# Concerns with the newer TFFC Rebels.



## Brlux (Jul 19, 2007)

I mentioned in a differnent thread that I had some problems with the newer rebels which are manufactured using the TFFC (Thin Film Flip Chip)process. 
https://www.candlepowerforums.com/posts/2085634#post2085634
I have had quite a bit of experience with the older 50 lumen per Watt cool white Rebels as well as every other color Rebel made. I have hand soldered them to 1/32 copper sheets to be used as heatsinks with the power terminals hanging off the sheet for easy soldering of wires. I had found this an easy way to use and evaluate these parts. 
Well I recently received some 100L Cool White and 70L Neutral White Rebels and immediately damaged the first 3 I worked with. The problem left me quite baffled but I have now figured out that it is a result of thermal damage due to the hand soldering of the thermal pad. The symptoms are strange and for me started to present themselves in the first 15 minutes when being driven at 350ma and even sooner at higher currents. The first thing I noticed is a strange oscillation in the brightness kinda like you have a bad wire or switch somewhere in the circuit. I hooked up an oscilloscope and noticed the Vf of the LED was fluctuating about 20mv. It looked like a bunch of random noise on the AC coupled scope. I hooked up older 50L Rebels and there was absolutely no fluctuation, I also tried several other Lux1 emitters which showed no fluctuation in output or Vf. This eliminated my power source as a culperate. I tried using a Li-ion CR123 with a dropping resistor and an LM317 based current source. The next thing I noticed is there was a minimum current that was required for the parts to emit light. I noticed it at 15ma and after running a damaged part for several hours at 800ma it had progressed to the point that it would not illuminate below 400ma. Mind you it still consumed the power below this point but t would not light. I next noticed dark spots on the LED die and as the parts degraded over the space of a few hours the LED got dimmer and dimmer.








It is my opinion that these newer parts are not capable of being hand soldered without being damaged. My Iron is a Metcal and I am quite good with it. I was using a 700F tip at the time. I have had success using a toaster oven as a reflow oven. I place a K type thermocouple inside and manually control the temperature to try and follow the profile of the reflow profile but I sued Pb solder which melts at a lower temperature so I didn't need to take it up to as high of a temperature as the PbFree profile called for. My reflow never exceeded 200C.







In the picture you can see some of my past experiments. On the bottom right you can see where I hand soldered a 100L to a small piece of copper, I used wire cutters and sniped the bottom of the part off to break the connections to the power pads on the bottom of the board. I then scraped the film off the top traces and soldered wires to the top of the part. I have not tried soldering to a coper sheet using the oven method but I think it would work. That set up would lend it's self to being great for the hotlops. Mag heatsink. The one face up in the middle is epoxied down to the Copper using AA and seems to work rather well.


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## Oznog (Jul 21, 2007)

If you have a fine-point engraver, you can Dremel out traces for the 2 power pins. You may need to use a wire brush to smooth out the foil edges so nothing juts up leaving the thermal pad raised up.
EDIT: oh wait that's a SOLID piece of copper I see. Can't make traces in that. Impressive dissipation though!
This presents a problem. The thermal mass of such a sink does not heat and cool all that quickly, and that's a problem regardless of whether you're hand soldering or using hot air/hotplate. It may be that bringing it up to the flow point means the temp will remain at this peak temp for longer than the Rebels can tolerate unless you dunk it in a bucket of water. Maybe heating it real quick and then blowing cold air over it would make it work better?

Your setup makes me cringe, that little emitter not being fully supported by the board. I wonder if mechanical flexing, esp under the heat stress, could have detached or otherwise increased the thermal resistance causing die overheating?

As I said in another thread I've been getting poor output out of three red-orange 0050 I mounted on a PCB with solder paste and a heat gun. I don't see any sort of visible damage to the emitter surface though.

I'm not sure I agree this is evidence to indicate that the hand soldering caused the failure. You have 3 bleeding edge devices and they all failed, right? Wouldn't that also suggest that the batch could be defective? This being a brand-new, unproven product improvement that's not unheard of.

