kmod's blog

11Sep/130

Toaster oven reflow soldering: first tests

I recently splurged and bought this $30 Target toaster oven ($36 online) to try my hand at reflow soldering.  I'd be holding off on this since most of the articles I saw online talked about hacking into the mains-level toaster oven circuitry to add custom temperature controllers -- didn't seem worth it.  But then I found this guy who seems pretty serious, who blogged about how he is able to reflow BGA's with an uncalibrated toaster oven.  Also, I heard about OSH Stencils, which offers solder paste stencils for the low price of $0.60 per square inch (most other places I've seen are an order of magnitude more expensive).  Newly-encouraged, I picked up a toaster from Target and started some experimenting.

My first test was with an old board that I had sent to Seeed Studio to try them out; it all went well, but they sent me 10 boards so I have a bunch of spares.  At some point in the past I had bought this solder paste in anticipation of wanting to do this at some point; this is a 0.5cc syringe, which is much smaller than the 5 or 10cc syringes that seem standard, but it was only $3.  Here's the board with the paste applied:

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And a closeup:

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Clearly, I didn't do a very good job of this.  I didn't want to be too OCD at this point because I wasn't sure how the rest of the process would turn out, so I just forged ahead and placed the components:

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Here are two pictures of the pcb inside the toaster; you can see that the paste has changed from a pastey grey to a milky white, which I think is due to the flux liquefying:

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And then the solder went through another change, which looks like the solder melting:

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I tried to copy the reflow profile from the blog post I mentioned earlier, but I found that my toaster did not heat the pcb as much as his apparently did.  One thing that might have been affecting this is that I had left the "drip tray" in the toaster, which probably blocked a large amount of the heat that came out of the bottom heating element.  In fact, the solder didn't melt at all until I had turned up the heat to "broil" setting and the top heating element came on.

Luckily, it seems like the solder is quite forgiving, and even though I left the board in the toaster for far longer than the recommended profile, nothing terrible seemed to happen.  I haven't checked the components, though; I'm going to have to test on a board that is more easily testable.

I went pretty quickly on the paste application + part placement, since I wanted to get through the first iteration quickly, so I'm actually pretty happy with the first results despite how they look:

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Yeah, some of the parts are completely not on their pads, but I feel like this is definitely a demonstration of feasibility, though there's a lot of tuning to do.

Second test

My next test was on the next iteration of this board, which is several times smaller, and has components on both sides.  Once I pulled out this board, I realized there's no way that people use these syringes to actually apply the solder:

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The nozzle is several times wider than the pads!  Apparently you're supposed to put a needle tip on the top... live and learn.  Anyway, the bare syringe actually worked for the 1206 capacitor pads, but for the 0603 leds and resistors I'd been using a hobby knife to apply small amounts at a time.  But that quickly broke down when I attempted the TSSOP parts, and I resorted to applying paste across all the pins and hoping for the best:

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Surprisingly, some of the pins do in fact have their own solder connections, though there are clearly a lot of bridges:

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I definitely used way too much solder, but it does seem like the process is fairly robust to these kinds of issues.

Third test

The last test was to reflow some of the components on the top side of the board, and see what happened to the bottom.  I was also interested in tweaking the reflow profile.  Here's what I put into the toaster (I used the bottom board as a raft since the top board was only barely larger than the grill spacing:

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You can see that I was pretty rushed about this too; the top pad, second from the right, definitely has too little paste on it, and the resistor ended up not sticking.  That wasn't my main focus though; from this test I learned that 1) I really do need to turn on the broiler setting to get the solder to melt, and 2) the bottom components seem fine even after the second reflow.  There was a fair amount of fumes when I opened up the toaster oven -- I'm hoping it was just the flux.  I was actually surprised how much the whole process smelled; I use a fume extractor+fan combo which means that I get almost none of the fumes when I hand solder.

Going forward

I'm pretty happy overall with the accessibility of this method: I spent about $40 and was able to get some proof-of-concepts despite rushing the whole process.  The downside, though, is that it was definitely more work and time than hand-soldering these components would have been, especially for the 2-terminal passives.  There are a couple things I want to do to continue experimenting:

  • Order some stencils from OSH Stencils, which should hopefully increase the speed of the method
  • Get a syringe tip so I actually have a chance of applying the paste by hand
  • Create some process test boards that let me test the process more quickly but with less labor
  • Once I do that, reflow a real board and test to make sure it actually works and I didn't damage the components
  • Get some DFN/QFN/BGA parts, and see if I can start using those

But again, I feel like I can report that this method is feasible and not just in "some guy on the internet did it once" territory.

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