Waterproof RGB LED Strip

Another package from halfway around the planet brought 5 meters of waterproof RGB LED strip, which may be useful for projects like longboard lighting. Not having worked with a waterproof strip before, I snipped off a segment:

Waterproof RGB LED Strip - one segment

Waterproof RGB LED Strip – one segment

The waterproof coating seems to be a soft silicone or acrylic pour with roughly the consistency of Gummy Bear tummy, so cutting it off requires a delicate touch to avoid slicing the flex circuit board:

Waterproof RGB LED Strip - peeled top

Waterproof RGB LED Strip – peeled top

It doesn’t actually bond to the circuit board, though, and if you get a sharp blade underneath, can be peeled away. I suspect this means water will eventually make its way into the circuitry and you shouldn’t expect to submerge the strip in a fish tank. I scraped the contacts clean, which probably isn’t the right way to do it:

Waterproof RGB LED Strip - end view

Waterproof RGB LED Strip – end view

The underside makes no pretension of being waterproof and you can peel / roll / rub the adhesive off the contacts:

Waterproof RGB LED Strip - peeled bottom

Waterproof RGB LED Strip – peeled bottom

Does anyone else doubt the authenticity of that 3M logo? The production values look rather low, but maybe it’s just me.

The trouble with soldering contacts to the bottom is the ensuing lump that prevents good adhesive bonding. The trouble with soldering contacts on the top is the surgery required to remove the coating. You can get punch-through contacts with snake-bite fangs, but even the vendors admit to about a 20% failure rate, which implies it’s pretty much a crapshoot.

  1. #1 by Red County Pete on 2012-11-26 - 12:22

    Rough guess is that the automagic assembly process uses a silver-bearing epoxy. The goop was developed for delicate chips and got used widely over the years. From what little I knew of it, you keep it cold till use and it would be cured at lowish (100 degrees C) temperatures. Not the kind of process I’d want to replicate on a small scale. If you really wanted, the stuff could be supplied in a pneumatic syringe for manual application.

    Watch out for delamination of the silicone when soldering. Guess I shouldn’t tell that story…

    • #2 by Ed on 2012-11-27 - 08:27

      a silver-bearing epoxy

      Not a chance of any silver in this thing!

      Heck, I’d be mildly astonished if the solder were RoHS-qualified as advertised…

      • #3 by Red County Pete on 2012-11-27 - 12:53

        Any silver would be used in the LED package, if they needed good contact to the back of the die (depends on the chip design). I was thinking silver epoxy for a production build using those strips. (What can I say, my former employer used to use gold plated leadframes in IC packages a lot longer than the rest of the industry…)

        • #4 by Ed on 2012-11-27 - 13:56

          in the LED package

          Can’t say, one way or the other. Squinting into the diode packages, the blue / green dies sit flush on their metal conductors and the red die sits on a mound of something atop the conductor, with wire bonds from the top as usual. The solder fillets on the board look gritty, but maybe that’s actual silver solder at work; I’ve done worse myself with 4% silver-tin solder.

          Speaking of QC: the beveled corner of the diode package is opposite of the beveled corner of the silkscreen pattern…
          Waterproof RGB LED Strip - LED detail

  2. #5 by Jetguy on 2012-11-26 - 17:27

    I just went through the same thing on my RGB strips, The hot tip (no pun intended) is you use the tip of the iron to melt away the coating. I’m sure it’s toxic, but gives a far nicer spot to solder to with no risk of cutting the tape. Besides, when you do solder it anyway, you burn some of the coating so it’s not like it’s that big of a deal. Trust me, this is the best way.

    • #6 by Ed on 2012-11-27 - 08:25

      use the tip of the iron to melt away the coating

      Between that and pulling the gunk off in chunks, I ought to get reasonable results… thanks!

  3. #7 by Jonathan Katz on 2012-11-26 - 21:15

    I’ve worked through perhaps 500m of similar strips and had to do a huge number of these joints. I’ve tried just about all the ways you could do this and have come up with something I use pretty consistently now. You cut with an xacto perpendicular to the board surface and the long axis of the lights right about the edge of the led nearest the solder joint. Using a slicing motion its not terribly hard to cut down close to the flexible board without touching it. Then you can essentially use friction to pull the adhesive off in one chunk which seems to always separate it cleanly from the board and its contacts. I just use RMA 63/37 to tin the contacts not dwelling longer than necessary. Definitely tin the wire too and use 22awg or smaller diameter wire. Strain relief is really needed for that joint, I use a very rubbery hot melt glue to fill in where the glue was cut off and bridge out to the wires and then cover it with heat-shrink.

    BTW its almost certainly not a silicone unless they advertise it as that, you generally have to pay a premium of around $10 per 5m roll to get the silicone type. The vast majority describe it as either glue drip or epoxy drip. Whatever it is its generally pretty crappy stuff that will off gas a lot and then yellow significantly in a 6-24month time frame. UV exposure will really wreck the stuff also so definitely go for the silicone if it will be in the sun any real amount.

    • #8 by Ed on 2012-11-27 - 08:23

      use friction to pull the adhesive off in one chunk

      That sounds about right! I failed miserably while trying to remove smaller pieces, because they were so tenacious; a larger block pulling a smaller surface should work fine.

      I have some acrylic caulk that may work for strain relief, if it hasn’t already cured in the tube.

      almost certainly not a silicone unless they advertise it as that

      And sometimes not even then, I suppose. [sigh]

      This glop looks to have been applied from a nozzle on the fly, with occasional slobbers over the edge and around to the adhesive cover strip. I’d call it Good Enough for prototyping and playing around, not much more than that.

      Thanks for the heads-up on sunlight exposure!

  4. #9 by Red County Pete on 2012-11-28 - 14:55

    That red LED looks like it got the conductive epoxy attach, while the others got some kind of solder or (dunno the metalurgy) eutectic die attach. (One of the joys of LED stuff-to change the color you change the composition of the diode and all its characteristics.) Love the mismatch between the silkscreen and the package. Do the strip terminals do what they say, or are they AFUed?

    • #10 by Ed on 2012-11-28 - 15:04

      Do the strip terminals do what they say

      The user interface runs exactly as specified… [grin]

      Given 12.0 V, the blue diode runs at 14 mA and the others at 17 mA, but I think that’s within spitting distance of the right numbers. They’re dealing with the small difference of big numbers across those ballast resistors and the diodes probably don’t come from the top-spec sorting bin.

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