Hi Carl! I’m really glad you are having fun with your B2 board and learning!
Of course the project is open source so you are free to make your own versions as long as you also release your project as open source and you don’t use the TinyFPGA name.
Ohh my gosh, good luck assembling the B2 board on your own, it is extremely difficult. I had a 50% success rate when I assembled my own prototypes . But it can be done!
The solder stencil is critical, if the paste doesn’t come off cleanly you can have some mean problems:
- Not enough paste: head in pillow. The solder balls may not flow with the solder paste into one big solder blob. It looks like a head laying down on a pillow. You will have poor connection.
- Too much paste: you’ll get shorts between solder balls.
My suggestion: just be slow and careful putting the paste on and especially when removing the stencil. Try to get all the paste on in one swipe. If you do more than one swipe of your squeegee you can push more paste into the holes and between the stencil and the board.
You also need to be very careful about alignment of the stencil. The holes need to be directly on the pads. Tape everything down to keep them from moving. I used a cheap USB microscope to verify alignment of the stencil on the board.
Placing the FPGA into the board was the most challenging. The pitch of the balls is 0.4mm, so you need to place the FPGA within 0.2mm of correct alignment. This was made harder because we can’t rely on the solder mask for placement. I suggest looking at landmarks on the PCB near the FPGA footprint and use those to guide your placement. Once the FPGA is placed you don’t want to push it down or move it around.
Finally, you’ll want a good toaster oven reflow kit to reflow the boards.
One more thing, when you get the boards back you will need to inspect the alignment of the solder mask vs the copper layer. The B2 prototype boards I got back from oshpark did not have good alignment and the BGA pads were either fully or partially covered. You may need to increase the soldermask pull-back to 0.05mm. When I got boards made at PCBWay they did this for me.
Good luck! It’s a challenge but it sure is satisfying to see them work!!