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Resolder Surface Mount Device

 
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pcaouolte
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Joined: Dec 27, 2006
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Location: South Lincs, UK.

PostPosted: Fri Dec 28, 2012 7:22 pm    Post subject: Resolder Surface Mount Device Reply with quote

Hi,

I have a circuit board with a Surface Mount LED on it which is not lighting. If I put my finger nail on the device it will illuminate so I assume that it has come adrift from the PCB at one end. I cannot see any movement when I touch it. Currently the fingernail prod has made the device work even after I remove the finger nail from it but I do not expect this to be a lasting repair.

I can solder wires and old fasioned through-hole devices to PCBs but have never soldered a Surface Mount Device.

If I just shove a hot soldering iron onto the end of the device is this likely to stick it back on or destroy it? Is there a better method?
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M8TJT
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Joined: Apr 04, 2006
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Location: Bexhill, South Sussex, UK

PostPosted: Fri Dec 28, 2012 8:52 pm    Post subject: Reply with quote

I have resoldered SM devices, but I think that I was a bit more circumspect than 'shoving a hot soldering iron on the end'. But that is pretty much the technique that I used but with a jolly small, hot soldering iron and a quick 'dab'. After all, if it's broke, it's difficult to broke it much more Very Happy . Having said that, I have seen some people destroy slightly broke stuff Rolling Eyes
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Andy_P
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PostPosted: Fri Dec 28, 2012 10:12 pm    Post subject: Reply with quote

I've re-soldered the odd surface mount device (just simple two-ended stuff like capacitors and resistors) with nothing more than a soldering iron and solder, but it's pretty much "dab it and see", not very satisfying!

When replacing components completely, you can improve matters a lot by using solder paste. This is great for sticking components in place on the board and melts quickly when touched with the iron.
I think it might be good for your situation too as it flows under the component onto the pcb pads well. I think Maplin and CPC will sell it in small tubes.

Be aware though that to get a good joint you should really use the SAME SORT of solder as used originally, and for any recent equipment that means lead-free.

Lead-free solder is a bit trickier to use than traditional 60/40 lead/tin and needs a slightly higher temperature on the iron.
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pcaouolte
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PostPosted: Sat Dec 29, 2012 9:37 am    Post subject: Reply with quote

Thank you both. I'll give it a go. I'm hoping that the existing solder will re-melt and give a joint. If not I'll get some lead free solder, as it is a recent board, and give that a try.

Nothing much to loose, I got the board for nothing as it was faulty.
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mikealder
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PostPosted: Sat Dec 29, 2012 10:52 am    Post subject: Reply with quote

Paul, Get some solder paste from Here

Put a small amount on using the tip of a knife then apply heat with a fine tipped iron, this stuff melts and flows at a much lower temperature and is therefore better suited to repairing SMD that have become detached (By the way they don't seem to sell this stuff in Maplins for some strange reason) - Mike
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pcaouolte
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Location: South Lincs, UK.

PostPosted: Sat Dec 29, 2012 11:29 am    Post subject: Reply with quote

Thank you Mike.

Just to be clear does the solder paste work without adding any solder or do I also need to add some solder to the joint?
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mikealder
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PostPosted: Sat Dec 29, 2012 1:15 pm    Post subject: Reply with quote

Just the paste followed by the heat to get it to flow, you will only need a tiny amount to solder an LED leg down to its pad, if you know of any electronics labs local to you they might let you have a small amount of paste for next to nothing, shame Maplins don't stock it as they tend to have a branch local to everywhere these days, CPC (Where I linked to earlier) is a good supplier, local to me and I use them all the time - Mike
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pcaouolte
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PostPosted: Sat Dec 29, 2012 1:26 pm    Post subject: Reply with quote

Thanks for your help Mike. CPC are a long way from me but I'm sure I'll find a supplier locally.
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