# Patent

## Title
Package on packaging structure and methods of making same

## Abstract

A package on packaging structure provides for improved thermal conduction and mechanical strength by the introduction of a sold thermal coupler between the first and second packages. The first package has a first substrate and through vias through the first substrate. A first set of conductive elements is aligned with and coupled to the through vias of the first substrate. A solid thermal coupler is coupled to the first set of conductive elements and to a die of the second package. A second set of conductive elements is coupled to the die and a bottom substrate is coupled to the second set of conductive elements. The thermal coupler may be, e.g., an interposer, a heat spreader, or a thermal conductive layer.

## Background
BACKGROUND
Package on Package (PoP) is becoming an increasingly popular integrated circuit packaging technique because it allows for higher density electronics. But increasing die power of a bottom package in a POP not only results in thermal dissipation inefficiency, but leads to severe thermal stress and warpage caused by thermal expansion mismatch between the components of the package.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIGS. 1a through 1e are cross sectional views of steps in the manufacture of a first embodiment device;
FIGS. 2a through 2e are cross sectional views of steps in the manufacture of a second embodiment device; and
FIGS. 3a through 3d are cross sectional views of steps in the manufacture of a third embodiment device.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
Various steps in the formation of Package on Package (PoP) will be described with reference to FIGS. 1 through 3. It should be appreciated that the materials, geometries, dimensions, structures, and process parameters described herein are exemplary only, and are not intended to be, and should not be construed to be, limiting to the invention claimed herein. Many alternatives and modifications will be apparent to those skilled in the art, once informed by the present disclosure.
A first embodiment package will be discussed with reference to FIG. 1 (which includes FIGS. 1a through 1e). FIG. 1a illustrates a top package 1 to be employed in the PoP package. Top package 1, which may be formed using a plastic ball grid array (PBGA) package assembly process or the like, includes a plurality of stacked die 2, which may be wire bonded to top substrate 10 by way of contacts 16 (on respective stacked die 2), bond wires 6, and contacts 12 (on top substrate 10). Individual stacked die may comprise a memory chip, a logic chip, a processor chip, or the like. Although FIG. 1a illustrates three stacked die this is for illustration only. Likewise, the use of wire bonding is merely illustrative and other approaches for electrically connecting the stacked die are within the contemplated scope of the present disclosure.
Top substrate 10 may be a laminated circuit board comprised of alternating layers of non-conductive polymers, such as bismaleinide-triazine (BT), and patterned (or non-patterned) conductive layers. As discussed above, top substrate 10 has contacts 12 on a first side (referred to herein sometimes as a top side for convenience) for electrical connection to stacked die 2. Top substrate 10 further has contacts 24 on a second side (sometimes referred to as a bottom side) for electrical connection to other components as will be detailed further below.
FIG. 1a further illustrates ground pad 18 on the bottom side of top substrate 10. As will be explained in greater detail with regard to FIG. 1e, ground pad 18 provides a path for thermal conduction of heat from a bottom package that will be subsequently attached to top package 1. Electrical and/or thermal contact to ground pad 18 may be made by way of thermal vias 22, which may be formed of copper, solder, aluminum, or other conductive materials.
