Assembly Defect
Partial component lifting during reflow soldering results in a surface mount passive component standing vertically on one terminal. Occurring when unbalanced surface tension forces pull a component upward, a tombstoning defect leaves one end completely disconnected from its PCB pad. Surface tension imbalances arise when solder paste on one pad reflows earlier than the opposite pad, or when pad sizes and copper connections are unequal.
Component termination geometry, solder paste volume, and profile ramp rates influence defect occurrence.
Mechanics of Failure
Unequal heating rates across component pads create surface tension differentials in liquid solder. The formation of a tombstoning defect occurs when the pulling force of molten solder on one end exceeds the component weight and opposite-end solder adhesion. Differential pad trace connections act as heat sinks, causing asynchronous solder paste reflow.
Solder paste printing accuracy and symmetric pad layout designs reduce tombstoning occurrences on small chip components.
Defect Boundary
Automated optical inspection systems detect lifted components during post-reflow inspection. Uncorrected tombstoning leads to open circuits and complete functional failure of the assembly. Layout rules enforce symmetrical thermal relief and pad dimensions for small passive packages.
Correct reflow profile tuning ensures simultaneous solder paste reflow across all surface mount terminations.
