SMD LED LTST-E681VEWT Datasheet - Size 2.8x3.5x1.9mm - Voltage 2.2V - Power 196mW - Red Color - English Technical Document

1. Product Overview

The LTST-E681VEWT is a high-brightness, surface-mount LED designed for modern electronic applications requiring reliable and efficient indicator lighting. This device utilizes an AlInGaP (Aluminum Indium Gallium Phosphide) semiconductor material to produce a vibrant red light output. It is housed in a compact, industry-standard package that is compatible with automated assembly processes, making it suitable for high-volume manufacturing.

The core advantages of this LED include its compliance with RoHS (Restriction of Hazardous Substances) directives, ensuring environmental safety. It is packaged on 8mm tape wound onto 7-inch diameter reels, which is the standard for automated pick-and-place equipment. The device is also designed to be compatible with infrared (IR) reflow soldering processes, which is the predominant method for assembling surface-mount technology (SMT) boards. Its primary target markets include consumer electronics, industrial control panels, automotive interior lighting, and general-purpose indicator applications where space is at a premium and reliability is key.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

These ratings define the limits beyond which permanent damage to the device may occur. Operating the LED under conditions exceeding these values is not recommended.

• Power Dissipation (Pd): 196 mW. This is the maximum amount of power the LED package can dissipate as heat at an ambient temperature (Ta) of 25°C. Exceeding this limit risks overheating the semiconductor junction, leading to reduced lifespan or catastrophic failure.
• Peak Forward Current (I_FP): 100 mA. This is the maximum allowable pulsed forward current, specified under a 1/10 duty cycle with a 1ms pulse width. It is significantly higher than the DC rating, allowing for brief, high-intensity flashes.
• DC Forward Current (I_F): 70 mA. This is the maximum continuous forward current that can be applied to the LED under normal operating conditions.
• Operating Temperature Range: -40°C to +85°C. The LED is designed to function correctly within this ambient temperature range.
• Storage Temperature Range: -40°C to +100°C. The device can be stored without degradation within this wider temperature range when not in operation.

2.2 Electrical & Optical Characteristics

These parameters are measured at a standard test condition of Ta=25°C and I_F=50mA, unless otherwise noted. They define the typical performance of the device.

• Luminous Intensity (I_V): 900 to 2800 mcd (millicandela). This is a measure of the perceived power of light emitted in a specific direction. The wide range indicates a binning system is used (detailed in Section 3). The measurement uses a sensor filtered to approximate the human eye's photopic response (CIE curve).
• Viewing Angle (2θ_1/2): 120 degrees. This is the full angle at which the luminous intensity drops to half of its value measured on-axis (0°). A 120° angle indicates a wide, diffused light pattern, suitable for applications requiring broad visibility.
• Peak Emission Wavelength (λ_P): 632 nm (typical). This is the wavelength at which the spectral power distribution of the emitted light is at its maximum. It falls within the red region of the visible spectrum.
• Dominant Wavelength (λ_d): 624 nm (typical). Derived from the CIE chromaticity diagram, this is the single wavelength that best represents the perceived color of the LED. It is the key parameter for color specification.
• Spectral Line Half-Width (Δλ): 20 nm (typical). This is the width of the spectral emission at half its maximum power. A value of 20nm is characteristic of AlInGaP red LEDs, indicating a relatively pure color.
• Forward Voltage (V_F): 2.2 V (typical) with a tolerance of ±0.1V. This is the voltage drop across the LED when driven at the specified 50mA current. It is crucial for designing the current-limiting circuitry.
• Reverse Current (I_R): 10 μA (max) at V_R=5V. This parameter is tested for quality assurance only. The LED is not designed for reverse bias operation and applying a reverse voltage in circuit could damage it.

3. Binning System Explanation

To ensure consistency in mass production, LEDs are sorted into performance bins. The LTST-E681VEWT uses a binning system based on luminous intensity at 50mA.

The bin codes (V2, W1, W2, X1, X2) represent ascending ranges of minimum and maximum luminous intensity. For example, bin code X2 contains LEDs with intensity between 2240 mcd and 2800 mcd. A tolerance of ±11% is applied within each bin. This system allows designers to select the appropriate brightness grade for their application, balancing cost and performance. The datasheet does not indicate separate bins for dominant wavelength or forward voltage for this specific part number, suggesting tight control on those parameters during manufacturing.

