LED lighting, part 2
Thursday, 18 March 2010
My first attempt at using the tiny Luxeon Rebel LEDs taught me several things, among them how difficult it is to hand-solder them. I also realized that I would need to experiment more with the different white LEDs that are available, or potentially using red green and blue LEDs to produce the white I’m looking for (or a combination thereof). So I designed two new PCBs… a new six-LED board with individual control over each of the LEDs, and a tiny single-Luxeon Rebel breakout board so I can mix-and-match different combinations quickly and easily. To both designs I also added very tiny dots at the corners of each LED location to help position the LEDs. These designs are two-sided, with the back side consisting of a large copper field to help transfer heat to the aluminum heat sink.
For this batch of boards I decided to finally try my hand at reflow soldering, using the skillet method described by the Sparkfun guys. I bought an inexpensive skillet at Target, an infrared thermometer at Lowes, and some no-clean solder paste. The solder paste came in a syringe package but didn’t come with any needles, so I squeezed a little paste onto a paper towel and carefully dabbed it onto the PCBs with a toothpick. Using a pair of tweezers I placed each LED into position and pressed it into the paste, which held the component fairly well.
Before trying any soldering I looked up the reflow profiles for both the solder paste and the LEDs, and experimented with the skillet to see what settings would yield the target temperatures. I may one day build an Arduino temperature control for the skillet to more precisely control the profile, but I think these reflow characteristics are pretty flexible and for now it’s working just fine.
I put the PCBs into the middle of the skillet (I got wildly different temperature readings from different spots on the skillet) and turned it up to “LOW”, watching the temperature with the thermometer. As the temperature leveled off at around 165°C the boards began to smoke and I turned it up to “MED”. Within a minute or so the solder liquified and flowed nicely, in some cases shifting the LED into perfect alignment with the solder pads (apparently a result of the solder’s surface tension).
I made breakout boards of several different LEDs: “warm white“, “ANSI 2700K white” (an even warmer white), “amber“, and red, green and blue. After the skillet reflowing I hand-soldered on a couple of header pins for the breadboard.
This breadboard setup allows me to swap out different combinations of LEDs to evaluate the color of the light (here I have six 2700K Rebels installed). For this mock-up I used a 24VDC desktop power supply powering a BuckPuck LED driver, switched with a momentary pushbutton. I’m getting closer to the color temperature I want, but right now these are all on (full power) or all off. The next step will be PWM control over individual LEDs or groups of two or more to start precisely dialing in the settings.