WO2014037738A1

WO2014037738A1 - Led thermal management

Info

Publication number: WO2014037738A1
Application number: PCT/GB2013/052341
Authority: WO
Inventors: James Reeves; Andrew Young; Elwyn Wakefield
Original assignee: Litecool Limited
Priority date: 2012-09-07
Filing date: 2013-09-06
Publication date: 2014-03-13
Also published as: GB201216024D0

Abstract

According to a first aspect there is provided alight emitting diode (LED) package. The LED package comprises a copper heat spreader having a thickness of between 200µm and 700 µm, one or more LED die mounted on a first surface of the heat spreader, and an electrically insulating substrate layer attached to a second surface of the heat spreader.

Description

LED THERMAL MANAGEMENT

The present invention relates to light emitting diodes and relates particularly but not exclusively to an LED package arrangement having improved cooling capabilities.

A light-emitting diode (LED) is a p-n junction semiconductor diode that emits photons when a current is applied. Figure la illustrates an example of a conventional LED die comprising p and n- type semiconductor layers, a substrate and electrical contact points. Before it can be used in a practical application, an LED chip or die must be packaged. Figure lb illustrates an example of an LED package comprising the conventional LED die of Figure la, a packaging substrate/case, primary electrical connections and commonly a primary optic in the form of a lens. One or more LED packages can then be connecting physically and electrically to a circuit board to form an LED module, such as that illustrated in Figure lc. One or more LED modules can then be assembled into an LED device, referred to as luminaire or lamp. Figure Id illustrates an example of an LED luminaire or lamp comprising an LED module, a heat sink, a reflector, and secondary optic (secondary lens).

LED devices are a very efficient way of providing light and whilst a very large proportion of the input current is converted to light there remains a significant portion that is converted to heat and this heat must be dissipated if the LED device is to function correctly and have an acceptable lifespan. Whilst there exists a number of ways of cooling LED devices they all make use of some sort of heat dissipation device in the form of a heat sink, as illustrated in Figure Id. Generally, within an LED module an LED package is attached to an upper surface of a circuit board and some form of at sink is then provided on the lower surface of the board. It is also possible to provide a path for heat dissipation immediately below the LED package in the form of one or more thermal vias in the circuit board connecting the LED package to the heat sink. Whilst such arrangements do provide a good degree of cooling they remain sub-optimal as the connection to the heat sink must be taken through the circuit board and the vias have limited thermal transmission characteristics.

In view of the above, there exists a demand for an improved LED package arrangement which reduces the problems associated with the prior art.

According to a first aspect there is provided a light emitting diode (LED) package. The LED package comprises a copper heat spreader having a thickness of between 200μιη and 700 μιτι, one or more LED die mounted on a first surface of the heat spreader, and an electrically insulating substrate layer attached to a second surface of the heat spreader.

It will be appreciated that the above LED package will enhance the heat transfer characteristics associated with the LED die / heat spreader interface and, thus, allow the led to operate at higher outputs whilst remaining at an acceptable temperature.

The copper heat spreader may have a thickness of between 300μιη and 700 μιη. Alternatively, the copper heat spreader may have a thickness of between 400μιη and 700 μιη.

The substrate layer comprises a layer of any of fiberglass reinforced epoxy laminate (FR-4), aluminium oxide (Al₂0₃), metal core printed circuit board (MCPCB), aluminium nitride direct bonded copper (AIN DBC) , grapheme, and synthetic diamond. The LED package may further comprise a first electrical contact and a second electrical contact on the first surface of the heat spreader that allow for the application of an electrical current to the one or more LED die. The first electrical contact 5 may be connected to the LED die 3 by a wire bond 7 and may be separated from the copper heat spreader 2 by an electrical insulation layer 8. The second contact 6 may be connected to the LED die 2 via the copper heat spreader 2.

The copper heat spreader may include a recess within the first surface, and the one or more LED die mounted within the recess. The first surface of the copper heat spreader may then include one or more first electrical supply tracks wherein the one or more LED die are connected to the first electrical supply tracks for the supply of electricity thereto. The first surface of the heat spreader may then also include one or more second electrical supply contacts wherein the heat spreader is electrically connected to the one or more second electrical supply contacts. The one or more LED die may be connected to the one or more first electrical supply tracks by wire bonds.

