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Old 04-30-2019, 12:49 PM   #1
brew99
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Default Recommended hot air soldering Technique

So I'm a newbie using the hot-air station (cheapie from China), and am possibly having issues properly soldering on a QFN package (i.e. BQ24780S) on a laptop motherboard, where most of the pad is under the chip.

When I look at the repair under a scope it looks fine to me, but when I apply power, I'm getting various different results each time I try to re-flow or replace it. I know it takes practice, and am willing to do it, just want a good reference video to verify I'm doing it correctly.

Does anyone have a link to a favourite YT video that they think is the proper way to replace a chip similar to this?
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Old 04-30-2019, 01:19 PM   #2
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Default Re: Recommended hot air soldering Technique

watch rossmann do them.

https://www.youtube.com/user/rossmanngroup/videos
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Old 04-30-2019, 05:49 PM   #3
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Default Re: Recommended hot air soldering Technique

What's your opinion on this video?

https://vimeo.com/219303789

I'm begining to wonder if my solder technique is not so terrible, but that this particular board I'm working on has damage to some of the internal layer vias!
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Old 04-30-2019, 06:19 PM   #4
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Default Re: Recommended hot air soldering Technique

Yes, that's a good video.
Perhaps something else connected to the BQ24780S is faulty ?
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Old 05-01-2019, 10:31 AM   #5
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Default Re: Recommended hot air soldering Technique

Yes, you are probably correct. At one point I had this board working from Ac adapter or battery, but it would not charge the batt. I know the battery is good as it works in another laptop.

In my multiple attempts at re-flowing or replacing the BQ chip, I must have done something else, as now it doesn't power on with either. Will continue to investigate areas around the IC.

More on topic... I did find that using the hot air at a distance from the board for about a minute to act as a pre-heat, seemed to help with the overall smoothness of the soldering experience, so I'm learning
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Old 05-08-2019, 05:44 PM   #6
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Default Re: Recommended hot air soldering Technique

What is your opinion on the "cheap china hot air station", 858 I think it is, compared to the Quick 861DW. I find the 858 has a hard time with the original leaded solder, and I wonder if I am damaging the board in the process. I find I need the temp. turned all the way up or else it struggles with removing a BQ chip in an appropriate amount of time. Each time I try, I think its making a good solder connection as I can see flowing, but after cooling down, I'm getting many pins which in diode mode show different readings to that of a good board that has not had rework. My process is similar to the video I asked about in an earlier post. Tin the board pads with lead-free, add no clean flux, place chip and do a pre-warm of the baord from about 6" away for about a min., then come in closer to chip to get the solder flowing.

I've never used any other hot air station, so don't have a comparison to go by, but I'm try to decide if its worth the almost $400 CDN for it. I'm just a hobbiest, that buying broken laptops and fixing to resell, so not as a main line of work.

Last edited by brew99; 05-08-2019 at 05:47 PM..
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Old 05-09-2019, 03:59 AM   #7
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Default Re: Recommended hot air soldering Technique

858 can reach 400' - that's 50 more than you need for lead-free solder.
your problem is most likely flux, airflow setting or not having a pre-heater.
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Old 05-09-2019, 08:37 AM   #8
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Default Re: Recommended hot air soldering Technique

Depending on the type of laptops that you are working on or type of board you're working on, it can be difficult to use more inexpensive stations because you are dealing with many many layers of copper on the board sinking the heat away. In particular, chips on boards on laptops like Apple laptops tend to have a decent number of via's and such to purposely sink heat away.

Have you tried preheating the board you were working on in a temperature-controlled toaster oven? It doesn't have to be too warm, but preheating aboard can help drive out moisture from it and also make doing rework on chips a lot easier. Drying the board out helps keep things like solder balls from popcorning out of place as they melt and means that the Via points and other mechanisms for heat sinking tend to not draw energy away from the part you are working on as easily.
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Old 05-09-2019, 09:58 AM   #9
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Default Re: Recommended hot air soldering Technique

These are Asus i5-7th gen, so maybe 2017/18 manufactured. Probably a few layers I'm sure. There are a few via in the centre pwr grd pad, but not to much other than that.

No, I have not tried pre-heating the board in say a toaster oven. I've only hit it with some heat from the hot air station (maybe for 1 minute at about 6" away in large circles) before coming in closer to the actual chip.

I have a heat gun (the type for stripping paint), so maybe I can set something up that would blow on the board that way before attacking the chip. Set a thermocouple up to see the temp at the board.
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Old 05-09-2019, 05:44 PM   #10
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Default Re: Recommended hot air soldering Technique

There should be really no need to preheat a board to remove a QFN nor keep the area hot with a heat gun.

Perhaps confirm your station isn't faulty by testing on a scrap board.
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Old 05-09-2019, 06:00 PM   #11
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Default Re: Recommended hot air soldering Technique

Thanks for all the suggestions.....The problem with me testing on a scrap board is I really don't know if all the pins are making proper contact, as I can't power it up and test signals.

It will visually look like its making contact, but when I use diode mode to test, I'm getting different results than a known good board. I know it takes practice, I'm just wondering if I'm making it more difficult using this 858 station as apposed to the quick 861DW. I guess the only way to test that theory is to buy one!!
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Old 05-09-2019, 08:02 PM   #12
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Default Re: Recommended hot air soldering Technique

Quote:
Originally Posted by brew99 View Post
It will visually look like its making contact, but when I use diode mode to test, I'm getting different results than a known good board.
The motherboard might have other broken components and therefore misleading you to focus over and over again on the QFN part...
Plus after so many attempts you may have damage other components and /or the pcb.
How about testing your skills replacing a similar QFN part on known working motherboard...?
Quote:
Originally Posted by brew99 View Post
I know it takes practice, I'm just wondering if I'm making it more difficult using this 858 station as apposed to the quick 861DW.
It's up to you where you spend your money... but expensive stuff doesn't necessary comes with better results
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Old 05-10-2019, 12:23 AM   #13
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Default Re: Recommended hot air soldering Technique

Ya it wouldn't surprise me if I've damaged the board as I've probably tried 5 different BQ chips, and reheated each chip a few times. I think I might declare this one a parts board, and work on another I have. Live and learn I guess
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Old 05-10-2019, 10:30 AM   #14
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Default Re: Recommended hot air soldering Technique

AH! I'm sorry, I should had read things more carefully. >.<; For some reason I thought you were trying to work on some proper bga chips, not a little QFN package. Also, if you can see the solder melting and, as Rossman calls it, "can make it dance" then the solder should be reflowed well generally.

What Rossman means by "make it dance" is that, if you move your hot air gun back and forth after the solder has melted, you generally can see the chip moving back and forth slightly/floating on molten solder.
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Old 05-10-2019, 12:12 PM   #15
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Default Re: Recommended hot air soldering Technique

I can make it "dance" for sure. Usually tap it with the tweezers and it falls back into place, so I would think that is good. Do you usually also press down on it, or just let it float?
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Old 05-11-2019, 03:25 AM   #16
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Default Re: Recommended hot air soldering Technique

well rossman presses down,
lets it cool and removes the excess solder with an iron.
then re-heats with air again to let it remove any bridges.
which makes sense - you want it down on the board - but without hidden shorts.
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Old 07-29-2019, 07:29 PM   #17
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Default Re: Recommended hot air soldering Technique

Quote:
Originally Posted by diif View Post
Yes, that's a good video.
Perhaps something else connected to the BQ24780S is faulty ?
Guys, Help me. I have rework station SV550 SHUTTLESTAR, but have problem about of temperature, i need pieces for this station,
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