US20080057333A1 - Heat dissipation substrate for electronic device

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Abstract

Description

Claims

US20080057333A1

Publication number: US20080057333A1
Application number: US11/803,039
Authority: US
Inventors: Fu Hua Chu; David Shau Chew Wang; Jyh Ming Yu; En Tien Yang; Kuo Hsun Chen
Original assignee: Polytronics Technology Corp
Current assignee: Polytronics Technology Corp
Priority date: 2006-08-30
Filing date: 2007-05-11
Publication date: 2008-03-06
Also published as: TWI302372B; TW200812025A; JP2008060535A

A heat dissipation substrate for an electronic device comprises a first metal layer, a second metal layer, and a thermally conductive polymer dielectric insulating layer. A surface of the first metal layer carries the electronic device, e.g., a light-emitting diode (LED) device. The thermally conductive polymer dielectric insulating layer is stacked between the first metal layer and the second metal layer in a physical contact manner, and interfaces therebetween include at least one micro-rough surface with a roughness Rz larger than 7.0. The micro-rough surface includes a plurality of nodular projections, and the grain sizes of the nodular projections mainly are in a range of 0.1-100 μm. The heat dissipation substrate has a thermal conductivity larger than 1.0 W/m·K, and a thickness smaller than 0.5 mm, and comprises (1) a fluorine-containing polymer with a melting point higher than 150° C. and a volume percentage in a range of 30-60%, and (2) thermally conductive filler dispersed in the fluorine-containing polymer and having a volume percentage in a range of 40-70%.

BACKGROUND OF THE INVENTION

(A) Field of the Invention
The present invention relates to a heat dissipation substrate, and more particularly to a heat dissipation substrate for an electronic device.
(B) Description of the Related Art
In recent years, white LEDs have become a very popular new product attracting widespread attention all over the world. Because white LEDs offer the advantages of small size, low power consumption, long life, and quick response speed, the problems of conventional incandescent bulbs can be solved. Therefore, the applications of LEDs in backlight sources of displays, mini-projectors, illumination, and car lamp sources are becoming increasingly important in the market.
At present, Europe, the United States, Japan, and other countries have a consensus with respect to energy conservation and environmental protection, and actively develop the white LED as a new light source for illumination in this century. Currently, energy is imported in many countries, so it is worthwhile to develop the white LED in the illumination market. Based on the evaluation of experts, if all the incandescent lamps in Japan are replaced with the white LEDs, electric power generated by two power plants can be saved each year and the indirectly reduced fuel consumption will be one billion liters. Furthermore, carbon dioxide exhausted during electrical power generation is also reduced, thereby reducing the greenhouse effect. Therefore, countries in Europe, America, and Japan have devoted a lot of manpower to white LED development. It is predicted that the white LEDs can be substituted for conventional illuminating apparatuses within ten years.
However, with regard to a high power LED for illumination, merely 15-20% of the input power of the LED is converted into light, and the rest of the input power is converted into heat. If the heat cannot be dissipated into the environment in time, the temperature of the LED device will become so high that the luminous intensity and service life are negatively affected. Therefore, the heat management of the LED device attracts a lot of attention.
Generally, for single conventional LED, the working current of a single conventional LED is about 20 to 40 mA, which produces a small amount of heat, so the heat dissipation problem is not a serious problem. Therefore, a common FR4 printed circuit board (PCB), which exhibits a heat dissipation coefficient of about 0.3 W/m·K, is sufficient enough to dissipate heat. However, for backlight display and bright illumination applications, a plurality LED devices are mounted to a circuit substrate. Due to the demand of higher current (>1 A) and the resulting much higher heat generation from LED devices, the circuit substrate should play the role not only as a carrier for LED devices, but also as a heat sink for heat dissipation. The common FR4 PCB cannot satisfy the heat dissipation requirement.

