Goofed up a BGA layout, tips? - Page 1
Forum section: electronics / projects

author: cubeflake
author_rank: Contributor
timestamp: October 26, 2020, 04:12:29 pm
content: Hi guys (and girls),I've designed a board around the Texas Instruments DLPC6401 DLP controller. It's 6-layer board, controlled impedance, had it assembled, it cost a good amount of money to get one prototype. Unfortunately, I've seem to have messed up my component footprint and connected 3 ground pads to the 1v2 line, shorting the 1v2 line to ground through the DLPC chip. The pads are in the middle of a copper pour on the top layer, so there's no way I can cut a trace or something like that.Has anyone dealt with something like this before? I had the idea of making an "interposer board" that I'll put in the board in place of the BGA chip, which corrects the footprint. The DLPC6401 will then go on top of the interposer board. So far it seems I only messed up a few power pins and no high speed traces, so an additional 1mm shouldn't really influence the signal integrity too much.Does this sound like a sensible idea? Any other ideas on how to deal with a goofed up BGA layout?Cheers,Jim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2020, 04:36:02 pm
content: Can you just mask off or drill out the affected pads? They're usually redundant, that is, if there are enough additional connections to 1V2 and GND, it's probably fine to leave out a few. Whether that's enough to affect performance, or cause glitches or worse, who knows.You can make an interposer, but you don't have much routing room on it. I guess that would be good enough here. More aggressive mistakes might need blind vias to solve.It's also a pain to solder down: the whole interposer has to exceed soldering temperature, to have any chance of soldering to the main board. Which has a lot of heatsinking (pours), so will need generous preheat for that to work without blistering the poor thing.And, do you have access to an x-ray machine to verify correct soldering on both parts?Tim

author: nctnico
author_rank: Super Contributor
timestamp: October 26, 2020, 10:00:17 pm
content: I have drilled holes from the back of BGAs to correct design mistakes. I used a milling bit to get near the pad and do the last bit with a small drill. If you are lucky you can simply drill a hole in order to remove the pad.

author: sd
author_rank: Unknown
timestamp: October 27, 2020, 12:51:17 am
content: You could also take a new IC and yeet away the offending balls before soldering it.

author: cubeflake
author_rank: Contributor
timestamp: October 27, 2020, 02:06:18 pm
content: Great suggestions, thanks guys.I think drilling might actually the best solution, since the grid is relatively coarse (1.0 mm), even though it feels a bit wrong drilling in a 1k$ board. This way I don't have to take the IC off, and I don't have to deal with any heating issues or risk burning any surrounding components.edit:Btw, no I unfortunately don't have access to an x-ray machine. More reason to use the drilling approach because I can check just check with a DMM if the short is gone.

author: cubeflake
author_rank: Contributor
timestamp: October 28, 2020, 12:46:44 pm
content: The result:https://imgur.com/a/tPTpzdTThe short is gone, so it seems to have worked so far.. now l'm just hoping that I didn't something in the process.