I use your 21.6x 0.5 circuit boards and am very pleased with them. I soldering LEDs from the diode base flush with the board ignoring the leg node soldering point indicator in the belief that the metal coated through holes provide sufficient heat sink to protect the diode. I would like to be sure that I am not damaging the diodes by applying heat closer to the diode than the leg nodes prescribe. What is the official wisdom in this regard, about soldering LEDs? Can you also explain the process of surface mount led soldering to a circuit board?
Surface mount LED soldering to a circuit board can be a bit tricky. However, it can be accomplished with only the basic soldering tools, and does not require any advanced or expensive machinery. Normally, surface mount led soldering to a circuit board requires the application of solder paste to the circuit board pads by using a solder paste stencil. Then the surface mount LEDs would be placed onto the pads, resting on top of the solder paste. In the final step for surface mount led soldering to a circuit board, the boards are passed through a reflow oven, where the temperature will normally reach in excess of 200 degrees Celsius. As the boards pass through the oven and reach the peak oven temperature, the solder paste will begin to reflow. The reflow process occurs only during a brief period as the boards pass through the oven. Before the actual reflow process, the boards are carefully brought up to the reflow soldering temperature range, as they go through the first area of the reflow oven. The time in the oven and temperature are both important aspects when working with surface mount led soldering to a circuit board. The process just described for surface mount led soldering to a circuit board, can be fully automated, or semi-automated, depending on the complexity of the machinery utilized. For those without the convenience of owning a real surface mount reflow oven, a basic soldering iron will work. Start by tinning one pad for each LED. For example, the SMT LED contains two pads, corresponding with the anode and cathode. Apply a very small amount of solder to the circuit board (not the LED) where these pads will rest. Remember to only apply solder to one pad, not both! Next, place the LED down onto the circuit board just beside the solder pads on the board. Do not place it on top of the solder pads yet. The next step for surface mount led soldering to a circuit board is the manual reflow process. With one hand, you must reflow, or heat up the solder on the circuit board pad (where previously applied). As you re-heat this solder, slide the LED onto the center of the pads with your opposite hand. This is where a pair a tweezers may come in handy. Your result should be an LED almost perfectly in place, and held to the board by one of the two available solder pads. The final step for surface mount led soldering to a circuit board is to solder the opposite pad on the LED normally, with your iron. Since the LED is already being held in place, you may use your opposite hand to feed the solder into the joint. This method only works on LEDs that contain wrap around surface mount contacts. These types of contact on the LED body allow you to manually heat and apply solder, using a regular soldering iron. However, some LEDs only feature flat solder pads on the bottom of the body. In this case, you will have no access with your soldering iron tip, and surface mount led soldering to a circuit board will become almost impossible. To overcome this challenge, you can use a regular toaster oven to complete the reflow process. This method works best if you first apply solder paste to the pads on the board. For those who may not be as familiar with solder paste, it is a paste like substance during the reflow process, and can be purchased at many electronic retail outlets. Do not confuse solder paste with regular solder. Apply the solder paste to the circuit board pads with an applicator, place the LED on the pads, and finally reflow. Be sure to pre-heat your toaster oven before placing the board inside. Watch the solder joints through the oven window so you can determine when they have reached the reflow point. When reflow occurs, remove the board, being careful that the LEDs do not come off of the pads. When the board initially comes out of the toaster oven, it the solder will remain in liquid state for several seconds. It may be difficult to mimic the recommended reflow solder profile exactly using a standard toaster oven, but try not to exceed the recommended period by leaving the board in your oven longer than specified on the LED data sheets. The information for surface mount led soldering to a circuit board can normally be found under a section of the LED data sheets labeled as reflow profile or standard reflow profile. By following these basic tips, you can fine tune your assembly skills and perfect surface mount LED soldering to a circuit board.
Surface mount led soldering to a circuit board is very different from through-hole soldering. The plated holes in the circuit board will help transfer the heat. However, to achieve a good solder joint, the surface temperature of your LED lead must reach a certain point. This means that you may need to hold your iron tip onto the soldering surface a bit longer, to allow both metal surfaces to heat up and accept the solder. Here is an extreme example. Imagine trying to solder an LED lead to a 1 inch metal pipe. It would take a very long time to heat the pipe before the solder reflows, thus damaging the LED in the process. However, if your board pads have thermal spokes, this should not be a problem at all. This is only one aspect of the soldering process. When you solder before the solder tabs, you are respecting the protective margin designed into the package. When soldering beyond the tabs, there is no guarantee. One option is to use a slightly lower temperature setting on your iron, which may help prevent thermal shock. Also consider a thinner solder wire, such as 0.20" or 0.15", which will reflow much faster and is much easier to work with. A no-clean flux can also help. I have seen numerous LED products that have been soldered beyond the solder tabs, and they have apparently had no known issues. In fact, our LED tubes feature LEDs that have been soldered beyond the tabs. The quality of the LED also has allot to do with reliability. I highly recommend purchasing form a well known respectable company such as Nichia, Cree, or Lumileds.