Microcoaxial chip interconnect structure and its application based on copper-based additive manufacturing process

Abstract

This invention discloses a micro-coaxial chip interconnect structure and its application based on copper-based additive manufacturing technology, belonging to the field of microwave radio frequency technology. It includes a coaxial horizontal-to-vertical connection structure and a connector-like interconnect structure. The coaxial horizontal-to-vertical connection structure includes a horizontal rectangular coaxial interface and a vertical circular coaxial interface connected by a first rectangular coaxial line. The outer side of the first rectangular coaxial line is an outer conductor. The connector-like interconnect structure includes a circular coaxial outer conductor, an inner core with protruding outer conductors at both ends, an inner core mating structure, and a dielectric support structure. The inner core has two segments connected by the inner core mating structure. The outer side of the dielectric support structure is a circular coaxial outer conductor. The inner core end faces connected to the external interface are respectively provided with structures where the inner core protrudes outer conductors at both ends. This allows direct contact with standard connectors, resulting in a compact structure that does not require soldering. It offers advantages such as flexibility and ease of assembly, which is beneficial for the packaging and use of micro-coaxial chips.

Description

Technical Field

[0001] This invention belongs to the field of microwave radio frequency testing technology, and particularly relates to a micro-coaxial chip interconnect structure and its application based on copper-based additive manufacturing process. Background Technology

[0002] With the increasing miniaturization of microwave systems, the interconnect structure within radio frequency (RF) systems plays a crucial role in system integration. Traditional 3D interconnect technologies, such as those based on microstrip lines, striplines, and waveguide transmission lines, have been widely researched and applied. However, in recent years, due to the advantages of microcoaxial devices in ultra-wideband, low insertion loss, and low crosstalk, their application in RF microsystems has attracted increasing attention. Microcoaxial devices have demonstrated superior performance and have achieved efficient integration and testing in certain applications.

[0003] Nevertheless, research on microcoaxial devices, especially their multilayer interconnect structures, remains relatively limited, particularly in high-frequency bands (such as the W-band). Currently, various broadband, low-loss microcoaxial interconnect structures have been proposed, exhibiting good electrical performance across different frequency ranges. However, these structures are often limited to specific testing environments or restricted frequency ranges, thus limiting their application in practical multilayer chip interconnects.

[0004] For example, a proposed micro-coaxial-GSG adapter structure can operate at 170 GHz, but is only suitable for probe testing; another micro-coaxial-CPW adapter structure possesses reliable mechanical properties and can be connected to other MMICs via gold wire bonding or flip-chip bonding, but its operating frequency is limited to below 40 GHz. Furthermore, most research on interconnects between micro-coaxial chips relies on solder balls or liquid metal as the interconnect medium. These methods are prone to short circuits or operational difficulties in practice, affecting the reliability and stability of the interconnect structure.

[0005] To address these challenges, especially to achieve ultra-wideband three-dimensional interconnection of microcoaxial devices in the W-band, a microcoaxial chip interconnection structure based on standard connectors and connector-like devices is needed. Summary of the Invention

[0006] To address the aforementioned problems and achieve the aforementioned objectives, this invention provides a micro-coaxial chip interconnect structure and its application based on copper-based additive manufacturing technology, aiming to achieve effective and convenient chip interconnection.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is: a micro coaxial chip interconnection structure based on a connector-like structure, comprising a coaxial horizontal to vertical connection structure and a connector-like interconnection structure. The coaxial horizontal to vertical connection structure includes a horizontal rectangular coaxial interface and a vertical circular coaxial interface connected by a first rectangular coaxial line, with the outer side of the first rectangular coaxial line being an outer conductor. The connector-like interconnection structure includes a circular coaxial outer conductor, an inner core with the outer conductor protruding at both ends, an inner core matching structure, and a dielectric support structure. The inner core is provided in two segments, which are connected to each other through the inner core matching structure. The outer side of the dielectric support structure is a circular coaxial outer conductor, and the inner core end faces connected to the outside are respectively provided with structures where the inner core protrudes from the outer conductor at both ends.

[0008] Furthermore, the coaxial line end face at the rectangular coaxial interface is connected to a matching structure, which has a rectangular cross-section.

