WO2019034988A1

WO2019034988A1 - Air cavity package with improved thermal conductivity

Info

Publication number: WO2019034988A1
Application number: PCT/IB2018/056097
Authority: WO
Inventors: Raymond S. Bregante; Ger Reuvers; Alex ELLIOT; John Ni; David Lam
Original assignee: Rjr Technologies, Inc.
Priority date: 2017-08-15
Filing date: 2018-08-13
Publication date: 2019-02-21

Abstract

The present disclosure is directed to an air cavity package with increased thermal conductivity and capable of withstanding higher reworking temperatures comprising a flange, a leadframe, and a lower ring but no upper ring. In some embodiments, an upper ring and a lower ring are present. The rings may comprise polymers such as LCP, metals such as copper, and other suitable materials. The lower ring may have a reduced thickness to increase thermal conductivity. The air cavity package with increased thermal conductivity may comprise a leadframe adhered directly to a flange. The leadframe may further comprises a copper layer adhered directly to the flange and the flange may be adhered directly to the copper layer with a solder.

Description

AIR CAVITY PACKAGE WITH IMPROVED THERMAL CONDUCTIVITY

Inventors: RAYMOND S. BREGANTE, GER REUVERS, ALEX ELLIOT, JOHN

Nl, DAVID LAM

[0001 ] The present disclosure relates generally to air cavity packages with structure and mechanisms that have improved thermal conductivity for the removal of heat.

BACKGROUND

[0002] Electronic devices are ubiquitous in consumer and commercial products and devices throughout the world. Many include circuitry which are often comprised of materials such as silicon, gallium arsenide, and other similar "semi-conductor" materials, and are commonly referred to in industry as "dies" or "chips." Any given die may be capable of containing a multitude of circuit elements for performing various functions. In use, these dies are often incorporated into packages known as air cavity packages (ACPs) generally comprised of a housing surrounding a volume for containing the dies and various electrical components that provide for a variety of functions. These air cavity packages are sometimes classified based on their reliability and/or performance, as well as whether they are fully, near, or non-hermetic.

[0003] Some conventional ACPs, particularly in RF packages, have dies that generate substantial heat when they are biased. The output lead(s) in particular can become very hot and lead to reliability issues if the temperature is not controlled or removed by dissipating the heat through a cooling feature such as a heatsink. Additionally, new and more demanding ACP manufacturing processes require the application of increasingly higher temperatures, sometimes exceeding 300°C (e.g., 320°C and higher die soldering). Similarly, there is a need to remove the heat generated at such temperatures.

[0004] Thus, for some ACPs, particularly those where the heat cannot be removed from the leads via a heatsink, alternative mechanisms and designs for removing the heat are desired.

SUMMARY

[0005] While the ways in which the present disclosure address the disadvantages of the prior art will be discussed in greater detail below, in general, the present disclosure is directed to air cavity packages with increased thermal conductivity and that are capable of withstanding higher reworking temperatures. Some ACPs of the present disclosure comprise a flange, a leadframe, and a lower ring but no upper ring, though in accordance with other aspects of the present disclosure, both an upper ring and a lower ring are present and within the scope of the present disclosure.

[0006] In accordance with various aspects of the present disclosure, the rings may comprise various types of polymers such as a liquid crystal polymer (LCP), ceramic(s), metals such as copper, and other suitable materials. In accordance with various aspects, the lower ring has a reduced thickness to increase thermal conductivity between the flange and the leadframe.

