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Mounting LEDs flush to PCB, OK over temperature?

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.

Reply to
markp
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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

-- Deep Friar: a very philosophical monk. Website:

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Reply to
Tim Williams

Bivar

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-- Boris

Reply to
Boris Mohar

I've seen flush mounting done lots of times with no problems. Make sure they're loose in the holes before soldering; LED plastic tends to soften up during soldering, so that's the worst stress situation.

A short spacer wouldn't help much anyhow.

John

Reply to
John Larkin

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

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
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.

Reply to
markp

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