Tombstoning During Pcb Board Assembly
Tombstoning is a common and potentially fatal manufacturing defect that occurs during the pcb board assembly process. The problem arises when one end of a component’s wetting force is greater than the other, which causes the component to rise above its anchor points during reflow soldering. To deal with tombstoning, a PCB’s designers must optimize the placement machine, layout, and components to minimize the risk of this defect. The article from Rush PCB Inc discusses how to address tombstoning and prevent the issue.
PCB designers must consider many factors that can contribute to tombstoning, including pad size and shape differences, placement force, reflow profiles, and more. They can reduce the likelihood of this issue by making sure their designs are properly optimized prior to production, and they should avoid using smaller passives if possible, as these tend to tombstone more easily than larger ones.
Keeping the pad sizes and shapes of a particular component consistent helps as well, as it ensures that the pads are sized to match. It is also important to note that the copper area around a pad must be larger than the footprint, which is crucial to maintaining even heating and cooling throughout the wave soldering process.
How to Deal With Tombstoning During Pcb Board Assembly
Another factor that can cause tombstoning is a change in the thickness of a track, solder mask, or legend layer. When a track or solder mask is thicker than the height of copper pads, it can lead to an imbalance in the wetting forces between adjacent tracks. For example, when pads are positioned at different heights on a circuit board, it’s easy for Pad 1 to wet more quickly than Pad 2. This imbalance can create the lifting force that causes tombstoning.
Other issues that can lead to tombstoning include uneven heat distribution, a difference in the temperature profile between the two ends of a component, and poor stencil design. Assemblers can avoid these problems by preheating boards to the correct temperature, using an accurate thermocouple to monitor actual temperatures during the reflow process, and adding soak zones to help control temperature variations. Moreover, they can use additional anchor points like nearby SMD resistors to act as tombstoning “anchors” to hold parts down.
In addition to these proactive steps, PCB assemblers can implement reactive mitigation techniques that can significantly lower the chances of tombstoning. For instance, a reflow process that uses a lower ramp rate can help prevent thermal shock, which often leads to tombstoning. Likewise, a good thermal relief can help prevent the formation of tin beads that can lift a component from the bare surface of the PCB. As a result, these mitigation methods help to eliminate tombstoning from the production line and improve quality. Moreover, they also help to ensure that the finished product will work as intended.