# Patent

## Title
Graphitic substrates with ceramic dielectric layers

## Abstract

Different kinds of printing pastes or inks are utilized in various combinations to develop multiple ceramic dielectric layers on graphitic substrates in order to create effective dielectric ceramic layers that combine good adhesion to both graphitic substrates and printed copper traces, and strong insulating capability. The pastes or inks may comprise a high thermal conductivity powder.

## Background
This application claims priority to U.S. Provisional Application Ser. No. 61/603,479, which is hereby incorporated by reference herein.


TECHNICAL FIELD
This application relates in general to electronic circuitry, and in particular, to conductivity of heat in electronic circuitry.
BACKGROUND INFORMATION
Thermal management materials with high thermal conductivity, high thermal diffusivity, machineability and low coefficient of thermal expansion (“CTE”) at low cost are desirable. For many electronic applications it would be beneficial if the material was not electrically conductive so that electronic components could be assembled directly onto the high thermal conductivity material. However, materials with high thermal conductivity are also typically electrically conductive. For example, carbon-based materials, such as graphite and graphene, typically have high thermal conductivity and are also electrically conductive. It would be desirable to have high thermal conductivity ceramic dielectric layers on graphitic substrates (i.e., a pure graphite substrate or a graphite-metal composite substrate) for fabrication of a high-performance circuit board through printing copper (“Cu”) circuit traces on the top of the ceramic dielectric layers.
Besides providing high thermal performance and sufficient dielectric strength, the dielectric layers should also possess a strong, adhesion with graphitic substrates, be compatible with Cu paste printing and curing processes that require a curing temperature over 400° C., and possess a strong adhesion with the Cu printed circuit traces.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an embodiment of the present invention;
FIG. 2 illustrates a digital image of copper traces printed on a wafer structure as illustrated in FIG. 1, showing that there is not any delamination after a taping test;
FIGS. 3A-3B show digital images of a circuit board configured with a wafer structure as illustrated in FIG. 1 with an LED soldered thereon;
FIG. 4 illustrates an embodiment of the present invention;
FIGS. 5A-5B show digital images of as circuit board configured in accordance with the embodiment illustrated in FIG. 4 with LEDs soldered thereon; and
FIG. 6 shows a digital image of a RFID circuit board configured with a wafer structure in accordance with the embodiment illustrated in FIG. 4, with a size of 85 mm×70 mm.



DETAILED DESCRIPTION
In order to create effective dielectric ceramic layers that combine good adhesion to both graphitic substrates and printed Cu, and strong insulating capability, multiple ceramic layer structures on graphitic substrates are implemented in embodiments of the present invention.
Generally, compared with polymeric dielectric layers (e.g., epoxy, silicone, polyimide), ceramic-type dielectric layers (e.g., made with ceramic powders, such as aluminum nitride (“AlN”), boron nitride (“BN”), silicon carbide (“SiC”), etc.) have much higher thermal conductivity and a lower CTE that matches well with typical semiconductor materials (e.g., Si, GaN, GaAs).
In embodiments of the present invention, different kinds of printing pastes or inks are utilized in various combinations, some of which are more specifically described herein, to develop multiple ceramic dielectric layers on graphitic substrates. The pastes or inks may comprise a high thermal conductivity powder (e.g., AlN, BN, SiC, or graphite micron powders) and a nonorganic binder. Water may be used as a vehicle to adjust the viscosity. The pastes or inks are further described in Table 1.











TABLE 1







Composition
Advantages
Disadvantages



















AlN paste
AlN powders +
Good adhesion to
Poor adhesion to


or ink
Lithium polysilicate
printed Cu traces (no
graphitic substrate



binder + vehicle
delamination occurred
(delamination occurred



(e.g., water)
during the taping test).
during the taping test).




Intrinsically had good




thermal conductivity




and insulating properties.


BN paste
BN powders +
Intrinsically had good
Poor adhesion to both


or ink
monoaluminum
thermal conductivity
graphitic substrate and Cu



phosphate binder +
and insulating properties
traces (delamination occurred



vehicle (e.g., water)

during the taping test).


