SMD LED LTST-108KSKT Datasheet - Package 3.2x2.8x1.9mm - Voltage 1.8-2.4V - Yellow Color - 72mW Power - English Technical Document

1. Product Overview

This document provides the complete technical specifications for a surface-mount device (SMD) Light Emitting Diode (LED). This component is designed for automated printed circuit board (PCB) assembly processes, making it suitable for high-volume manufacturing. Its miniature form factor is ideal for applications where space is a critical constraint. The LED is constructed using Aluminum Indium Gallium Phosphide (AlInGaP) semiconductor technology, which is known for producing high-efficiency light in the amber to red spectrum. The specific variant covered here emits yellow light.

1.1 Core Advantages and Target Market

The primary advantages of this LED include its compact size, compatibility with standard automated pick-and-place equipment, and its suitability for infrared (IR) reflow soldering processes, which are standard in modern electronics manufacturing. It is RoHS compliant, meeting environmental regulations. The device is packaged on 8mm tape wound onto 7-inch diameter reels, facilitating efficient handling in production lines.

Its target applications are broad, encompassing status indicators, backlighting for front panels, and signal or symbol illumination in various electronic equipment. Typical end-use markets include telecommunications devices (e.g., cordless and cellular phones), office automation equipment (e.g., notebook computers), network systems, home appliances, and indoor signage.

2. In-Depth Technical Parameter Analysis

A thorough understanding of the electrical and optical characteristics is essential for proper circuit design and ensuring long-term reliability.

2.1 Absolute Maximum Ratings

These ratings define the limits beyond which permanent damage to the device may occur. They are specified at an ambient temperature (Ta) of 25°C.

• Power Dissipation (Pd): 72 mW. This is the maximum amount of power the LED package can dissipate as heat without exceeding its thermal limits.
• Continuous Forward Current (IF): 30 mA DC. The maximum steady-state current that can be applied.
• Peak Forward Current: 80 mA. This is permissible only under pulsed conditions with a 1/10 duty cycle and a pulse width of 0.1ms. Exceeding the DC current rating, even briefly, can cause overheating.
• Operating Temperature Range: -40°C to +85°C. The ambient temperature range over which the device is guaranteed to function.
• Storage Temperature Range: -40°C to +100°C. The temperature range for storage when the device is not powered.

2.2 Electrical and Optical Characteristics

These are the typical performance parameters measured at Ta=25°C and a forward current (IF) of 20mA, unless otherwise noted.

• Luminous Intensity (Iv): Ranges from 180 mcd (minimum) to 450 mcd (maximum), with a typical value within that range. Intensity is measured using a sensor filtered to match the human eye's photopic response (CIE curve).
• Viewing Angle (2θ1/2): 110 degrees (typical). This is the full angle at which the luminous intensity drops to half of its value measured on the central axis. A 110-degree angle indicates a wide viewing pattern.
• Peak Emission Wavelength (λp): Approximately 591 nm. This is the wavelength at which the spectral output is strongest.
• Dominant Wavelength (λd): Specified between 584.5 nm and 594.5 nm. This is the single wavelength perceived by the human eye that defines the color (yellow). The tolerance is ±1 nm per bin.
• Spectral Line Half-Width (Δλ): Approximately 15 nm. This indicates the spectral purity; a smaller value means a more monochromatic light.
• Forward Voltage (VF): Ranges from 1.8V (minimum) to 2.4V (maximum) at 20mA. The typical value lies within this range. A current-limiting resistor must be calculated based on the actual VF and the supply voltage.
• Reverse Current (IR): Maximum of 10 μA when a reverse voltage (VR) of 5V is applied. This device is not designed for reverse bias operation; this parameter is for test purposes only.

3. Bin Ranking System Explanation

To ensure color and brightness consistency in production, LEDs are sorted into performance bins. Designers can specify bins to match application requirements.

3.1 Forward Voltage (VF) Binning

Units: Volts @ 20mA. Tolerance per bin: ±0.10V.

• Bin D2: 1.8V (Min) to 2.0V (Max)
• Bin D3: 2.0V (Min) to 2.2V (Max)
• Bin D4: 2.2V (Min) to 2.4V (Max)

3.2 Luminous Intensity (Iv) Binning

Units: millicandelas (mcd) @ 20mA. Tolerance per bin: ±11%.

• Bin S1: 180 mcd (Min) to 224 mcd (Max)
• Bin S2: 224 mcd (Min) to 280 mcd (Max)
• Bin T1: 280 mcd (Min) to 355 mcd (Max)
• Bin T2: 355 mcd (Min) to 450 mcd (Max)

3.3 Dominant Wavelength (WD) Binning

Units: Nanometers (nm) @ 20mA. Tolerance per bin: ±1 nm.