[0007] In accordance with various aspects of the present disclosure, the air cavity package with increased thermal conductivity may comprise a leadframe adhered to a flange with an electrically insulative layer (e.g., polyimide or the like) therebetween. The leadframe may further comprises a copper layer (or other suitable metal) adhered to the flange and the flange may be adhered to the copper layer with a polyimide layer, solder, epoxy, glue and or any other suitable adhesive or attachment method now known or as yet unknown.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure, and together with the description serve to explain the principles of the disclosure, wherein like numerals denote like elements and wherein:

[0009] Figure 1 is a cross-sectional view of a prior art air cavity package with a conventional flange, leadframes and molded rings;

[0010] Figures 2A and 2B are close-up cross-sectional views of a prior art air cavity package;

[001 1 ] Figures 3A and 3B are close-up cross-sectional views of an air cavity package with no upper ring;

[0012] Figures 3C and 3D are close-up cross-sectional views of another air cavity package with no upper ring ;

[0013] Figures 4A and 4B are close-up cross-sectional views of an air cavity package with no upper ring and a metal lower ring;

[0014] Figures 4C and 4D are close-up cross-sectional views of another air cavity package with no upper ring and a metal lower ring;

[0015] Figures 5A and 5B are close-up cross-sectional views of an air cavity package with no upper ring and a molded lower ring with a recess for a flange projection;

[0016] Figures 5C and 5D are perspective views of flanges with flange projections; [0017] Figures 6A and 6B are close-up cross-sectional views of an air cavity package with no upper ring or lower ring;

[0018] Figures 6C and 6D are close-up cross-sectional views of another air cavity package with no upper ring or lower ring;

[0019] Figures 6E is close-up cross-sectional view of another air cavity package with no upper ring or lower ring and a metal cap;

[0020] Figures 7 A and 7B are close-up cross-sectional views of another air cavity package with no upper ring or lower ring; and

[0021 ] Figures 8A through 8H are close-up cross-sectional views of various embodiments of air cavity packages with various ring, flange and leadframe configurations.

DETAILED DESCRIPTION

[0022] Persons skilled in the art will readily appreciate that various aspects of the present disclosure can be realized by any number of structures, components, and systems configured to perform various functions disclosed herein. Stated differently, other such structures, components, and systems can be incorporated herein to perform the intended functions. It should also be noted that the accompanying drawing figures referred to herein are not all necessarily drawn to scale, and may be exaggerated to illustrate various aspects of the present disclosure, and in that regard, the drawing figures should not be construed as limiting.

[0023] As noted above, the high temperatures encountered during manufacturing or "reworking" and/or resulting from the output of leads of certain ACPs can be detrimental as such temperatures may impact the resistance to failure of the ACP and its components (e.g., solder joint failure) and/or the performance of the components of the ACP. Moreover, as noted above, many conventional packages, particularly RF packages, have dies that generate heat when under power, exacerbating these effects. Accordingly, in accordance with various aspects of the present disclosure, the design of an ACP is adapted to provide improved thermal conductivity (e.g., greater than 2 W/mk) for transferring heat from the flange to the leadframes and out of the ACP. For example, the rings, leadframes, and flange components the ACP may be configured to enhance thermal conductivity and facilitate the removal of heat from the ACP.

[0024] The forgoing being noted, with reference to Figures 1 and 2A (unassembled) and 2B (assembled), an illustration of components of a conventional ACP 100 is shown. The illustrated ACP 100 includes a flange 102, leadframes 104, and a molded ring 106, which may be comprised of two pieces, an upper ring 106a and a lower ring 106b. Prior art molded rings 106 generally comprise conventionally known polymers and similar materials with generally low thermal conductivity, particularly with conventionally known shapes and geometries. The components of the ACP 100 may be bonded to one another with conventionally known or as yet unknown epoxies and/or adhesives 1 10 and/or films such as a variety of epoxies and die adhesive films (DAF), as well as various solders or other bonding materials. Within the molded ring 106 are one or more of a variety of dies 108 such as those described above. The illustrated conventional ACP 100 may suffer from various limitations related to thermal conductivity and heat transfer between the flange 102 and the leadframe 104 by virtue of the relatively low thermal conductivity of the molded ring 106. This can also limit the upper bounds of the rework temperatures used during assembly of the ACP 100. [0025] In contrast, using specifically configured designs of the rings, leadframes, and flange components as disclosed herein, ACPs have enhanced thermal conductivity between the flange and leadframes, increasing the efficiency in removing heat from the package. In accordance with various embodiments and as described herein, the ACPs may comprise any number of components, such as two- piece, three-piece, four-piece or more, and may comprise various combinations having upper and lower rings, one or the other, or neither upper and lower rings. Moreover, the materials of the various ACP components may be varied amongst the various embodiments described herein, as well as those not described but nonetheless falling within the scope of those described herein.

