Blunder!

I've made an awful blunder in the design of my drive board for Uno. I really wanted to use these nice dual LED holders that I salvaged from a circuit board that TomG gave me. However, I was so excited to get to the part where I used the solder paste that I neglected to do any final fit measurements with the parts on paper or even on the un-soldered board.


The IR leds did not nest as deeply as the original leds and thus stick further out than I was expecting. The result is that my board is 7mm too wide per the Mini-Sumo rules. Now I am looking at desoldering as many components as I can so that I can remount them on a redesigned board. If I break too much stuff during the process I'll have to place another order. Before I start desoldering, however, I think I will power the board up and see if I can get it to work.

Solder Paste is Cool

So I gave solder paste shot. I really like it. I bought a syringe of no-clean from Howard Electronics Instruments. The syringe comes with a fairly wide tip but the syringe and tip are luer lock type so you can buy all sorts of different width tips for the syringe. I wasn't 100% certain that the syringe would be a luer lock type when I bought it so I skipped buying extra tips. I found that even though the supplied tip is imprecise when the paste reflows even significant errors seem to shrink right up to the pad dimensions. What's better is that if the geometry of your pads is not disturbed by large intersecting traces the part will sqaure up automatically with the pads. (As you'll see below one cap with one pad pretty much engulfed by a wide high-current trace is a bit skewed.. but still not too bad.) Nevertheless I'll probably buy a couple/three boxes of tips.

So below is my board with un-reflowed paste on it.



(Ok I forgot a couple components )

Now here is the after photo


I used the techniue described on Cash Olsen's site (which seems to be down now). I set a skillet to around 100 degrees C, put the circuit on it and let it heat for a few moments. I let the paste flatten out a bit under the effect of the skillet heat. Then I hit each component with a $20 emobossing heat gun at a ~3-4 inch range until the paste reflowed and actually turned silver. This generally took just a few seconds for the small components.

As you can see in the after photo I left the heat gun on the 7805 a little too long and singed the edge of the circuit board. I probabyl got the gun too close. I should have been more patient from a farther distance.

I still need a better pick and place mechnism but that will be for the next board. One step at a time. The solder paste dramaticly speeds the process of soldering components down on the board and I am quite pleased with the technique.