Signal transmission board and method for manufacturing the same

A signal transmission board includes a substrate, a conductive via, a cavity and a connecting hole. The substrate has a first external surface and a second external surface. The conductive via penetrating through the substrate has a first end and a second end. The first end is disposed on the first external surface, and the second end is disposed on the second external surface. The cavity is disposed in the substrate and penetrated by the conductive via. The connecting hole disposed on the substrate has a third end and a fourth end. The third end is disposed on the first external surface, and the fourth end communicates with the cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 103143727 filed in Taiwan, R.O.C. on Dec. 15, 2014, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The disclosure relates to a signal transmission board and a method for manufacturing the same.

BACKGROUND

With development of technology of high-level servers, the transmission speed and quality of signals transmitted by a signal transmission board, e.g., a printed circuit board (PCB), are highly demanded.

A circuit board includes a transmission wire and a plated through hole (PTH). The transmission wire is used for signal transmission in a horizontal direction. The PTH is used for signal transmission in a vertical direction such that the signal can be transmitted through several stacked layers. However, the PTH in vertical direction has impedance mismatch with the transmission wire in horizontal direction, which reduces the speed and quality of the signal transmission. Specifically, when the PTH has impedance mismatch with the transmission wire and a signal is transmitted through several layers in the transmission wire and the PTH, the signal may be undesirably reflected due to the differences between the impedances of the PTH and the transmission wire. Thus, this causes loss and noise of the signal transmission.

SUMMARY

One embodiment of the disclosure provides a signal transmission board including a substrate, a cavity, a conductive via and a connecting hole. The substrate has a first external surface and a second external surface that are opposite to each other. The cavity is disposed in the substrate. The conductive via has a first end and a second end that are opposite to each other. The conductive via penetrates through the first external surface of the substrate, the cavity and the second external surface of the substrate in sequence. The first end of the conductive via is disposed on the first external surface, and the second end of the conductive via is disposed on the second external surface. The connecting hole has a third end and a fourth end that are opposite to each other. The connecting hole is disposed on the substrate. The third end of the connecting hole is disposed on the first external surface, and the fourth end of the connecting hole communicates with the cavity.

Another embodiment of the disclosure provides a method for manufacturing a signal transmission board including the following steps. A thermal decomposable material is disposed in a substrate with a first external surface and a second external surface. The thermal decomposable material is separated from the first external surface and the second external surface by distances, respectively. A through hole extending through the substrate and the thermal decomposable material is formed. The through hole has a first end disposed on the first external surface of the substrate and a second end disposed on the second external surface of the substrate. An inner conductive film is plated on a sidewall of the through hole to form a conductive via. A connecting hole extending from the first external surface of the substrate toward the thermal decomposable material is formed. The thermal decomposable material is removed through the connecting hole by vaporization so as to form a cavity in the substrate, and the conductive via penetrates through the cavity.

DETAILED DESCRIPTION

Refer toFIG. 1A,FIG. 1B,FIG. 2andFIG. 3,FIG. 1Ais a cross-sectional view of a signal transmission board according to an embodiment of the disclosure, andFIG. 1Bis a top view of the signal transmission board according to an embodiment of the disclosure,FIG. 2is a top view of a signal transmission board according to another embodiment of the disclosure, andFIG. 3is a cross-sectional view of the signal transmission board according to another embodiment of the disclosure. As shown inFIG. 1A, the signal transmission board10can be applied in a field of printed circuit boards (PCBs). Traces, i.e., circuits, can be printed on a surface thereof and electronic components can be disposed on the surface. Thus, signals generated by the electronic components can be transmitted through the traces and PTHs of the signal transmission board10.