[0026] For example, with reference to Figures 3A (unassembled) and 3B (assembled), an ACP 300 with only a lower ring 306b (no upper ring 306a) adhered to a flange 302 and a leadframe 304 with an adhesive 310 such as epoxy, DAF or other suitable adhesive in accordance with the present disclosure is illustrated. The lower ring 306b may be a molded LCP. By eliminating the upper ring 306a, there is notable improvement in the upper limits of the rework temperatures (i.e., 315°C and higher).

[0027] Alternatively, Figures 3C (unassembled) and 3D (assembled) illustrate a configuration similar to that illustrated in Figures 3A and 3B, but without DAF between the leadframe 304 and the lower ring 306b. In such an embodiment, the lower ring 306b and the leadframe 304 may be adhered by FAM molding. Again, by eliminating the upper ring 306a, there is notable improvement in the upper limits of the rework temperatures. In accordance with various aspects of the present disclosure, an opening in the leadframe 304 may be provided function as a mold lock and for connecting a top ring (when present) to the lower ring 306b. [0028] With reference now to Figures 4A (unassembled) and 4B (assembled), in addition to increasing the rework temperatures by removing an upper ring, the overall thermal conductivity can be increased by using a lower ring comprised of a material with a high thermal conductivity. For example, a copper ring 406 results in a very high thermal conductivity, though other metals and materials with conductivity higher than typical molded polymer rings may likewise be used and fall within the scope of the present disclosure.

[0029] In the presently described embodiment, the copper ring 406 may be adhered to the leadframe 404 and the flange 402 by any now known or as yet unknown adhesives 410, including epoxies or DAF. The copper ring 406 may be configured with various shapes such as a surface are maximizing profile to increase bonding strength and/or to increase thermal conductivity (i.e., by increasing the surface area). For example, the copper ring 406 may comprise steps 412. Upon assembly, the adhesive 410 flows around the steps 412 to fill the void left by the steps 412 and increase the bond strength. An ACP 400 such as the forgoing thus demonstrates very high increases in both rework temperatures and thermal conductivity.

[0030] With reference now to Figures 4C (unassembled) and 4D (assembled), similar to the embodiment of Figures 4A and 4B, this embodiment also does not use an upper ring and also uses a copper ring 406 to result in very high rework temperatures and very high thermal conductivity. As illustrated in Figures 4C and 4D, however, the copper ring 406 is adhered to the flange 402 using a solder 414 such as 360°C solder. In accordance with some aspects of the present disclosure, the solder 414 may be screen printed on the copper ring 406 for design controlled flow-out, though other methods of solder application now known or as yet unknown may be used. As with the embodiment described in connection with Figures 4A and 4B, the copper ring 406 may be configured with various shapes to increase bonding strength and/or to increase thermal conductivity (i.e., by increasing surface area). An ACP 400 such as the forgoing thus demonstrates very high increases in both rework temperatures and thermal conductivity. In accordance with various aspects of the present disclosure, a solder wetting barrier 409 may be provided in the flange to act as a barrier for solder wetting and to define the solder surface.

[0031 ] In various embodiments, the flange and the lower ring may both comprise a high conductivity material such as copper with an insulative material between the leadframe and the flange. Still further, in various embodiments such as that disclosed in Figure 6E, the upper ring may be eliminated with a lid adhered to the leadframe.

