What am I doing wrong? (soldering) - Page 1
Forum section: electronics / beginners

author: Technoguy3
author_rank: Contributor
timestamp: June 28, 2012, 07:01:02 pm
content: So I have this motherboard with bad capacitors. Replace the capacitors and easy fix, right? Well, for some reason, I cannot for the life of me remove the old solder. I've tried everything: I correctly tinned the iron (tried two different irons to make sure one wasn't bad), used extra flux on the joint, etc.For some reason the original solder will not come off! I tried to put some extra solder on it to increase thermal conductivity, but when I go to suck away the solder with either the wick or the solder-sucker, it just sucks up the new solder and leaves most of the old. I've been able to get most of the solder out of a couple holes now after hours (and hours, and hours) of fiddling, but I still have, well, most of them to go. The biggest problem is the solder in the holes that refuses to be sucked out no matter what.What am I doing wrong? I've been soldering for about a year now, and I am not particularly good at desoldering, but I haven't run into anything that I couldn't do after a few hours of fiddling. :/

author: DavidDLC
author_rank: Frequent Contributor
timestamp: June 28, 2012, 07:04:18 pm
content: Are you trying to desolder on ground planes with no thermals ? It is usually difficult to solder-desolder on them.Try to use both irons at the same time maybe you need extra heat.David.

author: benemorius
author_rank: Regular Contributor
timestamp: June 28, 2012, 07:31:16 pm
content: Motherboards are the worst. Multiple layers of huge planes of copper are sucking all your heat away and dissipating it to the air before the lead-free solder can get hot enough to flow. This is not insignificant - you're going to need more heat to counter this massive heat-sinking effect.Preheat the board if you can. A hot air gun, or even a hairdryer blowing on the board will make a huge difference. Temperature difference is like voltage potential - if you raise the temperature of the board you're working on, then there is a smaller temperature difference between the iron/solder and the board and less heat will flow away to the board.Crank up the heat on your iron. The tip temperature takes a nose dive as soon as it touches the joint anyway so you may as well start with as much heat as possible. Just be sure to turn the temperature back down below 180C in between soldering so your tip doesn't meet a premature end.Use the biggest tip that is practical on the joint you're working on. The thermal mass will help.If you're using a 15/30 watt firestarter iron, then this may be the excuse you need to pick up an adjustable bench iron like a hakko fx888. The hakko still won't make all your problems disappear, but it's a big help.Ultimately, preheating the board is probably what will make your problem go away.

author: david77
author_rank: Frequent Contributor
timestamp: June 28, 2012, 07:42:45 pm
content: Stand the board on it's edge, perpendicular to you and your work surface. Grab the component with one hand and heat the joints at the same time. Gently pull the offending cap out, you'll likely have to wiggle it from side to side a bit heating both joints in turn to get it out. When the cap is out place the sucker on one side and heat the solder from the other. This way you should be able to clear the holes.Be extra carefull not to damage or overheat the board.

author: T4P
author_rank: Super Contributor
timestamp: June 28, 2012, 07:55:30 pm
content: Quote from: david77 on June 28, 2012, 07:42:45 pmDon't burn somebody's hands ... use a plier

author: david77
author_rank: Frequent Contributor
timestamp: June 28, 2012, 08:02:14 pm
content: Oh, don't be a whimp. You'll grow a thick skin on your fingers when soldering anyway.

author: M. András
author_rank: Super Contributor
timestamp: June 28, 2012, 08:13:19 pm
content: ohh dont forget some motherboards has a nice thick coating.

author: rr100
author_rank: Frequent Contributor
timestamp: June 28, 2012, 08:55:06 pm
content: Quote from: david77 on June 28, 2012, 08:02:14 pmI still have a solder mark on one finger from when I was young (as in single digit age). And now I'm ... well... old.A friend had so thick skin from holding components bare hand while soldering that he would have trouble detecting heated components; you know how you "probe" sometimes components with your hand to check if they're heating up; "check, check, check; darn, I'm using the wrong hand for that".

author: david77
author_rank: Frequent Contributor
timestamp: June 28, 2012, 10:43:05 pm
content: I once knocked my iron off the table and out of reflex caught it with my hand. Of course I got hold of the wrong end...ouch. That taught me something.

