GB2480428A

GB2480428A - PCB with metal core having extended heatsink bosses for mounting LEDs

Info

Publication number: GB2480428A
Application number: GB201007891A
Authority: GB
Inventors: Nikola Tomislav Vicente Nikolich
Original assignee: QUANTA LIGHT IND Ltd
Current assignee: QUANTA LIGHT IND Ltd
Priority date: 2010-05-11
Filing date: 2010-05-11
Publication date: 2011-11-23
Also published as: GB201007891D0

Abstract

A thermal core printed circuit board, MCPCB, whose. core 1 comprises a planar thermally conductive substrate, such a metal, from which project mounting bosses that provide a heatsink to components mounted on the PCB. The bosses 10 project through an overlying electrically insulating layer 2 on which circuit traces 3 can be provided. The bosses and core may both be of the same material and are preferably metal. The bosses are suitable for mounting high-power light emitting diodes LED, and may comprise either a single element or plural elements (figs 4, 5). The boss may be produced by laser engraving the core substrate layer. These PCBs when mounting LEDs can be used to assemble lamps.

Description

THERMAL CORE PRINTED CIRCUIT BOARD AND

DEVICE FORMED THEREON

This invention relates to a thermal core printed circuit board, and to a lamp or other electrical device comprising a PCB, as well as to methods of making these products.

In this specification, the term "thermal core PCB" is intended to mean a printed circuit board having a core or substrate which is thermally conductive so that it can extract heat from devices mounted on it. It may be of metal, ceramics or carbon, for example.

The thermal management of an electronic system including devices such as light emitting diodes or other heat-generating circuits has involved the use of a metal core printed circuit board, MCPCB, whose metal core is used to dissipate the generated heat. An MCPCB typically consists of a number of layers including a dielectric, or electrically insulative layer, sandwiched between two metal layers. One of the metal layers, typically a copper foil, acts as a circuit pattern or layer for electrical connections to the electrical devices, whilst the other layer, a substrate or core, acts as a heat spreader. The metal core may for example be aluminium, which gives a lighter weight with gentle heat dissipation; alternatively, a copper core provides greater heat conductivity, at the expense of extra weight. Iron allOy or even carbon are also available, as are ceramics. In order to maximise the thermal conductivity from the device through the electrically insulative layer to the underlying core, the device being mounted on the electrically insulative layer, various alternative materials are proposed for the insulative layer. Examples include FR4 (fire retardant plastics) and polyimide, both of which have a thermal conductivity of about 0.3 Wm1K1. An alternative is PTFE ceramic with a thermal conductivity of 0.5 to 0.66 Wm1K1. A further alternative is a thermally conductive dielectric, which would have thermal conductivity of between 1.0 and 4.0 Wm1K1. The most commonly used dielectric in the thermally conductive laminate is a special filler-matrix composite, which offers a low thermal resistance path for heat conduction, acts as a bonding medium, and acts also as an insulation layer between the circuitry and the heat spreader. Typical thermal conductivity of this special insulation dielectric is 4 to 16 times higher than conventional FR4 dielectric.

Compared with ceramic substrates such as alumina, aluminium nitride and beryllium oxide, which can also be used as heat dissipation media, the MCPCB offers lower cost and better mechanical strength, even though its thermal conductivity is typically lower than that of ceramic substrates.

Light emitting diode lamps have been provided, mounted on MCPCBs, of approximately 2cm diameter. These devices typically conduct heat at between 3 and 5 Wn11 Ic1, in the case of an aluminium substrate, and 8 Wm1K1 in the case of a copper substrate. This has been found to be inadequate for meeting the demand for higher power lamps which may contain multiple LED units, and which are required for example to replace conventional tungsten filament lamps running on domestic mains lighting circuits. Manufacturers of LEDs recommend maximum temperatures for the semiconductor junctions, since excessive temperature reduces lumen output (brightness) and decreases lifespan.

Accordingly, the present invention provides a thermal core printed circuit board whose core comprises a planar, thermally conductive substrate from which project, through an overlying electrically insulative layer, a plurality of bosses of the same material as the remainder of the core, and an electrically conductive pattern layer overlying the electrically insulative layer, for the mounting thereon of electrical devices using respective bosses as heat sinks and elements of the electrically conductive pattern as electrical circuit connections.

The invention also provides electrical apparatus comprising a plurality of electrical devices fixed in direct thermal contact to respective metal bosses on the substrate of a thermal core printed circuit board, the electrical terminals of each device being connected electrically to a common conductive pattern layer formed on the substrate with an electrically insulative layer between the substrate and the conductive pattern layer, the bosses projecting through the insulative layer.

Further, the invention provides a method of manufacture of such apparatus comprising forming a thermal core printed circuit board with a plurality of bosses projecting from its substrate through an electrically insulative layer formed over the substrate, forming a common electrically conductive pattern layer over the electrically insulative layer, and connecting the electrical devices thermally to the respective bosses and electrically to the conductive pattern layer.

