Desoldering large QFP for PCB repair, best approach? - Page 1
Forum section: electronics / repair

author: Vincent
author_rank: Regular Contributor
timestamp: May 04, 2025, 02:23:08 pm
content: Here's the situation:I have an original PlayStation 1 (SCPH-7501, motherboard PU-22) which won't start. The issue is an open trace between the CD controller (CXD2938Q) and two other chips (BIOS and CPU if I recall). It turns out there was the tiniest bit of oxidation around pin 138 of the CXD2938Q. Continuity tests confirmed my suspicions. My guess at this stage is that the trace section that's under the chip is open. I don't think it'll be a difficult repair if I can manage to desolder the chip without damaging it (or the PCB for that matter).Which brings me to my question. The challenging part here is that both the chip and PCB will be reused, so the desoldering process must introduce as little heat as feasible. I feel that trying to desolder a large QFP208 IC with hot air using the standard nozzles would be painfully difficult, and using no nozzle at all would introduce too much heat. The default approach in this context would probably be to get a special nozzle for QFP packages (around 30mm X 30mm). But I have another idea:I've got one of those chunky Weller soldering guns laying around. What if I craft a special square-shaped tip using heavy-gauge copper wire and use it to literally apply heat to all the pins at once, but ONLY the pins? In theory I don't see why it wouldn't work, but I wonder if the current flowing through the tip might damage the delicate chip? It's probably very low voltage but anything CMOS won't take much at all without being damaged. Or perhaps ESD might be a problem?

author: alpher
author_rank: Frequent Contributor
timestamp: May 04, 2025, 03:14:42 pm
content: Two words: Chip Quik.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 04, 2025, 03:37:04 pm
content: pre heat the board ... use hot air on the part to remove

author: pcprogrammer
author_rank: Super Contributor
timestamp: May 04, 2025, 03:44:49 pm
content: Or just leave the chip on the board and solder a small gauge wire to the pins to restore the connection.

author: Vincent
author_rank: Regular Contributor
timestamp: May 04, 2025, 05:16:20 pm
content: Okay let me clarify a few things here:I am aware of the existence of ChipQuik but I would rather avoid that option unless there's a way to entirely clean it, and I do mean ENTIRELY. Apparently this special alloy can make subsequent solder joints brittle.The open trace isn't on the opposite side of the chip, it's literally UNDER the chip. Hence why I need to remove it. See attachment.

author: Grandchuck
author_rank: Super Contributor
timestamp: May 04, 2025, 05:29:51 pm
content: Your soldering gun idea could work. There will be an ac magnetic field associated with the removal but I doubt that will cause any ill effects. Disclaimer: I never tried such an approach.

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 04, 2025, 05:30:35 pm
content: If you know the trace is open, it implies you can probe both ends somehow without needing to go under the chip. At least that’s how I read your original post and if that’s the case as already has been said, no way would I remove a big qfp like that to repair one open trace.If it was mine to do, I’d let that area marinate in deoxit d-100 for couple of days, brush and clean well with isopropyl then repair the broken trace.

author: Vincent
author_rank: Regular Contributor
timestamp: May 04, 2025, 06:03:15 pm
content: Quote from: WattsThat on May 04, 2025, 05:30:35 pmCorrect. Easy enough by checking continuity between the pins that are supposed to be connected together. More precisely, with pin 18 of the BIOS chip and pin 111 of the CPU.The difficulty here is that the trace originating from the CXD2938Q's pin 138 directly goes under the chip, then emerges on the opposite side thru a via to the other chips. Of course if there was a visible section of the trace right next to the pin, I would've easily repaired the trace and this thread wouldn't even exist.Tacking jumpers sounds... I don't know. It's a data line, wouldn't the jumpers ruin the signal timings? It ain't a multi-GHz architecture by any means but still...

author: nctnico
author_rank: Super Contributor
timestamp: May 04, 2025, 06:24:04 pm
content: I'd tack on some thin (hair thin) enamel wire onto the pins and restore the connection that way. Some super-glue can be used to fixate the fix afterwards. I think that is the lowest risk fix. A wire will do just fine for data looking at the rest of the layout.If you attempt to remove that chip you might end up doing more damage. If you still insist on removing the chip, hot air is the way to go. Make sure to heat the board the most as the board needs most of the heat. Placing the chip back is easiest by adding some flux and using hot air to make the solder flow. It is not a good idea to try and remove tin from the pads as these are typically very easy to damage. When placing the same chip back on the same pads, the ideal amount of tin will already be there. The tin just needs to melt and flow.

