Popcorning SMDs - Page 1
Forum section: electronics / beginners

author: tony359
author_rank: Frequent Contributor
timestamp: March 30, 2022, 04:28:34 pm
content: Hi all,Can I please have the community's input on my SMD techniques.I am basically practicing with some old boards and my equipment as I would like to be a bit more successful when reflowing/reballing larger SMD chips.I am currently experimenting on some older XBOX360 I got for free or very little.I have a T8280 preheating plate and I've recently got a Quick 861DE as hot air station with SMD nozzles.I've been doing my "homework" for a while, taking temperature measurements and timing the temperature variations to try and follow a reflow profile.Today I went ahead and removed the GPU from the board. This is what I've done- I "baked" the board for a few hours - I think 2-3 hours, the board was at 100 degrees C.- I then prepared the board immediately after, set the preheater so that the board would get to 160 degrees approx (the actual board, measured with laser thermometer, not the set temp on the pre-heater). When that temp was reached, I started the Quick at 320 degrees and 120L speed.- Around 1 minute later the chip was loose and I removed it with a sucker- I carefully removed the old solder from both the PCB and the Chip- For the PCB, I kept it on the pre-heater so the PCB was at around 100 degrees while I was slowly using my 5mm chisel soldering tip to remove the solder- for the GPU, I put it on a metal surface, placed it on the preheater at the same temperature used for the PCB. I also aimed the hot air set to 200L/m so to help keeping the chip nice and warm while removing the solder. The chip was around 120 degrees while working on it.- The soldering iron was at 400 degrees.I ended up with two small blisters on the pin side of the GPU.Should have I baked it for longer time? Maybe 400C are too much for the soldering iron? I can't think what I am doing wrong as my temps are pretty mild, particularly when comparing with lots of online videos where I see them setting the hot air station at "max power"Indeed the board was sitting in my garage for a while. Maybe the website suggesting 8 hours in an oven before reflowing was not kidding?Can someone please share their experience please?Thank you!

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 30, 2022, 05:48:47 pm
content: I wouldn't blast the GPU with tons of 320ºC air while cleaning it, there's no escape for the heat like when it's soldered to the board, you might overheat it very fast.Definitely 400ºC is a lot and will cause damage to the pcb pretty easily, also that temperature will instantly burn the flux and make the rework actually harder.My advice is to set hot air+plate to 200-220ºC and the soldering iron to 320-350ºC, having a preheater+hot air should make things much easier, not requiring those high temperatures.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2022, 06:22:21 pm
content: Yeah, most likely decomposition of the laminate, not moisture. Bake-out is probably fine. Well, maybe more depending on plane area, I forget what's recommended in that case, but yeah just look that up to check.Use only what heat you need; with that much preheating, the chip was probably at the melting point already? Could turn the iron down to a hair above MP and still be fine to work.For reference, even on non-preheated boards (general purpose, like pins, SMT chips, no heavy copper), I run about 350C on the iron. I'll push up to 400C or more, as needed, for lead-free, heavier copper/lots of planes, etc. Or get some preheat in.Are you keeping lead-free the whole way, or cleaning the pads and replacing with leaded? You should only need a little extra heat to get everything initially soaked with lead, and then can turn it down for the remainder.Tim

author: wraper
author_rank: Unknown
timestamp: March 30, 2022, 06:27:08 pm
content: Blisters/popcorning are result of moisture and overheating. With higher temperature you need drier parts to avoid this.

author: thm_w
author_rank: Super Contributor
timestamp: March 30, 2022, 09:40:13 pm
content: Quote from: tony359 on March 30, 2022, 04:28:34 pmPlease post a photo, so its more clear what you are describing.

author: tony359
author_rank: Frequent Contributor
timestamp: March 31, 2022, 11:19:40 am
content: Thanks allI appreciate all your comments.I checked on the video I took and I believe the chip was popcorned before I started cleaning - I'm not 100% sure but I think I see the blister on the video. If that is the case, I assume that moisture is maybe the culprit here. I believe I read on a datasheet to bake the boards for 8 hours - maybe that is what I should do?Here they mention the default cycle for populated boards is 10 days at 90C!https://www.aeri.com/10-tips-to-avoid-popcorning-of-ics/This page seems to provide some more useful information:https://www.heavidesign.com/2020/04/29/moisture-sensitivity-levels-and-baking-pcbs-parts/I do not have a 5% humidity space so I would need to bake over 100C - depending on the thickness, times vary between 3 and 96 hours. It seems reasonable - but time consumingWhen cleaning, the hot air station was several cm away, I check on the video I took and the chip was only reading 112C when I started working on it with the soldering iron - the GPU was not really get much air at that distance in my opinion. But I see your point, better to avoid those temps altogether.For the iron, I kind of assumed that a higher temp - and a large chisel - was better to avoid lifting pads but maybe that was my main mistake.I agree that next time I can keep plate+hot air at a more reasonable temp and also the iron at the usual 350 degrees and that will probably work fine.@T3sl4co1lI will replace with leaded balls.@thm_wPhotos attached