In order that the invention may be better understood, a preferred embodiment will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: Fig. I is a cross-section through part of a typical conventional MCPCB; Figs. 2a to 21 illustrate the sequential processes of manufacture of a lamp on an MCPCB such as that of Fig. 1; Fig. 3 is a cross-section through part of a lamp embodying the present invention and formed on a modified MCPCB in accordance with the present invention; Fig. 4 illustrates in plan view a large scale MCPCB; and Fig. 5 illustrates in plan view part of a lamp including multiple light emitting units such as those of Fig. 3.

A typical lamp formed from a light emitting diode unit on an MCPCB is the LUXEON (registered trade mark) emitter manufactured by Philips as part of their Lumileds group of products. The structure of part of this lamp is shown in cross-section in Fig. 1, omitting the LED. The MCPCB consists of an aluminium substrate 1, over which is formed an epoxy layer 2, over which is formed a copper circuit pattern layer 3. Solder is used to connect wires from the LED to the copper circuit pattern 3, and Fig. I shows one solder pad, as well as an adjacent layer of solder mask. The MCPCB in this example is 1.6mm thick, approximately.

The LED light emitting device is manufactured in the steps shown in. Figs. 2a to 21. A planar MCPCB shown in Fig. 2a consists of a metal substrate 1, an electrically insulative layer 2 and a copper foil 3 from which the circuit pattern is.to be formed. As shown in Fig.2b, the layout is created and then copied via a photoresist 4 and etching process. This process selectively etches through the copper layer 3, as shown in Fig. 2c. A solder mask 5 is added, as shown in Fig. 2d, to protect the traces in the copper layer during the hot bar soldering process described below with reference to Fig. 2j. As shown in Fig. 2e, solder paste 6 and SMD components are applied to the surface, and as shown in Fig. 2f reflux soldering of the SMD components is carried out, together with the plating of tin on the solder pads. Thermally conductive glue 7 is dispensed as shown in Fig. 2g, and solder flux 8 is dispensed on the solder pads,, as shown in Fig. 2h. As shown in Fig. 2i, a fight emitting diode unit 9, comprising a dome over a circuit block from which extend wires 10, is picked and placed over the MCPCB using a vacuum nozzle, and is fixed to the electrically insulative layer 5 with the thermally conductive glue 7. The connector wires 10 have end tabs which overlie respective solder pads. Using a hot bar soldering machine, as shown in Fig. 2j, the wires are soldered to the pads. The emitter units are then tested functionally as shown in Fig. 2k, to check that the LED emits light, and the final product is shown in Fig. 21, as the glue is cured in an oven.

As explained above, the present invention improves thermal conductivity from the electrical device, such as the LED 9, through to the metal substrate 1. An embodiment of the invention is shown in partial cross-sectional view in Fig. 3, in which the same numerals are used to describe equivalent components to those in Figs. I and 2. The LED device 9 is shown schematically, without the underlying circuit block. In accordance with the invention, a boss 10 of the same metallic material as the substrate 1 projects through the overlying layers 2 and 3, and is in direct thermal contact through the glue layer 7 with a base of the LED device 9. Accordingly, the boss has a width slightly greater than that of the LED device, in this example. Although only one LED device 9 is shown in Fig. 3, two or more such devices can be formed on the same MCPCB, for example in a regular pattern as shown in Fig. 4.

In the example of Fig. 3, the metal core I has sufficient thickness to establish the necessary rigidity of the structure, and may for example be between 1 mm and 1cm thick. The copper layer 3 is typically about 40p thick, and the intermediate layer 2, for example of epoxy resin, is typically lOp thick. Although not shown inthis example, the MCPCB may finally be coated with a finish layer of insulation, typically lOp thick, so that the combined thickness of the three layers formed over the substrate I would be 6Op thick. The boss 10 is formed typically flush with the surface of the surrounding ayer, so that the boss would also be typically about SOp thick.

For the MCPCB structure shown in Fig. 3, the thermal conductivity for the LED through to the substrate I is typically 190 to 200 Wm1K1 in the case of an aluminium substrate 1, 10; and as much as 380 -400 Wm1K1 for a copper substrate 1, 10.

Although in this example the structure is an MCPCB, a ceramic or a carbon or carbon compound substrate could be used instead of the metal substrate 1, and ceramic materials described above could be used. In the case of the ceramic oxide material Ceramtec, the thermal conductivity can be 200 Wm1K1, and in the case of the ceramic oxide Alunit, the thermal conductivity can be 30 Wm1K1.

The metal or ceramic substrate 1 is preferably formed by engraving a planar substrate.

-Manufacturing processes are becoming available which enable planar substrates to be laser engraved, with a pattern such as that shown schematically in Fig. 4. The bosses 9 are formed as lands, projecting from the engraved surface of the substrate 11.

