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I am a rookie in this area so please excuse my language as I don't know all terms.

The solder paste I am using does not either melt or stick and just looks dry when I try to solder a LED onto a MCPCB.

I am using this solder paste (Sn63/Pb37) https://www.mouser.com/datasheet/2/73/TS391AX10-1150148.pdf

I don't have a reflow oven and I am instead using a hotair reworkstation which I tried with different temperatures (150, 180, 210 degrees celcius) and trying to make it "wet" and stick to the to the solder joints.

I also tried heating it up slowly waiting all from 30s to 3 min holding the airgun on different distances. I am getting the same result all the time.

I can see on the product specification that it's called thermally stable (which to me is an unknown term) - is that the fault?

I cleaned the MCPCB as well between tries with chemically pure isopropanol

This is the result every time (pic without LED)

result

Is the solder paste old? Am I doing something wrong?

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    \$\begingroup\$ Have you read page 2 of the data sheet? \$\endgroup\$
    – Andy aka
    Commented Nov 13, 2020 at 16:43
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    \$\begingroup\$ Is there a pad beneath that solder blob? \$\endgroup\$
    – Lior Bilia
    Commented Nov 13, 2020 at 16:43
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    \$\begingroup\$ You're not delivering enough heat, probably both because your air source temperature is set too low, and because high power LED PCB's have lots and lots of metal to shed heat. You may need to preheat on a hot plate and use the hot air tool at at least 300 C (maybe even more) from above. It's quite possible you want to do this entirely with a hot plate from below - let the heat soak through, then turn it off and let it cool, or set it on something like a piece of aluminum you can use to gingerly transfer it to some other heat tolerant surface for cooling. \$\endgroup\$ Commented Nov 13, 2020 at 16:46
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    \$\begingroup\$ @Noxious well, it's encouraged to post your own answers here! Post the picture from the datasheet, explain what you did. \$\endgroup\$ Commented Nov 13, 2020 at 16:51
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    \$\begingroup\$ I agree with Marcus @Noxious - post your answer and well-done for getting it working. \$\endgroup\$
    – Andy aka
    Commented Nov 13, 2020 at 16:53

2 Answers 2

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Although you've got the solder to flow, you should be cautious. The 300°C air temperature is way above what will damage the semiconductor, and it's likely that 300°C air directed at the top will heat the LED way hotter than the aluminum core PCB.

For example, here are the soldering instructions for a Cree XLED and lead-free solder (which is more difficult than your Sn63Pb37 eutectic solder).

enter image description here

No more than 40 seconds at no more than 245°C

If you have a way to preheat the board to 150°C and could use a high power temperature-controlled soldering iron it might be safer than hot air, especially if you can blow room temperature air over the board with a fan after the solder flows.

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After help I understood that the graph on second page of the specification was the temperature profile to use when soldering. I tested with 300 C and it worked like a charm.

enter image description here

Thank you everyone for quick and prompt responses

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  • \$\begingroup\$ This is the graph of the temperature the solder paste needs to achieve; but with a board designed to be a heatsink, you can't do that by supplying air at only such a temperature unless you can entirely engulf the board in such airflow; with more limited airflow and none to the back, you need a higher temperature to drive heat flow, or a contact source like a hot plate. \$\endgroup\$ Commented Nov 13, 2020 at 17:00

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