SMD LED LTST-T680TBWT Datasheet - Blue Diffused - 20mA - 80mW - English Technical Document

1. Product Overview

This document details the specifications for a surface-mount device (SMD) LED. This component is designed for automated printed circuit board (PCB) assembly processes and is suitable for applications where space is a critical constraint. The LED features a diffused lens, which provides a wider, more uniform light distribution compared to clear or water-clear lenses, making it ideal for indicator and backlighting purposes where glare reduction is desired.

1.1 Core Advantages and Target Market

The primary advantages of this LED include its compliance with RoHS (Restriction of Hazardous Substances) directives, making it suitable for global markets with strict environmental regulations. It is packaged on 8mm tape wound onto 7-inch diameter reels, which is compatible with standard automated pick-and-place equipment used in high-volume electronics manufacturing. The device is also designed to be compatible with infrared (IR) reflow soldering processes, which is the industry standard for SMD assembly. Its I.C. (Integrated Circuit) compatible drive characteristics simplify circuit design. The primary target markets for this component are telecommunications equipment, office automation devices, home appliances, and industrial equipment, where it is commonly used for status indication, signal and symbol illumination, and front panel backlighting.

2. In-Depth Technical Parameter Analysis

This section provides a detailed breakdown of the LED's operational limits and performance characteristics under standard test conditions (Ta=25°C). Understanding these parameters is crucial for reliable circuit design and ensuring the longevity of the component.

2.1 Absolute Maximum Ratings

The absolute maximum ratings define the stress limits beyond which permanent damage to the device may occur. These are not conditions for continuous operation.

• Power Dissipation (Pd): 80 mW. This is the maximum amount of power the LED package can dissipate as heat. Exceeding this limit can lead to overheating and accelerated degradation of the semiconductor material.
• Peak Forward Current (I_FP): 100 mA. This is the maximum allowable instantaneous forward current, typically specified under pulsed conditions (1/10 duty cycle, 0.1ms pulse width). It is significantly higher than the continuous current rating and is relevant for brief, high-intensity flashes.
• DC Forward Current (I_F): 20 mA. This is the recommended maximum continuous forward current for normal operation. Driving the LED at or below this current ensures optimal performance and lifespan.
• Operating Temperature Range (T_opr): -40°C to +85°C. The device is guaranteed to operate within specifications across this ambient temperature range.
• Storage Temperature Range (T_stg): -40°C to +100°C. The device can be stored without degradation within this temperature range when not powered.

2.2 Electrical and Optical Characteristics

These parameters describe the typical performance of the LED when operated within its recommended conditions (I_F = 20mA, Ta=25°C).

• Luminous Intensity (I_V): 140.0 - 450.0 mcd (millicandela). This is a measure of the perceived power of the emitted light. The wide range indicates the device is available in different brightness bins (see Section 3). Intensity is measured using a sensor filtered to match the human eye's photopic response (CIE curve).
• Viewing Angle (2θ_1/2): 120 degrees (typical). The viewing angle is defined as the full angle at which the luminous intensity is half of the intensity measured on-axis (0 degrees). A 120-degree angle indicates a very wide beam, characteristic of a diffused lens.
• Peak Emission Wavelength (λ_P): 468 nm (typical). This is the wavelength at which the spectral power distribution of the emitted light is at its maximum. It is a physical property of the InGaN (Indium Gallium Nitride) semiconductor material used.
• Dominant Wavelength (λ_d): 465 - 475 nm. This is the single wavelength that best represents the perceived color of the light, derived from the CIE chromaticity diagram. It is the parameter used for color binning.
• Spectral Line Half-Width (Δλ): 20 nm (typical). This is the spectral bandwidth measured at half the maximum intensity (Full Width at Half Maximum - FWHM). A value of 20nm is typical for a blue InGaN LED.
• Forward Voltage (V_F): 3.3 V (typical), 3.8 V (maximum). This is the voltage drop across the LED when driven at 20mA. It is a critical parameter for designing the current-limiting circuitry (e.g., selecting a series resistor or constant-current driver).
• Reverse Current (I_R): 10 μA (maximum) at V_R = 5V. LEDs are not designed for reverse bias operation. This parameter indicates the very small leakage current if a reverse voltage is accidentally applied. Applying a reverse voltage beyond the maximum rating can cause immediate failure.

