Fixing a QFN ground pad (forgot the via) - Page 1
Forum section: electronics / projects

author: Pack34
author_rank: Frequent Contributor
timestamp: January 20, 2016, 02:30:29 pm
content: I'm looking at trying to salvage a small handful of boards that I had made. I had unfortunately left the ground pad floating on a QFN package. The ground pad is fairly large (3.3mm) and the internal layers of the PCB are clear with no components or traces on the opposing side.To fix this and salvage the boards I was considering to pop off the chip, drill a small hole on the pad and then put it back on. I was thinking I could try something like this or to try to bridge the exposed pad to some of the external leads. There's a grouping of them in one of the corners.I modified the board and rushed it out. I can't believe I missed the DRC error. I rushed too much to get the gerbers out.Has anyone else here screwed up like this before? If so, were you able to fix it?

author: bxh
author_rank: Contributor
timestamp: January 20, 2016, 03:21:17 pm
content: Drilling a hole seems like it'll get really tedious really quickly, especially if you have all the other components already mounted.Still, I can't imagine trying to reliably bridge the central pad to the edge pads without messing anything else up for more than a couple of boards.Maybe you can try soldering a tiny piece of copper foil as a bridge before reflowing the QFN chip on it?If I didn't absolutely need those boards, I'd definitely consider ordering a new revised batch.

author: dcarr
author_rank: Regular Contributor
timestamp: January 20, 2016, 06:06:17 pm
content: I bet you can fix these using your plan. If it were me, I'd make the hole pretty big. Soldering your jumper wire onto the paddle is likely the trickiest bit. Perhaps something like:- Remove IC- Drill hole (big enough for a 30 AWG wire and your smallest soldering tip)- Re-install IC- Squirt some flux into the hole and work through the hole to put a small solder blob on the IC exposed paddle- Tin your tiny wire, add more flux in the hole, and solder the wire to the paddle- Attach other end of wire to ground and feel good about it!David

author: ajb
author_rank: Super Contributor
timestamp: January 20, 2016, 07:08:12 pm
content: How many you do you need to fix, and how reliable do you need to be? Just about any fix you could apply carries some risk of reduced reliability.It may be easiest to drill ~4 small holes in the pad, then thread some really fine wire (like #34 or so) up through one hole then down through the next etc, kind of like sewing a button, except don't cross thestreamswires. This will space the QFN slightly up off the pad, but shouldn't be too much that you can't make the connections to the perimeter pads, and saves you the trouble of trying to solder a wire to the pad through a hole in the board.

author: TheDirty
author_rank: Frequent Contributor
timestamp: January 20, 2016, 08:54:52 pm
content: I have done this for two boards for a LPC1114 that I only had single sided boards and wanted all the pins. It worked fine. I like the idea of the 2 holes and run a wire through, but not certain how well that would work trying to keep it flat while soldering it on. Basically I just drilled the hole (fairly large), hot aired it on, then soldered the wire underneath.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 21, 2016, 02:03:42 pm
content: I have success! I shied away from actually drilling the board. I'm working with aMicrochip USB3300and the pad is ~3.30mm wide. Which is definitely possible to do, but I was concerned about the internal layers. There are no traces underneath the pad BUT there is an internal power layer.Instead I was able to bridge the exposed pad to one of the leads using a small wire and then reflowing it back on. The hard parts were getting the wire fine enough and then reconnecting all of the pads. Aligning the chip was easy but because part of the chip is elevated I had to add some additional solder and flick the soldering iron tip upwards to reconnect all the pads.Luckily, so far everything seems to work fine with this mod. I just need to do this another 19 times on Saturday... This is why I always prefer to do a prototype run in-house before even approaching anything turn-key. But, ordering is out of my hands.