Ball junction impedance matching

Abstract

Methods and apparatus for providing an interconnect comprising a pile of wire joint balls having a selected impedance. The wire joint balls may have a size that includes a radius configured for the selected impedance. The pile may include a plurality of wire joint balls configured for the selected impedance and / or may include a material selected for the selected impedance. In embodiments, the selected impedance is primarily resistive (e.g., 50 ohms) so as to minimize the total reactance.

Description

Background Technology

[0001] As is known in the art, circuit boards, components, connectors, and other structures may need to be mated together. Mating circuit boards may require connections from one circuit board to another. Circuit traces between monolithic microwave integrated circuits (MMICs) and circuit card assemblies (CCAs) are typically simulated before manufacturing to ensure that the ball junctions are well-matched to the system. As frequency increases, impedance mismatch effects also increase. If the CCA has many transitions, matching each trace to a wire-joined MMIC / CCA can be time-consuming. Furthermore, modeling all interfaces may not be feasible. Additionally, if the simulation is inaccurate or the physical connections do not match the simulation, the CCA must be re-spinned, which can also be time-consuming.

[0002] Embodiments of the disclosure provide a method and apparatus for using wire joining equipment to form a series of piled balls, which may be referred to as stud bumps, to additionally form interconnections on the surface of a printed wiring board, a circuit card, an MMIC, etc., and the stud bumps may have a selected impedance. For example, the number of piled balls and / or the size of the piled balls may be selected to tune the capacitance of the connection, which may have an inductive component, such that the impedance is substantially resistive.

[0003] Examples of wire splicing equipment allow for a high level of control over specific parameters to enable precise impedance matching and impedance adjustment. For example, capacitance can be increased by increasing the diameter of the balls on the trace. Multiple stacked balls may be selected to achieve a desired impedance. In some embodiments, ball stacks may be used to provide impedance matching for a coaxial interface. In other embodiments, ball stacks may be used to provide impedance matching for a connection to an MMIC. Furthermore, if there are manufacturing issues resulting from non-specification connection impedance, the ball splicing parameters may be adjusted to compensate for manufacturing errors to achieve compliance.

[0004] In one embodiment, the assembly comprises an interconnection comprising a pile of wire joint balls having a selected impedance. The assembly may further comprise one or more of the following features, wherein the interconnection comprises a wire joint, the wire joint balls have a size configured for the selected impedance, the size includes a radius, the pile comprises a plurality of wire joint balls configured for the selected impedance, the wire joint balls comprise a material selected for the selected impedance, the wire joint balls comprise a material selected for mechanical properties, the selected impedance is primarily resistive such that the selected impedance minimizes reactance, the interconnection further comprises a first end configured to connect to the wire joint and an integrated circuit, and the interconnection further comprises / or comprises a second end configured to connect to a circuit card, or the interconnection comprises an RF coaxial connection and a wire joint.

[0005] In another embodiment, the method comprises the step of using an interconnection comprising a pile of wire joint balls having a selected impedance. The method may further comprise one or more of the following features, wherein the interconnection comprises a wire joint, the wire joint balls have a size configured for the selected impedance, the size includes a radius, the pile comprises a plurality of wire joint balls configured for the selected impedance, the wire joint balls comprise a material selected for the selected impedance, the wire joint balls comprise a material selected for mechanical properties, the selected impedance is mainly resistive such that the selected impedance minimizes reactance, the interconnection further comprises a first end configured to connect to the wire joint and an integrated circuit, the interconnection further comprises and / or comprises a second end configured to connect to a circuit card, or the interconnection comprises an RF coaxial connection and a wire joint. Brief explanation of the drawing

[0006] The aforementioned features of this invention, as well as the invention itself, can be more fully understood from the following description of the drawings: FIG. 1a is a cross-sectional view and FIG. 1b is a plan view of an exemplary RF interconnect having wire-joint ball dummy impedance matching according to exemplary embodiments of the disclosure; FIGS. 2a and 2b are schematic representations of wire splicing equipment for forming ball piles such as the ball piles of FIGS. 1a and 1b; FIG. 2c is an illustrated representation of an exemplary ball pile formed by a wire splicing device; FIGS. 3a through 3f illustrate wire joint connections between a circuit board trace and an MMIC having various ball joint configurations according to exemplary embodiments of the disclosure; Figure 4 is a graphical representation of the impedance for wire joint balls of various radii. Specific details for implementing the invention