[0009] Furthermore, a horizontal rectangular coaxial-to-vertical circular coaxial conversion structure is provided at the circular coaxial interface. The horizontal rectangular coaxial-to-vertical circular coaxial conversion structure is columnar in shape. A circular coaxial interface is provided at the top of the horizontal rectangular coaxial-to-vertical circular coaxial conversion structure, and one end of the first rectangular coaxial line is connected to the side, while the other end is a rectangular coaxial interface.

[0010] Furthermore, a cavity is provided at the rectangular coaxial interface to reduce parasitic coupling; the outer conductor has a five-layer structure, with the first rectangular coaxial line located in the middle layer, and the outer conductor is removed along the rectangular cross-section near the matching structure at the top and bottom layers of the outer conductor to form a cavity.

[0011] Furthermore, it also includes a coaxial horizontal connection structure, including an inner conductor, an outer conductor outside the inner conductor, the inner conductor being a second rectangular coaxial line, matching structures at both ends of the inner conductor, and a cavity at the connection point with the coaxial line of the outer conductor.

[0012] Furthermore, an auxiliary connection structure is provided at the connection between the coaxial connector and the rectangular coaxial line. The auxiliary connection structure consists of a circular coaxial transmission line with the inner conductor recessed to the inner side of the end face by a set length and an SU-8 photoresist support structure. The circular coaxial transmission line is connected to the matching structure 7 at the discontinuity of the inner core.

[0013] Furthermore, the inner core protrudes 20 μm from the outer conductor at both ends.

[0014] Furthermore, the bottom of the circular coaxial interface has an SU-8 photoresist support structure, and the SU-8 photoresist support structure is evenly spaced between the first rectangular coaxial line and the outer conductor. Positioning holes are opened on both sides of the middle of the coaxial horizontal to vertical connection structure.

[0015] On the other hand, the present invention also provides an application of the above-mentioned micro coaxial chip interconnection structure based on connector-like connectors, wherein the horizontal rectangular coaxial interface and the vertical circular coaxial interface are both directly connected to standard coaxial connectors.

[0016] Furthermore, it also includes a coaxial horizontal connection structure, including an inner conductor, an outer conductor outside the inner conductor, the inner conductor being a second rectangular coaxial line, matching structures at both ends of the inner conductor, a cavity at the connection point with the coaxial line, and standard coaxial connectors at both ends of the coaxial horizontal connection structure.

[0017] Compared with existing technologies, this invention has at least the following advantages: This invention provides two connection interfaces for connecting microcoaxial chips to standard connectors, including a horizontal connection interface and a vertical connection interface. These interfaces can directly contact standard connectors, are compact, and do not require soldering, offering flexibility and ease of assembly, which is beneficial for the packaging and use of microcoaxial chips. Furthermore, simulation results show that both interfaces exhibit good return loss and low insertion loss when connected to connectors, demonstrating good transmission performance in the DC-110GHz frequency band. In addition, the design of the vertical interconnect structure and the coordination with the microcoaxial chip interface design enable inter-layer interconnection of the microcoaxial chip, offering advantages such as compact structure, easy assembly, ultra-wideband performance, and low insertion loss, thus contributing to the realization of compact and low-loss microcoaxial systems. Attached Figure Description

[0018] To more clearly illustrate the embodiments of the present invention or the prior art, the accompanying drawings used in the embodiments or the prior art will be briefly described below. It should be noted that the drawings described below are merely some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0019] Figure 1 An isometric view of a coaxial horizontal-to-vertical connection structure based on copper-based additive manufacturing process provided in an embodiment of the present invention;

[0020] Figure 2 A front view detail of a coaxial horizontal to vertical connection structure based on copper-based additive manufacturing process provided in an embodiment of the present invention;

[0021] Figure 3 A schematic diagram of a coaxial horizontal to vertical connection structure based on copper-based additive manufacturing process connected to a standard coaxial connector, provided in an embodiment of the present invention;

[0022] Figure 4 An isometric view of a coaxial horizontal connection structure based on copper-based additive manufacturing process provided in an embodiment of the present invention;

[0023] Figure 5 This is a detailed diagram of a coaxial horizontal connection structure based on copper-based additive manufacturing technology, provided in an embodiment of the present invention.

