S/M LED storage

Move your production line to Arizona ?:-) ...Jim Thompson

If all you want to do is to get rid of absorbed water, it might be worth investing in a vacuum desicator.

It's basically a chamber you can pump down below the local vapour pressure of water - about 15mm of mercury at room temperature - so that any water vapour that comes out of the LEDs can move directly to your dried silica gel, pot of anhydrous phosphoric acid or liquid nitrogen cooled cold finger, without having to diffuse through an inert atmosphere of oxygen and nitrogen. It's the same idea that makes heat pipes work.

Warming the LEDs a bit speeds up the process.

Beats me.

-- Bill Sloman, Nijmegen

Are they Kingbright by any chance. ? We have found these( RGB 3.2 x 2.8) to be VERY static sensitive, especially the red die. :(

Epoxy encapsulated parts are, in general, subject to this problem. Especially thin parts.

There should be a rated MSL for the parts. IPC/JEDEC J-STD-020 is the relevant standard. Once you remove the parts from the envelope there is a time a time during which they must be used.

Level Floor life

Floor life (out of bag) at factory ambient of 30°C/60% RH or as stated

You can put them in a vacuuum sealed bag with some dessicant just like the manufacturer usually does. Or use humdidity controlled cabinets.

This isn't usually a problem with prototypes that are hand soldered- it's more of a production problem.

Best regards, Spehro Pefhany

I've had very good luck with Kingbright LEDs, though I haven't used very many reds. I've found they're the most tolerant of the RoHS solder profile, so use them almost exclusively.

Coincidence?

Yes, Kingbright KAA-3528 3.2 x 2.8mm red/green. And it is normally the red that fails.

I think I'm panicing now. The whole process is in a proper ESD save area in accordance with IEC 61340-5-2:2008. And we do monitor to make sure foot straps, wrist straps etc are used.

Thanks.

We had some HP leds from Farnell in a bag with check boxes for all that stuff but none of it filled in.

Nice low tech solution.

The plastic used to package LEDs isn't nearly as good as standard IC packaging epoxy because - surprise! - it has to be transparent. At ROHS temperatures, it may literally melt when being soldered. I like double-moulded LEDs that have a hard, opaque outer shell.

John

SNIP

I would say not a static problem in your case. Our problem was a faulty soldering Iron. We've moved on! I suggest a 60 degree bake for 24/48 hours, then use within 12hrs.....

How is the moisture absorption prevented once they are installed?

I have been using Lumex 1206 size with water soluble flux without any problem for years. I do not go out of my way drying them out after wash. Just blow dry and test the next day.

-- Boris

The problem comes in when the parts are reflowed. Any absorbed water boils and cracks the package, sometimes rather violently but usually unseen. After successful reflow, absorption isn't a problem (rework might also cause problems, however).

How MANY failures are you seeing?

I have a couple reels of 1210-size LEDs, mostly red, and have been storing them in the open for a year or so. I have a pretty conservative reflow profile, ramping up to about 238 C, and have had not a SINGLE failure that I can recall. They are not Kingbright, but I don't recall the brand off the top of my head. I am started on my second reel of them, so i have used at least 3000 so far with no trouble. So, at least this particular brand I am using has not had a moisture absorption problem.

Jon

About 5% failure rate when we have problems.

Lead free temperatures are higher, 260C from memory.

That's rather high for LEDs. The Kinbrights I've used are 250C for 10 seconds, IIRC. Many other manufacturers specify no more than 235C, and still call them RoHS. :-(

But of course. I forgot because I assemble small runs by hand.

-- Boris

Certainly not for LEDs! Many components have a 250 C cap on reflow. I have found I get decent lead-free reflow with an actual temp on the board of about 238 C. I can go a bit lower on the Pb/Sn boards, somewhere around 230 C.

I have a thermocouple that I poke into a through-hole on the board, so I am measuring actual PCB temperature, at least one spot on the board.

Jon

The temperature profiles are pretty well defined. We got crap connections until we got the reflow temperature up to the recommended 250C for 10sec. Of course there is a lot more to a proper reflow profile than the peak temperature, too. LEDs usually have the recommended profile in the datasheet. ...for a reason.

The Kingbright spec for reflowing our LED is 260C max for 10 seconds.