FLIP-CHIP BALL GRID ARRAY-TYPE INTEGRATED CIRCUIT PACKAGE FOR VERY HIGH FREQUENCY OPERATION

Abstract

The invention relates to a flip-chip integrated circuit package of the ball array type, wherein: the underside of the package includes a plurality of receiving pads for signal, ground and solder balls; stacks of signal and ground vias, electrically connected to respective receiving pads, pass vertically through the package's dielectric body forming a quasi-coaxial structure. In an upper part of the package body: the signal vias are electrically connected to a lesser number of signal conductive bumps protruding from the upper surface of the package's dielectric body; and at least two ground vias are connected by means of conductive projections to respective ground conductive bumps, forming a ring around the signal conductive bumps.

Claims

1. A flip-chip ball grid array type integrated circuit package comprising: a dielectric body having an upper face and an underside in which: the underside of the dielectric body comprises a plurality of receiving pads for means of connection, comprising a central signal receiving pad and peripheral ground receiving pads surrounding the signal receiving pad; a plurality of stacks of signal vias, electrically connected to the signal receiving pad, and passing vertically through the dielectric body; a plurality of stacks of ground vias, electrically connected to the ground receiving pads, passing vertically through the dielectric body and forming a ring around the signal vias; wherein, in an upper part of the dielectric body: the signal vias are electrically connected to a lesser number of signal conducting bumps projecting from the upper face of the dielectric body; and at least two ground vias are connected, by means of conductive projections, to respective ground conductive bumps, projecting from the upper face of the dielectric body and forming a ring around signal conducting bumps.

2. The package according to claim 1, wherein the dielectric body consists of a stack of dielectric layers.

3. The package according to claim 2, wherein: conductive ground planes are arranged between at least some of the dielectric layers, the ground planes having openings that have substantially the same diameter and are aligned in a vertical direction, perpendicular to the ground planes; the stacks of signal vias, electrically connected to the signal receiving pad, pass vertically through the dielectric layers corresponding to a central region of the openings of the ground planes; the stacks of ground vias, electrically connected to the ground receiving pads, pass vertically through the dielectric layers outside of the openings of the ground planes; and the ground vias connected, by means of conductive projections, to respective conductive bumps, are located on either side of the opening of the ground plane.

4. The package according to claim 3, wherein all the ground vias are connected by means of the conductive projections, to respective conductive bumps located inside the opening.

5. The package according to claim 1, wherein the signal vias are interconnected by conductive bridges.

6. The package according to claim 4, wherein the signal vias are interconnected by conductive bridges.

7. The package according to claim 6, wherein the stacks of ground vias do not extend through a lower part of the package, the electrical connection with the ground receiving pads being provided by other stacks of vias, forming a ring around the signal vias having a larger diameter than the diameter of the ring formed by the stacks of ground vias.

8. The package according to claim 1, wherein the stacks of ground vias do not extend through a lower part of the package, the electrical connection with the ground receiving pads being provided by other stacks of vias, forming a ring around the signal vias having a larger diameter than the diameter of the ring formed by the stacks of ground vias.

9. The package according to claim 8, wherein the dielectric body is made of ceramic material.

10. The package according to claim 8, wherein the dielectric body is made of organic material.

11. The package according to claim 8, comprising a single signal conducting bump.

12. The package according to claim 8, wherein the diameter of the openings of the ground planes, the number of stacks of signal vias, the diameter of the ring(s) of ground vias around the signal vias, the number of ground conductive bumps and the diameter of the ring that the ground conductive bumps forms around the signal bump or bumps are chosen to ensure impedance continuity between the pads for solder balls and the conductive bumps projecting from the upper face of the dielectric body.

