What Is PCB Tombstoning?
Tombstoning PCB, also known as the Manhattan effect or crocodile effect, is a common defect in Surface-Mount Technology (SMT) assemblies.
It occurs when one end of a surface-mounted component lifts vertically from the PCB during reflow soldering due to uneven soldering. This issue not only leads to open circuits but also affects production rate and product reliability.
What Causes PCB Tombstoning in Assembly?
Tombstoning happens when there is an imbalance in mechanical and thermal forces during soldering. As the oven temperature increases, the solder paste melts and solder joints form. However, if the solder paste is applied unevenly, heats up at different rates, or doesn’t adhere properly, the forces on each side of the component become unbalanced. The imbalance can cause one end of the component to lift up, making it stand upright on the PCB.
Key Contributing Factors
- Uneven Wetting Forces – Differences in solder joint wettability can cause an imbalance in soldering forces on both sides of a component
- Solder Paste Behavior – Surface tension changes when molten can directly affect component stability.
- Temperature Gradients – The difference in PCB temperature of different areas can cause inconsistent soldering rates, leading to misalignment or lift-up.
PCB Design & Material Considerations
Pad Size & Spacing
Pad design directly impacts soldering consistency. If pads are too large for the component footprint (e.g., placing a 0402 component on 0805 pads), excessive solder tension can lift the component. Additionally, improper pad spacing can reduce the quality of solder paste print, increasing the risk of defects.
Using PCB Assembly service allows customers to receive expert support during the component selection and design stages, helping to minimize the risk of tombstoning.
Thermal Conductivity & Heat Distribution
Uneven heat dissipation across the PCB can cause temperature imbalances during the second reflow, increasing the likelihood of tombstoning.
- If one pad is connected to a larger copper area than the other, it heats at a different rate.
- Unequal trace widths between pads can lead to inconsistent heat distribution (e.g., R3, R4), exacerbating tombstoning risk.
Solder Paste Performance & Printing Quality
Solder Paste Activity & Consistency
- Highly active solder paste can cause excessive wetting, leading to component displacement.
- Uneven solder paste thickness affects joint stability, triggering tombstoning.
Printing Accuracy & Stencil Design
- Stencil thickness and aperture size determine paste print quality. Thick stencils lead to excessive solder deposits, increasing larger tension.
- Precise squeegee pressure and angle control prevent solder paste from misalignment, deviation of thickness, or incomplete printing.
PCB Assembly uses strictly tested soldering materials and advanced equipment to ensure the accuracy of solder paste printing.
Reflow Soldering Process & Parameter Optimization
Reflow Profile & Airflow Control
- Rapid heating rates cause uneven melting between pads, leading to solder imbalance.
- Airflow turbulence in the reflow oven can physically shift components.
Nitrogen Reflow Considerations
While nitrogen reflow reduces oxidation and enhances solder joint quality, it can increase tombstoning risks due to increase of wettability. The usage of Nitrogen should be carefully weighed upon process demand.
Some images are sourced online. Please contact us for removal if any copyright concerns arise.
