LeafLabs Garden

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1. 

   MediumKahuna
   Member

   A BGA on a mini has one thing going for it: you only need to connect to 1/3 of the pins on the MCU.
   This solves the problem of getting the signals away from the BGA.
   Also, with with so many pins to choose from, you can make the ones you do use, less contentious.
   Even so, it will be hard to get a BGA on a DIY board.
   I know you guys are into making DIY boards and I will be too, on my next board but, I have a coupon (if you will) to get some boards made cheap. I get two trys (2 batches of boards).
   I going to use it to make a Mini with a full boat STM32F4, 100BaseTX, and any thing else I can cram on there. I think I can do it by putting a BGA on a 4 layer board with plated through holes.
   If you make all the balls sit on plated through holes (or at least the ones you need to connect to) shouldn't you be able to reflow a ball from the back side?

   I'm back to trying to fit some RAM on there too.

   Posted 3 years ago #
2. 

   gbulmer
   Moderator

   MediumKahuna -

   Even so, it will be hard to get a BGA on a DIY board.

   Yes, it is hard to make good plated-through vias using DIY techniques (can be done, but expensive), and without vias it will be very difficult to route the 176 BGA.

   Unless there is a great insight, I wouldn't try. I'd use a commercial PCB.

   If you make all the balls sit on plated through holes (or at least the ones you need to connect to) shouldn't you be able to reflow a ball from the back side?

   AFAIK, industrial PBCs use 'microvia' technology to put a via through a BGA pad. I believe microvia diameter is 0.006" or less, and it is drilled with a laser. I don't believe they use normal drilled vias within BGA pads.

   The articles I've read use high-quality PCB tolerances to route the outer rows of the grid on one layer, then inner rows with blind or buried vias on multi-layer boards.

   AFAIK, they aren't reflowed from the back side, but the top as normal.

   If, as you suggest, some pins are unused, then it might be practical to get power into the middle of the 176 BGA on only a double sided board. If it's doable (for testing purposes) on a double sided board with fine tolerances, then a 4-layer board could have pure power and ground planes, and no buried, blind or micro vias, which would keep costs down, and make it easier to test.

   I found these references:
   http://pcdandf.com/cms/magazine/95/3760
   http://www.fairchildsemi.com/an/AN/AN-5026.pdf
   http://www.altera.com/literature/an/an114.pdf
   http://www.ti.com/lit/an/sprabb3/sprabb3.pdf

   Posted 3 years ago #

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