Reflow Soldering MC-E

[kuksul08]

Hmm, a lot to take in...


Who knows what the black surface is in the first picture?

 

[mudman cj]

The LED will center itself, the solder pulls it into position. The issue with what you describe is no matter what control you have of the heating surface temperature, this will result in variable temperature of the solder depending on what you are reflowing. I suppose you could use an infrared thermometer to determine that, but it starts getting more and more elaborate and complex than necessary to just do an LED or two.

I have had XP-E packages self-align very well, but the heavier MC-E does not move as freely. Hey, I'm not saying dropping the LED onto the MCPCB won't work or can't be done; I just want the OP to be aware of their options. Again I have to side with Linger in having difficulty understanding your point about controlling the temperature of the solder. Either method under discussion would have the same solder melting temperature. The difference is that the method I described adds a second, lower temperature surface for preheating the emitter as shown in the graph that is posted above.

 

[bshanahan14rulz]

The difference is that the method I described adds a second, lower temperature surface for preheating the emitter as shown in the graph that is posted above.

Ah, I misunderstood you I think. Yes, it is always best to follow the manufacturer's suggested soldering times.

 

[kuksul08]

UPDATE:

I followed your guys' instructions and attempted to reflow solder the MC-E for the first time just now. I will try to list exactly my steps so someone can follow them in the future.

1. First I bought the bare LED emitter, star boards, and solder paste. I used:
-MC-E M bin from digikey, http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MCE4WT-A2-0000-000M01CT-ND
-individual star boards from DX, http://www.dealextreme.com/details.dx/sku.16545
-Mechanic solder paste from DX, http://www.dealextreme.com/details.dx/sku.7952

2. Then I waited a long time :whistle:

3. I used a small straight-slot screwdriver to carefully spread the solder paste onto all the pads like this. I tried to keep it on the designated pad (not touching eachother) and a relatively thin layer. I also applied a very very thin layer to the bottom of the emitter thermal pad to ensure it adheres properly.

4. Then I carefully placed the emitter on the board and pressed down gently, so the paste squeezed out a little bit, making sure the chamfer on the LED was facing the positive terminals.

5. I prepared this piece of aluminum with thermal paste on it, so I could place the emitter/board on it to cool quickly after the heating process

6. I used a little piece of 1/16" aluminum atop my space shuttle tile (insulating material) and touched my soldering iron to it. There is some solder in between to help conduct the heat.

7. I placed the board on and watched carefully. After about 15 seconds, the paste started moving, then suddenly it all liquified. The emitter was sucked down onto the board and centered itself perfectly. Excess solder from underneath flowed out of the side and formed balls. I removed the soldering iron, and moved the emitter/board to the heatsink and began blowing on it to cool it down.

8. That was that. It looks really good, and all the connections are solid. The little balls of solder were removed easily.

I hope this helps someone!! Thanks for the help so I could do it!

 

[J_C]

Good to see it turned out well, it looks good.

One caution though, be careful moving the assembly to the heatsink, be sure the solder has solidified before doing so as movement can upset the alignment and (especially if the solder is non-eutectic) cause fractured solder joints if it is still in a molten state when being moved manually.

I've even had Cree XR-E LEDs that became intermittent on the LED die-module (factory integrated PCB baseplate) from an intermittent joint, but I wasn't following best practices at the time, had been soldering a wire to that PCB baseplate itself instead of using a star and had to go back and reflow one side again to restore the factory solder connection.

 

[kuksul08]

Good to see it turned out well, it looks good.

One caution though, be careful moving the assembly to the heatsink, be sure the solder has solidified before doing so as movement can upset the alignment and (especially if the solder is non-eutectic) cause fractured solder joints if it is still in a molten state when being moved manually.

I've even had Cree XR-E LEDs that became intermittent on the LED die-module (factory integrated PCB baseplate) from an intermittent joint, but I wasn't following best practices at the time, had been soldering a wire to that PCB baseplate itself instead of using a star and had to go back and reflow one side again to restore the factory solder connection.

