of the junction part 14f can be visually inspected through the other end to which the solder 4k is not applied. For example, the amount of the solder 4k and the wettability of the solder 4k with the metal stein 3 can be inspected. The land 4z may be formed at a portion adjacent to the non-filling portion 14g, and an inner front portion of the assistance through-hole 4g may be plated. For example, even when the land 4z and the assistance through-hole 4g are formed as described above, by forming the assistance through-hole 4g having a shape sufficiently long in the second direction D2 by making the length in the major axis direction twice or more the length in the minor axis direction (the third direction D3), it is possible to form the junction part 14f in which the solder 4k is applied to only one end inside the assistance through-hole 4g. A small-size assistance through-hole 4g provides a small-size non-filling portion 14g, which may be less useful for the above described visual inspection. Not forming the land 4z at the portion adjacent the non-filling portion 14g and not plating the inner front portion of the assistance through-hole 4g help such a small-size non-filling portion 14g to get larger.

Second Embodiment

Next, an optical module according to a second embodiment will be described. Some configurations of the optical module according to the second embodiment are the same as some configurations of the above-described optical module 1. Therefore, in the following description, the description overlapping with the description of the optical module 1 is denoted by the same reference numerals and is appropriately omitted. Referring to FIG. 6, an optical module according to the second embodiment includes a plurality of metallized pins 23a. The metallized pins 23a may include a signal pin 23b and a ground pin 23c. For example, a metal stein 23 has a plurality of ground pins 23c. Two of the ground pins 23c are formed, for example, at two respective positions sandwiching a centerline L2 in a plan view of the outer surface 3B of the main body 3a. The centerline L2 is a straight line passing through the center O of the metal stein 23 and the signal pin 23b and extending in the second direction D2. As an example, the plurality of ground pins 23c are formed at positions line-symmetric to each other with respect to the centerline L2 in a plan view of the outer surface 3B of the main body 3a. For example, the two ground pins 23c are arranged such that when a virtual straight line passing through the centers of the ground pins 23c is orthogonal to the centerline L2, the distances of the centers of the ground pins 23c from the centerline L2 are equal to each other. The centerline L2 extends, for example, along the second direction D2.

FIG. 7 is a cross-sectional view of the metal stein 23 showing ground pin 23c. As shown in FIGS. 6 and 7, the metal stein 23 has a protrusion 23d protruding from the main body 3a of the metal stein 23 in the first direction D1 and a hollow 23h surrounding the protrusion 23d. The protrusion 23d functions as the ground pin 23c. For example, the metal stein 23 has a plurality of protrusions 23d, and the plurality of protrusions 23d include a first protruding portion 23p and a second protruding portion 23q. An FPC 40 is connected to the metal stein 23 as shown in FIGS. 8, 9, and 10. The FPC 40 has the ground through-hole 4c that includes a first ground through-hole 4P and a second ground through-hole 4Q, similar to the above-described FPC 4. In other words, the FPC 40 is, for example, the same as the FPC 4 except that the FPC 40 has the first ground through-hole 4P at the position of the first junction part 4p and the second ground through-hole 4Q at the position of the second junction part 4q shown in FIG. 2. The FPC 40 may further have a plurality of through holes corresponding to the metallized pins 23a shown in FIG. 6. Setting the outer diameter of the first ground through-hole 4P larger than the outer diameter of the first protruding portion 23p allows the first protruding portion 23p to pass through the first ground through-hole 4P. Similarly, setting the outer diameter of the second ground through-hole 4Q larger than the outer diameter of the second protruding portion 23q allows the second protruding portion 23q to pass through the second ground through-hole 4Q.

For example, each of the protrusion 23d and the hollow 23h has a cylindrical shape extending along the first direction D1. However, the protrusion 23d may have a shape other than the cylindrical shape, and may have, for example, a prismatic shape such as a quadrangular prism shape or a columnar shape having an oval bottom surface. The shape of the hollow 23h can also be changed as appropriate. The protrusion 23d passes through (pierces) the ground through-hole 4c of the FPC 40 when the FPC 40 is attached to the metal stein 23 (see FIG. 10). The protrusion 23d has a flat head 23g having a top surface side 23f intersecting the first direction D1 at the tip of the protrusion 23d. The top surface 23f is a flat surface, for example extending along a plane orthogonal to the first direction D1. That is, the top surface 23f extends in the second direction D2 and the third direction D3. The top surface 23f may be parallel with an outer surface 3B of the metal stein 23.