13. The package according to claim 1, wherein the dielectric body is made of organic material.

14. The package according to claim 1, comprising a single signal conducting bump.

15. The package according to claim 3, wherein the diameter of the openings of the ground planes, the number of stacks of signal vias, the diameter of the ring(s) of ground vias around the signal vias, the number of ground conductive bumps and the diameter of the ring that the ground conductive bumps forms around the signal bump or bumps are chosen to ensure impedance continuity between the pads for solder balls and the conductive bumps projecting from the upper face of the dielectric body.

16. The package according to claim 1, comprising carrying a flipped semiconductor chip, electrically connected to the conductive bumps projecting from the upper face of the dielectric body.

17. The package according to claim 15, comprising carrying a flipped semiconductor chip, electrically connected to the conductive bumps projecting from the upper face of the dielectric body.

18. A printed circuit board comprising: a signal conductor; and at least one ground plane forming a transmission line for microwave signals, the printed circuit board carrying a package according to claim 1, the signal receiving pad of the package being connected to the signal conductor by means of a solder ball and the ground receiving pads of the package also being connected to the ground plane(s) by means of solder balls.

19. A printed circuit board comprising: a signal conductor; and at least one ground plane forming a transmission line for microwave signals, the printed circuit board carrying a package according to claim 3, the signal receiving pad of the package being connected to the signal conductor by means of a solder ball and the ground receiving pads of the package also being connected to the ground plane(s) by means of solder balls.

20. A printed circuit board comprising: a signal conductor; and at least one ground plane forming a transmission line for microwave signals, the printed circuit board carrying a package according to claim 8, the signal receiving pad of the package being connected to the signal conductor by means of a solder ball and the ground receiving pads of the package also being connected to the ground plane(s) by means of solder balls.

Description

[0030] Other characteristics, details and advantages of the invention will become apparent upon reading the description given in reference to the appended drawings provided as an example and that represent, respectively:

[0031] FIG. 1 is a perspective view of a package according to the embodiment;

[0032] FIG. 2 is a first cross-sectional view of the package in FIG. 1 according to a first embodiment of the invention, based on ceramic technology;

[0033] FIG. 3 is a second cross-sectional view of the package in FIG. 1 according to said first embodiment of the invention;

[0034] FIG. 4 is a third cross-sectional view of the package in FIG. 1 according to said first embodiment of the invention;

[0035] FIG. 5 is a transparent view of the package in FIG. 1 according to said first embodiment of the invention;

[0036] FIG. 6, is a first cross-sectional view of the package in FIG. 1 according to a second embodiment of the invention, based on organic technology;

[0037] FIG. 7, is a second cross-sectional view of the package in FIG. 1 according to said second embodiment of the invention;

[0038] FIG. 8, is a transparent view of the package in FIG. 1 according to said second embodiment of the invention; and

[0039] FIG. 9, is a detailed view of a stack of vias according to said second embodiment of the invention.

[0040] FIG. 1 shows an FCBGA package 2 mounted on a printed circuit board (PCB) 5 by means of a solder ball array 4 on its underside and bearing a flip-chip 1 on its upper face.

[0041] FIG. 2, FIG. 3 and FIG. 4 show three cross-sectional views—according to sections A-A, B-B and C-C respectively—of package 2 according to a first embodiment made of ceramic material. FIG. 5 is a transparent view of this same package 2.

[0042] PCB 5 comprises a dielectric body 7, inside which is arranged a signal line 18 arranged between an upper ground plane 29a, on the upper surface of the PCB, and a lower ground plane 29b, on the bottom surface of the PCB, and is framed by screening vias 21 connecting the two ground planes. The signal line 18 is connected by means of a micro-via 17 to an interconnection area (“land”) 19 located on the upper face of the PCB and insulated from the ground planes; a so-called “signal” solder ball 16 is arranged on this interconnection pad 19. So-called “ground” solder balls 27 are arranged on the upper ground plane 29a so as to surround the signal ball 16. Corresponding to the ground balls 27, ground vias 28 connect the upper ground plane 29a to the lower ground plane 29b.