In this embodiment, the signal transmission board10includes a substrate11, a conductive via12, a cavity13and a connecting hole14. Specifically, the substrate11has a first external surface111and a second external surface112that are opposite to each other. In this and some other embodiments, the first external surface111and the second external surface112are a top surface and a bottom surface of the substrate11, respectively. Moreover, in this and some other embodiments, the substrate11is a multilayer substrate. The substrate11includes, from the first external surface111to the second external surface112in order, a first insulation layer113, a first electrically conductive layer114, a base layer115, a second electrically conductive layer116and a second insulation layer117. In detail, the first external surface111of the substrate11is a surface of the first insulation layer113away from the base layer115, and the first electrically conductive layer114is disposed between the first insulation layer113and the base layer115. The second external surface112of the substrate11is a surface of the second insulation layer117away from the base layer115, and the second electrically conductive layer116is disposed between the second insulation layer117and the base layer115. As shown inFIG. 1, for example, the base layer115is made of an electrically insulated material.

In this embodiment, for example, the signal transmission board10further includes a first trace15and a second trace16. The first trace15is disposed on the first external surface111of the substrate11. The second trace16is disposed on the second external surface112. Multiple electronic components can be disposed on the first external surface111and the second external surface112of the substrate11, and the electronic components can be electrically coupled to each other through the first trace15and the second trace16. However, the disclosure is not limited to the configurations of the first trace15and the second trace16. In other words, in other embodiments, there are no the first trace15and the second trace16disposed on the first external surface111and the second external surface112. In other embodiments, a plurality of first traces15and a plurality of second traces16are disposed on the first external surface111and the second external surface112of the substrate11, respectively.

The conductive via12penetrates through the substrate11, and the conductive via12has a first end122and a second end123that are opposite to each other. As shown inFIG. 1AthroughFIG. 3, a through hole121extends through not only the substrate11but also the first trace15and the second trace16. That is, the first end122of the through hole121is disposed on the surface of the first trace15, and the second end123of the through hole121is disposed on the surface of the second trace16. In other embodiments, when no first trace15and second trace16are disposed at the through hole121, the first end122of the through hole121is disposed on the first external surface111of the substrate11, and the second end123of the through hole121is disposed on the second external surface112of the substrate11.

In an embodiment shown inFIG. 1AthroughFIG. 3, in terms of the conductive via12, the conductive via12may be formed by any suitable method, such as drilling a through hole121in the substrate11and plating the through hole121with an inner conductive film124. The conductive via12extends through the first insulation layer113, the first electrically conductive layer114, the base layer115, the second electrically conductive layer116and the second insulation layer117. Furthermore, the first trace15can be connected to the inner conductive film124disposed on the first end122of the conductive via12. The second trace16can be connected to the inner conductive film124disposed on the second end123of the conductive via12. Since the first trace15and the second trace16are respectively connected to the inner conductive film124of the conductive via12, the electronic components (not shown) disposed on the first external surface111of the substrate11are electrically coupled to the electronic components (not shown) disposed on the second external surface112. In one embodiment, the quantity of the conductive via12disposed on the substrate11can be one or more than one, and the positions where the conductive via12is disposed can be properly arranged according to different designs, and the disclosure is not limited thereto.

The cavity13is disposed in the substrate11and separated from the first external surface111and the second external surface112by distances, respectively. In this embodiment, in detail, the cavity13can be disposed in the first electrically conductive layer114, the base layer115and the second electrically conductive layer116of the substrate11, and the cavity13can be disposed between the first insulation layer113and the second insulation layer117. In other words, the cavity13is disposed within the first electrically conductive layer114, the base layer115and the second electrically conductive layer116and does not extend through the first insulation layer113and the second insulation layer117. The inner conductive film124is not in direct contact with the first electrically conductive layer114, the base layer115and the second electrically conductive layer116. Also, the cavity13and the first external surface111are separated by a thickness of the first insulation layer113, and the cavity13and the second external surface112are separated by a thickness of the second insulation layer117. In other words, the cavity13is disposed in inner layers of the substrate11and does not expose to the outside of the signal transmission board10.

