# Best method for attaching LED to star?



## dnlwthrn (Oct 21, 2008)

So I've ordered an MC-E (from Endeavor) and since it doesn't come mounted to a star, I ordered a series-wired star for it from DX. I also have an XR-E that I got as a sample from Cree (gotta love working for an engineering firm) that needs to be mounted to a star.

What's the best method? I'm assuming they're soldered when they come mounted alread, but there's a LOT of material to heat up on the star before you could get a good solder connection. Can someone point me to a reference/tutorial? I've done quite a bit of soldering, but not on anything with that much mass to heat up...

Thx!


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## R33E8 (Oct 21, 2008)

I assume you can use a bit of solder paste? Just mount the star on a hotplate or something and carefully place the LED on..


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## dnlwthrn (Oct 21, 2008)

That's kind of what I figured. I'll probably give it a go with the XR-E (since it was a freebie) and see what I find. I'm guessing you need to use as little heat as possible for this...


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## Oznog (Oct 21, 2008)

Yeah you need solder paste. Actually this is hard to find in the US, you can get it cheap from DealExtreme BUT be careful to get the lead solder paste not the solder flux paste without the solder in it. DX is often unclear on that but look at the comments.

I've used a heatgun but you have to be careful not to overheat the board. There's an insulating later between the aluminum and top traces (which needs the heat) and that insulator can be damaged by high temps. I kind of wonder if the toaster oven isn't a better choice since it circulates heat to the top, except for the problem that the LED package can't tolerate extremely high temps either.


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## dnlwthrn (Oct 21, 2008)

Oznog said:


> Yeah you need solder paste. Actually this is hard to find in the US, you can get it cheap from DealExtreme BUT be careful to get the lead solder paste not the solder flux paste without the solder in it.



So I checked DigiKey, a supplier we often use at work for electronics stuff, and was shocked. Most of the solder paste they carry is non-stock, special order. The one they did have as a stocked item was for 500g and was over $50!

DX is now sending me some, at a measly $3.73 for 50g.

Since I have some extra stars for the XR-E, I may play around with heating them to see what's going to work the best while I wait for my solder paste.


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## odessit (Oct 21, 2008)

What is the number for the paste and the star? I am in the same situation as you.


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## Nake (Oct 22, 2008)

I use this paste with good results. 

http://www.solder-it.com/solderpaste.asp


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## dnlwthrn (Oct 22, 2008)

The soldering paste I ordered is SKU 04711, and the stars (5 pack) are SKU 16544.

I may get some of the stuff Nake listed as well... I like the idea of a syringe-like applicator.


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## Cemoi (Oct 22, 2008)

Nake said:


> I use this paste with good results.
> http://www.solder-it.com/solderpaste.asp



Thanks for the advice.
Can it be used to solder electronic components, especially tiny ICs otherwise hard to solder by hand with a soldering iron?
Is it available in retail stores?


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## LED Boatguy (Oct 22, 2008)

I've had good luck with a heat gun and DX's solder paste. Probably done it fifty times and not a failure yet, and that includes some pretty small SMD chips. Heat slowly from the bottom. When you see the paste kick off, give it about 5 seconds more heat then back the heat gun away. Let it cool naturally. At least this works for me.

I just got a syringe and an assortment of needles from Zephpaste. Have not used it yet though. $$$! Ouch.

http://www.zeph.com/zephpaste.htm

Also have a new IR toaster oven and PID controller on order so I can do controlled temp reflow.


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## Nake (Oct 22, 2008)

Cemoi said:


> Thanks for the advice.
> Can it be used to solder electronic components, especially tiny ICs otherwise hard to solder by hand with a soldering iron?
> Is it available in retail stores?


 
Can't say about the components, I've only done Cree emitters to boards. 

I don't see any mention of retail sales locations on their site. I ordered directly.


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## evilc66 (Oct 22, 2008)

Solder paste like that is perfect for smd work. I have built many prototype boards by hand using a syringe filled with paste (very tiring on the hands after a while) and a butane heat gun. Very easy to use. Try and get one with a water soluble or no-clean flux. Makes life a little easier on cleanup.


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## mds82 (Oct 22, 2008)

I did this myself once just using silver solder and a hot plate

I heated the hot plate and then melted some silver solder right onto the MCB ( the star) . after that lowered the temp a bit and placed the LED right into place - i havent had any problems with it at all. it was very easy.


