bare emitter soldering what a nightmare !!

Well as some of you may of guessed i've been trying to build an xml light, all was going well until i came to soldering the emitter.

First i scraped of the white squares to reveal the copper underneath then i added a small amount of flux and tried to tin the board but no matter how hard i try i can't get the solder to stick. what am i doing wrong ?

may be not enough heat in the board

its possible the solder is melting and cooling as soon as it touches the board

there is a fine line between not enough and to much heat as i have learned over the last few months myself

That's what I thought. It's nearly impossible to heat the star enough with a 30 to 60 W soldering iron. I would use a stove, like these guys in the videos here:

I've used an old toaster oven for reflow, though obviously you wouldn't want to use that for food afterward.

I would strongly advise against using water to speed up cooling as shown by that first video. You don't want to thermally shock your components by cooling them too rapidly.

Thanks guys i'm not trying to heat a star i just have a bare emitter...

I have already mounted it to the heat sink i'm guessing that's what's causing the problems.

Yes, you are right. As far as I know, it's even better to put them on surface that doesn't drain the heat to fast.

At the very least, you now know that the thermal interface between the LED and heat sink is effective.

Yeah it works bloody well the heat sink was getting nice and warm.

i'll have another go tonight and try with some thiner solder.

What you need is a much hotter iron. at least a 60-80+ watter. Get it nice and hot and do it quick, before the head can get drawn out. How thick is the wire your soldering? Try to pre-tin and then pre-heat the wire and emitter, bit of heat from a second (smaller iron is fine), then when your happy, use the big really hot iron and reflow the 2 together.

Slow and not enough heat just wont work in your situation. Yeah, its great to see the heatsink working really well!