author: Gyro
author_rank: Super Contributor
timestamp: May 04, 2025, 06:32:54 pm
content: Is the data line specific to the bios chip or a wider spread bus. If just the bios chip then it isn't going to be moving that fast (as you imply, nothing on a PS1 is movingthatfast). There look to be plenty of untented vias, just pick the nearest convenient one and thread a wire down that to swap sides. The wire doesn't have to be that thin, there are vias that will easily take 30AWG Kynar (it's fairly easy to tack onto 0.5mm pitch leads).

author: nctnico
author_rank: Super Contributor
timestamp: May 04, 2025, 06:42:09 pm
content: I still recommend to use thin enamel wire. Surface tension will suck it right to the center of the solder joint making the job surprisingly easy.

author: langwadt
author_rank: Super Contributor
timestamp: May 04, 2025, 07:01:28 pm
content: Quote from: nctnico on May 04, 2025, 06:24:04 pmyeh, and there is plenty of holes to feed the wire through to get to the opposite side

author: Vincent
author_rank: Regular Contributor
timestamp: May 04, 2025, 08:00:47 pm
content: Thanks for the inputs everybody. I guess at the end of the day it won't hurt anything to just give the jumpers a try (one to the BIOS and one to the CPU).Another idea occurred to me.What about using a regular hot air nozzle but in combination with DIY heat shields? Like one surrounding the chip and a second one covering the chip itself to shield the heat away from it and especially deflect it towards the pins. They should be easy enough to craft using thin sheet metal, say the bottom of an old coffee can.

author: nctnico
author_rank: Super Contributor
timestamp: May 04, 2025, 09:12:52 pm
content: My experience with that is poor. You have to realise that for hot air to reach a place, the air it replaces needs to go somewhere. Blowing air into a confined space does nothing as the air doesn't get replaced.

author: Shock
author_rank: Super Contributor
timestamp: May 04, 2025, 09:49:00 pm
content: Kapton the surrounding area and sensitive components. Then foil apron on that and tin plate heat shield with foil walls.Underside preheating ideally and then topside refliow. Legs must be fluxed and if corrosion may pay to flux and reflow them a few times with an iron and nudge to see if stuck legs.I think you have a good chance to clean up a low temp alloy as long as you wick off and reapply and clean off what you intend to solder with. It makes sense if the intermetallic layer has transitioned it should be reversible with further applications of your soldering alloy.Practice and tweezers or a wooden spudger stick (sharpened wood chopstick) is required to flip it off once it budges.

author: MathWizard
author_rank: Super Contributor
timestamp: May 06, 2025, 06:57:03 am
content: I would try a jumper wire 1st regardless. But I also have 1 of those big old high powered Weller's, without tips anymore, so I was using 14AWG copper wire. I have some old PCBs with junk chips on them. I must try to remove some big chip some day. I'd put lots of solder all the way around first. And if the copper wire "loop" stayed nice and flat, it might work ok and not kill the chip.Now another way that can work, including for BGA removal, for some people anyways, is to use a high-power lamp, like a 500W Halogen bulb. You can find video's of it. They use lots of flux, and ideally monitor some temperature on the PCB. But it looked pretty easy, just mount the lamp over the PCB.That reminds me, I should be using a lamp I have, just to pre-heat a desoldering job I'm doing a on big thick PCB. It's too big for my reflow-toaster oven.

author: u666sa
author_rank: Frequent Contributor
timestamp: May 06, 2025, 02:37:23 pm
content: I don't know why you want to desolder that chip. If the idea is to fix that little oxidized leg, then I would use toothbrush with cleaner to clean it and if needed solder a jumper -- you'd need a microscope for that.But back to your original question at hand:To desolder QFP208 you need to apply Rose's Metal all around its legs, and then hit it with hot air from the top. That should be quick and painless. Then you clean Rose's from legs and pads using solder wick.Alloy is available from eBay -- (link)

author: langwadt
author_rank: Super Contributor
timestamp: May 06, 2025, 02:45:42 pm
content: how would you repair a track under the chip without making it impossible to remount the chip? you can't mount the chip on top of a bodge wire

author: u666sa
author_rank: Frequent Contributor
timestamp: May 06, 2025, 03:00:20 pm
content: Quote from: langwadt on May 06, 2025, 02:45:42 pmYou'd be surprised. We can lift BGA and repair a track underneath tiny BGA chip.