[0024] Figure 6 A schematic diagram of a coaxial horizontal connection structure based on copper-based additive manufacturing process connected to a standard coaxial connector, provided for an embodiment of the present invention;

[0025] Figure 7 A detailed diagram of the auxiliary connection structure of a coaxial horizontal connection structure based on copper-based additive manufacturing process provided in an embodiment of the present invention;

[0026] Figure 8 A simulation result diagram of a coaxial horizontal to vertical connection structure based on copper-based additive manufacturing process provided in an embodiment of the present invention;

[0027] Figure 9 This is a simulation result diagram of a coaxial horizontal connection structure based on copper-based additive manufacturing process provided in an embodiment of the present invention;

[0028] Figure 10 This is a schematic diagram of a coaxial interconnection structure provided in an embodiment of the present invention;

[0029] Figure 11 This is an isometric schematic diagram of a micro-coaxial chip interconnection structure based on a connector-like structure provided in Embodiment 1 of the present invention;

[0030] Figure 12 for Figure 11 A frontal view diagram. Detailed Implementation

[0031] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. It should be noted that the described embodiments are merely some embodiments of this invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0032] In the description of the embodiments of the present invention, it should be understood that the terms "vertical", "horizontal", "vertical direction", "horizontal direction", "top", "bottom", "left side", "right side", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present invention and simplifying the description, and should not be regarded as indicating that the indicated element or device is a specific orientation.

[0033] In the description of the embodiments of the present invention, the structural dimensions given are preferred parameters. Referring to the embodiments of the present invention, modifying the dimensional parameters of each component can further obtain the actual required performance.

[0034] This invention proposes a micro-coaxial chip adapter structure based on a standard connector, designed to achieve horizontal or vertical connection between the micro-coaxial chip and the connector structure in both directions. This includes the electrical and mechanical design of the inner guide shaft, outer coaxial shaft, and SU-8 support structure. Based on these two adapter structures, a connector-like interconnect structure is designed, including an inner conductor, an outer conductor, and a dielectric bead, ultimately achieving vertical interconnection of two layers of micro-coaxial chips.

[0035] Both the microcoaxial-connector horizontal and vertical transition structures utilize an equivalent connector, i.e., a circular coaxial transmission line structure, for excitation at the interface. Furthermore, the design of the inner and outer conductors of the microcoaxial chip is used to perform impedance matching at discontinuities, enabling effective transition between the microcoaxial line and the standard coaxial connector. In addition, a type of connector structure is used as an intermediate structure to achieve low-loss, ultra-wideband interconnection between the two layers of microcoaxial chips.

[0036] Example 1: This invention provides a micro-coaxial chip interconnect structure based on a connector-like structure, including a coaxial horizontal to vertical connection structure and a connector-like interconnect structure. The coaxial horizontal to vertical connection structure includes a horizontal rectangular coaxial interface 4 and a vertical circular coaxial interface 1 connected by a first rectangular coaxial line 3. The outer side of the first rectangular coaxial line 3 is an outer conductor. The connector-like interconnect structure includes a circular coaxial outer conductor 12, an inner core with structures 13 protruding from the outer conductor at both ends, an inner core matching structure 14, and a dielectric support structure 15. The inner core is provided in two segments, which are connected to each other through the inner core matching structure 14. The outer side of the dielectric support structure 15 is a circular coaxial outer conductor 12, and the inner core end faces connected to the outside are respectively provided with structures 13 protruding from the outer conductor at both ends.

[0037] Example 2: This invention provides a coaxial horizontal connection structure, including an inner conductor and an outer conductor. The inner conductor is a second rectangular coaxial line 8, with matching structures 7 at both ends. A cavity 10 is provided at the connection point between the inner conductor and the coaxial line. A cavity 10 is provided at the connection point between the second rectangular coaxial line 8 and the coaxial connector to reduce parasitic coupling. The outer conductor has a five-layer structure, with the first rectangular coaxial line 3 located in the middle layer. The outer conductor is removed along the rectangular cross-section at the top and bottom layers of the outer conductor near the matching structures 7 to form the cavity 10. Both ends of the coaxial horizontal connection structure can be connected to coaxial connectors.

