A Look at Factors Causing Tombstoning

There is one SMD defect that everyone likes to focus on, including new designers: tombstoning. This simple defect results when an SMD component lifts off one of its pads and stands up on the board. Tombstoning doesn’t necessarily mean the component stands up vertically; even slight liftoff or slight skew can disconnect a component from its pad and leave an open circuit. It is one of those defects that has to be caught in assembly and flagged for rework, otherwise the end product being shipped will not function.

Tombstoning is one of those assembly defects that is communicated as being more common than it really is. The defect does occur, but the solution is not a simple matter of “always use thermal reliefs and you will never have tombstoning.” So in this post, we’ll look deeper at the various factors that lead to tombstoning, some of which have to be addressed by an assembler before volume production.

Factors Leading to Tombstoning

The main factor causing tombstoning is lack of wetting on one side of an SMD component .The problem is most common in SMD passives like resistors and capacitors due to their low weight, although technically larger components could experience minor tombstoning or skew that leads to open circuits. The points discussed below will refer to SMD passives in standard packages (0805, 0402, etc.).

When solder on one side of the component wets earlier than the other side of the component, the component can lift off its pads as shown below. This can occur because of an uneven temperature distribution, which is why design guidelines often recommend using thermal reliefs to prevent heat flow away from one pad during soldering.

Liftoff from an SMD pad during reflow. The pad on the left has undergone wetting before the pad on the right, resulting in pulling force on the component and tombstoning.

The point here is not to say that thermal reliefs are “bad” or that they don’t work. Rather, the reality is more complex and there is more than a single factor at play when tombstoning occurs.

Incorrect PNP Placement or Bad Stencils

If there is a mismatch between PNP placement and solder placement for an SMD component, then tombstoning and/or skew is almost guaranteed to occur. One of the pads could have insufficient solder or no solder, or one of the components could be placed off the SMD pad. The result would be liftoff from the pad and tombstoning during assembly. An example is shown in the images below, where a component is placed on a board and passed through reflow in situ.

Initially the component was incorrectly placed on the SMD pads, leaving very little contact between the solder and one of the component leads.

As expected, the component lifts up due to the pulling force during wetting, and the component stands up on its side. Although the pads begin to wet at the same instant, the lack of solder on one of the pads results in very low adhesion force and thus tombstoning of the component.

This should underscore the importance of having clean and working equipment used in assembly. Assembly QC teams need to track defect counts and periodically inspect production equipment:

• Make sure the stencils are periodically cleaned and replaced if they are damaged
• Make sure the PNP machine is correctly calibrated and periodically inspected for accuracy
• Check any fixtures used to secure boards being passed into the PNP machine

Bad Reflow Oven Regulation

This might sound like an overly simplistic problem, but it is related to tombstoning and should be checked during assembly. If the temperature distribution inside a reflow oven or reflow line is uneven, it’s possible that this would lead to uneven wetting. Smaller reflow ovens might not have this problem, but it can be problematic in larger ovens where many boards are moving through the reflow process simultaneously. The reflow oven temperature needs to reflect the required temperature profile to ensure accurate soldering, and this should be inspected if defect counts start to rise.

symmetric Pads or Paste

If the pads in a PCB footprint are asymmetric, then the component can have excess solder on one pad compared to the other pad. The issue here is that the paste mask aperture in the stencil will tend to match the pad size, so one pad will have much more solder than the other. The result could be insufficient wetting on one side of the pad, or more pulling force on one side of the pad, leading to tombstoning.

The paste mask apertures on this row of passives are all symmetric, which will help ensure even wetting occurs during reflow.

To solve this, ensure your footprints always have even paste mask apertures, even if the pad sizes are mismatched. It is also a good idea to set the solder mask opening to match on both sides of the component as this will ensure confinement of solder to the same area on the pads. A simple solution here is to copy the same outline into the paste mask and solder mask layers so that the mask apertures always match.

Package Profile

SMD packages only standardize the length and width of the package size, but not the package height. The z-axis profile influences the probability of tombstoning. You would think that a flatter component would have a lower probability of tombstoning, but actually the opposite is true.

This is because, for a given package size, a thicker package will have a larger surface for solder to grab the component, which creates greater force on the component when the solder begins to wet and later solidify. When optimizing for high-volume assembly, consider swapping SMD passives with thinner components.

No matter which of the solutions to tombstoning you want to implement, you can use the CAD tools in OrCAD from Cadence to build your PCB for volume assembly. OrCAD is the industry’s best PCB design and analysis software with utilities covering schematic capture, PCB layout and routing, and manufacturing. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.

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