The LED package may further comprise a first common connection for connecting the one or more first electrical supply tracks to a source of electricity. The first common connection may comprise a projection which projects above the first surface of the heat spreader.

The LED package may further comprise a second common connection for connecting the one or more second electrical supply contacts to a source of electricity. The second common connection may comprise a projection which projects above the first surface of the heat spreader.

The LED package may further comprise an encapsulation layer over one or more of the one or more LED die. The encapsulation layer may extend over all of the one or more LED die and at least a portion of the first surface of the heat spreader surrounding a periphery of the recess.

According to a second aspect there is provided a LED module. The LED module comprises a circuit board having one or more apertures therethrough, and one or more LED packages according to the first aspect mounted to a surface of the circuit board. The first surface of each LED package faces the surface of the circuit board and each of the one or more LED die are aligned with one of the one or more apertures.

The LED module may further comprise a lens mounted on an opposite surface of the circuit board to that on which the one or more LED packages are mounted. The lens may extend over one or more of the one or more apertures.

The LED module may further comprise one or more heat sinks, each of the one or more heat sinks being attached to the substrate layer of one or more of the LED packages.

The above and other features associated with the present invention will now be more particularly described by way of example only with reference to the accompanying drawings, in which:

Figure la illustrates a cross-sectional view of an example of a conventional LED die;

Figure lb illustrates a cross-sectional view of an example of a conventional LED package comprising the conventional LED die of Figure 1;

Figures lc illustrates a cross-sectional view of an example of a conventional LED module comprising one or more of the conventional LED packages of Figure lb; Figure Id illustrates a cross-sectional view of an example of a conventional LED device comprising

Figure 2 illustrates a cross-sectional view of an example of an improved LED package as described herein;

Figure 3 is a graph illustrating the effect of varying the thickness of a copper heat spreader layer on the junction-to-case thermal resistance (R_jc) of an LED package for a selection of electrically insulating substrate types;

Figure 4 is a cross-sectional view of an alternative embodiment of an improved LED package as described herein;

Figure 5 is a cross-sectional view of a portion of an LED module comprising the LED package of Figure 4; and

Figure 6 is a plan view of the LED module of Figure 5.

It has been recognised by the present inventors that heat dissipation efficiency can be improved by providing an LED package in which the LED die is mounted onto a copper heat spreader the thickness of which has been optimised, wherein the heat spreader removes heat from the LED die by thermal conduction and also spreads the heat from the smaller area of the LED die to the a larger heat sink. In particular, the present inventors have determined that increasing the thickness of the copper heat spreader layer can actually lower the thermal resistance, by improving its ability to transfer heat (i.e. to act as a heat spreader). However, the present inventors have also determined that there is a limit to this benefit, as the thermal resistance will eventually begin to increase again as the thickness of the copper layer increases.

In contrast, conventional LED packages are manufactured so as to be as small and thin as possible. This is largely done in order to minimise cost (i.e. by minimising the amount of material used) and to minimise the volume occupied by LED packages. In addition, in conventional LED packages it is considered desirable to minimise the thickness of any material that otherwise separates the light emitting element (i.e. p-n junction) of an LED die from any attached heat sink.

