Bridging is characterized by solder unintentionally attaching to adjacent pads. Bridging is typically a print issue, but can be caused by ramp rates that are too slow. A possible solution is to increase the ramp rate to 2.5-3°C/sec and a 150°C soak. Obviously, the down side is that you will have less activity when the paste reaches reflow.
Insufficient Wetting (Dry Lead Toe & Heels, Poor Fillets) occurs when solder does not flow fully into a joint, typically as a result of too short of a soak (insufficient cleaning time) or too-long of a soak (all flux cleaners are used up and new oxides form). The main purpose of the flux in the solder paste is to prep the surfaces in order to create a good solder joint. If the activator (flux) is burned off too quickly, you will see wetting problems. Ramp can also factor into poor wetting.
Microcracks occur at the interface between ball and pad when incompletely formed. This may be caused from too little time at reflow or too low of peak temperatures (head-in-pillow). Microcracks are also associated with moisture in your components, similar to the causes associated with popcorning.
Popcorning is when components (especially ICs) break open due to expansion of trapped moisture, typically because the ramp rate is too fast. Controlling your ramp rate through 100°C and improving your storage conditions are possible solutions.
Solder Balls are formed from oxidized solder paste that fail to coalesce from ramp rates that are either too slow or too fast. In the case of a ramp rate that is too fast, volatiles in the paste are driven off before the solder becomes molten. Solder balls can also be caused by letting the paste sit too long on the PCBs before reflow.
Solder Beading should not be confused with solder balls. Solder Beading is really a printer issue, and not directly attributed to reflow. Solder Beading is typically caused by no clean solder pastes. In this case, the apertures have not been reduced in the stencil and there is too much paste volume. The paste is actually squeezed out under discrete parts and creates a mid-chip ball that appears to be attached to the part. A home plate design is one way to reduce the amount of paste.
Tombstoning is characterized by discrete parts “standing up” on end. There are numerous causes for tombstoning, the most common due to poor wetting on the opposite end of the component. In addition, an inert atmosphere can actually introduce tombstoning into your reflow process, due to cooling rates. Large Δ T’s in your reflow process may cause tombstoning right before and after reflow. Component and/or paste mis-registration may also cause tombstoning.