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November/December 2018

Reflow Oven Opinions Needed

Anybody doing SMT reflow soldering at home? Trying to make the switch from TH to SMT. I was wondering what homebrew oven setups people are using and any tips and advice for a newbie.

Joey Dampier
Reston, VA

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Yes, I do SMT soldering using an electric skillet. Search the web for "solder skillet," there is a good write-up from the folks at SparkFun. Five to six minutes at 400 degrees Farenheit works great. Parchment paper under the board provides a way to lift it in and out of the skillet. Also, reasonably-priced hot air rework stations are now available, check CircuitSpecialists.

When I first started SMT, solder stencils were very expensive so I used the individual component stencils from Chip Quik. Some PCB houses now can provide stainless steel stencils along with your board for ~$30. For closely placed components you'll want a custom stencil.

Kerry Imming
Rochester, MN

A toaster oven can be used for reflow soldering work. I have one where one of the coils is broken, but the remaining coil gets it hot enough to melt the solder. I use an oven thermometer to monitor the temperature and approximately follow the recommended temperature/time profile.

Edward Alciatore
Beaumont, TX

I went down the path that you are, Joey. I was fortunate to have found SparkFun Electronics and even more fortunate that they had a kit with code ready to go. They’ve since discontinued that product, but there is a schematic and the code still on that site. I could simply regurgitate what is there, or not plagiarize by directing you there.

One thing that merits repeating is that you will need a toaster oven with at least 1500 watts (I used an Oster convection toaster oven) but 1800 watts would be better, especially if you’re going to use lead-free. That having been said, if I had this to do all over again, I’d not even use the timing aspect of this and simply experiment with how long it takes to melt the solder because it’is nearly impossible to replicate the Kester profile (that is published and out there if you look for it) with a toaster oven. Or, you can use a hot air gun (800-1000 degrees) to simply heat the board. It’s not as sexy, but it does the job just fine.

A word of caution on the toaster oven without using the timing function. DO NOT walk away, answer the phone, send a text, look out the window, chat with a friend... Do NOTHING except watch your work or you will have some very expensive, non-edible, toast.

Brendan Ames
Albuquerque, NM

It’s fairly easy and not expensive. People have successfully used toaster ovens, stoves, and kitchen hot plates. I use a $100 laboratory hot plate and monitor temperature with an infrared thermometer. The tricky part is laying down the paste for small SMT components. I strongly recommend an inexpensive stencil from oshstencils.com instead of doing it by hand.

Mike Hasselbeck
Albuquerque, NM

I worked for a while at a startup company where I (amongst other duties) built prototypes and very short runs, up to about 10 boards, that were nearly all surface mount technologies. If memory serves, at least one of the boards was about 8”x10” and had roughly 100 components on it, while others were a couple of inches on a side and only had a dozen or so components.

To build these boards, we would usually order a custom solder mask from Pololu (www.pololu.com) to use solder paste on the board. One word of caution is that solder paste has a relatively short shelf life, maybe 6 months, but that can be extended to at least 18 months if you keep it in the refrigerator. Take steps to make sure NOBODY tries to eat it!

Due to the nature of the solder paste, we wouldn’t start the process unless we knew we could finish it that day. We’d use the mask to put solder paste on just the spots we wanted. There are several techniques to hold the mask and board in alignment, then using good tweezers place all of the parts in their proper locations. Then we would “cook” them to “reflow” the solder.

Rather than using an oven, we used an electric griddle at that company. (DO NOT use the same one for cooking food later!) Once we had the boards ready to cook, we’d heat up the griddle using a non-contact infrared thermometer to check the temperature and for “hot spots.” We would use a flat metal spatula to transfer the boards onto the griddle, with something like a pair of tweezers or other small tool to slide the boards off the spatula and watch them closely to observe the solder becoming molten.

When all of the solder was molten, we’d wait another 5 to 10 seconds to make sure any hidden solder was also molten, then very carefully slide the boards back onto the spatula and transfer them to a heat-proof place where they could cool. I would usually only do one board at a time, though others sometimes would do two or three if they were small.

This technique works well for one or two of a kind boards that have parts that are manually “pick and placed.” A professional, time-temperature sloped oven is required for high-reliability boards, e.g., those going into space, but few hobbyists have this need.

I’ve never tried it with any ball-grid-array parts. We would send those out to assembly houses that also had x-ray inspection machines that could verify the BGA pins were all soldered.  I also found that it was best to leave off things like 100 lead quad flat-packs, and solder them on by hand. They would seem to invariably get jostled out of position and need to be removed and resoldered by hand anyway.

The larger and thinner the spatula the better. Don’t even think about trying it with any sort of plastic spatula — they’re all way too thick.

I also recommend that you practice on a board with only a dozen or so parts, none of which have more than 24 leads and accept the fact that you’re going to ruin a few before you get the hang of it.

Once you do get the hang of it, you’ll wonder why people are so scared of trying it — it’s really not all that hard!

Clark Jones
Gilbert, AZ

I converted a Black & Decker toaster oven into a reflow oven. Had to split off the cooling fan AC driver to a separate control line. I use leaded solder paste because of the lower melt temp & the eutectic parts to pads alignment. I only do design concept layouts & proof of concept boards.

Gail Schooley
via email