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How to avoid PCB tombstones defects in circuit board assembly

PCB Tombstone

PCB Tombstone

In the realm of PCB assembly, open defects and tombstones are commonly encountered issues that can significantly impact the functionality and reliability of electronic devices. Tombstoning, also known as the Manhattan effect or the Stonehenge effect, occurs when one end of a surface mount component is soldered to the PCB pad while the other end remains unattached, causing the component to stand upright like a tombstone.

Open defects, on the other hand, refer to incomplete or broken electrical connections, resulting in open circuits. Addressing these defects is crucial to ensure the proper operation of PCB assemblies. In this comprehensive guide, we delve into the causes and preventive measures to mitigate open defects and tombstones during PCB assembly.

Introduction

The tombstone phenomenon, also known as PCB tombstone, is observed during the welding process of Multi-layer Ceramic Capacitors (MLCCs) and PCBs. It occurs when one end of the MLCC leaves the welding area and stands upright or is inclined. This issue is primarily caused by unbalanced wetting force at both ends of the MLCC during the welding process. The main factors contributing to this unbalanced force include:

  1. Asymmetric Heating: Both ends of the MLCC cannot be melted at the same time, leading to uneven surface tension during the melting process.
  2. Unreasonable Pad Design: The pad design on the PCB can contribute to the tombstone phenomenon if it is not appropriately designed to ensure even wetting of both ends of the MLCC.

To mitigate the tombstone phenomenon, it is essential to keep the surface of the MLCC clean and pay attention to the pad design on the PCB. This includes ensuring that the solder paste activity is not weakened and that the MLCC melts evenly at both ends during the welding process. By implementing these measures, the tombstone phenomenon can be effectively prevented, leading to improved production results and reduced costs.

Analysis of the cause of the PCB tombstone phenomenon

The tombstone phenomenon, also known as PCB tombstone, is observed during the soldering process of MLCCs and PCBs, and can be attributed to several factors leading to uneven solder adhesion. The movement of MLCCs can be categorized into three main types:

Self-alignment: During the placement process, the placement head of the placement machine quickly positions the MLCC on the solder paste pads based on X and Y coordinates. However, due to pad unevenness or solder paste sliding, the MLCC may be offset by an angle (θ). When both solder joints melt simultaneously, the uniform tin-dipping force pulls the MLCC back to its correct position, correcting the alignment.

Skewing: If the two solder joints do not melt at the same time or if the tin-dipping forces at the two points differ significantly, one of the solder pads may pull the MLCC more diagonally, causing it to skew.

Tombstoning: This occurs when there is a significant difference in tin-dipping forces at both ends of the MLCC, especially in smaller MLCCs. The surface tension can cause one end of the MLCC to be pulled up, resulting in the tombstone phenomenon.

Modern placement machines can monitor and correct both the X and Y coordinates as well as the θ angle, reducing the occurrence of self-alignment issues. Improvements in conveyor belt smoothness have also minimized deflection before welding. However, to prevent skewing and tombstoning, it is crucial to ensure that the solder joints melt uniformly and that there is balanced wetting force at both ends of the MLCC during the soldering process.

Measures to Prevent Tombstones occurred in PCB

The tombstone phenomenon in PCB assembly is a common issue that can lead to significant quality and reliability issues in electronic devices. It occurs when one end of a surface-mounted component, such as a chip resistor or capacitor, lifts off the PCB during the reflow soldering process, resembling a tombstone. This problem can result in electrical opens, affecting the functionality of the circuit and potentially leading to costly rework or component replacement.

Preventing tombstoning requires careful attention to various factors, including solder paste application, pad design, component placement, and reflow soldering parameters. By understanding the root causes of tombstoning and implementing preventive measures, manufacturers can improve the yield and reliability of their PCB assemblies. To prevent PCB tombstoning, consider the following methods:

Stencil Design Optimization

Aperture Size and Shape

The stencil design plays a pivotal role in preventing tombstones during the solder paste printing process. Optimizing the aperture size and shape is essential to ensure uniform solder paste deposition and achieve balanced wetting forces on both ends of the component. Bridged Aperture Entries (BAE) stencils are generally preferred over Periphery Opened Ratio (POR) stencils, as they exhibit better performance in limiting tombstone defects, particularly with smaller component pitches and newer solder paste formulations.