SUMMARY OF THE INVENTION

The present invention is mainly directed to providing a heat dissipation substrate with superior heat dissipation, the ability to withstand high voltage, and dielectric insulation, a flexible mechanical structure, in which metal layers are well bonded with thermally conductive polymer dielectric insulating layers, so that it can be applied to a high power LED device, for example, a portable mobile phone.
In accordance with the present invention, a heat dissipation substrate for an electronic device comprises a first metal layer, a second metal layer, and a thermally conductive polymer dielectric insulating layer. The surface of the first metal layer carries the electronic device such as an LED device. The thermally conductive polymer dielectric insulating layer is in physical contact with and stacked between the first metal layer and the second metal layer. The interfaces between the thermally conductive polymer dielectric insulating layer and the first and second metal layers comprise at least one micro-rough surface with a roughness Rz larger than 7.0 according to JIS B 0601 1994. The micro-rough surface comprises a plurality of noduar projections with a grain size mainly in a range of 0.1-100 μm. The thermally conductive polymer dielectric insulating layer has a thermal conductivity larger than 1 W/m·K, and a thickness smaller than 0.5 mm. The thermally conductive polymer dielectric insulating layer comprises (1) a fluorine-containing polymer with a melting point higher than 150° C. and a volume percentage of 30-60%, and (2) thermally conductive filler dispersed in the fluorine-containing polymer and having a volume percentage in the range of 40-70%.
The heat dissipation substrate of this invention comprising a thermally conductive polymer dielectric insulating layer, a first metal layer and a second metal layer. The thermally conductive polymer dielectric insulating layer comprises (1) a fluorine-containing polymer with a melting point higher than 150° C. and a volume percentage in a range of 30-60%; and (2) thermally conductive filler dispersed in the fluorine-containing polymer and having a volume percentage in a range of 40-70%. The first metal layer has a micro-rough surface which is in direct physical contact with one surface of the thermally conductive polymer insulating layer, and consists essentially of nodules which protrude from the surface by a distance of 0.1 to 100 microns with roughness Rz larger than 7.0. The second metal layer has a micro-rough surface which is in direct physical contact with the other surface of the thermally conductive polymer insulating layer, and consists essentially of nodules which protrude from the surface by a distance of 0.1 to 100 microns with roughness Rz larger than 7.0. The heat dissipation substrate has a thermal conductivity larger than 1.0 W/m·K, and the total thickness of the substrate is smaller than 0.5 mm.
The fluorine-containing polymer is preferably selected from poly vinylidene fluoride (PVDF) or polyethylenetetrafluoroethylene (PETFE), and has a melting point higher than 150° C., and preferably higher than 220° C. The conductive filler is selected from thermally conductive ceramic materials such as nitride and oxide. The filler can be pretreated with silane coupling agent to improve the bonding with the fluorine-containing polymer.
The fluorine-containing polymer is known for its non-stick and lack of adhesion characteristics. The most common practice to bond fluorine-containing polymer to the metal surface is to apply a tie-layer between them. However, the commonly used tie-layer for fluorine-containing polymer is not a good thermal conductive material. Even a thin layer of the tie-layer could drastically deteriorate the thermal conductivity of the system. It is a great challenge to bond fluorine-containing polymer to metal substrate and to simultaneously maintain good thermal conductivity. This invention shows the application of nodulized metal foil to bond to the highly filled thermal conductive fluoro-polymer without using tie-layer to achieve good flexibility, good thermal conductivity, and good voltage withstanding capability.
The heat dissipation substrate can be irradiated by 0-20 Mrad, so as to make the thermally conductive polymer dielectric insulating layer cross-linked and cured, and has a favorable thermally conductive and insulating effect. Further, if the thicknesses of the first metal and the second metal are designed to be smaller than 0.1 mm and 0.2 mm, respectively, and the thickness of the thermally conductive polymer dielectric insulating layer is designed to be smaller than 0.5 mm (preferably 0.3 mm), the heat dissipation substrate may pass the flexural test in which a 1 cm wide test substrate is bent to be a column with a diameter of 5 mm, and the surface neither ruptures nor cracks, thereby being applicable to a folded product.
Furthermore, the fluorine-containing polymer material usually has a high melting point (for example, PVDF has a melting point of 165° C. and PETFE has a melting point of 260° C.), and has the advantages of being flame retardant and able to withstand high temperature. Therefore, the fluorine-containing polymer is valued for safety applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a heat dissipation substrate according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an LED device 10 is carried by a heat dissipation substrate 20. The heat dissipation substrate 20 includes a first metal layer 21, a second metal layer 22, and a thermally conductive polymer dielectric insulating layer 23 stacked between the first metal layer 21 and the second metal layer 22. The LED device 10 is disposed on the surface of the first metal layer 21, and the interfaces between the first and second metal layers 21 and 22 and the thermally conductive polymer dielectric insulating layer 23 are physically contacted, wherein at least one interface is a micro-rough surface. The micro-rough surface has a plurality of nodular projections with a grain size mainly in a range of 0.1-100 μm, thereby increasing the tensile strength therebetween.
The method of fabricating the heat dissipation substrate 20 is described as follows. The feeding temperature of a batch-type blender (HAAKE-600P) is set to be 20° C. higher than the melting point (T_m) of the material, and the pre-mixed materials of the thermally conductive polymer dielectric insulating layer 23 are added, and raw materials are placed in a steel cup and uniformly stirred with a measuring spoon. Initially, the rotation speed of the batch-type blender is 40 rpm, and 3 minutes later, the rotation speed is increased to 70 rpm. The materials are blended for 15 minutes and then taken out, thereby forming a heat-dissipating composite material.
The heat-dissipating composite material is put into a mould in a longitudinally symmetrical manner. The mould uses a steel plate as an outer layer and has a thickness in the middle of, for example, 0.15 mm. Teflon mold release cloths are disposed on the upper and lower sides of the mould, respectively. First, the heat-dissipating composite material is pre-heated for 5 minutes, and then pressed for 15 minutes under a pressure of 150 kg/cm²and a temperature equal to the blending temperature. Afterwards, a heat dissipation sheet of a thickness of 0.15 mm is formed.
The first metal layer 21 and the second metal layer 22 are disposed on the upper and lower sides of the heat dissipation sheet and then pressed again, pre-heated for 5 minutes, and then pressed for 5 minutes under a pressure of 150 kg/cm²and a temperature equal to the blending temperature, so as to form the heat dissipation substrate 20, in which the thermally conductive polymer dielectric insulating layer 23 is in the middle and the first metal layer 21 and the second metal layer 22 are respectively attached onto the upper and lower sides of the polymer dielectric insulating layer 23.
Table 1 shows experimental results of the tension and withstand voltage test of different roughness of metal layers. The thermally conductive polymer dielectric insulating layer 23 uses polyvinylidene fluoride (PVDF) with the melting point of 165° C. as a base material, and thermally conductive fillers Al₂O₃and AlN are dispersed in the PVDF, wherein the volume percentages of the two are 60% and 45%, respectively. In this embodiment, the thickness of the thermally conductive polymer material layer 23 is smaller than 0.3 mm. The adhesion test is performed according to JIS C6481 specification for testing the peeling strength of the interfaces.

