Wednesday, August 14, 2013

What not to do while reflow soldering

I had bought a few small Cree LEDs a while back, with the eventual intention of turning them into bike headlights (and also learning a bit more about LEDs). Unfortunately, they were surface mount, and with a thermal pad on the bottom of the package, solder paste and reflow soldering was pretty much mandatory.

So I etched some breakout boards on double-sided FR4 (sorry for somewhat blurry photos):

I added lead-free solder paste (which was all that was available) and mountedthe LEDs with a pair of tweezers.

Right into MITERS' sketchy reflow toaster oven it went, with a 2-minute preheat at 150° C and a final temperature of 260° C or so (as recommended by the datasheet). I left the dial at 260° C, and left briefly to take a call with the oven on.

I came back 4 minutes to the smell of FR4 fumes (which has this odd, slightly pleasant smell). I shut the oven off, waited for things to cool down, and this was the result (again, sorry for blur)
Baked to a crisp.


The fiberglass is noticeably darkened to a really dark brown, and the copper oxidized into this iridescent gold color (which admittedly, looks pretty cool). Fortunately, the LEDs worked just perfectly. The fumes though, were probably toxic (your usual FR4 contains quite a lot of halogens).

So yeah, lessons from this:
1. Don't leave things unattended. There probably would've been a lot less fumes and blacked boards if I wasn't an idiot and sat next to the oven the whole time and caught it earlier. 
2. Toaster ovens probably have really crappy temperature regulation. I set the dial of the oven to 260 degrees C (as recommended by the Cree datasheet for lead-free soldering). Considering the board blackened and the copper oxidized, it was probably significantly higher than that. Definitely don't trust the dial.
3. FR4 boards probably don't like high-temperatures. Just because you can solder it fine at 400 degrees C probably doesn't mean it can take several minutes of 260 C without giving off fumes. Reflow soldering may require special FR4 that can handle higher temperatures. That, or use leaded solder paste.

I'll need to mount the LEDs to a heatsink, and also build a driver circuit for them. I intend to run it off a single li-ion cell, so it's probably going to be some buck-boost switching affair.