Tombstoning

What is it?

Tombstoning is the name given to the effect whereby surface mount components such as chip resistors and capacitors stand vertically during assembly with only one termination actually soldered to the substrate.

Why does it happen?

The effect occurs when one end of the component is wetted by the melting solder paste before the other end. When this happens, surface tension effects can pull the component into an erect position. There are various causes of this phenomenon including temperature differentials across the circuit board during reflow soldering, poor placement of the component with respect to the solder pads and differences in wettability of terminations at each end of the component.

Lead-Free solders tend to exhibit higher surface tensions than conventional tin-lead solders and can contribute to the increasing occurrence of tombstoning.

Nitrogen inserting during soldering has been reported to lead to an increase in the incidence of tombstoning.

Although tombstoning has been known for some time and is not specific to Lead-Free solders, it may be an increasingly important problem. It is more likely to occur with smaller components and the move to finer features and increasing levels of miniaturisation on circuit boards. Smaller devices have less mass and thus the forces required to produce tombstoning are also smaller.

What problems can it cause?

If tombstoning does occur, at best it will mean a re-work of the board or at worst the board and components will have to be scrapped. This will lead to a reduction in productivity and the associated financial costs.

How can it be prevented?

The main way of avoiding tombstoning is to use of alloys with melting ranges rather than a specific melting point. With the correct choice of alloy it is possible to artificially extend the time the solder is liquid during the reflow operation. This increases the time available to wet each end of a component and reduces the tendency for tombstoning to occur. It can also be avoided by modifying the reflow profile.