FIG. 1b illustrates the attachment of interposer 30 to top package 1. Interposer 30 may be composed of a semiconductor material such as silicon, germanium or gallium arsenate. Interposer 30 may have a thickness of about 100 microns to about 300 microns. Through vias 32, which may be composed of copper, or may be composed of tungsten, aluminum, solder, or the like, pass through interposer 30. Through vias 32 may aligned with solder bumps 26, which electrically and thermally connect interposer 30 to top package 1. Alternately, through vias 32 may be not aligned with solder bumps 26. More particularly, through vias 32 are electrically and/or thermally connected to ground pad 18 by way of solder bumps 26 and thermal vias 22. Underfill 28 might surround solder bumps 26. Although solder bumps are illustrated, connection between interposer 30 and top package 1 could be made by way of solder balls, copper bumps, copper pillars, or other connector elements. Hence, the term connector element may be used herein to refer to solder bumps or any other form of connector elements 26.
Through vias 32 might have a pitch of about 0.1 mm to about 0.3 mm. Through vias 32 have an aspect ratio defined as the ratio of the thickness of interposer 30 to the width of through vias 32, where the aspect ratio is about 2 to about 6. It is believed that this aspect ratio provides the most advantageous range for improving thermal conductivity of the resulting package. Through vias 32 also may have a pitch (i.e., center to center spacing) in the range of about 0.1 mm to about 0.3 mm for advantageous thermal conduction. Through vias 32 may be composed of a copper or aluminum surface, which may be filled with a highly thermally conductive material, such as copper, tungsten, aluminum, or a polymer. In addition to providing for improved thermal conduction, interposer 30 also provides mechanical stiffening to the resulting package. In this way, interposer 30 provides stiffness and resistance to warping that might otherwise occur as a result of thermal coefficient of expansion (CTE) mismatch between top package 1 and bottom package 34 (illustrated in FIG. 1d).
Continuing with FIG. 1c, solder balls 36 are attached to bottom contacts 24 of top substrate 10. Solder balls 36 allow for electrical and/or thermal connection between top package 1 and bottom package 34 (not shown, but illustrated in FIG. 1d). In the illustrated embodiment, solder balls 36 provide for electrical conduction of signals and power to stacked die 2. Again, other connection components, such as conductive bumps, conductive balls, conductive pillars, and the like, could be employed in lieu of solder balls 36.
Bottom package 34 is attached to top package 1, as illustrated in FIG. 1d. Bottom package 34 includes die 37, which is flip chip attached to bottom substrate 38, and which is electrically connected thereto by way of connector elements 39. Electrical connection between die 37 and an underlying mother board or other circuitry (not shown) is provided by through vias 40 aligned with connector elements 39 on one side of bottom substrate 38 and connector elements 42 on the other side of bottom substrate 38. Likewise, electrical connection between top substrate 10 and an underlying mother board or other circuitry is provided by connector elements 36, through vias 40, and connector elements 42.
An advantageous feature of the illustrated package of FIG. 1d is a thermal conduction path provided between die 37 (in bottom package 34) and a thermally conductive layer 27, such as a ground plane (illustrated in FIG. 1e) in top substrate 10. This thermal conduction path is provided as follows. Die 37, which is flip chip connected to bottom substrate 38 has its so-called back side attached to interposer 30. As was described above, interposer 30 includes a plurality of through vias 32 that provide for good thermal conduction. A layer of thermal interface material 48 may be applied to die 37. Thermal interface material 48 may comprise a solder paste, a high conductivity adhesive or thermal grease. In this manner, heat may be conducted away from die 37 and toward top substrate 10, to which interposer 30 is attached.
FIG. 1e provides a more detailed view of interposer 30 and top substrate 10. Top substrate 10 may be a laminated substrate having multiple layers of dielectric layers and conductive layer. Layer 27, which may be a ground plane, a power plane, or the like, is an illustrative example of a conductive layer. In the illustrated embodiment, heat may be conducted away from die 37 by the heat conduction paths illustrated by arrows 29. For instance, heat may be conducted from die 37 to interposer 30 through thermal interface material 48. As addressed above, through vias 32 provide for good thermal conduction through interposer 30. Heat is conducted away from interposer 30 through connector elements 26 (solder bumps, solder balls, copper bumps, copper pillars, or the like) to thermal vias 22 and thence to conductive layer 27. Conductive layer 27 may be a large continuous layer of conductive material that operates as a heat sink and or a heat conduction path.
