Extreme Surface Mount Soldering

Step 2 The Hot Plate Soldering System

Lots has been said about surface mount soldering ( SparkFun tutorials; Scott Driscoll's web site and article in Make 16) but I think two things are critical that these sources don't address adequately (IMHO). Eventually, you'll want to solder really small SMT parts (QFN) and there's no reason you shouldn't. They're just incredibly tiny and their pads are underneath the part body making them virtually impossible to get to with a soldering iron. The technique I'll show you handles these with no problem. They also usually have a large central pad. Soldering it is critical for a lot of ICs - especially switching regulators. Since these are some of the most useful ICs, it's crucial to learn to solder them. Amazingly, the technique I describe solves both these issues: soldering tiny parts and soldering the central paddle.

Although both the Make article and the SparkFun tutorials say you can solder the pads on lots of SMT parts with a very fine tip soldering iron and then clean up shorts with desoldering braid, this seems mostly unnecessary to me and demands a level of patience (and perhaps skill?) that I just don't have. If forced to I would try this method, but I think my way is better. Putting solder on with a stencil, placing all the parts, and soldering everything in one, 10 minute pass on a hot plate just seems a whole lot simpler!

Even the Schmartboard method is a lot of work and doesn't really address how to build a board with multiple SMT components and keep it small. It's fine for prototyping and testing new parts quickly, but it won't yield really small boards.

SMT soldering using an electric skillet is a technique described in a tutorial by SparkFun. When I first read about this, it struck me as a great way to go. I had two concerns. First, I was using lead free solder which melts at a higher temperature than solder with lead. (420F minimum; temperatures up to 500F are used in the process. These are at the very end, or just beyond, the range of most electric skillets.) Also, the temperature profile for heating and soldering seemed marginal using the skillet and would be even more marginal with lead free solder. A hot plate would solve both these problems since it could get a lot hotter a lot quicker.

So a few dollars changed hands at the local department store (pick your favorite) and I was attempting to solder with a hot plate. (Picture 3) Although the unit itself had more than enough heating power to melt the solder, the heat control system was totally inadequate. After severely burning one test board I realized that a better controller was needed. Some alternatives were considered, but I quickly concluded that what was needed was a solid-state power control unit and Pulse Width Regulation to drive it. Creating and controlling the pulses was easy - I just used an Atmel ATtiny2313 and a couple of I2C Port Expanders, plus a few DIP switches. I'll describe this system in more detail and provide schematics and source code for it.

While Pulse Width Regulation is a great way to create control pulses, a little harder problem was how to use the 5V pulse output to control the 115VAC to the hot plate. More research turned up a simple solution for this also. I'll present all the details and explain how to build the AC Control Unit in the sections to follow. After doing a moderate amount of research, this seemed like the simplest and best approach. It has proven to work just as I hoped.

The Hot Plate Soldering System consists of three parts: the hot plate, the Pulse Width Regulation Unit (Picture 4), and the AC Control Unit (Picture 5). The latter two units form the Hot Plate Control System. I'll explain the details of the Hot Plate Control system and show you how to build the two units that comprise it. Finally, I'll detail the process of using this system for soldering tiny surface mount devices.