Optical module

An optical module includes a wiring board on which a first opening and a second opening are provided having, between the first and second openings, a traversing portion on which a signal wire is arranged, an optical component that is mounted on a first plane side of the first opening and the second opening of the wiring board and that generates heat, a heat sink arranged on a second plane side, which is on a reverse of the first plane side, of the first opening and the second opening of the wiring board, and an anisotropic heat dissipation sheet that is provided between the traversing portion and the heat sink, and the optical component, and that has a thermal conductivity higher in second directions, which cross the traversing portion in a plane orthogonal to first directions, than in the first directions, which are thickness directions of the wiring board.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-267147 filed on Dec. 6, 2012, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an optical module including a wiring board and an optical component.

BACKGROUND

In recent years, accompanying the speeding up and sophistication of optical communications, optical modules used for optical communications have also advanced in speed and density. As the speeding up and density growth are accelerated, the difficulty in dissipating heat caused by increased power consumption and higher density has been becoming more unignorable than before.

At the same time, communication methods have been becoming complicated in order to realize high-speed optical communications. Partly because better characteristics can be attained by gathering modules for performing high-speed processes in one portion, optical components, represented mainly by high-speed optical components, that have a high performance and have complicated configurations have been developed. Such optical components are often large in size, and it is desirable to fully utilize large components that generate a large volume of heat in order to realize high speed communications. It is difficult to meet this demand, which is not consistent with the density growth.

As described above, the development of the optical transmission technology has increased the transmission volume of signals year by year. Against that background, modules that transmit and receive optical signals (optical modules) have been generating a larger amount of heat from a portion for converting optical signals into electric signals and processing them (optical reception component).

Today, standardization at the level of 100 Gbps is in progress; however, there is a large optical component of 60 mm×41 mm according to an industry standard (OIF-DPC-RX-01.0) discussed by the Optical Internetworking Forum (OIF). Such an optical component often varies its optical characteristics depending upon temperature, and generates a very large volume of heat.

Accordingly, in order to maintain the optical characteristics, the ability to dissipate heat is important. The above industry standard partially specifies heat dissipation; however, this is because components at the current technology level often generate heat in specified parts particularly and it is a specification from the viewpoint of priority, and there is a possibility that the heat generation at the center portion will become relatively intense.

A wiring board has a signal wire directed to the lead portion for the connection with a lead pin that is disposed in the lateral direction of an optical component. Accordingly, as a configuration of a wiring board, a configuration having a large notch and an opening is known (see Patent Document 1, for example).

It is a widely accepted practice to bring a large optical component into close contact with a case that has received precision flattening and to fix the component on it with a screw so that heat is dissipated from the flat bottom plane of the large optical component. Also, there is a large hole (opening) in a board so that a component that has to dissipate its heat can dissipate the heat to a heat sink (see Patent Document 2 for example). As described above, methods have been employed in which an optical component itself is wired to the wiring board while providing a large opening in a board so as to cause heat coupling between a large heat sink and the optical component.Patent Document 1: Japanese Laid-open Patent Publication No. 07-30223Patent Document 2: Japanese National Publication of International Patent Application No. 04-502385Patent Document 3: Japanese Laid-open Patent Publication No. 2007-115868Patent Document 4: Japanese Laid-open Patent Publication No. 2007-19130

SUMMARY

According to an aspect of the embodiments, an optical module includes a wiring board on which a first opening and a second opening are provided having, between the first and second openings, a traversing portion on which a signal wire is arranged, an optical component that is mounted on a first plane side of the first opening and the second opening of the wiring board and that generates heat, a heat sink arranged on a second plane side, which is on a reverse of the first plane side, of the first opening and the second opening of the wiring board, and an anisotropic heat dissipation sheet that is provided between the traversing portion and the heat sink, and the optical component, and that has a thermal conductivity higher in second directions, which cross the traversing portion in a plane orthogonal to first directions, than in the first directions, which are thickness directions of the wiring board.

DESCRIPTION OF EMBODIMENTS

FIG. 11is a perspective view illustrating an optical component112in amounted state of an optical module according to a referenced technique.

FIG. 12is a perspective view illustrating a situation where the optical component of the optical module according to the referenced technique is not mounted.

FIG. 13is a perspective view illustrating a wiring board111of the optical module according to the referenced technique.

As illustrated inFIG. 12andFIG. 13, the wiring board111is provided with an opening111a.

