Optimizing the Lifetime of High-Brightness LEDs: ESD Protection Considerations

Along with improvements in brightness and efficiency, extended lifetime has become one of the main factors driving the rapid development of solid-state lighting designs based on high-brightness light-emitting diodes (HB-LEDs). However, not all HB-LEDs are equal in these aspects, and the way in which manufacturers apply electrostatic discharge (ESD) protection can be a critical factor affecting the field lifetime of HB-LEDs. In this article, Vidya Premkumar of ON Semiconductor will explore the importance of ESD protection and explain the latest advanced protection technologies that HB-LED module manufacturers are using to ensure that designs maximize lifetime and quality potential.

Introduction: Threats Outside the Evolution Curve

The commercialization of green and blue LEDs, combined with the steady and rapid increase in average light output per device achieved in recent years, has opened up a large number of new application markets for solid-state lighting. The price and performance of HB-LEDs have surpassed Haitz's Law, which is similar to Moore's Law for transistor density; according to this law, the light output level of LEDs will double every 2 years, and the average cost per lumen of light output will decrease 10 times every 10 years.

In fact, the light output of LEDs now doubles every 18 months or even less. Devices with luminous efficacy of 120lm/W are now available on the market, and leading laboratories have even demonstrated LEDs with luminous efficacy of 200lm/W. While the light output capacity and cost of HB-LEDs have been greatly improved, they also face the same problem as today's advanced integrated circuits (ICs) - they are easily severely damaged by ESD.

IC manufacturers have identified ESD damage as a major threat to the field reliability of CMOS devices because it can damage brand image and hinder market acceptance of new technologies. In response to this problem, the industry has actively worked to optimize integrated ESD protection architectures with subsequent new process nodes, although this work has received little, if any, media attention. Similarly, leading HB-LED manufacturers have identified ESD as a significant threat to the solid-state lighting opportunity and are working with ESD professionals to develop appropriate protection measures. Although total light output increases provide the most exciting storyline, many effective ESD protection measures have already emerged and are being integrated into HB-LEDs promoted by well-known manufacturers.

Susceptible to ESD damage

The integration of the sapphire substrate and the epitaxial layers used in manufacturing green and blue emitters results in devices that are more susceptible to ESD damage than, for example, red LEDs. Since the sapphire substrate is a pure insulator, a large amount of static charge accumulates when the device is processed during production. In addition, the epitaxial layers tend to be more susceptible to ESD damage than the epitaxial layers used in the manufacturing process of red LEDs, most likely due to effects such as defects introduced during the manufacturing process.

In CMOS devices, ESD events that occur during manufacturing may remain undetected until they are put into field applications, resulting in unexpected and costly failures in the field. Common consequences of ESD damage to LEDs are dark spots on the die surface, which results in reduced LED light output and can cause LED bulbs to fail soon after use. High ESD damage rates during LED manufacturing can hurt production yields and actually increase the price of good products. Since the long operating life of HB-LEDs is a major advantage of solid-state lighting over traditional lighting, effective ESD protection for HB-LEDs is clearly essential.

If the LED module does not contain suitable protection, customer engineers may need to apply discrete protection at the board level, which can be costly in terms of bill of materials (BOM) cost and loss of printed circuit board (PCB) space, and board-level ESD protection is far from sufficient to protect the LED die. Integrating effective ESD protection in the package is a more desirable approach and is favored by many of today's major HB-LED manufacturers. ESD protection can be applied as an additional die next to the LED emitter die, or as a submount or sidemount on which the LED emitter die is bonded in a more compact layout.

Integrated protection

The following two integrated ESD configurations have emerged in the industry. The side-mount configuration shown in Figure 1 applies the transient voltage suppressor (TVS) diode to the same package as the LED emitter die. The diode can be connected using wire bonding or flip-chip technology, depending on the specific application requirements. The rated ESD level varies depending on the die size and is usually between 8kV and 15kV human body model (HBM).

Figure 1: LED protection using side-mounted TVS diodes

图2显示了可以怎样藉在LED和引线框之间应用硅次级贴装来更紧密地集成ESD保护。这种构造使LED外形尺寸更紧凑；次级贴装替代侧面贴装LED模块中使用的传统衬底，提供的ESD保护等级超过15kVHBM.硅次级贴装的良好热传导性也帮助LED缓解由于LED与引线框热膨胀系数不同导致的应力。

Figure 2. ESD protection provided by silicon submount.

Both approaches are available with a variety of top and back gold plating processes to suit most manufacturing requirements, such as options with top aluminum coating (AuAl, CuAl), gold or copper processes for higher reflectivity, and gold or gold-tin (AuSn) back gold process options.

Protection performance

The most critical parameters of integrated ESD protection diode arrays include low dynamic resistance (Rdyn) and low input capacitance (Cin), so that the ESD protection device can respond quickly to ESD spikes and dissipate most of the current to avoid damage to the LED die. ON Semiconductor's ESD protection technology products provide extremely low dynamic impedance in the secondary mount protector, only in the 0.2 to 0.4Ω level. ON Semiconductor's secondary mount protection devices inherently provide better dynamic resistance than competing products, which translates into lower and better clamping voltages during transient events such as ESD, which in turn means that the LED or LED string is protected at a higher level. In addition, the secondary mount protector also provides surge protection, which is very important in applications where the LED is exposed to power surges or lightning spikes.

Most solid-state light modules consist of HB-LED die connected in series and in a common package. This approach is often used in white light LED (WLED) sources to provide a high total light output. In backlighting applications or special effects lamps, LED arrays may contain red, blue, and green die to allow color mixing and fine-tuning of the color emitted by the LEDs. Since each LED has a finite forward voltage drop of around 3.5V, modules containing a large number of LEDs may require a high DC voltage to be applied. ON Semiconductor's side-mount and sub-mount ESD protection manufacturing processes can be adjusted for breakdown voltages between 6V and 110V, making them suitable for LED strings of any feasible length. Top- and bottom-gold processes are available in different combinations and compositions; side- and sub-mount also have different packaging options, ranging from flip-chip to top and bottom.

Regardless of the type of mounting, a variety of options and configurations are available: A TVS diode connected in parallel across the LED string provides the specified level of ESD protection (see Figure 3a), while a pair of diodes connected in series across the LED string (Figure 3b) enables lamp manufacturers to apply reverse bias testing during production, identifying and isolating defective modules before they reach the customer. More complex diode arrays provide pairs of diodes connected in parallel across each LED in a string containing multiple LEDs, allowing lamp modules to continue to operate even if individual LEDs fail open circuit.

Figure 3 (a) and (b) Different ESD protection configurations.

in conclusion

In addition to their high efficiency and small size, the long life cycles that HB-LEDs are expected to provide are proving to be a major contributor to the huge success of solid-state lighting. The availability of HB-LEDs in a variety of colors, especially white as well as green, blue and red, opens exciting new avenues for designers targeting applications such as energy-efficient backlights, streetlights, interior lighting, task lights, electronic signs and automotive brake and headlights. Since HB-LED specifications can vary greatly from manufacturer to manufacturer, a comprehensive understanding of the ESD protection approach taken by HB-LEDs can be of great benefit in providing the best field life. Each original equipment manufacturer (OEM) has a very specific set of criteria in terms of packaging, process technology and material composition that need to be considered. Therefore, in order to obtain the optimized performance parameters required, they need to work closely with suppliers who are not limited to a few different LED protection solutions, but rather suppliers that offer a wide range of options and support the best match.