SiC paste
SiC powders +
Good adhesion to both
Weak instating capability


or ink
monoaluminum
graphitic substrate and
(i.e., a weak electrically



phosphate binder +
Cu printed traces (no
insulating capability)



vehicle (e.g., water)
delamination occurred




during the taping test).




Intrinsically had good




thermal conductivity.


Graphite paste
Graphite powders with
Good adhesion to
Reacted with Cu


or ink
Al2O3 addition +
graphitic substrate (no
printed traces during



silicate binder +
delamination occurred
Cu curing process.



vehicle (e.g., water)
during the taping test).
Electrically conductive




Intrinsically had a
material and cannot be




good thermal
used as the sole




conductivity.
insulating layer.









A SiCl/BN/SiC/Graphitic-waier structure 100 in accordance with embodiments of the present invention is illustrated in cross-section in FIG. 1. As indicated in Table 1, the bottom SiC layer 102, possesses a strong adhesion to the graphitic wafer 101, the middle BN layer 103 offers sufficient electrical insulating strength, and the top SiC layer 104 enhances the adhesion to the Cu traces 105. Each of the layers 102-105 may he deposited by ink or paste printing or spraying techniques well known in the art.
FIG. 2 shows a LED circuit board made according to this SiC/BN/SiC/Graphitic-wafer design 100. From FIG. 2, it can he seen for both the ceramic layers and Cu traces, no delamination occurs after a taping test (an adhesive tape is pressed on the layer or trace and pulled off; no peeled off Cu or ceramic fragments were left on the tape), indicating a good adhesion strength of this structure.
The Cu traces 105 on the ceramic dielectric layer are configured to be solderable. As shown in FIGS. 3A-3B, a chip can be tightly soldered onto the Cu traces (e.g., using lead-free tin solder). FIG. 3B shows that the structure was successful by the activation of the LED. Note that the lighter colored layer in FIGS. 3A-3B is the middle BN layer 103.
An AlN/Graphite/Graphitic-wafer structure 400 in accordance with embodiments of the present invention is illustrated in cross-section in FIG. 4. The bottom graphite layer 402 may be printed or sprayed on the graphitic wafer 401. Using a diluted graphite paste may provide, a stronger adhesion to the graphitic water. The AlN layer 403 may be printed or sprayed onto the graphite layer 402. The copper (“Cu”) traces 405 may be printed or sprayed onto the AlN layer 403.
FIGS. 5A-5B show a LED circuit hoard manufactured based on the foregoing AlN/Graphite/Graphitic-wafer 400. No delamination occurred after a taping test, and also the LED chips can be tightly soldered onto the Cu traces 405. FIG. 5B shows that the structure assembly was successful by the activation of the LEDs.
FIG. 6 shows a RFID circuit board built on an AlN/Graphite/Graphitic-wafer substrate 400, wherein two RFID patterned circuits 405 were printed on the top of the AlN layer 403.

## Claims

1. A structure for electrical circuitry comprising:
electrically conducting traces;
a graphitic substrate; and
a plurality of layers separating the electrically conducting traces from the graphitic substrate, wherein the plurality of layers are selected from the group consisting of:
(i) a plurality of different ceramic dielectric layers,
(ii) a graphitic layer and a ceramic dielectric layer, and
(iii) a graphitic layer and two or more different ceramic dielectric layers,

wherein the plurality of layers are a first silicon carbide layer deposited onto the graphitic substrate, a boron nitride layer deposited onto the first silicon carbide layer, and a second silicon carbide layer deposited onto the boron nitride layer, wherein the electrically conducting traces are deposited onto the second silicon carbide layer.

