LTST-S110KRKT SMD LED Datasheet - Side View - Red - 20mA - 2.4V - English Technical Document

1. Product Overview

The LTST-S110KRKT is a surface-mount device (SMD) light-emitting diode (LED) designed for applications requiring a side-emitting light source. Its primary application is in LCD backlighting modules where space is constrained and light needs to be directed laterally. The device utilizes an Ultra Bright AlInGaP (Aluminum Indium Gallium Phosphide) semiconductor chip, which is known for high efficiency and brightness in the red color spectrum. The package is water-clear, allowing for maximum light output without color tinting from the lens material.

Key advantages of this LED include its compliance with RoHS (Restriction of Hazardous Substances) directives, making it an environmentally friendly "Green Product." It is packaged on 8mm tape wound onto 7-inch diameter reels, compatible with standard EIA (Electronic Industries Alliance) packaging and automated pick-and-place assembly equipment. This compatibility ensures efficient, high-volume manufacturing. The device is also designed to withstand common soldering processes, including infrared (IR) and vapor phase reflow, which are standard in modern electronics assembly.

2. Absolute Maximum Ratings

The absolute maximum ratings define the limits beyond which permanent damage to the device may occur. These ratings are specified at an ambient temperature (Ta) of 25°C. The maximum continuous forward current (DC) is 30 mA. For pulsed operation, a peak forward current of 80 mA is permissible under specific conditions: a 1/10 duty cycle and a pulse width of 0.1 ms. The maximum power dissipation is 75 mW. To ensure reliable operation at higher temperatures, a derating factor of 0.4 mA/°C is applied linearly from 50°C upwards. This means the allowable forward current decreases as the temperature increases beyond 50°C.

The device can withstand a reverse voltage of up to 5 V. The operating and storage temperature range is specified from -55°C to +85°C, indicating suitability for a wide range of environmental conditions. For soldering, the LED can tolerate wave soldering at 260°C for 5 seconds, infrared reflow at 260°C for 5 seconds, and vapor phase reflow at 215°C for 3 minutes. Adhering to these limits is crucial for maintaining device integrity during the assembly process.

3. Electro-Optical Characteristics

The electro-optical characteristics are measured at Ta=25°C and an operating current (IF) of 20 mA, which is the standard test condition. The luminous intensity (Iv), a measure of perceived brightness, has a typical value of 54.0 millicandelas (mcd) with a minimum of 18.0 mcd. The viewing angle (2θ1/2), defined as the full angle at which the intensity drops to half its axial value, is 130 degrees, providing a very wide beam pattern suitable for backlighting.

The peak emission wavelength (λP) is 639 nanometers (nm), placing it in the red region of the visible spectrum. The dominant wavelength (λd), which defines the perceived color, is 631 nm. The spectral line half-width (Δλ) is 20 nm, indicating the spectral purity of the emitted light. The forward voltage (VF) typically measures 2.4 V with a maximum of 2.4 V at 20 mA. The reverse current (IR) is a maximum of 10 microamperes (μA) at a reverse voltage (VR) of 5 V. The device capacitance (C) is 40 picofarads (pF) measured at zero bias and a frequency of 1 MHz.

4. Binning System

The luminous intensity of the LEDs is categorized into bins to ensure consistency in brightness for production applications. The binning is based on the minimum and maximum luminous intensity values measured at 20 mA. The bin codes and their corresponding ranges are as follows: Bin M (18.0-28.0 mcd), Bin N (28.0-45.0 mcd), Bin P (45.0-71.0 mcd), Bin Q (71.0-112.0 mcd), and Bin R (112.0-180.0 mcd). A tolerance of +/- 15% is applied to each intensity bin. This system allows designers to select LEDs with a guaranteed brightness range for their specific application, ensuring uniform illumination when multiple LEDs are used.

5. Soldering and Assembly Guidelines

5.1 Reflow Soldering Profiles

The datasheet provides suggested infrared (IR) reflow profiles for both standard (tin-lead) and lead-free (Pb-free) solder processes. For the lead-free process, which typically uses SnAgCu solder paste, the profile must stay between the assembly line and the heat-resistance line of the component. Adherence to these temperature-time profiles is critical to prevent thermal damage to the LED package, such as delamination or cracking, while ensuring proper solder joint formation.

5.2 Cleaning

Cleaning the LEDs after soldering requires caution. Unspecified chemical liquids should not be used as they may damage the plastic package. If cleaning is necessary, it is recommended to immerse the LED in ethyl alcohol or isopropyl alcohol at normal room temperature for less than one minute. Prolonged exposure or the use of aggressive solvents can degrade the lens material or the epoxy encapsulant.

5.3 Storage and Handling

For long-term storage, LEDs should be kept in an environment not exceeding 30°C and 70% relative humidity. If removed from their original moisture-barrier packaging, LEDs should undergo IR reflow soldering within one week. For storage beyond one week outside the original packaging, they should be placed in a sealed container with desiccant or in a nitrogen ambient. LEDs stored this way for more than a week must be baked at approximately 60°C for at least 24 hours before assembly to remove absorbed moisture and prevent "popcorning" during reflow.

6. Mechanical and Packaging Information

The LED is supplied in a tape-and-reel format compatible with automated assembly. The tape width is 8mm, and it is wound on a standard 7-inch (178mm) diameter reel. Each reel contains 3000 pieces. For quantities less than a full reel, a minimum packing quantity of 500 pieces is specified for remainders. The packaging follows ANSI/EIA 481-1-A-1994 specifications. Empty component pockets on the tape are sealed with a top cover tape. The maximum number of consecutive missing components (empty pockets) allowed is two, ensuring feed reliability in automated machines. Detailed dimensional drawings for the tape, reel, and the suggested soldering pad layout on the PCB are provided to aid in PCB design and assembly process setup.

