LED light source

The present invention relates to a method for producing an LED light source of high light intensity with at least two LED dice, the LEDs being arranged on a thermally conductive circuit board so that the heat is dissipated through the circuit board during operation.

From US-5936353-A it is already known to arrange LEDs with high density on a printed circuit board made of ceramic or metal. This has the advantage that the heat can be dissipated through the circuit board during operation. It is thus possible to put a heat sink on the back. Suitable materials are BeO, AI₂O_3l Copper or aluminum or AIN.

The purpose of this measure is, of course, to obtain the highest possible luminance, and this also requires an optimization of the fastening and electrical contacting, as will be explained below.

LEDs processed on lead frames based on dice and wire bonding processes and recently with flip-chip technology are state of the art.

The currently most efficient blue and green LEDs are based on GaN on sapphire substrates. Due to the non-conductive substrate in this construction, both contacts are arranged on the top of the LED dice. However, the light emission that occurs within the GaN layer of the LEDDice is considerably restricted by the contact surfaces. In this way, the external power output of this LED dice can be increased by a factor of 2 if it is processed with the contact surfaces facing downwards using flip-chip technology (for flip-chip technologies, see e.g. US-5438477-A, US-5111279-A ). Similar advantages can be achieved with red and orange dice, which are based on transparent substrates, provided they are face down or be bonded to one side surface so that the emission occurs through the opposite surface. In the latter case, electrical contact is made via the end faces,

This essential advantage of the flip-chip technology for processing the LED dice with regard to the generation of high brightness in a small space can be increased if LEDs are applied directly to the circuit board in array form using flip-chip technology.

A particularly important aspect with this arrangement is the thermal management. The contact areas between the die and the contact pads of the circuit board are smaller compared to conventional die / wire bonding technology. For this reason, the thermal conductivity of the connection material between the LED dice and the circuit board is even more essential than with conventional chip-on-board technology. Contact bumps according to the status of

Technology are about as long and wide as they are high, see for example US-5909057-A, where spherical bumps are drawn and described. Their diameter is very small (about 0.1 mm), so that there is a correspondingly small cross-sectional area (about 0.01 mm²) results

In addition, high demands are placed on the structuring of the conductor tracks. The maximum structuring width is in the range of 50-100 pm. Since the maximum structure width typically correlates with the layer thickness of the conductor tracks due to technological limitations, this may also have a maximum of these values. With regard to good thermal dissipation, good heat dissipation of the conductor tracks, which is determined by the type of conductor track and its layer thickness, is essential. Additional heat dissipation from the die to the printed circuit board can be achieved by using a thermally conductive potting compound that is inserted between the dice and the printed circuit board (underfill).

Furthermore, special precautions must be taken that when contacting the dice no

Short circuit occurs between the two contact points. A pure electrical isolation from

Soldering points between two circuit boards as a replacement for a connector strip with an epoxy layer is known from US Pat. No. 5,060,844

It is the object of the present invention to improve the method of the type mentioned at the beginning in order to realize LED arrays with the highest possible brightness and the smallest possible size.

This object is achieved according to the invention by a method of the type mentioned at the outset in that the method includes the following steps:

- the circuit board - with the exception of the contact surfaces - is provided with an insulating layer,

- Either contact bumps or contact layers with a maximum height of 100 μm consisting of a solder material with good thermal conductivity are applied to a surface of the contact electrodes of the LED dice, or contact layers with a thickness of 1-50 μm are applied to the contact surfaces of the circuit board ,

- The LED dice are pressed by means of a flip-chip technique or a related technique with the contact surface facing downwards onto the contact surfaces of the circuit board or in their immediate vicinity and at the same time or subsequently - by means of ultrasound or thermally soldered, with each LED dice having a minimum metallization area between 0.5 and 5 mm² is assigned.

Contact surfaces are therefore produced on the conductor tracks, with an insulating layer in between. The LED dice is then pressed onto these surfaces and attached. In this way, a short circuit between the positive and negative contact zones can be prevented. The specified dimensions ensure adequate heat dissipation.

