DIY SMT Reflow: Toaster Oven, Skillet or?

Question

I've read quite a few write ups on converting toaster ovens or skillets for reflowing SMT components. The important parts appeared to be:

• Attempting to match a reflow profile (temperature pattern to ramp up and then down that gets hot enough to reflow but not hot enough to damage components/PCB)
• To make applying paste easier to specific areas: use a template (laser cut appears to be popular)

SparkFun even offers a kit to control an oven or skillet but apparently due to liability concerns, the kit uses a 12v relay instead of a 110/120v or 220v relay (adapting it to full voltage is left up to the DIYer).

What is your personal experience with this? It would be great to hear actual experience on what works and what to avoid.

Answer 1

From a hobbyist perspective:

I just picked up a toaster oven with a convection bake mode, no mods at all. The convection bake mode is important, as it more evenly distributes the temperature in the oven which prevents hot spots from frying components or cold spots from forming cold solder joints. I have used older toaster ovens, and they work just fine for hobbyist work, but if you're going to pick up a new one then spend the extra $10 or so to get convection.

I have successfully reflowed many boards with different ICs and have never had an issue. I can't remember the specifics, but I usually give it roughly 90 seconds to come to a "warm-up" temperature, then jack it up to my final baking temperature (I think it spends about a minute or two in that phase). Check your datasheets to make sure your components can handle whatever temperature your solder paste melts at, and for how long. I pretty much eyeball it when I first do a board of a given type to see when different parts reflow based on the amount of solder paste I use, but it's all been pretty similar in my smallish projects.

As far as templates/stencils go, I don't bother unless I have lots of fine-pitched ICs. Get yourself a solder paste kit with a syringe and different tips, and play with those (I used this one from Celeritous: http://www.celeritous.com/estore/index.php?main_page=product_info&products_id=47 there's also a lead-free version). All you need is a sufficient blob of solder paste, surface tension really does 90% of the work if you don't put too much on. I figured out "how much" through trial and error on a few spare boards.

I think if you're not trying to sell stuff and you're not working with sensitive components, no mods are required. In my experience, components are pretty darn robust all things considered, and nothing beats the learning process of watching solder paste reflow on some old boards (and SMT components, if you have some to spare). You'll know when you really need to address the issues you brought up.

Answer 2

I've found a cheap hotplate to be good:

• The Sparkfun guys said that even with a cheap commercial reflow oven, some of their components with plastic housings (a USB type B connector from memory) would melt. This problem was fixed by using a skillet.
• Skillets tend not to be very flat and change shape in the heat. I've found the hot plate to be slightly better here.

As others have mentioned, most components are quite durable and controlling the temperature profile is not critical for hobbyist work, however be sure to raise the heat slowly at first and keep and eye on the paste. As the board is being heated from one side, it may bow upwards and you'll need to rock it to ensure even heat across the board.

Applying the solder paste by hand works fine for small batches and one offs, but a stencil does drastically speed up the process when you're doing a few boards with fine pitch components. Laser cut mylar (plastic) stencils are cheap and effective - I'd also suggest getting a stencil with a few common SMT footprints for microcontrollers etc (32 & 48 pin QFN; BGA; SOIC) as you often have one of these components on a board that you can stencil and then do the rest by hand.

Answer 3

I have used the skillet method at a friends house. It works very well. He made a stencil by etching thin brass using the same process he uses to make PCBs. We spread the paste on the PCB using the stencil. I placed the parts on the PCB. After the parts are placed I carefully pushed the PCB+solder+parts onto the preheated skillet using a dowel. When the solder started to melt I pushed the PCB off the skillet. It is important to have the infeed area and outfeed area at the same height as the skillet.

A negative of the skillet is that you can only solder components on one-side. With a toaster oven you could do both sides.

A big positive of the skillet is that you are heating through the PCB. The component body will be at a lot lower temperature than in the oven method.

There is a fellow I now that laser cuts Kapton stencils. $25 for an 8.5 x 11 stencil. Some PCB manufacturer will also sell you a stencil.

Answer 4

I have used the skillet method... without a skillet.

I set the burner (electric stovetop) to low and place a 1/4" thick, 6"x6"aluminum slab with the boards on it directly on the burner. After about 2 minutes to get everything to warm up evenly, I crank up the heat to high. When the solderpaste starts to flow, I shut off the heat and slide the slab off. There's enough residual heat to reflow everything and being aluminum, it heats up and cools down evenly.

I've done this a few times and it works much better than hand soldering. Eventually I could get a skillet, I guess. But I also have an old toaster oven I will try out one of these days.

Answer 5

I've done dozens of proto and production boards in a manually controlled toaster oven with no problems. for 0.5mm QFPs I tend to carefully line it up and then tack a few pins down. Afterwards there may be the odd short but that's mostly down to the paste stencilling not the reflow and easily touched up afterwards. Even if I didn't have a stencil, if there's more than a handful of parts I'd always manually paste, place and reflow in preference to hand soldering - it's quicker and gives more consistent results.

Answer 6

I use a small toaster oven modded with temperature (profile) control. Seems to work nice so far. Smallest parts on these boards are FT232RL chips. A typical run is 10 boards. Doing it by hand would be perfectly possible, but would take much more time. A simple (plastic) solder paste sheat saves a lot of time compared to hand-applying solder paste.

Answer 7

OMG!

Look, if it works for you, great, but I wouldn't touch this kind of approach with a 10' (3m ;-) pole. Stick to hand-soldering.

I guess the exception to this is BGA parts or other devices with pads under the chip. For power devices with a single pad, some sort of solder paste + heat gun might work, I guess. For a BGA I wouldn't bother, as the quality control issues involved with BGAs are out of range of DIY techniques in my opinion.

Answer 8

Pay a short-run assembly company to do it for you. Much more reliabile, especially with small parts where it just isn't worth the hassle, unless your time is free. For example Advanced Assembly and Screaming Circuits. Disclaimer: I haven't used them, nor know anyone who has.