Hello, it's been a while.
You'll recall I've upgraded hundreds of Seoul emitters into luxeon based flashlights. I still have several dozen of them.
I just wanted to post an alert regarding something you may notice with Seoul emitters as they get older. The metallic base of the emitter will turn from a nice white tin color to yellow and orange. This occurs whether you use the flashlight or not.
I know it's not pretty, probably looks burnt to your eyes, or maybe you think the silicone dome is contaminated. I'm here to tell you it's normal and will not cause your emitter any loss of performance.
Here's the explanation.
The base metal used in the emitter is mostly copper. In order to keep the surface clean and processable during assembly of the emitter, they plate the bare copper alloy with an immersion tin finish. This is a very thin coating, 40 microinches or less. It is a finish commonly used in the printed circuit board industry.
Anyone in the PCB industry knows that an immersion tin board can't sit on the shelf too long or it will lose it's solderability properties. The pads will turn dark and brown looking after a matter of months. Some of this is oxidation, but there is something else going on.
There is a process occuring known as "intermetallic diffusion". When the immersion tin is applied, it "sticks" to the copper by producing molecular mixtures of copper and tin. Near the copper, the intermetallic is mostly copper, a little tin. Further away from the copper the intermetallic is a higher concentration of tin. Finally, on the surface pure "white" tin builds on the surface, little or no copper is present.
Over time and temperature, the copper works it's way up through the tin, building more copper intermetallic, and the thickness of pure tin on the surface thins. Eventually, after a few months the copper will make it's way through the tin to the surface, forming lots of intemetallic and diluting the pure tin. The tin will no longer appear white, it will be yellowed. As a stronger concentration of copper is present, the color will be orange, getting closer to the color of copper.
The problem with printed circuit boards is that a copper-tin intermetallic won't take solder.
However, for the inside of an electronic component, or for boards already soldered, there is no reliability issue at all. Any copper diffusion through the tin is purely cosmetic.
So when you notice this physical color change to the base metal in the Seoul emitter, please don't be alarmed. Don't think your flashlight was left on and you burned out your emitter. Don't think the manufacturer of your flashlight forgot to use heat sink epoxy or compound. It's not an indication of heat-damage.
Perhaps SSC could use immersion silver as a plating instead of tin. Immersion silver does not have the intermetallic diffusion issue. However, it's less common in the industry and a little more expensive. It is also more prone to tarnish than tin, which could be a cosemetic issue too.
Another choice could be ENIG (Electroless Nickel with Immersion Gold). This would have stable color. The cost might be hard to justify.
For those of you who purchased one (or more) of my Seoul flashlights, please continue to enjoy them. I'm sorry the appearance of the emitter will probably change in time, it's unavoidable for any emitters made with immersion tin inside.
I've had this color change happen to some of mine, and it's very dramatic, but it hasn't reduced the output or changed the light color in any way.
You'll recall I've upgraded hundreds of Seoul emitters into luxeon based flashlights. I still have several dozen of them.
I just wanted to post an alert regarding something you may notice with Seoul emitters as they get older. The metallic base of the emitter will turn from a nice white tin color to yellow and orange. This occurs whether you use the flashlight or not.
I know it's not pretty, probably looks burnt to your eyes, or maybe you think the silicone dome is contaminated. I'm here to tell you it's normal and will not cause your emitter any loss of performance.
Here's the explanation.
The base metal used in the emitter is mostly copper. In order to keep the surface clean and processable during assembly of the emitter, they plate the bare copper alloy with an immersion tin finish. This is a very thin coating, 40 microinches or less. It is a finish commonly used in the printed circuit board industry.
Anyone in the PCB industry knows that an immersion tin board can't sit on the shelf too long or it will lose it's solderability properties. The pads will turn dark and brown looking after a matter of months. Some of this is oxidation, but there is something else going on.
There is a process occuring known as "intermetallic diffusion". When the immersion tin is applied, it "sticks" to the copper by producing molecular mixtures of copper and tin. Near the copper, the intermetallic is mostly copper, a little tin. Further away from the copper the intermetallic is a higher concentration of tin. Finally, on the surface pure "white" tin builds on the surface, little or no copper is present.
Over time and temperature, the copper works it's way up through the tin, building more copper intermetallic, and the thickness of pure tin on the surface thins. Eventually, after a few months the copper will make it's way through the tin to the surface, forming lots of intemetallic and diluting the pure tin. The tin will no longer appear white, it will be yellowed. As a stronger concentration of copper is present, the color will be orange, getting closer to the color of copper.
The problem with printed circuit boards is that a copper-tin intermetallic won't take solder.
However, for the inside of an electronic component, or for boards already soldered, there is no reliability issue at all. Any copper diffusion through the tin is purely cosmetic.
So when you notice this physical color change to the base metal in the Seoul emitter, please don't be alarmed. Don't think your flashlight was left on and you burned out your emitter. Don't think the manufacturer of your flashlight forgot to use heat sink epoxy or compound. It's not an indication of heat-damage.
Perhaps SSC could use immersion silver as a plating instead of tin. Immersion silver does not have the intermetallic diffusion issue. However, it's less common in the industry and a little more expensive. It is also more prone to tarnish than tin, which could be a cosemetic issue too.
Another choice could be ENIG (Electroless Nickel with Immersion Gold). This would have stable color. The cost might be hard to justify.
For those of you who purchased one (or more) of my Seoul flashlights, please continue to enjoy them. I'm sorry the appearance of the emitter will probably change in time, it's unavoidable for any emitters made with immersion tin inside.
I've had this color change happen to some of mine, and it's very dramatic, but it hasn't reduced the output or changed the light color in any way.