Returning for a moment to FIG. 1d, another advantageous feature of the illustrated embodiment is shown. As illustrated, die 37 has a width, indicated as W1. As such, it is sufficient for making thermal contact to die 37 that interposer 30 has a width roughly equivalent to W1. Note, however, that in the illustrated embodiment, interposer 30 has a width W2 which is larger than W1. In some embodiments W2 is from about 1.2 to about 1.5 times W1. In this way, interposer 30 provides an additional role of providing mechanical stiffness to the resulting package in addition to conducting heat away from die 37. It is believed that this mechanical stiffness prevents, or at least reduces the severity of warpage resulting from, e.g., thermal expansion mismatch between the components of the resulting package.
A second embodiment package will be discussed with reference to FIG. 2 (which includes FIGS. 2a through 2e). Where appropriate, like or analogous elements are referenced with a common reference numeral in FIGS. 1 (including 1a through 1e) and 2 (including 2a through 2e). Beginning with FIG. 2a, top package 1, to be employed in the PoP package, is illustrated. Top package 1 in FIG. 2a may be formed by the same processes as top package 1 in FIG. 1a, and top package 1 in FIG. 2a may include the same elements as package 1 in the first embodiment.
FIG. 2b illustrates the attachment of heatspreader 46 to top package 1. Heatspreader 46 may be composed of a conductive material such as aluminum, copper tungsten, copper, or tin, although it may be composed of silicon carbide or graphite. Heatspreader 46 may have a thickness of about 500 microns. Heatspreader 46 is electrically and/or thermally connected to ground pad 18 of top package 1 by way of conductive layer 44, which may be solder, or may be another thermally conductive material, such as highly conductive adhesive or thermal grease.
In addition to providing for improved thermal conduction, heatspreader 46 also provides mechanical stiffening to the resulting package. In this way, heatspreader 46 provides stiffness and resistance to warping that might otherwise occur as a result of thermal coefficient of expansion (CTE) mismatch between top package 1 and bottom package 34 (illustrated in FIG. 2d).
Continuing with FIG. 2c, connector elements 36 are attached to bottom contacts 24 of top substrate 10. As in the first embodiment, connector elements 36 allow for electrical and/or thermal connection between top package 1 and bottom package 34 (not shown, but illustrated in FIG. 2d).
Bottom package 34 is attached to top package 1, as illustrated in FIG. 2d. As in the first embodiment, bottom package 34 includes die 37 which may be attached to an underlying mother board or other circuitry (not shown). Bottom substrate 38 contains through vias 40 which are aligned with connector elements 39 on one side of bottom substrate 38 and connector elements 42 on the other side of bottom substrate 38. Electrical connection between top substrate 10 and an underlying mother board or other circuitry is provided by connector elements 36, through vias 40, and connector elements 42.
As in the first embodiment, an advantageous feature of the illustrated package of FIG. 2d is a thermal conduction path provided between die 37 (in bottom package 34) and a thermally conductive layer 27, such as a ground plane (illustrated in FIG. 2e) in top substrate 10. This thermal conduction path is provided as follows. Die 37, which is flip chip connected to bottom substrate 38 has its so-called back side attached to heatspreader 46. As was described above, heatspreader 46 has a high thermal conductivity. A layer of thermal interface material 48 may be applied to die 37. Thermal interface material 48 may comprise solder paste, highly conductive adhesive or thermal grease. In this manner, heat may be conducted away from die 37 toward top substrate 10, to which heatspreader 46 is attached.
