Where to find a dense-packed, narrow LED strip?

[AaronD]

A few years ago, I replaced the fluorescent backlight in an old LCD monitor, with an RGB LED strip.  I went as densely-packed as I could find - 144/m - and that turned out to be necessary.  I ended up with an individually addressable strip, though I don't really need that, simply because it's all I could find in 7mm wide or less with that density.  (the original tray for the fluorescent tubes is 7mm)

That dense packing is necessary because I tried dithering every other one off or 1 LSb (effectively half the density, or 72/m) to reduce the overall intensity even more in low ambient light, and that allowed me to see the individual diodes that were still on at the edges of the screen.  So it does need to be 144/m or thereabout.

Now, one of the "pixels" has developed a problem of not passing the data through correctly to the rest of the string.  So the rest of the string is WAY TOO BRIGHT and completely the wrong color, which washes out what the other 3/4 or so are still doing correctly.  For example, dithering 1/2-LSb as above, red only, and alternating pixels for each refresh so I don't see the individual diodes, now results in a bright strobing blue!


So I need to replace them now.  Still 144/m or so, in 7mm wide or less.  And I think I need to have a "dumb" strip this time, that is all the same color with each component bussed directly to the outside.  That'll allow me to go even dimmer still while keeping a similar max level, perhaps by making it physically non-linear, or by adding a master channel, or whatever.  (or maybe 10-bit PWM is enough, compared to the 8-bit resolution that I have now?)

Anyway, in all of my searching so far, I've come across lots of strips that are *either* ~144/m *or* <=7mm wide, but rarely anything with both.  And the few that are, are all individually addressed.  Does anyone know of a "dumb bussed RGB strip" with that density and width?

[engineer_in_shorts]

Since this is replacing a flori tube I assume you do not need it flexible. Therefore why not make your own using a standard thin PCB. (e.g) jlpcb do 0.4mm thick PCB, 350x7mm for $50. Then you can choose your own LEDs and design for exact fit. You could sell the excess PCB as a kit?

[AaronD]

Hmm...now there's an idea.  I hadn't thought about that.  And you're right about not being flexible.

I do have the CAD tools, and a toaster oven with a reflow controller on it, and there are prototype fab shops that'll do the assembly too.  If it turns out that such a thing doesn't exist on a mass-production economy-of-scale, I might have to do that.

[Ian.M]

If your strip isn't encapsulated, why not simply replace the bad LED?  Scope DOUT of the one before it to be certain which one is bad.

[engineer_in_shorts]

How long does it have to be? If 1.6mm standard PCB fits it gets cheaper.  7mm would allow two rows of 0805 LEDs on say, 2mm pitch spacing in between you could pack a lot in and still be hand solderable.

[AaronD]

If your strip isn't encapsulated, why not simply replace the bad LED?  Scope DOUT of the one before it to be certain which one is bad.

It's not encapsulated, but I suspect a thermal problem too, so I want to either fix that while I'm in there or at least make something that can tolerate a few thermal failures without becoming unusable.  If I've cooked one, then a few others are probably close as well.

As it is now, I have a flexible self-adhesive strip on double-sided foam tape, both to be a guaranteed electrical insulator between the metal tray that it's in and the occasional factory-extension solder joint on the back, and to actually have it stick at all!  My experience with non-foam self-adhesive is pretty depressing.
I'll probably have to dig that foam off no matter what I do.

Some prototype fab shops offer an aluminum substrate instead of FR4.  I wonder if that's worth considering?  It would probably have excellent heat transfer from the diodes to the back of the PCB, but how well can I connect that to the thin sheet steel of the monitor?  And can I guarantee electrical insulation around both edges of the PCB (or between the diodes and the substrate) without killing the thermal performance?

How long does it have to be? If 1.6mm standard PCB fits it gets cheaper.  7mm would allow two rows of 0805 LEDs on say, 2mm pitch spacing in between you could pack a lot in and still be hand solderable.

455mm (almost 18 inches)