What is Tombstoning?
A tombstone-sometimes called a Manhattan skyline, crocodile, leaning tower or space rocket-is a chip component that has partially or completely lifted off one end of the surface of the pad. Tombstones may be caused by solderability variations on terminations; volume of paste; surface area of pad; variations in thermal demand of pads; solder mask thickness; paste under parts; limited placement force; and nitrogen usage.
To understand how tombstoning can occur, watch the reflow process. A small board can be printed, placed and reflowed, on a hot plate or under a rework station, simulating the conditions of the reflow process. If you are lucky, you will see the component lift. As components get smaller, the surface of the wettable area of the parts becomes significant and can develop high surface tension forces during reflow. The lifting of parts is becoming more of a problem as products continue to be miniaturized.
This defect is often referred to as tombstoning, draw bridging or Manhattan skyline, particularly if there are many parts all standing vertically. The defect is caused by uneven wetting where one termination solders before the second causing the surface tension of the solder to pull the device in one direction. It can be exaggerated by the amount of paste on small parts, 0805 chips and below or incorrect pad sizes increasing movement during reflow.