Figure 2 therefore illustrates an example of an LED package 1 that provides for improved heat dissipation when compared with conventional LED packages by making use of a copper heat spreader layer of optimised thickness directly beneath the LED die. The LED package 1 comprises a copper heat spreader 2 having a thickness of between 200μιη and 700 μιτι, an LED die 3 mounted on a first (obverse) surface 2a of the heat spreader 2, and an electrically insulating substrate layer 4 attached to a second (reverse) surface 2b of the heat spreader. The first surface 2a and the second surface 2b face opposite directions (e.g. lower and upper surfaces respectively). Figure 2 also illustrates a first electrical contact 5 and a second electrical contact 6 that allow for the application of an electrical current to the LED die 3. In this example, the first electrical contact 5 is connected to the LED die 3 by a wire bond 7 and is separated from the copper heat spreader 2 by an electrical insulation layer 8. The second contact 6 is connected to the LED die 2 via the copper heat spreader 2. The LED package 1 may also optionally be provided with an encapsulation layer 9 that covers the LED die 3 mounted on the first surface 2a of the heat spreader 2. Figure 3 is a graph illustrating the effect of varying the thickness of a copper heat spreader layer provided beneath a 1mm² LED die on the junction-to-case thermal resistance (R_jc) for a selection of electrically insulating substrate types (i.e. that form the electrically insulating substrate layer 4). In this regard, the junction-to-case thermal resistance (R_jc) is the thermal resistance between the light emitting element (i.e. p-n junction) of the LED die and the outer surface of the packaging substrate/case. The graph shows that, whilst the thermal resistance is greatly affected by the type of substrate layer, for all substrate types there is a minimum in the thermal resistance and therefore an optimum thickness of the copper heat spreader layer between 200μιτι and 700 μηι.

The optimum thickness of the copper heat spreader layer (i.e. the thickness at which the minimum in the thermal resistance occurs) will vary depending upon a number of factors, such as the total footprint of the heat source or heat sources (i.e. LED die), their location and separation on the heat- spreader, the footprint of the heat spreader, etc. Table 1 below therefore provides the approximate upper and lower limits for the optimum thickness of the copper heat spreader layer for a selection of substrate types.

FR-4 is composite material composed of woven fiberglass sheets/cloth bonded with a flame resistant epoxy resin that meets the internationally recognised standard defined by the National Electrical Manufacturers Association (NEMA). Metal-core Printed Circuit Board (MCPCB) is a laminated material, and a simple MCPCB typically comprises a top layer of solder mask above a layer of copper, a dielectric layer beneath the copper layer, and a metal core base layer. The metal core base layer is usually made out of aluminium, copper, or steel, although aluminium is commonly selected due to its cost, weight and thermal characteristics. Aluminium Nitride Directly-bonded Copper (AI N DBC) is composed of a tile of Aluminium Nitride ceramic with sheets of copper bonded to one or both sides by a high-temperature oxidation process.

Figure 4 then illustrates a further example of an LED package arrangement 10 that comprises one or more LED die 12 positioned within a recess 14 provided on a first (upper) surface 16 of a copper heat spreader 18 and an electrically insulating substrate layer 41 provided on a second (lower) surface 20. It will be appreciated that any number of die 12 may be placed on the first surface 16 of the heat spreader. In the embodiment of Figure 4, each of the one or more die 12 are electrically bonded to the heat spreader by means of a die attachment layer 22 formed of, for example, solder or electrically conductive adhesive, thereby to allow for the passage of electrical current between the LED die 12 and the heat spreader 18 as and when required. The first surface 16 of the heat spreader 18 may be provided with one or more first electrical supply tracks 24 for receiving electrical current and being connected to one or more of the one or more LED die 12 by means of one or more wire bonds 26 which are well known to those skilled in the art and, therefore, not described further herein. An insulation layer 28 is provided between the first electrical supply tracks 24 and the first surface 16 such as to electrically insulate them from the heat spreader 18.

Also shown in figures 4, 5 and 6 are one or more first common electrical contacts 30 to which each of the one or more first electrical supply tracks 24 are connected such as to provide a point of electrical contact for the LED package 10. The first common connector 30 may comprise a projection 30p which projects beyond the first surface 16 of the heat spreader 18 such as to allow for a connection to be made between the connection 30 and a further electrical power supply on another component whilst not interfering with the other electrical tracks on the first surface 16.

The first surface 16 may also be provided with one or more second common electrical contacts 32 which are each electrically connected to the heat spreader 18 such as to allow the passage of electrical current between the LED die 12 and the second electrical contact(s) 32 via the heat spreader 18. It will be appreciated that the first supply tracks 24 should be electrically insulated from the second electrical contacts and this may be done by simply spacing the two from each other on the upper surface as shown in figure 4of the attached drawings.

