Cannot desolder QFN due to epoxy(?) - Page 1
Forum section: electronics / repair

author: FixyMacFix
author_rank: Newbie
timestamp: November 11, 2022, 07:44:32 pm
content: Hi group,Hoping for some tips. I'm going insane.I'm repairing a defect Ubiquiti Edgeswitch. The buck converter in the picture has failed.But I can't for the life of me get it loose. I have an identical board which is defect (probably) beyond repair and have heated it to insane temperatures without any results. The solder is boiling but I can't lift it. (I've heated it hot & long enough to be certain it isn't due to heat being sunk by a copper layer on the belly side.)I've worked on their equipment before and had similar problems. The only explanation I can think of is them using some sort of extremely tough epoxy to hold components in place, as it's difficult to remove SMD resistors etc as well.Any ideas on how to soften this epoxy/glue somewhat?Thanks in advance

author: wraper
author_rank: Unknown
timestamp: November 11, 2022, 07:52:15 pm
content: QuoteSolder cannot possibly boil unless heated to temperature which would cause PCB instantly burn in flames. Likely you used some flux which is boiling.QuoteNot used under QFN, however there is a large pad in the middle which is soldered to the ground plane.

author: FixyMacFix
author_rank: Newbie
timestamp: November 11, 2022, 08:19:18 pm
content: QuoteYou're obviously right.QuoteHm. I've heated it on the other (identical) board for 10 minutes straight, which comes in addition to heating the board itself for several minutes. The solder points of all components around are liquid as well and plastic pads for neighbouring SMD caps are melting. I'm using the hot air gun at the 400°C setting.Any suggestions as to how to go about getting it loose then? Can't imagine I'd get any ChipQuick under it either...Afraid I'll damage the thing (not mentioning the other components) with the amount of heat required.

author: wraper
author_rank: Unknown
timestamp: November 11, 2022, 08:55:30 pm
content: Airflow matters, so be sure to set maximum airflow. If PCB has a lot of copper layers, low airflow station may have trouble desoldering this. Preheating from the bottom side makes a huge difference. Maybe use hair drier at max temperature and low airflow setting on bottom side if you don't have anything better. Upside down clothes iron will do the job too.

author: Bud
author_rank: Super Contributor
timestamp: November 11, 2022, 10:25:50 pm
content: If glue was used, other components would be glued too. Did you try to lift other components?What else i am thinking is this may be a multilayer board with power plane and ground plane. Unsoldering components from such boards is difficult because the large internal copper planes suck a lot of heat. Can try heating the entire board in a toaster oven.

author: Rasz
author_rank: Super Contributor
timestamp: November 14, 2022, 03:37:40 pm
content: nobody uses glue on ground pads

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 14, 2022, 04:02:36 pm
content: Probably the bottom pad is soldered to a heavy ground plane.Set the oven to 150ºC, pre-heat until the thermostat turns off, turn off the oven, wait 2 minutes to let the temperatures equalize, put the board in, take out after 5 minutes and quickly blast it with hot air.Better to wrap the board with some isolation cloth to keep the temperature.If you have one of those 2000W heat guns made to scrape the paint off, will also work great, just don't burn the board down!Apply the heat from 10-15cm in circles around the whole board for 30s-1 minute, then proceed with desoldering.