Summarize with AI:
A small SMD component suddenly stands upright or at an angle on the PCB after reflow soldering. At first glance, the defect may look minor. In practice, however, it can cause an open electrical connection, rework and quality issues. This defect is known as tombstoning.
Tombstoning is a known failure mode in SMT assembly. It occurs when a small two-terminal SMD component lifts on one side during the reflow soldering process. The component then stands at an angle or almost vertically on the PCB, similar to a small tombstone.
It mainly affects small passive SMD components such as resistors, capacitors and inductors. Since these components are lightweight and have only two terminations, unbalanced forces during soldering can cause one end to lift off the pad.
In short: What happens during tombstoning?
Tombstoning occurs when a two-terminal SMD component is pulled upward on one side during reflow soldering. The cause is an imbalance between the two component ends. This imbalance can result from different pad geometry, uneven solder paste volume, uneven thermal coupling of the pads, placement deviations or an unsuitable reflow profile.
If one termination is not soldered correctly, an open electrical connection is created. As a result, the assembly may malfunction or fail completely.
How does tombstoning occur?
Tombstoning is caused by an imbalance of forces during the soldering process. The critical moment is when the solder paste on the two pads of a component melts, wets the termination surfaces and spreads between the pad and the component termination.
The capillary effect also plays a role here. It describes how liquids can be drawn into very narrow gaps, tubes or spaces without external pressure. This happens because of the attraction between the liquid and the surface of the solid material. A simple example is water being absorbed by kitchen paper or rising inside a very thin glass tube.
In soldering, this means that once the solder is liquid and wets both the pad and the component termination, it can be drawn into the small gap between the two surfaces. This helps the solder spread where the solder joint is supposed to form. Together with surface tension, this creates forces that act on the component.
Ideally, both pads are heated as evenly as possible. The solder paste on both sides becomes liquid almost at the same time. The solder wets both terminations in a similar way, spreads evenly and the resulting forces largely balance each other out. The component remains flat on both pads.
It becomes problematic when wetting starts earlier or more strongly on one side than on the other. In that case, the liquid solder on one side already pulls on the component while there is not yet an equal counterforce on the opposite side. Especially with small and lightweight two-terminal SMD components, this force imbalance can be enough to lift one end of the component.
Put simply: The capillary effect helps liquid solder spread between the pad and the component termination. If this happens evenly on both sides, a stable solder joint is formed. If one side pulls earlier or more strongly, the component can lift up — and tombstoning occurs.
Typical signs of tombstoning
Tombstoning is usually easy to identify visually. Typical signs include:
The component does not lie flat on the PCB.
One end of the component is tilted or lifted.
One termination is not soldered correctly.
An electrical test may show an open connection.
The defect often occurs with small two-terminal SMD components.
The defect may repeatedly occur at the same positions on a PCB.
If the defect repeatedly appears at specific component positions, this is an indication that the layout, solder paste printing, placement or thermal conditions should be checked more closely.
Typical causes of tombstoning
Tombstoning usually does not have just one single cause. It often results from a combination of several factors, including PCB layout, solder paste printing, component placement and the reflow profile.
It is useful to distinguish between design-related and process-related causes.
Design-related causes
1. Different pad or land geometry
The geometry of the pads has a major influence on the force balance during soldering. If the two pads of a component are different in size or shape, different forces can occur at the two component ends.
For this reason, pads for two-terminal SMD components should be designed as symmetrically as possible. In practice, the recommended footprints from the component manufacturer or recognized layout guidelines should be followed.
2. Uneven thermal coupling of the pads
The thermal coupling of the pads also plays an important role. If one pad is connected more strongly to large copper areas, wider traces or other heat-conducting structures than the other, the two sides can heat up at different rates.
As a result, the solder paste on one side may melt earlier, or wetting may start earlier on one side. This timing difference can trigger tombstoning.
3. Vias near the pad
Vias close to a pad can influence soldering behavior. They can conduct heat away or contribute to solder paste not staying where it is needed.
This is especially critical if only one side of a two-terminal component is affected. It can create an imbalance between the two component ends.
Process-related causes
1. Uneven solder paste volume
Uneven solder paste volume on the two pads can contribute to tombstoning. If there is more or less solder paste on one side than on the other, the forces during melting can develop differently.
That is why consistent solder paste printing is important. The solder paste should be applied to both pads as evenly and reproducibly as possible.
2. Inaccurate component placement
Inaccurate component placement can also promote tombstoning. If a component is not positioned correctly on the pads, the forces during solder paste melting do not act evenly.
Molten solder can compensate for small placement deviations to some extent. However, if the deviation is too large, the component can shift, rotate or lift on one side.
