A prototyping lesson learned

I collected a cool purple envelope from our PO Box, containing 3 cool purple T-Board-328 PCBs. These were the first iteration in the prototyping process, and I learned again how important this process is.

Like a real beginner, I made an error in translating the self-etched design into a 2-layer layout.  For the self-etched board, I needed to mirror the pins of the SMD voltage regulators so that they were then mirrored back to the correct  layout during the toner-transfer process.  Unfortunately in my excitement to gat a 2-layer prototype in my hands, I didn’t check, check and check again!

The result was that the voltage regulators had a GND pad under the VCC pin, and the VCC pad under the GND pin.

A soldering iron to the rescue

Fortunately the 3.3v regulator could be flipped so that the pins were over the right pads.  I carefully bent the pins on the SOT223 package so that they contacted teh pads and soldered them to the pad.

Unfortunately the 5V DPAK regulator couldn’t be flipped and bent, as the output pad is underneath the package and not as a tab. I was able to jimmy a very clunky work-around by soldering it at an angle and using a jumper-wire.

The disconnected capacitor

What?  Another error?  The second error I found was an SMT capacitor pad that didn’t connect to anything.  Again, not paying attention and routed a trace on the bottom layer of the board to a cap that was on the top layer.  Funny, they aren’t connected!  Fortunately it was a decoupling cap, and I tend to overdo those, so I was able to test the fundamental workings.

The value of prototyping

The board passed all tests with flying colours.  Evertything that should have blinked, blinked.  The ICSP header programmed the chip.  The FTDI header gave me serial data.  The pins were at the right pitch to actually plug into the breadboard.

So the prototyping exercise did what it was supposed to: it proved that the concept and overall design worked, and highlighted areas that needed work.  More than that, it is a good audit of the effectiveness of the design and layout process – highlighting areas that need special attention, and a reminder not to take even basic layouts for granted.

Next: T-Board ready to launch