Stencil Thickness

The thickness of the stencil is another critical factor influencing solder paste release and tombstone formation. A stencil thickness ranging from 4 to 8 thou (0.1016 mm to 0.2032 mm) is recommended to adequately hold the solder paste and facilitate reliable printing. Additionally, the stencil thickness should accommodate at least five solder particles spanning the smallest aperture to ensure consistent paste transfer.

Solder Mask Optimization

Solder Mask Thickness

The solder mask thickness plays a crucial role in preventing oxidation and tombstoning. An excessively thick solder mask can lead to the formation of solder beads, increasing the risk of tombstones. Therefore, it is essential to maintain an appropriate solder mask thickness to achieve optimal solderability.

Pad Design

The design of the PCB pads significantly impacts the occurrence of tombstones. Ensuring that the pads cover more than 50% of the component’s terminals and minimizing the spacing between the pads can reduce the likelihood of tombstone formation during the reflow soldering process.

Component Placement and Orientation

Balanced Thermal Conductivity

Uneven thermal conductivity across the PCB can contribute to tombstoning. To mitigate this issue, it is essential to place components evenly and maintain similar orientations and trace widths. This approach promotes uniform heating during the reflow process, reducing the risk of uneven wetting forces that lead to tombstones.

Component Selection

Selecting smaller and lighter-weight components can help minimize the occurrence of tombstones. These components are less susceptible to imbalanced wetting forces caused by uneven solder paste melting or variations in thermal conductivity.

Solder Paste Printing Process Optimization

Paste Thickness and Uniformity

Ensuring consistent solder paste thickness and uniformity across the PCB is crucial for preventing tombstones. Calibrating solder paste printing machines and maintaining proper process parameters, such as squeegee pressure, speed, and separation, can contribute to achieving uniform paste deposition.

Paste Formulation and Rheology

The formulation and rheology of the solder paste can significantly influence its wetting behavior and propensity for tombstoning. Selecting a solder paste with good solderability and wetting characteristics, as well as appropriate metal load and viscosity, can help mitigate tombstone formation.

Reflow Soldering Process Control

Thermal Profiling

Implementing a well-controlled and optimized thermal profile during the reflow soldering process is essential to prevent tombstones. A gradual and uniform temperature ramp-up across the PCB reduces the risk of localized heating and uneven wetting forces that can lead to tombstoning.

Preheat Stage

Proper preheating of the PCB surface is crucial to minimize significant temperature differences that can result in the formation of tin beads and subsequent tombstoning. Maintaining a uniform preheat temperature across the PCB ensures consistent solder paste melting and minimizes the risk of uneven wetting forces.

Tombstone PCB

PCB Tombstone

Inspection and Quality Control

In-Process Monitoring

Implementing in-process monitoring and inspection procedures can aid in identifying potential issues that may lead to open defects or tombstones. Real-time monitoring of solder paste deposition, component placement, and reflow profiles can help detect and address anomalies before they result in defective assemblies.

Post-Assembly Inspection

Conducting thorough post-assembly inspections is essential to identify and address any open defects or tombstones that may have occurred during the assembly process. Visual inspection, automated optical inspection (AOI), and electrical testing can be employed to detect and rectify defective components or connections.

Conclusion

Preventing open defects and tombstones during PCB assembly requires a multifaceted approach that encompasses various aspects of the manufacturing process. By optimizing stencil design, solder mask properties, component placement and selection, solder paste printing, reflow soldering parameters, and implementing robust inspection and quality control measures, manufacturers can significantly reduce the occurrence of these defects and improve the overall reliability and functionality of their PCB assemblies. Continuous monitoring, process optimization, and adherence to industry best practices are key to achieving high-quality PCB assemblies free from open defects and tombstones.

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