Soldering/Desoldering problem. - Page 1
Forum section: electronics / repair

author: RayRay
author_rank: Frequent Contributor
timestamp: March 22, 2015, 07:28:40 pm
content: Hey guys!I'm new here and hope you could help me out.Anyhow, on to the problem, so I needed to replace a fried LED light on a circuit board the other day, so as usual, I added some fresh solder to the joints, and used my solder wick to remove it all and then just used my fingers to remove the LED. Here's the problem, after I inserted the new one and tried to apply fresh solder, it just didn't stick to the board! (like at all!) and it was just loose (I've desoldered/resoldered components many times before and this never happened to me before!) anyways, so I tried a workaround, I added solder from both sides of the board (LED legs from the other end too) and it finally sticked! But then when I turned on the device, the LED did NOT turn on (while the other ones did! - it's a simple cable tester) and I know for a fact the cable is fine (continuity tested) and I got the LED polarity correctly done too (it's marked on the board, also I checked the LED with a multimeter on diode mode and used a marker to mark the positive leg of it) Few more things I'd like to mention:1. I use a 0.8MM 60/40 soldering wire, which according to the label, has some flux in it, but probably not much.2. I never used a standalone flux (or soldering paste as some may call it) as I didn't had the use for it.3. Normally, after I use my wick for desoldering, the surface remains smooth and silver in color, this time however, the surface had a different color, more like honey, or light brown (flux possibly?)So basically, I think I may have somehow managed to suck out the internal solder, or flux from the joint.I'm thinking of buying flux, then putting it around the joints, plus dip the LED legs in it, and then resolder it and hope for the best!My question is, what are the odds that this will correct the internal contacts?Did any of you ever encountered this problem before?All help would be greatly appreciated.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: March 22, 2015, 07:47:02 pm
content: I always add flux. It's a little more cleaning to do, but the soldering is always much easier, so I reduce the risk of damaging things with longer dwell times. Get a flux pen or needle so you don't add much.Do a continuity check from each LED leg to someplace else on the board (like a resistor or something) that should be connected to it. If no continuity, then there's your problem. If the LED appears electrically connected to everything it's supposed to be, then I suggest testing the old LED (with a current limited power supply, for example); chances are the LED was out for some other reason than a bad LED.It does sound like something went wrong with the soldering, though. Just because you can't wiggle the part any more doesn't mean the solder actually wet to the pad and the LED. I couldn't tell from your description whether the solder had trouble with the pad or the LED. Is it even a plated through hole? Or is it just a pad? You might have damaged the pad when you desoldered the old LED. Did the old LED come out with smooth leads, or was there a solder blob left where the LED met the pad? A picture might help, too, if you can get a good macro shot of the joint and the old LED.

author: RayRay
author_rank: Frequent Contributor
timestamp: March 22, 2015, 08:12:56 pm
content: Thanks for your help Galaxyrise!Just did a continuity check, and I only got a hit from one of the LED legs, the other one just doesn't make contact with anything else on the board! So I guess it is a soldering issue!Anyhow, the solder sticked to the LED legs just fine, just not to the joint.I use a gentle (25W) soldering iron, replaced LEDS/Capacitors/Resistors many times and never had a problem til now (this is just some cheaply made circuit board, what can I say!)And as I said before, I applied fresh solder to the joints, and then used soldering wick to remove all the solder.The old LED came out smoothly when I pulled it out with my fingers. there was no "blob" or other residue on it.Gonna get me that soldering flux tomorrow, and hope for the best!Thanks again.

author: Shock
author_rank: Super Contributor
timestamp: March 23, 2015, 12:29:36 am
content: Tin the pad and check continuity first. If it tins it will solder, if it doesn't you can use an abrasive rubber to remove the oxide (carefully). You should use IPA and clean between operations. Then you don't get flux on your rubber, debris in you joint etc.

author: sleemanj
author_rank: Super Contributor
timestamp: March 23, 2015, 01:17:52 am
content: Is the pad actually there at all? Just it sounds to me like you have pulled the pad, or through hole plating off and have not noticed. Post a picture of the problem area.

author: edy
author_rank: Super Contributor
timestamp: March 23, 2015, 01:31:49 am
content: Yes, I agree, I find the pads sometimes burn off the board. Then no amount of solder will stick because it's just PC wafer under there. You have to then trace the circuit and find copper somewhere else to tie into (sometimes with a "bodge wire"). Happened to me a few times on some repairs. The copper pads sometimes pull with the old component or just too small or old and get damaged. If you have multilayered PC it gets really hard.