best way to solder leads to stars ?

I'm new to working with these surface-mount LEDs. After they're reflowed to a "star", what is the easiest way to solder leads to the '+' and '-' terminals on the star ? I'm using solid wire, probably 20ga I'd guess. I have tried tinning the pad (on the star) and the wire first, and that seems to help, but I'm still not getting very good joints. Thanks.

Are you using flux, or have you rosin core solder? One or the other is good. I had trouble too until I used a 45W Weller Iron with a flat tip. You need to heat the pad area for aboutt 15 secs before attaching the wire, as the heatsink pulls a lot of heat away. I've been soldering since before I could walk, and still have trouble with these. I'm impressed you did the reflow - too scared.
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I think you'll need to provide more info. Pictures, perhaps?

You might also want to tell us what you don't like about these solder joints.

you have to work with proper soldering iron - normally i work with head with 80W and flat tip (ERSA-i-CON ) good it is also to preheat a star a little to cca 60°C)

solid wire ? you need a wire with flux inside http://www.amazon.com/dp/B00ARBEMCE/?tag=cpf0b6-20 or attach liquid flux on http://www.amazon.com/dp/B016G4DHRG/?tag=cpf0b6-20

How do you manage to get a nice shiny dome over the joint?
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Flux flux flux! Make sure you heat thoroughly and then don't move the joint until solder has hardened. Remember RoHOS solder will often look gray or foggy compared to lead based solder.

The problem with stars is that they are designed to suck heat away from whatever is hot. Thus it is very difficult to get the joints hot enough to solder properly. Everyone mentions flux, and that's important, but if you can't get the joint hot enough, you'll never get a good joint no matter how much flux you use. If you can solder other things well but have trouble with stars, the problem is heat not flux. It helps to have a big beefy soldering iron with a fat tip in good condition. It also helps a lot if the star is not yet connected to the heatsink.

Lucca mentioned wire with flux inside. He was referring to the solder wire, not the copper wire you are trying to solder. Most (not all) solder for electronics has flux in the core. Plumbing solder never has flux inside, AFAIK.

flux, flux, flux.... that and isolate the star from any other metal. Have a 50+ watt iron, use the broad side of the chisel tip. Avoid silver bearing solder [higher melting point]

I use Kester 951, but any flux should work.

to echo DIWdiver, you'll never be able to properly solder wires to a star that is mounted to a suitable heatsink. That was one reason I suggested providing photos... it would provide us with a lot of incidental info that could provide hints to what was wrong.
In case it's not clear, always solder wires to the stars *before* attaching the star to the heatsink.

and speaking of what is wrong, my suggestion to the OP to describe what is perceived as being wrong with the solder joint still applies.

Illum said:

flux, flux, flux.... that and isolate the star from any other metal. Have a 50+ watt iron, use the broad side of the chisel tip.

Click to expand...

I think that's where I've been going wrong, I always rest the star on a metal table to solder it as I was worried that the heat from the iron would damage the emitter. Wrong?
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The emitter was reflowed at a temperature that's was much higher than your soldering iron will provide while soldering the contact pads. I solder my star leads on a rubberized anti-static mat, never had a failure.

Thanks for that Fritz.
Looks like I'm filling in for the Op on this one!.
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Sorry, I tuned out for awhile...

I have a lot of experience soldering. I am using rosin-core solder (Tenma 21-1045) and a nice soldering station (Tenma 21-7930). I have epoxied the star to a small piece of FR4 (making some home-made wedge-base bulbs) at right-angle, and holding that in a vise; so I don't think much heat is getting sucked away.

The thing I don't like about my joints is I don't see a nice solder fillet. It's more like the wire is just kind of stuck to the pad on the star.

I think I'm starting to get the hang of it though. I first solder the wire to the FR4, then bend it so spring tension holds it against the star. That seems to be working better. As opposed to first soldering it to the star, and having to try holding it in place using pliers.

A small drop of RMA flux on the pad, pre-tinned wire, and a 80 watt or higher iron with chisel tip.

I hold the wire down with a Teflon tool, wet the iron tip, make contact for 2-3 seconds and wait another 10 seconds for the solder to solidify.

You need mass for the solder tip, otherwise the heat sink of the Star will suck all the heat away.

The Weller iron I'm using is sold for lead stained glass work.

RustyShackleford1 said:

Sorry, I tuned out for awhile...

I have a lot of experience soldering. I am using rosin-core solder (Tenma 21-1045) and a nice soldering station (Tenma 21-7930). I have epoxied the star to a small piece of FR4 (making some home-made wedge-base bulbs) at right-angle, and holding that in a vise; so I don't think much heat is getting sucked away.

The thing I don't like about my joints is I don't see a nice solder fillet. It's more like the wire is just kind of stuck to the pad on the star.

I think I'm starting to get the hang of it though. I first solder the wire to the FR4, then bend it so spring tension holds it against the star. That seems to be working better. As opposed to first soldering it to the star, and having to try holding it in place using pliers.

Click to expand...

My guess would be you were causing mechanical fault in the joint when holding it with pliers. This can cause both ugly and "cold" solder joints.
Often enough a "third hand" tool works well to prevent this but in the future you may want to solder the leads before mounting the star.

Like Don said. HEAT your work prior to applying the soldering iron. Makes things so much easier. You'd be surprised to see how well you can solder by elevating the item temp by just a 20 or 30 degrees C.

I.E. break out the GF hair dryer or get yourself that belated X-mas gift. A heat shrink gun!

m4a1usr said:

Like Don said. HEAT your work prior to applying the soldering iron. Makes things so much easier. You'd be surprised to see how well you can solder by elevating the item temp by just a 20 or 30 degrees C.

I.E. break out the GF hair dryer or get yourself that belated X-mas gift. A heat shrink gun!

Click to expand...

I've personally never needed to preheat a LED star, unless it's a specific job where it's already mounted on a heatsink. With proper soldering technique and tools it's not necessary. Just use proper wattage iron, flux OR flux core solder and do it off a heatsink and you'll be fine. Just make sure everything is tinned, apply heat for 10-15 seconds and then hit it with solder.

Great tips guys, this is a super thread. Subscribed!

as i have previously said ... solder on small regulated stowe with temp around 60-70°C . But all soldering have to be done in few seconds






PS : that is nothing special -rework machines have preheating plates

I've soldered to a lot of stars, with lead-free solder, and I've rarely had any difficulties even when they're fixed to decent heatsinks. No added flux, no pre-heating.

I use a temperature-controlled iron, with a reasonable-sized bit (2.3mm wide oval). Usually I tin the pad first, then solder the wire later in a quick action.
I normally use PTFE-coated wire, since though it's a bit harder to strip, it gives a neat result with fine wires with no insulation melt-back, and the wire I use has silver-coated copper inside which solders really nicely and very quickly.

That said, having used Luxeon/Seoul/Crees for the last 12 years, I recently had to solder to some LEDEngin stars for a UV project, and they were all a real pain - it was far harder to solder to those stars resting on a soldering mat than to other stars well-fixed to big heatsinks.