An optical fiber115is inserted into the optical component112illustrated inFIG. 11via a rubber boot112a.

Leads112bof the optical component112are arranged in, for example, a bilaterally symmetric manner.

The optical component112is fixed to a case113on fringes112cwith screws112dat the four corners.

As illustrated inFIG. 12, a projection part113athat projects upward from the case113is inserted from below into the opening111aof the wiring board111. This makes the case113function as a heat sink.

The contact between the projection part113aand the optical component112dissipates heat of the optical component112through the projection part113a.

In this type of optical module, the large opening111ais formed in the wiring board111, imposing limitations on the designing of the wiring board111. First, the component mounting space is restricted severely. In optical modules for which the reduction in the number of parts has become difficult because the advanced sophistication and the density growth has advanced further, reduction in the effective area for the wiring board111directly leads to failure of the mounting designing.

The large optical component112is expected to become further larger in years to come, and thus there is a demand that the area on which the case113(projection part113a) and the large optical component112are in close contact be secured, the efficiency be reviewed, and the effective area for the wiring board111be secured while maintaining the heat dissipation performance.

When the optical component112is an optical reception component, it is arranged next to an optical transmission component in the wiring board111, and it is usually arranged in a region close to an edge of the wiring board111as illustrated inFIG. 11.

The leads112bfrom the optical component112are usually provided on both sides of the optical component112. Accordingly, signals on the edge side of the wiring board111from among signals output from the lead112bare transmitted on a route that bypasses the opening111aas illustrated inFIG. 13.

Accordingly, there are many bypass wires, causing a problem to arise in which the designing for wiring of the wiring board111to the leads112bis very difficult. Bypass wires have to pass through a region on the wiring board111that wires would otherwise not pass through, and naturally there are components and wires thereof in this region too. Bypass wires increase the wiring density of the bypass route and the difficulty of wiring, making it impossible to perform wiring in some cases. Also, the pattern length becomes longer because of bypass wires. Noise is involved in patterns more often, deteriorating the transmission quality of the patterns at large.

The large optical component112to be connected processes, at a high speed, signals having small amplitudes, and accordingly the noise involvement as described above may lead to a situation where the characteristics of a large optical component are not utilized sufficiently and instead, only its unstable characteristics emerge.

Aside from the above, there are conditions that are characteristic of the heat dissipation structure of the optical component112. One is that stresses have to be made smaller. Stresses are applied for fixing components, and it is ideal that stresses be applied evenly to the plane that is to be fixed, which means that a structure that receives a strong stress locally is not easy to employ.

It is also difficult to employ heat-dissipating grease, heat-dissipating adhesive, or the like. It is difficult to remove adhesive, making repair operations etc. difficult. Heat-dissipating grease lacks in long-term reliability. When a lot of silicone oil is used, it tends to enter surrounding optical components including the optical component112, often causing a decrease in the optical levels by polluting prisms or optical waveguides inside.

As described above, it is desirable to solve, at a high level while taking characteristics conditions into consideration, both the problem of the reduction in the effective area in the wiring board111and the problem about the heat dissipation, which is, from the beginning, the reason for providing a large notch.

Hereinafter, first through fourth embodiments will be explained.

First Embodiment

FIG. 1is a perspective view illustrating an example of an optical module1.

FIG. 2is a sectional view illustrating an internal structure of the optical module according to the first embodiment.

FIG. 3is a perspective view illustrating a wiring board11according to the first embodiment.

As illustrated inFIG. 1, in the optical module1, an optical component12, which will be explained later, and the like are accommodated in a case13, and a reception-side optical fiber15and a transmission-side optical fiber16are inserted into the case13.

The optical module1illustrated inFIG. 1is an example, and any components including the wiring board11, the optical component12, case13, and an anisotropic heat dissipation sheet14, which will be explained later, can be applied to the present embodiment.

As illustrated inFIG. 1throughFIG. 3, the optical module1includes the wiring board11, the optical component12, the case13as an example of a heat sink, and the anisotropic heat dissipation sheet14.

As illustrated inFIG. 2andFIG. 3, on the wiring board11, a first opening11aand a second opening11bare provided having between them a traversing portion11con which signal wires are arranged. The number of dividing openings may be three or more; however, as the number of divisions increases, the area in contact with the case13decreases sharply, and the number is desirably two in view of securing the heat dissipation.