In addition, the conductive via12is disposed within the cavity13. In an embodiment, as shown inFIG. 1B, the cavity13and the conductive via12are coaxial. Accordingly, the inner wall of the cavity13and those of the conductive via12are separated by a constant distance. However, the disclosure is not limited to the coaxial structure of the cavity13and the conductive via12. In other embodiments, as shown inFIG. 2the cavity13is eccentric to the conductive via12, and the conductive via12is still disposed within the cavity13. Moreover, as shown inFIG. 1A, the connecting hole14has a third end141and a fourth end142that are opposite to each other. The connecting hole14extends from the first external surface111of the substrate11toward the cavity13. The third end141of the connecting hole14is disposed on the first external surface111of the substrate11, and the fourth end142of the connecting hole14communicates with the cavity13. Accordingly, the cavity13can communicate with the external environment outside the substrate11through the connecting hole14.

The connecting hole14in one embodiment extends through the first insulation layer113and a part of the first electrically conductive layer114, such that the connecting hole14communicates with the cavity13. However, the disclosure is not limited to the communication between the connecting hole14and the cavity13. In other embodiments, the connecting hole14can directly extend through the first insulation layer113to communicate with the cavity13, as shown inFIG. 3. In other embodiments, the connecting hole14can extend through the first insulation layer113and the first electrically conductive layer114such that the fourth end142is disposed at a contact surface between the first electrically conductive layer114and the base layer115. In other embodiments, the connecting hole14can extend through the first insulation layer113, the first electrically conductive layer114and a part of the base layer115. In other embodiments, the connecting hole14can extend through the first insulation layer113, and the first electrically conductive layer114and the base layer115such that the fourth end142of the connecting hole14is disposed at a contact surface between the base layer115and the second electrically conductive layer116. In other embodiments, the connecting hole14can extend through the first insulation layer113, the first electrically conductive layer114, the base layer115and a part of the second electrically conductive layer116. The disclosure is not limited to the extension of the connecting hole14. The connecting hole14communicating the cavity13and the external environment outside the substrate11falls within the scope of the disclosure. In addition, the disclosure is not limited to the quantity and position of connecting hole14. In other embodiments, the quantity of the connecting hole14can be more than one. Alternatively, the third end141of the connecting hole14is disposed on the second external surface112. The connecting hole14extends through the second insulation layer117and the second electrically conductive layer116in order to make the fourth end142of the connecting hole14communicate with the cavity13.

In the disclosure, the phrase “A communicates with B” is defined that two rooms A and B are connected to each other such that air or certain gas is able to circulate in the two rooms A and B.

Furthermore, in an embodiment, the cavity13is full of air. Since the air has relatively low dielectric constant (i.e., permittivity), the air inside the cavity13can enhance the characteristic impedance of the conductive via12. That is, the impendence of the conductive via12matches with the transmission wire, i.e., the first trace15and second trace16. Hence, when signals are transmitted through the signal transmission board10, the impedances of the vertical paths, i.e., along the conductive via12, or horizontal paths, i.e., along the first trace15and second trace16, are substantially the same, which reduces the discontinuity of impedances of the transmission path.

In one embodiment, the disclosure is not limited to the material disposed in the cavity13being air. For example, the cavity13can be filled with a material with a low dielectric constant or another material with a high dielectric constant. In one embodiment, the low dielectric constant is defined as being lower than that of the first insulation layer113or that of the second insulation layer117. The high dielectric constant is defined as being higher than that of the first insulation layer113or that of the second insulation layer117. For example, a dielectric material with low dielectric constant is air or other materials with low loss tangent. For example, a dielectric material with high dielectric constant is SiO2, Ta2O5, BaTiO3or other dielectric material with high dielectric constant. When the cavity13is filled with a dielectric material with a low dielectric constant, the characteristic impedance of the conductive via12is enhanced. When the cavity13is filled with a dielectric material with a high dielectric constant, the characteristic impedance of the conductive via12can be reduced. Thus, the signal transmission board10can be applied in different kinds of fields. The dielectric material in the cavity13can be a solid, liquid or gas.