Yes, I worry about upsetting the solder as it is still cooling. I made sure to remove heat, then wait a moment before moving it. The surface tension (?) of the solder does a good job of holding the LED in position either way.

I wanted to cool it down quickly so the emitter didn't sit there at such a high temperature for so long, it can't be very good for it.

 

[VanIsleDSM]

We shouldn't be trying to heat and cool the LED so quickly.

Thermal shock is what will kill the LED during reflow soldering, from being heated too quickly, that's why CREE recommends a maximum of 3C/s on warmup, and 6C/s on cool down.

It should take a minimum of 2 minutes to get to the point where solder melts, 6 minutes max. Don't rush this step, the part needs to be heated gradually.

Same goes for cooling, moving the emitter and star to a heatsink after the solder melts is not what you want to do, you want it to cool gradually as well.

Just look at the datasheet and try to simulate the relflow table already posted.

 

[trout]

Here is a little video of a heat gun set up I used to replace some xpes

watch carefully at 4.30 and you can see the leds self align as the solder melts .

http://www.youtube.com/watch?v=NN3tXzrJUE0

 

[mudman cj]

We shouldn't be trying to heat and cool the LED so quickly.

Thermal shock is what will kill the LED during reflow soldering, from being heated too quickly, that's why CREE recommends a maximum of 3C/s on warmup, and 6C/s on cool down.

It should take a minimum of 2 minutes to get to the point where solder melts, 6 minutes max. Don't rush this step, the part needs to be heated gradually.

Same goes for cooling, moving the emitter and star to a heatsink after the solder melts is not what you want to do, you want it to cool gradually as well.

Just look at the datasheet and try to simulate the relflow table already posted.

I always set the just-soldered assembly onto a piece of glass to cool slowly. It will still cool rather quickly from the danger zone that damages the LED die while reducing the thermal shock, though I can't say that it complies with the 6C/min specification because I haven't measured it.

 

[kuksul08]

We shouldn't be trying to heat and cool the LED so quickly.

Thermal shock is what will kill the LED during reflow soldering, from being heated too quickly, that's why CREE recommends a maximum of 3C/s on warmup, and 6C/s on cool down.

It should take a minimum of 2 minutes to get to the point where solder melts, 6 minutes max. Don't rush this step, the part needs to be heated gradually.

Same goes for cooling, moving the emitter and star to a heatsink after the solder melts is not what you want to do, you want it to cool gradually as well.

Just look at the datasheet and try to simulate the relflow table already posted.

I can see how the rapid heating and cooling could be bad for it. If you go at 3C/s warm-up though, then it should take about 1 minute to get to the melting point of the solder.

 

[HarryN]

Hi, thanks for all of the posts and pics on reflow soldering. I am following this as I have some SMT LEDs that need to be reflowed as well. It is quite an interesting, and remarkably simple concept.

Just an FYI, if the goal is just to get some premium MC-Es on a star board, I ordered some from etgtech, and they mounted them on a star board for me for 50 cents. I had them in about 1 week. Unfortunately, I decided to use a different LED for this project, so they are still sitting in a box with no good home. :ohgeez:

 

[kuksul08]

Hi, thanks for all of the posts and pics on reflow soldering. I am following this as I have some SMT LEDs that need to be reflowed as well. It is quite an interesting, and remarkably simple concept.

Just an FYI, if the goal is just to get some premium MC-Es on a star board, I ordered some from etgtech, and they mounted them on a star board for me for 50 cents. I had them in about 1 week. Unfortunately, I decided to use a different LED for this project, so they are still sitting in a box with no good home. :ohgeez:

I looked on their site and don't see where to actually buy them?

The goal was partially to get a premium MC-E on a star board, but it's also fun learning about the process. I can only learn so much by reading stuff online. When you actually do it yourself, a lot of other questions arise, so you learn more.


