LED lighting: why the connection counts

Everywhere you turn, you can find LEDs in almost any lighting form factor, and connections are more challenging than a screw-in light bulb. Andrew Bogaczyk* of Phoenix Contact explains how to connect LEDs and why the connection counts.

LEDs continue to dominate the lighting marketplace; everywhere you turn, you can now find LEDs in almost any form factor. LEDs allow designers incredible flexibility when it comes to designing light and fixtures: the beauty of this simple technology is that you now have the ability to create numerous colours and designs.

The challenge comes in how to connect your fixture. Because an LED can be designed to look like anything, connections are no longer as simple as a screw-in light bulb. The connections must be flexible, like the LEDs themselves. We need connectors that can be placed in a variety of locations, and some may need to be wired in the field or in a factory.

There are also different types of PCBs used with LEDs. Some use a rigid or solid board made of FR4 or a metal core; these boards will only allow for processing on the top side of the PCB. Those that want even greater flexibility may choose the flex-strip LED boards, which allow the actual PCB to bend around corners and other structural design elements. Both of these applications require a unique type of connection to ensure maximum reliability.

The real value of LEDs is the lower power consumption and the long-lasting life of the light. Therefore, the connector we place on the fixture must be able to outlast the life of the luminaire itself. The connector will need to handle vibration, ambient temperature, humidity and mechanical security to the board.

Rigid or solid-core boards

The most common type of lighting in a building is probably the light above your head, known as downlighting. The purpose of downlighting is to cast illumination on objects below the luminaire. These fixtures are typically integrated into the ceiling, which is generally flat. A rigid or solid core board works perfectly.

The other (maybe more important) reason rigid boards are used is their ability to drive heat away from the LEDs. The biggest drain on LED life is heat. With proper design, the solid board, with a good heat sink on its back, can drive away heat.

Solid boards make great sense for LEDs, but when it comes to connecting power, there are some challenges. The first option is to directly solder wire to the PCB. Soldering requires someone who knows how to really step in and solder correctly. The challenge is that due to the small size of the fixtures, the soldering pads on the PCB may be too close together, and you don’t want the wires to short from one position to the next.

There is also a concern about vibration and its effects on the solder joint. A crack in the solder joint could lead to excess resistance and cause the device to fail or, in a worst case, cause a fire. Soldering wires to the board also means eliminating the chance to repair later. If something goes wrong on the PCB, you will need to clip the wires and throw the module away.

Traditionally, connectors have been thru-hole components. The solder pins would go through the PCB, and a wave solder process would be used to secure the connectors to the PCB. With the proliferation of solid boards in lighting, these thru-hole connectors can no longer be used. The connector now must be mounted and soldered to the top side of the board through surface-mount technology (SMT).

SMT is more commonly used for other components on the board, such as ICs, but not commonly used for connectors. Connector manufacturers must create something that is field-wireable, but also mechanically secure enough to handle the force of wiring a connector. One good way to prevent the product from delaminating from the board is to utilise anchors on the side of the connector. Anchors supply no electrical contact and ensure these connectors are secure while taking stress off the electrically necessary solder joints.

Connector options for rigid boards

Once you’ve decided to connectorise your application, you must choose the style of connection you need - fixed or pluggable. A fixed connection means a one-piece connector that solders to the board and accepts the wire all in one block. Typically with this type of connector, you terminate the wire once and you’re done. The wiring on the PCB is easy to access. Most fixed connectors are available so that wire entry can be horizontal, angled or vertical to the PCB.

Pluggable connectors are great when you need to troubleshoot your boards, swap modules or have a difficult-to-access wiring interface. With pluggable connections, you have two pieces - a plug connected to the wire and a header soldered to the PCB. Pluggable connectors also give you a lot of options when it comes to orientation of wire coming to the PCB. Header (board-mount side) can be mounted to the PCB horizontally or vertically. A few manufacturers also have a thru-board design that allows the plug to connect on the bottom side of the board, moving the connector mass below the PCB and away from the LEDs. The plug would then be wired and connected to the header.

Some manufacturers also allow you to use the same connector family to do board-to-board connections. When you have LED modules that you want to stack end to end, you can do it securely with a connector on each board and slide the modules together. There is a connector series that allows this to be done without the use of wires. Use an ‘inverted’ header (a header with plug-mating interface) with a standard header, and you can slide the connectors into each other without the need to wire the boards together.

Flex boards

LED lighting designers are getting more creative because of the flexibility LEDs offer. One of these options is the ability to use the flex-strip LEDs to allow lighting to wrap around almost anything. Many consumers are using these in their homes because of the simplicity of installation. Buy a kit, unwind the reel of LEDs, peel the adhesive off the back of the strip, stick the strip wherever you want and simply plug the strip into an outlet.

But what happens if you are designing something beyond the at-home application? You’ll need more than a standard reel of 16.4 feet, which is inexpensive and meant for light use. There is little concern if the product breaks, because the cost of the product is not high. When designing a product for a professional installation, however, you want everything to work all the time, as well as to look aesthetically pleasing.

The flexibility of these strips is actually a challenge. Combining circuitry and flexing was not always successful in the past. Engineers have created a valuable product with these flexible LEDs. The flexibility, however, makes it very difficult to find a connector that will do the job consistently. Directly soldering a wire is an option, but if any flex happens at the solder joint, the solder could crack and create significant resistance on the electrical connection.

There are a number of connectors available on the market that improve this process a bit by connecting with the solder pads without the solder. These connectors often have a pigtail of loose wires coming out of the backside of the flex-strip connector to allow for connections to power or RGB control. Connectors can be a great option here, but many of the connectors secure to the board with a pressure fit to the flex strip. This connection may not be enough to secure the connector to the board if the flex strip has any movement. To maintain a secure connection, there are connectors that have a locking pin that pierces through the flexible PCB. The pin does not damage any circuitry and ensures that the strip and the connector stay connected.

What’s next for LEDs and connectors?

The LED lighting industry is moving forward fast and furious, and it seems that every year something new comes to the market. Connector manufacturers have to evolve with the market as well. A great connector introduced three years ago may be all but obsolete today, due to the incredibly high pace of technological advancements.

What is next for LEDs and what will they require of connector manufacturers? As buildings and facilities get smarter and more connected, will we see a need for more data connections to every light? As renewable energies continue their proliferation at business sites, will we begin to see a DC electrical system in the ceiling put in place for lighting? As LED lighting continues to evolve, will we ever get away from the Edison light bulb concept?

Some of these trends may be coming soon and some might be further off in the distance. One thing we do know is that LED lighting technology is constantly evolving. The connector industry is doing its best to keep up with new innovative connectors, allowing lighting engineers to create truly unique designs.

*Andrew Bogaczyk is a product marketing specialist, device connection technology, at Phoenix Contact USA Inc.

For more information, please contact Robin Pearce, Bishop & Associates, via email at rpearce@bishopinc.com.

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