[0007] FIGS. 1a and 1b illustrate exemplary interconnects using ball piles formed, for example, by wire splicing equipment, and the ball piles have impedance matching according to exemplary embodiments of the disclosure. Exemplary interconnects may be formed for DC printed circuit boards (PCBs), RF boards, digital PCBs, MMICs, coaxial connections, etc. As used herein, the term “circuit card” should be interpreted broadly to include boards with or without ICs, radiators, PWBs, substrates such as alumina substrates, microelectronics / microcircuits, etc.

[0008] It should be understood that any actual number of ball piles, through holes, wire joints, etc., may be used to meet the needs of a specific application. A preferred number of offset ball piles with balls of a selected geometric structure may be used to provide impedance matching for a given connection.

[0009] FIG. 1a illustrates a cross-sectional view, and FIG. 1b illustrates a plan view of a low-profile RF interconnection system (100) having impedance matching according to exemplary embodiments of the disclosure. In the illustrated embodiment, a coaxial connection is formed. A printed wiring board (PWB) (102) includes a conductive layer (104) between a first dielectric layer (106) and a second dielectric layer (108). A first ball pile (110) is formed in a first hole (112) in the first dielectric layer (106) that forms a connection to the conductive layer (104). A first wire joint (114) is connected to the top of the first ball pile (110) by a trace (116) on the top surface (118) of the PWB. In the embodiments, the trace (116) provides a signal path for an RF signal for coaxial interconnection.

[0010] As illustrated in FIG. 1a, a second ball pile (120) may be formed in a second hole (122) in the first dielectric layer (106). A plated through hole (124) in the second dielectric layer (108), e.g., a via, may provide a connection to a ground plane (125) below the second dielectric layer (108). A second wire joint (126) may be connected from the top of the second ball pile (120) to a ground pad (128).

[0011] As best illustrated in FIG. 1b, the ground pad (128) can form an arc with a first ball pile (110) located in the middle of the arc, which partially surrounds the first ball pile (110) of the signal-transmission. In the illustrated embodiment, additional ball piles (110) similar to the second ball pile (120) and additional wire joints (132) similar to the second wire joint (126) are connected to the arc-shaped ground pad (128). It will be understood that the arc in FIG. 1b is conceptual, and the same ground effect can be created by using pads instead of the arc.

[0012] In the embodiments, the ball piles (110, 120, 130) may be formed as described to efficiently and accurately form connections for interconnections such as the illustrated low-profile RF interconnect. As used herein, the term “ball pile” refers to a pile of at least two balls that are piled on top of each other and bonded together, from balls generated from molten wire to form balls by wire bonding equipment as described in detail below.

[0013] In the embodiments, the geometry of the balls in the ball piles (110, 120, 130) and the number of balls in the ball piles are selected to achieve a desired impedance, as described in more detail below. The height of the ball pile may correspond to the number of balls in the ball pile.

[0014] It is understood that any suitable material may be used for the wire used by the wire splicing equipment to form balls in the ball piles. Gold is an exemplary material that may be used to form the ball piles. In some embodiments, the wire may be doped, such as with palladium. Specific materials for the ball piles may be selected to achieve specific impedance and / or mechanical properties. Exemplary mechanical properties include tensile strength / elasticity, for example, where larger piles may require materials with higher tensile strength.

[0015] FIGS. 2A and 2B illustrate a part of an exemplary wire splicing apparatus capable of forming ball piles or stud bumps, and FIG. 2C illustrates an exemplary ball pile formed by a wire splicing system having specific impedance characteristics for a given interconnection. A wire (200) is fed through a needle-shaped tool (202) which may be referred to as a capillary. A high-voltage charge is applied to the wire (200) to melt the wire at the tip of the capillary (202). The tip of the wire (200) forms a ball (204) due to the surface tension of the molten metal. The ball (204) solidifies rapidly, and the capillary (202) typically [is at least 125] It is lowered onto the surface of the chip that is heated. Then, the machine presses down on the capillary (202) and applies ultrasonic energy with an attached transducer. The combined heat, pressure, and ultrasonic energy create a weld between the metal ball and the surface of the chip. A series of balls, which may be referred to as ball piles (210) or stud bumps, can be stacked on top of each other as shown in FIG. 2c. The stud bumps can be formed to have any practical dimensions to meet the requirements of a specific application.

