AU5365500A - Heat generating electrical circuit device array with heat dissipation - Google Patents

Description

P/00/01i1 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Heat generating electrical circuit device array with heat dissipation The following statement is a full description of this invention, including the best method of performing it known to me/us: Freehills Carter Smith BeadleMELC600370397.7 CD/00370378.3 1 HEAT GENERATING ELECTRICAL CIRCUIT DEVICE ARRAY WITH HEAT DISSIPATION Field of the Invention The present invention relates to heat generating electrical circuit device arrays and in particular to an improved method of dissipating heat generated by the devices. The invention has particular application to light emitting diode (LED) arrays but is not so limited.

Background of the Invention The technology of LED devices is continually being developed and current 10 LED's are displaying higher intensities and uniformity than their earlier counterparts. Together with the development of multicoloured LED's, these devices are moving away from being used solely as indicating means towards lighting means. LED's are programmable, giving them the potential to replace neon and fluorescent lighting in many applications where intricate lighting patterns l*oe and displays are desired. LED's are favoured for their high efficiency, their ***reliability and long operating life, their resistance to shock and vibration, and their versatility. LED's are finding application in traffic signals, cellular phones, automobiles, and electronic signs, for example. When used for electronic signs, the LED devices are typically provided as an array. In this way it is possible to 20 achieve a high brightness over a large area by attaching a large number of LED devices to a substrate.

An inherent deficiency of LED devices, and many electrical circuit devices, is their generation of heat, the problem being amplified when the devices are provided in an array. Most of the electrical power provided to LED's, as with all light sources, is dissipated as heat. When the LED's are closely spaced, this heat generation can substantially increase the temperature of the substrate and the temperature of the LED devices. This undesirable increase in temperature of the LED devices reduces the luminous flux or brightness of the LED's, affects the dominant wavelength or perceived colour of the LED's, and can be destructive both to the LED material and to the encapsulating material. The latter is typically CD00/00370378.3 2 but not necessarily epoxy resin. Conversely, in order to avoid substantial increases in temperature, limits are placed on the current that can be "safely" applied to the LED devices, in turn limiting their brightness.

Conventional printed circuit boards (PCB's) may include multiple vias, i.e.

fine holes through the PCB, lined with copper to provide conductive paths from one side of the PCB to the other. The leads of the circuit devices are connected to planar copper pads in the case of surface mounted components, or soldered directly into or through holes drilled into the substrate material. In use of the array, the vias can act as a heat sink, often resulting in solder work on the surrounding PCB becoming molten and unusable. Conventional practice now therefore is to locate the vias as far away as possible from the circuit devices.

i Many methods of dissipating the heat generated in an LED array have been suggested. Large metal pads may be provided on the PCB to act as heat sinks.

Leads from each LED are connected to the metal pads in order to direct heat away from the devices and to the heat sink. Manufacturers have also devised various specific arrangements of package and lead design in order to reduce the *"thermal problems.

It is therefore an object of the invention to provide an improved means of dissipating heat from an array of heat generating electrical devices.

Summary of the Invention In a first aspect, the invention provides a printed circuit board for mounting a plurality of heat generating electrical circuit devices in an array, the printed circuit board including a plurality of apertures arranged in an array such that each of said heat generating electrical circuit devices is located adjacent at least one of said apertures, said apertures being lined with a conductive material and sized such that respective ventilation paths for heat generated by said devices are provided through each of said apertures.

In one application, the heat generating electrical circuit devices are light CD/00370378.3 3 emitting diodes (LED's).

Advantageously, each of the heat generating electrical circuit devices are in thermal contact with the conductive material lining the adjacent apertures, either directly via contacting leads and/or conductive panels or indirectly via radiation.

The contacting leads may be electrical circuit elements. In this manner, heat is directed away from the LED devices to the conductive material where it is largely dissipated by means of the ventilation path through each of the apertures.

Preferably, the conductive material lining the apertures is copper which may be plated with solder, tin, or gold so as to prevent corrosion.

10 The apertures may be any shape with oval, round or rectangular preferred.

Preferably, the apertures are sized substantially larger than conventional vias. More preferably, the aperture diameter is at least 1:1 the thickness of the printed circuit board, preferably 2:1.

*The devices and apertures may be arranged on the printed circuit board in S. 15 an ordered array. Advantageously, the devices and apertures are arranged in respective rows and columns on the printed circuit board.

Brief Description of Drawings The invention will now be described by way of example, with reference to the accompanying illustrative drawings, in which: Figure 1 is a plan view diagram of a surface-mounted LED device located between two apertures in accordance with an embodiment of the invention; and Figure 2 is a cross-sectional side view of the LED device of Figure 1.

Description of Preferred Embodiments Referring to Figure 1 of the drawings, a single, surface-mounted LED 12 is CD/00370378.3 4 illustrated, located between apertures 20, 22 in printed circuit board (PCB) LED 12 includes a light emitting junction structure encapsulated in an epoxy package or dome. It will be understood that LED 12 is one of an array of LED devices provided on PCB 10 and that a plurality of apertures 20, 22 are also provided in PCB 10. It will also be understood that while the invention is described with reference to LED devices, the invention may be applied to any heat generating electrical circuit devices.

LED 12 is mounted on LED substrate 14 which in turn is connected to PCB via solder panels 16, 18. LED 12 is located between a pair of apertures 20, 22 10 provided in PCB 10. Apertures 20, 22 are illustrated as rectangular with rounded ends but they may take any shape. The apertures 20, 22 are substantially larger e.than conventional vias so as to provide ventilation paths for heat generated by the :eeeLED devices through PCB As is illustrated in Figure 2, the inner walls of apertures 20, 22 are lined with 15 a conductive material, preferably copper 24, 26. The copper lining 24, 26 extends through the apertures 20, 22 and on to each surface of PCB 10 surrounding the apertures 20, 22.

Leads 28, 30 of LED 12 are connected to the copper lining 24, 26 as shown in Figure 2. Leads 28, 30, together with solder panels 16, 18, are a means of directing heat from LED 12 to the copper lining 24, 26. In this manner, leads 28, solder pads 16, 18, and copper lining 24, 26 form an integral thermally conductive network. Heat may also be transferred to the conductive material via direct radiation. Heat in copper lining 24, 26 is substantially dissipated by the ventilation paths provided by each of the apertures 20, 22.

It will be appreciated that apertures 20, 22 in combination with copper lining 24, 26 have been designed with the purpose of acting as heat sinks. They may also be used to provide a conductive path from one side of the PCB to the other.

Conventional vias may also be provided on the PCB as described above.

CD/00370378.3 Alternatively, additional solder-filled holes may be provided in the PCB, through which the leads from the LED devices are passed through the PCB to electrical connections on the other side of the PCB. Apertures 20, 22 are still provided in the PCB with copper leads 28, 30 being used for thermal purposes only.

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