Patent Application
20090079042
An interconnect is a conduit for passing an electrical signal from one device to another, for example an integrated circuit to an external device. The most widely used interconnect is a wire bond. Interconnects include, but are not limited to, single or multiple round wires, ribbon wires and wire mesh bonds.
A common disadvantage of a wire bond is its parasitic inductance. The inductance is a complex and unpredictable combination of the length of the wire bond and the bonding material used to secure the wire bond to an electrical device.
In high frequency electronic applications, for example those operating in excess of 20 GHz, it is desirable to minimize the parasitic inductance of the interconnects to ensure signal integrity.
Accordingly, there is a need for an improved way to interconnect an integrated circuit to a point outside without degrading the integrity of the signal.
An embodiment of the invention is an interconnect between two high frequency nodes (‘node’). The embodiment comprises a center conductor, conductive bumps, and a bonding process. As an example, the invention can replace a wire bond for connecting an electrical pad on a circuit board to an external connector. The node can be any conducting surface to which a center conductor can be bonded to. Thus the node can include a surface of the conductive bump, a surface of a electrical pad, a bonding pad or an electrical component.
The invention has the advantage of lower inductance over conventional interconnects (for example, wire bonds, ribbon wire or wire mesh bonds). This characteristic is particularly beneficial in electrical systems operating at frequencies in excess of 20 GHz. The invention also improves the consistency (compared to the prior art) of the connections between nodes, thereby lowering manufacturing costs. Additionally, unlike the prior art, shear strength of the bonds between the center conductor and the node is not compromised and does not lead to additional structural failures in the field.
In general the center conductor is elongated and can have various cross sections. For example, it can have a shape substantially resembling a circular cylinder. The elongated section of center conductor can be generally straight or have multiple bends to fit between nodes. In other embodiments, the center conductor can have a cross section resembling an ellipse, a polygon, or other shapes.
The center conductor of this embodiment is shown with an elliptically shaped cross-section. The center conductor 201 can have a length 227 of at least a half inch and a minor axis radius 207 of approximately 10 mils or less.
Generally the ratio of the length 227 of the center conductor to the smallest (‘minimum’) radius 207 can be used to characterize the center conductor of the present invention. In the embodiment shown in
In one embodiment, the core 211 can be made from brass. A brass core exhibits low Young's Modulus (a measure of the stiffness of a given material) and allows for deformation (Plastic Region) at low stress levels. In addition, a brass core is able to withstand a heat-treated bonding process. These advantages result in fewer tension-induced pull failures at the interconnect and subsequently lower failures in the field.
In other embodiments the core 211 can be made from beryllium copper or other materials.
The plating 213 of the center conductor can be made of Type III Class A gold purity. In one embodiment, soft bondable gold of at least 50 microinches can be used to plate the center conductor. At frequencies in excess of 20 GHz, gold plating reduces ‘skin effect’ on non-plated conductors. Skin effect generally leads to a loss of signal integrity.
The overall bonding process can comprise two stages. In the example of the center conductor being affixed to the electrical pad 205 (in
Bonding the center conductor to the stud bump is accomplished by a thermosonic process (described in detail below). This process maintains the cross-sectional properties of the center conductor when bonded (reference numeral 219 of
A 2460-V Palomar automated ball bonder or a MEI Thermosonic ball bonder are examples of a ball bonders that will facilitate the ball bond process and the coining off process of the stud bump embodiment. The Mechel bonder modifications results in a 2.5 mil free air ball and a 3.5 mil wide 1.2 mil high bonded ball with a 35% aspect ratio.
In block 305, center conductors are placed onto the nodes (with stud bumps already bonded thereon). The center conductor rests within the confines of the stud bump. This requires aligning the center conductor within +/−0.5 mil placement accuracy of the center of the stud bump in a horizontal plane 209 of the node as illustrated in
A bond tool with a 3.2 by 10 mil tungsten carbide wedge foot and a 90-degree electrical discharge machining (EDM) groove cut into a bond tip is an example of a bonding tool. The bond tool is capable of transferring ultrasonic energy through the center conductor and into the stud bump attached to the node.
The circuit board is then sent into a thermosonic assembly to electrically connect the center conductors to the gold stud bumps and pads (block 307).
The thermosonic process is set to ambient temperature, 100 gram force, 16 microinches of transducer excursion and 300 milliseconds of bond time. This results in an electrical connection from the node to the center conductor.
The steps in blocks 303-307 are repeated for the remaining center conductors (block 309).
A center conductor aligned within the requirements described above will bond to the stud bump in a desired form 219 in
This shear strength derived from a thermosonic bond is equal in magnitude to that of a wire bonding process. Destructive pull tests conducted on a bonded center conductor (using the method embodiment described in
The stud bump may be bonded to the node using either a manual or an automated process. In the case of an integrated circuit, the stud bump can be bonded to a pad on the integrated circuit either while it is still part of a wafer, or after the wafer is diced into individual circuits. Alternatively, the integrated circuit may be fabricated to include the stud bumps, in which case subsequent bonding of the stud bumps to the pad is not needed. Similarly, alignment of the center conductor onto a node may be performed manually or accomplished by automated equipment. Likewise, the center conductor to stud bump bonding may be either a manual or automated thermosonic process.
In this simplified illustration of the microcircuit, the integrated circuits are interconnected with a bus of four transmission lines 409 to carry lower frequency signals, for example control signals. The wire bonds 421 can also connect the passive component 433, e.g. a capacitor or resistor, to the integrated circuit 405.
A high frequency signal can pass between the integrated circuits and the passive component 431 through the center conductors 461. External connectors 411-415 provide external access to the integrated circuits and other components, and are connected to various parts of the microcircuit using center conductors 451 and 453. Direct connectivity between integrated circuits 405 and 407 is provided for by center conductor 465.
While the embodiments described above constitute exemplary embodiments of the invention, it should be recognized that the invention can be varied in numerous ways without departing from the scope thereof. It should be understood that the invention is only defined by the following claims.
