B2 Product Feedback


I just got my B2 board a few weeks ago. I really want to thank for you for making this available. I have been studying every aspect of it – the circuit schematic, board layout, and USB bootloader software. It’s quite educational for me. I have a Lattice IceStick as well, and it got me going with Verilog and FPGAs in general. Your board is a good next step. The schematic and board design are quite approachable. The fact that your board exists gives me some confidence that I can produce one, too. (Where I will steal all your ideas.)

I’m using the IceStorm tools with it, and so far I have some blinking LEDs. Next I’m going to interface it to a microcontroller, and transfer some data via SPI.

I have the blank board from OSH Park, and I’m awaiting the components from Mouser. I have access to a laser cutter, so I made a mylar solder stencil with it, and I’ll try my luck assembling and reflowing a board in a toaster oven. If it works, I’ll post a picture.

Thanks again!


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 :flushed:. 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!!


Should I use solder paste under the BGA? I was planning to rely on the solder balls alone.

Looking at my boards under the microscope. the alignment of the solder mask looks good.


Good point, I hadn’t tried that myself. You’ll still need flux though.


FWIW, I cover the area with flux, no solder paste, and use a cheap hot air tool (butane with a hot air tip, actually http://www.engineer.jp/en/products/sk50e.html Only the size of a large pen). We have IR reflow tables and ovens… I never use them.

I find preheat is very important, especially with large amounts of flux.

Also FWIW, my colleague, who assembles many more prototype boards than I do, just takes them to an assembly house for the BGAs… avoids the risk and hassle. Drop it off, pick it up the next day, or pay a little more and wait.


I’m very interested to see how it goes. I use a controleo2 for reflowing my prototypes. For fine pitch BGA parts I now use my PCBA house to do it…but it can take a long time. I’ve been waiting on two batches of prototypes for a few weeks now.


So I assembled some TinyFPGA BX prototype boards this last weekend and I remembered this post. Just some flux and the hot air tool did the trick. The FPGAs reflowed perfectly. I would recommend this method in the future for such fine pitch BGA parts.

Next time I’ll be getting stencils made with no holes over the BGA footprint. I’ll reflow all the non-BGA parts using solder paste and an oven. Then I’ll reflow the BGA package(s) using flux and hot air tool.