The optical component12is a component that generates heat, and is mounted on the side of a first plane11dof the first opening11aand the second opening11bof the wiring board11. The optical component12is, for example, an optical reception component. An optical fiber15on the reception side is inserted into the optical component12via a rubber boot12a.

Similarly to the optical component112illustrated inFIG. 11, it is desirable that the optical component12include leads112barranged in, for example, a bilaterally symmetric manner, and that the optical component12be fixed to the case13with the screws112don the fringes112cat the four corners.

As illustrated inFIG. 1andFIG. 2, the case13accommodates the wiring board11, the optical component12, and the anisotropic heat dissipation sheet14. The case13also functions as a heat sink that is provided on the side of the first opening11aand a second plane11eof a second opening11bof the wiring board11. The second plane11eis a plane positioned on the reverse side from the first plane11d.

The case13includes a first projection part13athat projects while being inserted into the first opening11aof the wiring board11and a second projection part13bthat projects while being inserted into the second opening11b. In the present embodiment, the first projection part13aand the second projection part13bare integrated into the case13.

The anisotropic heat dissipation sheet14is provided between the traversing portion11cand the first and second projections12aand13b, and the optical component12.

The anisotropic heat dissipation sheet14has a higher thermal conductivity in the second directions D2than in first directions D1, which are the thickness directions of the wiring board11, the second directions D2crossing the traversing portion11cin the plane that is orthogonal to the first directions D1. The second directions D2are directions orthogonal to the longitudinal directions of the traversing portion11cin embodiments. Note that the traversing portion11cextends in a belt-like manner; however, the shape thereof is not limited particularly.

As an example of the anisotropic heat dissipation sheet14, known sheets such as a graphite sheet, a sheet of a carbon nano tube, or the like may be used (For example, Japanese Laid-open Patent Publication No. 2007-019130 and Japanese Laid-open Patent Publication No. 2007-115868).

However, when there is a heat insulation layer on a heat dissipation path as in conventional techniques, it is not possible to connect the body of the optical component12to the signal ground (SG), weakening the effect of the ground, and therefore this configuration is not suitable for the heat dissipation of optical components that have to operate stably at high speeds.

The problem whereby the anisotropic heat dissipation sheet14having low thermal conductivity in the thickness directions is not capable of dissipating heat to the plane on the reverse side from the wiring board11only emerges when the anisotropic heat dissipation sheet14is extremely thick, and this problem may be ignored when a thin sheet is used such as a commercially available graphite sheet having a thickness of, for example, 0.1 mm or smaller because the resultant thermal resistance becomes low. The thermal resistance may be ignored also when the anisotropic heat dissipation sheet14has a thickness of 0.1 mm or greater, and it is exceptional that the anisotropic heat dissipation sheet14is not capable of dissipating heat to the plane on the reverse side from the wiring board11, and thus excluding the thickness of several millimeters or greater as the thickness of the anisotropic heat dissipation sheet14is not intended.

However, it is desirable that the anisotropic heat dissipation sheet14be a sheet that is as thin as possible and that has thermal conductivity in the second directions D2be about ten times as high as that in the first directions D1.

In order to reliably dissipate heat from the optical component12to the case13, the wiring board11is pressed onto the optical component12so that the anisotropic heat dissipation sheet14is brought into close contact with the traversing portion11cand the optical component12.

Specifically, the top surfaces in the projection direction of the first projection part13aand the second projection part13bare on the same plane as the first plane11dof the wiring board11. When, for example, the first plane11dof the wiring board11is set to be at a level that is, for example, 0.2 mm higher than the top surfaces in the projection direction of the first projection part13aand the second projection part13b, the contact between the traversing portion11cand the optical component12can be improved due to the elasticity of the wiring board11because the wiring board11is at a level that is slightly higher.

However, when there is too great a height gap, there is a possibility that too strong a stress is applied locally, deteriorating the characteristics of the optical component12. Accordingly, it is desirable that too great of a height gap not be created. The internal layers between the first plane11dand the second plane11eof the traversing portion11ccan be used as wiring regions so that the length of wires for the optical component12can be reduced.

It is desirable that no patterns be made on the first plane11dof the traversing portion11cor that a wide-area pattern11fthat has the same potential as the body of the optical component12be disposed on the first plane11dof the traversing portion11cas illustrated inFIG. 4.

It is desirable that the first plane11dof the traversing portion11cbe in close contact with the anisotropic heat dissipation sheet14; however, the anisotropic heat dissipation sheet14often has a high electric conductivity and the body of the optical component12is often made of signal ground (SG).

