PATENT DOCUMENT

Publication Number: US-10470307-B2
Application Number: US-201816016610-A
Country: US
Kind Code: B2

Title: Circuit substrate with embedded heat sink

Abstract:
An apparatus includes a main substrate, a device, and a heat spreader. The main substrate is configured for mounting the device in a mounting location thereon and having a cavity located below the mounting location. The device is mounted in the mounting location, and the heat spreader is fitted into the cavity and coupled to the device and to a heat sink. The heat spreader is configured to conduct heat from the device to the heat sink and to provide electrical insulation between the device and the heat sink.

Claims:
The invention claimed is: 
     
       1. An apparatus, comprising:
 a main substrate, configured for mounting a component in a mounting location thereon and having a first cavity located below the mounting location; 
 a secondary substrate that is coupled to the main substrate and has a second cavity therein at the mounting location; 
 a device, mounted in the second cavity; and 
 a heat spreader comprising a ceramic compound, which is fitted into the first cavity and attached to the secondary substrate, and which is coupled to the device and to a heat sink, wherein the heat spreader is configured to protrude from the first cavity, beyond a surface of the main substrate that faces the heat sink. 
 
     
     
       2. The apparatus according to  claim 1 , wherein the main substrate is configured to provide electrical interconnections to the device. 
     
     
       3. The apparatus according to  claim 1 , wherein the device is configured to emit light. 
     
     
       4. The apparatus according to  claim 1 , wherein the main substrate comprises a printed circuit board (PCB). 
     
     
       5. The apparatus according to  claim 1 , wherein the ceramic compound comprises aluminum nitride (AlN). 
     
     
       6. The apparatus according to  claim 1 , wherein the secondary substrate is configured for mounting a peripheral device. 
     
     
       7. The apparatus according to  claim 6 , wherein the secondary substrate is configured to provide electrical interconnection between the peripheral device and the main substrate. 
     
     
       8. The apparatus according to  claim 6 , wherein the secondary substrate is configured to provide electrical interconnection between the device and the main substrate. 
     
     
       9. A method for production, comprising:
 producing a main substrate having a mounting location for mounting a component thereon and having a first cavity located below the mounting location; 
 coupling a secondary substrate to the main substrate, wherein the secondary substrate has a second cavity therein at the mounting location; 
 mounting a device in the second cavity; and 
 fitting a heat spreader comprising a ceramic compound into the first cavity, attaching the secondary substrate to the heat spreader, and coupling the heat spreader to the device and to a heat sink, wherein the heat spreader conducts heat from the device to the heat sink and protrudes from the first cavity, beyond a surface of the main substrate that faces the heat sink. 
 
     
     
       10. The method according to  claim 9 , wherein producing the main substrate comprises providing on the main substrate electrical interconnections to the device. 
     
     
       11. The method according to  claim 9 , wherein mounting the device comprises mounting a light emitter. 
     
     
       12. The method according to  claim 9 , wherein producing the main substrate comprises producing a printed circuit board (PCB). 
     
     
       13. The method according to  claim 9 , wherein the ceramic compound comprises aluminum nitride (AlN). 
     
     
       14. The method according to  claim 9 , and comprising mounting a peripheral device on the secondary substrate. 
     
     
       15. The method according to  claim 14 , wherein coupling the secondary substrate comprises providing on the secondary substrate electrical interconnection between the peripheral device and the main substrate. 
     
