Got very good answers so far, including Guy's unsuported holes. Thanks a lot.

Why are the LEDs failing... according to the lab, there is a trend in the industry, they have right now 3 customers with the same problem. The failure mode is not the bonding wire breaking, not the bondings detaching, but the semiconductor die detaching from the joint that bonds it to one of the leads.
The culprit is most likely the CTE mismatch between the casing (epoxy) and the leadframe (steel), the CTE of the epoxy is 5x bigger than the one of steel. This seems to be consistent with the thermal processing limitations recommended by the manufacturer.

I guess we are having just another component that works well, but it is not solderable ;-)

Best regards,

Ioan

-----Message d'origine-----
De : Rex Waygood [mailto:[log in to unmask]] 
Envoyé : 20 novembre 2008 03:38
À : TechNet E-Mail Forum; Ioan Tempea
Objet : RE: [TN] Low temp solder alloy

It strikes me that whoever wrote the hand soldering spec either doesn't
understand hand soldering or is doing a CYA. 
In doing the rework I would ignore the 35W 'command' as that will just
get you into difficulties. 280C from a 90W iron is the same 280C as from
a 35W iron but my guess is time for soldering with a 90W iron will be a
lot less than a 35W iron and therefore potentially less damaging. If
Cree are making assumptions about thermal impedance of a 35W vs a larger
iron they are in dangerous territory (35W laser?). I would preheat the
board assembly prior to reworking with the iron to the maximum you think
you can get away with.
If you wanted to be really clever and enthusiastic you could calibrate
the 'Vbe' of the led vs temp and monitor that during a rework to
characterise your process. Making connection would require a bit of
inventiveness! :-)

More of interest, why are the leds failing?
Rex


Rex Waygood
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