• Bin H: 584.5 nm (Min) to 587.0 nm (Max)
• Bin J: 587.0 nm (Min) to 589.5 nm (Max)
• Bin K: 589.5 nm (Min) to 592.0 nm (Max)
• Bin L: 592.0 nm (Min) to 594.5 nm (Max)

A full part number typically includes codes for VF, Iv, and WD bins (e.g., LTST-108KSKT-D3T1K).

4. Performance Curve Analysis

Graphical data provides deeper insight into device behavior under varying conditions.

4.1 Current vs. Voltage (I-V) Characteristic

The I-V curve for an AlInGaP LED shows a forward voltage that is relatively stable but increases slightly with rising junction temperature. The curve is exponential near the turn-on voltage, becoming more linear at higher currents. Designers use this to determine the dynamic resistance and to model power dissipation.

4.2 Luminous Intensity vs. Forward Current

This relationship is generally linear within the recommended operating current range (up to 30mA). Increasing current increases light output, but also increases heat generation. Operating beyond the absolute maximum ratings leads to efficiency droop (decreased light output per watt) and accelerated degradation.

4.3 Spectral Distribution

The spectral output curve centers around 591 nm (peak) with a typical half-width of 15 nm. The dominant wavelength, which defines the perceived color, will fall within the binned range (e.g., 589.5-592.0 nm for Bin K). The spectrum is relatively narrow, characteristic of AlInGaP materials, resulting in a saturated yellow color.

4.4 Temperature Dependence

Key parameters are affected by temperature:

• Forward Voltage (VF): Decreases as junction temperature increases. This has a negative temperature coefficient, typically around -2 mV/°C for AlInGaP.
• Luminous Intensity (Iv): Also decreases with increasing temperature. The derating curve is important for applications operating at high ambient temperatures to ensure sufficient brightness.
• Dominant Wavelength (λd): May shift slightly with temperature, usually towards longer wavelengths (red shift).

5. Mechanical and Packaging Information

5.1 Package Dimensions

The LED is housed in a standard surface-mount package. Key dimensions (in millimeters) are:

• Length: 3.2 mm (tolerance ±0.2 mm)
• Width: 2.8 mm (tolerance ±0.2 mm)
• Height: 1.9 mm (tolerance ±0.2 mm)

Detailed mechanical drawings should be consulted for pad spacing, lens shape, and cathode/anode identification mark. The cathode is typically indicated by a green marking on the package or a chamfered corner.

5.2 Recommended PCB Land Pattern

For reliable soldering, the PCB pad design is critical. The recommended pattern includes two rectangular pads for the anode and cathode, sized to provide sufficient solder fillet for mechanical strength and electrical connection while preventing solder bridging. The pad design is optimized for both infrared and vapor phase reflow soldering processes.

5.3 Tape and Reel Packaging

The components are supplied in embossed carrier tape with a protective cover tape. Key specifications:

• Carrier Tape Width: 8 mm
• Reel Diameter: 7 inches (178 mm)
• Quantity per Reel: 4,000 pieces
• Minimum Order Quantity: 500 pieces for remainder reels
• Pocket Pitch: As per the dimensional drawing
• Standards: Compliant with ANSI/EIA-481 specifications.

6. Soldering and Assembly Guidelines

6.1 IR Reflow Soldering Profile (Lead-Free)

The device is compatible with lead-free (Pb-free) solder processes. A recommended reflow profile, compliant with J-STD-020, includes:

• Preheat: Ramp from ambient to 150-200°C over a maximum of 120 seconds.
• Soak/Activation: Maintain between 150-200°C to allow flux activation and temperature equalization.
• Reflow: Ramp to a peak temperature not exceeding 260°C. The time above 217°C (liquidus for SnAgCu solder) should be controlled.
• Cooling: Controlled cool-down phase.

The specific profile must be characterized for the actual PCB assembly, considering board thickness, component density, and solder paste specifications.

6.2 Hand Soldering

If manual soldering is necessary, extreme care must be taken:

• Soldering Iron Temperature: Maximum 300°C.
• Soldering Time per Lead: Maximum 3 seconds.
• Attempts: Only one soldering attempt per pad is recommended to avoid thermal stress.

6.3 Cleaning

If post-solder cleaning is required, only specified solvents should be used to avoid damaging the plastic lens or package. Acceptable cleaners include ethyl alcohol or isopropyl alcohol. The LED should be immersed at normal temperature for less than one minute. Harsh chemical cleaners must be avoided.

7. Storage and Handling Cautions

7.1 Moisture Sensitivity

The plastic LED package is moisture-sensitive. As delivered in a sealed moisture-barrier bag (MBB) with desiccant, it has a shelf life of one year when stored at ≤30°C and ≤70% RH. Once the original bag is opened, the components are exposed to ambient humidity.