[0032] With reference now to Figures 5A (unassembled) and 5B (assembled), in addition to increasing the rework temperatures by removing an upper ring, the overall thermal conductivity can be increased by using a molded lower ring with a configuration that reduces the thickness (T) of the ring material in an area proximate an interface between the flange 502 and the leadframe 504. For example, in the presently described embodiment, the molded lower ring 506b may be molded or otherwise machined with a recess 515 configured to at least partially surround a flange projection 516 extending from the top surface of the flange 502. With momentary reference to Figures 5C and 5D, perspective views of flanges with flange projections 516 are illustrated. Because the thickness of the molded lower ring 506b is reduced, for example, from about 0.5mm/20 mils to about 0.1 mm/4 mils, the thermal conductivity between the leadframe 504 and the flange 502 is increased. Other thicknesses of the molded lower ring 506b may be appropriate depending on the particular use or application

[0033] Moreover, in addition to increasing the thermal conductivity, by selecting certain shapes, the molded lower ring 506b may increase bonding strength by increasing surface area. For example, the recess 515 and the flange projection 516 increase the surface area for bonding (e.g., with solder, epoxy or DAF) between the molded lower ring 506b and the flange 502. An ACP 500 such as the forgoing thus demonstrates high increases in both rework temperatures and thermal conductivity.

[0034] With reference now to Figures 6A (unassembled) and 6B (assembled), similar to embodiments described above, this embodiment also does not use an upper ring. Additionally, this embodiment also does not use a lower ring. Instead, the leadframe 604 comprises a copper layer 620 (such as a copper clad or solid copper layer) adhered to the leadframe by a polyimide layer 618. The copper layer 620 may be single or double sided. The copper layer 620 and polyimide layer 618 may be in the range of about 25 microns thick, though the thickness may vary depending on the application. The copper layer 620 may be adhered to the flange 602 by a solder 622 such as 360°C solder. The solder 622 may be screen printed on the copper layer 620 or the flange 602 for design controlled flow-out, though other methods of solder application now known or as yet unknown may be used. As shown in Figure 6D, the flange 602 may be made thicker to compensate for the lack of a lower ring. For example, the flange 602 may have a flange thickness between about 0.5mm/20 mils and 2.0 mm/80 mils, though other thicknesses may be appropriate depending on the particular use or application. An ACP 600 such as the forgoing thus demonstrates very high increases in both rework temperatures and thermal conductivity. The ACP 600 is also near hermetic.

[0035] With reference now to Figures 6C (unassembled) and 6D (assembled), similar to the embodiment of Figures 6A and 6B, this embodiment also does not use an upper ring or a lower ring and similarly, the leadframe 604 comprises a copper layer 620 adhered to the leadframe by a polyimide layer 618. The copper layer is again adhered to the flange 602 by a solder 622 such as 360°C screen printed on the copper layer 620 or the flange 602 for design controlled flow-out, though other methods of solder application now known or as yet unknown may be used. In the presently described embodiment, a solder wetting barrier 624 in flange 602 may be provided. Figure 6E illustrates an ACP 600 with a metal cap 626 adhered thereto with conventionally known or as yet unknown adhesives 628 and films such as epoxies and DAF. An ACP 600 such as the forgoing thus demonstrates very high increases in both rework temperatures and thermal conductivity, and further, are also near hermetic.

[0036] With reference now to Figures 7 A (unassembled) and 7B (assembled), similar to the embodiment of Figures 6A, 6B, 6C and 6D, this embodiment also does not use an upper ring or a lower ring. However, this embodiment also does not comprise a copper layer or polyimide adhesives. Rather, a layer of glue 726 is used to adhere the leadframe 704 to the flange 702 in conjunction with an epoxy 710. In this embodiment, solder wetting barriers 728 in flange 702 may be provided. An ACP 700 such as the forgoing thus demonstrates very high increases in both rework temperatures and thermal conductivity. The ACP 700 is also near hermetic.

[0037] With reference now to Figure 8A, an ACP 800 design in accordance with the present disclosure is illustrated showing a clamped leadframe 804 wherein the overall thermal conductivity can be increased by using a molded lower ring 806 with a thickness (T) that has been reduced, for example by a factor of two or more, between the flange 802 and the leadframe 804. Additionally, the flange 802 is configured with an elevated portion 830 to raise the flange 802 while retaining the die at the same level while the elevated portion 830 of the flange 802 compensates for the lack of the reduced thickness of the molded lower ring 806 in that area. Because the thickness (T) of the molded lower ring 806b is reduced, the thermal conductivity between the leadframe 804 and the flange 802 is increased.