The following describes the method for manufacturing the signal transmission board10shown inFIGS. 1A and 1B. Please refer toFIG. 4andFIG. 5AthroughFIG. 5I.FIG. 4is a flow chart of a method for manufacturing the signal transmission board ofFIG. 1A, andFIG. 5AthroughFIG. 5Iare the steps of a manufacturing process of the signal transmission board ofFIG. 4. As shown inFIG. 5AthroughFIG. 5C, a thermal decomposable material17is disposed in the substrate11. At Steps S401, S402and S403, the thermal decomposable material17is disposed in the substrate11.

Firstly, at Step S401and as shown inFIG. 5A, the first electrically conductive layer114and the second electrically conductive layer116are disposed on two sides of the base layer115that are opposite to each other, respectively. The first electrically conductive layer114and the second electrically conductive layer116can be made of cooper or other conductive materials. Afterwards, at Step S402and as shown inFIG. 5B, a penetration hole118is formed and extended through the first electrically conductive layer114, the base layer115and the second electrically conductive layer116. Then, at Step S403and as shown inFIG. 5C, the penetration hole118is filled with the thermal decomposable material17.

Next, at Step S404and as shown inFIG. 5D, the first metal layer15′ is formed on the first insulation layer113, and then the first insulation layer113and the first metal layer15′ are laminated on the first electrically conductive layer114. The second metal layer16′ is formed on the second insulation layer117, and then the second insulation layer117and the second metal layer16′ are laminated on the second electrically conductive layer116. The first metal layer15′ and the second metal layer16′ can be made of cooper or other conductive materials. In detail, at Step S404, the first metal layer15′ is formed on the first insulation layer113, and then the first insulation layer113and the first metal layer15′ are laminated on a side of the first electrically conductive layer114, which is away from the base layer115, together. In other words, the first electrically conductive layer114is disposed between the first insulation layer113and the base layer115, and a part of the first insulation layer113is disposed between the first metal layer15′ and the first electrically conductive layer114. Also, the first insulation layer113covers the first electrically conductive layer114and one side of the thermal decomposable material17. In addition, the second metal layer16′ is formed on the second insulation layer117, and then the second insulation layer117and the second metal layer16′ are laminated on a side of the second electrically conductive layer116, which is away from the base layer115, together. In other words, the second electrically conductive layer116is disposed between the second insulation layer117and the base layer115, and a part of the second insulation layer117is disposed between the second metal layer16′ and the second electrically conductive layer116. Also, the second insulation layer117covers the second electrically conductive layer116and the other side of the thermal decomposable material17. That is, the thermal decomposable material17is encircled by the first electrically conductive layer114, the base layer115and the second electrically conductive layer116, and the thermal decomposable material17is disposed between and in contact with the first insulation layer113and the second insulation layer117. Consequently, the thermal decomposable material17is disposed in the substrate11.

Then, Step S405is performed. As shown inFIG. 5E, the first metal layer15′ and the second metal layer16′ are patterned to form the first trace15and the second trace16according to design of circuitry. The first trace15and the second trace16can be made of cooper or other conductive materials. However, the disclosure is not limited to the step of disposing the first metal layer15′ and the second metal layer16′. In other embodiments, the first metal layer15′ and the second metal layer16′ are not formed, which means Steps S404and S405can be omitted.

Afterwards, Step S406is performed. As shown inFIG. 5F, the through hole121is formed on the substrate11and extended through the substrate11and the thermal decomposable material17. In the embodiments shown inFIG. 5F, the through hole121not only extends through the substrate11but also the first trace15and the second trace16such that the first end122and the second end123of the through hole121are disposed on the first trace15and the second trace16, respectively. In other embodiments, as the first trace15and the second trace16are not disposed on the first insulation layer113and the second insulation layer117, respectively, the first end122and the second end123of the through hole121are disposed on the first external surface111and the second external surface112of the substrate11, respectively. In the embodiments shown inFIG. 5F, the through hole121extends through the first trace15, the first insulation layer113, the thermal decomposable material17, the second insulation layer117and the second trace16.