In many cases, accordingly, by using the wide-area pattern11fas the SG and omitting the application of solder resist, the SG of the optical component12can be reinforced. When the potential is different, a short circuit due to the difference in potential can be avoided by not making patterns on the first plane11dof the traversing portion11c, as illustrated inFIG. 2.

Also, on the portion where the anisotropic heat dissipation sheet14exists between the case13and the optical component12, it is possible to make the potentials equal by bringing those members into close contact because of the electricity conductivity of the anisotropic heat dissipation sheet14. It is desirable that the case13and the anisotropic heat dissipation sheet14entirely cover the first plane11dof the traversing portion11cand the top surfaces of the case13and the second projection part13band be in a size equal to or slightly smaller than the bottom area of the optical component12(the area of the plane facing the first plane11dof the traversing portion11c).

This is because there is a possibility that uncovered portions of the anisotropic heat dissipation sheet14touch surrounding electric devices to cause a short circuit due to the above described electric conductivity. It is ideal that the anisotropic heat dissipation sheet14cover the fringes112cfor the optical components112illustrated inFIG. 11and that holes be made on the portions of the anisotropic heat dissipation sheet14through which the screws112dpass. This structure is advantageous in respect of the reduction of stresses applied to the fringes112cfor the optical components112due to the height gap of about 0.2 mm, which is the thickness of the sheet.

When the second plane11eof the traversing portion11cdoes not have components mounted thereon, it is possible and desirable to raise the level of the portion of the case13that faces the second plane11eof the traversing portion11cso as to make the portion of the case13closer to the second plane11eof the traversing portion11c. This accelerates the heat dissipation toward the second directions D2because the level of the portion of the case13facing the second plane11eof the traversing portion11cis higher while heat received from the top surfaces of the first projection part13aand the second projection part13bmoves downward inFIG. 2. Because metals have an almost isotropic heat conductivity, the temperature decreases at the hot spot (the high temperature portion on the surface of the case13because of the thickness of the case13). As a result, this contributes to a decrease in the temperature of the optical component12.

It is also possible to employ a solution in which the first plane11dof the traversing portion11cis not brought into contact with the anisotropic heat dissipation sheet14, the level of the top surfaces of the first projection part13aand the second projection part13bare made to be higher than the first plane11dof the traversing portion11c, and the anisotropic heat dissipation sheet14in the vicinity of the first plane11dof the traversing portion11cis adhered to the optical component12with heat dissipation grease, heat dissipation adhesive, or the like.

However, it is difficult to remove adhesive from the beginning, making it difficult to replace the anisotropic heat dissipation sheet14in repair operations, etc. Heat dissipation grease is not problematic in view of removal; however, it lacks long-term reliability because its heat dissipation characteristics are sometimes subject to change by the drying of silicone oil, and further when it contains much silicone oil, the silicone oil enters the optical component12around it, often causing a decrease in the optical levels by polluting prisms or optical waveguides inside. The absence of the necessity of heat dissipation grease and heat dissipation adhesive is one advantage of the present embodiment.

Note that the traversing portion11ccan be connected by a flexible board such as an FPC, which is different from other portions of the wiring board11. However, the addition of a different material is disadvantageous in view of cost for materials or assembling, and also troublesome, decreasing the productivity. Accordingly, it is desirable that the traversing portion11cbe provided in such a manner that it is integrated with the wiring board11.

In the above described embodiment, the first opening11aand the second opening11bare provided having, between them, the traversing portion11con which a signal wire is arranged. The optical component12is mounted on the side of the first plane11dof the first opening11aand the second opening11bof the wiring board11, and generates heat. A heat sink, exemplified by the case13, is provided on the side of the second plane11e, which is on the reviser side from the side of the first plane11dof the first opening11aand the second opening11bof the wiring board11. The anisotropic heat dissipation sheet14is provided between the traversing portion11cand the heat sink (case13) and the optical component12, and has a higher thermal conductivity in the second directions D2than in the first directions D1, which is the thickness direction of the wiring board11, the second directions D2crossing the traversing portion11cin the plane that is orthogonal to first direction D.

This makes it possible to provide a signal wire on the traversing portion11cand to prevent the influence of noise that would be caused when signal wires bypass the openings. Also, the anisotropic heat dissipation sheet14can conduct, to the side of the case13, heat that is propagating to the traversing portion11cfrom the optical component12, which will be explained in detail in the fourth embodiment. Accordingly, the present embodiment can increase the resistance property of signal wires against noise while suppressing the reduction in heat dissipation efficiency.

