# Patent

## Title
Integrated circuit chip packaging

## Abstract

An electrical circuit device that includes a circuit board with an integrated circuit chip in a cavity that extends from a surface of the circuit board to an embedded conductor, and an electrical connection between the integrated circuit chip and the embedded conductor.

## Background
This invention was made with Government support under Contract No: H98230-04-C-0920 awarded by the National Security Agency. The Government has certain rights in this invention.


BACKGROUND OF THE INVENTION
Field of the Invention
The present invention generally relates to integrated circuit chip packaging. More particularly, the present invention relates to integrated circuit chip packaging connecting an integrated circuit chip to a conductive layer embedded between layers of a printed circuit board.
Description of the Related Art
Conventional methods and systems for connecting an integrated circuit chip to a printed circuit board have relied upon a surface attachment. A surface attachment restricts the launch to a conductive layer that is on the surface of a printed circuit board and if a signal from the integrated circuit chip must be launched into a conductive layer that is between the layers of the printed circuit board (i.e., an embedded conductor), then the signal is subject to problems, which may be caused by vias that are used to transfer the signal from the surface to the embedded conductor.
Conventionally, an integrated circuit chip may be placed into a carrier (e.g., a package) and the carrier may then be mounted onto a surface of the printed circuit board. Exemplary conventional techniques are shown in FIG. 1.
FIG. 1 shows a conventional surface mount package 102 and a conventional flip chip package mounting 104. The conventional flip chip package mounting 104 connects to the printed circuit board using two via configurations. A first configuration includes a via 106 that has a long stub 107 that extends past an embedded conductor layer 108. The flip chip 104 is electrically connected to the embedded conductor layer 108.
The second configuration includes a buried via 110 that does not include a lengthy stub and, therefore, avoids the signal degradation associated with a lengthy stub. However, a buried via 110 is more expensive to fabricate. The conventional surface mount package 102 also shows connection to another embedded conductor 111, which connects to the distal end of via 109. This configuration, referred to as a “through via,” minimizes the length of the stub, but the presence of the via still leads to signal distortion.
In addition, if many signals try to connect to embedded conductors at the same level of a layer, then wiring congestion may result.
Integrated circuit chips that operate at high frequencies often rely on flip chip packaging to minimize the parasitics that are associated with conventional chip packages. For best performance, conventionally, the highest speed signals use conductive surface layers or buried vias in the printed circuit board in order to reduce a via stub effect that may compromise signal transmission.
SUMMARY OF THE INVENTION
In view of the foregoing and other exemplary problems, drawbacks, and disadvantages of the conventional methods and structures, an exemplary feature of the present invention is to provide an integrated circuit chip package in which a cavity in a printed circuit board receives an integrated circuit chip to connect to a conductor that is embedded between layers of a printed circuit board.
In a first exemplary aspect of the present invention, a printed circuit board has a cavity extending into a surface to a conductor that is embedded between layers of a printed circuit board, an integrated circuit chip is in the cavity, and an electrical connection connects the integrated circuit chip to the embedded conductor.
In a second exemplary aspect of the present invention, a method of mounting an integrated circuit chip to a printed circuit board includes providing a cavity extending from a surface of the printed circuit board to a conductor that is embedded between layers of a printed circuit board, placing the integrated circuit chip into the cavity, and electrically connecting the integrated circuit chip to the conductor.
In a third exemplary aspect of the present invention, a printed circuit board may include a plurality of insulating layers, a conductor embedded between two of the plurality of insulating layers, and a cavity extending from a surface of the printed circuit board to the embedded conductor.
The inventors of the present invention discovered that more direct access to a conductor that is embedded between layers of a printed circuit board is desired, without having to rely upon a surface mount or a via to connect to an embedded conductor.
