I have received the PCBs and assembled the first one. Everything expect connections to external components, that is.
Here's a picture. It looks pretty much like the rendering. And I know: some of the soldering joints could have been made better but bear in mind that this is my first SMD soldering attempt so I'm actually pretty pleased with the result.
Schematic and layout revised again. I added the 10 µF capacitor (that really ought to be there) and a LED with accompanying resistor for debugging purposes.
Here a print-it-out-and-see-if-it-fits test. And it does fit. Note that all the resistors are the same value (doesn't matter of course since all 1206 resistors are the same size) and that I've placed a 1206 resistor instead of a 1206 LED. Also, the B sized 10 µF capacitor isn't placed on the printout but it shouldn't pose any problems.
Until now I have only made through-hole circuits.But the Airsoft Rounds Counter project's demand for a small PCB made me decide to use SMT. The components are a lot smaller so you can fit a lot more onto a small board but they are more difficult to solder by hand. (But easier and faster to assemble automatically. And cheaper.)
So I have to practise my SMT soldering. Fortunately there are a couple of great tutorials on the net. I used these:
- SparkFun's tutorials
- Excellent video tutorial
- SMD Soldering Guide by Infidigm
- This guide from fpga4fun.com about SMD was also helpful
And it's actually not that hard. I have – so far – had much success with flux-pen-and-solder-on-the-tip-of-the-iron method. A flux pen if your friend – trust me.
Here's an example of my handiwork (the ting at the top is a millimeter scale ruler):
The resistor in the image above is a fairly large SMD component: a 1206 sized resistor. The smallest I will be using to begin with are 0805 sized capacitors and SO-20 and -24 ICs. You can see the various SMD sizes here:
(Note: SMD means "surface-mount devices" and SMT means "surface-mount technology". I use the terms pretty much interchangeably. Which is wrong. I know.)
Oops. The photo interrupter's collector wire wasn't connected to INT0 on the ATTiny. That has been fixed now. Also, the board is now 1 mm wider so there is enough clearing from the segment A wirepad to the edge of the board.
And the VCC and GND traces are now 24 mils and all other traces are 12 mils wide. Refer to the first layout if you can't remember what the various wirepads are for.
Here's the first draft of an SMD PCB layout for the Airsoft Rounds Counter project.
The wire pads are as follows:
9V power input
- AN, CA
Anode and cathode the the photo interrupter LED
- CO, EM
Collector and emitter for the photo interrupter transistor
- A-G, DP
Segment anodes on the display
Digit cathodes on the display
- RST/PRG SW
Reset and program tact switch
Actual size is 49.53 x 20.32 mm.