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## dnlwthrn (Oct 22, 2008)

If I get some time at work tomorrow, I'll try that... I thought maybe it would work, and the solder I have melts at a fairly low temp. Time is the issue right now...


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## Cemoi (Oct 23, 2008)

Nake said:


> I don't see any mention of retail sales locations on their site


I contacted them, and they answered:

This product is available at Lowes
Their minimum shipping fee (which I haven't found on their website) is $10


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## Cemoi (Oct 23, 2008)

mds82 said:


> I did this myself once just using silver solder and a hot plate


What do you mean by a _hot plate_?
A few pictures or the URL of a tutorial would be welcome for a nube like me.


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## mds82 (Oct 23, 2008)

Well not really a hot plate, i just used the stove top burner on my oven. I help the MCPCB Star with tweezers and kept turning up the heat until it was hot enough to melt the solder. I ended up putting the burner somewhere around medium, and that was hot enough to melt the solder onto the star. After that i dropped the LED into place, gave it a little push to make sure it stuck, and then took it off and let it cool down.


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## dnlwthrn (Oct 23, 2008)

I actually have ceramic hot plates here at work. Similar in idea to an electric range, but with a smooth ceramic top. It has a dial for controlling the surface temp. If I get around to trying things this week (looking unlikely at the moment) I'll take a few photos...


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## Oznog (Oct 24, 2008)

LED Boatguy said:


> I've had good luck with a heat gun and DX's solder paste. Probably done it fifty times and not a failure yet, and that includes some pretty small SMD chips. Heat slowly from the bottom. When you see the paste kick off, give it about 5 seconds more heat then back the heat gun away. Let it cool naturally. At least this works for me.



That's a little off though. Pastes have 2 phases, one where the solvent bubbles off and the second where the solder reflows. If you do it too fast, the larger bits of solder still have a lot of evaporative cooling going on. And it's a little violent.

Ideally, you'd watch for the paste to bubble lightly and you can see it dry out and take on a dull appearance. When I do this with a heat gun I back off on the heating as soon as I see this to let it finish without getting any hotter. Then when it all settles in a few seconds put the heat close again and the solder joints flash over into reflow.

The cooling period is also an important 3rd stage, which affects the strength of the joint. 

Though for the most part just heating it until it melts is "good enough".


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## dnlwthrn (Oct 24, 2008)

Well, I tried it... A couple of thoughts. I used traditional rosin-core solder, and this may be part of the issue, but it took quite a bit of heat from the hotplate to get the solder to melt. I actually cut off a small length of solder, coiled it up, and set it on the center pad. When it started to melt, I added solder to the + and - tabs as well. When setting the LED onto the puddle of solder, you have to have REALLY steady hands, or you'll put things on crooked. I need to double check continuity, as I'm afraid the solder from the center pad may have run over onto the electrical connections when I put the LED on. Also, be careful not to get the LED too hot! Duh. I'm just glad I practiced on the free sample that I got, as I'd hate to damage my new MC-E...


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## dom (Oct 25, 2008)

You don't need all that much solder.
I did a Cree emitter the other day -just tinned the pads of the MCPCB,used a flux pen on the emitter.

Held the emitter maybe 1mm above the board while using a blow torch under the board -pressed the emitter on just as the tinning melted.

The tinning squished out as beads and the emitter stuck well.

Of course it would be better with jigs and a press -but good enough for a home job.


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## Nake (Oct 25, 2008)

Solder paste flows at about 100 degrees less than wire solder. Less heat to the LED or components on a board.


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## dnlwthrn (Oct 25, 2008)

Yep, but I'm waiting on my paste from DX. Maybe by next weekend it will be here.


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## Cemoi (Nov 11, 2008)

Oznog said:


> Ideally, you'd watch for the paste to bubble lightly and you can see it dry out and take on a dull appearance.