A printed circuit board may include alternating stacks of patterned conductors, such as, for example, copper planes. Usually those planes are copper, in which case they may include ground planes or power planes. Alternatively, the planes are patterned into narrower traces to form conductors, which are sandwiched between layers to provide controlled impedance transmission lines. These conductors are, thus, embedded between layers of the printed circuit board and are particularly well suited to transmit high fidelity signals. Other signals, such as power, control connections, or the like, do not need controlled impedance lines and may have varying widths depending upon their functional needs.
An embedded conductor is superior to a surface conductor because an embedded conductor has better signal fidelity and an embedded conductor is completely shielded, while a surface conductor is not completely shielded. Therefore, an exemplary embodiment of the present invention is advantageous because the integrated circuit chips no longer need to connect to surface conductors, rather the integrated circuit chips may attach more directly to embedded conductors.
An exemplary embodiment of the present invention connects a chip to a conductor embedded between layers of a printed circuit board without using a via.
In an exemplary embodiment of the present invention, the printed circuit board may be milled to expose an embedded conductor into which a signal from an integrated circuit chip may be launched.
In an exemplary embodiment of the present invention, the milling may create a cavity that may be shaped to receive the integrated circuit chip.
In an exemplary embodiment of the present invention the cavity may be deep enough such that, when the cavity receives the chip, the top of the chip is substantially co-planar with an embedded conductor. In this manner, very short ribbon bonds may be connected between contacts on the chip to the substantially co-planar embedded conductor.
It is advantageous to minimize the length of a wire bond and/or a ribbon bond, because a bond represents a discontinuity. Therefore, milling the cavity such that the top of the chip is substantially co-planar with the launch, minimizes the length of a wire/ribbon bond and, as a result, reduces the adverse affects of the discontinuity.
A launch into an embedded conductor is generally capable of carrying a signal that incorporates frequency components that exceed 40 Ghz. Thus, an exemplary embodiment of the present invention is capable of handling signals incorporating a 40 Ghz component.
In an exemplary embodiment of the present invention, the printed circuit board includes a step-shaped (e.g., a terraced) side surface that may reveal launch points into a plurality of embedded conductors and non-controlled impedance traces, such as those used for control and power.
In general, the best launch exposed by the step-shaped structure is the one that is substantially co-planar with the top of a chip. The remaining launches may be used to handle slower and/or less critical signals.
In an exemplary embodiment of the present invention two integrated circuit chips may be connected by a high speed embedded conductor by placing each of these chips into cavities such that the tops of both chips are substantially co-planar with the same embedded conductor. In this manner, the present invention obviates the necessity of providing vias to connect the two chips to each other.
An exemplary embodiment of the present invention may use a precision engraving machine, rather than just a generic milling machine.
Further, an exemplary embodiment of the present invention may be adapted to either wire/ribbon bonding or to a flip chip implementation. This packaging technique offers even higher performance than the conventional techniques mentioned above while at the same time being low in cost.
These and many other advantages may be achieved with the present invention.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other exemplary purposes, aspects and advantages will be better understood from the following detailed description of an exemplary embodiment of the invention with reference to the drawings, in which:
FIG. 1 is a cross-sectional view of a printed circuit board with two conventionally mounted integrated circuit chip packages;
FIG. 2 is a cross-sectional view of an exemplary embodiment of the present invention;
FIG. 3 is a cross-sectional view of the exemplary embodiment of FIG. 2 in a passivated configuration;
FIG. 4 is a cross-sectional view of another exemplary embodiment of the present invention; and
FIG. 5 is a flowchart of a exemplary method in accordance with the present invention.



DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
Referring now to the drawings, and more particularly to FIGS. 1-5, there are shown exemplary embodiments of the method and structures of the present invention.
As explained above, there are many topologies conventionally available for implementing transmission lines in circuit boards (e.g., printed circuit boards (PCBs)). Internal embedded conductors, which may include an embedded conductor sandwiched between solid planes of a reference ground plane, offer many advantages over surface conductors. Embedded conductors support less dispersive transverse electric and magnetic modes which may be advantageous for wide band operation. An embedded conductor may also be self-shielding and, since they may undergo fewer processing steps than surface wiring they may be easier and less costly to fabricate.
Connecting an integrated circuit chip to embedded conductors conventionally requires the use of vias to connect from the surface of a printed circuit board where components are traditionally attached. However, if the overlying layers are removed from the printed circuit board in accordance with an exemplary embodiment of the present invention (e.g. by milling) these embedded conductors may permit a more direct attachment of these embedded conductors to high speed nets on a chip. A chip may be connected to an embedded conductor using a wire bond, a ribbon bond, or the like, or may be flip chip connected using solder balls, or the like.
As illustrated by FIG. 2, in accordance with an exemplary embodiment of the present invention, layers in a printed circuit board 200 may be milled back to create a cavity with a step-shaped side surface 202. As shown, the side surface may have a plurality of steps.
In other words, the outer surface of the printed circuit board may be milled to form a cavity that extends down to a desired embedded conductor. Further, a side of such a cavity may have a terraced profile (step-shaped side surface) wherein each step of the terraced profile exposes a surface of an embedded conductor.
In another exemplary embodiment of the present invention, after a cavity is formed in the printed circuit board, an exposed surface of the embedded conductor may be treated to facilitate bonding. For example, the surface may be plated with a metal (e.g., a noble metal such as gold, platinum, silver, and the like) to improve wire bonding and/or ribbon bonding.
In an exemplary embodiment of the present invention, the cavity in the printed circuit board extends deep enough into the printed circuit board such that a top of an integrated circuit chip positioned in the recess would be substantially co-planar with an embedded conductor. The embedded conductor may then be provided the highest speed signal more directly from the integrated circuit chip.
For example, in FIG. 2 the top surface 204 of the chip 206 is substantially co-planar with an embedded conductor 208. In this manner, an exemplary embodiment of the present invention offers a launch from the chip 206 into the printed circuit board more directly to a desired embedded conductor 208. The top of the chip 204 may be connected to the highest performance embedded conductor 208 with a ribbon bond 210 because a ribbon bond may offer a higher performance potential.
The step-shaped surface of the cavity may also reveal the surfaces of other embedded conductors, which are not substantially co-planar with a top of the chip. In general, as the distance from the top of the chip 204 to an embedded conductor increases, a longer connection will be required. A longer connection will generally offer poorer performance than a shorter connection and, therefore, the printed circuit board may be designed such that, as the distance between an embedded conductor and the top of the chip increases, the less critical a signal will be carried by that respective embedded conductor. For example, most low speed control lines do not require controlled impedance and, therefore, are insensitive to the longer distances that need to be bridged by a wire bond.
Further, the less critical connections may use lower performance connections such as, for example, a wire bond and/or a ball bond as opposed to a ribbon bond. However, one of ordinary skill in the art understands that any type of connection may be used to establish electrical communication between a chip and an embedded conductor and still practice the invention.
In the exemplary embodiment illustrated by FIG. 2, the embedded conductor 212, which is the next closest to the top of the chip 204, and a ground plane 214 are also connected to the chip using ribbon bonds 210.
Further, the next closest embedded conductors 216 to the top of the chip 204 are connected by ball bonds 218 to the chip 206.
FIG. 2 also illustrates a flip chip package 220, which may be connected to the printed circuit board 200 in a conventional manner.