Then, Step S407is performed. As shown inFIG. 5G, an inner conductive film124is formed on the inner sidewall of the through hole121to form the conductive via12. The inner conductive film124can be made of cooper or other conductive materials. A part of the inner conductive film124disposed at the first end122of the conductive via12is electrically coupled to the first trace15on the first external surface111. Another part of inner conductive film124disposed at the second end123of the conductive via12is electrically coupled to the second trace16on the second external surface112. Thus, the first trace15and the second trace16are coupled to each other through the conductive via12. Consequently, the signals can be transmitted and received from the electronic components disposed on the first external surface111to the electronic components disposed on the second external surface112through the conductive via12.

Then, Step S408is performed. As shown inFIG. 5H, the connecting hole14is formed on the substrate11, and the connecting hole14extends from the first external surface111to the thermal decomposable material17. In detail, the connecting hole14has the third end141and the fourth end142. The third end141of the connecting hole14is disposed on the first external surface111, and the fourth end142of the connecting hole14is connected to the thermal decomposable material17. The disclosure is not limited to the above-mentioned position, quantity and deepness of the connecting hole14. The connecting hole14falls in the scope of the disclosure is that the fourth end142of the connecting hole14is connected to the thermal decomposable material17, and the third end141of the connecting hole14can be exposed to the outside of the substrate11to communicate with the outside of the signal transmission board10.

Afterwards, Step S409is performed. As shown inFIG. 5I, the thermal decomposable material17is removed through the connecting hole14by vaporization. Thus, the cavity13, which is penetrated by the conductive via12, is formed in the substrate11. At Step S409, the vaporization temperature of the thermal decomposable material17is higher than the temperatures of pressing and curing of the first insulation layer113and those of the second insulation layer117. The thermal decomposable material17is heated to gradually convert to be gas state. The thermal decomposable material17flows to the outside of the substrate11through the connecting hole14. After the thermal decomposable material17is vaporized, the space which originally stores the thermal decomposable material17becomes the cavity13. Accordingly, the manufacturing of the signal transmission board10according to the embodiments of the disclosure is finished. For example, the vaporization temperature of the thermal decomposable material made by Empower Company is 300 degrees Celsius. Different kinds of thermal decomposable materials have different vaporization conditions. However, the disclosure is not limited to the vaporization conditions, e.g., the vaporization temperature in this embodiment.

In one embodiment, the cavity13is full of air. Because the conductive via12is surrounded with the air in the cavity13, the characteristic impedance of the conductive via12can be enhanced. Thus, the impedance of the conductive via12matches with that of the horizontal transmission wire, e.g., the first trace15and the second trace16. Accordingly, the quality and speed of signal transmission of the signal transmission board10are improved.

Please refer toFIG. 6, which is a cross-sectional view of a signal transmission board according to yet another embodiment of the disclosure. The signal transmission board10ashown inFIG. 6includes the substrate11, the conductive via12, the cavity13and the connecting hole14, these components are similar to those described in the above-mentioned signal transmission board10. Also, the configuration of the signal transmission board10ais similar to that of the signal transmission board10. The difference between this embodiment and the signal transmission board10shown inFIG. 1Ais that the signal transmission board10afurther includes an external conductive film18. The external conductive film18is formed on a sidewall of the cavity13surrounded by the first electrically conductive layer114, the base layer115and the second electrically conductive layer116. The external conductive film18can be made of cooper or other conductive materials. Moreover, the external conductive film18is electrically coupled to the first electrically conductive layer114and the second electrically conductive layer116such that the first electrically conductive layer114is electrically coupled to the second electrically conductive layer116.

