Reflow Profile for 5050 RGB SK6812 LEDs

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caponsi
 
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Reflow Profile for 5050 RGB SK6812 LEDs

Post by caponsi »

Hi,

My team is attempting to assemble a PCB using the SK6812 RGB LEDs supplied by Adafruit, and we were wondering if there are any special considerations with this component when it comes to reflow soldering. Is there any more specific information about the temperatures these LEDs can withstand? I was not able to find much on the SK6812 datasheet, nor could I confirm that these components are RoHS compliant. We will be using leaded solder paste, so we will be staying on the lower range of temperatures for reflow. Should a standard protocol suffice?

For context: this is for our senior capstone project, and none of us are electrical engineers. We want to make sure we learn everything we can about reflow procedures and the components we are using before we proceed. Any advice would be appreciated. Thank you!

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adafruit_support_mike
 
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Re: Reflow Profile for 5050 RGB SK6812 LEDs

Post by adafruit_support_mike »

caponsi wrote:nor could I confirm that these components are RoHS compliant
They probably aren't. Many LEDs are made from gallium arsenide, which automatically fails RoHS because of the arsenic content.
caponsi wrote:We will be using leaded solder paste, so we will be staying on the lower range of temperatures for reflow
That would also be an automatic fail for RoHS.
caponsi wrote:For context: this is for our senior capstone project, and none of us are electrical engineers. We want to make sure we learn everything we can about reflow procedures and the components we are using before we proceed.
Reflow isn't really part of electronics.. it's more metallurgy and materials science. The deep parts of the subject won't be an issue as long as you use a standard commercial solder. The people who made the solder have already done the hard stuff.

For what you're doing, the main thing you'll need to understand is the reflow curve:

The first stage is to preheat and dry the parts. Different materials expand by different amounts in response to heat, and can pull themselves apart if they heat and cool too quickly. Many plastics also absorb water from the air, which will turn into steam when heated above 100C. If the steam can't escape quickly enough, the plastic can crack or explode.

Parts that can absorb water are shipped in sealed bags with moisture-indicating cards and desiccant packs. Parts that need serious drying should be kept in a 90C oven for 24 hours, but even normal parts need a short drying period before being heated above 100C. Allow 5 minutes as an initial guess, then extend the time if you see any problems with parts coming out damaged.

The next stage is a steady ramp to a few degrees below the temperature where the solder melts. The standard heating rate is 2C per second. If you heat more slowly, the flux can start to spread out across the PCB surface and spoil joints. If you heat more quickly, the volatile solvents in the flux can get trapped and create bubbles inside the solder joints. You want to find the ramp where the solder and flux stay on the pads, and the joints are clean and smooth.

Then you want another dwell period to make sure all the solder is up to temperature. If the dwell is too short the solder on the pads will melt at different times, which can lead to problems like tombstoning: the solder at one end of a resistor melts and pulls the other end up into the air before the solder at the other end melts. If the dwell time is too long, too much of the flux will burn off and you'll start to see oxidation on the joints.

The actual reflow is a sharp rise in temperature.. usually 20C to 30C above the melting point of the solder, and lasting 30 seconds to 90 seconds. The sudden increase in heat will make all the solder melt at the same time, and you want it to stay liquid long enough to form good bonds with the pins and the pads. You want the shortest peak that yields 100% melted solder and good joints.

The last phase is cooldown. You want the solder to resolidify quickly to get fine grain structure. Solder that cools too slowly can form large crystals called 'cold joints', but cooling too quickly below the solder's melting point can cause thermal shock that damages the parts. If you're using lead-based solder try to get a 61% lead, 39%-tin 'eutectic' alloy.

A eutectic is a liquid solution that freezes into two different solids at the same temperature. In non-eutectic alloys one metal will start to crystallize while the other is still fully molten, creating the metallic equivalent of slush. Eutectic solder has the best cooldown behavior, and will save you a lot of fussing around with the reflow-cooldown timing.


All of those conditions are constrained by the reflow oven you use, the size and locations of the heating elements, the size of your circuit board, the size and locations of your components, etc. The profile that works on one line can be a disaster on different machines. Reflow is a mass-production process, so it's standard operating procedure to do test runs to tune the profile for each kind of board you send through.

Operators who spend a lot of time running different boards through a reflow line will learn to estimate what parameters are most likely to work for a new board, but it still takes a few test runs to get production-ready.

You won't have that luxury, so accept the fact that you'll need to do test runs before making the final boards. Start with pad-per-hole board to get your basic melting and cooling parameters, then add resistors to make sure joints at both ends form at the same time. A reel of 0805 resistors only costs $5 or so, so you get lots of practice without spending much. You should only need to do one or two test runs with parts like the SK6812s before you're comfortable doing the final boards.

Since you're making a small batch you'll also have the option to rework any boards that don't come out perfectly. In many cases, all it takes is a few seconds with a hot air gun to fix a badly reflowed joint. That's unacceptably expensive for a production house, but okay for one-offs and short runs.

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