Doing solder reflow work can be expensive and difficult, but thankfully there exists a simple and elegant solution: Toaster Ovens. This project shows my preferred setup and the tricks that make the process run smooth. In this example I'll focus on doing reflow of a BGA (ball grid array).
Step 1: Find a toaster oven.
You're looking for two main things, an adjustable temperature knob, and a timer that will time down. The more precision you can get in the timer the better.
Also, if you can get it, some sort of forced air flow will improve the uniformity of the oven temperature, but you have to make sure that the air flow isn't powerful enough to move your components around.
Step 2: Get a thermometer and timer.
Even though the toaster oven has a temperature set point and an integrated timer, you still want to get some more accuracte readings. Get a cheap oven thermometer and toss it inside the oven and get a timer with an alarm to remind you to check on your baking PCBs.
Step 3: Make your PCBs.
In this exampe I'm working with an ADXRS300 which is a 1 axis Gyrometer made by Analog Devices. It comes in a ball grid array package with the balls already attached to the bottom of the component. The PCB needs to be designed with pads for each of the balls, along with a silk screened outline to make it easy to align the component (which is critical when you can't actually see the pads). Also, duh, make sure you mark the location of Pin 1.
Step 4: Add flux to the PCB.
The balls in the BGA don't have flux so you *absolutely* have to put down flux on the board prior to doing the reflow. If you don't add flux then the oxide on the top of the pads will keep the balls from flowing and you'll end up with slightly squished balls that are not actually connected to the underlying PCB.
Step 5: Align the components on the PCB.
Position the PCB on the tray of the toaster oven, preferably oriented so that you can keep an eye on it through the window of the oven. Precisely position the component on the PCB using the silk screened outline to do the alignment. You don't have to be exactly accurate since the solder reflowing will actually pull the component into alignment, but you should try to get it as close as possible. Worst case scenario would be having the component offset by more than half of the ball spacing pitch which would cause the component to shift over by one set of pads. Not good.
Step 6: Start 'em cooking.
Close the toaster oven door, (make sure you don't bump the component out of alignment.) Set the temperature dial for somewhere around 450 and start the timer at around 20 minutes. Later on once you've determined the characteristics of your particular toaster oven then you can start using exact values. But for right now we're going to use our oven thermometer and the external timer to keep track of what's happening.
Step 7: Watch the temperature.
Keep on eye on the thermometer. You'll have to check the reflow profile for your particular components to know what temperature you're trying to reach. In my case, the solder balls would start to melt at 183C and I wanted to hit a top temperature of 210C. If you go beyond 230-240C you'll start to incinerate your PCBs, which although amusing, is probably not what you want.
Step 8: Turn off the toaster oven.
As soon as the oven hits the top temperature you're aiming for, turn it off!
Step 9: Let it cool, and don't move anything!
You can speed up the cooling process by opening up the front door of the toaster oven... BUT, make sure you don't jostle the components or move them in any way. The solder is still liquid at this point and if you poke at the component you will shift it around and ruin it. This is the time to just walk away. Once the temperature drops below 100C (or 50C if you're paranoid) you can feel free to move things around.
Step 10: Inspect and enjoy.
You should make sure that all of the balls are connected and that the component is strongly attached to the PCB. This image shows 3 of the reflowed BGAs integrated together in a 3-axis Inertial Measurement Unit.