Please refer toFIG. 7andFIG. 8AthroughFIG. 8J.FIG. 7is a flow chart of a method for manufacturing the signal transmission board ofFIG. 6, andFIG. 8AthroughFIG. 8Jare the steps of a manufacturing process of the signal transmission board ofFIG. 7. The difference between the method of manufacturing the signal transmission board shown inFIG. 7and that inFIG. 4is that the method in this embodiment further comprises the step of plating the external conductive film18.

Firstly, Step S701is performed. As shown inFIG. 8A, the first electrically conductive layer114and the second electrically conductive layer116are disposed on two sides of the base layer115that are opposite to each other. The first electrically conductive layer114and the second electrically conductive layer116can be made of cooper or other conductive materials. Then, at Step S702and as shown inFIG. 8B, the penetration hole118is formed and extends through the first electrically conductive layer114, the base layer115and the second electrically conductive layer116. Next, at Step S703and as shown inFIG. 8C, the external conductive film18is plated on the sidewalls of the penetration hole118. The external conductive film18is electrically connected to the first electrically conductive layer114and the second electrically conductive layer116. That is, the external conductive film18can be electrically coupled to the first electrically conductive layer114and the second electrically conductive layer116. Afterwards, at Step S704and as shown inFIG. 8D, the penetration hole118is filled with the thermal decomposable material17.

Then, at Step S705and as shown inFIG. 8E, the first metal layer15′ is disposed on the first insulation layer113, and then the first insulation layer113and the first metal layer15′ are laminated on a side of the first electrically conductive layer114, which is away from the base layer115. The second metal layer16′ is disposed on the second insulation layer117, and then the second insulation layer117and the second metal layer16′ are laminated on a side of the second electrically conductive layer116, which is away from the base layer115. The first metal layer15′ and the second metal layer16′ can be made of cooper or other conductive materials. In detail, at Step S705, the first metal layer15′ is formed on the first insulation layer113, and then the first insulation layer113and the first metal layer15′ are laminated on the first electrically conductive layer114. The first electrically conductive layer114is disposed between the first insulation layer113and the base layer115, and a part of the first insulation layer113is disposed between the first metal layer15′ and the first electrically conductive layer114. Thus, the first insulation layer113covers the first electrically conductive layer114and one side of the thermal decomposable material17. Similarly, the second metal layer16′ is formed on the second insulation layer117, and then the second insulation layer117and the second metal layer16′ are laminated on a side of the second electrically conductive layer116, which is away from the base layer115. The second electrically conductive layer116is disposed between the second insulation layer117and the base layer115, and a part of the second insulation layer117is disposed between the second metal layer16′ and the second electrically conductive layer116. Thus, the second insulation layer117covers the second electrically conductive layer116and the other side of the thermal decomposable material17. The thermal decomposable material17is disposed between the first insulation layer113and the second insulation layer117.

At Step S706and as shown inFIG. 8F, the first metal layer15′ and the second metal layer16′ are patterned to form the first trace15and the second trace16according a design of circuitry, which is similar to Step S405as shown inFIG. 4. For example, the method for pattering includes a general manufacturing method of printed circuit boards and semiconductors, such as, etching, electroplating and deposition. The first trace15and the second trace16can be made of cooper or other conductive materials. Moreover, Steps S707through S710andFIG. 8GthroughFIG. 8Jare similar to Steps S406through S409andFIG. 4FthroughFIG. 4I, so the repeated description is not described again hereinafter.