[0038] With reference to Figure 8B, in another embodiment, the lower ring 806b is configured with a lower ring recess 807 thus causing the thickness (T) of the ring 806b at the point of adhesion to the flange 802 to be thinner. The leadframe 804 has a leadframe depression 805 (e.g., created by stamping or the like) which corresponds to the lower ring recess 807 and the upper ring 806a has an upper ring projection 809 which corresponds to the shape of the leadframe 804 proximate the leadframe depression 805 so that the lower ring 806b, the leadframe 804, and the upper ring 806a correspond to one another upon assembly. Because the thickness (T) of the lower ring 806b is reduced between the leadframe 804 and the flange 802, the thermal conductivity between the leadframe 804 and the flange 802 is increased.

[0039] With reference now to Figure 8C, in another embodiment, the flange 802 is configured with a flange protrusion 803 and the lower ring 806b is configured with a lower ring recess 801 corresponding to the flange protrusion 803, thus causing the thickness (T) of the ring 806b at the point of adhesion to the flange 802 to be thinner, thereby increasing thereby increasing the thermal conductivity between the leadframe 804 and the flange 802. [0040] With reference now to Figure 8D, in another embodiment, the leadframe 804 is bent so it is closer to the flange 802, with a gap 813 in the leadframe 804 functioning as a mold lock. Thus, the thermal conductivity between the leadframe 804 and the flange 802 is increased.

[0041 ] With reference now to Figure 8E, in another embodiment similar to that of Figure 8C, the lower ring 806b is configured with a lower ring recess 801 . However, instead of a flange protrusion, a molded insert 81 1 comprising a metal (or other material more thermally conductive than the material of the lower ring 806b) corresponding to the lower ring recess 801 and extending at least partially therefrom is provided. The molded insert 81 1 may be press fit or otherwise adhered to the flange 802 and by virtue of the reduced thickness (T) of the lower ring 806b proximate the leadframe 804 and the thermal conductivity of the molded insert 81 1 , the overall thermal conductivity is increased. Figure 8F illustrates an embodiment similar to that of 8E, but the molded insert 81 1 has a stepped shape which, as discussed above, may further increase surface area and thermal conductivity.

[0042] With reference now to Figure 8G, in another embodiment, similar to the embodiment of Figure 8A, the thickness (T) of the lower ring 806b is reduced and the flange 802 is configured with an elevated portion 830, thereby increasing the thermal conductivity between the leadframe 804 and the flange 802. Additionally, the flange 802 has an inner recess 832 and the inner end of the leadframe 804 is bent towards the inner recess 832 so that the leadframe 804 is closer to the flange 802 in that area.

[0043] With reference now to Figure 8H, the thickness (T) of the lower ring 806b is reduced in only a portion 834 proximate the interior of the lower ring 806b, thereby increasing the thermal conductivity between the leadframe 804 and the flange 802. Additionally, the inner end of the leadframe 804 is bent back in upon itself to fill the volume left by the thinner portion of the lower ring 806b and thereby increasing the thermal conductivity between the leadframe 804 and the flange 802.

[0044] Finally, the foregoing description emphasizes particular embodiments and examples of the contemplated disclosure. However, as those skilled in the art will recognize, however, the scope of the present disclosure extends as well to variations and modifications of the above, in terms of materials, operating conditions, operating procedures, and other parameters and their components and of procedures for their assembly.

