Reflow LEDs

[Kamlesh Barot]

I have started with 2 red and 5 blue LEDs reflow, till date they are working fine. Recently I reaffixed them on a heatsink and added 2 white Sunlight brand 3 watts, 10500 CCT kelvin LEDs on the same driver, without a dedicated fan over my fish tank.

Additionally I purchased 35 new white Sunlight brand 3 watts, 10500 CCT kelvin LEDs and reflowed them on two heat sinks, 9 each with two dedicated computer type fans with new drivers of 10x3 w LEDs capacity.

May I know the reason why ever day, at least one or two LEDs blow out either becoming open so that I have to reflow a new LED or becomes dead without any light, but maintaining the continuity which doesn't require to fix a new LED, but reduces the count of LED on each heat sink from 9 to 8 to....?

Ultimately all my 35 LEDs have got replaced and now out of them only 15 are lighting. Surprisingly the blue red strip with two new white from this lot of 35, are working continuously without any problem in the same tank hood!

Can't understand what am I doing wrong, is it that the Sunlight brand LEDs are bad or the driver or the fan cannot keep them cool enough? But the two red and white strip proves everything wrong!

Thanks,

Kamlesh

 

[RoGuE_StreaK]

I'm a little unclear; are you sure you are actually using a heatsink? Normally reflow is done onto a star; a star's purpose is to make the electrical connections, and to help spread the heat, but they are NOT to be regarded as a heatsink! The star then needs to be affixed to a proper heatsink of some sort, otherwise your LEDs will cook themselves in very short order.
Google images for "LED star", and then "LED heat sink", and let us then know what your exact setup is and how things are adhered.

Strip LEDs tend to run at much lower power per LED, and the LEDs are more spread out, so sometimes the strip by itself may be capable of dispersing enough heat to keep them OK.

 

[ShineOnYouCrazyDiamond]

Can you explain that driver a bit? Is it 10 LEDs in series or parallel. Does it saw that it will drive all LEDs at 3 Watts each or 3 Watts total?

If the driver is designed for 10 LEDs in series (30 Watts total) and you are only using 9 it is possible that the LEDs are being overdriven. That would explain them burning out so fast.

 

[DIWdiver]

When you look at the failed LEDs, do they appear discolored? I'll bet they do.

It really sounds like the LEDs are not being sufficiently cooled. I'm going to guess that your heatsinks don't get too hot (too hot to hold your finger on for a second is around the threshold of too hot, though some will certainly argue that), which would mean there isn't good conductivity of the heat from the back of the LED to the heatsink.

 

[Kamlesh Barot]

Yes, I have soldered the LEDs on a star with a heat compound between the LED & the Star body. After the soldering of the LED, I have put the heat compound between the Star body and the heatsink body and then applied the epoxy (Araldite) glue to fix it.

I have a module LED of three 0.24watts eeach, total of 150 number LEDs too, but since they are low power they do not emit as much heat. My requirement is to have high power and high Kelvin to reach the bottom of my 3 feet tank for the plant carpet to grow.

 

[Kamlesh Barot]

Yes the driver is 10 LEDs in series so each LED gets 3 watts each from the 30 odd watts power.

The 9 LEDs have now become 8 and are stable but were blowing out when all 10 were connected?

 

[Kamlesh Barot]

When you look at the failed LEDs, do they appear discolored? I'll bet they do.

It really sounds like the LEDs are not being sufficiently cooled. I'm going to guess that your heatsinks don't get too hot (too hot to hold your finger on for a second is around the threshold of too hot, though some will certainly argue that), which would mean there isn't good conductivity of the heat from the back of the LED to the heatsink.

Yes but only one is discolored, the rest look the same as when they were bought.

I have two fans that suck and push cool air on the heat sink. The heat sink gets a bit hot to touch, but now that the 8+7 LEDs are not blowing any further, does it mean that the ones blown out were not conducting heat and burnt out, the ones working are well conducting?

Also I wanted to ask if anybody can answer if I can test whether these LEDs are 10500 CCT Kelvins without buying an expensive meter? Would my photographer's light meter borrowed for just checking the CCT suffice, if it has that capability or could my smart phone do it?

 

[ShineOnYouCrazyDiamond]

Yes, I have soldered the LEDs on a star with a heat compound between the LED & the Star body. After the soldering of the LED, I have put the heat compound between the Star body and the heatsink body and then applied the epoxy (Araldite) glue to fix it.

I have a module LED of three 0.24watts eeach, total of 150 number LEDs too, but since they are low power they do not emit as much heat. My requirement is to have high power and high Kelvin to reach the bottom of my 3 feet tank for the plant carpet to grow.