		Roughness	Filler	Filler	Thickness	Strength	Conductivity	Withstanding
Experiment	Type	(Rz)	Type	Vol %	(mm)	(N/cm)	(W/m · K)	Voltage Test

Example 1	1 oz Cu	7.0–9.0	Al₂O₃	60	0.21	14.3	1.7	>5 kV
Example 2	2 oz Cu	9.5–11.5	Al₂O₃	60	0.24	16.8	1.6	>5 kV
Example 3	4 oz Cu	10.0–12.0	Al₂O₃	60	0.22	17.5	1.7	>5 kV
Example 4	1 oz Ni	9.5–11.5	AlN	45	0.21	15.4	1.2	>5 kV
	plated
	Cu
Example 5	1 oz Ni	9.5–11.5	Al₂O₃	60	0.23	16.9	1.7	>5 kV
	plated
	Cu
Example 6	2 oz Ni	10.0–12.0	Al₂O₃	60	0.23	17.8	1.6	>5 kV
	plated
	Cu
Example 7	1 oz Ni	10.0–12.0	Al₂O₃	60	0.24	18.1	1.6	>5 kV
Comparative	1 oz Cu	3.0–4.5	Al₂O₃	60	0.23	7.5	1.6	>5 kV
Example

As shown in Table 1, the surface roughness (Rz) of the Comp. case is in a range of 3.0-4.5, and is lower than those in the Examples 1 to 7. The adhesion of the Comparative Example case is 7.5 N/cm, which is far less than those of the Examples 1 to 7. It is obvious that a larger roughness can increase peeling strength between the thermally conductive polymer dielectric insulating layer and the first and second metal layers. Moreover, all experimental cases can pass the withstand voltage test of 5 kV or at least higher than 3 kV, and the thermal conductivity is larger than 1.0 W/m·K.
Table 2 shows a test comparison table of different types of high polymers.

TABLE 2

		Thickness
		of Thermal
		Conductive
	High	Polymer	Thermal	Peel
Serial	Molecular	Material	Conductivity	Strength	Flexibility	Withstanding
Number	Polymer	Layer (mm)	(W/m · K)	(N/cm)	(5 mm)	Voltage Test

Example 1	PVDF	0.22	1.6	14.5	PASS	>5 kV
Example 2	PETFE	0.24	1.7	16.8	PASS	>7 kV
Comparative	HDPE	0.21	1.7	15.7	PASS	<2 kV
Example 1
Comparative	EPOXY	0.20	1.6	22.1	FAIL	>5 kV
Example 2