In the future if I were to do this again, I would do the same thing, except move it to a smaller heatsink afterward, with no thermal paste. Everything else seemed to work great. I tested the LED and all 4 dies function.

 

[Linger]

Glad you gave it a go.
It is fun, isn't it? The moment the emitter is sucked down into place is so gratifying, nothing like a perfectly centered emitter to make you feel you've really up'ed your game.

Before you do too many more, consider a practice with a bit of wire and on a board - maybe start with 1/2 as much paste and work down. I'd like you to get a feel for how much of the solder paste is flux and how much joint remains after cooling. Using too much paste could compromise electrical isolation and may bridge an unwanted connection.

 

[kuksul08]

Glad you gave it a go.
It is fun, isn't it? The moment the emitter is sucked down into place is so gratifying, nothing like a perfectly centered emitter to make you feel you've really up'ed your game.

Before you do too many more, consider a practice with a bit of wire and on a board - maybe start with 1/2 as much paste and work down. I'd like you to get a feel for how much of the solder paste is flux and how much joint remains after cooling. Using too much paste could compromise electrical isolation and may bridge an unwanted connection.

Haha yes, quite gratifying. I experimented with the paste a little bit on an old star board before doing it to a good emitter. It seems about half of the paste is flux. I was surprised how much it flattened out once the flux separated.

It was strange on the MC-E how the excess solder was squeezed out and formed those balls. There's no way to tell what it looks like underneath the emitter, but as it sits there is no electrical continuity between the heatsink and electrical contacts.

 

[HarryN]

I looked on their site and don't see where to actually buy them?

.

Hi - it is not listed, I found out when I was talking to them. I have a couple if you want them - I will send a PM.

Harry

 

[blesbok]

Just adding my DIY reflow experience to the mix.... I first tried the toaster oven approach, but couldn't consistently get the ramp rate and dwell right. I now heat the boards by holding them with pliers or tongs a few inches over an electric range on med high. I tack a thermocouple to an unused pad near the LED using high temp solder, so it will stay in place during reflow, and modulate the heat by moving the board closer or further from the burner. Works well every time for single and multi emitter boards.

 

[sigsour]

Keep in mind that the profile is more or less for the paste and not for the LED. The soak zone is for the paste to get rid of certain volatiles and to begin the activation of the flux. When anyone receives the solder paste be sure to stir it up for 30 seconds or so for everything to be mixed properly. Also remember that is very critical to stay in liquidous state for at least 45 to 60 seconds so no cold solder is formed. The maximum temperature is what needs to be monitored for the LED but most SMT devices can withstand 225 C without any issues.

 

[kuksul08]

Keep in mind that the profile is more or less for the paste and not for the LED. The soak zone is for the paste to get rid of certain volatiles and to begin the activation of the flux. When anyone receives the solder paste be sure to stir it up for 30 seconds or so for everything to be mixed properly. Also remember that is very critical to stay in liquidous state for at least 45 to 60 seconds so no cold solder is formed. The maximum temperature is what needs to be monitored for the LED but most SMT devices can withstand 225 C without any issues.

45-60 seconds seems pretty excessive. The solder clearly liquified almost instantly. I gave it about 5 seconds to ensure the flux was removed and everything seated properly, but there's no doubt all the solder particles melted.

 

[HarryN]

Just adding my DIY reflow experience to the mix.... I first tried the toaster oven approach, but couldn't consistently get the ramp rate and dwell right. I now heat the boards by holding them with pliers or tongs a few inches over an electric range on med high. I tack a thermocouple to an unused pad near the LED using high temp solder, so it will stay in place during reflow, and modulate the heat by moving the board closer or further from the burner. Works well every time for single and multi emitter boards.

Thanks for that useful insight. I was wondering about something like this, or the alternative - putting the board in an aluminum frying pan. Since my wife is probably slightly less excited about stove-top soldering, I was even wondering about doing it out on the gas gril.