The flat head 23g has a diameter d2 greater than the diameter d1 of the base 23j of the protrusion 23d. For example, the length (maximum value) of the flat head 23g in the second direction D2 may be larger than the length (maximum value) of base 23j in the second direction D2. As an example, the diameter D1 of the base 23j is greater than or equal to 0.2 mm and less than or equal to 0.3 mm. The diameter d2 of the flat head 23g is, for example, not less than 1.5 times and not more than 2.5 times (for example, about twice) the diameter d1 of the base 23j. When viewed along the first direction D1, the diameter d3 of the hollow 23h is, for example, not less than 1.5 times and not more than 2.5 times (for example, about 2 times) the diameter d1 of the base 23j. As an example, the diameter d3 of the hollow 23h may be greater than the diameter d2 of the flat head 23g. For example, the length (depth) T1 of the hollow 23h in the first direction D1 may be ¼ or more and ¾ or less of the length (thickness) T of the main body 3a of the metal stein 23 in the first direction D1.

For example, as illustrated in FIG. 7, the protrusion 23d has a T-shape in which a flat head 23g provided at an upper end of a base 23j has both sides each extending outside along the second direction D2 in a cross-sectional view along the second direction D2. However, the protrusion 23d may have an L-shape in which the flat head provided at the upper end of the base has only one side extending outside along the second direction D2. For example, a distance K1 (third distance) between the signal pin 23b and the far end of the flat head 23g from the signal pin 23b in the second direction D2 may be set greater than a distance K2 (fourth distance) between the signal pin 23b and the far end of the base 23j of the protrusion 23d from the signal pin 23b in the second direction D2. In this case, the ground through-hole 4x of the FPC 40 can be hooked to the flat head 23g at the upper end of the base 23j. It is noted that the inside diameter of the ground through-hole 4c of the FPC 40 should be larger than the outside diameter d2 of the flat head 23g of the metal stein 23.

Next, a method of manufacturing the optical module according to the embodiment will be described with reference to FIGS. 8, 9 and 10. First, as shown in FIG. 8, a ground through-hole 4c is provided in the FPC 40. At this time, the ground through-hole 4c penetrating the FPC 40 along the first direction D1 is formed in the FPC 40 (a step of forming a ground through-hole). Next, the protrusion 23d of the metal stein 23 is inserted into the ground through-hole 4c. Then, as shown in FIG. 9, an edge of the ground through-hole 4c is hooked on the flat head 23g formed on the tip of the protrusion 23d (step of hooking the edge of the ground through-hole). For example, the FPC 40 through which the protrusion 23d is inserted is slid along the second direction D2 to bring an inner side of the ground through-hole 4c into contact with the base 23j of the protrusion 23d. Thereafter, as shown in FIG. 10, the solder 4k is applied to the hollow 23h and the protrusion 23d in a state where the edge of the ground through-hole 4c is caught by the flat head 23g (applying step). Then, heat N is applied to the solder 4k via the protrusion 23d. For example, the solder 4k is heated by bringing a solder iron into contact with the top surface side 23f of the flat head 23g. The solder 4k filled and melted in the hollow 23h forms junction among the base 23j and the flat head 23g of the protrusion 23d and the ground through-hole 4c. As a result, the metal stein 23 and the ground layer 4t of the FPC 40 are joined to each other by the solder 4k after cooling down, and a series of steps is completed.

Next, operational effects of the optical module according to the second embodiment will be described. In the optical module according to the second embodiment, the metal stein 23 has a protrusion 23d penetrating the ground through-hole 4c of the FPC 40. Therefore, since the metal stein 23 has the protrusion 23d passing through the ground through-hole 4c, when the FPC 40 is connected to the metal stein 23 by the solder 4k, the protrusion 23d can be efficiently heated by using a heating tool such as a solder iron. Efficient heating by the protrusion 23d at the time of melting the solder 4k reduces excessive heat conduction to the periphery of the hollow 23h and prevents the entire metal stein 23 from reaching a high temperature, so that the heating of the solder 4k can be performed for a short time. The high temperature and long heating may cause misalignment of the optical semiconductor element 2.

The protrusion 23d may have a flat head 23g at the tip of the protrusion 23d that has a top surface 23f that intersects the first direction D1. In this case, when the flat head 23g of the protrusion 23d is heated at the time of melting the solder 4k, heat can be efficiently transferred to the solder 4k filled between the base 23j and the hollow 23h.

The metal stein 23 may further include a hollow 23h surrounding the protrusion 23d. Since, for example, the solder 4k enters the hollow 23h, heat is transferred mainly to the solder 4k via the base 23j, and heat conduction to the metal stein 23 is reduced. Therefore, in this case, when the protrusion 23d is heated by the solder iron, the hollow 23h formed around the protrusion 23d reduces excessive heat conduction to a portion other than the hollow 23h of the metal stein 23. More specifically, the heat of the base 23j is primarily transferred to the solder 4k and then transferred to the outside of the hollow 23h. Therefore, the solder 4k can be efficiently heated, and the entire metal stein 23 can be more reliably prevented from reaching a high temperature. Thus, the protrusion 23d and the hollow 23h enable effective heating of the solder 4k and reduce the thermal influence on the metal stein 23.