It is also possible to eliminate bypass wires without greatly reducing spaces for mounting components of the wiring board11. Also, the wiring density of the wiring board11decreases, which can decrease the difficulty of designing. It is also possible to reduce the length of patterns on the wiring board11by eliminating bypass wires. Thereby, it is possible to suppress the reduction in the effective area of the wiring board11and to solve the problem of heat dissipation, which is the reason for providing a large opening from the beginning.

Also, in the present embodiment, a heat sink, exemplified by the case13, includes the first projection part13a, which is inserted into the first opening11aof the wiring board11, and the second projection part13b, which is inserted into the second opening11bof the wiring board11. This facilitates the heat conductance from the optical component12to the case13, leading to a suppression of the reduction in the heat dissipation efficiency.

Also, in the present embodiment, the top surfaces in the projection direction of the first projection part13aand the second projection part13bare on the same plane as the first plane11dof the wiring board11. This can bring the anisotropic heat dissipation sheet14into close contact with the optical component12, the traversing portion11c, the first projection part13a, the second projection part13b, and the like in a simple configuration, leading to the suppression of the reduction in heat dissipation efficiency.

In the present embodiment, the heat sink is the case13that accommodates the wiring board11, the optical component12, and the case13. Accordingly, it is possible to suppress the reduction of the heat dissipation reliably by a simple configuration.

Second Embodiment

FIG. 5is a sectional view illustrating an internal structure of an optical module according to a second embodiment.

The present embodiment is different from the above first embodiment in that a first projection part23aand a second projection part23bof a case23, which is an example of a heat sink, project from the first plane11dof the wiring board11after passing through the first opening11aand the second opening11b, and that an insulation sheet17, which is an example of a sheet-shaped member, is disposed between the traversing portion11cand the anisotropic heat dissipation sheet14, and these embodiments are roughly similar to each other in other respects. Accordingly, detailed explanations will be omitted.

The insulation sheet17is pasted on the first plane11dof the traversing portion11c. A first plane side pattern11gis provided to the first plane11dof the traversing portion11c. The insulation sheet17is, for example, polyimide heat-resistant tape, etc., and is for example approximately 0.07 mm in thickness. The insulation sheet17is provided on the first plane side pattern11g, and thus it is possible to estimate the total thickness of the insulation sheet17and the first plane side pattern11gto be about 0.1 mm.

Accordingly, when a thickness t1 of the projecting portions of the first projection part23aand the second projection part23bof the case23above the first plane11dof the wiring board11is similarly about 0.1 mm, the anisotropic heat dissipation sheet14and the traversing portion11c(insulation sheet17) can be brought into close contact with each other.

Note that polyimide heat-resistant tape is an example, and other materials having an electricity insulation property can be used as the insulation sheet17.

Some types of the anisotropic heat dissipation sheet14include an insulation sheet or an insulation film pasted thereon. The insulation property of these sheets can be used as the insulation sheet17; however, this naturally means that insulating objects are on the first projection part23aand the second projection part23bof the case23, increasing the thermal resistance.

When taking the contact into consideration, it is desirable that a sheet-shaped member (insulation sheet17) be an elastic sheet, and a thermal resistance material having cushioning characteristics (elasticity) is the best selection. An alternative example is an insulative thin silicone heat dissipation sheet. A silicone heat dissipation sheet is suitable as an alternative to heat-resistant tape because of its thermal resistance against 100° C. or higher and softness.

The reactive force of a member having cushioning characteristics when it is being pressed increases gradually in accordance with the pushing-up dimension. When the dimension tolerance is taken into consideration, the reactive force to the optical component12can be reduced, and it is a configuration recommended to be used in the present embodiment, in which stresses are applied locally from the traversing portion11cto the optical component12.

Also, a sheet-shaped member, exemplified by the insulation sheet17, can reduce the downward distortion of the traversing portion11cinFIG. 5. This is true regardless of whether the sheet-shaped member is elastic or not.

Also, some types of the wiring board11have insulative solder resist applied thereon, and when the electric insulation property of the solder resist is perfect, it may be used in place of the insulation sheet17. However, as a general rule, solder resist has problems in its insulation property and strength, and it is better to paste an insulation sheet on solder resist.