I bought some Solder-It and gave it a try, not to solder an emitter on a star, but to solder a tiny CMS on a circuit. I expected the solder paste to have a creamy consistency, in fact it is more like moist sand. Doesn't really flow nor adhere to my circuit nor component legs. I placed dabs of solder on each leg, and heated it by putting my soldering iron very close to the solder paste, but nothing happened even after a long time.
So I lade a test by putting a bigger amount of solder paste on a metal plate, and again heated it by putting the bit of my solder iron very close (one mm) to the surface of the paste. My iron was set to 400°C = 750°F which should be enough since the package says "flows smoothly at 430°F, re-melts at 650°F". After a while it looked like some liquid (the flux?) was going out of the solder paste, but no bubbles even after a very long time. If I put the soldering iron closer (touching the solder paste) it burnt the solder paste.
After cooling down, the solder paste didn't get hard, it was still like moist sand.
What am I doing wrong?


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## VanIsleDSM (Nov 11, 2008)

If your solder paste is like moist sand then it's either dried out and no good, or you need to stir it up.

The real key to remember when SMT soldering is that temperature alone is not what is going to kill your LEDs. The most likely failure will be from thermal shock. Heating the star until the solder is melted and then placing the led onto the hot star with molten solder is not advisable.

If you look at any of the reflow soldering graphs in LED datasheets (or any SMT product for that matter) You'll see that you need to slowely heat and cool in a controlled manner.

Another thing to note before SMT soldering XR-E, and I assume MC-E, though I haven't checked.. is that you should bake them at 80C to remove all moisture before reflowing them. Moisture left in the LED before relow can adversly affect tint and lumen output.


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## Nake (Nov 11, 2008)

VanIsleDSM said:


> If your solder paste is like moist sand then it's either dried out and no good, or you need to stir it up.


 
I don't think it's dried out. I have the same product and that's what it looks like. I've never used a soldering iron to activate it though.


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## VanIsleDSM (Nov 11, 2008)

Well could be, but all the soldering paste I've used has been.. well, paste-like.

Sometimes some near the top dries out and I would describe it as moist sand. Once you mix it back down into the flux it's all good again.


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## NetKidz (Nov 11, 2008)

VanIsleDSM said:


> If your solder paste is like moist sand then it's either dried out and no good, or you need to stir it up.



If it dried out, could I mix it with liquid no-clean flux? 

Thanks.


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## Cemoi (Nov 12, 2008)

VanIsleDSM said:


> all the soldering paste I've used has been.. well, paste-like.


From your first picture it looks liquid enough for you to drop these tiny drops on your circuit board, which would be totally impossible with my "moist sand".



> Sometimes some near the top dries out and I would describe it as moist sand. Once you mix it back down into the flux it's all good again


This may be the reason, but how to mix paste contained in a closed syringe? Would heating the syringe slightly and shaking it do?


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## VanIsleDSM (Nov 12, 2008)

I imagine you could mix it with another type of flux, as long as it didn't react badly with whatever flux is left over in your dried paste.

As for stirring a syringe I'm not sure. None that I've ever bought has come in a syringe, Usually something like a pill bottle or film canister.

Keeping solder paste in a fridge is a good way to store it so it won't dry out or separate as quickly. Always best to pull it out and wait for it to hit room temp before you use it though.


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## ichoderso (Nov 13, 2008)

Hi, is it possible to take Arctic Silver Thermal Adhesive ( or arctic alumina..)?


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## Cemoi (Nov 18, 2008)

VanIsle:

I'd like to be able to build the same nice circuit like yours on the above picture.
What solder paste do you use and where can it be purchased?
What do you heat it with, and do you do it with the components in place, or heat the dabs and put the component once the solder has melt?


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## VanIsleDSM (Nov 18, 2008)

That board was done by hand with the LodeStar solder paste from DX. Place the solder paste first, and then put the components down, then use a soldering iron. Hold the part down with a toothpick for the first joint, and then it's solid for any other soldering.


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## Cemoi (Nov 21, 2008)

Thanks VanIsle for the solder paste info.
In the meantime I have contacted the manufacturer of Solder-It. From my description he said my paste must have been spoiled from too long on the shelf (I bought it at Lowe's), so he will send me some new paste. Thumbs up to Solder-It for their service!



VanIsleDSM said:


> Place the solder paste first, and then put the components down, then use a soldering iron.



How close to the solder paste do you put your soldering iron tip, and how long?


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## VanIsleDSM (Nov 21, 2008)

You stick it right into the paste. If I'm soldering an IC I touch the board and paste first, and then move up the pin of the device and pull away.. all done quite quickly, about 1 second, as the small SMT pins don't take much to heat up.

I wouldn't use a soldering iron for an SMT LED though.


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