[0038] Please see Figure 1 An isometric projection of a coaxial horizontal-to-vertical connection structure based on copper-based additive manufacturing technology provided in this embodiment of the invention. Figure 2 This is a front detail view of a coaxial horizontal to vertical connection structure based on copper-based additive manufacturing technology provided in an embodiment of the present invention. It includes a circular coaxial interface 1, a horizontal rectangular coaxial-to-vertical circular coaxial conversion structure 2, a first rectangular coaxial line 3, a rectangular coaxial interface 4, and positioning holes 5 for chip and fixture positioning. The inner conductor of the first rectangular coaxial line 3 is supported by a first SU-8 photoresist support structure 6. The circular coaxial interface 1 and the rectangular coaxial interface 4 have an impedance of 50 ohms and are used to connect to a standard coaxial connector. One end of the coaxial horizontal to vertical connection structure is semi-circular, and the other end is rectangular. The semi-circular end is provided with the horizontal rectangular coaxial-to-vertical circular coaxial conversion structure 2, and the rectangular end is provided with the rectangular coaxial interface 4. One end of the first rectangular coaxial line 3 connects to the horizontal rectangular coaxial-to-vertical circular coaxial conversion structure 2, and the other end extends to the rectangular coaxial interface 4. Positioning holes 5 are located on both sides of the middle of the coaxial horizontal to vertical connection structure.

[0039] Please see Figure 3 The present invention provides a schematic diagram of a coaxial horizontal to vertical connection structure based on copper-based additive manufacturing process connected to a standard coaxial connector. In this embodiment, a circular coaxial line 11 is used as a simplified standard coaxial connector, which is connected to a circular coaxial interface 1 and a rectangular coaxial interface 4 respectively. The circular coaxial line 11 connects the horizontal rectangular coaxial to vertical circular coaxial conversion structure 2 and the rectangular coaxial interface 4.

[0040] Please see Figure 4 This is an isometric view of a coaxial horizontal connection structure based on copper-based additive manufacturing technology provided in an embodiment of the present invention. Figure 5 This invention provides a detailed diagram of a coaxial horizontal connection structure based on copper-based additive manufacturing technology, including a matching structure 7 at the discontinuity of the inner core, a second rectangular coaxial line 8, a second SU-8 photoresist support structure 9, and a cavity 10. The two ends of the second rectangular coaxial line 8 are respectively connected to the matching structure 7 at the discontinuity of the inner core. The outer side of the second rectangular coaxial line 8 is an outer conductor, and cavities 10 are formed at both ends of the outer conductor. The matching structure 7 at the discontinuity of the inner core is used to connect to a standard coaxial connector. The size of the matching structure 7 for the coaxial horizontal connection structure in the DC-220GHz band is 0.05mm-0.2mm. The cavity 10 is a rectangular cavity hollowed out at the top and bottom layers of the outer conductor near the matching structure 7, used to reduce parasitic coupling between the horizontal connection structure B and the standard coaxial connector. The size of the cavity 10 for the coaxial horizontal connection structure in the DC-220GHz band is 0.05mm-0.5mm.

[0041] Figure 6 This is a schematic diagram illustrating the connection between a coaxial horizontal connection structure based on copper-based additive manufacturing technology and a standard coaxial connector, as provided in an embodiment of the present invention. Figure 7This is a detailed diagram of an auxiliary connection structure for a coaxial horizontal connection structure based on copper-based additive manufacturing process provided in an embodiment of the present invention. The circular coaxial line 11 is used as a simplified standard coaxial connector. The auxiliary connection structure 16 consists of a circular coaxial transmission line 17 with the inner conductor recessed 0.1mm inward from the end face and an SU-8 photoresist support structure 9. It is connected to the matching structure 7 at the discontinuity of the inner core, thereby protecting the matching structure 7 from damage by the standard coaxial connector.

[0042] Please see Figure 8 and Figure 9 The figures shown are simulation results of a coaxial horizontal-to-vertical interconnect structure and a coaxial horizontal interconnect structure based on copper-based additive manufacturing process provided in the embodiments of the present invention. The echoes in the DC-110GHz frequency band are both less than -20dB, which can realize the function of interconnecting chips and has little impact on the accuracy of testing.