In the present embodiment, the insulation sheet17has to be prepared as an additional member; however, the electric insulation of the first plane11dof the traversing portion11ccan be secured, making it possible to use the first plane11dof the traversing portion11cas a wiring region.

Also, a through hole11iand an interstitial via hole (IVH) are provided for the connection between wire layers between the first plane side pattern11gand a second plane side pattern11hof the traversing portion11c, and an annular ring is provided on the first plane11d.

When the insulation sheet17is not provided such as in the first embodiment, it is not possible, in order to prevent short circuit, to provide an annular ring on the first plane11d, and accordingly it is practically impossible to connect between layers. By contrast, the present embodiment uses the insulation sheet17, making it possible to connect between layers.

This makes it possible to make wires cross each other in the wiring board11, dramatically enhancing the degree of freedom in designing. It is better to set the portion facing the second plane11eof the traversing portion11cof the case23to be at a high level as long as a short circuit with the second plane side pattern11his not caused. It is best to set the above facing portion of the case23to be at the limit of the mechanical height level to contact the insulation sheet17by also pasting the insulation sheet17to the second plane11eof the traversing portion11c.

For the same configuration as that in the first embodiment, the above second embodiment can attain the same effect as that attained by the configuration of the first embodiment, i.e., the enhancement of the noise resistance of signal wires while suppressing the reduction in the heat dissipation efficiency, etc.

Also, according to the present embodiment, the first projection part23aprojects from the first plane11dof the wiring board11after passing through the first opening11aand the second projection part23bprojects from the first plane11dof the wiring board11after passing through the second opening11b. Accordingly, it is possible to provide a space between the traversing portion11cof the wiring board11and the optical component12. This makes it possible to provide to the space the insulation sheet17, an elastic sheet, or a sheet-shaped member such as a sheet having the properties of both of them.

Also, according to the present embodiment, a sheet-shaped member exemplified by the insulation sheet17is disposed between the traversing portion11cand the anisotropic heat dissipation sheet14. Accordingly, it is possible to prevent the traversing portion11cfrom being distorted.

Also, the present embodiment employs the insulation sheet17as a sheet-shaped member, and this leads to a higher degree of freedom in designing, permitting the provision of the first plane side pattern11gon the first plane11dof the traversing portion11c, the provision of the through hole11ion the traversing portion11c, etc.

Also, the present embodiment can reduce the reactive force applied from the traversing portion11cto the optical component12when a sheet-shaped member is an elastic sheet.

Third Embodiment

FIG. 6is a sectional view illustrating an internal structure of an optical module according to a third embodiment.

The present embodiment is different from the above second embodiment in that a traversing portion21cof a wiring board21is formed at a level lower than a first plane21d, and is roughly similar to the first and second embodiments in other respects. Accordingly, detailed explanations will be omitted.

The wiring board21according to the present embodiment includes, similarly to the wiring board11of the second embodiment, a first opening21a, a second opening21b, a traversing portion21c, a first plane21d, a second plane21e, a first plane side pattern21g, a second plane side pattern21h, and a through hole21i.

Further, the traversing portion21cof the wiring board21has a recess21j(the depth is t2) that is open to the side of the wiring board21and that is formed on, for example, the entirety of the traversing portion21c. Thereby, the traversing portion21cis formed at a position lower than the first plane21d.

Note that in the present embodiment, similarly to the second embodiment, the first projection part23aand the second projection part23bof the case23project from the first plane21dof the wiring board21(the thickness of the projecting portion is t1); however, it is possible to employ a configuration where, similarly to the first embodiment, they do not project.

An insulation sheet27(which is an example of a sheet-shaped member) according to the present embodiment is, for example, an insulation silicone sheet, and is thicker than the insulation sheet17of the second embodiment. Accordingly, the insulation sheet27tends to cause wrinkles on the anisotropic heat dissipation sheet14. When the anisotropic heat dissipation sheet14is fixed with wrinkles on it, the anisotropic heat dissipation sheet14becomes prone to bending or breakage.

Accordingly, it is ideal that there be no height differences between the surface of the insulation sheet27and the top surfaces of the first projection part23aand the second projection part23b. In order to realize this, the recess21jis formed over the entire traversing portion21cand the traversing portion21chas a reduced thickness.

Also, some types of the wiring board21have insulation solder resist applied thereon; however, the present embodiment assumes that a thick insulation sheet is pasted on the solder resist.

