Ball bond impedance matching

Abstract

Methods and apparatus are disclosed for providing interconnects including a stack of wire bond balls having a selected impedance. The wire bond balls can have dimensions configured for the selected impedance, and the dimensions can include a radius. The stack can include a number of wire bond balls configured for the selected impedance and / or can include a material selected for the selected impedance. In an embodiment, the selected impedance is primarily resistive (e.g., 50 ohms), thereby minimizing overall reactance.

Description

[Background technology]

[0001] As known in the art, circuit boards, components, connectors, and other structures may need to be connected together. Connecting circuit boards may require connections from one circuit board to another. Typically, prior to manufacturing, circuit traces between a monolithic microwave integrated circuit (MMIC) and a circuit card assembly (CCA) are simulated to ensure that ball bonds fit the system. As frequencies increase, the effects of impedance mismatch also increase. If there are many transitions in the CCA, matching each trace to the wire-bonded MMIC / CCA may be time-consuming. Furthermore, modeling all interfaces may not be practical. Also, if the simulation is not accurate or the physical connections do not match the simulation, the CCA may need to be respun, which may be time-consuming. Summary of the Invention

[0002] In an embodiment of the present disclosure, a method and apparatus are provided for additively forming interconnects on a surface of a printed wiring board, circuit card, MMIC, etc., using wire bonding equipment to form a series of laminated balls, which may be referred to as stud bumps, where the stud bumps may have a selected impedance. For example, the number and / or dimensions of the laminated balls may be selected so that the impedance is substantially resistive to tailor the capacitance of the connection, which may have an inductive component.

[0003] Exemplary wire bonding tools allow for precise impedance matching and tuning with a high degree of control over certain parameters. For example, the diameter of the balls on the traces can be increased to increase capacitance. The number of stacked balls can also be selected to achieve a desired impedance. In some embodiments, ball stacks can be used to provide impedance matching for coaxial interfaces. In other embodiments, ball stacks can be used to provide impedance matching for connections to MMICs. Additionally, if there are manufacturing issues that result in out-of-spec connection impedances, ball bond parameters can be adjusted to compensate for manufacturing errors to achieve compliance.

[0004] In one aspect, an assembly includes an interconnect including a stack of wire bond balls having a selected impedance. The assembly may further include one or more of the following features: the interconnect includes wire bonds, the wire bond balls have dimensions configured for the selected impedance, the dimensions include a radius, the stack includes a number of wire bond balls configured for the selected impedance, the wire bond balls include a material selected for the selected impedance, the wire bond balls include a material selected for mechanical properties, the selected impedance is primarily resistive, such that the selected impedance minimizes reactance, the interconnect further includes wire bonds and a first end configured to connect to an integrated circuit, the interconnect further includes a second end configured to connect to a circuit card, and / or the connection includes an RF coaxial connection and includes wire bonds.

[0005] In another aspect, a method includes using an interconnect including a stack of wire bond balls having a selected impedance, the method may further include one or more of the following features: the interconnect includes wire bonds, the wire bond balls have dimensions configured for the selected impedance, the dimensions include a radius, the stack includes a number of wire bond balls configured for the selected impedance, the wire bond balls include a material selected for the selected impedance, the wire bond balls include a material selected for mechanical properties, the selected impedance is primarily resistive, such that the selected impedance minimizes reactance, the interconnect further includes wire bonds and a first end configured to connect to an integrated circuit, the interconnect further includes a second end configured to connect to a circuit card, and / or the connection includes an RF coaxial connection and includes wire bonds. [Brief description of the drawings]

[0006] The foregoing features of the present invention, as well as the invention itself, may be more fully understood from the following description of the drawings, in which:

[0007] [Figure 1A] 1 is a cross-sectional view of an exemplary RF interconnect having wire bond ball stack impedance matching according to an exemplary embodiment of the present disclosure. [Figure 1B] FIG. 2 is a top view of an exemplary RF interconnect having wire bond ball stack impedance matching according to an exemplary embodiment of the present disclosure. [Figure 2A] 2A and 2B are schematic diagrams of a wire bonding tool for forming a ball stack such as the ball stack of FIGS. 1A and 1B. [Figure 2B] 2A and 2B are schematic diagrams of a wire bonding tool for forming a ball stack such as the ball stack of FIGS. 1A and 1B. [Figure 2C]1 is a photographic representation of an exemplary ball stack formed by a wire bond tool. [Figure 3A] 3A-3F show wire bond connections between MMICs and circuit board traces having various ball bond configurations according to exemplary embodiments of the present disclosure. [Figure 3B] 3A-3F show wire bond connections between MMICs and circuit board traces having various ball bond configurations according to exemplary embodiments of the present disclosure. [Figure 3C] 3A-3F show wire bond connections between MMICs and circuit board traces having various ball bond configurations according to exemplary embodiments of the present disclosure. [Figure 3D] 3A-3F show wire bond connections between MMICs and circuit board traces having various ball bond configurations according to exemplary embodiments of the present disclosure. [Figure 3E] 3A-3F show wire bond connections between MMICs and circuit board traces having various ball bond configurations according to exemplary embodiments of the present disclosure. [Figure 3F] 3A-3F show wire bond connections between MMICs and circuit board traces having various ball bond configurations according to exemplary embodiments of the present disclosure: Graphical representations of impedance of wire bond balls having various radii; [Figure 4] 1 is a graphical representation of the impedance of wire bond balls having various radii. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] 1A and 1B show an exemplary interconnect using a ball stack formed, for example, by a wire bonding tool, according to an exemplary embodiment of the present disclosure, where the ball stack has impedance matching. Exemplary interconnects may be formed for DC printed circuit boards (PCBs), RF boards, digital PCBs, MMICs, coaxial connections, and the like. The term "circuit card" as used herein should be interpreted broadly to include substrates such as ICs, radiators, PWBs, alumina substrates, and substrates with or without microelectronics / microcircuits, and the like.

