What are the uses of LED circuit boards

How LED circuit boards can be constructed

Matrix headlights adapt the light to the driving behavior and the environment. Human Centric Light systems create biodynamic light to make you feel good indoors. Intelligent street lighting improves the quality of light and lowers energy and operating costs. While modern light-emitting diodes open up undreamt-of possibilities for lighting designers, thermal management for ultra-bright LEDs presents hardware developers and circuit board designers with new challenges. For reasons of cost alone, it makes sense to integrate the LED driver electronics on the same circuit board instead of connecting two boards with cables, plugs or SMT jumpers.

Hotspots and costs

Designing control electronics and LEDs on a printed circuit board saves space, connection technology and / or assembly steps. In addition, the weight and volume of the assembly are reduced. However, the circuit board must not limit the performance of the LEDs. The circuit board design, which largely determines the function of the entire assembly, must therefore optimally cover five key requirements:

  • 1. Highly efficient heat dissipation and even heat spreading for maximum performance and reliability of the LEDs as a basic requirement
  • 2. Realize high packing densities in a small installation space (miniaturization)
  • 3. Intelligent lighting control on the same circuit board as the LEDs in order to achieve different lighting moods and / or to adapt the lighting to the respective situation
  • 4. Flexible implementation of light control and lighting design for function and aesthetics
  • 5. Reduce system costs

There is no one-size-fits-all solution. Every development project has different electrical, optical, mechanical, design and economic goals, a different starting point and different boundary conditions. At KSG, FR4 multilayers are the basis for a large number of different solution concepts for LED projects. This means that all options for intelligent sensor-based light controls based on a cost-effective standard circuit board and the further processing of these in established manufacturing processes in assembly production are available.

The key to success is co-engineering, in which the application engineers of the circuit board manufacturer work together with the developer / designer to develop a design that is optimized for the respective LED lighting - because, as is well known, the subsequent production costs are determined in the design. The application team draws on various circuit board technologies and has a lot of experience. Together with the hardware designers, the circuit board experts can optimize the thermal concept, taking into account the technical and economic requirements as well as the installation situation. At the same time, the use of multi-dimensional constructions can reduce the installation effort and technical lighting effects can be achieved.

For applications in which the focus is exclusively on light intensity, the basic function of a circuit board is often sufficient to ensure efficient heat management for reliable operation of the LEDs. In extreme cases, developers have to combine various measures in a precisely coordinated manner in order to exceed the apparent limits of feasibility.

Requirements for printed circuit boards for lighting controls with LEDs. KSG

Efficient thermal management

Efficient thermal management is the basic requirement for temperature-sensitive semiconductor components, as most very bright LEDs only offer a small area for heat dissipation. The solution is a thermally optimized circuit board design with a layer structure adapted to the LED performance class and the respective application. Depending on the design and requirements, the layer structure is combined with microvias, thermal vias, blind vias (blind holes) and buried vias (buried vias). To ensure a long service life for the LEDs, the waste heat from the LEDs can also be decoupled from the control electronics using a well thought-out circuit layout.

Thermovias, bores through-plated with copper near the hotspot, support heat dissipation from the LED via the circuit board to the heat sink. Plugged thermal vias or copper-filled microvias with integrated solid copper elements achieve a higher thermal performance than classic Thermovia FR4 circuit boards. The thermal vias filled with thermal paste can be placed directly below the LEDs, which reduces the thermal resistance.

The heat dissipation as well as the heat spreading increase massive copper elements in the multilayer inner layers directly under an LED hotspot. After pressing the multilayer layers, the copper is about 60 µm below the surface of the printed circuit board. For maximum performance and reliability, the copper elements are combined with a thermally optimized layer structure and correspondingly placed vias.

Reduce system costs

With a well thought-out circuit board design, the overall costs of the assembly can also be reduced because there is no need for later assembly or connection with connectors and cables. Semiflex circuit boards are an inexpensive alternative to soldering or wiring rigid circuit boards with static bending loads. An FR4 remnant web and conductor tracks establish the connection.

The starting point is a rigid FR4 circuit board. In the later flexible areas, the base material is milled off to a thickness of approx. 150 µm and the conductor tracks placed on it are coated with flexible varnish or a cover film. The Semiflex circuit board is laid out as a two-dimensional circuit board, manufactured and assembled. During assembly, installation in a housing or on a heat sink, the assembly is then bent into shape and fixed in this position.

Principle of heat dissipation via an FR4 circuit board. KSG

An example: DLYX Simple Lighting is the name of the event lighting developed by Klaus Schober, which combines miniaturization with strong light output and is robust and durable. The multi-colored headlights themselves are connected to each other and to the operating unit and controller with just one line, which transmits both energy and data. In the original solution, separate energy and data lines (voltage distribution and DMX, network or bus system) were required to control the headlights and LEDs. The cabling of such a solution turns out to be relatively complex in practical handling when setting up event lighting, just like a theoretical control of the lights by radio can often be error-prone. Here the solution scores with just a single line. However, in order to accommodate the necessary control electronics directly next to the LEDs in the narrow luminaire housing, four individual circuit boards had to be mounted over the heat sink and manually connected to one another by soldering, which resulted in high production costs.

The solution, newly designed in co-engineering with KSG, which directly eliminates this problem, is a 4-layer multilayer with a base area of ​​137.24 mm x 100.2 mm and 10 mm x 10 mm copper profiles and three Semiflex bending points, which the replace manual soldered connections directly. The circuit board is populated on one side and then bent around the heat sink. The heat sink takes over the heat from the high-performance LED with 10 watts via the integrated copper profile. This structure optimally cools the high-performance LED (4 colors). The lights are only wired to one another using a cable that is connected directly to a Wago terminal. Thus, this application could be optimally simplified by the use of a 3D multilayer circuit board with integrated thick copper and the system and production costs could be reduced significantly.

Three-dimensional structure

The internal copper profiles for the heat dissipation of the power LEDs also offer lighting designers structural advantages. The solid copper profiles are even suitable for one-time bends when installing the assembly. They are self-supporting and thus enable multi-dimensional printed circuit boards to build up targeted lighting. At the same time, the heat is transported over the bending edge and spread in the circuit board. Individual constructions can direct the light of several LEDs in certain directions, i.e. focus, scatter or simulate the beam angle of an incandescent lamp.

Notch millings are carried out in the FR4 at the bending point defined by the customer. After the assembly process, the segments are given their future shape. Bending angles of up to ± 90 ° are possible. With this solution, too, a two-dimensional circuit board is laid out, manufactured, equipped and only then brought into the three-dimensional form for assembly of the assembly.

Up to now, four rigid circuit boards were assembled together after they had been populated (left). The new solution uses three semiflex connections (right). KSG

The following example from vehicle electronics uses this construction. The developers had three tasks to solve:

  • 1. Three LEDs must light up in three different directions offset by 90 °.
  • 2. The circuit board is designed to dissipate the heat from the three LEDs to the rectangular heat sink inside the assembly.
  • 3. The three LEDs and the control electronics must be on a circuit board and each LED must be individually controllable.

Instead of mechanically connecting three circuit boards after assembly, a circuit board with integrated copper elements was developed and bent in two places. One board thus solves all three tasks. The copper elements in combination with micro or thermal vias spread the heat through the circuit board and conduct it away to the heat sink. The LEDs are brought into the required spatial directions via two bending points and the assembly is folded around the heat sink thanks to the self-supporting construction.