High-Performance Multi-Functional LED Headlamps
Vehicle headlamps are among the last automotive lighting applications to successfully adopt LED technology. Compared to other lamps such as rear lights, brake lights, turn signals and Daytime Running Lamps (DRLs), headlamps have proved more difficult to realize. Although powerful white LEDs on the market today are able to provide sufficient brightness for use in main headlamp units, recent advances in thermal-management and LED assembly at the light-engine level have proved critical in enabling the latest generation of LED-based headlamps to enter production.
High-brightness white LEDs for use in headlamps promise several important advantages over current tungsten-halogen and xenon lamps. The light emitted is at a color temperature close to that of natural daylight, which can improve visibility for the driver. Moreover, the illumination pattern can be controlled accurately to prevent dazzling other drivers. In addition, given proper thermal management, reliability can be extremely high, allowing the lamp to last the lifetime of the vehicle.
An LED headlamp can also be much smaller than a conventional headlamp, giving greater freedom to car stylists, and innovative features can be built in such as selectable illumination patterns for various driving conditions. Finally, it is possible to produce all-in-one light units, which combine LED turn signal, DRL and headlamp high/low beam in a convenient sealed unit that can simplify vehicle assembly and repair.
Despite the many advantages LED headlamps have to offer, thermal design is critical to allow driving them at high current for optimum brightness while simultaneously ensuring reliability for the lifetime of the vehicle. This calls for careful choices with regard to material technologies, especially the substrate technology, as well as assembly techniques such as soldering, mechanical fixings, and thermal adhesives. Accurate and precise placement of light emitters during assembly is also required, to ensure correct optical alignment relative to the lenses and reflectors in the headlamp unit so as to maximize light output and directional control of the beam.
Optimising Thermal Management
Various types of thermally-enhanced substrates can be used, depending on performance requirements and cost constraints. Metal core PCB (MCPCB) technology is widely used and understood, and can offer adequate thermal performance at a reasonable overall cost. Recently, engineers at TT Electronics‘ Salzburg technical center have been able to push beyond the performance limitations of MCPCB by developing thick-film technology on ceramics like aluminium (Al2O3) and aluminium nitride (AlN). TT has over 30 years’ experience working with ceramic substrates and the associated technologies, including thick-film deposition of microelectronic circuitry and surface-mount assembly.
Figure 1 compares MCPCB, Aluminium oxide ceramic and Aluminium nitride ceramic substrates suitable for headlamp light-engine applications, and shows that Aluminium nitride (AlN) offers a superior solution when the best possible thermal performance is required.
AlN has thermal conductivity of 180-230W/mK, which is close to that of pure aluminium and promotes efficient extraction of heat from soldered LEDs into an attached heatsink to be dissipated into the surrounding ambient environment. This extremely high thermal efficiency helps to simplify headlamp design and minimize the size of the heatsink required.
TT Electronics’ expertise in the use of thermal adhesives contributes further to achieving high overall thermal performance. Whereas conventional techniques for attaching the substrate to the heatsink may rely on mechanical fixings or a thermal interface material such as a gap pad, thermal adhesive simplifies assembly and enables a very low bondline thickness typically in the range 70-100µm as shown in figure 2. Automated Optical Inspection (AOI) of dispensed adhesive is performed on 100% of units, before the substrate and heatsink are bonded together.
High-Accuracy Assembly
In addition, the TT Electronics team in Salzburg has developed processes that allow placement of individual LEDs on the thermal substrate with extremely high precision using a patented technique that is able to position a reference point on the emitter chip accurately to within ±50µm, at high levels of repeatability. The reference point may be an edge of the chip, or the center, depending on the requirement of the application. This technique gives superior control over the positioning of the light source without referencing LED-package fiducials, thereby eliminating any inaccuracies in the LED assembly.
This gives a number of advantages that ultimately enable the headlamp to produce a very crisp edge to the illumination pattern, while allowing the headlamp assembly to be produced quickly, efficiently and cost effectively. Because the position of the chip can be controlled so precisely, there is no need to fine-tune the position of the light engine relative to the optics (reflector or lens) when assembling the headlamp. Hence the headlamp can be designed without any need for adjustable mechanisms, as seen in figure 3, thereby simplifying design and saving cost. In addition, the cost of performing the adjustment on the production line can also be eliminated.
The View Ahead
The engineering team at TT Electronics‘ Salzburg technical center has already brought various LED headlamp modules to market successfully, and has also worked with a leading tier-1 headlamp supplier on a concept for the latest generation of matrix LED headlamps. These new multiple-segmented headlamps take advantage of TT Electronis’ accurate LED placement capability to allow multiple selectable lighting patterns.
The lighting patterns offered can minimize the glare experienced by oncoming drivers while enhancing illumination in areas where the driver’s vision needs to be optimized. During town driving, for example, the driver can choose a mode that boosts illumination of peripheral areas so that pedestrians or signage can be seen, while on rural roads alternative modes can prioritize illumination of any hazards ahead.
This new type of lighting will be fitted to a number of high-end models soon to be introduced by premium German car brands.
Conclusion
The headlamp is the final automotive exterior lighting application to benefit from the improved reliability, functionality and performance achievable using LED technology.
Innovative ceramic substrate technology by TT Electronics enables headlamp manufacturers to significantly improve their light systems in weight, performance and cost.
TT Electronics is a leader in the advanced thermal engineering and high-accuracy assembly processes needed to build light engines that will meet the demands of premium car brands and their discerning customers.
About the author: Peter Stefan Kühleitner is LED Applications Manager, TT Electronics (www.ttelectronics.com)
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