author: Technoguy3
author_rank: Contributor
timestamp: June 29, 2012, 02:06:54 pm
content: Quote from: DavidDLC on June 28, 2012, 07:04:18 pmYou mean the ones that have no thermal pads? Yup, they're those.I don't have an adjustable temp. iron, but the 40 watt Weller I have supposedly goes up to 900 degrees Fahrenheit. I can't imagine how that would not be enough, unless the packaging is just lying to me.

author: T4P
author_rank: Super Contributor
timestamp: June 29, 2012, 02:11:48 pm
content: Quote from: Technoguy3 on June 29, 2012, 02:06:54 pmIs that a firestarter? And no, 40 watts from such an iron would purely wouldn't be enough

author: Psi
author_rank: Super Contributor
timestamp: June 29, 2012, 02:15:12 pm
content: I found the best method is to-Use the biggest tip you can find. A tip big enough to touch both the component legs at the same time.-Apply a good amount of new solder so there is a good thermal joint.-Wait ~5 seconds.-Gently pull/wiggle on component from the other side.When you have the component out use the iron and desoldering tool to to clear outoneof the holes. (One will be easy to clear and one will be almost impossible due to being the ground plane).Then cut the new capacitor legs so the longest leg is the leg for the clean hole. Push it in until the other leg hits the unclean hole then heat that hole up until the capacitor slides in. Then tidy up both joints.

author: nukie
author_rank: Frequent Contributor
timestamp: June 29, 2012, 02:44:12 pm
content: Firstly, use a pen knife try scraping off a tiny bit of solder from the part you are trying to desolder. If you get to cut bits of solder, try melting them with your firestick.(who started the term firestick anyway??)If it melts then congrats, you have a workable solder joint. These boards are highly armoured and it just laugh at you whenever you poke it with your firestick.As mention by others, preheating is the answer to your problem. A regular heatgun used for stripping paint, preferably with temperature adjust will work very well. It's best to preheat to 150c and attack with your firestik. Don't overheat as the board may wrap. If higher preheating temperature is required then its best to preheat the whole board rather than localised heating to prevent wrapping.tapatalk

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: June 29, 2012, 02:53:12 pm
content: Quote from: david77 on June 28, 2012, 07:42:45 pm^ This, OR, clip the leads to the caps and use the same process but use pliers to 'tug' on the remaining leads until they come loose while applying heat (the caps themselves can sometimes act as heat-sinks).

author: T4P
author_rank: Super Contributor
timestamp: June 29, 2012, 05:27:23 pm
content: Quote from: nukie on June 29, 2012, 02:44:12 pmIt was started after some peopleused DSE firesticks and they caught fire ... so they are now called firesticks

author: benemorius
author_rank: Regular Contributor
timestamp: June 29, 2012, 08:13:04 pm
content: Quote from: Technoguy3 on June 29, 2012, 02:06:54 pmThere is more to thermodynamics than temperature. What you have to consider is that the tip may be 900 degrees before you touch it to anything, but when you touch it to a really good heatsink (like a motherboard with several power planes) then lots of heat begins to flow away and the temperature of the tip/solder will drop if the flow of heat away from the joint is greater than the flow of heat in to the joint. In your case, it is dropping below the ~420 degree melting point of lead-free solder. That's a big drop from 900 degrees, but itishappening.The steady-state condition that you're left with at that point is a (perhaps) 400 degree soldering iron pumping 40 watts of heat through a motherboard to the air. 40 watts is not very much heat and a modern motherboard will not have much trouble dissipating it. What it means for something to dissipate heat "without much trouble" is that the temperature does not rise very much with respect to whatever it's dissipating heat to (the air in this case.)Thus, you need to add more heat. Nottemperature, butheat. You'll have twice as muchheat(80 watts) if you use a second iron, but again I'd recommend the hot air approach assuming that you do have access to some kind of hot air device.You may not think that a hair dryer gets very hot, and you'd be right, but this istemperature. We're interested inheat. I just ran and checked my wife's hair dryer. It generates 1800 watts of heat. That is quite a lot of heat. Apply that much heat to a motherboard and it will solder about as easily as anything else.