SMD LEDs in Your PCB and Your Bill of Materials

January 2, 2019 Altium Designer

LED array in a theatrical can

 

Unless you refuse to get out of the house on a Friday night (like me), you’ve probably been at a concert and had the privilege of being bathed in multi-colored lights from the stage. LED arrays provide a great concert experience, but I think that learning what makes them tick is far more interesting.

 

Kingbright (KPKF) surface-mounted LEDs are probably my favorite LED to use on a PCB. They look clean on your board and they are available from a number of distributors. These LEDs are a great choice for some LED lighting arrays as they are quite durable, although there are some design choices you’ll need to make before building an LED lighting array.

SMD LEDs in Your PCB

Many devices that include LEDs on a PCB using surface-mounted components. Through-hole components can bend slightly if the wires are very thin, making the device look cheap from the outside. SMD LEDs, if soldered properly, remain rigid on the board. They can also be placed behind a screen, whereas cheaper LEDs that use a bulb will stick out through the packaging. SMD LEDs can be placed behind a small screen in your packaging, giving your device a cleaner look and feel.

 

Most PCBs that include LEDs are built using multi-layered FR4 substrates. You’ll need to include a pattern of closely-spaced filled and/or plated through vias under each component to transmit heat, as well as reach your power and ground layers.

 

VIPPO vias can be used if your LEDs have a small footprint and are surface-mounted. Solder can wick into these vias during assembly if these vias are not filled or plated over, resulting in a weak solder joint or even tombstoning. This problem with wicking is one reason why it is better to just use SMD LEDs on an LED lighting array. VIPPO is a better choice for other PCBs that include surface-mounted components.

 

A single LED with decent power output will not cause undue thermal damage to your PCB. But if you are building a system for lighting applications, the PCB that supports your LEDs will need to withstand a significant amount of heat, and these boards are difficult to cool via traditional means. You can’t really attach a heatsink anywhere as the individual LEDs are too small, and a heatsink would block the emitted light anyways.

 

Due to the thermal demands, PCBs with a metal core are usually used for LED lighting applications thanks to their ability to dissipate a large amount of heat. Aluminum is often used as the metal core in PCBs that are used for LED lighting applications. Among all the possible metal core PCBs (sometimes called MCPCBs) aluminum is the most common metal used as the core. Copper and iron are also used for metal core PCBs.

 

SMD LED on a green PCB

SMD LED on a green PCB

 

Metal Core Boards for LED Lighting Arrays

Just like the case of ICs that generate a significant amount of heat, you should try to place your LED array near the center of the board. Placing any component that generates a significant amount of heat at the edge of the board causes heat to be trapped at the edge and leads to a larger temperature rise. Placing these components at the center of the board allows heat to dissipate away from the components and conduct towards the edges.

 

With LED arrays, this may not be possible as they tend to be the only components on the board, thus the board size is reduced to the size of the array. Therefore, using a metal core PCB to dissipate heat is the best option for preventing a large temperature rise.

 

One important difference between a standard PCB and a metal core PCB is how the materials work together to transfer heat away from the LED array. Typically, the components are placed on copper foil bonded to the top layer of an FR4 board. The metal core is not really a core, it is just bonded on the backside of the board. The board can then be mounted to the device housing, and any heat will be dissipated through the metal core and into the housing.

 

Thermal vias or thermal pads are normally used to dissipate heat away from the top side of a board. If you’re using a metal core PCB for an LED array, you should not use any plated through hole vias and only use SMD LEDs. Remember that the bottom of the layer structure is a thick piece of metal. SMDs LEDs in lighting arrays are placed in parallel, so any pair of plated through hole vias will short the entire board.

 

The high thermal conductivity of the metal core eliminates the need to vias entirely. The metal core should also span the entirety of the LED array. If you have other components that need to be included for your lighting array that are not surface-mounted or that require vias to form interconnects, you’ll want to place them on a separate board and wire the boards together.

 

SMD LEDs on a lighting array

SMD LEDs on a lighting array

What to Include in Your Bill of Materials

Any SMD LEDs that appear directly on your PCB should be included in the bill of materials that is sent to your PCB fabricator/assembler. You’ll also need to generate Gerber files for your manufacturer as they show the location of your SMD pads and other areas filled with copper throughout your board.

 

Your PCB design software plays an important role here. Your design software needs to interface directly with your component libraries and your bill of materials tools. If you’re using a popular SMD LED like a Kingbright LED, you could find sourcing information for this component online. The better choice is to use design software that provides comprehensive sourcing information and life-cycle information for all of your components.

 

If you opt to design a metal core PCB for a lighting system, you’ll need to define the metal core as a metal fill when you build your layer stack. This information will be incorporated into your Gerber files for your manufacturer. Most PCB design packages define these layers as copper by default, but a great PCB package will allow you to define this layer as aluminum if you so choose.

 

Unifying your schematic, layout, component data, and deliverable generation tools in a single interface helps you avoid time-consuming redesigns that can arise from including unavailable components in your PCB. The information you need for your bill of materials and other deliverables appears in a single program, and you won’t have to rely on your manufacturer or an external third-party service to source your components.

 

Altium Designer gives you access to the best design tools and extensive component libraries for your next PCB. It also contains tools that make it easy to generate deliverables for your manufacturer directly from your design data. The ActiveBOM tool, component libraries, and other design features in Altium Designer let you create high quality layouts, generate a thorough bill of materials and other deliverables, and manage component sourcing and life-cycle information.

 

If you want to learn more about how Altium Designer can help you select the right components for your PCB and bring your next device to production, talk to an Altium expert today.

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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