How to Prevent Tombstoning During PCB SMT Assembly
Tombstoning During PCB SMT Assembly
Tombstoning is a common issue that occurs during the pcb smt assembly process. Basically, it is when one end of a component’s connector stands upright, rendering the component unusable. The problem can be caused by a number of issues, including inappropriate placement pressure or z-height, mismatched trace widths, solder paste chemistry, and more. In order to prevent tombstoning, it is necessary to be aware of the factors that can contribute to this issue and take steps to correct them before production.
The first step in preventing tombstoning is to look at the inner layer of your circuit board. If the copper area on the inner layer is not evenly distributed, this will cause uneven heating of the pads and increase the chances of tombstoning. The solution is to edit your layout to ensure that the pads are covered by a uniform amount of copper and that each pad is connected in the same direction.
Another factor that can lead to tombstoning is the surface finish of your PCB. Many manufacturers use hot air solder leveling (HASL) on their boards, which can leave an uneven surface on the pads that makes it difficult for the solder to wet properly. This problem can be eliminated by using other surface finishes such as immersion tin or immersion gold, which are more compatible with soldering.
How to Prevent Tombstoning During PCB SMT Assembly
In addition to the surface finish, the thickness of the solder mask on the top of your board can also lead to tombstoning. If the mask is too thick, it will raise it above the pads on the board and create an uneven heating surface that can cause the solder to melt or flow differently than it should. To avoid this, the designer should make sure that the solder mask thickness is consistent and well below the pads on the board.
It is also possible for tombstoning to occur if the pads on your board are not sized correctly. This is particularly common in smaller components like 0402 and 0201 packages. Make sure that all of the pads are sized appropriately for their intended use and that they have the same shape to help keep them from being able to move during the soldering process.
Other common causes of tombstoning include poor wetting on the components and chemical inconsistencies in the solder paste. If the paste is too thick, it may not be able to fully wet the component terminals, leading to an imbalance of pulling forces between the ends of the component. In addition, different chemistries of solder paste can contribute to this issue by having different vapor pressures at reflow temperatures.
Tombstoning is a common problem that can be hard to diagnose, and the best way to prevent it is through careful planning. By taking the time to carefully review your design before it goes into production, you can ensure that tombstoning does not become a problem for your pcb smt assembly. By following these simple tips, you can be sure that your boards will come out of the oven with all of their connections intact.