[0043] The body of package 2 consists of a stack of dielectric layers (of ceramic material, in the embodiments of FIG. 2-FIG. 4). Optionally, all or some of these dielectric layers, and in particular the upper dielectric layer of the stack, carry on their upper face a conductive ground plane 23. These conductive ground planes have vertically-aligned central openings 31. The underside of the package 2 carries conductive receiving pads corresponding to the solder balls: a central signal receiving pad 15 for the interconnection with the signal ball 19 and peripheral ground receiving pads 26 surrounding said signal receiving area 15 and intended for interconnection with the ground balls 27. The upper face of the package carries conductive bumps 9, 20 intended to allow interconnection with the active face 8 of the flip-chip 1. More specifically, one or more signal conductive bumps 9 are surrounded by ground conductive bumps 20. Vias extending through the dielectric stack connect the signal conductive bump or bumps 9 to the signal receiving pads 15 on the underside, and the ground conductive bumps 20 to the ground receiving pads 26. The space between the upper surface of the package 2, the conductive bumps and chip 1 is filled with an “underfill” resin 10.

[0044] The signal propagating along signal line 18 is vertically routed directly to chip 1 by a plurality of stacked signal vias 11, two in the illustrated embodiment, interconnected by means of conductive bridges 12. In the upper part of the stack (which is to say in the upper dielectric layer) these vias—identified by reference 13—have a reduced radius for feasibility reasons.

[0045] The use of bridges 12, which is non-essential, makes it possible to increase the frequency of a resonance that limits the interconnection's RF bandwidth, this resonance being produced by the signal rebounding between a possible impedance fault at the crossing of the PCB—package interface and at the crossing of the package—chip interface, marked by the surface current density rotating around a horizontal axis inside the plurality of signal vias 11 then acting in the manner of a resonant cavity. Indeed these bridges reduce the resonant cavity's volume and thus the resonant frequency's wavelength, and thus increase the package's maximum frequency of use.

[0046] The stacks of signal vias 11 are surrounded by an ideally-concentric ring composed of a plurality of stacked ground vias 22, eight of them in the illustrated embodiment. The number of stacks of signal vias 11 and the diameter (more generally: the lateral dimensions) of the ring of stacked ground vias 22 are chosen such that the characteristic impedance of the quasi-coaxial structure 11, 22 is substantially identical to that of the transmission line 18, 29a, 29b, typically 50 Ohm. More specifically, for a given value of characteristic impedance, the higher the number of stacks of signal vias 11, the further apart the stacks of ground vias 22 should be.

[0047] If the dielectric layers of the package body carry ground planes 23 with central openings 31, the stacks of ground vias 22 are located outside of said openings, for example in a position tangential to their edges. The stacks of signal vias 11, on the other hand, are found in the center of said openings.

[0048] The spacing of the conductive ground bumps 20, which corresponds to that of the pads on the active face 8 of the chip 1 (not shown), is typically tighter than that of the stacks of ground vias 22. If this were not the case, the chip would have to have an unnecessarily increased surface area, which would increase the cost. As can be seen in FIG. 3, the stacks of ground vias 22 stop before reaching the top layer (or, more generally, a group of top layers) of the dielectric stack. This last layer is crossed through by ground vias 24 which also form a ring around the signal vias 11, but of a smaller diameter to correspond to the positioning of respective pads on the active face 8 of the chip 1. More specifically, the vias 24 are inside the opening 31 of the ground plane 23 located between the upper dielectric layer and the one that is immediately below it, and are connected to this ground plane (or to the upper end of the stacks of vias 22) by conductive projections 30. The ground conductive bumps 20 are arranged corresponding to the upper ends of the vias 24.

[0049] The narrowing of the circular ring of ground vias tends to reduce the characteristic impedance of the quasi-coaxial structure. To avoid impedance breakdown, a single signal conducting bump 9 is laid on the top layer's bridge 12 (more typically, there will be a lesser number of signal bumps 9 than there are stacks of signal vias 11).

