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 B...
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).
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.