desoldering circuit board. Solder won't melt.  - Page 1
Forum section: electronics / beginners

author: mt007
author_rank: Newbie
timestamp: September 09, 2017, 09:50:16 pm
content: This is not first time to desolder circuit board but I am not pro in soldering/desoldering. circuit board won't desolder. the solder just won't melt for every solder in the board. I am using good solder I tried going up to 480 C but still didn't melt it. I cleaned the tip and tried soldering gun just to make sure it is not from the Iron soldering. I tried desoldering other circuit at less 480 C actually way less at around 300 C/350C I was able to melt different CBs except for this one. The circuit belongs to alpha cordex rectifier it will help. I attached photo to the circuit. any tip for the novice here.

author: IanB
author_rank: Super Contributor
timestamp: September 09, 2017, 09:58:34 pm
content: This may happen if heat is being conducted away by a copper layer in the board, acting as a big heat sink.One thing that may help is to melt fresh solder between the iron and the joint to act as a bridge and ensure good heat conduction.You might also try a high power soldering gun instead of an electronics iron. Temperature is not so much the issue here. The issue is how fast the iron can add heat to the joint to replace the heat that is being conducted away.

author: Ian.M
author_rank: Super Contributor
timestamp: September 09, 2017, 10:22:52 pm
content: Also, if possible preheat the board. Hot air or infrared from underneath heating it to 105 deg C reduces the heat the iron must deliver by up to 50%. If you don't have a preheat system, improvise with a variable temperature hot air gun, a type K thermocouple kapton or foil taped to the underside of the board to monitor the temperature so you don't cook it, and whatever you have handy to contain and direct the airstream e.g. aluminum foil and bricks or wood blocks or even a cardboard box (but make sure the gun nozzle isn't hot enough to char it). Don't forget to protect your bench surface! Another possibility is a shallow metal tray of sand on a hotplate with a simmerstat or even direct IR from the hotplate to a board suspended in a frame over it, + a sheet of ply cut to fit round your board so you don't burn yourself on the edge of the tray or the hotplate.

author: Sarajouksa
author_rank: Newbie
timestamp: September 09, 2017, 10:23:21 pm
content: It looks to me like the board has a coating on it, in which case you need to scrape away some of the coating on the joint to get it going. Applying fresh solder/flux to the tip and the joint will also help to get rid of the remaining coating.

author: wraper
author_rank: Unknown
timestamp: September 09, 2017, 10:29:01 pm
content: Too small tip and/or too weak soldering iron. I hope you use large enough chisel tip and not some conical crap. Preheating the board will help as well.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: September 10, 2017, 01:10:22 am
content: Are you using lead free solder? Generally, lead free stuff has a higher melting point than the leaded one.

author: alpher
author_rank: Frequent Contributor
timestamp: September 10, 2017, 01:11:54 am
content: This will helphttps://americanbeautytools.com/Soldering-Irons/16

author: sasa
author_rank: Regular Contributor
timestamp: September 10, 2017, 07:07:58 am
content: When desoldering during repair, I have experience several issues...Some joints will not melt easily, especially ticker wire (1 or 2 mm). re-flowing fresh solder usually helps.However, sometimes it seems joints are covered with some kind of plastic as additional protection or some other impurity, including oxidation. When try to desolder that, or re-flow fresh solder over that, things usually get worts and impurity with solder may glue inside hole....I usually first use a bit of liquid flux to clean joint a bit. Perhaps carefully clean surface of larger soldering point with acetone first if suspect it is plastic film, or even fine sand paper, may helps,however never tried.Avoid high temperature and long exposure!Use enough temperature that lead or lead-free solder start to melt and make sure that point and area around get cooled for another attempt. Otherwise, excessive temperature may oxidize traces below soldering mask, pull it up or destroy near ICs and other sensitive parts.

author: Rerouter
author_rank: Super Contributor
timestamp: September 10, 2017, 07:39:52 am
content: To clean the solder out of a hole to remove a larger chip, Requires that the hole has some space around it, and is not too snug to the pin (surface tension of solder is a pain)My method is, If the joint is a bit old, i add a dot of gel flux to the joint, then heat and feed in fresh solder until the surface is reflecting like a mirror. (pushes impurities to the edge of the joint) I always rework with lead based solder, which if it was lead free means it becomes a mixture that melts at a lower temperature, and is then easier to work with,While the Iron is hot to the pin, i nudge the pin in the hole with my tweezers, once the pin is moving in the hole i know the entire joint has melted.I Lift the iron and place desolder braid over the joint and then press down with the iron again (generally under 1 second, but its part of my rythm)The majority of the solder begins wicking in the wick, and i slowly move it across to keep fresh wick exposed, generally i keep the fresh wick about 2-3mm infront of the iron, while the braid between the iron and joint contains a large amount of solder it will flow out of the hole quite easily.Once it slows to a near stop, i swap out for the tweezers and inspect the hole before it really solidifies, if its just got a few fine strands holding it then i'll wiggle the pin as it dries to keep it free, if not I'll add a small amount of solder to melt down to where the remaining is (again using tweezers to gauge when the pin is free, and hit it with the wick a second time.This generally takes me about 15 seconds max per pin, but i havent ripped or de-adhesed a pin yet, I use a Hakko 888 (non D) set at 330C at all times and have never had an issue.!. When using desolder braid, do not move the braid unless the solder in the braid looks liquid. and if you feel resistance to motion, try adding solder. a dry braid can solidify to the pad beneath and not conduct enough heat to remelt it.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: September 10, 2017, 08:23:45 am
content: Using some low melting point solder paste can be very efficient like this onehttp://www.ebay.com/itm/lieomo-100g-Low-Temperature-Lead-free-SMT-Melt-Point-138-C-Melting-Solder-Paste-/253065686377?epid=2145390022&hash=item3aebe3ed69:g:lQ0AAOSwTyZZeaSiyou just put a little bit of solder paste and heat. It will amalgamate with the solder on the board and lower its melting point. The propagation is quite rapid through the solder that is on the board.It makes then very easy to melt all solder and remove the chip.

author: alecto
author_rank: Contributor
timestamp: September 10, 2017, 09:04:26 am
content: if you don't care about the pcb...dremel could help...I had the same problem once...but I finally got the part