Sometimes you need a longer strip of LEDs than you have and you need to connect more than one strip end-to-end. Or, perhaps one of your pixels has broken or gone bad. If one pixel breaks, all the pixels "downstream" will stop working, so it's great to know how to trim out a pixel and replace it.
How Can I Tell if a Pixel is Bad?
If your strip only lights up to a certain point but then the rest of the strip is dark, the first thing to check is your code. The most common reason for only having half a strip light up is because your code is only looking for 30 pixels when you actually have 50. So check the code first before you start cutting up your strip!
If your code is fine and you still have a strip that either goes completely dark or starts showing inaccurate colors or flickering after a certain point, you likely have a bad pixel or damaged strip. This happens in costumes a lot, especially at bend-points. These LED strips are really flexible in one direction but very delicate if you try to bend them sideways. Luckily it's not all that hard to repair.
Step 1: Prepare Your Strip
If you've got a bad pixel, use your flush cutters to carefully trim it out of the strip. You don't need to cut through the center of the copper pads - you can leave almost the entire pad on either side of the pixel in place. This will make it much easier to reattach the pixels.
Be sure your copper pads are clean and solder-free. If you're attaching additional strip, double check and make sure you've got the "in" end -- the arrows should be pointing away from the microcontroller end. Trim the silicone casing back so you have plenty of room to work. Take a deep breath.
Step 2: Heat Shrink
Slide on a piece of clear 3/4" heat shrink tubing that's about an inch long. We'll use this to seal up and reinforce our joint when we're done testing it.
Line up the strips so the copper pads are overlapping each other and are perfectly straight. I like to tape them down to my work surface so they don't move around.
If needed, trim the strips carefully with your flush cutters until the alignment is really perfect and all three pads are connecting with their mates.
Step 3: Solder
Heat up your soldering iron to 750 degrees (or for 3-5 minutes, if yours doesn't have a temperature gauge). Make sure the tip is nice and clean and hot.
Touch the soldering iron to the middle copper pad on the upper strip and wait a few seconds until it gets nice and hot. Touch your solder to this pad as well and let it flow, as though you were tinning the pad for a wire connection. Be a little more generous with the solder. You're looking for a nice little solder dome here.
Remove the solder and soldering iron and let the solder dome cool. Take another deep breath. Then, touch your soldering iron to the exposed copper pad on the lower strip. Get it nice and hot.
Then, reach over with your soldering iron to the solder dome. Use the tip of your iron to push the solder blob over so that it spans both strips, making a good solid connection between the strips. You may need to add a little more solder. Take your time, this isn't as easy as it looks.
Repeat the process with the two outer pads.
Before you go any further, connect your project to power and test to see if it all works.
Step 4: Seal it Up
Once you're sure the joint is rock-solid, slide your clear heat shrink over the window you cut in your silicone sleeves. Slip the nozzle on your hot glue gun in under the heat shrink and squirt a little glue in there, then use a heat gun to shrink your heat shrink in place. This will make a stiff spot in your strip, but will keep your solder joint connected since it's all encased in plastic and won't break again.