A printed circuit board may be constructed by laminating many different layers together using, for example, an epoxy resin. That lamination may be done under a high temperature and a pressure to cure the resin. The resin essentially flows between the layers in a pattern sensitive manner depending upon what copper features happen to be nearby. Thus, the surfaces between the layers of a printed circuit board may not necessarily be planar. Rather, the surfaces of the layers may incorporate a bit of waviness depending upon the copper patterns.
Thus, when using a conventional milling machine and open-loop programming on the milling machine to mill down to a certain level into a printed circuit board, there is a likelihood that the milling might not reach a level that corresponds to the level of a desired embedded conductor. The thickness of a patterned copper layer of an embedded conductor is typically about 1 mil and the waviness of a reasonably thick layer of a printed circuit board is typically more than 1 mil. Therefore, a conventional milling machine may cut entirely through the embedded conductor, thereby, destroying the embedded conductor in some places, while simultaneously not even reaching the same embedded conductor in another place.
In an exemplary embodiment of the present invention, a precision milling machine may sense an electrical contact between a cutting edge of the milling machine and a stripline. In this manner, the milling machine may incorporate a closed-loop feedback system that regulates the depth of the milling into the printed circuit board.
In another exemplary embodiment of the present invention, a closed-loop feedback might not electrically sense the patterned layer of the desired embedded conductor in order to control the depth of the milling process. Rather, a calibration structure that may closely track the local internal waviness may be provided which provides a desired feedback control signal. This may be accomplished either electrically, with optical recognition, or by analyzing the chips as they are received from the milling operation. When copper chips are detected, then the desired target layer has been reached.
Before connecting the leads, but after milling, an embedded conductor may have a bare surface. In an exemplary embodiment of the present invention, the bare surfaces of the embedded conductor may be plated with a material, which facilitates bonding. A material for plating may include, for example, gold and the like, which may be electro-lessly plated onto a surface of an embedded conductor.
Alternatively, if a thicker gold layer is required, then a sacrificial plating web may be patterned in the copper and subsequently milled away.
Although not shown in the Figures, ground planes in a printed circuit board may be electrically connected to each other using a via in close proximity to the milled cavity in order to maintain tight coupling between the planes for embedded conductor integrity.
FIG. 3 illustrates an exemplary embodiment of a printed circuit board 300 in accordance with the present invention. The step-shaped surface 302 and the chip 304 are incorporated into the printed circuit board 300 in a final passivated configuration. The printed circuit board 300 includes an underfill material 306 which fills the recess 308 in the printed circuit board 300 to protect the chip 304 and the connections 310 from the chip 304 to the embedded conductors 312 within the printed circuit board 300.
The printed circuit board 300 also includes a thermal slug 314 mounted to a top surface 316 of the chip 304 and a heat sink 318 mounted to an outer surface 320 of the thermal slug 314 to conduct thermal energy away from the printed circuit board 300.
In another exemplary embodiment in accordance with the present invention (not shown), the chip may be accessed from the backside for thermal management.
As illustrated by FIG. 4, an exemplary embodiment 400 of the invention may permit a chip 402 to connect with an embedded conductor 404 using a flip chip connection 406.
The other vias 408 supporting the non-embedded conductor signals from the chip 402 are not buried vias, but are typical through vias that have pads 410 at the level of the embedded conductor 404.
In accordance with an exemplary embodiment of the present invention, these pads 410 may be revealed in the course of milling a cavity into the printed circuit board. This exemplary method maintains the low cost of through via construction by avoiding the use of buried vias.
FIG. 5 illustrates a flowchart 5 for an exemplary method in accordance with the present invention. The flowchart 500 starts at step 502 and continues to step 504 where a cavity is milled into a circuit board to expose an embedded conductor. The flowchart 500 continues to step 506 where an integrated circuit chip is positioned inside the cavity. Next, in step 508, the integrated circuit chip is electrically connected to the embedded conductor that was exposed in step 504. The flowchart ends at step 510.
While the invention has been described in terms of several exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modification.
Further, it is noted that, Applicant's intent is to encompass equivalents of all claim elements, even if amended later during prosecution.