3. Binning System Explanation

To ensure consistency in mass production, LEDs are sorted into performance bins after manufacturing. This allows designers to select components that meet specific brightness, color, and voltage requirements for their application.

3.1 Forward Voltage (V_f) Binning

LEDs are sorted based on their forward voltage drop at 20mA. The bins (D7 to D11) have a tolerance of ±0.1V within each bin. For example, bin D9 includes LEDs with a V_f between 3.2V and 3.4V. Selecting LEDs from the same V_f bin can help ensure uniform brightness when multiple LEDs are connected in parallel with a common current-limiting resistor.

3.2 Luminous Intensity (I_V) Binning

This is the brightness binning. The bins range from R2 (140.0-180.0 mcd) to T2 (355.0-450.0 mcd), with an 11% tolerance on each bin. Applications requiring specific brightness levels can specify the desired intensity bin code.

3.3 Dominant Wavelength (W_d) Binning

This is the color binning. For this blue LED, the bins are AC (465.0-470.0 nm) and AD (470.0-475.0 nm), with a tight tolerance of ±1nm. This ensures a consistent shade of blue across all LEDs in an assembly, which is critical for aesthetic and signaling applications.

4. Performance Curve Analysis

While specific graphs are referenced in the datasheet (e.g., Figure 1, Figure 5), their typical implications are analyzed here. These curves are essential for understanding performance under non-standard conditions.

4.1 Forward Current vs. Forward Voltage (I-V Curve)

The I-V characteristic of an LED is exponential. A small increase in forward voltage beyond the knee voltage results in a large increase in current. This non-linear relationship is why LEDs must be driven by a current source or with a current-limiting resistor; a constant voltage source would lead to thermal runaway and destruction. The typical V_F of 3.3V at 20mA represents a point on this curve.

4.2 Luminous Intensity vs. Forward Current

Luminous intensity is approximately proportional to forward current within the operating range. However, efficiency (lumens per watt) may peak at a current lower than the maximum rating. Driving the LED at the maximum continuous current (20mA) provides the highest output but may reduce efficacy slightly compared to a lower drive current.

4.3 Temperature Dependence

LED performance is temperature-sensitive. As the junction temperature increases:
- Forward Voltage (V_F) decreases. This can cause an increase in current if driven by a simple resistor from a constant voltage supply.
- Luminous Intensity (I_V) decreases. The light output drops as temperature rises, a phenomenon known as thermal droop.
- The dominant wavelength may shift slightly, causing a subtle color change.
Proper thermal management (e.g., adequate PCB copper area for heat sinking) is therefore essential for maintaining consistent performance.

4.4 Spectral Distribution

The spectral output curve shows a single peak centered around 468 nm with a typical half-width of 20 nm. This is characteristic of a blue InGaN LED. There is minimal emission in other parts of the visible spectrum, resulting in a saturated blue color.

5. Mechanical and Package Information

5.1 Package Dimensions and Polarity

The LED is housed in a standard industry SMD package. The cathode is typically marked by a green dot on the top of the component or a notch/chamfer on one side of the package body. Correct polarity must be observed during placement. The package is designed to be compatible with infrared reflow and vapor phase soldering processes.

5.2 Recommended PCB Attachment Pad

The datasheet includes a recommended land pattern (footprint) for the PCB. Adhering to this pattern is crucial for achieving reliable solder joints, proper self-alignment during reflow, and effective heat transfer from the LED to the PCB. The pad design typically includes thermal relief connections to balance solderability and heat dissipation.

6. Soldering and Assembly Guidelines

6.1 IR Reflow Soldering Profile

The component is rated for lead-free (Pb-free) soldering processes. A suggested reflow profile compliant with J-STD-020B is provided. Key parameters include:
- Pre-heat/Soak: Ramp from 150°C to 200°C, held for a maximum of 120 seconds to activate flux and minimize thermal shock.
- Reflow (Liquidus): Peak temperature should not exceed 260°C, and the time above 217°C (typical liquidus temperature for SAC solder) should be limited to recommended values (e.g., 30-60 seconds).
- Cooling: Controlled cooling rate to minimize stress on the solder joints and component.
It is critical to characterize the profile for the specific PCB assembly, as board thickness, component density, and oven type affect the thermal profile seen by the LED.