[0043] Please see Figure 10 This is a schematic diagram of a coaxial interconnect structure provided in an embodiment of the present invention. It includes a circular coaxial outer conductor 12, an inner core with structures 13 protruding from the outer conductor at both ends, an inner core matching structure 14 at discontinuities in the inner core, and a dielectric support structure 15 for fixing the position of the inner core. The inner core has two segments connected by the inner core matching structure 14. The dielectric support structure 15 is coaxially arranged on the outside of the inner core matching structure 14, and the outer side of the dielectric support structure 15 is a circular coaxial outer conductor 12. The inner core end faces connected to the outside are respectively provided with structures 13 protruding from the outer conductor at both ends. The inner core matching structure 14 is used to reduce reflections caused by the dielectric support structure 15. The protruding inner conductor structure 13 allows the inner core of the coaxial interconnect structure to undergo a certain elastic deformation when interconnected with the vertical interface of a horizontal-to-vertical coaxial horizontal-to-vertical connection structure. This ensures sufficient contact force when the two are in contact, guaranteeing tight contact and enabling effective signal transmission. Since the presence of the dielectric support structure 15 will worsen the echo and insertion loss of the coaxial interconnect structure, the inner core is designed with coplanar compensation where the dielectric exists in order to achieve impedance matching of the structure, so that the radius of the inner core is discontinuous.

[0044] This invention discloses a transition structure based on microcoaxial chips and between chips and standard connectors, belonging to the field of microwave RF technology. It includes a microcoaxial horizontal-connector transition structure, a microcoaxial vertical-connector transition structure, and an interconnection structure between microcoaxial chips. The rectangular microcoaxial line includes an outer conductor, an inner conductor, and a support structure. The interconnection structure includes an outer conductor, an inner conductor, and a dielectric bead. The coaxial structure portion where the connector contacts the chip includes an inner conductor and an outer conductor, wherein the inner and outer conductors protrude by 20 μm to ensure increased reliability of contact between the inner conductors when interconnecting with the microcoaxial chip. The structure portion where the microcoaxial chip contacts the connector in the horizontal direction includes an inner conductor, an outer conductor, and an SU-8 support structure. The matching structure at discontinuities in the inner conductor, including changes in the width of the inner conductor and a protruding structure, can improve the interconnection effect and reduce reflection. The structure portion where the microcoaxial chip contacts the connector in the vertical direction includes an inner conductor, an outer conductor, and an SU-8 support structure. Increasing the thickness of the SU-8 support structure can improve the mechanical strength of the microcoaxial chip during interconnection. The micro-coaxial interconnect structure provided by this invention is applicable to operating frequency bands covering DC-67GHz, DC-110GHz, DC-145GHz, and DC-220GHz.

[0045] Based on the above description, the microcoaxial structure includes a horizontal-connector transition structure, a vertical-connector transition structure, and an interconnection structure between microcoaxial chips. The rectangular microcoaxial line includes an outer conductor, an inner conductor, and a support structure. The interconnection structure includes an outer conductor, an inner conductor, and a dielectric. The coaxial structure portion where the connector contacts the chip includes an inner conductor and an outer conductor, with the outer conductor protruding 20μm to ensure increased reliability of contact between the inner conductors when interconnecting with the microcoaxial chip. The structure portion where the microcoaxial chip contacts the connector in the horizontal direction includes an inner conductor, an outer conductor, and an SU-8 support structure. The matching structure at discontinuities in the inner conductor, including changes in the width of the inner conductor and a protruding structure, can improve the interconnection effect and reduce reflection. The structure portion where the microcoaxial chip contacts the connector in the vertical direction includes an inner conductor, an outer conductor, and an SU-8 support structure. Increasing the thickness of the SU-8 support structure can improve the mechanical strength of the microcoaxial chip during interconnection. The micro-coaxial interconnect structure provided by this invention is applicable to operating frequency bands covering DC-67GHz, DC-110GHz, DC-145GHz, and DC-220GHz.

