ROHM's automotive LED technology

1 Introduction

Nowadays, the application of LED in various fields has become commonplace, and the automotive field is no exception. As the light source of indicator lights such as brake lights and speedometers, the application of LED is increasing year by year. (Figure 1)

Figure 1. Location of LEDs in car interiors

Since ROHM began producing LED lights in 1973, it has been at the forefront of the industry in product development for 45 consecutive years. ROHM's greatest advantage is that it can conduct comprehensive and strict quality management and use a vertically integrated production system (strictly implementing product concepts from the component manufacturing stage) to develop unique products. In the process of LED development, ROHM always adheres to its corporate purpose of "quality first", such as adopting chip designs that are easy to manufacture in the assembly process, implementing traceability management for ultra-small products, and managing processes in accordance with the quality requirements of automotive-grade products. ROHM attaches great importance to production technology and quality management, so that it can provide customers with miniaturized, highly reliable, original LED products to meet the diverse needs of customers.

2. LED for speedometer indicator light source

In recent years, the use of small LED lamps as indicator lights for speedometers has increased. However, in order to adapt to the harsh temperature changes in cars, a space is generally set up to prevent the light shielding wall from contacting the PCB board. The "leakage" of LED light from this space to adjacent parts has always been an issue that needs to be solved urgently. In addition, as LEDs are increasingly used in various fields, especially in the fields of automobiles and industrial equipment with harsh operating environments, products with high reliability that have taken measures against aging caused by environmental stress are needed.

In this context, ROHM has developed the "CSL0901/0902 Series", a small, high-output lens-type surface mount LED for the indicator light source of the speedometer of a car that may be used in harsh environments. This series of products has the following three features:

① Solve the problem of light leakage to adjacent parts when using small LEDs

As mentioned earlier, conventional speedometer light sources use relatively large reflective LEDs (3.5×2.8 t=1.9mm). However, as applications become increasingly demanding in terms of space saving, smaller LEDs are being explored. On the other hand, the light source position of common small molded LEDs is designed to be very low, at only 0.18mm, which may cause light leakage to adjacent parts.

In response to this, ROHM raised the position of the light source to 0.49mm, solving the light leakage problem. This enables the use of small LEDs, which are only 1/18 the volume of previous reflective LEDs, greatly helping to save space in applications. (Figure 2)

Figure 2. By adjusting the position of the light source, light leakage can be prevented and space can be saved.

①Prevent the light decay of short-wavelength and high-brightness products when powered on at high temperatures

In addition, automotive LEDs are required to have stable light output performance even in harsh vehicle environments. On the other hand, especially in short-wavelength products such as blue, as the brightness increases, the light decay caused by high-temperature power supply has become an issue that needs to be solved urgently. This time, the use of ROHM's newly developed molding resin has successfully improved the light decay problem when high-temperature power is supplied, even for short-wavelength, high-brightness products. For example, in the high-temperature power-on acceleration test of blue LEDs (Ta=85℃, IF=20mA, power supply 1000Hr), the luminous flux has improved by about 80% compared to previous products. (Figure 3)

图3. 高温通电导致的光衰显著改善

① Anti-sulfurization measures for LED

To ensure reliability, anti-sulfurization measures are also essential. Because in harsh usage environments, in automotive applications where electronic components are used, hydrogen sulfide in the air (also contained in exhaust gas from cars and factories) acts on the surface of metal materials such as silver, causing corrosion and darkening of the surface. This sulfidation has become a major cause of aging over the years.

Previously, ROHM's automotive LEDs, like other discrete products, met the global standard "AEC-Q101" for the reliability of automotive electronics products. Now, the CSL090x series is being evaluated to meet the "AEC-Q102※1" special standard for optoelectronic components promulgated and implemented in March 2017. The new standard also includes content related to anti-sulfurization testing, so this series of products is also a qualified product in this sulfurization test. (Figure 4)

Figure 4. Effect of anti-sulfurization measures of the CSL090x series

※1）AEC-Q102

AEC is the abbreviation of Automotive Electronics Council, which is a reliability standard for automotive electronic components jointly developed by large automobile manufacturers and large electronic component manufacturers in the United States. Q102 is a standard specially developed for optoelectronic components.

Product lineup of CSL090x series

This series lineup includes 18 models, including the "CSL0901 series" with normal brightness and the "CSL0902 series" with higher brightness. (Figure 5)

Figure 5. CSL090x Series Product Lineup

3. LED for automotive interior lighting

As target applications become more multifunctional, LEDs used as lighting sources for displaying icons and illuminating the entire panel in automotive interior equipment, including the speedometer, are also required to be miniaturized.

ROHM has been promoting the development of LEDs suitable for such applications. Product features are as follows:

①Small package, higher brightness

As target applications become more multifunctional, the installation space for LEDs is decreasing year by year. In addition, in order to increase design flexibility, the display area is showing a trend of reducing the light transmittance of the panel to make it darker when the light is not on. Therefore, LEDs are required to be both small and have higher brightness. ROHM continues to develop small and high-brightness LEDs by improving the package shape, reflector material, frame shape and surface coating.

② Fine chroma support

照明光源由LED和导光板共同组成，以照亮整个面板。但是，当光通过导光板时，色调会因导光板的材质和形状而变化。因此，所使用的LED需要针对每种应用进行精细地色度调整。ROHM通过精细控制元件和荧光粉，可显著减少传统的色度变化。

This allows the large package LEDs that were once used as automotive interior lighting sources to be unified into a small package of 1608 size. (Figure 6)

Figure 6. Development trend of LED for automotive interior lighting

4. Future Outlook

Nowadays, LED has become an indispensable part of our lives. LED has been widely used in various applications, including not only car taillights, but also digital displays and indicator lights of display panels of various equipment in the field of automotive instruments, ambient lighting, industrial equipment, and functional displays in the field of consumer electronics, etc., and the demand for LED is expected to grow further in the future.

In the future, ROHM will use its own advantages - vertically integrated production system to ensure reliability, and strive to develop new products for applications at home and abroad that value LED added value such as reliability and design flexibility.