Tombstone Effect with SMD in Electronic PCB

Hello! My name is Dmitrii. In this article, I’ll explain the “Tombstone Effect”, also known as the Manhattan effect, chip lifting, Stonehenge effect, or drawbridge effect. That effect occurs during the reflow process of solder paste on a PCB in an oven. With this effect, an SMD component – a resistor, capacitor, or any other - lifts on one contact pad and detaches from the other pad. I’ll tell you six causes of this defect and how to prevent it.

In the reflow oven, different profiles are used for leaded and lead-free solder pastes. In one of my upcoming videos, I’ll show you how to build a reflow profile. In this case, the conveyor speed is too high, and the PCB does not spend enough time in the oven’s second zone to stabilize its temperature. You need to align the profile with the solder paste manufacturer’s recommendations. Then test the actual temperatures in the oven zones using a thermal profiler. The correct profile will preheat the PCB and components evenly by the time the solder paste melts.

One pad heats up faster than the other because it’s connected to a large copper area that dissipates heat. This prevents both pads from reaching the same temperature. The component is pulled by the surface tension of the molten solder to the contact pad where the solder melts faster. To fix this, the PCB must be revised and a new version released.

When a via is placed on one of the contact pads, heat leaks into the metallized through hole, disrupting uniform heating. When designing, place vias outside the pads and provide thermal relief. Additionally, solder may leak into the via, shifting the surface tension difference to the other pad, which then pulls the component toward it. The solution is to redesign the PCB.

If one pad receives a larger volume of solder paste, it creates excessive surface tension that pulls the component toward it. This problem is often caused by the stencil used to apply the solder paste. Make the stencil apertures the same for this SMD component.

Different pad sizes can cause a slight temperature difference. Another common mistake is different stencil apertures for the pads of a symmetrical component, resulting in an unequal volume of solder paste. These need to be corrected at the PCB design stage.

Low-quality stencils and apertures can cause solder paste to get stuck, resulting in incomplete paste application to the PCB. Here a difference in surface tension is created between the two contact pads. The pad with a larger volume of molten solder paste has a higher surface tension.

In Conclusion… by making PCB corrections at the planning stage, you can eliminate future PCB soldering issues, avoid costly repairs, and significantly improve the quality of your PCBs. Be sure to check out my other videos on the channel to further improve your skills.

Thank you for reading!