Stovetop solder reflow oven

billsea

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Nov 12, 2004
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17
All of the talk about the new Cree is contagious. I haven't ordered any yet, but probably will soon. One of the problems with doing a mod with these led's appears to be soldering the emitter to a heat sink. I though I would try to figure out a way to do the soldering before figuring out the mod.

Here is the setup. 10" cast iron frying pan with lid, oven thermometer, two aluminum foil spacers, aluminum foil workpiece tray.

oven.jpg


Process
1. Fold some aluminum foil to form two spacers about 2" square. The spacers should be sturdy enough to support the weight of the thermometer, and be open on two sides to allow air to flow. They should sit flat and level and ½" off the bottom of the pan.
2. Put the two spacers and oven thermometer in the pan and cover.
3. Find a setting on your stove that will stabilize the temperatue in the pan at about 220-230C. On my stove that was halfway between medium and medium low. It may take a half hour for the heavy pan to get up to temperature. Be patient. If you keep adjusting the knob the temperature will never stabilize. Don't keep opening the lid to look inside.
4. Fold some foil to make a sturdy workpiece tray. Apply some solderpaste to the led and heatsink. When the temp is stabile, quickly remove the cover, place the workpiece tray on the second spacer and recover.
5. Cook for 4 minutes. Remove the cover and turn off the heat. Allow the solder to cool slowly in the pan to room temperature.


Here are some pics of some soldering with this setup. It shows a piece of FR4 with copper foil and a copper penny with solder reflowed on top. Also a piece of copper pipe that was split and flattened, two pieces were soldered together. I tried to solder an old Lux I to the cooper. Forgot that slug was aluminum Duuhh!

solder.jpg


A glass cover would be better so you can monitor the process. I quickly lifted the cover every minute to peek inside. I took about 90 seconds for the solder to flow. This means that the 2 degreesC/sec. temperature rise specified in the Cree spec is not exceeded. I do soldering in production reflow ovens all the time, and this is not very different from the heat profile that would normally be used. This will also prevent the led from being overheated.

Thoroughly clean the pan and cover before using it for food because it may be contaminated with solder and flux. Hope this helps. Now I have to figure out this whole reflector thing.
 

cy

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Dec 20, 2003
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good results!

but personally I'd never use that cast iron pan for food again.
 

Anglepoise

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Nov 4, 2004
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I am aware of the benefits of having a little extra flux when conventional soldering but have never done any re flow soldering.

Must one use this paste?
Or can one just melt a piece of conventional multi core in a 'sandwich'

If one has to use the solder paste, are there any tips as to storage, use etc.
 

Kinnza

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Nov 15, 2005
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223
Location
Spain
Thanks for the tip, billsea.

It seems a good way to home reflow. But still remains the problem of lateral electric contacts. From the method explained for Newbie,https://www.candlepowerforums.com/posts/1674815&postcount=19, kapton supports 356F (180ºC) in continous use. It will support 4 minutes at 230ºC?

I like this sytem in order to reflow solder some leds at a time.
 
Last edited:

billsea

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Nov 12, 2004
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17
Anglepoise,
Regular flux core solder wire would not work. It must be solder paste. Solder paste is a mix of small solder balls and flux. I get mine from where I work. I don't know a good place to get it retail. Maybe someone else does. As far as storage goes, Solder paste normally has a shelf life of 6 months if kept in a refrigerator. But, I think that it will be OK for much longer than that. The paste that I used for this test is a year old, and it worked just fine.

Kinnza,
My idea was to drill holes in the copper heatsink so that the +/- pads on the bottomside of the emitter would not touch the heatsink. The solder could flow over the entire heatsink. Then I could hand solder wires to the bottomside emitter pads. No kapton tape would be necessary. Or, grind off the bottomside pads and solder wires to the topside pads. But, if you use just a small amount of solderpaste the solder should stay just on the center solderpad of the emitter.
 

vacuum3d

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Feb 26, 2004
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553
Location
Bay Area, CA
The problem with contact pads on the bottom of the led has already been solved. Anglepoise showed us how. I don't have the url handy. Do a search on Anglepoise's threads. Just clip the corners of the XR-E.


ernest
 

KWillets

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Nov 1, 2006
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San Francisco, CA
There's actually a kaptontape.com. They say 260C is OK; I didn't check how long. Other SMD heatsinks use Kapton for (electrical) insulation, so I assume it's good for reflow.
 
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