6.2 Hand Soldering

If hand soldering is necessary, it should be performed with extreme care. The recommendation is to use a soldering iron at a maximum temperature of 300°C, with the soldering time limited to 3 seconds per pad. This should be done only once to avoid thermal damage to the plastic package and the internal wire bonds.

6.3 Cleaning

Post-solder cleaning should only be performed with specified solvents. Isopropyl alcohol (IPA) or ethyl alcohol are recommended. The LED should be immersed at normal temperature for less than one minute. Harsh or unspecified chemicals can damage the plastic lens and package material.

6.4 Storage and Moisture Sensitivity

The LEDs are packaged in a moisture-barrier bag with desiccant. Once the original sealed bag is opened, the components are exposed to ambient humidity. It is strongly recommended to complete the IR reflow soldering process within 168 hours (7 days) of opening the bag. For longer storage after opening, the LEDs should be stored in a sealed container with desiccant or in a nitrogen environment. If components have been exposed for more than 168 hours, 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) during reflow.

7. Application Notes and Design Considerations

7.1 Drive Method

An LED is a current-driven device. The most common and simplest drive method is a series current-limiting resistor connected to a voltage supply. The resistor value is calculated using Ohm's Law: R = (V_supply - V_F) / I_F. For example, with a 5V supply, a V_F of 3.3V, and a desired I_F of 20mA: R = (5V - 3.3V) / 0.02A = 85 Ohms. A standard 82 or 100 Ohm resistor would be suitable. For applications requiring multiple LEDs, connecting them in series ensures identical current through each LED, promoting uniform brightness. Parallel connection is possible but requires careful matching of V_F or individual resistors for each LED to prevent current hogging.

7.2 Thermal Management

Although the power dissipation is relatively low (80mW max), effective heat sinking is still important for longevity and color stability. Using the recommended PCB pad with adequate thermal connection to copper planes helps dissipate heat. Avoid placing the LED in enclosed spaces without ventilation.

7.3 Application Limitations

This component is designed for general-purpose electronic equipment. It is not specifically qualified for applications where high reliability is paramount and failure could jeopardize safety (e.g., aviation, medical life-support, transportation control). For such applications, components with appropriate qualifications should be sourced.

8. Packaging and Ordering Information

8.1 Tape and Reel Specifications

The LEDs are supplied on embossed carrier tape with a protective cover tape. The tape width is 8mm. The reels have a 7-inch (178mm) diameter. Each reel contains 2000 pieces. The packaging conforms to ANSI/EIA-481 standards to ensure compatibility with automated assembly equipment. The tape has orientation pockets to ensure correct polarity during pick-and-place.

9. Frequently Asked Questions (FAQ)

Q: Can I drive this LED with a 3.3V supply without a resistor?
A: No. The typical V_F is 3.3V, but it can vary from 2.8V to 3.8V depending on the bin. Connecting it directly to a 3.3V supply could result in excessive current for low-V_F units or no light for high-V_F units. A series resistor or constant-current driver is always required.

Q: What is the difference between Peak Wavelength and Dominant Wavelength?
A: Peak Wavelength (λ_P) is the physical peak of the light spectrum. Dominant Wavelength (λ_d) is the single wavelength perceived by the human eye, calculated from color coordinates. λ_d is used for color specification and binning.

Q: Why is there a 168-hour floor life after opening the bag?
A> SMD plastic packages absorb moisture from the air. During the high-temperature reflow soldering process, this moisture can turn to steam rapidly, causing internal pressure that can crack the package (\"popcorning\"). The 168-hour limit is based on the moisture sensitivity level (MSL) of the component.

Q: How do I achieve uniform brightness in a multi-LED array?
A> The best method is to connect LEDs in series, ensuring the same current flows through each one. If a parallel configuration is necessary, use LEDs from the same V_F and I_V bins and consider using an individual current-limiting resistor for each LED to compensate for V_F variations.