FIG. 2e provides a more detailed view of heatspreader 46 and top substrate 10. As in the first embodiment, top substrate 10 may be a laminated substrate having multiple layers, including dielectric layers and conductive layers, such as conductive layer 27. In the illustrated embodiment, heat may be conducted away from die 37 by the heat conduction paths illustrated by arrows 29. For instance, heat may be conducted from die 37 to heatspreader 46 through thermal interface material 48. As addressed above, heatspreader 46 provides for good thermal conduction. Heat is conducted away from heatspreader 46 through conductive layer 44 to thermal vias 22 and thence to conductive layer 27.
Returning for a moment to FIG. 2d, another advantageous feature of the illustrated embodiment is shown. As illustrated, die 37 has a width, indicated as W1. As such, it is sufficient for making thermal contact to die 37 that heatspreader 46 has a width roughly equivalent to W1. Note, however, that in the illustrated embodiment, interposer 30 has a width W2 which is larger than W1. W2 might be from about 1.2 to about 1.5 the width of W2. In this way, heatspreader 46 provides an additional role of providing mechanical stiffness to the resulting package in addition to conducting heat away from die 37. It is believed that this mechanical stiffness prevents or at least reduces the severity of warpage resulting from, e.g., thermal expansion mismatch between the components of the resulting package.
A third embodiment package will be discussed with reference to FIG. 3 (which includes FIGS. 3a through 3d). Where appropriate, like or analogous elements are referenced with a common reference numeral in FIGS. 1 (including 1a through 1e) and 3 (including 3a through 3d). Beginning with FIG. 3a, top package 1, to be employed in the PoP package is illustrated. Top package 1 in FIG. 3a may be formed using the same processes as top package 1 in the first embodiment. Top package 1 in FIG. 3a may include the same elements as package 1 in the first embodiment.
Continuing with FIG. 3b, connector elements 36 are attached to bottom contacts 24 of top substrate 10. As in the first embodiment, connector elements 36 allow for electrical and/or thermal connection between top package 1 and bottom package 34 (illustrated in FIG. 3c).
Bottom package 34 is attached to top package 1, as illustrated in FIG. 3c. As in the first embodiment, bottom package 34 includes die 37 which may be attached to an underlying mother board or other circuitry (not shown). Electrical connection is provided by through vias 40 aligned with connector elements 39 on one side of bottom substrate 38 and connector elements 42 on the other side of bottom substrate 38. Electrical connection between top substrate 10 and an underlying mother board or other circuitry is provided by connector elements 36, through vias 40, and connector elements 42.
As in the first embodiment, an advantageous feature of the illustrated package of FIG. 3c is a thermal conduction path provided between die 37 (in bottom package 34) and a thermally conductive layer 27, such as a ground plane (illustrated in FIG. 3d) in top substrate 10. This thermal conduction path is provided as follows. Die 37, which is flip chip connected to bottom substrate 38 has its so-called back side attached to thermal interface material 48. Thermal interface material 48 has a thigh thermal conductivity. Thermal interface material 48 may comprise of solder paste, highly conductive adhesive or thermal grease. Thermal interface material 48 may have a thickness of about 50 microns to about 100 microns. In this manner, heat may be conducted away from die 37 toward top substrate 10, to which thermal interface material 48 is attached.
FIG. 3d provides a more detailed view of thermal interface material 48 and top substrate 10. As in the first embodiment, top substrate 10 may be a laminated substrate having multiple layers, including dielectric layers and conductive layers, such as conductive layer 27. In the illustrated embodiment, heat may be conducted away from die 37 by the heat conduction paths illustrated by arrows 29. For instance, heat may be conducted from die 37 to thermal interface material 48. As addressed above, thermal interface material 48 provides for good thermal conduction. Heat is conducted away from thermal interface material 48 through thermal vias 22 and thence to conductive layer 27.
Although the present embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

