Saturday, October 3, 2009

Lightuino V2.0 -- An Arduino compatible optimized for driving LEDs

The CCShield board was pretty successful and I had a lot of fun with it, so I decided to do another run. I wanted to attempt some surface mount work but at the same time not produce a lemon if it did not work, so I created the hybrid board you see above. The board has a complete Arduino-compatible subsection on it implemented using surface mount parts and the CCShield (plus some extra goodies) implemented using thru-hole parts. The idea is that the board can be used as a shield if the surface mount parts are not populated, or used standalone if the parts are.

Another cool feature is that you can stack them -- so I can experiment with running a bunch of processors simultaneously.

I'll be adding a couple of posts about this board. This first is about building up the surface mount parts. The image above is the board with the surface mount parts baked on and the minimum thru hole parts added to bring power and the ICSP programmer on.

I used the smallest part I could find for the CPU, QFN-32 and 0603 parts for the resistors and caps. The tricky part of the QFN32 package is that the leads are underneath the part, making it hard to repair solder bridges.

To make my surface mount station, I "rescued" an old toaster oven from the dump and "splurged" on some solder paste, rosin, and wick from dealsextreme dealextreme. Then I bought some .5mm mechanical pencils from Staples, some tweezers from Walgreens. Total cost about $20.

This supplemented my current thru-hole solder station that consists of a plastic clamp (Home Depot) to hold a circuit board, a radio-shack 40 watt soldering iron, and an old a magnifying glass/light on arm combo from the basement (really its the bright light that's important).

I then used the mechanical pencil to "paint" the solder paste on the leads. For the QFN part, I just laid the paste across the leads and then used an Exacto-knife to "cut" the spaces between them. It seems like the biggest pitfall is simply using TOO MUCH PASTE. You want just the thinnest layer, and do not even cover the entire pad!

Then I just placed the components in their positions using the tweezers, and did fine adjustment of the QFN32 part with the edge of the tweezers.

Baking was easy. I followed the temperature profile here. But not really... I used the thermostat on the oven, not a real temperature gauge. Basically, just set your oven for 170 C put the boards in and wait 3 minutes. Then raise the temperature to 220 C and shine a bright light in so you can see the boards clearly. After a minute or two the solder will go from grey to silvery. Wait another 30 sec to a minute to be sure that ALL the solder has gone silvery. Then turn off the oven (but leave the boards in there) and crack open the door. You want to cool the boards evenly. After a few minutes, open the door fully, and when they are cool to the touch, pop them out!

I did 4 boards and had 2 solder bridges on the QFN parts. Every other part worked without any issues.

If you have a bridge (you can see it), paint the entire side of the part with a gob of rosin. Then place your solder wick over all of the leads and heat it all up with a clean iron on top of the wick. The rosin will help the solder flow and it will either flow onto the pins, or onto your wick. Easy!

So if you are a DIYer who has been leery of using SMT parts, I'd say "go for it"!

[edit oct 15, 2009: I just did 12 successful boards without any solder bridges. Mix 50/50 paste and flux so the result is viscous like maple syrup]


  1. At this point you will have to email me at g dot andrew dot stone at gmail dot com (replace the word "dot" with . and "at" with @) and we can work out something through paypal, ebay, or another financial service. I am still working on productisation and putting up a commercial web site.