[0016] FIG. 3a illustrates a schematic representation of a component (300), which is illustrated as an MMIC in an illustrated embodiment having wire joint connections to traces on a circuit board (302). In the illustrated embodiment, the circuit board traces (304) include a conductive material in the form of a microstrip.

[0017] FIGS. 3B and 3C illustrate wire connection portions (306) from a pad (308) on an MMIC (300) to one of the traces (304) on a circuit board (302) without wire connection balls.

[0018] FIG. 3d illustrates a wire bonding connection (306) from a pad (308) on an MMIC (300) to a microstrip trace (304), and the connection at the pad (308) of the MMIC (300) includes a wire bonding ball pile (350). FIG. 3e illustrates a wire bonding connection from a pad (308) on an MMIC (300) to a microstrip trace (304), and the connection to the trace includes a wire bonding ball pile (352). FIG. 3f illustrates a wire bonding connection from a pad on an MMIC (300) to a microstrip trace (310), and the connection at the pad and the connection to the microstrip trace of the MMIC (300) each include a wire bonding ball pile (350, 352).

[0019] In the embodiments, the number of balls in the piles, e.g., 350, 352, and / or the geometry of the balls may be varied to achieve a selected impedance. It is understood that the expected frequency of operation may be considered to provide impedance matching for the interconnects.

[0020] FIG. 4 illustrates a graphical representation (400) of the impedance characteristics of wire joint balls having various radii. In the chart, complex reflections illustrate the results of the wire joint interface by an exemplary Smith chart. Since the wire joint connection impedance is a long, thin wire, the impedance has primarily inductive reactance. It is desirable to change the impedance so that it moves toward primarily resistive impedance by increasing the capacitance of the wire joint. This can be achieved by adding an area with a trace to which the wire joint is attached. Another method for increasing capacitance is by increasing the diameter of the wire joint ball joint, since an increase in the metallic area will cancel out the inductance. In one particular embodiment, it is desirable to achieve impedance matching for 50 ohms, i.e., 50 + j0, which is purely resistive. It is understood that matching can be performed for any actual impedance.

[0021] The first curve (402) illustrates the impedance for a wire joint ball having a radius of about 5 mil. As can be seen, the first curve (402) has the smallest ball radius and the largest inductance. The second curve (404) illustrates the impedance for a wire joint ball having a radius of about 10 mil, and the third curve (406) illustrates the impedance for a wire joint ball having a radius of about 15 mil. The fourth curve (408) illustrates the impedance for a wire joint ball having a radius of about 20 mil. As can be seen, the fourth curve has the largest ball radius and the smallest inductive, e.g., most resistive, impedance. Generally, wire joint balls can be increased by the machine's capability.

[0022] In the embodiments, the wire-joined balls may have a diameter that can range from about 1 mil to about 30 mils. In exemplary embodiments, the piled balls have an average diameter of about 20 mils. In the embodiments, the height of the exemplary ball piles may range from about 2 mils to about 90 mils.

[0023] In exemplary embodiments, ball piles are metallic and directly attached to the substrate metallization. Since the ball piles are created using conductive materials, they do not negatively affect the electrical or thermal performance of the attached structures. The ball piles can be thermosonically welded to form strong metallurgical bonds that are difficult to dislodge without significant applied force. Generally, ball piles can be formed at a relatively low cost and a relatively high rate to manufacture compared to conventional alternatives. Furthermore, ball piles do not require separate components such as washers, stand-offs, and physical connector components that may be required for conventional alternatives. Moreover, the geometry of the ball piles can be controlled by changing the wire size, process parameters, and ball joining tools. Once developed, the ball characteristics are extremely repeatable.

[0024] Relative terms such as "vertical," "up," "down," "lower," "upper," "left," and "right" may be used to facilitate understanding of exemplary embodiments, but these terms do not limit the scope of the claimed invention in any way. These terms, and any similar related terms, should not be interpreted as limiting in any way, but rather as terms of convenience for describing embodiments of the invention.

