On the Reprap forums we have been discussing various ways to make/use heated beds to prevent warping of plastic parts as they are extruded.An inexpensive way to make a very flat, controllable hot plate would be to take a plate of aluminum about ¼" or 6 mm thick a little bigger than the biggest board you want to tin. Attach either power resistors or nichrome wire to the underside, along with a thermocouple or thermistor, and use a power MOSFET to turn on and off 12volt power. You can either use an op-amp circuit to cycle power on and off at an adjustable set point, or a microcontroller chip to read the temp and drive the MOSFET either on/off (bang-bang mode) or with PWM to get a very stable temperature. This would double as an SMD mounting hot plate.