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 31, 2022, 01:27:38 pm
content: I keep saying 400ºC will cause a carnage extremely easily.If you're already blasting it with 320ºC air, you don't need more temperature, that will already melt any solder alloy.This is not when you set 400ºC because the tip can't handle the thermal load, in those cases the temperature that reaches the pcb is way lower due the thermal impedance.In your case, 400ºC applied to a 320ºC board will completely obliterate it.Just as example, I soldered very few BGAs in my life.Few months ago I modded an old linux box, soldering small wires to the unpopulated SATA pads under the CPU.It took me 3 tries, each one lifting the cpu, cleaning both board/chip, reballing and soldering it back, no preheating.I just wrapped the pcb in some isolation cloth to prevent heat escaping, and lifted it using my $20 hot air station.I first tried with some scrap boards, and I was able to easily cause blisters do that when going too high with the temps, around 380ºC and higher.Keeping the temps at 360ºC did a good job. I also upgraded the RAM:

author: tony359
author_rank: Frequent Contributor
timestamp: March 31, 2022, 01:50:04 pm
content: Thanks for your input.I can assure you the chip was as 112C when I started removing the solder - see attached picture to confirm the hot air was basically just warming it up!But I agree that I should not use those temps at all. I just did not want to remove the nozzle and "keep the BGA warm". When I realised the chip was going over 150C I lowered the temp of the hot air to 260.As you say, I used 400C as I thought the chip would suck away most of the heat - but considering I had preheating+hot air, maybe that was not necessary at all.At times I could feel the solder wick getting stuck on the BGA so I am confident I was not overheating the chip.Still, I agree that I should not use 400 degrees at all.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 31, 2022, 01:59:43 pm
content: Nah, the chip might absorb some heat for the first seconds, then it will heat up and the job will be done easily, not even close to what happens when you solder a heavy ground plane on a big multi-layer pcb.

author: tony359
author_rank: Frequent Contributor
timestamp: March 31, 2022, 02:19:41 pm
content: I'll keep that in mind for my next tests, thank you!BTW: the mod you did, with wires UNDER the BGA...

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 31, 2022, 03:09:42 pm
content: Being Gbit, it was kinda useless being limited to USB 2.0 speeds, so I had nothing to lose, if it died, it diedBut now is running again as a small NAS, reaching 60MB/s with an old laptop hdd over NFS.

author: tony359
author_rank: Frequent Contributor
timestamp: March 31, 2022, 03:50:22 pm
content: Whatever the purpose was, the idea of soldering something UNDER a BGA requires skills which I admire! I hope I can do that too one day!

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 31, 2022, 04:24:30 pm
content: Nah, 0.8mm bga is pretty easy to work with.

author: tooki
author_rank: Super Contributor
timestamp: March 31, 2022, 05:21:46 pm
content: Quote from: tony359 on March 31, 2022, 11:19:40 amThe higher the temperature, the more likely you are to lift a pad, because the bond between pad and board substrate is weakened dramatically when hot. So what you want is a way to heat the whole board as little as possible overall, while still the same time, above all avoiding spot heating to damaging temperatures.Hence, the whole point of preheating is toavoidneeding to use aggressive temperatures on the iron. The idea is to being the whole board closer to the melting point of the solder, so that you don’t have to add as much heat with the iron, since you don’t have a ton of cold board wicking the heat away. (Yeah, you can sometimes overcome a cold board by cranking up the heat on the iron, but the spot in direct contact with the iron risks being heated well beyond the point where damage occurs.)A large tip is also preferable, since it can be at a lower temperature and still have sufficient heat flow to the joint.

author: tony359
author_rank: Frequent Contributor
timestamp: March 31, 2022, 05:34:15 pm
content: gotcha. I shall change the temps on my next test.Glad to hear we are all on the same page - I am always so confused when I see youtubers (even famous ones) cranking up their hot air stations "to maximum" to remove chips from boards...

author: tooki
author_rank: Super Contributor
timestamp: April 03, 2022, 07:58:13 pm
content: Quote from: tony359 on March 31, 2022, 05:34:15 pmNothing wrong with that IF they’re keeping enough distance from the board. Then the hot air is getting “diluted” with cool ambient air, but over a larger area. Or if they’re moving the handpiece over a larger area quickly, so that it’s still heating slowly and evenly.Unlike a soldering iron, temperature control with a hot air handpiece is extremely imprecise. Sure, the air may be exactly the right temperature when it exits the nozzle, but it won’t be that temperature when it touches the board, and even minor differences in angle, distance, airflow rate, and dwell time make a big difference in the heat transferred to a particular component (or part of a component!).

