How did the tombstone phenomenon of SMT come about?

Tombstone is also known as the suspension bridge and the Manhattan phenomenon. So how did it come about? How to solve it?

The root cause of tombstone is the imbalance of wetting forces on both sides of the component. The production of tombstones is directly related to solder paste, components, substrate design, and reflow soldering. For example, after passing through the high-temperature reflow soldering furnace, the tin at one side melts faster while the other side melts slowly, which will results in the force unbalanced on both sides and one side to tilt up, thus the tombstone phenomenon occurs. We can avoid the tombstone phenomenon in designing, by reducing the inner distance of the pads, and minimizing the distance between the pads at both sides without causing a short circuit, so that the solder paste at the slower melting side has more space to be stuck and avoid standing up, so as to avoid the phenomenon of tombstone. In addition, in the production process, by slowing down the temperature rise of reflow soldering, the temperature of all circuits on the PCB can reach a consistent level, achieving the effect of melting tin at the same time, which can effectively avoid the tombstone phenomenon.