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How to (properly) glue a SMD leds to (slightly flexible) plastic? Answered

Hi people or robots,

For a project of mine I need to glue SMD leds to a transparant piece of plastic. the LEDs need to be glued face down on the plastic so the soldering contacts are facing up. I've already tried super glue (CA glue) but this didn't hold, the LEDs would pop of when the plastic was flexed a bit.
What would you (the awesome instructables community) recommend?
I've ordered some two part clear epoxy to give that a try, would that work?


thanks for all the replies!

I'll give clear silicone a try. Maybe I'll also try to chemically weld a LED, hope this won't damage it.

Loca glue also sounds interesting but after soldering I won't be able to paint it. So unfortunately this won't be an option.

How can I tell how hard the epoxy will be?

I've done some tests and (cheap Chinese) epoxy is the winner.

Acetone is hard to get right, it makes the plastic foggy and I didn't get a good bond

Silicone looked good but didn't bind well to the LED and plastic

The cheap Chinese epoxy bonded well and remained slightly flexible. Only thing I encountered was that it took longer that the advirtised 20 minutes to harden.

The best "solution" for this I can offer comes from the mobile world.
For prototyping we use Loca Glue", the stuff used to repair displays.
It is thin like warm honey and cures under UV or direct sunlight.
To use it for your purpose:
Place a drop of Loca Glue on some plastic and use tweezers to dip you SMD part in the glue - only dip so much that you get glue on the surface you want to glu, do not submerge.
Now place your part where you need it and continue.
If the glue is very fluid you can us a little UV lamp to set the glue a bit before curing all when done.
Please make sure to add a layer of paint when you are done soldering as the loca glue remains sticky and will otherwise collect dust and dirt.

Interesting. What's it like when it sets then ? Vaguely rubbery ?

If used like that it is best to press the parts onto the surface as the glue stays like rubber or silicone when cured.
But that is also the good part as it allows for massive flex without letting go of surface or parts.
Wherever superglue fails or is not viable Loca Glue is my next choice unless I can substitute with silicone - which I can not recommend for electronics.

Plastic is made to repel things from sticking to it most of the time you can only get plastic to stick to plastic by welding in most cases, and the LEDs are plastic.

Sand the plastic with 600 grit paper then use a clear flexible epoxy the softer the better, you might even use clear silicone.

If the epoxy is stiff or hard the LEDs and other things will just pop off when you flex the plastic.