[0050] The decoupling of the radius of the ring of ground vias 24 on the upper part of the body of the package from the radius of the ring of ground vias 22 on the central part of the body of the package thus makes it possible to avoid an impedance fault appearing near the chip's interconnection. It should be noted that the electrical connection between the ground vias 22 and 24 is made by a small width of copper (projections 30) to avoid creating a capacitive fault (decreased capacitive impedance) that would be produced by a narrowing of the opening 31, and therefore a rapprochement between the ground plane 23 and the signal vias 11.

[0051] In the lower part of the package body, the ring of ground vias 22 of the central part gives way to another ring of ground vias 25, of larger diameter. This makes it possible, on the one hand, to align, relatively, with the spacing of the ground balls 27, and, on the other hand, to compensate for the capacitive fault caused by the proximity between the ground balls' receiving pads 26 and the signal ball's receiving pad 15; this proximity, in turn, is due to the significant size of these receiving pads compared to the balls' footprint, which is typical of ceramic substrate technology.

[0052] FIG. 6 and FIG. 7 show two cross-sectional views—according to sections A-A, and B-B respectively—of package 2 according to a second embodiment, made of organic material. FIG. 8 is a transparent view of this same package.

[0053] As in the case of the ceramic package in FIG. 2-FIG. 5, the body of package consists of a stack of dielectric layers, this time made of organic material. More specifically, it is possible to distinguish a central part 34 consisting of thicker layers, or even one single thick layer, and upper and lower end parts 33 consisting of thinner layers. A solder-mask layer 32 can cover the upper and lower faces, revealing only the electrical interconnection regions.

[0054] Compared to the embodiment made of ceramic material, the package made of organic material in FIG. 6-FIG. 8 can have receiving pads 15, 26 for smaller-dimension solder balls. For this reason, it is unnecessary to provide an enlargement of the ring of ground vias in the lower part of the package. In the upper part of the package, the signal can be conveyed by a single stack of vias 13.

[0055] The vias 22, 11 that pass through the package body made of organic material can be produced by laser ablation (as illustrated) or mechanically. The various stacks of ground vias 25, 24 and of signal vias 13, 11 can be broken up so that the vias are not strictly stacked but slightly offset in relation to their counterpart in the top and/or bottom layer, ideally offset by a capture pad diameter, in order to improve the assembly's reliability and, more specifically, the thermomechanical stress resistance. FIG. 9 illustrates the structure of such a stack of vias 200 (which may comprise a stack of ground vias 25, 24 and/or signal vias 13, 11).

[0056] Certain features of the package in FIG. 6-FIG. 8 do not directly result from the use of an organic material, but constitute design choices. It basically involves that the opening in the ground planes is larger and square-shaped, and above all that only some of the ground solder bumps are brought closer to the signal solder bump, the other ground solder bumps forming a ring of the same lateral dimensions as the quasi-coaxial structure of vias 22. More specifically, three ground solder bumps on either side of the signal bump 9 are brought closer to the signal bump 9 (it would have been possible to bring together only two, but it is preferable to “split” [or duplicate] one to provide a passage for a signal line). The interconnection's sizing (determining the number of ground bumps to bring closer to the signal bump and their exact position) must be done in such a way as to ensure characteristic impedance continuity between the PCB and the flip-chip.

[0057] The invention has been described in reference to two particular embodiments, but variants can be considered. Different dielectric and conductive materials can be implemented, and different fabrication techniques can be employed. The number, spacing and relative position of the signal and ground vias in the various parts of the package body (lower, central, upper) can be determined by the designer according to the specific constraints of each application, while taking care to ensure the absence of impedance faults throughout the interconnection. Furthermore, the package's dielectric body can be solid, although a multi-layer structure is generally preferable. In addition, the package may not be intended to be connected to a printed circuit board via solder balls, but to be placed in an adapter “socket”, in which case the receiving pads 15, 26 make contact with spring connectors.