[0025] Now that exemplary embodiments of the invention have been described, it will be apparent to those skilled in the art that other embodiments incorporating their concepts may also be used. The embodiments held herein should not be limited to the disclosed embodiments, but should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

[0026] The elements of the different embodiments described herein may be combined to form other embodiments not specifically described above. The various elements described in the context of a single embodiment may also be provided individually or in any suitable sub-combination. Other embodiments not specifically described herein are also within the scope of the following claims.

Claims

Claim 1 A method for forming an interconnect having a selected impedance, comprising: providing a substrate having a first conductive layer, a first dielectric layer having a first hole in the first dielectric layer exposing a portion of the first conductive layer, and a conductive trace on the top surface of the first dielectric layer; selecting a number of balls from a ball pile to achieve the selected impedance, wherein the number of balls is at least two; determining the radius of the balls to achieve the desired capacitance and inductance characteristics of the selected impedance, wherein all the balls have the same determined radius; and forming the number of balls joined to each other on the top surface of the first conductive layer in the first hole, wherein the top of the ball pile is at the same level as the top surface of the first dielectric layer, and then using a wire joining device to melt a wire to form a single wire joining interconnect from the top of the ball pile to the conductive trace. Claim 2 A method according to claim 1, wherein the selected impedance is resistive such that the selected impedance minimizes reactance. Claim 3 In claim 1, the ball pile and the single wire joint interconnection are a method for connecting an integrated circuit and a circuit card. Claim 4 A method according to claim 1, wherein the selected impedance is 50 ohms. Claim 5 In claim 1, the radius of the wire joint balls is in the range of 0.5 mil to 15 mil. Claim 6 In claim 1, the method wherein the ball pile has a height ranging from 2 mils to 90 mils. Claim 7 delete Claim 8 In claim 1, the wire is a doped method. Claim 9 A method according to claim 1, wherein the radius of the balls is determined to increase the capacitance of the ball pile and offset the inductance of the single wire joint interconnect. Claim 10 A method for forming an interconnect having a selected impedance, comprising the steps of: providing a substrate having a first conductive layer, a first dielectric layer having a first hole in the first dielectric layer that exposes a portion of the first conductive layer, and a conductive trace on the top surface of the first dielectric layer; using a wire joining device to melt wires to form a plurality of balls joined to each other on the top surface of the first conductive layer having the same determined radius at the first hole to form a ball pile, wherein the top of the ball pile is at the same level as the top surface of the first dielectric layer; and then using the wire joining device to form a single wire joining interconnect from the top of the ball pile to the conductive trace. Claim 11 In claim 10, the number of balls achieves a selected impedance for the ball pile, and the radius of the balls achieves desired capacitance characteristics and inductance characteristics for the entire ball pile. Claim 12 In claim 10, the selected impedance is resistive, and the radius of the plurality of balls is determined to minimize the reactance. Claim 13 A method for forming an interconnection system having a selected impedance, comprising: providing a substrate having a first conductive layer, a first dielectric layer, a conductive trace on the top surface of the first dielectric layer, a ground pad forming an arc on the top surface of the first dielectric layer, a first hole in the first dielectric layer located at the center of the arc exposing a portion of the first conductive layer, and a plurality of second holes located between the first hole and the arc, spaced apart from each other in the first dielectric layer and exposing portions of the first conductive layer; selecting a number of balls and a radius of the balls to form a ball pile, wherein the height of the ball pile is equal to the depth of the first hole and the second holes, the number of balls determines the selected impedance, and the radius of the balls achieves the desired capacitance and inductance characteristics of the selected impedance for the ball pile; and a first hole in which the top surface of the first ball pile is at the same level as the top surface of the first dielectric layer. A method comprising the step of using wire joining equipment to melt wires to form a plurality of second wire joining interconnects from the top of each of the plurality of second ball piles to the ground pad at positions opposite to each of the second ball piles: 1 ball pile; a first wire joining interconnect from the top of the first ball pile to the conductive trace; a plurality of second ball piles in the plurality of second holes where the top surfaces of the second ball piles are at the same level as the top surface of the first dielectric layer; and a step of using wire joining equipment to form a plurality of second wire joining interconnects from the top of each of the plurality of second ball piles to the ground pad at positions opposite to each of the second ball piles. Claim 14 delete Claim 15 delete Claim 16 delete Claim 17 delete Claim 18 delete Claim 19 delete

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