Current developments and challenges in LED based vehicle lighting applications
Interior lighting – dashboard and ambient lighting
In the vehicle interior, a consolidation of the use of LED packages in favor of individual multifunctional diodes can be observed. The decreasing demand results from the increased use of TFT technology in the cluster / cockpit area, but also by RGB LEDs in the ambient lighting, which can be used to display all colors according to the customer’s requirements instead of a single LED. The main focus here is on the footwell, but also the side parts of vehicles are equipped accordingly.
For most vehicles fitted with ambient lighting, about 10 RGB LEDs per vehicle are used. Ambient lighting applications were first used in the upper-class vehicles. But now it has also become increasingly popular in medium-class vehicles.
However, in terms of the LED characteristics of ambient lighting with RGB LEDs are concerned, opinions [tend to] differ sharply. In general, the preferred/most wanted wavelength range of red is 620-633nm, green is 520-535nm, blue is 447-471nm. As far as the basic colors are concerned, the desired range is often limited to a maximum of +/- 4 nanometers. But for blue, there are differences: the shorter the blue wavelength, the lower the brightness. Some car manufacturers therefore value a blue over 456nm, some others request a significantly higher figure. For this reason, Everlight has launched four customer-specific RGB versions for different vehicle manufacturers which are available now.
Especially in the interior, also the requirements regarding white with its respective color grades rise sharply. Many suppliers are confronted with the problem that their plastic light conductors cause a shift in the white point, whereas the overall appearance in the vehicle interior is supposed to further ensure the highest possible uniformity.
This issue can often only be solved by qualifying a particular color location. For that, close communication between the supplier and the LED manufacturer is indispensable in order to meet the requirements of the vehicle manufacturer.
Even small manufacturing tolerances may cause that virtually identical light-emitting diodes produce a different color impression. Therefore, LED manufacturers are working tirelessly to improve both the materials used and the processes.
Up to now, a single-bin was the best possible way to get a limited color location at white. But even a nominal ‘single’ bin can, without any technical problems, be split into 4 ‘sub’ bins. To make sure that a so-called ‘fine white binning’ offer benefits for both sides, customer and manufacturer, an open, constructive discussion is necessarily and always an advantage.
Exterior lighting – the demand for Optoelectronics is strongly increasing
The exterior area of the vehicle is still dominated by red, yellow and white. Absolute sulfur resistance is now regarded as a prerequisite for a successful qualification. This is achieved by the use of gold in leadframe plating.
But there are different existing materials and methods. The most commonly used combination in the PLCC leadframe consists of a copper base, which usually receives a 0.4 – 2.0 μm coating of nickel and finally a corresponding gold coating. There are two methods, the galvanic or the chemical gilding, whereby the galvanic gilding receives about twice the gold layer thickness of the chemical one.
As in the interior, the tried and tested PLCC packages are often used as housings, especially for combination rear lights. However, ceramic housings are indispensable for high brightness and high operating currents. The area that is in between is interesting too. For such outdoor applications, it is recommended to use an EMC (Epoxy Molded Compound) housing, which provides an excellent price-performance ratio. The combination of high lumen values, coupled with high reliability and an attractive price level makes the EMC package absolutely essential.
In terms of Red (taillights, central high mounted stop lights) there is an increasing call also for darker reds with higher wavelengths of 626nm or even 633nm instead of the conventional used 618nm. Unfortunately, higher wavelengths are also associated with lower efficiency, which in extreme cases could lead to design changes.
Of course there is always the possibility of using more efficient light-emitting diodes, e.g. emitting diodes with a larger active chip area or increase the number of LEDs, but all simpler ways should be taken before changing the existing layout.
Amber yellow, used in the turn-lights, changes more and more to PC Amber (Phosphor Converted Amber). The advantage here is higher brightness, especially at higher temperatures. At the same time, PC Amber LEDs are also significantly more color-stable and have therefore become increasingly attractive in applications starting at 0.5 watt. For daytime running lights and headlights, there are new reliable solutions like a ready-to-assemble module as well as a component solution. The modules or LEDs are available in a 2, 3, 4 or 5 chip version.
In order to offer the most reliable LED measurement solutions to determine luminous flux, light intensity, color parameters, spectrum and radiation characteristics, it is essential to work with the largest manufacturers of LED measuring technology, especially in Europe. Almost every component qualification with its corresponding tests is carried out in our company’s own laboratories in Taiwan.
Zero defect rate
Everyone wants 0-defect rates, but why is this particularly important in the automotive sector? Because here, we are talking about a ‘0 ppm failure rate’, that is, Zero faulty parts per million. Especially in the automotive industry, the frequency of failures (defect rates) is expressed in ppm, for example in the electronics of climate control devices or cockpit instruments. The car manufacturers demand strict ppm rates from their suppliers. This means that only a given, relatively small number of a million finished modules may be faulty.
Let us look at a printed circuit board with various active and passive components. If every component supplier had his own small error rate here, it would add up quickly. For the fully equipped, complete PCB, however, even greater error rates would arise, which would entail a considerable financial expense for their elimination.
Everlight subjects its products to a comprehensive series of management system certifications, including TS-16949, OHSAS 18001 and ISO 14001 for manufacturing quality. All products are qualified according to AEC-Q101 (Automotive Qualification Requirements for Discrete Product).
A car life cycle, what is this? For many suppliers of LED modules 8.000 operating hours. Over this period of time, the characteristic changes only slightly, because LEDs do usually not fail suddenly. The luminous flux decreases gradually.
Aging depends on various criteria, e.g. the semiconductor device used, operating conditions such as temperature and current, color temperature changes / drift, mechanical stress on the LED during processing or use, obsolescence / clogging of the housing or potting material caused by gallium nitride as well as external influences like salt-containing, sulfur-containing or chlorine-containing environments and, in particular, by the effects of moisture on a component.
Everlight’s Automotive products are continually tested and improved to meet these criteria as well as the production processes.
About the author: Andreas Schimmelpfennig is Director Automotive Business, Everlight Electronics Europe.