A flexible circuit board can be used for this purpose on which at least one radiation source is mounted and that is composed of a film/foil system. The flexible circuit board includes a thermal connection to a heat sink, while the film/foil system is made from at least an insulating support layer and a metal foil. The insulating support layer has an opening at the site at which the thermal connection is to be created to the heat sink, and the metal foil is subdivided into different sections.
Recently, LEDs have increasingly fulfilled the requirements that are imposed for efficient light sources in terms of light spectrum and luminous efficacy. LEDs have a number of advantages compared to conventional light sources such as incandescent lamps, halogen lamps, and fluorescent tubes. Energy consumption is reduced for the same quantity of light; less heat is generated; LEDs are insensitive to shock, achieve significantly shorter switching times, and have a longer service life.
Due to their small overall size, LEDS furthermore have design advantages for the configuration of illumination modules, for applications to general room lighting, but also in industry, automotive technology, as well as in medical equipment technology, and other applications.
Special components for interconnecting individual LEDs to create larger illumination modules are required in order to implement these advantages in the above-mentioned applications, which components ideally satisfy the following requirements. A desirable factor is a low electrical resistance for electrical currents in the range of 100 mA up to several A, and voltages of typically up to 500 V. It is also desirable to be able to mount the illumination elements flexibly on as many 3-dimensional structures as possible so as to achieve a wide variety of lighting effects.
Thermal management is extremely important due to the high output over a very small area measuring less than a few mm2 for the LEDs. A low junction temperature is absolutely essential since an increase in the junction temperature results in a reduction in the service life and light output of the LED. The efficiency declines here as the temperature rises, with the result that the luminous efficacy declines at the limit of performance as a function of the type of cooling. For this reason it is critically important to create the most effective thermal connection possible to a heat sink so as to allow the heat generated in the LED to be dissipated as efficiently as possible. As a result, LEDs typically have, in addition to the two electrical (+/−) contacts, a “cooling connection” below the semiconductor junction that is responsible for generating the light and that must be connected as efficiently as possible to a heat sink, for example, by means of a thermally conductive adhesive or solder. To this end, a thermal resistance of 0.5-5 K/W must be achieved for high-performance applications.
Currently, so-called high-performance circuit boards are used to interconnect LEDs, for which circuit boards various technologies have typically come to be employed. Metal-core circuit boards have a metal core of copper or aluminum that dissipates heat. DE 10 2008 016 458 [U.S. Pat. No. 2,530,026] for example, discloses a rigid circuit board that is provided with at least one throughgoing hole in which a heat-dissipating element is mounted, at least one radiation source being mounted on the heat-dissipating element. The effective heat-removing heat-dissipating element, for example, is a block of metal, in particular copper. The disadvantages of these circuit boards are the high cost per surface area, as well as their size and weight. In contrast thereto, “Direct Copper Bond” (DCB) substrates are composed of copper conductive traces that are applied to ceramic substrates. These are employed primarily in power electronics for automotive applications, and in part in optoelectronics to connect laser modules. U.S. Pat. No. 4,563,383 discloses a three-layer copper foil that is embedded between two ceramic layers, which approach enables thin modules to be produced. The disadvantages for all rigid circuit boards are the restrictions on freedom of design since it is impossible to incorporate different three-dimensional structures into the rigid circuit boards and also not go below a certain size.
For this reason, flexible circuit boards have been developed, including ones based on polyimide films with high temperature stability, or also PET (polyethylene terephthalate) and PEN (polyethylene naphthalate) films. Flexible circuit boards are known from the prior art that use polyimide films as the base material on which a thin copper layer is applied. Acrylic adhesive can be used for dynamic flexible connections; epoxy adhesive also allows for a certain amount of dynamic flexibility. A flexible protective film can be pressed on, also by using an acrylic or epoxy adhesive. The flex circuits constructed thereby can be used in a space-saving approach by folding them into extremely tight structures, such as, for example, photographic or video cameras. Flexible connections are also used in applications undergoing long-term stresses, such as inkjet printers or laptops, when connecting main board and monitor. The disadvantage of flexible circuit boards of this constructive design is that it is impossible to efficiently dissipate heat. In addition, they are still too rigid to be installed on freely designed three-dimensional substrates.
Various techniques have been developed to overcome this problem. EP 1 874 101 [U.S. Pat. No. 8,071,882], for example, describes a flexible circuit board that is composed of alternating layers of metal foils and plastic films on which an LED light source can be mounted. These are well suited, in particular for use as an LED light source for a backlight of a liquid-crystal display. However, these circuit boards are multilayered and complex in terms of design, and have less efficient dissipation of heat due to the reduced thickness of the layers. In addition, they cannot be connected to three-dimensionally designed substrates without additional installation expense.