[0046] The micro-coaxial chip interconnect structure based on connectors described in this application can be used to realize ultra-wideband three-dimensional interconnection of micro-coaxial devices in the DC-67GHz, DC-110GHz, DC-145GHz, and DC-220GHz bands. It overcomes the problem that existing structures can only be applied to specific test environments or limited frequency ranges, and have limitations in practical multi-layer chip interconnection. It can be used for connection of interfaces in multiple orientations.

[0047] The above describes a micro-coaxial chip interconnection structure based on connectors and connector-like structures provided by the present invention. For those skilled in the art, based on the ideas of the embodiments of the present invention, there will be changes in specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A micro-coaxial chip interconnect structure based on a connector-like interface, characterized in that, The coaxial horizontal-to-vertical connection structure and the connector-like interconnection structure are included. The coaxial horizontal-to-vertical connection structure includes a horizontal rectangular coaxial interface (4) and a vertical circular coaxial interface (1) connected by a first rectangular coaxial line (3). The outer side of the first rectangular coaxial line (3) is an outer conductor. The connector-like interconnection structure includes a circular coaxial outer conductor (12), a structure (13) in which the inner core protrudes from the outer conductor at both ends, an inner core matching structure (14), and a dielectric support structure (15). The inner core is provided in two sections, and the two sections of the inner core are connected by the inner core matching structure (14). The outer side of the connection and dielectric support structure (15) is a circular coaxial outer conductor (12), and the inner core end face connected to the outside is respectively provided with a structure (13) where the inner core protrudes from the outer conductor at both ends; a horizontal rectangular coaxial-vertical circular coaxial conversion structure (2) is provided at the circular coaxial interface (1). The horizontal rectangular coaxial-vertical circular coaxial conversion structure (2) is columnar in shape. A circular coaxial interface (1) is provided at the top of the horizontal rectangular coaxial-vertical circular coaxial conversion structure (2), and one end of the first rectangular coaxial line (3) is connected to the side, while the other end is a rectangular coaxial interface (4).

2. The micro-coaxial chip interconnect structure based on a connector-like interface according to claim 1, characterized in that, A matching structure (7) is connected to the end face of the coaxial line at the rectangular coaxial interface (4), and the matching structure (7) is a rectangular cross section.

3. The micro-coaxial chip interconnect structure based on a connector-like interface according to claim 1, characterized in that, A cavity (10) for reducing parasitic coupling is provided at the rectangular coaxial interface (4); the outer conductor has a five-layer structure, the first rectangular coaxial line (3) is located in the middle layer, and the outer conductor is removed along the rectangular cross section at the top and bottom layers of the outer conductor near the matching structure (7) to form a cavity (10).

4. The micro-coaxial chip interconnect structure based on a connector-like interface according to claim 1, characterized in that, The inner core protrudes from the outer conductor at both ends (13) 20 μm from the outer conductor.

5. The micro-coaxial chip interconnect structure based on a connector-like interface according to claim 1, characterized in that, The bottom of the circular coaxial interface (1) has an SU-8 photoresist support structure. The first rectangular coaxial line (3) and the outer conductor are evenly spaced together by the first SU-8 photoresist support structure (6). Positioning holes (5) are opened on both sides of the middle of the coaxial horizontal to vertical connection structure.

6. A micro-coaxial chip interconnect structure based on a connector-like interface, characterized in that, The coaxial horizontal connection structure includes an inner conductor, an outer conductor outside the inner conductor, a second rectangular coaxial line (8) of the inner conductor, matching structures (7) at both ends of the inner conductor, and a cavity (10) at the connection point of the outer conductor with the coaxial line; auxiliary connection structures (16) are provided at both ends of the coaxial horizontal connection structure. The auxiliary connection structure (16) is composed of a circular coaxial transmission line (17) recessed to the inner side of the end face by a set length and an SU-8 photoresist support structure. The circular coaxial transmission line (17) is connected to the matching structure (7) at the discontinuity of the inner core.

7. The method of applying the micro-coaxial chip interconnect structure based on connector-like structures as described in any one of claims 1-6 as an interconnect structure, characterized in that, The horizontal rectangular coaxial interface (4) and the vertical circular coaxial interface (1) are directly connected to the standard coaxial connector or the two ends of the coaxial horizontal connection structure are connected to the standard coaxial connector.

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