Last time I tried soldering small resistors was an experience, I’ll tell you what. They were almost as likely to stick to my soldering iron as they were to stick to the board, and I burned plenty of them in half.
Admittedly, I don’t have the world’s best soldering station. In fact, what I have is a junky old pencil iron, not much better than what you’d buy at RadioShack—no adjustable temperature control and no replaceable tips. I also don’t have US $500 to invest in upgrading my station, but even if I did, it wouldn’t help much. I’m getting older—and so are my eyes.
The solution was obvious: Build a reflow oven!
First off, let’s get all the disclaimers out of the way: This project involves dangerously high voltage at high currents! Be careful!
The plan was a simple one: Design a controller. Put it in a toaster oven. I searched the Internet and found some similar projects. This was comforting, as it meant that the toaster oven would get warm enough to do the job.
I decided to use a PIC microcontroller (from Microchip Technology) and a type K thermocouple as the temperature-sensing device. I did some research on thermocouples. It turns out that I’d need an “ice reference” of some sort. I chose an Analog Devices AD595.
This thing was going to be fully buzzword 3.0 compliant, with three preset control curves (lead/tin, RoHS, and one manual-entry curve), an LCD on the front, and RS-232 communication to monitor the temperature profile from a PC. In other words, I would be able to flow standard lead-tin solder and the “green” solders, and do some custom curves for components that require it (like big, honkin’ FPGAs). I spent some time putting together block diagrams. I even got so far as selecting most of the parts. The total cost would be about $400.
I was just starting to put together schematics when a friend at work ruined the whole project (thanks a lot, Ryan). He pointed me to a company, Annex Depot, that sells PID ramp/soak controllers for about $75. Last time I looked at such things was a couple years back, and they didn’t go for less than about $500. Ain’t Moore’s Law great? Unfortunately, the documentation left something to be desired—it was poorly translated into English from Chinese. Also, the unit that would best fit inside the oven housing was out of stock.
I looked around some more and found another company, Auber Instruments, that sold a similar product (SYL-2352P) that was American-made and had much better documentation, for the same price. Auber also sells the thermocouples (with a 4-inch probe tip), 40-ampere solid state relays (SSRs) for plenty of margin, and SSR heat sinks, all cheaper than I found elsewhere.
Wow! Total cost (including a spare thermocouple and the toaster oven) would be right about $200. I spent $153.85 (including shipping) with Auber and a bit less than $50 at Walmart for a Black & Decker toaster oven with an internal convection fan (to minimize hot spots). It took me less than a day to integrate the system. It took me most of another day to get a reasonable control heating profile (lead-tin) out of it.
Removing the oven’s screws is one of the more interesting parts of this project. The bottommost screw on the back isn’t a Phillips, it’s a “star”—presumably to keep morons like me from cracking the case open and getting electrocuted.
The next trick is the LRF (little rubber feet). These little plugs pull out to access the screws underneath. My oven had a piece of bent metal that scratched my kitchen table. Fortunately, it’s not a new table, and I have kids, so my wife didn’t notice.