Figure 5 then illustrates the LED package 10 of Figure 4 connected physically and electrically to a circuit board 36 to form an LED module. In Figure 5, the circuit board 36 has a first (upper) surface and a second (lower) surface 40, and one or more apertures 42 that extend through the circuit board 36 in a direction perpendicular to the first and second surfaces. The LED package 10 is then located such that first surface 16 of the LED package 10 (i.e. on which the LED die 12 are mounted) faces the second surface 40 of the circuit board 36. The LED die 12 mounted on the first surface 16 of the LED package 10 are then aligned with an aperture 42 such that light emitted by the LED die 12 will pass through the aperture 42.

In Figure 5, the die 12 are each positioned such that they lie below the second surface of the circuit board 36 but it will be appreciated that they may be positioned to lie within the plane P of the circuit board 36 or indeed above it if so desired. To do this one simply needs to raise the portion of the heat spreader 18 on which the LED die 12 are mounted within the centre of the recess 14 to within or above the aperture 42 whilst maintaining the electrical connections associated with the first surface 16 in a position where they can still make the appropriate connections.

The lower surface 40 of the circuit board 36 may also be provided with +ve and -ve electrical circuits 50, 52 for the passage of electrical current to the LED package 10. The electrical contact between circuits 50, 52 and the first and second common electrical contacts 30, 32 may be provided by simply positioning the LED package lOappropriately below the circuit board 36 and securing the LED package lOsuch that contacts 30, 32 come into electrical contact with circuits 50, 52. A securing means (not shown) may be employed for this purpose. Suitable arrangements include screws. bonding with adhesive or securing through multiple solder points as electrical connections can also be structural.

The first surface 16 of the heat spreader 18 may also be provided with an encapsulation layer 54 over one or more of the one or more LED die 12 which may extend over all the LED die 12 and also a portion of the upper surface 16 of the heat spreader 18 surrounding the recess 14 such as to protect them and possibly also the wire bonds from the environment and from inadvertent damage.

It will be appreciated from figure 5 in particular that the circuit board 36 is provided with a plurality of apertures 42 and this will allow for the provision of a plurality of LED packages lOin association therewith. In the arrangement shown, a single LED package lOis positioned below each aperture 42 but multiple LED packages lOmay be positioned beneath each aperture. It will also be appreciated that when multiple apertures are provided each LED package lOwill need to be electrically connected to the circuit board 36 and, thus, the electrical supply circuits 50, 52 may be positioned to supply multiple modules.

The arrangement may also include one or more lenses over the one or more LED die, as shown in general at 60 in figure 5. Such a lens may be provided over each LED die 12 individually or may extend over a number of LED die 12 in which case it forms a compound lens. A single lens 60 may be provided for each aperture 42 or the arrangement may take benefit from a single lens or compound lens 60c over a number of apertures 42.

It will be appreciated that mounting the LED package lOsuch that the first surface 16 of the LED package 10 on which the LED die 12 are mounted faces a surface of the circuit board 36 leaves the second surface 18 of the LED package exposed/uncovered. It is therefore possible to attach a heat sink 70 to the second surface 20 of the heat spreader 18 using a thermal interface material 72, without the circuit board 36 interposed between them. Prior art arrangements typically require that the LED package is mounted onto the top surface of the circuit board, such that any heat sink must then be attached to the lower surface of the circuit board, thereby disrupting the thermal path and limiting the thermal dissipation. Complex heat transfer vias extending through the circuit board are then required which can be problematic and are costly to produce. The present arrangement allows for the LED package 10 to be directly connected to a heat sink 70 which may also serve as a common heat sink for a number of LED packages 10. This has the advantage of spreading the heat dissipation more widely and also allowing the heat dissipation capacity of one section of the heat sink to be used to support cooling of neighbouring LED packages 10 if an immediately associated LED package 10 is not being used.