The experimental cases 1 and 2 use PVDF and PETFE (Tefzel™) as polymeric base materials, respectively, and the thermally conductive filler is Al₂O₃. The polymers in the Comparative Examples 1 and 2 are selected from HDPE and EPOXY without fluorine. In the Examples and Comparative Examples, the volume percentages of the polymers and the thermally conductive filler are 40% and 60%, respectively, and the copper foils having the same roughness Rz in the range of 7.0-9.0 are used as the first metal layer and the second metal layer.
The Comparative Example of EPOXY comprises liquid EPOXY, Novolac resin, dicyandiamide, urea catalyst, and Al₂O₃. The liquid EPOXY selects Model DER331 of Dow Chemical Company, the Novolac resin selects Model DER438 of Dow Chemical Company, the dicyandiamide selects Dyhard 100S of Degussa Fine Chemicals, and the urea catalyst selects Dyhard UR500 of Degussa Fine Chemicals. The Al₂O₃has a grain size in the range of 5-45 μm, and is manufactured by Denki Kagaku Kogyo Kabushiki Kaisya.
The EPOXY is prepared according to the following steps. First, 50 phr of DER331 and 50 phr of DEN438 are mixed in a resin kettle at a temperature of 80° C. till becoming a homogeneous solution. Then, 10 phr of Dyhard 100S and 3 phr of Dyhard UR300 are added in the resin kettle and further mixed for 20 minutes at a temperature of 80° C. Subsequently, 570 phr of the Al₂O₃filler are added into the resin kettle and mixed till the filler is completely dispersed in the resin to form a resin slurry. The gas contained in the resin slurry is removed in a vacuum for 30 minutes. Then, the resin slurry is placed on a copper foil surface, and another copper foil is laid on the surface of the resin slurry, thereby forming a copper foil/resin slurry/copper foil composite structure. The copper foil/resin slurry/copper foil composite structure is placed in a metal frame with a thickness of 3 mm. A rubber roller is used to flatten the copper foil surface. The composite structure together with the metal frame is placed in a furnace at a temperature of 130° C. to be pre-cured for 1 hour. Then, the composite structure together with the metal frame is placed in a vacuum hot press machine with a vacuum degree of 10 torr and a pressure of 50 kg/cm²in order to be further cured for 1 hour at 150° C. The composite structure is cooled to be lower than 50° C. at a pressure of 50 kg/cm²and is removed from the hot press machine.
The test substrates used in the PVDF and PETFE experimental cases and the HDPE and EPOXY Comparative cases have passed the following tests.
1. Flexibility: an 1 cm wide test specimen is bent 180 degree along the exterior circumference of a metal rod which has a diameter of 5 mm, and the surface of the test specimen should has neither ruptures nor cracks.
2. Peel Strength Test: a 180 degree T-peel strength measurement is applied to the test specimen (1.0 cm×12 cm) by clamping the upper and lower metal foil at one end of the specimen, and testing the sample under a constant tensile speed of 3 cm/min in a tensile testing machine.
3. Dielectric Strength (insulation withstanding voltage) Test: it is a withstanding voltage test using Kikusui Model TOS5101 Withstanding Voltage Tester by applying 1 kV on the upper and lower electrodes of the 1″ diameter specimen for 30 seconds and applying a step increase 0.5 kV for each consecutive tests until the applied voltage exceeds the withstanding voltage of the insulation layer of the specimen.
As shown in Table 2, the Example 1 and Example 2 cases of the fluorine-containing polymers PVDF and PETFE have superior flexibility, and pass the withstanding voltage test since these two could withstand a voltage higher than 5 kV. On the contrary, the Comparative Example 1 using HDPE as the polymer passes the flexibility test, however, fails the withstanding voltage test since the HDPE system could only withstand a voltage lower than 2 kV, which is obviously lower than those in the Example 1 and Example 2. Comparative Example 2 using EPOXY as the polymer passes the withstanding voltage test, however, fails the flexibility test.
Furthermore, the fluorine-containing materials such as PVDF and PETFE are not easily caught on fire and do not support combustion (meeting UL 94 V-0), and are much more suitable for safety applications in comparison with HDPE or EPOXY.
The volume percentages of the fluorine-containing polymer and the thermally conductive filler can be adjusted to some extent and still have the same characteristics. The volume percentage of the fluorine-containing polymer is preferably in the range of 30-60%; the volume percentage of the thermally conductive filler is in the range of 40-70%, and more preferably in the range of 45-65%.
In addition to the aforementioned materials, the thermally conductive high molecular polymer can be selected from the group of poly(tetrafluoroethylene) (PTFE), tetrafluoroethylene-hexafluoro-propylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), perfluoroalkoxy modified tetrafluoroethylenes (PFA), poly(chlorotri-fluorotetrafluoroethylene (PCTFE), vinylidene fluoride-tetrafluoroethylene copolymer (VF-2-TFE), poly(vinylidene fluoride), tetrafluoroethylene-perfluorodioxole copolymers, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, and tetrafluoroethylene-perfluoromethylvinylether with cure site monomer terpolymer.
The thermally conductive filler can be nitride and oxide, wherein the nitride includes zirconium nitride (ZrN), boron nitride (BN), aluminum nitride (AlN), and silicon nitride (SiN) and the oxide includes aluminum oxide (Al₂O₃), magnesium oxide (MgO), zinc oxide (ZnO), and titanium dioxide (TiO₂).
Furthermore, in order to be used in the high power light-emitting devices such as LEDs, the first metal layer 21 carrying the LED device 10 can be made of copper so as to fabricate a relevant circuit of the LED device 10 thereon. The second metal layer 22 on the bottom can be made of copper, aluminum, or an alloy thereof.
The heat dissipation substrate of the present invention has the advantages of high thermal conductivity, high withstanding voltage, high tensile strength, and flexibility, so that it can be applied in an LED module for illumination to dissipate heat. Furthermore, the heat dissipation substrate can further be used in more compact size portable devices, e.g., a notebook or a mobile phone, in which higher efficiency of heat dissipation is required.
The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims.