The above third embodiment can attain the same effect as that attained by the configurations of the first and second embodiments, i.e., the enhancement of the noise resistance of signal wires while suppressing the reduction in the heat dissipation efficiency, etc.

Also, in the present embodiment, the traversing portion21cis formed at a level lower than the first plane21dof the wiring board21. Accordingly, it is possible to provide a space between an optical component22and the traversing portion21cof the wiring board21. This makes it possible to provide to the space the insulation sheet27, an elastic sheet, or a sheet-shaped member such as a sheet having the properties of both of them.

Fourth Embodiment

FIG. 7is a sectional view illustrating an internal structure of the optical module according to the fourth embodiment.

The present embodiment is different from the above second embodiment in that an electronic component ilk is mounted on the second plane11eof the traversing portion11cand this electronic component11kis electrically connected to the first plane side pattern11g, which is a wire provided on the first plane11dof the traversing portion11c, and these embodiments are roughly similar to each other in other respects. Accordingly, detailed explanations will be omitted.

As illustrated inFIG. 7, the traversing portion11chas, similarly to the second embodiment, the first plane side pattern11g, the second plane side pattern11hand the through hole11iformed thereon, and the electronic component11kis provided on the second plane11eof the traversing portion11c.

In the present embodiment, because the electronic component11kis electrically connected to the first plane side pattern11g, it is desirable in view of maintaining insulation that the insulation sheet17illustrated inFIG. 7or the insulation sheet27of the third embodiment exist between the traversing portion11cand the anisotropic heat dissipation sheet14(optical component12).

FIG. 8is a sectional view illustrating directions of heat transfer according to the fourth embodiment.

FIG. 9is a sectional view illustrating calculation conditions according to the fourth embodiment.

FIG. 10illustrates a relationship between width L of the traversing portion and temperature increase Δt according to the fourth embodiment.

The thermal conductivity of the anisotropic heat dissipation sheet14illustrated inFIG. 8is approximately 7 W/mK in the first directions D1, which are the thickness directions of the wiring board11illustrated inFIG. 7, and is approximately 700 W/mK in the second directions D2that are crossing the traversing portion11cin the plane orthogonal to the first directions D1(actually they are identical in the plane orthogonal to the first directions D1) in the case of, for example, a graphite sheet, and is very advantageous in view of the thermal transfer in the second directions D2.

The anisotropic heat dissipation sheet14does not have such an excellent thermal conductivity in the first directions D1, however, it has a small thickness of approximately 0.10 mm from the beginning, and accordingly its thermal resistance is suppressed to approximately 0.03° C./W. InFIG. 8the heat dissipation from the portion of the optical component12facing the traversing portion11cto the portion contacting the case23is represented by arrows. As illustrated in the figure, the heat in the position of the optical component12facing the traversing portion11cis conducted to the first opening11aand the second opening11bvia the anisotropic heat dissipation sheet14, and thereafter is conducted to the first projection part23aand the second projection part23b.

Here, explanations will be given for calculation conditions in the fourth embodiment.

The case23is made by a cutting process performed on aluminum or a die-cast molding of aluminum. It is desirable that the flatness of the top surfaces of the heat dissipation surfaces, which are the top surfaces of the first projection part23aand the second projection part23b, be taken into consideration in the designing because the flatness has an influence on the contact with the optical component12.

When the case23is made of aluminum, it will have a thermal conductivity of approximately 150 W/mK through 237 W/mK; however, because it has an isotropic thermal conductivity, heat is not dissipated well through a portion having a thickness of about 1 mm. Accordingly, the case23is designed to have a large thickness even around the portion contacting the optical component12.

The optical component12is an integrated reception device according to OIF-DPC-RX-01.0; however, the optical component12can be applied to other optical components having highly similar configurations such as a Tunable Laser Diode (LD).

The body of an integrated reception device is made of kovar, which is not greatly influenced by changes in shape caused by heat. However, its characteristics are influenced by distortion of torque caused by a tightening of screws, and when the flatness of the contacting portion is low, a great distortion will be caused by the tightening of screws. It is desirable that special consideration be given to the attaching methods and tightening torque.

The wiring board11is made of an FR4 material, which has an excellent high-speed characteristic. The thickness is about 2 mm. In recent years, regarding FR4-related materials, the specific permittivity of glass cloth has become remarkably lower and the property of the dielectric tangent has become remarkably better, which makes it suitable for the wiring board11of an optical module.