[0009] It will be understood that any practical number of ball stacks, through holes, wire bonds, etc. may be used to meet the needs of a particular application. Any desired number of offset ball stacks with balls of selected geometric shapes may be used to provide impedance matching for a given connection.

[0010] FIG. 1A illustrates a cross-sectional view of a low-profile RF interconnect system 100 with impedance matching according to an exemplary embodiment of the present disclosure, and FIG. 1B illustrates a top view of the low-profile RF interconnect system 100 with impedance matching according to an exemplary embodiment of the present 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 stack 110 is formed in a first hole 112 in the first dielectric layer 106 and forms a connection to the conductive layer 104. A first wire bond 114 is connected to a top of the first ball stack 110 and to a trace 116 on a top surface 118 of the PWB. In the embodiment, the trace 116 provides a signal path for an RF signal of the coaxial interconnect.

[0011] 1A, a second ball stack 120 may be formed in a second hole 122 in the first dielectric layer 106. A plated through hole 124, e.g., a via, in the second dielectric layer 108 may provide a connection to a ground plane 125 below the second dielectric layer 108. A second wire bond 126 may be connected from the top of the second ball stack 120 to a ground pad 128.

[0012] As best shown in Figure 1B, ground pad 128 may form an arc with first ball stack 110 disposed approximately in the center of the arc, and the arc partially surrounding first ball stack 110, which carries the signal. In the illustrated embodiment, an additional ball stack 130, similar to second ball stack 120, and an additional wire bond 132, similar to second wire bond 126, are connected to arc-shaped ground pad 128. It will be understood that the arch shape of Figure 1B is conceptual, and that non-arched pads can be used to create the same ground effect.

[0013] In embodiments, ball stacks 110, 120, 130 may be formed as described to efficiently and accurately form connections for interconnects such as the illustrated low profile RF interconnects. As used herein, the term "ball stack" refers to a stack of at least two balls stacked on top of each other and interconnected, the balls being produced by melting wires to form the balls with a wire bonding tool, described in more detail below.

[0014] In embodiments, the geometry of the balls in ball stacks 110, 120, 130 and the number of balls in the ball stacks are selected to obtain a desired impedance, as described in more detail below. The height of the ball stack may correspond to the number of balls in the ball stack.

[0015] It will be appreciated that any suitable material may be used for the wires that the wire bond tool uses to form the balls of the ball stack. Gold is an exemplary material that may be used to form the ball stack. In some embodiments, the wire may be doped with palladium, or the like. A particular material may be selected for the ball stack to obtain particular impedance and / or mechanical properties. Exemplary mechanical properties include tensile strength / elasticity, e.g., larger stacks may require a material with higher tensile strength.

[0016] 2A and 2B show a portion of an exemplary wirebonding tool capable of forming a ball stack or stud bump, and FIG. 2C shows an exemplary ball stack formed by a wirebonding system with a particular impedance characteristic at a given interconnect. A wire 200 is fed through a needle-like tool 202, which may be referred to as a capillary. A high voltage charge is applied to the wire 200, melting it 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 rapidly solidifies, and the capillary 202 is lowered to the chip surface, which is typically heated to at least 125° C. The machine then presses down on the capillary 202 and applies ultrasonic energy with an attached transducer. The combination of heat, pressure, and ultrasonic energy forms a weld between the metal ball and the chip surface. A series of balls, which may be referred to as a ball stack 210 or stud bump, may be stacked on top of each other, as shown in FIG. 2C. The stud bumps may be formed in any practical dimensions to meet the requirements of a particular application.

[0017] 3A shows a schematic diagram of a component 300, which in the illustrated embodiment is shown as an MMIC, with wirebond connections to traces on a circuit board 302. In the illustrated embodiment, the circuit board traces 304 comprise a conductive material in the form of a microstrip.

