VQFN desoldering temperature - Page 1
Forum section: electronics / repair

author: R-1125F
author_rank: Regular Contributor
timestamp: January 13, 2024, 02:29:30 pm
content: I may need to replace a 24 pin VQFN package TI bq24170 charge controller chip. I am a bit confused about the hot air temperature I should use. I have seen advice not to exceed 260C and others say 360C. I only have one chance to do this and I don't want to damage the board. What hot air temperature should I use?

author: coromonadalix
author_rank: Super Contributor
timestamp: January 13, 2024, 03:48:54 pm
content: when i play with theses vqfn i'm around 360but the secret sauce is more the solder, you have some around 235 260 ... and for the pcb density, 4 layers 8 layers ? thats why we have infrared heating beds to help a bit i dont have this, but i use another heating soltution under the pcb to help a bitnow you have some who will need lower settings, and that way you dont overheat the part, but things who may help, is before putting the part try circular movements on the pcb surface around the chip to heat a bigger region "but dont abuse", not just the part pads

author: ataradov
author_rank: Super Contributor
timestamp: January 13, 2024, 04:13:30 pm
content: 350-400. The temperature of the air does not matter if you remove the IC when the solder melts. By that time the board will get as hot as it needs to get.Plastic parts and connectors nearby may change that a bit ,but generally go high. And QFN-24 it tiny and it will get up to heat fast. Plus it is easy to keep even heat on it.

author: R-1125F
author_rank: Regular Contributor
timestamp: January 14, 2024, 03:33:07 am
content: Thank you. I agree that this chip is small and thin and should transmit heat where it needs to go. I have some concern about getting the thermal pad soldered correctly. Do do you have any advice on what type of solder to use?

author: ataradov
author_rank: Super Contributor
timestamp: January 14, 2024, 03:42:38 am
content: Any, it does not matter, same principle applies. My usual process for soldering QFNs is this:1. You will never get the "right"amount of solder. So, make sure there is enough of it to make contact with the pad, but not too much.2. Heat and make sure the solder melts.Chances are you have more solder than necessary and the chip "floats".3. Press on it, the excess solder will squeeze out as solder balls on the side.4. Collect the balls with a soldering iron.5. Heat again to make sure that it seats properly and the pins also have a chance to flow. This also cleans up potential shorts caused by pressing the chip.You can skip all the pressing if you've got the right amount of solder, but it is not that easy to do. And this process if fast enough to not worry about it.

author: R-1125F
author_rank: Regular Contributor
timestamp: January 14, 2024, 03:44:21 am
content: Should I tin the thermal pad on the chip?

author: ataradov
author_rank: Super Contributor
timestamp: January 14, 2024, 03:50:19 am
content: It is not normally needed, but you can. Again, the concern here is excessive solder on the pad. It is very hard to get the exact right amount. Adding more solder to the chip will not make it better.Make sure to use plenty of flux though.

author: MarkT
author_rank: Frequent Contributor
timestamp: January 14, 2024, 11:13:57 pm
content: Start cool and gradually increase the temp - that way you won't risk damaging the part (its easy to do - different nozzles may perform differently too, larger one's help heat the surroundings and work at a lower temp as a result.Yes, use lots of flux, make sure the solder is covered with flux all the time or it will oxidize.

author: R-1125F
author_rank: Regular Contributor
timestamp: January 15, 2024, 10:25:53 pm
content: Thanks to all. I was able to get the chip out but it took a lot of heat to loosen the thermal pad. I ended up preheating the board from below and the used about 390C air to actually get it off.I lifted one pad but I think I can manage.

author: MathWizard
author_rank: Super Contributor
timestamp: January 16, 2024, 05:25:03 am
content: I need to make some holders for thermocouple wires. I hate trying to have them stay put. I think some metal coat hangers could work ok, maybe with some insulation of some kind. But IDK if anyone even sells them around here any more, most went to plastic.