Flexible Printed Circuit Board Assembly
Tombstones are a common sight in cemeteries, but they’re not welcome in your printed circuit boar. In PCB assembly, tombstoning refers to a surface mount component that partially lifts off of its pad during the soldering process. This causes the component to stand up on end and resemble a tombstone, which is not desirable in a product. The good news is that this can be prevented by following best practices and using a contract manufacturer with industry experience.
The most common cause of tombstoning is different wetting force between two terminals of a component. When this occurs, one end of the component liquefies faster than the other, which exerts a stronger pull on the end and lifts it up from its pad. This phenomenon is most commonly associated with two-pin surface mount components, such as resistors and capacitors. However, it can also occur with four-pin surface mount chip packages.
flexible printed circuit board design is a primary factor in preventing tombstoning. Designers must carefully match the shape and size of pad to pad, which is a crucial step in ensuring that they’re properly aligned during the wave soldering process. Additionally, designers should ensure that the pads are sufficiently separated and have equal copper areas. This helps to prevent the pads from heating up at uneven speeds, which can cause them to wet at different times and lead to tombstoning.
How is Component Tombstoning Prevented During Flexible Printed Circuit Board Assembly?
Another potential cause of tombstoning is the presence of a via or PTH close to a component pad. When this happens, the via or PTH draws molten solder away from the pad, which can slow down the wetting process and make it harder for both sides to reach liquidus at the same time. In addition, it’s important to avoid having a plane under a pad because this can also wick solder away from the pad and increase the risk of tombstoning.
Incorrect solder mask thickness is another common cause of tombstoning, as it can restrict the flow of molten solder and lead to uneven heating of the pad. In addition, if the pad is not correctly aligned with the metal plane, it can cause an imbalance in the force that pulls the component up from its pad.
Other factors that can lead to tombstoning include solder paste printing errors and an improper reflow profile. The former can lead to uneven torque on the component and the latter can cause one side of the component to be pulled up more than the other, which can result in a tombstone-like appearance.
Finally, a final cause of tombstoning is the use of an insufficiently heated reflow oven, which can lead to a higher temperature gradient on the board and make it more likely that the wetting process will be uneven. To prevent this from happening, it’s important to choose a contract manufacturer that has a proven track record of using a hot air reflow oven with a stable temperature profile. This will help ensure that the reflow oven is set at the appropriate temperature for every run and that the wetting process is evenly distributed across the pad surface.