Mounting LEDs flush to PCB, OK over temperature?

[markp]

Hi All,

I've got some 8mm LEDs and would like to mount them flush to the PCB. I'm a
bit concerned about thermal expansion (it's a commercial project but I'm
assuming storage might be down to -20degC to +40 degC). Problem I'm
concerned about is different thermal expansion co-efficients of the PCB
laminate and the metal of the legs introducing thermal stresses in hthe
epoxy shell of the LED, possibly causing it to crack. Anyone had any
experience of this? I can use a spacer, but only a few mm thick, not sure
where to get them though.

Mark.

 

[Tim Williams]

Surface mount LEDs make me think 'duh'. They are smaller though.

I've taken apart plenty of equipment with LEDs and transistors supported by
nylon spacers, but I've never seen them in catalogs.

Tim

 

[Boris Mohar]

Surface mount LEDs make me think 'duh'. They are smaller though.

I've taken apart plenty of equipment with LEDs and transistors supported by
nylon spacers, but I've never seen them in catalogs.

Tim

Bivar

http://www.bivar.com/default.asp?contentID=90

 

[Spehro Pefhany]

Hi All,

I've got some 8mm LEDs and would like to mount them flush to the PCB. I'm a
bit concerned about thermal expansion (it's a commercial project but I'm
assuming storage might be down to -20degC to +40 degC). Problem I'm
concerned about is different thermal expansion co-efficients of the PCB
laminate and the metal of the legs introducing thermal stresses in hthe
epoxy shell of the LED, possibly causing it to crack. Anyone had any
experience of this? I can use a spacer, but only a few mm thick, not sure
where to get them though.

Mark.

Bivar and others make spacers, but I've had much better results by
machining up a jig to hold the LEDs while they are being
hand-soldered. You can get them accurate that way. They'll slop all
over the place in a spacer. You can even get water-soluble spacers
that give the same effect (except for the sloppy LEDs pointing in
every cross-eyed direciton).



Best regards,
Spehro Pefhany

 

[markp]

Bivar and others make spacers, but I've had much better results by
machining up a jig to hold the LEDs while they are being
hand-soldered. You can get them accurate that way. They'll slop all
over the place in a spacer. You can even get water-soluble spacers
that give the same effect (except for the sloppy LEDs pointing in
every cross-eyed direciton).



Best regards,
Spehro Pefhany

I agree, a jig wuld be best. I'd like it to be machine soldered if possible.
I've come up with a posible solution of using an M4 nylon washer as a
spacer, it's only 0.5mm thick but is soft enough to absorb any tension if
thermal expansion is really a problem. What i don't want is to make 1000s of
the things and find some fail after they've been thermally cycled in the
field. I was also wondering whether RTV could be used to hold them in place
although I'd have to make sure it doesn't percolate through the holes.
Actualy that might work, a couple of blobs either side (they are 8mm LEDs
with 0.1" pitch leads) and that might hold them even while machine
soldering.

Mark.