PCB Tombstoning occurs due to a mismatch in the solder solidification time during the PCB assembly process. The term solder solidification or ‘Wetting’ comes in to the picture when surface mount components are soldered to their respective pads. To do so, solder is applied on the PCB board, after which the component is placed on the board and then passed through a reflow oven, where it melts and creates a strong joint between a component lead and pad.
However, the problem arises when one pad (see the figure above) starts to solidify before the other. When this happens, the solidified pad pulls the component towards itself, resulting in the component tilting – This is called Tombstoning. Tombstoning is the result of component tilting towards the solidified pad. It happens due to the uneven wetting of the solder on two different pads associated with the same component. It results in a dysfunctional PCB with an open circuit.
Let us understand how tombstoning occurs in a practical design. Let’s assume an SMT component is connected to two different traces having different widths. When a PCB is passed through a reflow oven, the pad associated with the wider trace will generate more heat than that connected to the thinner trace. So, pad with a wider trace will complete its wetting process before the other and will also likely to pull the pin from the thinner pad side. This is what happens results in tombstoning.
Factors responsible for Tombstoning
- Uneven temperature of the reflow oven: It causes the solder to begin and completes its wetting process at different times.
- Non-uniform application of the solder paste is also responsible for different solidification time.
- Difference in pad size and trace width also affects the accuracy of solder application