## Claims

1. An electrical circuit device, comprising:
a printed circuit board with a cavity extending from a top surface of said printed circuit board to a top surface of an embedded conductor, said printed circuit board contacting an upper surface and a lower surface of said embedded conductor inside the printed circuit board;
an integrated circuit chip that is disposed on a lower surface of said cavity inside the printed circuit board such that said printed circuit board extends below the integrated circuit chip and covers a side surface of the integrated circuit chip;
an electrical connection between said integrated circuit chip and said embedded conductor;
a plurality of connections each electrically connecting said integrated circuit chip to a corresponding one of a plurality of embedded conductors, which are embedded within the printed circuit board in addition to the embedded conductor;
a ground plane embedded within the printed circuit board;
a flip chip package connected to the printed circuit board;
a thermal slug disposed on a top surface of said integrated circuit chip, the top surface of said integrated circuit chip being located above the side surface of the integrated circuit chip; and
a heat sink mounted to an outer surface of the thermal slug for transferring a thermal energy away from the printed circuit board, the heat sink extending above the top surface of said printed circuit board,
wherein the embedded conductor and each of the plurality of embedded conductors are disposed on a same side of the integrated circuit chip such that said each of the plurality of connections and said each of the plurality of embedded conductors are located above the top surface of the embedded conductor, said each of the plurality of connections and said each of the plurality of embedded conductors being located between the side surface of the integrated circuit chip and the flip chip package,
wherein said electrical connection between said integrated circuit chip and said embedded conductor comprises a ribbon bond, and
wherein an upper one of said plurality of connections comprises a ball bond that connects said integrated circuit chip to an upper one of the plurality of embedded conductors, among the plurality of embedded conductors, the upper one of the plurality of embedded conductors being located with a longest distance from the top surface of the embedded conductor.

2. The device of claim 1, wherein the top surface of said integrated circuit chip is substantially co-planar with said top surface of the embedded conductor.

3. The device of claim 1, wherein said cavity comprises a side surface with a step.

4. The device of claim 3, wherein a top surface of said step is aligned with another embedded conductor.

5. The device of claim 1, wherein each of said plurality of connections comprises one of the ball bond and the ribbon bond, and
wherein said electrical connection between said integrated circuit chip and said embedded conductor comprises the ribbon bond rather than the ball bond.

6. The device of claim 1, wherein said printed circuit board directly contacts a lower surface and an upper surface of said ground plane inside the printed circuit board.

7. The device of claim 1, further comprising:
another ground plane embedded within the printed circuit board above said embedded conductor, said printed circuit board abutting a lower surface and an upper surface of said another ground plane inside the printed circuit board.

8. The device of claim 1, wherein said embedded conductors comprise another embedded conductor disposed within the printed circuit board above said embedded conductor, said printed circuit board abutting a lower surface and an upper surface of said another embedded conductor inside the printed circuit board, and
wherein the device further comprises another ground plane embedded within the printed circuit board between said embedded conductor and said another embedded conductor, said printed circuit board abutting a lower surface and an upper surface of said another ground plane inside the printed circuit board.

9. The device of claim 1, wherein the plurality of connections is located above the electrical connection.

10. The device of claim 1, wherein the plurality of connections extends above an upper surface of the electrical connection to electrically connect said integrated circuit chip to the corresponding one of the plurality of embedded conductors.

11. The device of claim 1, wherein the plurality of embedded conductors is located above the embedded conductor and below the flip chip package.

12. The device of claim 1, wherein each of the plurality of embedded conductors is located above the embedded conductor and below the flip chip package.

13. The device of claim 12, wherein each of said plurality of connections comprises one of the ball bond and the ribbon bond, and
wherein said electrical connection between said integrated circuit chip and said embedded conductor comprises the ribbon bond rather than the ball bond.

14. The device of claim 1, wherein said printed circuit board directly contacts an entirety of a lower surface of said ground plane inside the printed circuit board.

15. The device of claim 1, wherein the heat sink extends above a top surface of the flip chip package.

16. The device of claim 1, wherein a bottom surface of the heat sink is disposed on an upper surface of the thermal slug and an upper surface of the flip chip package.

17. The device of claim 1, wherein the thermal slug comprises a single layer that extends from the top surface of said integrated circuit chip to a bottom surface of the heat sink.