What are SMD LEDs?

SMD (Surface Mounted Device) LEDs are the most common type used in LED PCBs. They are small, discrete LED components that are soldered onto the surface of the PCB. Typical sizes range from the tiny 0402 (1mm x 0.5mm) up to larger 5630 (5.6mm x 3.0mm) packages.

Advantages of SMD LEDs

• Wide variety of sizes and power ratings available
• Low cost due to high volume manufacturing
• Easy to assemble using standard SMT processes
• Can be individually controlled for RGB color mixing
• Allows flexibility in LED placement on the PCB

Disadvantages of SMD LEDs

• Lower power than COB/DOB LEDs
• Large arrays require many solder joints
• Light can appear dotty or pixelated
• Requires more complex PCB design for large arrays

Best Uses for SMD LEDs

• Backlighting LCDs and keypads
• Status indicator lights
• Small to medium channel letter signs
• Accent and decorative lighting
• Automotive interior lighting

What are COB LEDs?

COB (Chip-on-Board) LEDs consist of multiple LED chips bonded directly to a PCB substrate or ceramic base. The individual LED chips are connected in an array which is then coated with a phosphor layer. This creates what looks like a single illuminated panel.

Advantages of COB LEDs

• High lumen density in a small area
• Excellent heat dissipation
• Smooth, uniform light without hot spots
• Simplified assembly with just one component
• Thin form factor for space constrained designs

Disadvantages of COB LEDs

• Limited size range (typically 5-40mm)
• Higher cost than SMD LEDs
• Drive circuitry is more complex
• Color mixing requires multiple COBs
• Prone to catastrophic failure if damaged

Best Uses for COB LEDs

• High bay and low bay lighting
• Architectural lighting
• Street lamps and parking lot lights
• Stage and entertainment lighting
• Medical and dental exam lights

What are DOB LEDs?

DOB (Driver-on-Board) LEDs take COB technology a step further by integrating the driver circuitry directly onto the LED module’s PCB. This creates a compact, all-in-one solution that only requires a DC power input. The driver’s constant current output ensures stable brightness and longer lifespan.

Advantages of DOB LEDs

• Plug-and-play with minimal wiring
• Excellent thermal management
• High efficiency and lumen output
• Uniform lighting profile
• Longer lifespan than discrete solutions

Disadvantages of DOB LEDs

• More expensive than COB and SMD
• Larger size than equivalent COB
• Less design flexibility
• Limited wattage range

Best Uses for DOB LEDs

• Retrofit lamps and bulbs
• Downlights and spotlights
• High power flashlights
• Automotive headlamps
• Aquarium and horticulture lighting

Comparison Table

Feature	SMD LED	COB LED	DOB LED
Typical Size	0402 to 5630	5-40mm	20-100mm
Power Range	0.1 to 3W	1 to 200W	10 to 100W
Lumen Density	Low	High	High
Color Options	White & Mono	White & Mono	White & Adjustable
Cost	Low	Moderate	High
Design Flexibility	High	Moderate	Low
Ease of Assembly	Moderate	Simple	Plug-and-Play
Drive Complexity	Low	Moderate	Integrated
Heat Dissipation	Fair	Excellent	Excellent
Lifespan	Long	Longer	Longest

FAQ

1. Can I use regular PCB material for LED PCBs?

While regular FR-4 PCB material can work for low power LED applications, it is not recommended for high power LEDs. Instead, use a metal Core PCB (MCPCB) which has a dielectric layer bonded to an aluminum substrate for better heat transfer.

2. How do I decide between SMD, COB and DOB LEDs for my design?

The choice depends on your specific requirements such as power level, light distribution, available space, thermal constraints, and cost budget. Use the comparison table above to weigh the trade-offs for your application.

3. What are some common issues to watch out for with LED PCBs?

Some common pitfalls include inadequate heat sinking leading to overheating, incorrect current limiting causing reduced lifespan, and poor LED binning resulting in inconsistent color. Be sure to follow the LED manufacturer’s specifications and guidelines closely.

4. Are there any special design considerations for LED PCBs?

Yes, LED PCBs often require large copper pours for heat dissipation, electrically isolated thermal vias, and careful component placement to avoid shadowing. It’s also important to use at least a 4-layer PCB Stackup with dedicated power and ground planes.

5. How do I ensure good color consistency across my LED PCB?

The key is to pay close attention to the LED manufacturer’s binning and color sorting process. Use LEDs from the same bin and ideally from the same production lot. Also consider using optics and diffusers to mix the light and create a more uniform appearance.

By understanding the differences between SMD, COB, and DOB LEDs, you can make an informed decision about which type to use in your LED PCB designs. Careful component selection and attention to thermal management, drive circuitry, and layout techniques will help ensure a successful and long-lasting LED product.