Need some help - please

JB01245

Newly Enlightened
Joined
Oct 5, 2006
Messages
82
I just got some Cree R2's that I ordered from Cutters. I ordered them mounted on round MCPCB's knowing that they would be too large for my application, thinking that I would just trim down the MCPCB to the size needed. Now I am thinking it might be easier to just transfer the emitter from the large MCPCB to a smaller MCPCB that I have been using. Here is where I have no experience, transferring a SMT component. I have access to a SMT soldering iron (one with the tweezer end). I also have access to people with SMT experience, but no experience with SMT LED's. Do I just grab hold of the emitter until it comes off of the MCPCB? What temperature should I set the soldering iron for? Do I need to put flux and/or solder on the smaller MCPCB if an emitter was previously installed on it?

If there is already a thread with this information, please just point me to it. I am really excited to see if I can notice the difference between a P4 and my new R2's, but I am also afraid of damaging one of these expensive R2's that took 2 weeks to get here and are currently out of stock at Cutters. Any guidance would be greatly appreciated.
 
Did I post this in the wrong forum? I thought I would have gotten at least one response of some type by now.
 
i am not going to be any help, but yes it was the right forum.
i would just buy what your trying to get, buy more Raw Emitters off of star things, and use the stars somewhere else.
but i can tell you one thing, chances are good that you CAN get it removed without destroying it, or popping off the dome, they handle normal soldering temperatures (but not more) a lot better than one would think.
but then you still got a hunk of solder on the bottom, its on a heat removing device, which makes it lots harder,and you still have to stuff it onto another board with the same issues.
it can be done, but it seems like a lot of trouble and possible loss of items, and misalignment, or reduced heat contact, if not done right and all. for my money, and the clutz a thon, i would far be better off to grab some more and use the wrong ones elsewhere.

i wouldnt do it with a regular soldering gun, i would want to have "most" of the heat be on the star thing, then remove as Soon as the star peaked to solder movement temperature, and not try and heat the emitter thing itself. i got "lucky" soldering direct to heat sincs, but i had nothing to loose, did it on the stove. (and see that is why i cant tell ya, cause it worked but it was stupid.)
have your stuff "pre-Tinned" very lightly then prepare for the "rise of the mercury" as the emitter floats high on top of the solder, like a innertube on water, and has to be pressed down, and scoots, and you got 2 seconds to align it up.
something best done by some machine somewhere, or all the right tools, and temps. then ya probably loose one too :-( for the $14 bucks total savings is it worth it? when instead you will have $21 worth of leds for the next project.
 
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VidPro - Thank you very much for the honest reply. I had not thought about the MCPCB doing what it is supposed to do and pulll the heat away from the emitter. Going to have to take a step back and think about this a little more. Maybe I will try practicing on a $5 Cree instead of the $18 one. May even just remove the R2 emitter from the MCPCB and AA the emitter in place. Would just have to break the trace(s) on the bottom of the emitter. Once again, thank you very much for stepping up and helping out someone asking for a little knowledge transfer. I can't tell you how many Lux V's I have burnt up trying to learn this LED thing. Fortunately, the Lux V's were given to me as they were leftover prototypes from work.
 
If it were me, I woulnd't risk damaging the Cree by removing it. My vote would be for trying to trim down the existing heat sink with a dremmel and a diamond saw. I would score the diamater you are trying to get to on the bottom and then superglue the star to an aluminum shaft or wood dowel (perfectly centered) and abrade away until I had the right diameter. BTW, page 9 of XR-E's datasheet says the maximum temperature is 215-260C for a maximum of 30-40 seconds, plus the ramp up and ramp down curves, but that is for reflow soldering. It doesn't say anything about hand soldering or desoldering. Sorry I couldn't be any more help.
 
I've done it both ways a few times using common tools, and been successful in spite of being mechanically declined.

I tried once to cut down a regular star with mounted LED to the small size using a cutoff wheel on a Dremel tool. The star quickly got so hot that the solder melted and the emitter fell off. So I ended up having to solder one on anyway. Also, the particular star apparently had a little ceramic on the bottom for insulation, and this got goofed up in the process so I had to insulate the remainder of the star from the mount. Even so, it ended up working fine.

The next couple, I unsoldered the old LED -- intentionally! -- by holding an iron (at about 700 F/370 C) on the bottom side of the star until the solder melted. 700 - 800 F is about right for this. Then I cleaned the LED mounting pad with some solder-wick, and re-tinned it and the LED very lightly with fresh solder. Finally, I put the LED in place and again heated up the bottom of the star until the solder melted. I put a little gentle pressure on the LED while the solder was molten to make sure it was flat. Those worked also.

The most recent one was easy. I had a Q5 LED on a star, and the old star wasn't too much smaller than full size. So I trimmed the star with a file until it fit.

This process undoubtedly exceeds the specified temperature profile for the LED. I've measured the LED outputs and they're just what I expected. The abuse might lower the reliability some, but hey, I'll almost certainly be replacing them again well before they die, even if they die young for an LED. Overall, the Cree LEDs seem to be able to take a fair amount of abuse.

c_c
 
c_c, thank you for your input. I am also one of those people when someone tells me I can't or shouldn't do something, that just makes me want to do it! I am going to practice on some $5 Crees first.
 
I tried once to cut down a regular star with mounted LED to the small size using a cutoff wheel on a Dremel tool. The star quickly got so hot that the solder melted and the emitter fell off.
c_c

Plunk !
that is funny, i wondered about that, while burning my hands dremmeling and sanding things. the friction sure can do a heat up of the whole heat sinc, like when cutting PC type heat sincs down to some size, the whole thing goes balistic then holds the heat well too. i have to put it aside, or use ice or something to be able to work with it again.

even on the bandsaw, things will get really hot, but its easier to clip chunks off then round it later.
 
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Thanks to those who have responded. I got the R2's off of the MCPCB's without damaging anything. Used a heat gun on the backside of the MCPCB.

Now my next question. Can I just use Artic Silver to secure and isolate the emitter to the heatsink or do I need to cut (grind away) the traces on the bottom of the emitter also before securing with Artic Silver?

Thanks in advance.
 
Hey, thanks for all the help gang! I ended uping dremeling off the "power" traces on the bottom of the emitter and AA'd the emitter to the heatsink plate. These R2's are nice coming from a P4.
 
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