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 03, 2022, 09:29:51 pm
content: I remembered a HP inkjet printer where the **** cartridge leaked and messed everything up, had with my nails painted black/purple for 2 weeks, nothing removed that!Searching revenge, I intentionally tried to kill the small bga mcu from the logic board by roasting it with 400ºC while powered on.The mcu was completelty melted, moving slightly when poking it with tweezers, yet the the serial port was outputting the same stuff like nothing was happening.At some point, after >10 seconds of further intense heating, it stopped working, coming back to life 2-3 seconds later after releasing the heat source (But still melted).I kept the heat for like 2 minutes, still didn't die, reviving when cooling down!Finally got bored and shorted Vreg input with output (18V->3.3v), that did the trick

author: tony359
author_rank: Frequent Contributor
timestamp: April 04, 2022, 06:04:29 am
content: TookiOh yes I also use my Quick hot air station at full power (500C and 200L/m) to preheat PCBs - from a distance as you say, as a ‘replacement’ for my preheating plate for small tasks.But no, unfortunately I’m consistently seeing YouTubers setting their gear ‘at maximum’ to then get very close to components in order to remove them.I guess the ‘warmer for less time’ could be debatable for small component removals but I don’t believe one should blast 450 degrees to a component to try to overcome the thick PCB sucking all the heat away.

author: tony359
author_rank: Frequent Contributor
timestamp: April 12, 2022, 05:41:23 pm
content: I'm still testing and I have been more careful with temps.However I do believe that baking is not an optional step when working with BGA. I've tried to soldered an old Xbox GPU which had been on my desk for ages. Let's call it an exercise. I did not bake it, hot air was set to 320 (granted, I could lower it a little bit considering it's lead solder balls) and the whole process lasted 45 seconds. Still, the chip made a loud POP after 20 seconds - that's why it's called popcorning!Either XBOX's GPU's are notoriously bad, or dealing with large BGA chips is definitely not easy in a home environment.Any recommendations on baking? I found that standard procedure calls for 5% humidity, which is impossible at home. Hence someone suggested to bake at 125C for a few hours instead.

author: aazard
author_rank: Newbie
timestamp: July 21, 2022, 11:18:19 pm
content: Puhui T8280 is very good for the price. Can be found at "returns" liquidators for well under $100.The only consideration, other than mounting a decent hot air unit for additional top/spot heating, is that its thermometer probe is fairly "spotty".Like it holds temps and is rather large in area, but it doesnt detect true temps well. I found a lot better results using a stand-alone, confirmed accurate, temp probe.Amazing "1st/back-up" unit, with a tiny flaw in design, but at its price point it absolutely smacks all competitors.

author: bingo600
author_rank: Super Contributor
timestamp: July 23, 2022, 02:41:10 pm
content: Quote from: aazard on July 21, 2022, 11:18:19 pmMight wanna fix that one before use ....https://www.gov.uk/product-safety-alerts-reports-recalls/product-safety-report-puhui-ir-preheating-plate-t-8280-2207-0049

author: tony359
author_rank: Frequent Contributor
timestamp: July 31, 2022, 04:11:25 pm
content: Wow, government recall!I did spot the earthing issue in my review video (my videos don't make money, it's just a hobby!) but yes, please fix your 8280 ASAP!The thermocouple is just a guide. I use a laser temperature meter and found that when my board is at the desired temperature, the machine reads around 100 degrees higher. I was not expecting that probe to be accurate as it almost touches the plates! I see that more of "plate temperature" than "board temperature".and here some more earthing!

author: tony359
author_rank: Frequent Contributor
timestamp: July 31, 2022, 07:34:17 pm
content: Yes, that is what I am reading too. I have also prepared a "low humidity box" using some silica beads. The idea is that I can store the board I want to reflow in that box for a while (humidity there is 10%) and then bake it for 24 hours. Hopefully at that point I have better chances of success.I'm just a bit puzzled by so many videos online where I don't see this popcorning issue but I also don't see much baking and preparation before reflowing/reballing!

author: tooki
author_rank: Super Contributor
timestamp: July 31, 2022, 09:30:02 pm
content: Quote from: tony359 on July 31, 2022, 04:11:25 pmNot a recall. The page clearly says it’s a safety report of a device rejected at the border.