A tombstone defect is a common soldering issue that can occur during the surface mount technology (SMT) assembly process. It involves one end of a surface mount component (SMC) lifting or standing upright, resembling a tombstone, while the other end is properly soldered to the printed circuit board (PCB). This defect can negatively impact the functionality and reliability of electronic assemblies.
Causes of Component Tombstone:
The tombstone defect can result from various factors and process-related issues, including:
- Imbalanced Solder Paste: Uneven solder paste deposition on the pads of the PCB can cause one end of the component to lift during reflow soldering.
- Component Placement: Misalignment of the SMC during component placement on the PCB can lead to tombstoning. If one end of the component is not accurately positioned over its corresponding pad, solder paste may not make proper contact.
- Solder Paste Properties: The properties of the solder paste, such as its viscosity and tackiness, can affect its ability to hold the component in place during reflow.
- Reflow Profile: An improper reflow profile, including temperature ramp-up and ramp-down rates, can contribute to tombstoning.
- Component Size: If the surface mount component is not the correct size for the footprint on the PCB, only one side may touch a pad causing the component to tombstone during reflow.
- Component Solderability: If the component has poor solderability due to its age or surface finish, it will cause the solder not to wet to the end termination and so will tombstone.
- PCB Land-Pattern Design: If the copper difference between each side of a component footprint is uneven, such as if one side is connected to 'Ground', then they will heat up at different rates during preheat. This will cause the solder on one side to melt before the other and so the component will tombstone.
Impact of Component Tombstone:
Tombstoning can have negative consequences on the SMT assembly process and the quality of electronic assemblies:
- Electrical Connectivity: Tombstoning can result in poor electrical connectivity, as one end of the component may not make a reliable connection with its pad.
- Reliability: Solder joints formed during tombstoning may be weak and prone to failure over time, affecting the overall reliability of the assembly.
Preventing and Addressing Component Tombstone:
To prevent and address tombstone defects in SMT assembly, consider the following strategies:
- Solder Paste Printing: Ensure precise and uniform solder paste printing, with appropriate stencil design, squeegee control, and stencil cleaning.
- Component Placement: Implement precise and accurate component placement techniques to ensure proper alignment with PCB pads.
- Solder Paste Properties: Choose solder paste with suitable viscosity and tackiness to hold components in place during reflow. If necessary, tacky or no-clean fluxes can be used to enhance component adhesion.
- Reflow Profile Optimization: Carefully design and optimize the reflow profile to ensure uniform solder paste reflow and minimize the risk of tombstoning.
- Visual Inspection: Perform visual inspections of assemblies to identify any tombstone defects that may have occurred.
- Reflow Simulation: Some manufacturers use reflow simulation software to predict and prevent tombstone defects by analyzing the impact of different process parameters.
- Component Size: Ensure the correct size surface mount component is selected for the footprint size on the PCB to minimise risk of component tombstone defect during reflow soldering.
- Component Solderability: If the component is old and the solderability is uncertain, a solderability can be carried out to check that solder can wet to the end termination.
- PCB Land-Pattern Design: When copper balance is different between either side of a component footprint, consider using 'thermal-relief' pad design to minimise the effect during reflow soldering. Also, modify reflow soldering profile to increase time allowed for preheat to enable all areas of the PCB to reach the required temperature before entering reflow stage.
By implementing these strategies and maintaining strict process controls, manufacturers can significantly reduce the risk of tombstone defects and improve the overall quality and reliability of their electronic assemblies.