     
       16. The method according to  claim 14 , wherein coupling the secondary substrate comprises providing on the secondary substrate electrical interconnection between the device and the main substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/145,828, filed May 4, 2016, which claims the benefit of U.S. Provisional Patent Application 62/164,612, filed May 21, 2015, whose disclosure is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     Embodiments described herein relate generally to design and production of optoelectronic assemblies, and particularly to methods and systems for reducing thermal resistance in optoelectronic assemblies. 
     BACKGROUND 
     Optoelectronic assemblies are often designed to reduce thermal resistance between high-power devices mounted on the top surface of a substrate and a heat sink coupled to the bottom surface of the substrate. 
     For example, U.S. Pat. No. 6,936,855, whose disclosure is incorporated herein by reference, describes a bendable light emitting diode (LED) array that includes heat spreaders, dielectric material disposed above each heat spreader, and a bendable electrical interconnection layer disposed above these heat spreaders and electrically insulated from these heat spreaders by the dielectric material. At least one via passes through the dielectric material above each heat spreader, and at least one LED die is disposed above each via. 
     U.S. Pat. No. 6,156,980, whose disclosure is incorporated herein by reference, describes a circuit structure and method for conducting heat from a power flip chip. Heat is dissipated from a flip chip mounted to a PCB by conducting heat through conductive vias to the opposite surface of the PCB. The flip chip is equipped with two sets of solder bumps, one of which is registered with conductors on the PCB, while the second is registered with a thermal conductor layer on the PCB surface and electrically isolated from the conductors. 
     SUMMARY 
     An embodiment that is described herein provides an apparatus, including a main substrate, a device and a heat spreader. The main substrate is configured for mounting the device in a mounting location thereon and having a cavity located below the mounting location. The device is mounted in the mounting location, and the heat spreader is fitted into the cavity and coupled to the device and to a heat sink. The heat spreader is configured to conduct heat from the device to the heat sink and to provide electrical insulation between the device and the heat sink. 
     In some embodiments, the main substrate is configured to provide electrical interconnections to the device. In other embodiments, the device is configured to emit light. In yet other embodiments, the main substrate includes a printed circuit board (PCB). 
     In an embodiment, the heat spreader includes aluminum nitride (AlN). In another embodiment, the heat spreader is configured to protrude from the cavity, beyond a surface of the main substrate that faces the heat sink. In yet another embodiment, the apparatus includes a secondary substrate that is coupled to the main substrate and is configured for mounting a peripheral device. 
     In some embodiments, the secondary substrate is configured to provide electrical interconnection between the peripheral device and the main substrate. In other embodiments, the secondary substrate is configured to provide electrical interconnection between the device and the main substrate. 
     There is additionally provided, in accordance with an embodiment that is described herein, a method for production, including producing a main substrate having a mounting location for mounting a component thereon and having a cavity located below the mounting location. A device is mounted in the mounting location. A heat spreader is fitted into the cavity and the heat spreader is coupled to the device and to a heat sink. The heat spreader conducts heat from the device to the heat sink and provides electrical insulation between the device and the heat sink. 
     These and other embodiments will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, pictorial illustration of an optoelectronic assembly, in accordance with an embodiment that is described herein; and 
         FIG. 2  is a schematic side view of an optoelectronic assembly, in accordance with an embodiment that is described herein. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Electronic devices, such as high-power devices comprised in optoelectronic assemblies, are prone to produce undesirable residual heat. The high-power device (e.g., a light emitter) is typically mounted on a main substrate, such as a flexible printed circuit board (PCB). In principle, one may couple a heat spreader to the device so as to conduct the residual heat from the device to a heat sink connected to the back surface of the main substrate. In some cases, however, it is desirable to prevent electrical (Galvanic) connection between the device and the heat sink. For example, in some designs the heat sink is electrically connected to ground, and it is desired to keep the device package electrically floating rather than grounded. 
     Embodiments that are described hereinbelow overcome this limitation by evacuating heat away from the device, while at the same time electrically isolating the device from the heat sink. In some embodiments, the main substrate is patterned so as to form a mounting location for the device. An area below the mounting location, typically as large as or larger than a cross-section area of the device, is cut-out from the main substrate so as to form a cavity. An electrically insulating heat spreader is fitted into the cavity and coupled, at one end, to the back surface of the device and, at the opposite end, to the heat sink. The heat spreader may comprise any suitable material (or a multilayered stack) that spreads the heat without conducting electrical current between the device and the heat sink. The patterned main substrate further comprises electrical interconnections between the device and additional (e.g., peripheral) active and passive devices mounted on or connected to the main substrate. 
