How do you mount smd (smt) led's?

Firegod · 20 Mar 2008 at 19:28

Hello!

I've got some surface mounted led's that I need to attach to a PCB and I have no idea how these tiny things attach.

Normal led's I can deal with as the legs go through holes and you simply solder them in place.... but how do I solder surface mounted ones? :confused:

I tried searching on Google and couldn't find anything.

Cheers!!

JAMAL · 20 Mar 2008 at 19:36

Just place them on the pads and solder them on, you will probably need a low wattage iron.

If you want to do it properly then you need to apply solder paste to the pads, place the LEDs on and then send it down an oven which will melt the solder paste.

KotFM · 20 Mar 2008 at 19:38

Put a small amount of solder on one of the pads on the pcb. Hold the led with some tweezers and reflow the solder while holding the led in place. Then solder the other side of the led.

I can't remember how the anode/cathode is marked on smt leds though, so there may be some trial and error necessary...

edit: actually, if you have a multimeter with a diode tester you can use that to check.

Focus · 20 Mar 2008 at 19:39

Assuming that the PCB your soldering onto has a location for your LED, it is normally just a case of lining up the part and running solder between the pad and the component using a soldering iron.

Another way is to tin the pads with a small amount of solder then use a heat gun until the solder is molten and dropping the part into position.

Make sure you have correct orientation while your on. There is normally a small arrow on the back/side of the LED.

Firegod · 20 Mar 2008 at 19:45

Cheers peeps. I think I see what you are saying, I just couldn't picture how I'd heat the solder and place the pad at the same time. But I guess if you hold it just above the solder, as soon as it starts to flow I can put it in place.

So I take it there are only certain places the solder goes.... i.e. it's not on the entire bottom of the smd? I'll check when I remove the old one's as to where the solder is.

neodude · 20 Mar 2008 at 19:59

You are better not putting solder on the pads before hand as you want the part to sit flush to the board. If you are removing an old part then you will need to remove solder from the pads using solder braid and an iron (Don't leave the iron on too long or remove the iron before the braid or you will lift the pad and it will require repairing) . When placing the new part on use a good amount of liquid flux on the pads, hold the part in place with a pair of tweezers and then solder one side using an iron loaded with solder. Then you can do the other side.

KotFM · 20 Mar 2008 at 20:02

Yeah, one end will be the cathode, the other the anode. There will be two pads on the pcb, that look like this: [] []

The led will have a tinned section at each end that lines up with these pads.

Be careful if you use the heatgun method, it's very easy to burn the pcb or other components.

I find soldering one side at a time makes it easier to line up the component, but everyone seems to have their own method that works for them. I suppose it depends on whether you have solder paste/braid etc.