Hello,

Maybe this past experience could be helpful (if your supplier is as bad as
ours!):

We encountered once this kind of failure and found that the supplier had
"forgotten" to mark the reels as moisture sensitive (level 4) and do not put
components into convenient dry pack. 

The moisture cause failures after reflow that was difficult to detect
because it was only intermittently. The LED were likely to turn off when we
bended the board, on the contrary they turns on again when we pushed on the
top of the case (I assume it was cracks on the bonding or something
similar). 

Unless, the potting compound of the LED is sensitive to the solvent from the
conformal coating (leading to a mechanical stress inside the component) I
don’t think the conformal coating is likely to be the root cause.

Regards,

Cedric



-----Message d'origine-----
De : TechNet [mailto:[log in to unmask]] De la part de Lon Weffers
Envoyé : jeudi 15 juillet 2010 16:09
À : [log in to unmask]
Objet : [TN] FW: Failing leds due to conformal coating?

Hi,
 
For one of our customers we produced a board containing green smt leds on a
lead-free process.
The production took place last December.
Our customer has tested the boards on its functionality twice; before and
after conformal coating.
No errors were reported. (also december)
Since a week, several leds seem to be defect. While testing, several leds
don't lighten, if you push a little bit it lightens.
The soldering is oke.
 
My question is: can the led fail due to a conformal coating? And can it
appear after 3 or more months? Has someone experienced the same problems?
 
Regards,
 
Lon Weffers
Matas Electronics bv


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