Tombstoning and Placement Defects

What Tombstoning Looks Like

A tombstoned component stands on one end, with one termination soldered to its pad and the other lifted clear, like a headstone. It is common on small passive components, where the mass is low and the imbalance is easy to create, and it is a functional defect because the circuit is open. The mechanism is a torque around the component: one end of the solder melts first and its surface tension pulls the termination down onto the pad, while the other end is still solid and provides a pivot, and the component rotates upward before the second end can wet.

Why the Two Ends Heat Differently

If both terminations reach the melting point at the same moment, the component settles. Tombstoning happens when they do not, and the causes are typically geometrical. A pad connected to a plane heats more slowly than one connected to a thin track, because the plane conducts heat away. A pad with a larger copper area takes longer to reach temperature. A component placed slightly off centre, so that one termination overhangs its pad more than the other, changes the wetting and the balance. A nearby via in the pad increases the heat loss. Any asymmetry between the two ends creates the temperature difference that starts the rotation.

Pad Design and Its Effect

The layout is the primary lever. Pads that are longer than the termination encourage the paste to pull the component toward the centre; pads that are too short leave the termination overhanging and unsupported. Where one pad connects to a plane and the other to a track, the thermal asymmetry is built in, and the countermeasure is a thermal relief on the plane connection or a matched connection on both sides. The mask between the pads should be sufficient to prevent the two deposits from joining, and the pad width should match the termination so that the paste footprint is symmetric. Two identical pads are the goal, and any design that makes them different is a tombstoning risk.

Paste Volume and Placement

The paste volume per pad should be equal, and a difference between the two deposits is enough to start the rotation. That means the stencil aperture for both pads must be identical, the print must be uniform across the board and the paste must not have slumped. Placement accuracy contributes too: a component placed with a lateral offset, or with a rotation, changes how much of each termination rests on its paste. Placement pressure matters as well, since a component pressed too far into the paste displaces it and can tilt before reflow begins.

Reflow and the Thermal Profile

The profile determines the temperature difference that develops. A slow, even profile with an adequate soak lets the whole assembly approach a uniform temperature before the alloy melts, which reduces the difference between the two ends of a small component. A fast ramp heats the board unevenly, and the part of the board near the edge and the part above a plane behave differently. The preheat should be long enough to bring the copper up to temperature, and the peak should be reached without a rate that favours one side of a component. The profile does not create a tombstoning problem by itself, but it decides whether a marginal design produces defects.

Material and Component Factors

Not all of the cause is in the process. The terminations of the component have to be solderable, and a termination that has oxidised or that has a contamination will wet later than its partner. The paste alloy and the flux affect the wetting speed and the surface tension, and a paste with a slow activation can leave one end dry while the other is molten. The component’s own mass and geometry matter as well: a small part has less inertia to resist the torque, which is why the smallest passives are the most affected. Where a component type is repeatedly involved, it is worth checking its termination finish and its storage history.

Eliminating Tombstoning

The practical sequence is to make the pads symmetric, to equalise the paste volume, to provide thermal reliefs where a plane connection creates an imbalance, to verify the placement accuracy and to use a profile with an adequate soak. Where the problem persists on one component, the aperture for that part should be compared with its neighbours, and the placement program checked for an offset. Where it appears randomly across the board, the cause is likely to be the print or the profile rather than the design. Recording the position and the component of each tombstone is the quickest way to tell the two cases apart.

Lot Based Investigation

Where tombstoning appears across a production lot, the investigation is different from a single persistent case. The pattern across the panel, the position relative to the board edge, the feeder that supplied the part and the paste lot used for the run are all worth recording, because the cause is likely to be a change rather than the design. A comparison against the previous lot with the same setup usually identifies what moved. Where the defect rate rises gradually, the likely cause is a printer drift or a paste condition; where it appears suddenly after a change, the change is the answer.

FAQ

What is tombstoning? A small component standing on one end because one termination melted and pulled down before the other could wet.

Why does it happen? An asymmetry in heating, paste volume or wetting between the two terminations.

How does the layout contribute? Unequal pads, or one pad connected to a plane and the other to a track, create a thermal imbalance.

Does the reflow profile matter? Yes. A slow ramp with an adequate soak equalises the temperature and reduces the risk.

How do I diagnose a persistent case? Compare the aperture, the placement offset and the paste volume for the affected component against its neighbours.

Conclusion

Tombstoning is a balance problem, so make the pads and the paste symmetric, equalise the heat and check the placement. Design it out rather than screen it out. Pad and footprint rules belong to PCB design and layout, the print and reflow sit in SMT PCB assembly, and the acceptance criteria are part of quality management. Footprint selection for a new product is made during prototype PCB assembly in 2026.