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Soldering a LED to the 'Wrong' Side of a PCB

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Picture of Soldering a LED to the 'Wrong' Side of a PCB
Why would you want to mount LEDs on the copper side of a single sided board?  How about if you want to mount a board directly behind a front panel with the LEDs showing through holes in the panel.  You can't put them on the usual side because the other components would stop the board fitting flush to the front panel, and you can't solder to the component side because there's no track there (unless you have a plated-through hole PCB).  I have just such a project in mind and this is the solution I came up with.

All you need is a couple of strands from a multi-strand cable; A power cable from a PC strips down nicely.  Put the LED in position and wrap a couple of turns of wire strand around one leg, pushing it down to the PCB with needle-nose pliers or a scalpel.  Feed one free end through the adjacent hole in line with the track and cut off the other end.  Solder the wrapped strand to the LED with a fine-tipped iron - See picture 3.

Turning the board over, hold the wire strand in place with a finger along the line of the track and solder as close as you can to the LED, being careful not to solder over the next hole along.  Cut off the loose end - See picture 4.   Repeat this and the previous step for the other leg and any other LEDs.

You now have LEDs mounted on the 'wrong' side of the board, and can continue adding the other components to the 'right' side.  Drill the front panel to accept the LEDs using a step drill and a scrap piece of matrix board as a measuring guide.  Before you position the board on a metal front panel, make sure that nothing from the PCB will short out.  Make a card spacer drilled the same as the front panel and put over the LEDs as an insulator before securing the PCB.

The final photos show the actual project build and how tightly you can pack LEDs with this method.  There's 7 x 5mm LEDs in a 3/4" (2cm) square which will have additional components filling the rest of the PCB and be mounted directly into an Altoids Smalls box lid. 
i'm a total n00b, so bear that in mind when i ask, what is the purpose of the strands from the multi-strand cable?
AndyGadget (author)  kewpiedoll992 years ago
 
Normally you'd mount a LED on the same side as the other components and solder to the back side where the copper tracks are - Easy!

At the moment I'm making an electronic dice in an Altoids smalls tin and need to get 7 LEDs in the space of a square inch on the top of the box.  I'll have the LEDs poking out through the tin lid but to do this there will have to be  no other components on the same side of the board.
If I put the LEDs through the board the other way, I can't solder to the copper because the LED covers up the tracks where I want to solder it.

A single strand of multicore wire is very flexible so this is what I've used to wrap around each LED leg and pass through the board where I can solder it to the copper track so the current gets to the LED.  I can now get the board close to the tin lid as the other components are away from the LEDs.
Thanks AndyGadget! I'm learning a little at a time... :) This is a cool instructable.
randofo2 years ago
That is clever. I normally do that with more pain and frustration.
omnibot2 years ago
Nicely done. I usually just surfacemount them.
pwnag3 omnibot2 years ago
*nods*
ernie6662 years ago
Good idea, but I rather just bend the leads around and pulling it through to the other side with pliers, forming a U shape with the leads.
AndyGadget (author)  ernie6662 years ago
 
I thought of that, but none of the LEDS I had (a dozen different types) had wires thin enough to pull through without forcing.  Applying a lot of force would seriously stress the LED and possibly crack it.  Also, you'd lose a stripboard hole on each side of the LED which could be important if you're doing a compact build.

Just widen the holes slightly so the LED legs go in, then bend them back and pull through enough to solder them. Or sand/cut the upper part of the LED legs so they fit in the original hole.

I didn't think this was much of an instructable, I'll see if I can find one how to tie a knot, pour a glass of water or get out of bed in the morning. ;-)
AndyGadget (author)  e5frog2 years ago
 
I'd like to see you try that and have a working LED afterwards.
As for not being much of an Instructable, check out the competition title and the other 256 entries.
And tying a knot -  Which sort would you like?
That's what I meant, many of the other has shown ways to make use of it, you're showing how to mount a LED the wrong way in a difficult way. Still don't see the problem, if the LED legs can easily go though the holes enough to solder those thin wires to it, why can't they be bent twice and soldering be done directly on the LED pins. Only reason for doing it your way is if you just have one LED and the pins have been cut already for some reason.



Here's the constructive part of the criticism, a 30 second attempt that seem to do the same thing:
http://www.instructables.com/files/deriv/FMZ/6UQE/H673D2FE/FMZ6UQEH673D2FE.THUMB.jpg

Didn't feel like doing the whole circuit but I'm sure it still works, don't say your way is wrong but most LED pins bend quite easily.
Img_0555.jpg
AndyGadget (author)  e5frog2 years ago
 
That's fine if you've got the space to do it, but the projects I came up with this idea for pack the LEDs together quite tightly.  The one I mention in my reply to KewpieDoll needs 7 LEDs within a square inch.
Well, I thought your other instructables were better so I'll consider this particular one as the least interesting. As mentioned there are also surface mounted LEDs that might need two strands of wires if you mount them on the component side. ;-)
I look forward to your "get out of bed in the morning" instructable - something I find very difficult :-)
przemek2 years ago
I'd drill a LED-sized hole next to the pads, and bend the leads into a U-turn with pliers, so that the leads end up parallel to the LED body at a distance that allows inserting the whole thing from the component side---leads into the vias, and the body into the drilled opening.

For that matter, you could just design the board and the front panel in such a way that the LED indicators are just outside the board outline, and the LEDs are just bent 180deg around the edge of the board--no large hole drilling necessary. This is useful because f you have your boards made, extra/unusual size drills may cost extra.
pretty cool idea, also if you like making stripboard projects go HERE
rayshek2 years ago
But why dont you just solder the components on the wrong side, so then you dont get a blob of solder on the same side as the LED which means you cannot get the PCB flat to whatever your mounting it on.
AndyGadget (author)  rayshek2 years ago
 
I'm not quite sure what you mean, but the idea here is that the only components on one side (the copper side) are the LEDs.  This will let you mount the PCB onto a front panel with the LEDs almost all the way through holes drilled in the panel.  Any other components on the same side would get in the way.  It's most useful when you're trying to build a circuit in a small box.  I'll add another photo of this in action later today
I've Uploaded a Drawn Picture
http://www.instructables.com/files/deriv/F67/JCBE/H5Y9WEI9/F67JCBEH5Y9WEI9.THUMB.jpg
Solder.bmp
AndyGadget (author)  rayshek2 years ago
 
Hmmm . . . Good idea, but has some issues.  That would work if you were building a circuit from scratch on stripboard.  There would be problems with tracked PCBs as any multi-pin components (especially ICs) would be reversed.  The workaround, (if you were designing your own boards) would be to produce it as a mirror-image.  There would also be a problem with IC sockets as you couldn't solder the legs.
Thats true, well this idea would only really work if its just for the LED.
AndyGadget (author)  rayshek2 years ago
I could have used that idea here. As it was, I did it the usual way round and used PCB strips as spacers.  Your suggestion would have made it slightly thinner as space was pretty tight in there.
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