SMD Package Selection and Board Land Pattern Tolerances
Calculated land pattern dimensions matching IPC-7351 guidelines balance solder fillet volume and placement tolerance to prevent joint stress and yield loss.
Tombstoning
Assembly Defect
Reflow soldering sometimes causes small passive components to lift off the board at one end. This defect, known as tombstoning, occurs when the solder paste on one pad melts before the paste on the opposite pad. The surface tension of the liquid solder pulls the component upright, breaking the electrical connection at the raised end.
Tension Imbalance
Unequal heating of the two component pads is the primary cause of the surface tension imbalance. Larger copper traces connected to one pad can act as a heat sink, delaying the melting of the solder paste on that side. As a result, the solder on the hotter pad melts first and exerts a force that the unmelted paste cannot resist.
Trace Layout
Designers must balance the copper traces connected to the component pads to prevent uneven heating. Using thermal relief traces or matching the pad connections helps ensure that both pads reach the reflow temperature at the same time. These design measures reduce the risk of tombstoning during production.
Reflow Optimization
Gradual heating in the oven profile prevents uneven melting across the pads. This slow temperature rise ensures both pads reach the solder melting point simultaneously, eliminating tombstoning during production.