kmod's blog

4Apr/140

BGA update: some good news, mostly bad

I blogged a couple times about how I was attempting to do BGA soldering at home using my toaster oven.  The last post ended with me being stumped, so I create a few new boards: one with 3.3V JTAG circuitry in case that the previous 1.8V JTAG was the issue -- while I had designed my JTAG programmer to support a range of voltages using a voltage translator, I hadn't actually tested it on anything other than 3.3V, which is what the programmer itself runs at.  Then I created two more boards, which I like to call the non-bga bga tests: they're simply versions of the BGA boards but with QFP parts instead.  Other people might less-cheekily call them the controls.

Well, I soldered up the first control board, corresponding to the BGA board I've already been working with.  In what I suppose is good news for the BGA boards, the QFP board didn't work either....  After some testing, I discovered the issue was with my test setup, and not the board.  My test board has two rows of 0.1" headers, at the right spacing for plugging into a breadboard, but apparently I had simply not plugged it far enough into the breadboard, and certain critical connections weren't being made (in particular, the 1.8V power line).  After fixing that, the QFP board worked, so I excitedly plugged back in the BGA board and: nothing, still no results.

So I guess overall that's not a great thing, since the BGA board isn't working but the control board is, but I suppose there's a silver lining that maybe one of the previous iterations had worked and I didn't know it.  I feel like I'm getting better at producing BGAs that actually stick to the board; the "tricks" I've learned are:

  • Don't apply so much tack flux that the BGA can't reach the PCB.
  • Make sure to really, really carefully align it, and not bump it off while transferring to the toaster oven.
  • Wait to remove the PCB+BGA from the toaster oven until it's had time to cool and solidify.
  • [Update] Forgot to add -- make sure to use a longer, hotter reflow profile, since the BGA balls are 1) more thermally insulated due to the IC package above them, and 2) made out of unleaded solder rather than my typical leaded solder paste, which has a higher melting temperature.

All pretty simple things, but whenever I managed to do all of them I would at least get the BGA to be soldered (mechanically-speaking) to the PCB.  I'll have to stop here for tonight but I'll give this another go over the weekend.

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