Claims

A light emitting diode, LED, package (1, 10) comprising:

a copper heat spreader (2, 18) having a thickness of between 200μΓη and 700 μιτι; one or more LED die (3, 12) mounted on a first surface (2a, 16) of the heat spreader; and

an electrically insulating substrate layer (4, 41) attached to a second surface (2b, 20) of the heat spreader.

A light emitting diode package as claimed in claim 1, wherein the copper heat spreader has a thickness of between 300μιτι and 700 μιη.

A light emitting diode package as claimed in claim 1, wherein the copper heat spreader has a thickness of between 400μιη and 700 μιη.

A light emitting diode package as claimed in claim 1, wherein the substrate layer comprises a layer of any of:

fiberglass reinforced epoxy laminate, FR-4;

aluminium oxide, Al₂0₃;

metal core printed circuit board, MCPCB;

aluminium nitride direct bonded copper, AIN DBC;

graphene; and

synthetic diamond.

A light emitting diode package as claimed in any preceding claim, and further comprising: a first electrical contact (5, 30) and a second electrical contact (6, 32) on the first surface (2a, 16) of the copper heat spreader (2, 18) that allow for the application of an electrical current to the one or more LED die (3, 12).

A light emitting diode package as claimed in claim 5, wherein each of the one or more LED die (3, 12) are connected to the first electrical contact (5, 30) by a wire bond (7, 26) and the first electrical contact (5, 30) is separated from the copper heat spreader (2, 18) by an electrical insulation layer (8, 28).

A light emitting diode package as claimed in any of claims 5 or 6, wherein each of the one or more LED die (3, 12) are connected to the second electrical contact (6, 32) via the copper heat spreader (2, 18).

A light emitting diode package as claimed in any preceding claim, wherein the copper heat spreader (18) includes a recess (14) within the first surface (16), and the one or more LED die (12) are mounted within the recess.

A light emitting diode package as claimed in claim 8, wherein the first surface (16) of the copper heat spreader includes one or more first electrical supply tracks (24) and wherein the one or more LED die (12) are connected to the first electrical supply tracks for the supply of electricity thereto.

10. A light emitting diode package as claimed in claim 9, wherein the first surface (16) of the copper heat spreader includes one or more second electrical supply contacts (32) and wherein the copper heat spreader is electrically connected to the one or more second electrical supply contacts (32).

11. A light emitting diode package as claimed in claim 9, wherein the one or more LED die are connected to the one or more first electrical supply tracks by wire bonds (26).

12. A light emitting diode package as claimed in any of claims 9 to 11, and further comprising a first common connection (30) for connecting the one or more first electrical supply tracks to a source of electricity.

13. A light emitting diode package as claimed in claim 10, and further comprising a second common connection (32) for connecting the one or more second electrical supply contacts to a source of electricity. 14. A light emitting diode package as clamed in claim 12, wherein the first common connection comprises a projection (30p) which projects above the first surface of the heat spreader.

15. A light emitting diode package as clamed in claim 13, wherein said second common connection comprises a projection (32) which projects above the first surface of the heat spreader.

16. A light emitting diode package as claimed in any one of claims 1 to 15, and further comprising an encapsulation layer (9, 54) over one or more of the one or more LED die. 17. A light emitting diode package as claimed in claim 16 when appended to any of claims 8 to

15, wherein said encapsulation layer extends over all of the one or more LED die and at least a portion of the first surface of the heat spreader surrounding a periphery of the recess.

18. A light emitting diode, LED, module comprising:

a circuit board (36) having one or more apertures (42) therethrough; and one or more LED packages (10) as claimed in any of claims 1 to 14 mounted to a surface of the circuit board, the first surface of each LED package facing the surface of the circuit board with each of the one or more LED die (12) aligned with one of the one or more apertures.

A light emitting diode module as claimed in claim 18, and further comprising a lens (60) mounted on an opposite surface of the circuit board to that on which the one or more LED packages are mounted.

20. A light emitting diode module as claimed in claim 19, wherein the lens extends over one or more of the one or more apertures.

21. A light emitting diode module as claimed in any one of claims 18 to 20, and further comprising one or more heat sinks (70), each of the one or more heat sinks being attached to the substrate layer of one or more of the LED packages.