7.2 Floor Life and Baking

• Floor Life: After opening the MBB, components should be subjected to IR reflow soldering within 168 hours (7 days) under conditions of ≤30°C and ≤60% RH.
• Extended Storage: For storage beyond 168 hours outside the MBB, components must be stored in a sealed container with desiccant or in a nitrogen desiccator.
• Baking: If the 168-hour floor life is exceeded, a bake-out is required before soldering to remove absorbed moisture and prevent "popcorning" (package cracking during reflow). Recommended bake condition: 60°C for at least 48 hours.

8. Application Design Considerations

8.1 Current Limiting

A series resistor is mandatory to limit the forward current to a safe value, typically 20mA for optimal performance and longevity. The resistor value (R) is calculated using Ohm's Law: R = (V_supply - VF_LED) / I_desired. Always use the maximum VF from the datasheet (2.4V) for a worst-case design to ensure the current does not exceed limits.

8.2 Thermal Management

Although power dissipation is low (72 mW max), proper thermal design extends LED life and maintains brightness. Ensure the PCB has adequate copper area connected to the LED pads to act as a heat sink. Avoid placing the LED near other heat-generating components. For high ambient temperature applications, derate the maximum forward current.

8.3 Optical Design

The wide 110-degree viewing angle makes it suitable for applications requiring broad visibility. For focused or directed light, secondary optics (lenses, light guides) may be needed. The water-clear lens allows the intrinsic yellow color of the AlInGaP chip to be seen directly.

9. Comparison and Differentiation

Compared to other yellow LED technologies:

• vs. Traditional GaAsP: AlInGaP offers significantly higher luminous efficiency and better temperature stability, resulting in brighter and more consistent light output.
• vs. Phosphor-Converted White/Yellow: This is a direct-emission semiconductor, so it has a narrower spectrum (more saturated color) and does not suffer from phosphor degradation over time.
• Key Advantage: The combination of a standard EIA package footprint, compatibility with lead-free reflow, and high brightness in a miniature size makes it a versatile choice for modern electronics.

10. Frequently Asked Questions (FAQs)

10.1 What is the difference between Peak Wavelength and Dominant Wavelength?

Peak Wavelength (λp) is the physical wavelength where the LED emits the most optical power. Dominant Wavelength (λd) is a calculated value based on the CIE colorimetric system that represents the single wavelength the human eye perceives as the color. For a monochromatic source like this yellow LED, they are close but not identical. Designers concerned with color matching should use the Dominant Wavelength bin.

10.2 Can I drive this LED without a current-limiting resistor?

No. An LED is a diode with a non-linear I-V characteristic. Connecting it directly to a voltage source exceeding its forward voltage will cause current to rise uncontrollably, rapidly exceeding the maximum rating and destroying the device. A series resistor or constant-current driver is always required.

10.3 Why is there a storage and baking requirement?

The plastic epoxy used in the LED package can absorb moisture from the air. During the high-temperature reflow soldering process, this trapped moisture can vaporize rapidly, creating internal pressure that can delaminate the package or crack the die ("popcorning"). The storage and baking procedures control moisture content to prevent this failure mode.

11. Practical Application Example

Scenario: Designing a status indicator for a portable device powered by a 3.3V rail.

1. Current Selection: Choose 20mA for a good balance of brightness and power consumption.
2. Resistor Calculation: Using worst-case VF (Max) = 2.4V. R = (3.3V - 2.4V) / 0.020A = 45 Ohms. The nearest standard value is 47 Ohms. Recalculate actual current: I = (3.3V - 2.2V_Typ) / 47 = ~23.4mA (safe).
3. PCB Layout: Place the 47Ω resistor close to the LED. Use the recommended land pattern. Provide a small copper pour under the LED for heat dissipation.
4. Manufacturing: Ensure the assembly house follows the lead-free reflow profile guidelines. Keep opened reels in a dry cabinet if not used within 168 hours.

12. Technical Principle Introduction

This LED is based on Aluminum Indium Gallium Phosphide (AlInGaP) semiconductor material grown on a substrate. When a forward voltage is applied across the p-n junction, electrons and holes are injected into the active region where they recombine. In a direct bandgap semiconductor like AlInGaP, this recombination releases energy in the form of photons (light). The specific wavelength (color) of the emitted light is determined by the bandgap energy of the semiconductor material, which is engineered during the crystal growth process by adjusting the ratios of Aluminum, Indium, Gallium, and Phosphorus. The water-clear epoxy lens encapsulates the chip, providing mechanical protection, shaping the light output, and enhancing light extraction.

13. Industry Trends

The trend in SMD LEDs for indicator applications continues towards higher efficiency (more light output per mA), smaller package sizes for increased design flexibility, and improved reliability under harsh conditions (higher temperature, humidity). There is also a focus on tighter binning tolerances for color and brightness to enable more consistent aesthetic results in consumer products. The drive for miniaturization pushes the development of chip-scale package (CSP) LEDs, though standard packages like this one remain dominant for cost-sensitive, high-volume applications due to their mature manufacturing processes and compatibility with existing assembly infrastructure.