Reverse Mount SMD LED LTST-C21KGKT Datasheet - Green AlInGaP - 20mA - 2.4V - English Technical Document

1. Product Overview

This document provides the complete technical specifications for a high-brightness, reverse mount surface-mount device (SMD) light-emitting diode (LED). The device utilizes an Aluminum Indium Gallium Phosphide (AlInGaP) semiconductor chip to produce green light. It is designed for automated assembly processes and is compliant with RoHS (Restriction of Hazardous Substances) directives, making it an environmentally friendly component suitable for modern electronic manufacturing.

The primary application of this LED is in backlighting, status indicators, and panel illumination where space is limited on the top side of a printed circuit board (PCB). Its reverse mount design allows it to be soldered on the opposite side of the board from which light is emitted, enabling innovative and space-saving product designs.

2. Technical Parameter Deep-Dive

2.1 Absolute Maximum Ratings

The device must not be operated beyond these limits to prevent permanent damage. Key ratings include a maximum continuous forward current (I_F) of 30 mA at an ambient temperature (T_a) of 25°C. The power dissipation is rated at 75 mW. For pulsed operation, a peak forward current of 80 mA is permissible under a 1/10 duty cycle with a 0.1 ms pulse width. The maximum reverse voltage (V_R) is 5 V. The operating and storage temperature range is specified from -55°C to +85°C.

Soldering conditions are critical: wave or infrared reflow soldering should not exceed 260°C for more than 5 seconds, while vapor phase soldering should not exceed 215°C for more than 3 minutes. A linear derating factor of 0.4 mA/°C applies to the forward current for ambient temperatures above 50°C.

2.2 Electro-Optical Characteristics

Measured at T_a=25°C and a forward current (I_F) of 20 mA, the key performance parameters are defined.

• Luminous Intensity (I_V): Ranges from a minimum of 28.0 mcd to a maximum of 180.0 mcd. The typical value is not specified in the summary table, indicating it depends on the specific bin code (see Section 3). Measurement follows the CIE photopic eye-response curve.
• Viewing Angle (2θ_1/2): Defined as 70 degrees. This is the full angle at which the luminous intensity drops to half of its value measured on the central axis.
• Peak Wavelength (λ_P): Approximately 574 nm. This is the wavelength at which the spectral power distribution is at its maximum.
• Dominant Wavelength (λ_d): Ranges from 567.5 nm to 576.5 nm at I_F=20mA. This is the single wavelength perceived by the human eye that defines the color of the light, derived from the CIE chromaticity diagram.
• Spectral Half-Width (Δλ): Approximately 15 nm. This indicates the spectral purity of the green light.
• Forward Voltage (V_F): Ranges from 1.80 V to 2.40 V at I_F=20mA.
• Reverse Current (I_R): Maximum of 10 μA at V_R=5V.
• Capacitance (C): Typically 40 pF measured at 0 V bias and 1 MHz frequency.

3. Binning System Explanation

To ensure color and brightness consistency in production, LEDs are sorted into bins. This product uses two independent binning criteria.

3.1 Luminous Intensity Binning

Units are in millicandelas (mcd) at I_F=20mA. The bins are:

• Code N: 28.0 mcd (Min) to 45.0 mcd (Max)
• Code P: 45.0 mcd to 71.0 mcd
• Code Q: 71.0 mcd to 112.0 mcd
• Code R: 112.0 mcd to 180.0 mcd

A tolerance of ±15% applies within each intensity bin.

3.2 Dominant Wavelength Binning

Units are in nanometers (nm) at I_F=20mA. The bins are:

• Code C: 567.5 nm (Min) to 570.5 nm (Max)
• Code D: 570.5 nm to 573.5 nm
• Code E: 573.5 nm to 576.5 nm

A tight tolerance of ±1 nm applies within each wavelength bin. The full part number includes these bin codes to specify exact performance.

