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View Full Version : Can't repair PCB after desoldering part



ags0000
11-18-2012, 03:38 PM
I should have left well enough alone. I have several boards (a simple design for signal/power distribution) that worked fine last year. After assembly, I realized that I had mixed up my "water clear" green LEDs with the "water clear" red LEDs (the color is not evident until power is applied). Each board has one LED as a power indicator and half ended up with green (my intention) and the other half red. So this year I decided to fix that. I desoldered the incorrect LEDs and now cannot successfully solder in the correct ones. I probably used too much heat to remove the bad LEDs. When I try to solder the new LED it's like oil and water - the solder wants to stay on the lead and not on the board. I went to RadioShack and bought Flux and used loads of that and still no luck.

I haven't been able to get a picture with enough detail to be useful. I have soldered many boards and seem to do that well (but not desoldering, apparently). I have a Weller WES51 (analog) temperature controlled soldering station @ 650-750 degees. I'm using 63/37 2.2 rosin flux solder. I tried the RS non-spill rosin soldering paste flux here: http://www.radioshack.com/product/index.jsp?productId=2049774&znt_campaign=Category_CMS&znt_source=CAT&znt_medium=RSCOM&znt_content=CT2032236

I would greatly appreciate any suggestions (other than start with a new board).

Greg in Canby
11-18-2012, 03:47 PM
Take a section of wire and soldier it to the led, then trace the path to the next viable soldier point and bingo, bango, bongo . . . your back in business.

ags0000
11-18-2012, 03:55 PM
Take a section of wire and soldier it to the led, then trace the path to the next viable soldier point and bingo, bango, bongo . . . your back in business.

I actually damaged a trace from the cathode on one board and used that technique to bridge the trace to the next connection. If I need to do that for all I will, but that still leaves the LED not mechanically well attached to the board. I'd like to understand what is happening here. Is the likely explanation that with excessive heat I've removed all the metal at the througholes and am attempting to solder the the PCB substrate itself? (Note to self: you should not do this stuff at 2 AM after weeks of pushing to get the display ready). There appears to be metal left at the vias/throughholes.

LightUp
11-18-2012, 04:17 PM
Yes, excessive heat, or excessive time per joint.
I often use a solder sucker and/or solder wick. Once the solder is molten it must be quickly removed or it burns the flux off.

Greg's method is used to get past damaged tracks.

For LED mechanical hold, that is now damaged. I might use a dab of hot glue, after the soldering job. A dab of silicone might work too.

ags0000
11-18-2012, 10:44 PM
Yes, too much heat and/or time. I used a solder sucker and a desoldering wick. There seems to be a lot of information about soldering, but not nearly as much on proper technique for desoldering (without expensive equipment). I suppose if I spent almost $100 on a soldering station I could look into proper equipment for desoldering if necessary, but I was trying to avoid that extra expense. Anyone have experience/recommendations to share?

roberto88
10-03-2013, 01:42 PM
@ags0000: It seems to me that due to the excessive heat the pads of the PCB may be damaged that is why the solder does not want to stay on the board. The best thing you can do to fix this problem is; you can solder the leads of the LEDs to the route directly not through the pads because the solder will stay on the leads and the copper routes and help them to make the joint.

circuit card assembly (http://www.7pcbassembly.com/circuit-card-assembly.php)

markha1
11-10-2013, 10:02 AM
Cir-Kit Makes a repair kit that repairs both trace runs and thru hole damage. It takes practice but they work VERY reliably. The kit will have bushings that go thru the PWB and are swedged in place to provide a stable mount as well as reconnecting both top and bottom of PWB as well as whae is left of the original plating thru hole.
I use this kit many times in my profession and it is prety complete. It will require a good soldering tenique to obtain professional looks though. E-Mail me for additional information if you need it.

bolwire
11-10-2013, 10:13 AM
Make sure you are not using lead free solder. I've had nothing but trouble with it. As for a good mechanical connection you may be able to carefully scrape the silk screen from the trace running to the via then simply fold the lead over the newly exposed copper trace and resolder at that point. I haven't seen your board but it is very unlikely that you will need a new board. I to am familiar with the 2 am should have let it be. Walk away for a bit. Calm down and retry. Hope you get it repaired soon. Let us know how it turns out.

James

griffixdc
11-10-2013, 11:30 AM
i am not sure exactly what happened wrong, but if you damaged the traces or the pcb thats ok too. think of this way the pcb is a cooler bread board and it only holds the components and connects them where they need to be connected internally, if you were to loose that connection internally make it externally. use a jumper wire (according to trace size) and connect from point A to point B. follow where the trace goes and you will be able to determine this.

my first time desoldering i created problems like this....use a desoldering iron with a bulb to suck the solder out... this method prevents damage.

ags0000
11-13-2013, 04:45 PM
The end result was that I ended up using jumpers (wire) from the LED to another part of the trace that I exposed by scraping off the solder mask.

Lesson learned (he says after three days in a row of <3 hours sleep working on this year's new stuff...)

THurrle
11-13-2013, 07:30 PM
This item is worth the money and will save you head aches. I have not messed p one pad or trace using it. Just take your time and you can desolder about any component.
http://www.radioshack.com/product/index.jsp?productId=2062731. I have to give credit to Wayne J for telling me about it.

Wayne J
11-13-2013, 08:51 PM
This item is worth the money and will save you head aches. I have not messed p one pad or trace using it. Just take your time and you can desolder about any component.
http://www.radioshack.com/product/index.jsp?productId=2062731. I have to give credit to Wayne J for telling me about it.

Yep, this is a life saver. Inexpensive and works like a charm. You just have to let it get up to temperature before you try to use it (takes a bit longer than a soldering iron).

griffixdc
11-13-2013, 09:37 PM
Yep, this is a life saver. Inexpensive and works like a charm. You just have to let it get up to temperature before you try to use it (takes a bit longer than a soldering iron).

after my mistake...this year i bought this exact same de-soldering iron....sad this iron was cheaper than the solder sucker i bought thinking it was going to work better... took me about 2 hours to take the triacs of the renard24 with the solder sucker, plus a lot of stress put on the traces...when i used the de-soldering iron it took me less than 30 minutes for the same board and minimal or no stress on the traces...i am highly happy with this tool.