On the traversing portion11c, the recess21jillustrated inFIG. 6is not formed, and the number of layers in the traversing portion11cis the same as that of other portions of the wiring board11. The wiring board11has two openings, i.e., the first opening11aand the second opening11b, and the traversing portion11cserves as a bridge around the center.

The through hole11iis provided as a pattern of the traversing portion11cand there are also patterns on internal layers and on both of the top layers (the first plane side pattern11gand the second plane side pattern11h). The insulation sheet17used for the insulation between the anisotropic heat dissipation sheet14and the traversing portion11cis Kapton tape (polyimide tape) having a thickness of about 0.07 mm. Thereby, the side of the first plane11dof the traversing portion11cis insulated entirely. Also, the wiring board11has solder resist applied thereon by using a pre-flux process; however, Kapton tape is used for the insulation because there is a possibility that the solder resist will be peeled by the contact with the optical component12.

The anisotropic heat dissipation sheet14is a graphite sheet as described above. A graphite sheet is a sheet produced by carbonizing a polyimide sheet, and has a thermal conductivity of about 700 W/mK in the plane direction (in the plane orthogonal to the first directions D1, i.e., the directions including the second directions D2) due to the carbon molecular binding in the plane directions, and has a thermal conductivity of about 7 W/mK in the thickness directions (the first directions D1). The thickness is about 0.1 mm with the handling being taken into consideration. The shape is identical to the bottom surface of the optical component12, and the fringe112cas illustrated inFIG. 11is also brought into contact. Accordingly, through holes are provided on the graphite sheet so that the first planes11dpass through them at the four positions.

The effects of the present embodiment can be reviewed easily. Large scale calculations such as simulation are not necessary. First, the thermal resistance is calculated from the thermal conductivity, the area and length of the heat conducting path, and they are studied as a combined resistance of all pieces of heat resistance.

The thickness of the bottom plate of the optical component12is 2 mm, and there is a heat generating body. The width of the traversing portion11c(the second directions D2) is L. With L is center symmetric, and all the other portions are fixed to the case23of the aluminum with screws. It is assumed that the thermal conductivity of the case23is 237 W/mK (the calculation is based on an assumption that the aluminum is pure aluminum) and the thermal conductivity of the optical component12is 17 W/mK. The thickness of the GS (graphite sheet) is 0.1 mm. Treating the heat reference point as the temperature reference point illustrated inFIG. 9, the difference in temperature between this temperature reference point and the heat generating body is defined as ΔT. The temperature difference ΔT is set to be 10° C. or lower as a design requirement.

There is a design requirement also for width L of the traversing portion11c. When width L is too small, it is not possible to mount components on the bottom surface of the traversing portion11c, which is problematic. Width L is 10 mm or greater, including the wiring regions around the electronic component11k. The heat generating body is 4 W, and is mounted at the center of the optical component12.

When there is graphite as the anisotropic heat dissipation sheet14, it is possible to suppress temperature increases ΔT with respect to width L of the traversing portion11cas represented by the plot of the white triangles inFIG. 10in comparison with the plot of the black squares, which represents the case where there is no graphite as the anisotropic heat dissipation sheet14.

It is only when there is graphite and width L is 10 mm through 20 mm that the condition that width L is 10 mm or wider and the temperature increase ΔT is 10° C. or lower is met as the above design condition.

By setting width L of the traversing portion11cto be 20 mm in the designing of the wiring board11, an FET or a large A/D converter that are connected with the pins of the optical component12can be disposed. These components are mounted on the side not facing the optical component12, i.e., are mounted on the second plane11eof the traversing portion11c. Because it is desirable to use the through hole11ifor wiring, the above Kapton tape is used as the insulation sheet17.

The wiring board11has, for example, sixteen layers, and it is possible to perform wiring by using wide-area patterns on six of the layers for the connection with the above described SG, FET, and large A/D converter and to use all the other layers except for the top and the bottom layers, i.e., eight of the layers, for wiring for the connection with pins of the optical component12. Due to the effect of the graphite sheet (the anisotropic heat dissipation sheet14), width L can be as large as 20 mm, drastically improving the degree of freedom in designing.

As described above, at the designing stage, the high density of wires at the first opening11a, the second opening11b, and the ends of the wiring board11illustrated inFIG. 3is eliminated, making it possible to perform wiring on shorter routes. Also, during the reviewing of heat, it is possible to confirm that the temperature is within the rated temperature of a device. It is possible to confirm that the wiring aspect and heat dissipation aspect are compatible with each other.