The flat head 23g may have a diameter greater than a diameter of the base 23j of the protrusion 23d. In this case, since the edge of the ground through-hole 4c of the FPC 40 can be caught by the flat head 23g, the resistance (holding force) of the FPC 40 against the force X bending the FPC in the direction away from the metal stein 23 can be further enhanced. The ground through-hole 4c of the FPC 40 caught by the flat head 23g of the protrusion 23d brings a strong anchor effect. The resistance to the bending force X is greatly strengthened by the anchor effect. The anchor effect is obtained by the fact that the force required to deform the flat head 23g and peel off the FPC 40 becomes larger than the force required to peel off the FPC against the joining force of the solder 4k. The anchor effect is expected to be strengthened by increasing the diameter d2 of the flat head 23d relative to the diameter d1 of the base 23j.

The distance K1 between the signal pin 23b and the far end of the flat head 23g from the signal pin 23b in the second direction D2 may be greater than the distance K2 between the signal pin 23b and the far end of the base 23j of the protrusion 23d from the signal pin 23b in the second direction D2. In this case, since the distance K1 from the signal pin 23b to the far end of the flat head 23g is greater than the distance K2 from the signal pin 23b to the far end of the base 23j of the protrusion 23d, the edge of the ground through-hole 4c of the FPC 40 through which the protrusion 23d is inserted may be caught by the flat head 23g.

The ground through-hole 4c may include a first ground through-hole 4P and a second ground through-hole 4Q. In the plan view of the FPC 40, the first ground through-hole 4P and the second ground through-hole 4Q may be formed to sandwich the signal line 4h. The protrusion 23d may include a first protruding portion 23p and a second protruding portion 23q. The first protruding portion 23p may pass through the first ground through-hole 4P, and the second protruding portion 23q may pass through the second ground through-hole 4Q. The protrusion 23d may be formed as a part of the main body 3a of the metal stein 3, and may be configured by a conductive material and electrically and mechanically connected to the main body 3a. Accordingly, the protrusion 23d may be configured to provide a reference potential. In this case, since the first ground through-hole 4P and the second ground through-hole 4Q are arranged so as to sandwich the signal line 4h in the plan view of the FPC 40, the return current flows near the signal line 4h due to the first ground through-hole 4P and the second ground through-hole 4Q. Therefore, the frequency-response characteristics related to the signal line 4h can be further improved. In this case, the first protruding portion 23p and the second protruding portion 23q may be used as the ground pin 23c.

Further, in the method of manufacturing the optical module according to the embodiment, the ground through-hole 4c passing through the FPC 40 along the first direction D1 may be formed, and an edge of the ground through-hole 4c may be caught by the flat head 23g in a state in which the protrusion 23d is inserted into the ground through-hole 4c. Since the solder 4k is applied to the hollow 23h formed around the protrusion 23d in this state, it is possible to easily and firmly connect the FPC 40 to the metal stein 23.

Various embodiments and modifications of optical modules and methods of making optical modules according to the present disclosure have been described above. However, the present invention is not limited to the embodiments or modifications described above. That is, it is easily recognized by those skilled in the art that various modifications and changes can be made to the present invention within the scope of the gist described in the claims.

For example, in the above-described embodiment, the solder 4k is filled in the assistance through-hole 4g that electrically connects the ground layer 4t to the metal stein 3. However, for example, the ground layer 4t may be electrically connected to the metal stein 3 by a conductive adhesive instead of the solder 4k. In the above-described embodiment, an example has been described in which the optical semiconductor element 2 is the optical module 1 that is an optical transmission module that outputs an optical signal, and the signal pin 3b transmits an electrical signal to the optical semiconductor element 2. However, the optical module according to the present disclosure may be an optical receiver module in which a signal pin transmits an electrical signal output from an optical semiconductor element 2. For example, the optical semiconductor element 2 may include a light receiving element (for example, a photodiode), an optical signal incident on the lens R from the outside may be condensed and incident on the light receiving element, and the optical semiconductor element 2 may convert the incident optical signal into an electrical signal and output the electrical signal. The electrical signal may be transmitted to the signal line 4h of the FPC 4 via the signal pin 3b.

OPTICAL MODULE AND MANUFACTURING METHOD OF OPTICAL MODULE

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CPC

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