Neopixel LEDs and break out boards Moderators: adafruit_support_bill, adafruit

[astrayelmgod]

Hi --

I have a project requiring a large number of RGB LEDs, and would like to use your Neopixel 5050 surface mount devices with the break out boards. I have a sample of the LEDs and boards here, and have found that it takes an expert to hand solder without making a mess. I know that you recommend using your solder paste, and that brings me to the question. Assuming that I use the solder paste and a convection oven to solder the LEDs, how, and more to the point, when, do I solder the lead wires to the break out board? If I solder the wires after the LEDs are mounted, how do I avoid melting the solder on the LEDs? If I solder the wires before mounting the LEDs, how do I avoid melting those joints and having them fall apart? Would using lead-free, high temperature solder for the wires and putting them on before the LEDs work?

I should probably mention that most of the work will be done by high school students whose soldering skills are sketchy at best.

[adafruit_support_mike]

I have a sample of the LEDs and boards here, and have found that it takes an expert to hand solder without making a mess.

It isn't that bad once you've had some practice. The main trick is using the proper technique.

Instead of trying to solder all four pads at the same time, start by melting a small blob of solder onto one of the PCB pads. Then you can re-melt the solder on that pad as you place the pixel with tweezers in your other hand.

Once you have that first joint, you can reheat the solder and nudge the pixel around with a toothpick if it's out of alignment.

Once the pixel is where you want it, let that joint cool and solder the others. In the SMD world, the dominant physical force is the surface tension of the molten solder. The parts and pads are designed to form good joints automatically as long as you get well-fluxed solder near them.

Using plenty of flux is important though. It keeps the solder nice and fluid so it can flow easily and bond to the metal surfaces. Solder that's exposed to the air oxidizes, and the oxide turns into a powder that mixes in with the molten metal. You can tell flux has seen too much air when it starts to behave more like a paste than a liquid. Flux contains organic acids that are mostly inert at room temperature, but become aggressive enough to dissolve the oxide at soldering temperatures. They also form a layer that floats on the molten metal, isolating it from the air and preventing further oxidation. That layer burns off in a few seconds though, so you have to replenish the layer if you want to keep working the solder.

As a side note, DO NOT use plumber's flux for electronics. It contains zinc chloride, which decomposes to hydrochloric acid when heated. It's great for cutting through thick layers of corrosion, but will continue to attack any nearby metal after the solder cools. Worse yet, it never really goes away, but is highly hygroscopic. It can mix into humidity in the air and redeposit itself on any nearby surfaces. Once you have it, it's a slowly-expanding zone of corrosion that can last for years.

Assuming that I use the solder paste and a convection oven to solder the LEDs

That's also a valid option. You can also get good results with a piece of aluminum plate on a hotplate. The biggest issue with reflow is to monitor and control the temperature.

how, and more to the point, when, do I solder the lead wires to the break out board?

Solder the connecting wires after you've soldered the pixels. Even if you have to re-melt the joint for one of the pins to connect a wire, the other three unmelted joints will hold the pixel in place.

If I solder the wires after the LEDs are mounted, how do I avoid melting the solder on the LEDs?

That shouldn't be an issue if you're using our 5050 breakout:

https://www.adafruit.com/product/1762

The trace between the SMD pad and the hole for a wire acts as a thermal barrier. No matter how much solder you have at the hole, the SMD pad only sees heat coming from a small piece of copper foil. That doesn't mean you get complete isolation.. keep the pad at the hole hot long enough and the solder on the SMD pad will eventually melt.. but you have plenty of working time before that happens.

Would using lead-free, high temperature solder for the wires and putting them on before the LEDs work?

Technically yes, but don't try it. Working with different melt temperatures is a a moderately advanced skill. You have to develop a feel for all the different solders and learn how you can push their working ranges.

[astrayelmgod]

Thanks for the thorough reply. Maybe I’m just overthinking it. Wouldn’t be the first time. I’ll let you know how it goes.