Tombstoning Causes and Process Cures
Tombstoning is a chip component that has stood up on one end during reflow, and it happens because the two joints did not melt at the same time or did not apply the same force. The part is lifted by the surface tension of the joint that melted first.
The defect is a process symptom rather than a component fault, and it is more common on small bodies, on parts with unequal terminations and on boards where the two pads heat differently.
The Mechanism
When one joint melts before the other, the wetting force pulls the component towards that pad. If the part is light and the force is unbalanced, the component rotates and lifts its other end.
The force is small, which is why the defect appears on the lightest parts. A 0402 body weighs almost nothing, and the surface tension of a molten fillet is easily enough to move it.
Unequal Heating
The two pads of a chip part can heat differently if one is connected to a plane and the other to a trace. The thermal asymmetry is designed into the board by the routing, and it is the most common root cause.
The remedy is to balance the thermal connection, either by adding copper to the lighter pad or by connecting both pads to the same plane through similar paths. The change is made in the layout rather than in the profile.
Ramp Rate and Soak
A fast ramp reaches the melting point quickly, which reduces the time available for the two joints to equalise and increases the likelihood of one melting first. A gentler ramp and a longer soak give the assembly time to even out.
The profile change is often the fastest correction to try, and it should be verified with a measurement rather than by observation. Where the ramp is already gentle, the cause is in the layout or the paste.
Paste Volume and Deposit Position
An unequal paste volume makes one joint melt and pull before the other, and so does a deposit that is offset from the pad centre. Both are printing issues and both are correctable with the stencil or the printer.
A deposit that is placed towards the toe of one pad and the heel of the other creates a rotational force even when the volumes match. The aperture should be centred and symmetric.
Component and Termination Factors
A component with terminations of different mass, or one that has been stored badly so that one termination is oxidised, behaves differently at each end. The oxide delays wetting on that side and creates the imbalance.
The component should be checked for solderability and for storage condition, and the paste flux should be capable of reducing the oxide present. Where a batch is suspect, a solderability test settles the question quickly.
Placement Accuracy
A component placed off centre has more paste under one end than the other, which changes both the force and the timing. Placement accuracy therefore contributes to the defect as well as to the obvious alignment requirement.
The nozzle and the vision system should be verified when the defect appears in a cluster, since a drift in either produces a systematic offset rather than a random one.
Pad Geometry and Mask
The land pattern decides how much solder is available at each end. A pad with a larger extension on one side, or a mask opening that is asymmetric, produces the imbalance that starts the rotation.
Symmetric pads are therefore a design rule rather than a preference, and the rules for pad design standards apply directly. Where the package is asymmetric by construction, the pads may still be symmetric.
Prevention in Design
Keeping the two pads thermally similar, avoiding a trace on one side and a plane on the other, and using the recommended land pattern are the design measures. A part placed over a thermal relief or a via array is at higher risk.
Where an asymmetric connection is unavoidable, the layout can add a copper balance pad on the lighter side, which is a small change with a measurable effect.
Detection and Response
Tombstoning is visible to an optical inspection system and usually to an operator, so detection is not the difficulty. The response should include the position of the affected components on the board, because the pattern points to the cause.
A cluster around one component type suggests the component, a cluster in one area suggests the layout, and a spread across the board suggests the profile or the paste. This reasoning is the same one applied in board quality judgement.
Additional Considerations for This Build
Practical attention to placement offset pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating placement offset explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, tombstoning is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
Process Control and Verification
On a design of this kind, tombstoning is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
Does a slower ramp always fix tombstoning? It helps where the cause is thermal, and it does nothing where the pads are asymmetric or the paste deposit is offset.
Are smaller components more prone to it? Yes, because the mass is lower and the surface tension force is the same, so the balance is easier to upset.
Can tombstoning be reworked? It is usually reworked by removing the part, cleaning the pads and refitting with a corrected deposit.
Is nitrogen relevant? It improves wetting and does not correct the thermal or geometric imbalance that causes the defect.