Please refer toFIG. 9andFIG. 10.FIG. 9is a cross-sectional view of a signal transmission board according to yet another embodiment of the disclosure, andFIG. 10is a cross-sectional view of a signal transmission board according to yet another embodiment of the disclosure. As shown in the figures, each of the signal transmission boards10band10cincludes the substrate11, the conductive via12, the cavity13, the connecting hole14and the external conductive film18of the signal transmission board10ashown inFIG. 6. The configurations of the signal transmission boards10band10care similar to that of the signal transmission board10a. Compared to the signal transmission board10aofFIG. 6, the difference is that the external conductive film18in this embodiment is only electrically coupled to the first electrically conductive layer114or the second electrically conductive layer116. In the signal transmission board10bshown inFIG. 9, the external conductive film18is electrically coupled to the first electrically conductive layer114. The external conductive film18is electrically insulated from the second electrically conductive layer116. The external conductive film18, the first electrically conductive layer114and the second electrically conductive layer116can be made of cooper or other conductive materials. In the signal transmission board10cshown inFIG. 10, the external conductive film18is electrically coupled to the second electrically conductive layer116. The external conductive film18is electrically insulated from the first electrically conductive layer114. The external conductive film18, the first electrically conductive layer114and the second electrically conductive layer116can be made of cooper or other conductive materials.

Please refer toFIG. 11, which is a cross-sectional view of a signal transmission board according to yet another embodiment of the disclosure. The signal transmission board10dofFIG. 11includes the substrate11, the conductive via12, the cavity13and the connecting hole14of the signal transmission board10shown inFIG. 1A, and the configuration of the signal transmission board10dis similar to that of the signal transmission board10. Compared to the signal transmission board10shown inFIG. 1, the base layer115of the signal transmission board10dfurther includes an inner portion115aand an external portion115b. The inner portion115ais encircled by the external portion115b. The inner portion115ais made of a conductive material or a semi-conductive material. The external portion115bis made of an insulated material. That is to say, an exterior surface of the base layer115is made of an insulated material, which is suitable for being applied in the disclosure.

Please refer toFIG. 12, which is a cross-sectional view of a signal transmission board according to yet another embodiment of the disclosure. The signal transmission board10eofFIG. 12includes the substrate11, the conductive via12, the cavity13and the connecting hole14of the signal transmission board10shown inFIG. 1A. The configuration of the signal transmission board10eis similar to that of the signal transmission board10. Compared to the signal transmission board10shown inFIG. 1A, the difference is that the signal transmission board10efurther includes a filling material19. The through hole121is filled with the filling material19. The filling material19is made of a conductor or insulator. When the through hole121is filled with the filling material19, the structural strength of the conductive via12is enhanced.

Please refer toFIG. 13, which is a cross-sectional view of a signal transmission board according to yet another embodiment of the disclosure. The configuration of the signal transmission board10fis similar to that of the signal transmission board10shown inFIG. 1. The difference between the signal transmission board10fand the signal transmission board10shown inFIG. 1is that the signal transmission board10fin this embodiment has two connecting holes14aand14b. The opening of the connecting holes14aand14bare disposed on the first external surface111and the second external surface112of the substrate11, respectively. The connecting hole14aextends through the first insulation layer113to communicate with the cavity13. The connecting hole14bextends through the second insulation layer117to communicate with the cavity13. In this embodiment, the connecting holes14aand14bare disposed at the same side of the conductive via12, but the disclosure is not limited to the positions of the connecting holes14aand14b. In other embodiments, the connecting holes14aand14bcan be disposed at two sides of the conductive via that are opposite to each other. Alternatively, multiple connecting holes14aand14bare disposed on the substrate11.

Summarily, in the signal transmission board, since the connecting hole communicates with the cavity, the thermal decomposable material which is vaporized can be dissipated to the outside of the substrate. Also, the cavity is full of air with the low dielectric constant rather than the dielectric material, and the conductive via is disposed within the cavity. Thus, the characteristic impedance of the conductive via is enhanced. The impedance of the conductive via along the vertical direction matches with the transmission wire, i.e., traces, along the horizontal direction. Since the impedances of paths of signal transmission along the vertical and horizontal directions match with each other, the quality and speed of signal transmission of the signal transmission board are improved when signals transmit through several layers. Furthermore, in other embodiments, the cavity is filled with high dielectric material or the through hole is filled with the filling material, so the signal transmission board can be utilized for a wider variety of applications.