4. Performance Curve Analysis

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

• I-V (Current-Voltage) Curve: Shows the exponential relationship between forward current and forward voltage. The curve will have a distinct "knee" voltage around 1.8-2.0V, after which current increases rapidly with small voltage increases, highlighting why constant-current drive is essential.
• Luminous Intensity vs. Forward Current: Demonstrates that light output is approximately proportional to forward current, but may show saturation or efficiency roll-off at very high currents.
• Luminous Intensity vs. Ambient Temperature: Shows that light output decreases as the junction temperature increases. This is a critical consideration for applications operating in high-temperature environments.
• Spectral Distribution: A plot of relative power versus wavelength, showing a peak at approximately 632nm and a width of about 20nm at half the peak power.
• Viewing Angle Pattern: A polar plot illustrating the angular distribution of light intensity, confirming the 120° viewing angle with a Lambertian or near-Lambertian distribution due to the diffused lens.

5. Mechanical & Package Information

5.1 Device Dimensions

The LED conforms to an EIA standard SMD package. Key dimensions (in mm) are:

• Overall Length: 3.2 mm
• Overall Width: 2.8 mm
• Overall Height: 1.9 mm
• Lens Width: 2.2 mm
• Lens Length: 3.5 mm
• Lead Width: 0.7 mm
• Lead Length: 0.8 mm

Tolerance is ±0.2mm unless otherwise specified. A detailed dimensioned drawing is provided in the original datasheet.

5.2 Polarity Identification & Pad Design

The anode (positive) connection is identified. For reliable soldering, a recommended printed circuit board (PCB) attachment pad layout is provided, optimized for both infrared and vapor phase reflow soldering processes. Proper pad design is critical to prevent tombstoning (component standing up on one end) and to ensure a reliable solder joint with the correct amount of solder paste.

6. Soldering & Assembly Guidelines

6.1 Reflow Soldering Parameters

The device is compatible with lead-free (Pb-free) IR reflow soldering. The recommended profile should comply with JEDEC standard J-STD-020B. Key parameters include:

• Pre-heat: 150-200°C for a maximum of 120 seconds to gradually heat the board and components, activating the flux and preventing thermal shock.
• Peak Temperature: Maximum of 260°C. The time above liquidus (typically 217°C for lead-free solder) should be controlled.
• Total Soldering Time: Maximum of 10 seconds at peak temperature. Reflow should be limited to a maximum of two cycles.

It is emphasized that the optimal profile depends on the specific PCB design, components, solder paste, and oven, and should be characterized for each application.

6.2 Hand Soldering

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

• Iron Temperature: Maximum 300°C.
• Soldering Time: Maximum 3 seconds per lead, and only one soldering cycle is permitted to prevent excessive heat stress on the plastic package and the internal wire bonds.

6.3 Storage Conditions

LEDs are moisture-sensitive devices (MSD).

• Sealed Package: Store at ≤30°C and ≤70% Relative Humidity (RH). The shelf life is one year when stored in the original moisture-proof bag with desiccant.
• Opened Package: Components must be used within 168 hours (7 days) of exposure to ambient air (≤30°C / ≤60% RH). If this time is exceeded, a bake-out at approximately 60°C for at least 48 hours is required before soldering to remove absorbed moisture and prevent "popcorning" (package cracking due to vapor pressure during reflow). For long-term storage of opened packages, use a sealed container with desiccant or a nitrogen-filled desiccator.

6.4 Cleaning

If post-solder cleaning is required, only specified alcohol-based solvents like ethyl alcohol or isopropyl alcohol should be used at normal temperature for less than one minute. Unspecified chemicals may damage the plastic lens or package.

7. Packaging & Ordering Information

• Tape Specifications: The LEDs are supplied on 8mm wide embossed carrier tape.
• Reel Specifications: Tape is wound on a standard 7-inch (178mm) diameter reel.
• Quantity per Reel: 2000 pieces.
• Minimum Order Quantity (MOQ): 500 pieces for remainder quantities.
• Standards: Packaging complies with ANSI/EIA-481 specifications.
• Part Number: LTST-E681VEWT. The naming convention typically includes series code (LTST), package/style, color/wavelength code (E681VE), and possibly other variants (WT).

8. Application Notes & Design Considerations

8.1 Drive Circuit Design

LEDs are current-driven devices. To ensure stable and uniform brightness, especially when driving multiple LEDs in parallel, a series current-limiting resistor is mandatory for each LED. The resistor value (R) is calculated using Ohm's Law: R = (V_supply - V_F) / I_F. Using the typical V_F of 2.2V and a desired I_F of 20mA with a 5V supply: R = (5V - 2.2V) / 0.02A = 140 Ohms. A standard 150 Ohm resistor would be suitable. Driving LEDs directly from a voltage source without a current limit will result in excessive current and rapid failure.

8.2 Thermal Management

Although the power dissipation is relatively low (196mW), effective thermal management is still important for maintaining long-term reliability and consistent light output. Ensure the PCB has adequate copper area connected to the LED's thermal pad (if applicable) or leads to help dissipate heat. Avoid operating at the absolute maximum current and temperature limits for extended periods.