     In other embodiments, the assembly may comprise a secondary substrate coupled to the main substrate. The secondary substrate may comprise a cavity within which the high-power device is fitted, and mounting locations for the peripheral devices. The secondary substrate may further comprise electrical interconnections that form electrical paths between the devices and the main substrate. 
     The disclosed techniques can be viewed as separating the thermal path of the device from the electrical path. These techniques may allow, for example, activating a high-power vertical-cavity surface-emitting laser (VCSEL) device, whose package is floating and not grounded. 
     System Description 
       FIG. 1  is a schematic, pictorial illustration of an optoelectronic assembly  20 , in accordance with an embodiment that is described herein. Assembly  20  may be part of any suitable electronic equipment, such as a mobile phone or computing device. Assembly  20  comprises a main substrate, in the present example a flexible multilayer printed circuit board (PCB)  22 , a secondary substrate  30  mounted on PCB  22 , and a heat spreader  28  fitted into a cavity formed in PCB  22 . PCB  22  and heat spreader  28  are described in detail in  FIG. 2  below. 
     In some embodiments, secondary substrate  30  is fabricated from a ceramic compound, such as aluminum oxide. In alternative embodiments substrate  30  is made from any suitable material such as polymer or a mixture of ceramic and polymer materials. 
     Assembly  20  further comprises a high-power optoelectronic device  24 , such as a vertical-cavity surface-emitting laser (VCSEL) array, made from a gallium arsenide (GaAs) chip, mounted directly on heat spreader  28  which is attached to substrate  30  using an adhesive or solder. Heat spreader  28  is used for conducting heat from device  24  to a heat sink (not shown in the figure) located below the heat spreader. In an embodiment, device  24  is mounted directly on heat spreader  28  within a cavity  60  (shown in  FIG. 2 ), using any suitable mounting technique known in the art (such as solder or thermally conductive epoxy). The cross-section of cavity  60  has an area equal to or greater than a footprint of device  24 , so as to accommodate device  24 . 
     Assembly  20  may comprise active and passive peripheral devices that enable controlling the optoelectronic assembly and interfacing with external systems. Conductive pads  36  are patterned on substrate  30  in close proximity to device and additional devices, such as a surface mounting technology (SMT) device  26  that may serve, for example, as a controller of device  24 . Devices  24  and  26  are electrically connected to pads  36  via one or more wires  32 , using wire bonding or any other suitable techniques known in the art. For example, device  26  may be connected to substrate  30  and PCB  22  through bumps and redistribution layers (RDLs) (not shown) using flip-chip mounting technology. 
     Pads  36  are typically fabricated from copper (or another suitable conductive material optimized for wire bonding) and configured to provide electrical interconnections between the devices and PCB  22  via substrate  30 . The electrical interconnections are depicted in  FIG. 2  and described in detail below. In an embodiment, walls  41 - 44 , typically formed by punching and co-firing methods used in multi-layer ceramic (aluminum oxide) substrate technology, define a cavity  40  (shown in  FIG. 2 ) in substrate  30 . Cavity  40  defines a domain around the devices (e.g., devices  24  and  26 ) and typically is not covered so as to increase the amount of light emitted from assembly  20 . Typically, additional optical elements (not shown) are assembled on top of cavity  40  so as to focus the emitted light as desired in the application. 
     The embodiments described in  FIG. 1  describe PCB  22 , which is referred to herein as a main substrate of assembly  20 . The main substrate may alternatively comprise any suitable substrate such as organic (e.g., conducting and insulating polymers), inorganic (e.g., ceramic, glass), hybrid (e.g., polymer/ceramic, polymer/metal) or any combination of the above. Assembly  20  further comprises substrate  30 , which is referred to herein as a secondary substrate, on which components of the optoelectronic assembly (e.g., devices  24  and  26 ) are mounted. In the embodiments described herein substrate  30  is ceramic. Alternatively, however, the secondary substrate may comprise any other suitable material. 
     Thermal and Power Management in Power Devices 
     High-power devices, such as device  24 , typically produce a considerable amount of heat in assembly  20 . Heat management may be carried out using a heat spreader coupled to the back side of the high-power device. In some cases, the heat spreader may also provide electrical interconnection between the power device and the heat sink. The disclosed techniques allow independent management of the thermal path and the electrical path in assembly  20 , thereby enabling sinking the heat from device  24  without necessarily grounding the device. 
       FIG. 2  is a schematic side view of an optoelectronic assembly  80 , in accordance with an embodiment that is described herein. Assembly  80  may serve, for example, as assembly  20  of  FIG. 1  above. In some embodiments, assembly  80  comprises separate electrical and thermal paths from device  24 . An electrical path connects between device  24  and PCB  22 , and is separate from a heat sinking (thermal) path between device  24  and an external heat sink (not shown) located below heat spreader  28 . 
     Referring to the electrical path, which is configured to route electrical signals and electrical power supply between PCB  22 , via substrate  30 , and device  24 . Device  24  receives electrical signals and electrical power supply from PCB  22  through one or more vias  38 , pads  36  and wires  32 , all made from copper, or another suitable electrical conductor, and configured to conduct electrical current. Flexible PCB  22  comprises alternating layers of patterned metal, such as copper  48 , and a flexible dielectric material, such as polyimide (PI)  46 . Such a layered structure provides PCB  22  with mechanical flexibility. Electrical vias  49  etched in PI  46  and filled with conductive material (e.g., copper), and are configured to interconnect between copper layers  48 . The fabrication process of PCB  22 , including the formation of cavity  50 , are described below. 