4. Performance Curve Analysis

While specific graphs are referenced but not detailed in the provided text, typical curves for such devices would include:

• I-V (Current-Voltage) Curve: Shows the exponential relationship between forward voltage and current. The curve will have a specific knee voltage around 1.8-2.4V.
• Luminous Intensity vs. Forward Current: Demonstrates that light output increases with current, but not necessarily linearly, especially at higher currents due to heating effects.
• Luminous Intensity vs. Ambient Temperature: Shows the decrease in light output as the junction temperature increases. AlInGaP LEDs typically have a negative temperature coefficient for light output.
• Spectral Distribution: A plot showing the relative power emitted across wavelengths, peaking around 574 nm with a width of about 15 nm at half maximum.
• Viewing Angle Pattern: A polar plot illustrating the angular distribution of light intensity, which is typically Lambertian or side-emitter shaped for this package style.

These curves are essential for designers to predict performance under non-standard operating conditions.

5. Mechanical and Package Information

5.1 Package Dimensions

The LED conforms to an EIA standard SMD package outline. All critical dimensions (body length, width, height, lead spacing, etc.) are provided in millimeter-based drawings with a standard tolerance of ±0.10 mm unless otherwise noted. The lens is specified as "Water Clear."

5.2 Polarity Identification and Pad Layout

The component has anode and cathode terminals. The datasheet includes a recommended solder pad footprint diagram for PCB layout. Adhering to these dimensions is crucial for achieving a reliable solder joint, proper alignment, and effective heat dissipation during the reflow process. The pad design also helps prevent tombstoning (component standing up on one end) during soldering.

6. Soldering and Assembly Guidelines

6.1 Reflow Soldering Profiles

Two suggested infrared (IR) reflow profiles are provided: one for standard tin-lead (SnPb) solder process and one for lead-free (Pb-free) solder process, typically using SAC (Sn-Ag-Cu) alloys. The lead-free profile requires a higher peak temperature (up to 260°C) but must carefully control the time above liquidus to prevent damage to the LED's epoxy package. Pre-heating stages are critical to minimize thermal shock.

6.2 Storage and Handling

LEDs are moisture-sensitive devices. For extended storage outside the original moisture-barrier bag, they should be kept in an environment not exceeding 30°C and 70% relative humidity. If stored unpackaged for more than one week, a bake-out at approximately 60°C for at least 24 hours is recommended before soldering to remove absorbed moisture and prevent "popcorning" during reflow.

6.3 Cleaning

If cleaning after soldering is necessary, only specified solvents should be used. Immersing the LED in ethyl alcohol or isopropyl alcohol at room temperature for less than one minute is acceptable. Unspecified or aggressive chemicals can damage the plastic lens and package material.

6.4 Electrostatic Discharge (ESD) Protection

The LED is susceptible to damage from electrostatic discharge. Proper ESD controls must be in place during handling and assembly:

• Use grounded wrist straps and anti-static mats.
• Ensure all equipment and workstations are properly grounded.
• Consider using an ionizer to neutralize static charges that may accumulate on the plastic lens.

7. Packaging and Ordering Information

The LEDs are supplied in industry-standard packaging to facilitate automated assembly.

• Tape and Reel: Components are placed in 8mm wide embossed carrier tape.
• Reel Size: Mounted on 7-inch (178 mm) diameter reels.
• Quantity: Standard reel contains 3000 pieces. A minimum order quantity of 500 pieces is available for remainder stock.
• Packaging Standards: Complies with ANSI/EIA-481-1-A specifications. The tape has a cover seal, and a maximum of two consecutive empty pockets is allowed.

The full part number (e.g., LTST-C21KGKT) encodes the specific characteristics, including the bin codes for luminous intensity and dominant wavelength.

8. Application Notes and Design Considerations

8.1 Drive Circuit Design

LEDs are current-driven devices. For stable and uniform operation, especially when driving multiple LEDs in parallel, a series current-limiting resistor for each LED is strongly recommended (Circuit Model A). Driving LEDs directly in parallel without individual resistors (Circuit Model B) is not recommended due to variations in the forward voltage (V_F) from device to device. These variations can cause significant differences in current sharing, leading to uneven brightness and potential overstress of the LED with the lowest V_F.

The value of the series resistor (R_s) can be calculated using Ohm's Law: R_s = (V_supply - V_F) / I_F, where I_F is the desired operating current (e.g., 20 mA) and V_F is the typical or maximum forward voltage from the datasheet.

8.2 Thermal Management

Although power dissipation is relatively low (75 mW max), effective thermal management is still important for maintaining long-term reliability and consistent light output. The LED's light output decreases with increasing junction temperature. Ensuring a good thermal path from the LED's solder pads to the PCB copper planes helps dissipate heat. Avoid operating at the absolute maximum current and temperature limits for extended periods.

