BGA "Fanout" - Page 1
Forum section: electronics / beginners

author: homebrew
author_rank: Frequent Contributor
timestamp: December 09, 2017, 09:43:25 pm
content: Dear all,I would really like to use some tiny STM32F100 MCU in an 0.5mm BGA 8x8 package. However, my favourite PCB manufacturer is nowhere near the drill, restring or track/space requirements for that kind of package.So playing around with the package I might have found a workaround: If I would sacrifice some of the inner pads I could use some standard vias (0.2mm finished hole size with 0.125mm restring) for connecting GND and VCC. Of course the vias would be covered with solder mask. 4-Layer board with 0402 bypass caps directly under the chip.Luckily all important pins are on the outside perimeter (OSC, DBG, RESET ...) and I don't need a lot of Its for my application.I know that it is suboptimal but would you give it a try?P.S.: The fat pads on the right hand side of the picture are standard .1" header pins ...

author: homebrew
author_rank: Frequent Contributor
timestamp: December 11, 2017, 07:31:54 am
content: Nobody?

author: janekm
author_rank: Unknown
timestamp: December 11, 2017, 07:39:34 am
content: This may not solder reliably. BGA packages depend on all the solder-balls collapsing equally to ensure reliable connection between the pads on the PCB and on the base of the chip. If some of the balls don't have a pad they are in contact with, they will be effectively raised above the PCB a little compared to other pads.You might get lucky though, so if this is for a non-critical application it may be worth a try. You probably won't get 100% success rate in diy-soldering this package anyway.

author: Rick60
author_rank: Contributor
timestamp: December 11, 2017, 08:06:51 am
content: if you have the spare i/o route the 3v3 and 0v through the unused pins to the outside ,just remember never to define them as outputs

author: homebrew
author_rank: Frequent Contributor
timestamp: December 11, 2017, 08:25:03 am
content: Perfect! Thanks for the replies!Quote from: Rick60 on December 11, 2017, 08:06:51 amI thought of that, too. However, I don't feel very comfortable that bogus software could fry my board. Thus, using vias and traces, covered by solder mask, that risk is reduced ...Quote from: janekm on December 11, 2017, 07:39:34 amYeah, you are right - now the board is uneven. But (as you supposed) the board is just a prototype anyway. I'll just try to solder the component by hand using hot air.

author: AndyC_772
author_rank: Super Contributor
timestamp: December 11, 2017, 08:44:57 am
content: Having vias under the device with solder mask over them will prevent the chip from sitting flat against the board, I wouldn't recommend it.Routing to inside pads through unused outer ones isn't uncommon, though. Most I/O pins will survive a continuous short to power or ground, so I wouldn't worry too much about damaging the CPU during development.

author: Benta
author_rank: Super Contributor
timestamp: December 11, 2017, 03:11:36 pm
content: Instead of "Fanout", the technical term is "escape routing" or "escape pattern". This might give you better search results, I've even seen semiconductor manufacturers provide this as App. Notes.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 11, 2017, 04:07:11 pm
content: Quote from: AndyC_772 on December 11, 2017, 08:44:57 amSince this is a one-off hobby thing, he could use some wick to carefully remove those solder balls from the bottom of the chip before putting it on the board.

author: janekm
author_rank: Unknown
timestamp: December 12, 2017, 02:10:06 am
content: Quote from: suicidaleggroll on December 11, 2017, 04:07:11 pmTrying to use solder wick to remove individual balls from a 0.5mm pitch BGA would be... InterestingIt might be possible to just pry them off with the tip of some tweezers though. Still tricky without damaging something else at that pitch though.

author: homebrew
author_rank: Frequent Contributor
timestamp: December 12, 2017, 08:15:14 pm
content: Quote from: janekm on December 12, 2017, 02:10:06 amChallenge accepted! :-)Honestly I think that may chances are pretty slim but we will see ...