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Version 2 of ATtiny84a development board

A project log for ATtiny84a tiniest development board

A development board for the ATtiny84a which can be used on a bread board. Just all the basics to run the micronucleus bootloader.

Sander van de BorSander van de Bor 05/26/2019 at 05:203 Comments

Unfortunately the first prototypes did not work due to an error in the schematic, but I was still able to use these for some of my projects. I even used the micronucleus bootloader by adding some external resistors. Initially I had no LED on the board to determine if the chip is in bootloader mode or not and I was really missing that indicator light. So for my next set of boards I was going to fix all these issues and add the LED standard on the board.

There were some issues in the "production" as well. I make these boards by hand, and I was able to work around the issues for now, but this was the right time to improve the design for manufacturing as well. The panel did not leave enough room for the USB connectors and these were soldered on after the reflow process. Since most components were 0805, I had to place these on the back to make them all fit. For this next prototype I decided to go down to 0402 and keep all the components on the same side.

For the first prototype panel I used mouse bites to separate the boards. For this new design I use V-cuts and that is such an improvement!

Here are some pictures of the new panel design with 36 boards per panel:

The 0402 are small, but not impossible to place with a pair of tweezers. I tried the vacuum pick-up tool, but it sucked them up inside and they disappeared so that was no longer an option.

It all seems to work now! The micronucleus bootloader is working over USB and it accepts sketches directly from Arduino. The LED indicator light is a great help to determine the status or the program.

I will make these available on Tindie as well so that everybody can start using these in their own projects.

Discussions

Jan wrote 07/23/2019 at 20:02 point

Oh wow, those are really tiny boards then! 

Thanks for the complete reply. I hope you sell enough over tindy to at least cover your expenses! 

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Jan wrote 07/23/2019 at 10:35 point

Nice! I'd like to know: Are you using one big stencil to apply paste to all boards at once? Do you place components by hand and reflow them manually in a reflow oven?

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Sander van de Bor wrote 07/23/2019 at 17:57 point

I wouldn't say the stencil is big, because the panel of 36 boards is only 100mmx100mm, but yes, I do stencil them all at the same time. In the beginning I used to create boards individually, but aligning a stencil to a board that is only 10mm wide is very difficult! After I apply the paste I place the components by hand. For some components I use a camera to get a better picture of the position on my laptop, but in most cases enough light will help to place these with the bare eye. I have a cheap reflow oven which I modified with some custom firmware and some additional sensors, and it works on these small 100mmx100mm sheets. I have done boards up to 160mmx200mm, but it didn't do too well with the components closer to the corners and had to use a heat gun to finish those. After the reflow oven I usually have do some additional touch-up. Components might have moved and sometimes disappear (good luck finding the 0402!). And most of the time the VQFN has bridged pins. On my next project I will go to the thinnest stencil possible to avoid that bridging in the future.

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