SMD Oven

I started looking some time ago at a temperature controlled oven to do surface-mount device soldering.

An oven has a couple of advantages: It's much easier to get even temperature distribution, and it's easier to follow the temperature soak profile listed in the datasheet (Important for some of the more complex sensor devices).

The first point has been a serious pain to me: Devices with sharp edges are likely to see those edges heat up very quickly so when using a hot-air gun it's easy to burn the edge of the part before the solder is close to melting. (Very easy. Really, really easy. Especially with expensive sensors that have wierd things poking out. Or expensive ZIF ribbon connects. For example. I'm just saying).

So I put together a simple board to interface to a K-type thermocouple. It's just a MAX6675 which does a K-type thermocouple on one side and an SPI interface on the other. This connects to an NGW100 board, and the SPI drivers in the linux kernel take care of all the hardware interfacing.

A tiny userspace program handles reading temperature. The end result is ~ 0 - 1000 celsius with 0.25 degree precision and ~ 2 degree accuracy.

This is coupled with a computer-driven relay switching 240VAC that drives an SMD reflow oven (aka, an electric frypan I picked up at K-mart). I just drilled a hole in the side of the lid to insert the thermocouple into.

The frypan is turned up to maximum, and the element is pulse-width modulated. (Actually, it's sigma-delta modulated with minimum on and off times, but let's not split hairs).

From here, it's a trivial matter of writing some software to run closed-loop control of the heating element to walk the temperature through a profile.