[0018] 3B and 3C show a wirebond connection 306 from a pad 308 on the MMIC 300 to one of the microstrip traces 304 on the circuit board 302 without the use of a wirebond ball.

[0019] Figure 3D shows a wirebond connection 306 from a pad 308 on the MMIC 300 to a microstrip trace 304, where the connection at the pad 308 of the MMIC 300 includes a wirebond ball stack 350. Figure 3E shows a wirebond connection from a pad 308 on the MMIC 300 to a microstrip trace 304, where the connection to the trace includes a wirebond ball stack 352. Figure 3F shows a wirebond connection from a pad on the MMIC 300 to a microstrip trace 304, where the connection at the pad of the MMIC 300 and the connection to the microstrip trace include respective wirebond ball stacks 350, 352.

[0020] In an embodiment, the number of balls and / or ball geometry of a stack, such as 350, 352, may be varied to obtain a selected impedance. It will be appreciated that the expected operating frequency may be taken into consideration to effect impedance matching of the interconnect.

[0021] FIG. 4 shows a graphical representation 400 of the impedance characteristics of wire bond balls with various radii. In the chart, the complex reflections of the wire bond interface are shown by an exemplary Smith chart. Since the wire bond connection is a long thin wire, the impedance of the wire bond connection has mainly inductive reactance. It is desirable to change the impedance towards a mainly resistive impedance by increasing the capacitance of the wire bond. This can be achieved by adding an area to which the wire bond is attached relative to the trace. Another way to increase the capacitance is to increase the ball bond diameter of the wire bond, since increasing the metal area cancels out the inductance. In one particular embodiment, it is desirable to achieve an impedance match to 50 ohms, which is purely resistive, i.e., 50Ω+j0. It will be appreciated that matching can be performed to any practical impedance.

[0022] A first curve 402 shows the impedance of a wire bond ball having a radius of about 5 mils. As can be seen, the first curve 402 has the smallest ball radius and the largest inductance. A second curve 404 shows the impedance of a wire bond ball having a radius of about 10 mils, and a third curve 406 shows the impedance of a wire bond ball having a radius of about 15 mils. A fourth curve 408 shows the impedance of a wire bond ball having a radius of about 20 mils. As can be seen, the fourth curve has the largest ball radius and the smallest inductive impedance, e.g., the most resistive impedance. Typically, wire bond balls can be scaled to fit the capabilities of the machine.

[0023] In embodiments, the diameter of the wire bond balls can range from about 1 mil to about 30 mils. In an exemplary embodiment, the stacked balls have an average diameter of approximately 20 mils. In an exemplary embodiment, the height of the ball stack can range from about 2 mils to about 90 mils.

[0024] In an exemplary embodiment, the ball stack is metallic and is bonded directly to the metallization of the substrate. The ball stack is made using conductive materials and therefore does not adversely affect the electrical or thermal performance of the structure to which it is attached. The ball stack may be thermosonic welded to form a strong metallurgical bond that is unlikely to come off without significant force. In general, the ball stack is relatively inexpensive to manufacture and can be formed relatively quickly compared to traditional alternatives. Furthermore, the ball stack does not require separate components such as washers, standoffs, or physical connector components that may be required with traditional alternatives. Furthermore, the geometry of the ball stack can be controlled by varying the wire dimensions, process parameters, and ball bonding tools. Once developed, the ball characteristics are highly repeatable.

[0025] Relative terms such as "vertical," "top," "bottom," "lower," "upper," "left," and "right" may be used to facilitate understanding of exemplary embodiments, but such terms are in no way intended to limit the scope of the invention as claimed. These terms and any similar relative terms should not be construed as limiting in any way, but rather as terms of convenience in describing embodiments of the invention.

[0026] Although exemplary embodiments of the invention have been described, it will now become apparent to those skilled in the art that other embodiments incorporating these concepts may also be used. The embodiments contained herein should not be limited to the disclosed embodiments, but rather 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.

[0027] Elements of different embodiments described herein may be combined to form other embodiments not specifically described above. Various elements that are described in the context of a single embodiment may be provided separately or in any suitable subcombination. Other embodiments not specifically described herein are also included within the scope of the following claims.

Claims

1. 1. A method for forming an interconnect having a selected impedance, comprising: selecting a number of balls in a ball stack to achieve the selected impedance, the number of balls being at least two; determining a radius of the ball to achieve a desired capacitive and inductive characteristic of the selected impedance; using a wire bonding tool to melt wire to form said number of balls on top of each other having said determined radius, and then forming a single wire bond interconnect from the top of said ball stack; The method includes:

2. The method of claim 1 , wherein the selected impedance is predominantly resistive, such that the selected impedance minimizes reactance.

3. The method of claim 1 or 2, wherein the ball stack and the single wire bond connect an integrated circuit to a circuit card.

4. The method of claim 1, wherein the selected impedance is 50 ohms.

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