Tombstoning Prevented During Board PCB Assembly
Tombstoning is a common soldering defect that occurs on surface-mount components. It can lead to intermittent connections or even device failure, causing not only financial but also brand and reputational damage. Fortunately, it is preventable, thanks to comprehensive risk management techniques and effective solutions.
In board pcb assembly, surface-mount devices are placed on the circuit board using solder paste, a mixture of tiny metal particles and flux that helps it adhere to the circuit boards. Once the component is in place, it is heated to melt the solder, resulting in a secure electrical and mechanical connection between the board and the component. However, this process is not without its risks: if the components experience an unbalanced thermal profile during reflow, they may rise into the air, resembling tombstones.
The main reason for this unbalanced wetting is that the reflow process sometimes fails to completely melt the solder on both sides of the component pad. This causes the side that has more solder to wet more quickly, thereby exerting a pulling force that can lift the component into the air. Depending on the type of pad, this force may be enough to lift the component entirely into place. The resulting tombstoned component looks similar to the slanted slabs of granite used to mark graves.
This problem is particularly prevalent on thin, two-pin surface-mount devices with a high aspect ratio. Thicker devices can have the same issue if their terminal surfaces have different weight distributions or thermal characteristics, but it is most common with passives.
How is Tombstoning Prevented During Board PCB Assembly?
To prevent this, manufacturers need to ensure that the pads on their PCBs have the same size and shape, so that they can be wetted evenly. They must also make sure that the stencil used for printing the solder paste is properly designed.
Stencil design involves assessing the aperture dimensions, the height of the apertures on the PCB, and the thickness of the solder mask. An aperture that is too big or a stencil that is too thick can create inconsistent wetting, resulting in the occurrence of tombstoning.
The reflow process itself can also cause tombstoning. A low-quality reflow machine can distribute heat unevenly on the surface of the board, which can result in one side of the pad melting faster than the other. This can also happen if the reflow oven is set too high.
Another way that tombstoning can occur is if the copper layer on the inner side of the circuit board has unequal coverage. Since the copper helps to spread the heat applied to the board, an unequal coverage can result in uneven heating, which can also cause tombstoning. Finally, improper assembly can also result in tombstoning. This includes not using a proper through-hole component design or misplacement. It is important to keep in mind that a tombstoned component can cause a short circuit between the conductive traces on a PCB and other nearby components. In this case, the upright tombstoned component could become a deadly threat to the safety of any devices that use it.