3. Unsuitable reflow profile
The reflow profile affects how evenly the assembly is heated. If the temperature rises too quickly or if the two ends of a component heat up differently, the solder paste on one side may melt earlier than on the other.
A suitable reflow profile helps reduce temperature differences and allows wetting on both sides to occur as evenly as possible.
Why is tombstoning a problem?
The main issue is the open electrical connection. A component that is correctly soldered on only one side cannot perform its function reliably.
In production, tombstoning creates additional inspection and rework effort. Affected assemblies must be identified, evaluated and, if necessary, reworked or rejected.
Tombstoning is especially relevant in series production, because even low defect rates can result in many affected assemblies at higher production volumes.
How can tombstoning be detected?
Tombstoning is usually visible. The affected component does not lie flat on the PCB but stands at an angle or upright.
In production, the defect can be detected by visual inspection or suitable inspection systems. However, it is important not only to find the defect at the end of the process, but also to reduce its root causes in the layout and manufacturing process.
How can tombstoning be prevented?
Preventing tombstoning starts with the PCB layout and continues throughout the manufacturing process. The goal is to make the conditions on both component ends as balanced as possible.
Design pads symmetrically
The pads of a two-terminal SMD component should be as equal in size and shape as possible. Different pad sizes or asymmetric pad shapes can lead to different forces during soldering.
Pay attention to thermal balance
Both pads should have similar thermal coupling. Large differences in trace width, copper areas or heat dissipation should be avoided or compensated for in the design.
Apply solder paste evenly
Solder paste should be applied evenly to both pads. A stable solder paste printing process helps avoid different solder volumes and therefore different forces during melting.
Place components accurately
Components should be positioned as accurately as possible on the intended pads. The smaller the component, the more important placement accuracy becomes.
Check the reflow profile
The reflow profile should match the assembly, the solder paste and the components used. The goal is even heating so that both terminations wet as simultaneously as possible.
Troubleshooting checklist
If tombstoning occurs, a systematic check can help. The following questions are especially relevant:
Are both pads the same size and shape?
Are both pads thermally coupled in a similar way?
Are there large copper areas on only one component end?
Are there vias close to only one pad?
Is the solder paste volume even on both pads?
Is the component placed correctly and centered on the pads?
Does the reflow profile match the assembly and the solder paste?
Does the defect mainly occur with small two-terminal components?
Does the defect repeatedly occur at the same positions?
This checklist does not replace a full process analysis, but it helps narrow down the most likely causes in a structured way.
Preventing tombstoning: Design and process must work together
Tombstoning shows how closely PCB design and the manufacturing process are connected. An asymmetric layout, uneven solder paste volume, placement deviation or unsuitable reflow profile can individually or jointly cause a component to lift.
That is why the defect should not be viewed in isolation. Anyone who wants to reduce tombstoning should check both the PCB design and the SMT process.
The most important points are symmetrical pad geometries, thermally balanced pad connections, even solder paste printing, accurate component placement, a suitable reflow profile and targeted inspection of especially small two-terminal components.
Conclusion
Tombstoning is a known defect in SMT assembly. It occurs when unbalanced forces act on a small two-terminal component during the reflow soldering process. In most cases, this is related to the two terminations not wetting at the same time or not wetting evenly.
The most important influencing factors are pad geometry, thermal balance, solder paste volume, component placement and reflow profile. Small passive SMD components are particularly susceptible.
Tombstoning is best prevented through the combination of a clean PCB layout and a stable SMT process. Looking at both sides together helps reduce rework, scrap and quality risks in electronics manufacturing.
Frequently asked questions about tombstoning
What does tombstoning mean?
Tombstoning describes the one-sided lifting of a small two-terminal SMD component during reflow soldering. The component then stands at an angle or almost vertically on the PCB.
Which components are most affected?
Small passive SMD components with two terminations are especially affected, for example resistors, capacitors or inductors.
Why does tombstoning occur?
Tombstoning occurs because of an imbalance of forces during soldering. Common influencing factors include asymmetric pads, uneven thermal pad coupling, uneven solder paste volume, placement deviations or an unsuitable reflow profile.
How can tombstoning be detected?
The defect is usually visible. The affected component does not lie flat on the PCB but stands at an angle or upright.
How can tombstoning be prevented?
Important measures include symmetrical pads, thermally balanced pad connections, even solder paste printing, accurate component placement and a suitable reflow profile.
Is tombstoning only a visual problem?
No. If one termination is not soldered correctly, an open electrical connection is created. This can cause the assembly to malfunction or fail.