## Claims

1. A semiconductor device comprising:
a first package comprising a first substrate and through vias through the first substrate;
a first set of conductive elements aligned with and coupled to the through vias of the first substrate;
a solid thermal coupler coupled to the first set of conductive elements, wherein the solid thermal coupler comprises an interposer having through vias formed therethrough;
a die coupled to the solid thermal coupler;
a second set of conductive elements coupled to the die; and
a bottom substrate coupled to the second set of conductive elements.

2. The semiconductor device of claim 1, wherein the interposer comprises a semiconductor interposer.

3. The semiconductor device of claim 1, wherein the interposer has a thickness of from about 100 microns to about 300 microns.

4. The semiconductor device of claim 1, wherein the through vias of the interposer haves a pitch of from about 0.1 mm to about 0.3 mm.

5. The semiconductor device of claim 1, wherein the through vias of the interposer have an aspect ratio of from about 2 to about 6.

6. The semiconductor device of claim 1, wherein the solid thermal coupler comnrises a heat sink.

7. The semiconductor device of claim 1, wherein the thermal coupler has a width and the die has a width, wherein the width of the thermal coupler is from about 1.2 to about 1.5 times the width of the die.

8. A package comprising:
a first package having a first die formed on a first substrate, the first substrate having a plurality of thermal vias;
a second package having a second die formed on a second substrate; and
a solid thermal conductor interjacent the first package and second package and thermally coupling the second die to the first substrate, the solid thermal conductor being thermally coupled directly to the plurality of thermal vias, the solid thermal conductor passing no electrical signals between the first package and the second package.

9. The package of claim 8 further comprising:
a first set of connector elements electrically coupling the second die to the second substrate;
a second set of connector elements electrically coupling the first die to the second substrate; and
a third set of connector elements configured to electrically couple the second substrate to another electrical circuit.

10. The package of claim 8, wherein the thermal conductor comprises an interposer having through vias therethrough.

11. The package of claim 10, wherein the first substrate includes a thermally conductive layer coupled to the plurality of thermal vias, the package further comprising:
a fourth set of connector elements aligned with and coupling the plurality of thermal vias to respective ones of the through vias.

12. The package of claim 8, wherein the second die has a first width and the thermal conductor has a second width that is greater than the first width.

13. The package of claim 12, wherein the second width is about 1.2 to 1.5 times as great as the first width.

14. The package of claim 8, wherein:
the thermal conductor comprises a heat sink thermally coupled to the second die;
the first substrate includes a thermally conductive layer coupled to the plurality of thermal vias; and
wherein the second die is thermally coupled to the thermally conductive layer by way of the thermal vias.

15. The package of claim 14 further comprising a thermal interface material interjacent the heat sink and the first substrate.

16. The package of claim 8, wherein the thermal conductor is a thermally conductive material filling a gap between the second die and the first substrate.

17. A semiconductor device comprising:
a first integrated circuit electrically and thermally connected to a first substrate;
a first set of connectors thermally and electrically connecting the first substrate to a first side of a thermal coupler, the thermal coupler having a plurality of through vias;
a second integrated circuit electrically connected to a second substrate and thermally connected to a second side of the thermal coupler, wherein the second die has a width less than a width of the thermal coupler;
a second set of connectors electrically and thermally connecting the second substrate to the first substrate, electrical signals between the first substrate and the second substrate not passing through the thermal coupler.

18. The semiconductor device of claim 17 wherein the thermal coupler comprises an interposer having the plurality of through vias formed therein.

19. The semiconductor device of claim 18 wherein the plurality of through vias are respectively aligned with the first set of connectors.

20. The semiconductor device of claim 18 wherein the first substrate includes a ground plane that is thermally connected to the second integrated circuit.

21. A package comprising:
a first package having a first die formed on a first substrate;
a second package having a second die formed on a second substrate; and
a solid thermal conductor interjacent the first package and second package and thermally coupling the second die to the first substrate, the solid thermal conductor having a plurality of through vias and passing no electrical signals between the first package and the second package;
wherein the second die has a first width and the solid thermal conductor has a second width that is greater than the first width, wherein the second width is about 1.2 to 1.5 times as great as the first width.

22. The package of claim 21 further comprising:
a first set of connector elements electrically coupling the second die to the second substrate;
a second set of connector elements electrically coupling the first die to the second substrate; and
a third set of connector elements configured to electrically couple the second substrate to another electrical circuit.

23. The package of claim 21, wherein the thermal conductor comprises an interposer having the through vias therethrough.

24. The package of claim 23, wherein the first substrate includes a thermally conductive layer and thermal vias, the package further comprising:
a fourth set of connector elements aligned with and coupling the thermal vias to the through vias.

25. The package of claim 21, wherein:
the thermal conductor comprises a heat sink thermally coupled to the second die;
the first substrate includes a thermally conductive layer and thermal vias; and
wherein the second die is thermally coupled to the thermally conductive layer by way of the thermal vias.

26. The package of claim 25 further comprising a thermal interface material interjacent the heat sink and the first substrate.

27. The package of claim 21, wherein the thermal conductor is a thermally conductive material filling a gap between the second die and the first substrate.