LED reflow soldering question/problem

kosPap

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hi all!

Recently I have tried reflow soldering some LEDs on copper mcpcbs using a hotplate, but I am not happy with the results.

I got a small temperature adjustable hotplate and with the help of my DMM temp setting I got a good range of temperature (170-230 C)

Everything was fine when I was testing with some old LEDs, factory soldered on the mcpcbs. I could remove the LEDs, add some flux, put them back, wait a few, and get that bouncing effect when i moved them on the MCPCB

But it took me 4 tries to solder an SFT40 on a board. No bouncing effect. And no self-alligning on the traces. And used pleanty of flux.

So what I was doing wrong? Too little solder? I used quality solder paste, but I found it hard to spread it evenly. Maybe try pre-soldering the pads with an iron next time?

What do yo think?
 

scalpel_ninja

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Sep 16, 2020
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1,084
hi all!

Recently I have tried reflow soldering some LEDs on copper mcpcbs using a hotplate, but I am not happy with the results.

I got a small temperature adjustable hotplate and with the help of my DMM temp setting I got a good range of temperature (170-230 C)

Everything was fine when I was testing with some old LEDs, factory soldered on the mcpcbs. I could remove the LEDs, add some flux, put them back, wait a few, and get that bouncing effect when i moved them on the MCPCB

But it took me 4 tries to solder an SFT40 on a board. No bouncing effect. And no self-alligning on the traces. And used pleanty of flux.

So what I was doing wrong? Too little solder? I used quality solder paste, but I found it hard to spread it evenly. Maybe try pre-soldering the pads with an iron next time?

What do yo think?
Are the pads on the board oxidized? Poor flow of solder paste on a hot board sounds like the contact surface is not clean. Did you apply the flux first to burn off contaminants?

Also, I'm guessing the pads are matched to the footprint of the SFT40? I've never reflowed an SFT40, but I've heard of people putting LEDs on slightly mismatched pads before.
 

kosPap

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You probably have the wrong MCPCB. If the copper pads don't line up, you will experience what you are.
footprint is the correct one. MCPCB was a new one.
Did not clean the traces.
By self aligning I mean the LED sits off, and then magically takes its proper position by the action of the intenal tension of the solder.
 

vicv

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footprint is the correct one. MCPCB was a new one.
Did not clean the traces.
By self aligning I mean the LED sits off, and then magically takes its proper position by the action of the intenal tension of the solder.
Ya. I know what self aligning means. I've done quite a few reflows. I've not experienced the issues you're having. Except when I'm using a different footprint mcpcb
 

scalpel_ninja

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Messages
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footprint is the correct one. MCPCB was a new one.
Did not clean the traces.
By self aligning I mean the LED sits off, and then magically takes its proper position by the action of the intenal tension of the solder.
Photos will be helpful in troubleshooting.
 

kosPap

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Location
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Photos will be helpful in troubleshooting.
Sorry I couldn't...too much fuss

BTW, I found out the problem...a bit less solder and too much flux.
Once it was burnt off, the LED centered itself.

ALSO,
I tried what Vinh showed in one of his videos.
Soldered the copper MCPCB directly on the P60 brass pill.
(pre-tinned both tough)
Once temp was high enough on the hotplate, the two bonded and there was enough heat stored to solder the LED too!
Same process can be done with a powerful soldering iron with a wide tip.
 

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