The small size of the SMD (Surface Mount Device) component and the confined workspace of the PCB (Printed Circuit Board) are the challenging parts of SMD soldering. The best approach is to obtain the proper equipment and to plan out the process, before you actually begin. Working with an LED (Light Emitting Diode) is no different than working with any other SMD component, except that the LED is directional. Proper installation of the LED depends upon soldering the correct end of the component to the correct pad (point of attachment) on the PCB.
Check the documentation for the soldering iron and make sure the temperature capability applies to the type of solder you will be using. For example, the melting point of Sn99 lead-free solder is 227 degrees Celsius, but the SMD component and the PCB pad also have to be heated to that temperature. Due to heat transfer and dissipation, the iron will need to deliver much more than 227 degrees Celsius.
Place the proper size tip into the iron, according to the size of the PCB workspace and the pads. A narrow, flat tip is ideal for SMD soldering, but a smaller tip also means that less heat will be delivered to the solder joint. Conversely, a larger tip can transfer more heat, but it can be cumbersome and can damage the LED, the surrounding components and the PCB.
Obtain the correct solder. Match the type of solder to the solder on the PCB (if it is pretreated with solder). Lead-free solder often contains Silver (Ag) or Tin (Sn) and has a higher melting temperature than 40 per cent lead (Pb) solder. Also, a thinner diameter, flux-core solder is ideal for SMD components.
Identify the cathode of the LED by referring to the LED data sheet. There should be a line or other marking printed on one end of the LED, designating the cathode. Just like standard diodes, LEDs need to be oriented in a specific direction to work properly. You may need a magnifying glass to help you see the markings.
Verify that you will be soldering the cathode end of the LED on the correct pad on the PCB. Review your schematic or PCB layout for cathode information. Some PCBs may even have markings for components and you will see the cathode line.
Clean the pads of the PCB where the LED will be soldered. After putting on safety goggles, you can touch the heated soldering iron to them, melting the solder, or you can gently scrape them with a hobby knife. If the PCB has bare copper pads, you may have to apply a small amount of solder to them.
Pick up the LED with tweezers and place the cathode end on the correct pad on the PCB. Center each end of the LED on each pad. A small portion of each pad should still be showing.
Clean the iron tip and carefully apply a small amount of solder. While holding the LED with the tweezers, simultaneously place the iron tip on the end of the LED and on the pad. The solder should melt and flow between the LED and the pad. Slowly remove the iron tip from the joint. Keep holding the LED with the tweezers for a few more seconds. Remove the tweezers; the LED should stay on the PCB.
Place the iron tip on the other end of the LED, making sure it touches the LED and the pad at the same time. Slide a length of solder wire under the tip of the iron, at the junction of the LED and the pad. You should see the solder flow and form the joint. Slowly remove the iron and solder wire.
Reapply solder to the other side of the LED. The initial solder joint was just to hold the component in place and may not be totally solid.
Inspect your soldering. Ensure that each side of the LED is firmly connected to each pad, with no gaps. An ideal solder joint will look like a 45-degree slope from the pad to the top edge of the LED, with an inward curve.
Bring the soldering iron to the proper temperature before using. If you are new to SMD components, practice on a "scrap" PCB before attempting the real thing. Too much heat causes damage, and not enough heat causes poor solder joints. Get the iron tip properly "tinned" before using for the first time. Let it heat up, and let solder flow onto the bare tip. This allows proper heat transfer and protects the tip. Clean the iron tip. Dip it in tip cleaner or wipe it on a damp sponge. This removes impurities which can inhibit the soldering process. Normally, lead-based solder should look smooth and slightly dull. Lead-free solder may appear less smooth and shiny.
Wear ANSI-approved safety goggles at all times while soldering. Wear proper clothing to protect the skin from burns. Never leave a heated soldering iron unattended. Use an exhaust fan or have proper ventilation. Wear a smoke-rated face mask or respirator, if necessary. Wear chemical-resistant rubber gloves or use pliers to handle solder wire. Wash hands thoroughly after soldering. Disconnect the power going to a PCB, before attempting to solder.