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Sep
3
Written by:
Gary Holbrook
9/3/2008 4:17 PM
Over the Labor Day weekend, I could not bear to leave my computer at home. While my wife read in the evenings, I worked out the design for my newest circuit board. This board will incorporate an Atmega 2560, TQFP 100. That's 100 pins at 1/2 mm spacing. Good stuff.
Now, I've done a TQFP 144 board...but it was just a breakout board. This board actually has to do something. That's a bit more challenging. Getting the whole shebang to work on one layer may have been possible, but not very practical. I have big plans for this board! (Insert maniacal laughter) These big plans necessitated a two layer board. My first, as it turns out.
Although I have never created a two layer board, I have rehearsed the process in my mind many, many times. The big problem is getting the registration right between the two sides. My technique was pretty darn good. I would glue a paper printout of the circuit to the outside of board and drill holes through the board before removing the plastic sheeting and exposing it.
Thus proceeding, I used a center punch to mark four holes and used my drill to drill them through. Having rehearsed quite a bit, I drilled the top layer first because it is a much finer pitch. Better to have burrs stuck to the less sensitive bottom layer. A small problem arose when I laid the mask on the board. The mask is mirrored so the toner can be as close as possible to the board. The paper was also mirrored, but was glued on toner side up. I had to flip the board and put the mask on the "burred" side.
When I printed the masks I printed both mirrored. As it turns out, the bottom should not have been mirrored. Lesson number two learned. After exposing the top, I exposed the bottom and developed the board. Registration was nearly perfect! Unfortunately, I've been using a laser printer for my AT90USB board masks...those are 1mm spacing, not 1/2. The 600 DPI laser was not up to the task, and my first two layer board was useless.
This was frustrating for a lot of reasons. The biggest being that I only had two two layer boards on hand. My "play" account this pay period is already spoken for, and just the chip to put on the board is $17. A funk permeated my afternoon.
There were a lot of questions. Was it the laser printer? Did I under-expose the board (I use a timer set to 11 minutes, so this wasn't likely)? It has been a while since I have had a failure exposing and developing a board, so I didn't take this well.
Later in the evening, I printed another mask. Instead of using the laser printer, I used my wife's inkjet at 1200 DPI. The mask was almost perfect. Razor sharp. At the same time I also printed a non-mirrored piece of paper for the top layer. The same process was followed as I used on the earlier board.
After exposing both sides, the exposed board looks promising.
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