Introduction to SMD LEDs
Surface mount device (SMD) LEDs are increasingly being used in printed circuit boards (PCBs) due to their small size and ability to be easily mounted during the PCB assembly process. Choosing the right SMD LEDs and properly documenting them in your bill of materials (BOM) is key to a successful PCB design.
What are SMD LEDs?
SMD LEDs are LEDs (light emitting diodes) that are designed to be soldered directly onto the surface of a PCB, rather than using traditional through-hole mounting. The underside of the LED component has metal pads that line up with matching pads on the PCB. The LED is then soldered in place using solder paste and reflow or wave soldering techniques.
The benefits of SMD LEDs include:
- Smaller size - SMD LEDs take up less space on the PCB compared to through-hole LEDs. This allows for smaller and more compact PCB designs.
- Easier automated assembly - SMD LEDs can be efficiently placed and soldered by automated pick-and-place and reflow soldering machines during PCB assembly. This reduces manufacturing costs and time.
- Improved aesthetics - With no wire leads, SMD LEDs have a cleaner look when mounted on a PCB. The small size also allows them to be incorporated into innovative industrial designs.
- Design flexibility - SMD LEDs free up more PCB real estate for routing traces and placing other components. They also allow LEDs to be placed on both sides or internal layers of the PCB.
SMD LED Types
There are many varieties of SMD LEDs to choose from when designing your PCB. Some key types include:
0402 Sized LEDs - The 0402 size is one of the smallest surface mount LED packages available at just 1mm x 0.5mm. These micro-LEDs allow very compact and dense PCB designs but the light output is low.
0603 Sized LEDs - Slightly larger than 0402 at 1.6mm x 0.8mm, the 0603 size provides higher luminosity in a still small package. This is a very popular size for SMD LEDs.
0805 Sized LEDs - At 2mm x 1.25mm, the 0805 LED package offers a good balance of compact size while still producing bright, visible light. This is the most common size used.
1206 Sized LEDs - For applications needing high brightness, the 1206 LED size at 3.2mm x 1.6mm has a large light-emitting surface and is very bright.
Right Angle LEDs - These have a 90 degree angle in the packaging so the LED points outward from the edge of the PCB. This is useful when you need side emitting LEDs.
High Power LEDs - Designed for very bright illumination, these larger SMD LED packages contain power-handling circuitry and lenses for increased luminosity.
SMD LED Colors
SMD LEDs come in a rainbow of different color options. Some common colors include:
- Red - Used for indicator lights and backlighting.
- Green - Provides a bright green glow, often used for status indicators.
- Blue - Emits a cool blue color, popular for accent lighting.
- White - Offers a neutral white light for illumination purposes.
- Yellow - Emits a warm yellow glow useful as a spotlight color.
- RGB - Contains red, green and blue dies allowing generation of any color by mixing the intensities of each one. Useful for decorative lighting.
In addition to single color LEDs, there are also RGB SMD LEDs that contain red, green and blue dies in a single package. By controlling the intensity of each die, RGB LEDs can produce any color in the visible light spectrum, allowing dynamic and decorative lighting effects.
Choosing the Best SMD LEDs for Your PCB
When selecting SMD LEDs for your printed circuit board design, there are a number of factors to consider so you choose the right LED package style, size, luminosity, viewing angle, color, and forward voltage for your particular application.
Consider the Application and Environment
First, think about how much illumination is needed and the intended viewing angle for your LEDs. Is very high brightness required or will low-current indicator LEDs suffice? Do you need a wide 120+ degree viewing angle or will a narrow 30 degree beam work better to highlight a specific spot? What color and number of LEDs will produce the desired lighting effect? Also consider the operating environment - will the LEDs be exposed to dust, moisture or other contaminants that require a sealed package?
Choose the Right Size SMD LED
Consider the amount of PCB space available for the LEDs and any size constraints your PCB design may have. Generally, it's best to use the smallest LED that provides enough luminous intensity for your needs. But very tiny 0402 or 0603 LEDs can be challenging to hand solder if not assembling using reflow soldering. They also offer lower light output than mid-size 0805 or 1206 LEDs. Check your PCB assembly process capabilities when deciding on LED sizes.
Match the Forward Voltage
Pay attention to the LED's forward voltage spec which indicates the minimum voltage needed to light the LED and achieve the rated luminosity. Lower forward voltages around 1.8V to 3V allow direct interfacing with lower voltage CMOS or TTL logic circuits without the need for current limiting resistors or transistors. Higher voltage LEDs require drive circuitry to provide the proper current limiting.
