Optimizing the service life of HB LEDs: ESD protection components take on the heavy responsibility

As brightness and energy efficiency gradually increase, extending service life has become a new requirement to accelerate the development of high brightness light emitting diode (HB LED) solid-state lighting. By properly selecting electrostatic discharge (ESD) protection components and optimizing related circuit designs, HB LED solid-state lighting module manufacturers can effectively extend product service life and achieve better quality.

随着亮度和能效的提升，延长使用寿命已为促进基于高亮度发光二极体(HB LED)的固态照明设计快速发展的关键之一。然而，并非所有HB LED在这些方面皆旗鼓相当，制造商应用静电放电(ESD)保护的方式可能是影响HB LED现场使用寿命的关键因素。本文中将探讨ESD保护的重要性，阐释HB LED模组制造商藉着最先进保护技术来确保其设计将使用寿命和品质潜能提升至最优。

Threat signals hidden outside the evolution curve

The commercialization of green and blue LEDs, combined with the steady and rapid increase in average light output per component 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 (similar to Moore's Law for transistor density), according to which the light output level of LEDs doubles every 2 years and the average cost per lumen of light output decreases tenfold every 10 years.

In fact, the light output of LEDs is now doubling every 18 months (or even less), and there are now components on the market with luminous efficacy of 120 lumens per watt, and leading laboratories have even demonstrated LEDs with luminous efficacy of 200 lumens per watt. While HB LEDs have greatly improved their light output capabilities and costs, they are also susceptible to severe damage due to ESD, just like today's advanced integrated circuits (ICs).

IC制造商已经将ESD损伤确定为互补式金属氧化物半导体(CMOS)元件现场可靠性的一项主要威胁，它可能损害品牌形象并妨碍市场接受新技术。为避免此情况，业界积极努力用后续新制程世代来最佳化整合ESD保护架构。同样地，领先的HB LED制造商也将ESD确定为固态照明机遇的一项显著威胁，并与ESD专家协作制定适合的保护措施。当总光输出增加提供令人兴奋的题材，众多有效的ESD保护措施也应运而生，并被知名制造商并入到已在市场销售的HB LED之中。

HN LEDs are susceptible to ESD damage

The integration of the sapphire substrate with the epitaxial wafer used in manufacturing green and blue emitters results in components that are more susceptible to ESD damage than red LEDs. Since the sapphire substrate is a pure insulator, a large amount of static charge accumulates when processing the components during production. In addition, the epitaxial layer is often more susceptible to ESD damage than the epitaxial layer used in the manufacturing process of red LEDs, most likely due to effects such as defects introduced during the manufacturing process.

ESD will help extend the service life of HB LEDs and accelerate their popularization.

In CMOS components, 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 early failure of the LED bulb. High rates of ESD damage 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 key 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 on the circuit board, which may cause considerable losses in terms of overall bill of materials (BOM) cost and printed circuit board (PCB) space, and ESD protection is far from sufficient to protect the LED die.

Integrating effective ESD protection into 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 above the LED emitter die in a more compact layout.

藉整合ESD配置保护HB LED

The following two integrated ESD configurations have emerged in the industry. The side-mount configuration shown in Figure 1 uses a transient voltage suppressor (TVS) diode in 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, usually between 8k and 15kV human body model (HBM).

Figure 1: LED protection using side-mounted TVS diodes

Figure 2 shows how to more tightly integrate ESD protection between the LED and the leadframe by using a silicon bottom mount. This construction allows for a more compact LED; the bottom mount replaces the traditional substrate used in side mount LED modules and provides ESD protection levels exceeding 15kV HBM. The good thermal conductivity of the silicon bottom mount also helps the LED relieve stress caused by the different thermal expansion coefficients of the LED and the leadframe.

Figure 2 ESD protection provided by silicon bottom mounting

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.

Rdyn/Cin is the key parameter to avoid LED damage

The most critical parameters for integrating ESD protection diode arrays include low dynamic resistance (Rdyn) and low input capacitance (Cin), so that the ESD protection element can quickly respond to ESD spikes and dissipate most of the current, thereby avoiding damage to the LED die. ON Semiconductor's ESD protection technology products provide extremely low dynamic impedance of only 0.2 to 0.4 ohms (Ω) in the bottom-mount protector, which translates into lower and better clamping voltages during transient events such as ESD, and the LED or LED string is accordingly protected at a higher level. In addition, the bottom-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 in a common package. In white light LEDs, this approach is often used to provide a high total light output. In backlighting applications or special effects lamps, the LED array may contain red, blue, and green die to allow color mixing and fine-tuning of the color emitted by the LEDs. Modules containing a large number of LEDs may require a high DC voltage to be applied because each LED has a finite forward voltage drop of about 3.5 volts (V).

The industry's side-mount and bottom-mount ESD protection manufacturing processes can be adjusted for breakdown voltages between 6 and 110 volts, so they are suitable for LED strings of any feasible length. Top- and bottom-gold processes offer different combinations and compositions; side-mount and bottom-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 LED luminaire manufacturers to apply reverse bias testing during production, identify and isolate defective modules, and prevent them from reaching customers. More complex diode arrays provide pairs of diodes connected in parallel for each LED in an LED string containing multiple LEDs, allowing the lamp module to continue operating even if an individual LED fails to operate open circuit.

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

ESD prolongs HB LED life

In addition to high efficiency and small size, the expected long life cycle of HB LEDs has proven to be an important factor in the huge success of solid-state lighting. HB LEDs in a variety of colors, especially white light as well as green, blue and red light, have opened up a smooth road for designers to develop applications such as high-efficiency backlights, street lights, interior lighting, task lights, electronic signs and automotive lights and headlights.

Since HB LED specifications vary greatly from manufacturer to manufacturer, a comprehensive understanding of the ESD protection approach used by HB LEDs is of great benefit in providing optimal field life. Each original equipment manufacturer (OEM) has a specific set of standards in packaging, process technology, and material composition that should be considered. Therefore, to obtain the desired optimized performance parameters, designers need to work closely with suppliers that are not limited to providing a few different LED protection solutions, but rather suppliers that can provide a wide range of options and support to make the best match. (The author is the senior product marketing manager of the protection products division of ON Semiconductor)