Thermal Conductive

Metal Foil

Polymer Layer

1. A heat dissipation substrate for an electronic device comprising:

a thermally conductive polymer dielectric insulating layer comprising:

(1) a fluorine-containing polymer with a melting point higher than 150° C. and a volume percentage in a range of 30-60%; and

(2) thermally conductive filler dispersed in the fluorine-containing polymer and having a volume percentage in a range of 40-70%;

a first metal layer having a micro-rough surface which is in direct physical contact with one surface of the thermally conductive polymer insulating layer, and consists essentially of nodules which protrude from the surface by a distance of 0.1 to 100 microns with roughness Rz larger than 7.0; and

a second metal layer having a micro-rough surface which is in direct physical contact with the other surface of the thermally conductive polymer insulating layer, and consists essentially of nodules which protrude from the surface by a distance of 0.1 to 100 microns with roughness Rz larger than 7.0;

wherein the heat dissipation substrate has a thermal conductivity larger than 1.0 W/m·K.

2. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the first metal layer has a thickness smaller than 0.1 mm.

3. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the second metal layer has a thickness smaller than 0.2 mm.

4. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the fluorine-containing polymer has a melting point higher than 220° C.

5. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the thermally conductive filler has a volume percentage of 45-65%.

6. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein tensile strength between the thermally conductive polymer dielectric insulating layer and the first and second electrode layers is larger than 8 N/cm.

7. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein a surface of the heat dissipation substrate neither is ruptured nor has cracks when the heat dissipation substrate is 1 cm wide and is bent 180 degree along the exterior circumference of a metal rod with a diameter of 5 mm.

8. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the heat dissipation substrate has a withstand voltage larger than 3 kV.

9. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the fluorine-containing polymer is selected from the group consisting of poly vinylidene fluoride and polyethylenetetrafluoroethylene.

10. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the fluorine-containing polymer is selected from the group consisting of poly(tetrafluoroethylene), tetrafluoroethylene-hexafluoro-propylene copolymer, ethylene-tetrafluoroethylene copolymer, perfluoroalkoxy modified tetrafluoroethylenes, poly(chlorotri-fluorotetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene copolymer, poly(vinylidene fluoride), tetrafluoroethylene-perfluorodioxole copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, and tetrafluoroethylene-perfluoromethylvinylether with cure site monomer terpolymer.

11. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the thermally conductive filler is nitride or oxide.

12. The heat dissipation substrate for an electronic device in accordance with claim 11 wherein the nitride is selected from the group consisting of zirconium nitride, boron nitride, aluminum nitride, and silicon nitride.

13. The heat dissipation substrate for an electronic device in accordance with claim 11, wherein the oxide is selected from the group of aluminum oxide, magnesium oxide, zinc oxide, and titanium dioxide.

14. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the heat dissipation substrate is irradiated by 0-20 Mrads, so as to make the thermally conductive polymer dielectric insulating layer cross-linked and cured.

15. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the electronic device is a light-emitting diode device.

16. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the first metal layer comprises copper.

17. The heat dissipation substrate for an electronic device in accordance with claim 1, wherein the second metal layer comprises aluminum.