8.3 Application Scope

This LED is intended for general electronic equipment such as office appliances, communication devices, and household applications. It is not designed or qualified for safety-critical applications where failure could risk life or health (e.g., aviation, medical life-support, transportation control). For such applications, components with appropriate reliability certifications must be sourced.

9. Technical Comparison & Differentiation

The LTST-E681VEWT's key differentiators in its class include:

• Material Technology: Use of AlInGaP, which typically offers higher efficiency and better temperature stability for red and amber colors compared to older technologies like GaAsP.
• Brightness: With a maximum intensity of 2800mcd, it offers high brightness in a standard package size.
• Viewing Angle: The 120° wide viewing angle with a diffused lens provides excellent off-axis visibility, which is preferable for status indicators over narrow-beam LEDs.
• Process Compatibility: Full compatibility with automated SMT assembly and standard lead-free reflow profiles reduces manufacturing complexity and cost.

10. Frequently Asked Questions (FAQs)

Q: Can I drive this LED without a series resistor if my power supply is exactly 2.2V?
A: No. The forward voltage has a tolerance (±0.1V) and varies with temperature. A slight over-voltage would cause a large, uncontrolled increase in current, potentially destroying the LED. Always use a current-limiting mechanism.

Q: What is the difference between Peak Wavelength and Dominant Wavelength?
A> Peak Wavelength is where the most light energy is physically emitted. Dominant Wavelength is calculated from color coordinates and represents what the human eye perceives as the color. For monochromatic LEDs like this red one, they are often close, but Dominant Wavelength is the key parameter for color matching.

Q: My board will be washed after soldering. Is this LED compatible?
A> The datasheet specifies cleaning only with alcohol-based solvents (isopropyl or ethyl alcohol) for less than one minute. Many aqueous or aggressive flux cleaners may damage the package. Verify compatibility with your specific cleaning process.

Q: Why is there a 168-hour floor life after opening the bag?
A> The plastic package absorbs moisture from the air. During the high heat of reflow soldering, this moisture can turn to steam rapidly, causing internal pressure that can crack the package or delaminate internal layers ("popcorning"). The 168-hour limit and baking procedure manage this risk.

11. Practical Application Example

Scenario: Designing a power status indicator for a 12V DC router.
Design Steps:
1. Select Drive Current: Choose a conservative I_F of 15mA for long life and lower heat.
2. Calculate Resistor: Using typical V_F = 2.2V. R = (12V - 2.2V) / 0.015A = 653 Ohms. Use the nearest standard value, 680 Ohms.
3. Calculate Resistor Power: P_R = I_F² * R = (0.015)² * 680 = 0.153W. A standard 1/4W (0.25W) resistor is sufficient.
4. PCB Layout: Place the LED and its 680Ω resistor close together. Follow the recommended pad layout from the datasheet for reliable soldering.
5. Assembly: Use the JEDEC-compliant lead-free reflow profile. If the boards are assembled more than 7 days after the LED bag was opened, bake the LEDs first.

12. Operating Principle

Light emission in the LTST-E681VEWT is based on electroluminescence in a semiconductor p-n junction made of AlInGaP materials. When a forward voltage exceeding the junction's built-in potential is applied, electrons from the n-type region and holes from the p-type region are injected into the active region. When these charge carriers recombine, they release energy in the form of photons (light). The specific composition of the AlInGaP alloy determines the bandgap energy, which directly defines the wavelength (color) of the emitted light—in this case, red at approximately 624-632 nm. The diffused epoxy lens over the chip serves to extract the light from the semiconductor and shape its angular distribution into a wide 120-degree pattern.

13. Technology Trends

The optoelectronics industry for indicator LEDs continues to evolve. General trends relevant to devices like the LTST-E681VEWT include:

• Increased Efficiency: Ongoing material science improvements aim to produce more lumens per watt (lm/W), allowing for brighter output at the same current or the same brightness with lower power consumption and less heat.
• Miniaturization: While standard packages like this one remain prevalent, there is constant pressure to reduce footprint and height for ever-slimmer electronic devices.
• Enhanced Reliability: Improvements in packaging materials, die attach techniques, and wire bonding aim to extend operational lifetime and increase tolerance to thermal and mechanical stress.
• Color Consistency: Tighter binning tolerances and advanced manufacturing controls are reducing color and brightness variation from lot to lot, which is critical for applications using multiple LEDs.
• Integration: A trend towards integrating drive circuitry, protection features (like ESD diodes), or multiple LED chips (RGB) into a single package exists, though discrete LEDs remain fundamental for simplicity and cost in many applications.