     A patterned layer  51  located on the top surface of PCB  22  is typically made from copper and configured to connect PCB  22  with vias  38 . The electrical path may be applied to electrically connect between PCB  22  and additional devices, such as device  26  comprised in assembly  80 . 
     Referring to the thermal path formation process. Cavity  50  is formed by cutting out a section of PCB  22  located directly below device  24 . Cavity  50  is as large as or larger than the cross-sectional area of heat spreader  28  on which device  24  or any alternative device (e.g., a light emitting diode) that is to be mounted thereon. Heat spreader  28  is fitted into cavity  50  so as to conduct to heat from device  24  to the heat sink (not show) located directly below the heat spreader, thereby forming the thermal path. 
     After layers  46 ,  48 ,  51  and vias  49  have been patterned, the layers are fused together using a conventional lamination process. Cavity  60  and vias  38  are formed in substrate  30  using patterning methods known in the art. Vias  38  are filled with conductive material and pads  36  are formed using any suitable plating and patterning processes known in the art. 
     Typically, heat spreader  28  and devices  24  and  26  are mounted and wire bonded after substrate  30  and PCB  22  are completed. The sequence of the device mounting may vary to comply with design and process technology constraints. In some embodiments, heat spreader  28  is fitted within cavity  50  directly below the mounting location of device  24  in substrate  30 . Heat spreader  28  may comprise any suitable material, or stack of materials, with desired mechanical and thermal properties. 
     In an embodiment, heat spreader  28  is fabricated from a single bulk of a ceramic compound, such as aluminum nitride (AlN), which is thermally-conductive and electrically-insulating. In other embodiments, heat spreader  28  may comprise a stack of layers made from materials that provide thermal-conductance and electrical-insulation. The choice of an electrically-insulating or electrically-conductive heat spreader depends, inter alia, on whether or not electrical isolation is required. In an alternative embodiment, heat spreader  28  may be made of an electrically conductive material, such as copper, in case electrical insulation is not required. 
     Device  24  is fitted into cavity  60  and coupled (e.g., glued or soldered) to the top surface of heat spreader  28 , for example, using thermally conductive adhesive or solder (not shown). SMT devices  26  are typically mounted sequentially before or after device  24 , using pick and place techniques or other methods known in the art. Next, all the mounted devices are connected to pads  36  using wires  32  or any alternative wire bonding or bumping techniques known in the art. 
     In an alternative mounting sequence, device  24  may be fitted into cavity  60  first (typically sequentially with devices  26 , during the picking and placing of all devices on substrate  30 ), and heat spreader  28  may be fitted into cavity  50  and coupled to device  24  followed by the wire bonding process described above. 
     The cross-sectional dimensions of heat spreader  28  are typically 3×2 mm and the spreader is about 0.5 mm thick, but larger or smaller dimensions may alternatively be used depending, for example, on application requirements and the dimensions of device  24 . As shown in  FIG. 2 , it can be advantageous to make the embedded heat spreader thick enough to protrude below the lowest PCB layer, in order to facilitate coupling the external heat sink (not shown) to the heat spreader. 
     Although  FIG. 2  shows a particular flexible PCB design and configuration with a cavity for insertion of heat spreader  28 , other designs implementing the principles of this embodiment using different sorts of PCB and heat spreader materials, as well as other geometrical configurations, will be apparent to those skilled in the art after reading the present disclosure and are considered to be within the scope of the present invention. 
     Although the embodiments described herein mainly address optoelectronic assemblies, the methods and systems described herein can also be used in other applications, such as in any type of electronic assembly comprising any type of device that dissipates a large amount of heat. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and that the following claims are not limited to what has been particularly shown and described hereinabove. Rather, the scope includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.

Metadata:
Filing Date: 20180624
Publication Date: 20191105
Grant Date: 20191105
Priority Date: 20150521
Inventors: PYPER, DENNIS R.
RAJU, VENKATARAM R.
ALNAHHAS, YAZAN Z.
Assignee: APPLE INC
CPC Classifications: [{"code": "H05K1/141", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05K3/368", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/183", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/0204", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K2201/10416", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/0204", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/183", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/021", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K1/021", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K2201/10416", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K3/368", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/141", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05K1/144", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/144", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/144", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K2201/10416", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/141", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05K1/183", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/021", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K3/368", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K1/0204", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 57326076