IR transparency behaviour of epoxy and
IR transparency behaviour of epoxy and
(OP)
Hi, we've got a device with an optical transmission line made as follows:
1) an infrared transmission device containing a led surrounded by either Dow Corning Sylgard or Araldite D and a 0.4mm thick PES disk as "external window"
2) a 0.5mm free air path
3) a receiver with the same architecture of the tx
We must do a thermical cycle to validate the product: 3h@-20°C, then 3h@+70°C, all this for 2 times.
What happens is this:
a) without thermical cycle our electronics has a gain X on the optical amplifier
b) with Sylgard we see that after the first -20°C the gain need to be lowered (i.e. the optical attenuation diminish), after +70°C the effect goes on the same direction but is less important and another -20 does almost nothing so we can say that the first cycle tends to improve and stabilize the path loss
c) with Araldite D the effect is more intense and after every step, both -20 and +70, ther is a substantial improvement without stabilization.
We made the cycle on some specimens without filler and saw no variations at all, so it must be the polymer that makes the difference.
Can anyone please suggest an interpretation of this?
1) an infrared transmission device containing a led surrounded by either Dow Corning Sylgard or Araldite D and a 0.4mm thick PES disk as "external window"
2) a 0.5mm free air path
3) a receiver with the same architecture of the tx
We must do a thermical cycle to validate the product: 3h@-20°C, then 3h@+70°C, all this for 2 times.
What happens is this:
a) without thermical cycle our electronics has a gain X on the optical amplifier
b) with Sylgard we see that after the first -20°C the gain need to be lowered (i.e. the optical attenuation diminish), after +70°C the effect goes on the same direction but is less important and another -20 does almost nothing so we can say that the first cycle tends to improve and stabilize the path loss
c) with Araldite D the effect is more intense and after every step, both -20 and +70, ther is a substantial improvement without stabilization.
We made the cycle on some specimens without filler and saw no variations at all, so it must be the polymer that makes the difference.
Can anyone please suggest an interpretation of this?





RE: IR transparency behaviour of epoxy and
1. Cure the epoxy longer
2. Cure the epoxy at higher temperature
3. Use a catalyst to help the cure
4. Choose a more reactive epoxy
RE: IR transparency behaviour of epoxy and
He says that in his last try he removed the PES windows on both tx and rx, so there are just the diodes embedded in the polymer and the air gap between.
RE: IR transparency behaviour of epoxy and
Thanks for the info. I still think that it may well be the post curing of the epoxy stressing the diode and thereby changing the way light passes out.
RE: IR transparency behaviour of epoxy and
RE: IR transparency behaviour of epoxy and
RE: IR transparency behaviour of epoxy and
RE: IR transparency behaviour of epoxy and
What's the CTE match or mismatch? Even if it were perfectly matched, you could still have problems, since the LED body may be sufficiently small enough to self-relieve the compression/expansion stress, relative to the diode portion.
TTFN
RE: IR transparency behaviour of epoxy and
I agree that the variations may well be due to strain on the LED caused by the epoxy.
The fact that the magnitute of the magnitute of the variation lessens with each heating cycle may indicate that the epoxy is post curing.
One idea that could potentially help is to first coat the LED in a layer of curing silicone rubber. Then coat the silicone (after cure) with epoxy. The silicone (or other rubber) should act as a buffer when the epoxy contracts. So the epoxy will squeeze the silicone and not the LED! Another way to achieve the same effect is to add very small rubber particles to the epoxy or choose a lower moldulus epoxy.
Your idea of adding filler was a good one but it didn't work. I think that's because although adding filler reduces shrinkage on cure it also increases the modulus of the epoxy (relative to the unfilled) and thereby increases the force on the LED.