Consider the Viewing Angle
Most SMD LEDs emit light in a wide 120 to 130 degree viewing angle. For indicator applications, a narrow 30 or 45 degree beam angle LED focused on a specific spot on a panel may work better. Wide viewing angle LEDs are desirable for area illumination or decorative lighting. Consider the placement of your LEDs and the desired illumination pattern.
Evaluate Luminous Intensity
Brighter LEDs allow smaller packages but ensure any lens and encapsulant material will not degrade or discolor over the LED's lifetime at higher intensities. Also confirm the LED's rated luminosity matches your requirements. Mild underlighting may only require 10-30mcd while indicators may need 150-200mcd and bright spots up to 10000mcd intensity.
Check Current Rating
Drive current affects LED brightness but also impacts reliability and lifetime so don't exceed the maximum current rating. Most SMD LEDs can handle 10-30mA drive currents but LED packages can overheat at higher currents causing failures.
By evaluating these factors, you can select optimal SMD LED components tailored specifically for your PCB design requirements.
Documenting SMD LEDs in Your Bill of Materials
Clear documentation of every SMD LED used in your PCB design is essential for efficient assembly and sourcing of your board. Your bill of materials (BOM) should contain detailed information on each unique LED component.
Include LED Package Size
At minimum, list the package style such as the common 0603, 0805, or 1206 sizes which gives assemblers the necessary information to choose the right LEDs. For less common package sizes, include the dimensions in mm for length x width such as 1.6 x 0.8mm.
Specify Viewing Angle
Note the viewing angle of the LED whether it has a narrow 30 degree beam or standard 120 degree wide viewing cone. This ensures the chosen LEDs emit light in the desired pattern.
List Luminosity Rating
Include the LED's rated luminous intensity in millicandelas (mcd). This allows comparing specs to determine if LEDs with the required brightness are sourced.
Show Forward Voltage
The forward voltage indicates the minimum voltage needed to properly illuminate the LED. Listing this spec helps ensure the LED will interface properly with drive circuitry and logic levels. Common voltages are 1.8V, 2.0V, 3.3V.
Indicate Color and Wavelength
Identify the dominant wavelength or color such as 660nm for red, 525nm for green, 470nm for blue. This prevents incorrect colors from being installed. For RGB LEDs, list all three colors.
Include Current Rating
Showing the maximum drive current helps prevent damage from excessive current. Currents are typically in the range of 10-30mA but high power LEDs may draw 100mA or more.
Specify Temperature Range
List the operating temperature range and soldering temperature rating so assemblers know the LED's environmental limits for proper functionality. Common ranges are -40°C to +85°C and 260°C reflow soldering.
Provide Manufacturers and Part Numbers
When available, listing the manufacturer part number provides clear identification of the exact LED component that should be installed. Provide manufacturer names as well to aid component sourcing.
Give a Description
A concise description summarizes the key details of the LED in one place, for example: "SMD LED, 0805 package, 45 degree viewing angle, red color, 660nm wavelength, 2.0V forward voltage, 30mA current".
List Quantity Per PCB
Include the total quantity of that LED component needed to populate each assembled PCB. This ensures enough LEDs are sourced for the production run.
With careful documentation in your BOM, assemblers will have the full specifications needed to accurately source, purchase and install the correct SMD LED components on your PCB design.
Design Best Practices for SMD LEDs in PCB Layout
When laying out the PCB, there are some best practices to follow that will ensure your SMD LEDs are designed properly on the board.
Leave Adequate Clearance Around LEDs
Give SMD LED packages, especially high-power types, sufficient clearance from other taller components to allow proper light dispersion and prevent shadowing. LED datasheets specify recommended clearance areas.
Follow Manufacturer Recommended PCB Layouts
Many LED datasheets provide suggested PCB land patterns showing pad shapes and sizes. Following these closely prevents soldering issues like tombstoning. Provide sufficient pad area for heat dissipation.
Place Pads for Ease of Soldering
Position the LED pads so that the package sits parallel to the PCB edge. This allows maximum surface contact during reflow soldering to prevent tombstoning where one pad lifts off the board.
Include Pads for Multiple LED Pins
For high power LEDs with 3 or 4 pins, make sure to include pads for all pins even if not all are electrically connected. This prevents stress on solder joints from unsupported package weight.