Where the substrate 11 is sufficiently thin that it is flexible and can be formed around a roller, then a gravure cylinder engraving process may be used, using for example a fibre optic laser with a spot size of less than l0ii, to give an engraving resolution of up to 1i. Alternatively, chemical etching or die stamping or electroplating or SLS, selective laser sintering processes may be used to form the bosses 9 in a pattern.

The method of making the substrate may be similar to that for making a so-called ballard skin, such as a copper layer, formed over a gravure cylinder, by electroplating.

The ballard skin is then engraved by laser, or etched, with a pattern. Such a ballard skin has normally been retained on the cylinder and used in situ for gravure printing.

For implementing the present invention, it would be removed and flattened out.

A conventional PCB with integrated circuits or light-emitting units attached to it electrically could be brought into contact with the embossed substrate and then fixed to it by thermal glue.

The present invention may be used in the manufacture of lamps, for example lamps containing LED units and used to replace conventional tungsten filament lamps. The LED lamps may be formed in housings and with electrical contacts and external terminals for connection to a power socket for domestic lighting, without the need to provide different configurations for the sockets. Such lamps typically comprise a plurality or a multiplicity of semiconductor light emitter units such as LED units 13, formed on a substrate 12 as shown in Fig. 5. The number of light emitter units 13 is selected to provide the necessary brightness of the lamp. Heat generated in each unit 13 is dissipated through the substrate 12, which may be an MCPCB or a ceramic core PCB in accordance with the invention and illustrated in Fig. 3. To simplify manufacturing processes, the sub assembly shown in Fig. 5 may be separated from a large structure shown in Fig. 4, for example by machine cutting. Thus the assembly 11 of Fig. 4, complete with a multiplicity of light emitting units 9, may be used for forming multiple lamp sub-assemblies such as the one shown in Fig. 5. Clearly many different possibilities exist for the configuration and size of the lamp units, and the one shown in Fig. 5 which is circular and which shows six light emitting units 13 is only an example.

The invention decreases the cost of lighting, by increasing brightness and lifespan of light-emitting units, as a result of the cooling of their operating temperature. Similar advantages result from the cooling of integrated circuits or other devices that generate unwanted heat.

Claims

CLAIMS1. A thermal core printed circuit board whose core comprises a planar, thermally conductive substrate from which project, through an overlying electrically insulative layer, a plurality of bosses of the same material as the remainder of the core, and an electrically conductive pattern layer overlying the electrically insulative layer, for the mounting thereon of electrical devices using respective bosses as heat sinks and elements of the electrically conductive pattern as electrical circuit connections.
2. Electrical apparatus comprising a plurality of electrical devices fixed in direct thermal contact to respective metal bosses on the substrate of a thermal core printed circuit board, the electrical terminals of each device being connected electrically to a common conductive pattern layer formed on the substrate with an electrically insulative layer between the substrate and the conductive pattern layer, the bosses projecting through the insulative layer.
3 Electrical apparatus according to claim 2, in which each electrical device is a semiconductor light emitter unit.
4. Electrical apparatus according to claim 3, in which the semiconductor light emitter units are LED, light emitting diode, units.
5. Electrical apparatus according to any of claims 2 to 4, in which each electrical device has a planar base thermally connected as a heat sink to a heat generating element of the device for heat dissipation, the planar base being coupled thermally to a planar surface of the respective boss.
6. A method of manufacture of electrical apparatus according to any of claims 2 to 5, comprising forming a thermal core printed circuit board with a plurality of bosses projecting from its substrate through an electrically insulative layer formed over the substrate, forming a common electrically conductive pattern layer over the electrically insulative layer, and connecting the electrical devices thermally to the respective bosses and electrically to the conductive pattern layer.
7. A method according to claim 6, in which the bosses are formed by laser engraving the substrate to leave the bosses as lands.
8. A method according to claim 6 or claim 7, comprising connecting a multiplicity of the electrical devices to the thermal core printed circuit board and separating regions of the printed circuit board to form separate units each comprising a plurality of adjacent electrical devices.
9. A method according to claim 8, in which the electrical apparatus is a lamp comprising plural light-emitting units on the same thermal PCB, the method comprising connecting the lamp to a housing with external terminals for connection to a power socket for lighting.
10. A method of manufacture of a thermal core printed circuit board according to claim 1, in which the bosses are formed by laser engraving the substrate to leave the bosses as lands.
11. A lamp substantially as described herein with reference to Figs. 3 and 5 of the accompanying drawings.
12. A printed circuit board substantially as described herein with reference to Figs. 3 to 5 of the accompanying drawings.
13. A method of manufacture of a lamp, substantially as described herein with reference to Figs. 3 to 5 of the accompanying drawings.
14. A method of manufacture of a printed circuit board, substantially as described herein with reference to Figs. 3 to 5 of the accompanying drawings.