Killing blue led's

[Steve_88]

I've had many surface mount blue led's pass away after a few weeks of use.

I'm using the led as an activity light, flashing when my PIC16F876 starts up and flashing every few seconds to show it is alive.

I'm driving the led from the I/O of the PIC with a 750 ohm current limiting resistor.

I just re-read the data sheet for this part and noticed it talks about both ESD and moisture causing problems. The moisture may be the issue, as these are used outside and generally not well protected, though they are in enclosures.

With the latest LED's death, I scoped the LED and found that when the PIC I/O switched logic high (5 vdc) that I had only about 3/4 of a volt entering the led.

Thoughts?

Thanks, I'll attach the data sheet

[Pic_User]

I've had many surface mount blue led's pass away after a few weeks of use.

I'm using the led as an activity light, flashing when my PIC16F876 starts up and flashing every few seconds to show it is alive.

I'm driving the led from the I/O of the PIC with a 750 ohm current limiting resistor.

I just re-read the data sheet for this part and noticed it talks about both ESD and moisture causing problems. The moisture may be the issue, as these are used outside and generally not well protected, though they are in enclosures.

With the latest LED's death, I scoped the LED and found that when the PIC I/O switched logic high (5 vdc) that I had only about 3/4 of a volt entering the led.

Thoughts?

Thanks, I'll attach the data sheet

Hi Steve,
The moisture problem seem to be moisture creeping into the package. During the IR or Reflow soldering process, the heat turns the moisture into steam. The steam explodes the package integrity. Using the components in a outside environment (after assembly) looks like it is not so much of a problem.

Data Sheet
jedec level 4 moisture sensitivity

Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices
http://www.jedec.org/download/search/jstd020c.pdf]

Pasted from web
Delamination and package cracks during IR solder re flow are the predominant failure modes.

The data sheet also says:
Solder heat is a major cause of early and future failure. Please pay attention to your soldering process.

I would think about your soldering process, to see if it might be overheating the device.

-Adam-

[Steve_88]

Hi Adam,

Thanks for the help. I had to zoom in on the document to see the fine print about the soldering process. It seems hand soldering these parts is not a good idea, unless you warm them up before hand?

Maybe it is time I sprung for one of those small convection ovens which has been modified for reflowing?

http://www.articulationllc.com/main.sc

Thanks
Steve

[Pic_User]

Hi Steve,

Maybe too much heat too. The leads on Surface Mount are so short, that the heat is applied almost inside the device. No leads to cushion the thermal shock. The iron damages the inside of the LED.

I like that convection oven idea.
-Adam-