Ensure Trace Widths Can Handle Currents
Size PCB traces appropriately to carry higher LED drive currents without excessive heating or voltage drops. Wider traces or copper fills may be needed. Follow trace width guidelines based on current.
Provide Adequate Thermal Dissipation
Higher current SMD LEDs produce significant heat which must dissipate through the PCB to prevent failures. Use thermal pads,thermal vias, exposed copper fills, and other cooling methods in the PCB design.
Include Current Limiting Resistors
When driving LEDs directly from logic signals, add a current limiting resistor in series on the board. Size resistors to allow ~20mA through the LED. Verify resistor power rating as well.
Design For Ease of Troubleshooting
Arrange discrete LEDs in an orderly matrix pattern with adequate spacing to allow replacement if needed. Connect longer trace runs to groups of LEDs to localize problems.
Follow Assembly Process Requirements
Design board layout and component density considering your PCB assembly process capabilities. Very densely packed boards can cause issues for automated SMD assembly equipment.
By following SMD LED design best practices, your PCB layout will ensure successful illumination, simplify assembly, and prevent potential issues with your surface mount LED components.
Properly Testing SMD LED Functionality
Verifying that all SMD LEDs are installed correctly and operating on your assembled PCBs requires focused visual inspection and electrical testing tailored to LED components.
Visually Inspect LED Placement and Soldering
Use an inspection microscope to check that each LED package is properly aligned on its PCB pads, with pin 1 in the right orientation. Verify all LED pins are fully soldered with no bridging between pads.
Confirm Proper LED Color and Size
Visually check that the correct color LED is installed in each position by powering the board and qualifying the emitted color. Also verify that the LED package size matches the PCB footprint.
Electrically Test LED Functionality
Apply test voltages to illuminate each LED using an appropriate current limiting resistor. Vary the voltage to qualify brightness at different intensities. Check for open or shorted LEDs that fail to illuminate or draw excessive current.
Measure LED Forward Voltage
Using a multimeter, test the actual forward voltage needed to light each LED at low current. Compare to datasheet specs to detect defective LEDs requiring higher voltage.
Evaluate LED Brightness and Color
Qualify LED brightness and hue against a known color/intensity standard LED of the same type under identical drive conditions. Check for outliers.
Assess LED Beam Angle Coverage
Power the LEDs and examine the full light dispersion pattern. Verify wide angle LEDs properly illuminate a broad area and narrow beam LEDs focus light properly.
Test LEDs Under Temperature Extremes
Heat boards to high temperature and cold soak at low temperature per the LED rating while inspecting for luminous intensity or color shifts that may indicate marginal components.
Thoroughly testing SMD LEDs during PCB inspection and quality control confirms proper functioning, prevents field failures, and ensures your design's illuminated features operate as intended.
Frequently Asked Questions About SMD LEDs
What is the main advantage of using SMD LEDs instead of through-hole LEDs?
The small size of SMD LED packages allows dense placement on printed circuit boards, supporting miniaturization and portability of electronic devices. SMD LEDs also simplify automated PCB assembly compared to manual insertion of through-hole LEDs.
How important is an SMD LED's forward voltage rating when selecting components?
Forward voltage is very important for proper LED circuit design. Voltages under 2V allow direct logic level interfacing while higher voltages require current limiting drive circuitry and transistors. Always choose SMD LEDs with forward voltages that match your PCB's drive voltage capability.
What causes an SMD LED to "tombstone" during reflow soldering?
Uneven heating of the two contacts during soldering causes one pad to melt first, allowing one end of the LED to lift from the PCB. Following datasheet land patterns and placing LEDs along the board edge minimizes tombstoning.
What happens if too much current is applied to an SMD LED?
Excess current will overheat the semiconductor junctions inside the LED causing permanent damage. LED datasheets specify maximum drive currents to prevent failures. Always use current limiting resistors when powering LEDs directly from logic signals.
How long will SMD LEDs typically last in a PCB design?
Properly driven within ratings, SMD LED lifetimes often exceed 50,000 hours. Automotive-grade LEDs are rigorously tested for long lifetimes of 100,000+ hours. Following datasheet guidelines prevents premature LED failures.
Conclusion
SMD LED technology allows ingenious PCB lighting solutions, but only with the right LED selection, documentation, PCB design, assembly, and testing. By following the guidelines in this article, you can successfully incorporate tiny surface mount LEDs into your next printed circuit board design and enjoy bright, long-lasting LED illumination.
