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Aug
24
Written by:
Gary Holbrook
8/24/2008 10:14 AM
One of the things I discovered while working with the XBee boards is that I need another reliable USB-TTL serial adapter. Since the most recent incarnation of my USB board seems "Ok", I decided to print another one and use the chip that I rescued when I wrote about the Chip Quik.
When I finished the board, it looked really good. Almost perfect. It had nice traces with very little stippling due to over-exposure. I drilled the board, populated it, and plugged it in.
My computer complained about an "unrecognizable USB device". Great. I checked the traces and found one short, which I fixed. Still nothing. I swapped out the passive components on the board. Still no deals. Next, I tried a different crystal (a through hole instead of surface mount). Finally, I pulled the chip again and replaced it with a brand new one.
It still didn't work. My pretty board was now a mess. 
This was particularly crushing to me for a number of reasons. First, I've had great success with these boards. The pins are relatively large, the boards are simple, and there aren't a lot of pins. I have also started working on a board which will house a 100 pin TQFP chip, which added to my anxiety. If I couldn't make this board work, I had no chance with the next one.
The only thing I had not replaced was the USB connect. I plugged a cable in and checked connectivity between the pins on the cable, and the pins on the chip. One of them failed. I put down my Sherlock Holmes pipe and grabbed my eye lupe. When I assembled the board, the epoxy on one of the vias gave in and popped away from the board. Held in place by the wire, it only moved a few millimeters. After fixing it, the board immediately be gan behaving properly.
The lesson? I should have tested more thoroughly before moving components around. My gut told me that the board was the problem, but I ignored it. The second lesson? Glue down wire vias.
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