7. Application Notes and Design Considerations

7.1 Drive Circuit Design

LEDs are current-operated devices. To ensure uniform brightness when driving multiple LEDs in parallel, it is strongly recommended to use a current-limiting resistor in series with each LED (Circuit Model A). Driving LEDs directly in parallel without individual resistors (Circuit Model B) is not advised. Small variations in the forward voltage (VF) characteristic between individual LEDs can cause significant current imbalance, leading to noticeable differences in brightness and potentially overstressing some devices.

7.2 Electrostatic Discharge (ESD) Protection

The LED is sensitive to electrostatic discharge (ESD) and power surges, which can cause immediate or latent damage. To prevent ESD damage, proper handling procedures must be followed: Personnel should use conductive wrist straps or anti-static gloves. All equipment, workbenches, and storage racks must be properly grounded. An ionizer (ion blower) can be used to neutralize static charges that may accumulate on the plastic lens due to friction during handling. ESD-damaged LEDs may exhibit abnormal behavior such as reduced light output, increased leakage current, or complete failure.

7.3 Application Scope and Reliability

These LEDs are intended for use in ordinary electronic equipment, including office equipment, communication devices, and household appliances. For applications requiring exceptional reliability where failure could jeopardize life or health—such as in aviation, transportation, medical systems, or safety devices—additional consultation and qualification are necessary prior to use.

8. Performance Curves and Typical Characteristics

The datasheet references typical performance curves which graphically represent the relationship between various parameters. These curves, typically plotted against forward current or ambient temperature, include the forward voltage (VF) vs. forward current (IF), luminous intensity (Iv) vs. forward current (IF), and luminous intensity vs. ambient temperature. Analyzing these curves helps designers understand the device's behavior under different operating conditions. For instance, the luminous intensity typically decreases as the ambient temperature rises, which must be accounted for in thermal management. The forward voltage has a negative temperature coefficient, meaning it decreases slightly as the junction temperature increases.

9. Technical Comparison and Advantages

The use of AlInGaP technology for the red chip offers distinct advantages over older technologies like GaAsP (Gallium Arsenide Phosphide). AlInGaP LEDs generally provide higher luminous efficiency, better temperature stability, and longer operational lifetime. The side-viewing package geometry is a key differentiator, enabling light emission parallel to the mounting plane. This is essential for edge-lit backlight systems commonly found in LCD displays for consumer electronics, automotive dashboards, and industrial panels, where vertical space is extremely limited. The wide 130-degree viewing angle ensures good light diffusion and uniformity across the backlit area.

10. Frequently Asked Questions (FAQ)

Q: What is the difference between peak wavelength and dominant wavelength?
A: Peak wavelength (λP) is the wavelength at which the optical output power is maximum. Dominant wavelength (λd) is derived from the CIE chromaticity diagram and represents the single wavelength that best matches the perceived color of the light. For monochromatic LEDs like this red one, they are often close but not identical.

Q: Can I drive this LED at its maximum DC current of 30mA continuously?
A: While possible, it is not recommended for optimal lifetime and reliability unless necessary for the application. Operating at the typical 20mA condition or lower will reduce thermal stress and increase longevity. Always consider the derating above 50°C ambient.

Q: Why is a series resistor necessary for each LED in parallel?
A: The forward voltage (VF) of LEDs has a manufacturing tolerance. Without individual resistors, LEDs with a slightly lower VF will draw disproportionately more current, leading to brightness mismatch and potential overcurrent failure. The resistor acts as a simple, effective current regulator for each LED.

Q: Is baking always required before soldering?
A: Baking is only required if the LEDs have been removed from their original moisture-barrier packaging and stored in a non-controlled environment for more than one week. This process removes absorbed moisture to prevent vapor pressure damage during the high-temperature reflow soldering process.

11. Design and Usage Case Study

Consider designing a backlight for a small monochrome LCD display in a handheld medical device. The display requires even, red backlighting for night-time readability. The LTST-S110KRKT is selected for its side-emitting profile, fitting into a slim bezel. Four LEDs are placed along one edge of a light guide plate. Based on the required brightness and the light guide efficiency, the designer selects LEDs from Bin N (28-45 mcd) to ensure sufficient intensity. A constant current driver is used, with each LED having its own 100-ohm series resistor calculated for a 20mA drive current from a 5V supply. The PCB layout follows the suggested pad dimensions to ensure proper soldering and alignment. During assembly, ESD precautions are strictly followed, and the recommended lead-free reflow profile is used. The final product achieves uniform illumination with low power consumption and high reliability.

12. Operating Principle

An LED is a semiconductor p-n junction diode. When a forward voltage is applied, electrons from the n-type region and holes from the p-type region are injected into the junction region. When these charge carriers recombine, energy is released in the form of photons (light). The specific wavelength (color) of the emitted light is determined by the energy bandgap of the semiconductor material. The AlInGaP material system used in this LED has a bandgap corresponding to red light. The side-viewing package incorporates a molded plastic lens that shapes the emitted light, directing it laterally from the top surface of the component.

13. Technology Trends

The general trend in LED technology is toward higher efficiency (more lumens per watt), improved color rendering, and greater reliability. For indicator and backlight applications, miniaturization continues, with smaller package sizes becoming standard. There is also a focus on enhancing compatibility with advanced, low-temperature soldering processes to accommodate heat-sensitive substrates. Furthermore, the drive for higher brightness in smaller packages pushes advancements in chip design and thermal management within the package itself. The side-view LED format remains critical for ultra-thin display designs in mobile and wearable electronics.