The Rebel dies have a relatively high temp tolerance, 150C for whites. Did you check the temp profile on the spec sheet? 260C for 5-20 sec which seems nice and high to me for a prescribed cycle. 700F is higher but the die probably did not get as hot as the iron. I wonder if not going through these preheating stages could have thermal "shocked" it somehow?


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## Brlux (Jul 21, 2007)

Oznog said:


> I'm not sure I agree this is evidence to indicate that the hand soldering caused the failure. You have 3 bleeding edge devices and they all failed, right? Wouldn't that also suggest that the batch could be defective? This being a brand-new, unproven product improvement that's not unheard of.



All Feel it is evidence as the 3 I hand soldered failed within minutes and the 4 I have done in a reflow oven have been running for days without any problems. I also have one that was epoxied down to the copper sheet and it has had no problems either. There is defiantly something different between the older and newer Rebels with respect to thermal tolerances. It could possibly be related to the sapphire layer being removed from the die.


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## Oznog (Jul 21, 2007)

What temp profile did you use for reflow?
Was it on the copper sheet or a PCB?


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## Brlux (Jul 21, 2007)

The Rebels in the reflow oven were on a pcb. I preheated to 160C and waited 1 minute, then increased heat until I noticed the Pb solder paste melt around 185C then shortly there after I turned off the heat and by the time the temperature started to decline it was about 200C. All temperature ramp ups and down were well within the max and min recommendations.

When hand soldering you have no preheating for a prescribed soak period and you have no control over temperature ramp up or down. I know when I soldered to the copper sheets almost the instant the iron was removed the solder solidified which makes me think is is cooling far faster than the recommended maximum of 6C/second.


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## knabsol (Aug 9, 2007)

Hi!
What about soldering the power pads and glueing the thermal pad with thermally conductive glue? Is that something you could try out or maybe you´ve already done it?
I´m planning on buying and soldering some rebels soon, if it works.


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## dat2zip (Aug 9, 2007)

I'm just starting to play with the new Rebels and I've had success hand soldering them onto the thin PCBs I have. I have a 10mm thin single Rebel PCB and I've soldered one by hand and two using a hot plate. The hot plate rise/fall takes about 4 minutes to reach 235C and ramps down fairly slowely due to thermal mass of the hot plate itself.

Interesting is that I had one failure on a 6X MCPCB strip that I used on the hot plate. The rebel did not light up and I suspected a solder bridge across the anode/cathode and found none. Since it was in series with the other 5 and the whole string lit up the bad rebel was conducting, but, not lighting.

The bad rebel had a very low Vf of around 0.8V. I don't know if this means anything or not and I thought I'd share my experiences to date.

Wayne


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## Gryloc (Aug 20, 2007)

You know, I had the same problems with my Rebels by soldering. I wished that I noticed this area of CPF sooner, since all this happened about two-three weeks ago. Anyway, I had four of the 0090's out and I tried soldering them to a spare blank Cree XR-E star (four would fit perfectly  -just cut each + and - contact pad so two would be wired in series -see Figure 1). However, because I was using my soldering iron, the solder would not even melt enough, or for long enough, for the Rebel to actually stick any (both surfaces were tinned). The star dissipated the heat way too well. This was a thicker aluminum MCPCB I got from Cutter when I got my first Cree P3 and not like the ones I got from DX :sick2:.

Figure 1:

```
[FONT=Verdana] before:                  after:        \   cut traces (and cleared of trace material)
 ======                 =x==x=      /       V   V
 --------                --------     \      =x==x=   <--(XR-E star lead pad trace)
|          |               |          |    /      |  ||  |   <--(Rebel wiring)
 --------                --------     \      +  -+ -   <--(polarity orientation)
 ======                 =x==x=      /[/FONT]
```
Well, I tried a small portable butane powered soldering iron. It worked a little better, but I could not solder both the thermal pad or the two lead contacts with the same iron (it was either one or the other). I gave up with that idea and tried the small strip of copper. I was actually able to heat this enough to get things to stick well. I soldered the 4 Rebel 0090 cool whites to a small strip of copper (shown below in my beautiful text code -see Figure 2). They were overhanging so I could solder this very small diameter intercom wire between all the pads (for series hook-up).

