tombstoning
An unwanted effect in the manufacture of electronic circuit boards, in which a component stands up on end instead of lying flat.
tombstoning: component standing on its end during soldering
Tombstoning occurs when a surface-mount component tips up onto one end during the soldering process, instead of sitting flat on the printed circuit board. The component, usually a 0805 or 1206 resistor or capacitor, rotates 90 degrees so that one pad contacts the solder while the other end points upward, resembling a gravestone. The solder on one pad melts and cools faster than the other, creating an imbalance in surface tension that tips the part over.
The effect is almost entirely mechanical and thermal. When a reflow oven heats the board, solder paste under both pads of the component begins to melt at roughly 200 degrees Celsius. If the paste volume or composition differs between pads, or if one connection has slightly better wetting than the other, the liquid solder on the side with higher surface tension pulls the component upright. The board's exact positioning in the oven and air circulation patterns also influence the result.
Prevention and detection
Manufacturers reduce tombstoning by maintaining consistent solder paste deposition across all pads, using stencils with precise aperture sizes. Component placement machines must apply consistent downward pressure. Slight rotation of the board during preheating can help equalize thermal conditions. Boards must then pass through automated optical inspection, which flags any component standing upright so it can be reflowed or replaced before final testing.
A tombstoned component creates an open circuit and is immediately detectable during in-circuit testing. Rework is usually necessary, though some shops can salvage the board by heating the standing component and allowing it to fall flat again. The defect rate increases sharply when component sizes drop below 0603 or when designers place parts too close to board edges where convection differs.
The term entered electronics manufacturing slang sometime in the 1980s as surface-mount assembly became the standard. Its visual resemblance to cemetery monuments is obvious and stuck in the industry. Tombstoning remains one of the most common cosmetic and functional defects in high-volume board production, particularly in contract manufacturing where process parameters drift between production runs.