8.3 Application Scope and Limitations

This component is designed for general-purpose electronic equipment such as consumer electronics, office automation devices, and communication equipment. It is not specifically designed or qualified for applications where failure could lead to direct safety hazards (e.g., aviation control, medical life-support, transportation safety systems). For such high-reliability applications, consultation with the manufacturer for specialized products is necessary.

9. Technical Comparison and Differentiation

The key differentiating features of this LED are its reverse mount capability and its use of an AlInGaP chip for green emission.

• Reverse Mount vs. Standard Top-View SMD: This allows the LED to be mounted on the bottom side of a PCB while shining light through a hole or a light guide, freeing up valuable top-side real estate for other components. It enables slimmer product designs.
• AlInGaP vs. Traditional GaP or InGaN: AlInGaP technology offers higher efficiency and better temperature stability for red, orange, amber, and green wavelengths compared to older technologies. It typically provides higher brightness and more saturated color points.
• Water Clear Lens: Provides the true color of the chip without diffusion, resulting in a more focused and intense beam pattern compared to diffused lenses.

10. Frequently Asked Questions (FAQ)

Q1: What is the difference between peak wavelength and dominant wavelength?
A1: Peak wavelength (λ_P) is the physical wavelength where the LED emits the most optical power. Dominant wavelength (λ_d) is a calculated value based on human color perception (CIE chart) that best represents the perceived color. For a monochromatic green LED, they are often close, but λ_d is the more relevant parameter for color matching.

Q2: Can I drive this LED at 30 mA continuously?
A2: While the absolute maximum rating is 30 mA DC, optimal performance for longevity and stable light output is typically achieved at or below the test current of 20 mA. Operating at 30 mA will generate more heat, reduce efficiency, and may shorten lifespan. Always consult derating guidelines for elevated temperatures.

Q3: How do I interpret the bin codes in the part number?
A3: The part number suffix contains codes that specify the luminous intensity bin (e.g., R for highest output) and the dominant wavelength bin (e.g., D for mid-green). Selecting the appropriate bin codes is crucial for applications requiring consistent brightness and color across multiple LEDs.

Q4: Is this LED suitable for wave soldering?
A4: Yes, the datasheet specifies a wave soldering condition of 260°C for 5 seconds maximum. However, reflow soldering is the preferred and most common method for SMD components like this one.

11. Design and Usage Case Study

Scenario: Designing a status indicator for a portable medical device.
The device requires a bright, unambiguous green "power on/ready" indicator. Space on the top control panel is extremely limited. A reverse mount LED is chosen. It is placed on the bottom side of the main PCB. A small, precisely drilled aperture in the top panel allows the light to shine through. A light pipe or simple hole design can be used. The drive circuit uses a 3.3V supply. Calculating the series resistor: R_s = (3.3V - 2.2V_typ) / 0.020A = 55 Ohms. A 56 Ohm standard value resistor is selected. To ensure color consistency across all units, LEDs from the same wavelength bin (e.g., Code D) are specified in the bill of materials.

12. Technology Principle Introduction

This LED is based on Aluminum Indium Gallium Phosphide (Al_xIn_yGa_1-x-yP) semiconductor material grown on a substrate. When a forward voltage is applied, electrons and holes recombine in the active region of the chip, releasing energy in the form of photons (light). The specific ratio of aluminum, indium, and gallium in the crystal lattice determines the bandgap energy, which directly defines the wavelength (color) of the emitted light. For green emission, a specific composition is used to achieve a bandgap corresponding to light around 570-580 nm. The AlInGaP material system is known for its high internal quantum efficiency in the red-to-green spectral range.

13. Industry Trends and Developments

The trend in SMD LEDs for indicator and backlighting applications continues toward higher efficiency, smaller packages, and greater reliability. There is a strong drive for improved performance in lead-free and high-temperature reflow soldering processes. The demand for precise color control and tighter binning is increasing, especially in applications where color matching is critical across displays or panels. Furthermore, the integration of LEDs with built-in current regulation or control circuitry (like IC-driven LEDs) is a growing trend to simplify design and improve performance consistency, though this particular component is a standard, discrete LED.