Figure 2:

```
[FONT=Verdana]      ____
+-------|  |  |-------[/FONT][FONT=Verdana]+
|'     [/FONT][FONT=Verdana]#1 |O|O| [/FONT][FONT=Verdana]#2     '|<---copper strip
|'     [/FONT][FONT=Verdana]#3 |O|O| [/FONT][FONT=Verdana]#4     '|
+"------|_'|_'|-------+
              ^Two back-to-back Rebels (all in series)
[/FONT]
```
I soldered two back of them back (like Rebels 2 and 4 in the above "code"). Then I soldered the next two back to back (Rebels 1 and 3). It took me a little while to get them positioned well so each would butt up against each other (make them look nice and square/straight). When I would adjust one when the solder is melted, another would get nudged and become crooked. Unfortunately, because of my annoying, anal nature, things had to look too nice. As a result, the first Rebels (2 and 4), which were soldered first, were damaged. I guess that soldering the rough and quick way is best for these Rebels. If I only had a hotplate, oven, a range, or even a toaster here, I would have done it right the first time. Oh well...

Well, with the damage, it is not that major when the LEDs are driven at normal current levels. However, when small currents are applied, I noticed that weird "flickering" of the LED die. This was with both of my first two Rebels (2 and 4). It made me stare in awe as these were flickering like candles and I was using 4AA NiMH battery pack!!! :thinking: Yes these were damaged. 

I gave them a better treatment and hooked them to my Fatman driver circuit housed in a plastic enclosure. It was my best form of adjustable current LED tester. It has a lot of history and it has tested and burned in many LEDs. With this, I could measure the forward voltage of the LED at any current level (which is adjusted via a big external potentiometer), then with a flip of the switch, I could measure the current. I used the built on sense resistor on the Fatman for this and it is hooked up to my standard digital multimeter (on the voltage setting, of course). It is rather accurate and I can power my single LEDs with it, or my tri- and quad- clustered LED arrays easily, but as long as I had the right input voltage range. 

Anyway, at currents over 100mA, the two LEDs snapped out of their convulsions and they lit up solidly. I am sure they are wasting a bit more power because of this limp, but oh well. At higher currents (from 150mA to over 700mA) everything is dandy. I do notice some loss of flux when I held an aspheric lens over the cluster and looked at its projection on the ceiling. 

Well, this was put into my Fatman modded 3AA Streamlight Tasklight (it used to have the stock Lux I, then a Lux III, then the Fatman and LuxV, then a Cree XR-E P4). It is bright, but floody since I used a diffusing lens to remove the HUGE Duncan Donut from the beam. I still haven't opened the head further to measure current (squeezing that dang Fatman in there is a BEAR), but I think each LED is provided ~1.30W of light, assuming board efficiency of ~85%. This is based off of tail-cap readings and with not-so-fresh batteries.

Finally, I noticed the same issue when soldering two cool Rebel 0070 LEDs. Luckily, these are even cheaper than the 0090. Actually these two Rebel 0070s are in worse shape since they both are put to shame by a single Lux I. Mind you that I did take off the dome and I did make some unique modifications to the ceramic substrate of these 0070s. Luckily this was not my 0100 Rebel. 

Epoxying all my other Rebels (from white 0040s, to R/O 0050s, to my singe white 0100) have proved very successful. I still get decent heat dissipation with AA epoxy with the isolating trick by NYLYTE (used for SSC P4s). Check it out here:
https://www.candlepowerforums.com/threads/157064
I can actually overdrive my 0100 (past 1.5A) with no problems. The heatsink get warm quick and I am happy. This thing is just super bright at these currents!

So, if you have a wimpy soldering iron, little patience, or you need your Rebels soldered to a big chunk of copper (or attached to aluminum), then maybe epoxying is best. If you can reflow solder and have some a stove top or a hotplate and some solder paste, then you may be in luck with soldering. I hope I have helped with adding to the knowledge of mounting Rebels. I would like to see someone try the Cree XR-E hex board trick with 4 Rebels. It is viable, but not for me with my equipment. Good luck all with your Rebels! 


-Tony


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