[0045] Likewise, numerous characteristics and advantages have been set forth in the preceding description, including various alternatives together with details of the structure and function of the methods and systems described herein. The disclosure is intended as illustrative only and as such is not intended to be exhaustive. It will be evident to those skilled in the art that various modifications may be made, especially in matters of order, process, structure, elements, components, and arrangement including combinations of the same within the principles of the disclosure, to the full extent indicated by the broad, general meaning of the terms in which the appended claims are expressed. To the extent that these various modifications do not depart from the spirit and scope of the appended claims, they are intended to be encompassed therein.

Claims

CLAIMS We claim:

1 . An air cavity package with increased thermal conductivity comprising:

a flange;

a leadframe; and

a lower ring and no upper ring.

2. The air cavity package of claim 1 , wherein the lower ring comprises copper.

3. The air cavity package of claim 2, wherein the lower ring is adhered to the flange using a solder.

4. The air cavity package of claim 3, wherein the solder is screen printed on the lower ring.

5. The air cavity package of claim 1 , wherein the lower ring comprises a polymer.

6. The air cavity package of claim 5, wherein the lower ring has a thickness in an area proximate an interface between the flange and the leadframe between about

0.5mm/20 mils and about 0.1 mm/4 mils.

7. The air cavity package of claim 1 , further comprising an adhesive between the flange and the lower ring.

8. The air cavity package of claim 7, wherein the adhesive is at least one of DAF and epoxy.

9. The air cavity package of claim 1 , wherein the lower ring is configured with a surface area maximizing profile.

10. The air cavity package of claim 1 , wherein the lower ring further comprises a recess and the flange further comprises a flange projection, wherein the recess configured to at least partially surround the flange projection.

1 1 . The air cavity package of claim 10, wherein the lower ring has a thickness in an area proximate an interface between the flange and the leadframe between about 0.5mm/20 mils and about 0.1 mm/4 mils.

12. An air cavity package with increased thermal conductivity comprising a leadframe adhered directly to a flange.

13. The air cavity package of claim 12, wherein the leadframe further comprises a copper layer adhered directly to the flange.

14. The air cavity package of claim 13, wherein the flange is adhered directly to the copper layer with a solder, epoxy or adhesive.

15. The air cavity package of claim 13, wherein the copper layer is adhered directly to the leadframe with at least one of DAF and an epoxy.

16. The air cavity package of claim 13, wherein the copper layer is adhered to the leadframe with polyimide.

17. The air cavity package of claim 12, wherein a flange thickness is about 0.5mm/20 mils and 2.0 mm/80 mils.

18. The air cavity package of claim 12, further comprising a metal cap.

19. An air cavity package with increased thermal conductivity comprising:

a flange;

a leadframe;

a lower ring; and

an upper ring.

20. The air cavity package of claim 19, wherein the lower ring and the upper ring comprise at least one of a polymer and a ceramic.

21 . The air cavity package of claim 19, wherein the lower ring has a thickness in an area proximate an interface between the flange and the leadframe between about 0.5mm/20 mils and about 0.1 mm/4 mils.

22. The air cavity package of claim 19, further comprising an adhesive between the flange and the lower ring.

23. The air cavity package of claim 22, wherein the adhesive is at least one of DAF and epoxy.

24. The air cavity package of claim 19, wherein the lower ring is configured with a surface area maximizing profile.

25. The air cavity package of claim 19, wherein the lower ring further comprises a lower ring recess, the upper ring comprises an upper ring projection, and the leadframe comprises a leadframe depression, wherein the lower ring recess, the leadframe depression, and the upper ring projection correspond to one another upon assembly.

26. The air cavity package of claim 19, wherein the lower ring further comprises a lower ring recess corresponding to a flange protrusion.

27. The air cavity package of claim 19, further comprising a molded insert and wherein the lower ring further comprises a lower ring recess corresponding to the molded insert.

28. The air cavity package of claim 19, wherein an inner end of the leadframe is bent towards an inner recess.

29. The air cavity package of claim 19, wherein an inner end of the leadframe is bent back upon itself.

30. The air cavity package of claim 19, wherein the lower ring has a thickness in an area proximate an interface between the flange and the leadframe between about 0.5mm/20 mils and about 0.1 mm/4 mils.