To be honest - something doesn't sound right with your reflow method.

Are you using the same heat compound between the LED and star and the star and heatsink? If so, then that could be your problem.
LEDs should be reflowed with a solder or solder paste only on both the pos/neg contacts and the center heat transfer pad. The way you describe it it sounds like the center heat pad is connected with a heat compound?

 

[Kamlesh Barot]

To be honest - something doesn't sound right with your reflow method.

Are you using the same heat compound between the LED and star and the star and heatsink? If so, then that could be your problem.
LEDs should be reflowed with a solder or solder paste only on both the pos/neg contacts and the center heat transfer pad. The way you describe it it sounds like the center heat pad is connected with a heat compound?

Oh! I thought the pos/neg contacts of the LED & the star should only be soldered, while the center heat transfer pad does not have to be soldered cause there is no electric current flowing there! I have hence put the same heat compound that I put between the star and the heat sink, under the LED & the center heat pad of the star!

But I remember seeing on some reflow youtube videos that only the pos/neg were soldered?

 

[ShineOnYouCrazyDiamond]

Assuming you are using a LED that has positive, negative and a heat pad you should reflow all 3 onto the mPCB. This will allow sufficient heat transfer from the LED into the mPCB and not cause it to burn up.

Then, for connecting the mPCB to the heatsink I would use a different compound from using a heat sink compound and then an epoxy. Heat sink compounds are usually only for two components that are held together with pressure - such as a CPU and heatsink held with a spring. For the type of connection you are doing I would use a proper epoxy like Arctic Alumina or Arctic Silver which both bonds and provides heat transfer all in one.

 

[Kamlesh Barot]

Assuming you are using a LED that has positive, negative and a heat pad you should reflow all 3 onto the mPCB. This will allow sufficient heat transfer from the LED into the mPCB and not cause it to burn up.

Then, for connecting the mPCB to the heatsink I would use a different compound from using a heat sink compound and then an epoxy. Heat sink compounds are usually only for two components that are held together with pressure - such as a CPU and heatsink held with a spring. For the type of connection you are doing I would use a proper epoxy like Arctic Alumina or Arctic Silver which both bonds and provides heat transfer all in one.

Thank you so much!

 

[DIWdiver]

The thermal conductivity of aluminum is around 200 W/m-K. There's no need to understand the units, unless you want to make calculations. But it is important to compare to other materials with the same units.

For example, copper is around 400 W/m-K. Solders vary from 20-ish to 60-ish. IIRC, the best thermal compounds are around 7, common ones are around 1. Air is around 0.03.

The relative numbers here can give you an idea how important it is to eliminate any air gaps. They also show that thermal compounds are a great replacement for air, and that they suck as a replacement for solder, aluminum or copper.

In light of this, the following question seems pretty obvious:

does it mean that the ones blown out were not conducting heat and burnt out, the ones working are well conducting?

Yeah, it seems likely that given your assembly method, some LEDs have good compound between the LED and the star, while others have some air gaps. The ones with air gaps would likely burn out much faster than those without. If you were to replace the compound/air mixture with solder, the entire picture would change. As ShineOnYouCrazyDiamond has suggested, you would likely have vastly better results this way.

Of course, the only way to get good soldering between the thermal pad and the star is to reflow them...

That's why I buy LEDs mounted on stars. For the few that I do, it's not worth the effort to do it properly myself (and I certainly could).

 

[Kamlesh Barot]

The thermal conductivity of aluminum is around 200 W/m-K. There's no need to understand the units, unless you want to make calculations. But it is important to compare to other materials with the same units.

For example, copper is around 400 W/m-K. Solders vary from 20-ish to 60-ish. IIRC, the best thermal compounds are around 7, common ones are around 1. Air is around 0.03.

The relative numbers here can give you an idea how important it is to eliminate any air gaps. They also show that thermal compounds are a great replacement for air, and that they suck as a replacement for solder, aluminum or copper.

In light of this, the following question seems pretty obvious:



Yeah, it seems likely that given your assembly method, some LEDs have good compound between the LED and the star, while others have some air gaps. The ones with air gaps would likely burn out much faster than those without. If you were to replace the compound/air mixture with solder, the entire picture would change. As ShineOnYouCrazyDiamond has suggested, you would likely have vastly better results this way.

Of course, the only way to get good soldering between the thermal pad and the star is to reflow them...

That's why I buy LEDs mounted on stars. For the few that I do, it's not worth the effort to do it properly myself (and I certainly could).[/COLOR]

You're all a wonderful gang, Thanks!