This method can be modified so that it is not soldered, but that metallic contact bumps or contact layers, in particular made of gold, are used and that the LED dice are pressed exactly onto the free contact surfaces of the circuit board and fastened by means of a thermocompression process.

For LEDs with contacts on the end faces, the procedure includes the following steps:

- the circuit board - with the exception of the contact surfaces - is provided with an insulating layer,

- on the contact electrodes of the LED dice, contact bumps or contact layers with a maximum height of 100 pm consisting of a solder material with good thermal conductivity are applied to a surface,

- The LED dice are pressed with the contact surfaces perpendicular to the PCB plane with one side surface on the free contact surfaces of the circuit board or in their immediate vicinity and soldered to the contact bumps applied to the side, with each LED dice having a minimum metallization area between 0.5 and 5 mm² assigned.

In order to achieve a high luminance, it is useful if more than 4 dice, partly connected in series and partly in parallel, are pressed on.

For air cooling, a heat sink can be attached to the back of the circuit board, for example glued on. This is usually dimensioned in such a way that its maximum temperature in continuous operation is at most 45 ° C above the ambient temperature.

A thin adhesive mat coated on both sides can be used to glue the heat sink, but a copper foil coated on both sides can also be used to glue the heat sink.

Instead of gluing the heat sink, it can be soldered flat to the back of the conductor track. A vacuum soldering furnace can be used to solder the heat sink.

Instead of air cooling, electrical cooling can be applied to the rear of the circuit board.

It is useful for luminance and thermal management if LED dice with a base area between 300 and 700 μm² be pressed on.

The thermally conductive circuit board can contain at least one ceramic layer, or a metal core circuit board can be used as the circuit board.

According to another embodiment, a ceramic body can be used as the printed circuit board, on the upper side of which the conductor tracks are applied and the surface of which is shaped in such a way that it serves as a cooling body. In this way, no additional heat sink needs to be connected to the circuit board.

Finally, it is useful if the contact areas on the conductor track are oval or rectangular and have a length that approximately corresponds to the edge length of the LED dice. This results in the largest possible contact points, which is important for electrical, but especially for thermal conductivity.

The present invention is explained in more detail using an example. A ceramic circuit board (consisting of Al2O3, AlN, BN, BeO, ..) is used, which is coated with metals such as Cu, Ag / Pd, Au. Thick-film ceramic circuit boards, that is to say ceramic circuit boards in which the conductor tracks are applied by means of a screen printing technique, are preferably used here. Furthermore, printed circuit boards with electrodeposited metallizations or metallizations applied by means of thin-film technology can be used. The structuring of the circuit board is carried out using photolithographic techniques or, in the case of thick-film systems, directly during printing.

An insulating layer is applied to the conductor tracks, which does not extend continuously over the surface of the circuit board, but has recesses over the contact zones. In a preferred variant, these recesses are designed in such a way that they have a rectangular or oval shape in order to contact the largest possible area on the contact side of the LED dice.

Contact bumps (consisting of solder materials, NiAu, Au or similar) are then applied to the LED dice. These can be applied, for example, by means of laser welding technology or by means of wire bonding processes and consist of a low-melting solder material or Au. Contacts that are made via such bumps have the disadvantage, however, that the distance from the contact surfaces of the die relative to the circuit board is predetermined by the bump height (> 50 μm), which hinders good heat dissipation from the die to the circuit board. As an alternative to the bumps, contact layers are applied to the circuit boards, which serve as contacts. The layer thicknesses of these layers are chosen in the range of 5-50 μm.

The dice are then placed on the circuit board using a pick and place device. Then there is a targeted heating and subsequent cooling of the attached

Dice, so that the solder or Au melts and then solidifies, thereby fixing the dice on the circuit board. As an alternative to soldering, a thermocompression process is used to attach the LED dice to the circuit board. The LED dice are placed or the metallization is structured in such a way that each LED dice has a minimum metallization area of ​​between 0.5 and 5 mm² assigned

In order to prevent discharges via the LED dice when it is put on, a diode can optionally be placed in anti-parallel to the LED for ESD reasons.

It is of course possible that at least one LED dice has a different emission color than the 5 others and its operating current can be regulated independently of the operating current of the others, so that mixed colors can be generated.