Archives for posts tagged ‘reflow soldering’

LED lighting, part 2

tin plated 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.

I made the boards the same way as before, except that I tin-plated the finished boards with Tinnit to protect the copper from tarnishing and to improve the solderability.

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.

reflow skillet reflowing

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).

soldered assembled with headers

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.

My First Robot part 4 – Power Regulator PCB

With Adafruit’s lithium ion battery pack and USB/DC/Solar charger our power strategy is off to a good start. I chose those in part because I could easily add a solar panel later, but assuming the robot is running continuously, that would result in a possible output voltage range from 2.5V (the shutdown voltage of the LiIon battery pack) to 6V (the full-sun output of the solar panel). The Raspberry Pi (and ATMEGAs, which I’ll probably use as ‘standalone’ Arduinos) want a 5V supply, requiring either a buck or boost depending on the output voltage from the charger. I couldn’t find any turnkey solutions, but I did find this buck-boost DC-DC converter IC. The LTC3112 from Linear takes 2.7 – 15V in and provides 2.5 – 14V out, configurable with two resistors. Maximum current depends on Vout, at 3V in and 5V out I should get at least 1A. The documentation included some pretty thorough design guidelines, including a complete demo board schematic and layout, so I set about designing one of my own.


The design is pretty similar to the demo board. I removed a few things I didn’t need and added a bunch of output pads, and a second set of inputs so additional boards can be daisy chained. I’m told the component layout of boards like these is extremely sensitive, so I stayed as true to the demo board’s layout as possible.


I’ve been wanting to try outsourcing my PCB fabrication, and I think this finicky design was a good one to start with. The best value for this one seemed to be OSH-Park, and the process was really easy and smooth. They even take native Eagle files, so there was no messing with Gerbers, etc., and the instant preview made it easy to catch errors. Two weeks later I got three of these beauties in the mail. Ten bucks, delivered.

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Typically the next step for me would be dabbing tiny dots of solder paste onto the pads and mashing the components into them, but these boards look so nice and professional that I knew it was time to try a solder stencil. I recently gained access to a laser cutter, so I ordered some .004″ thick Mylar from McMaster and conducted a quick test.

I first exported the “Cream” and “Dimension” layers from Eagle as a 1200 dpi .png image, then brought that into Photoshop. I then selected all the pads (Select -> Color Range on black) and shrunk them down one pixel layer at a time using Select -> Modify -> Contract, Saving As every time until I had seven different versions, each one eroding the pads by about .0008″ all around. I cut these on the laser, using Raster settings of 30% power and 40% speed. Turns out the best one for this layout (with the fine pitch of that IC) was the “minus 4 pixels” version. By the way the fogginess of the Mylar there is the result of sanding off some very small ridges that had formed around all of the openings.

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Next I made a couple of fresh ones and cut a piece of 1/16″ acrylic to serve as a fixture to hold the board in alignment with the stencil.

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Here we go. First try, not bad! I made a little mess in the mounting holes, but generally the transfer was nice and clean.

2014-04-27 18.03.02I tweezered the components onto the board (my cheat sheet can be seen in the background) and fired up the reflow toaster oven.

2014-04-27 18.10.50About 6 minutes later…

2014-04-27 20.11.12

The inductor slid around a bit as the solder melted, but it’s still on its pads and not touching anything else. The IC looks perfect. You can see where R1 and R2 can be changed with thru-hole resistors to provide any output between 2.5 and 14V. The IOUT pad provides an analog voltage proportional to the current draw of the load, which I may end up using to monitor which systems are consuming the most power.

I works! 9V on the left, 3V on the right, identical output (and pretty darn close to 5V, too). I realized I had to hook up an LED to get a good output, probably because the multimeter was not drawing enough current and the IC went into a shutdown mode.


Robot is ready to go wireless!

UPDATE: Here’s the files and BOM to make your own:

OSH Park link

Eagle .sch and .brd files

Bill of Materials (Google Docs)