2. The structure as recited in claim 1, wherein the electrically conducting traces comprise copper.

3. A structure for electrical circuitry comprising:
electrically conducting traces;
a graphitic substrate; and
a plurality of layers separating the electrically conducting traces from the graphitic substrate, wherein the plurality of layers are selected from the group consisting of:
(i) a plurality of different ceramic dielectric layers,
(ii) a graphitic layer and a ceramic dielectric layer, and
(iii) a graphitic layer and two or more different ceramic dielectric layers,

wherein the plurality of layers are a graphite layer deposited onto the graphitic substrate,
and an aluminum nitride layer deposited onto the graphite layer, wherein the electrically conducting traces are deposited onto the aluminum nitride layer.

4. The structure as recited in claim 3, wherein the electrically conducting traces comprise copper.

5. A structure for electrical circuitry comprising:
electrically conducting traces;
a graphitic substrate; and
a plurality of layers separating the electrically conducting traces from the graphitic substrate, wherein the plurality of layers are selected from the group consisting of:
(i) a plurality of different ceramic dielectric layers,
(ii) a graphitic layer and a ceramic dielectric layer, and
(iii) a graphitic layer and two or more different ceramic dielectric layers,

wherein the plurality of different ceramic dielectric layers comprise a first silicon carbide layer deposited onto the graphitic substrate, a boron nitride layer deposited onto the first silicon carbide layer, and a second silicon carbide layer deposited onto the boron nitride layer, wherein the electrically conducting traces are deposited onto the second silicon carbide layer.

6. The structure as recited in claim 5, wherein the electrically conducting traces comprise copper.

7. A structure for electrical circuitry comprising:
electrically conducting traces;
a graphitic substrate; and
a plurality of layers separating the electrically conducting traces from the graphitic substrate, wherein the plurality of layers are selected from the group consisting of:
(i) a plurality of different ceramic dielectric layers,
(ii) a graphitic layer and a ceramic dielectric layer, and
(iii) a graphitic layer and two or more different ceramic dielectric layers, wherein the graphite layer and the ceramic dielectric layer comprise the graphite layer deposited onto the graphitic substrate, and an aluminum nitride layer deposited onto the graphite layer, wherein the electrically conducting traces are deposited onto the aluminum nitride layer.

8. The structure as recited in claim 7, wherein the electrically conducting traces comprise copper.

9. A method of making electrical circuitry comprising:
depositing a plurality of layers over a graphitic substrate, wherein the plurality of layers are selected from the group consisting of:
(i) a plurality of different ceramic dielectric layers,
(ii) a graphitic layer and a ceramic dielectric layer, and
(iii) a graphitic layer and two or more different ceramic dielectric layers; and

depositing electrically conducting traces on a top one of the plurality of layers, wherein depositing of the plurality of layers over the graphitic substrate comprises depositing a first silicon carbide layer onto the graphitic substrate, depositing a boron nitride layer onto the first silicon carbide layer, and depositing a second silicon carbide layer onto the boron nitride layer, wherein the electrically conducting traces are deposited onto the second silicon carbide layer.

10. The method as recited in claim 9, wherein the electrically conducting traces comprise copper.

11. The method as recited in claim 10, wherein the first and second silicon carbide layers are deposited as paste or ink formulations comprising silicon carbide powders, and
wherein the boron nitride layer is deposited as a paste or ink formulation comprising boron nitride powders.

12. A method of making electrical circuitry comprising:
depositing a plurality of layers over a graphitic substrate, wherein the plurality of layers are selected from the group consisting of:
(i) a plurality of different ceramic dielectric layers,
(ii) a graphitic layer and a ceramic dielectric layer, and
(iii) a graphitic layer and two or more different ceramic dielectric layers; and

depositing electrically conducting traces on a top one of the plurality of layers, wherein
depositing of the plurality of layers over the graphitic substrate comprises depositing a graphite layer onto the graphitic substrate, and depositing an aluminum nitride layer onto the graphite layer, wherein the electrically conducting traces are deposited onto the aluminum nitride layer.

13. The method as recited in claim 12, wherein the electrically conducting traces comprise copper.

14. The method as recited in claim 13, wherein the aluminum nitride layer is deposited as a paste or ink formulation comprising aluminum nitride powders, and
wherein the graphite layer is deposited as a paste or ink formulation comprising graphite powders.