What is Sunken Joint in PCB?

PCB Basics 
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Editorial Team - PCB Directory

Oct 18, 2023

Sunken Joints in Printed Circuit Boards (PCBs) are a distinct type of solder joint anomaly that can have significant repercussions for PCB functionality. Unlike standard solder joints, sunken joints exhibit a unique structural characteristic: they appear indented, often creating small cavities or gaps in the solder connection. This deviation from the norm can be attributed to several factors within the PCB assembly process.

Solder joints are the tiny, yet critical, connections that link electronic components to the PCB. They serve a dual purpose: not only do they establish the electrical pathways necessary for device operation, but they also provide the mechanical support needed to withstand the rigors of everyday use. These joints must be capable of enduring environmental stress, temperature fluctuations, and mechanical shocks while maintaining their integrity. Any compromise in the quality of solder joints can have far-reaching consequences.

Sunken joints represent a common challenge in PCB assembly. They are the solder joints that, for various reasons, fail to form as intended, resulting in depressions or gaps in the joint structure. The underlying causes of sunken joints can range from incorrect soldering techniques to component misalignment and insufficient solder material.

In this article, we will delve into the world of sunken joints in PCBs, exploring their causes, effects, and solutions. By understanding this phenomenon and its implications, we aim to shed light on the critical importance of achieving reliable solder joints in PCB assembly.

Sunken joints differ from standard solder joints primarily in their appearance and structural integrity. While standard solder joints are characterized by a smooth, uniform, and well-connected surface, sunken joints deviate from this norm by displaying depressions, voids, or incomplete solder coverage. These irregularities can hinder the electrical and mechanical performance of the joint, making them a cause for concern in PCB manufacturing.

The presence of sunken joints is a concern in PCB manufacturing for several reasons. Firstly, these depressions or gaps in the solder joint can lead to compromised electrical connections. Incomplete solder coverage may result in high electrical resistance or open circuits, disrupting the flow of electricity and impeding the device's functionality. Additionally, sunken joints can introduce mechanical vulnerabilities into the PCB, making it more susceptible to mechanical stress, vibration, and thermal cycling, which can ultimately lead to premature failure.

Causes of Sunken Joints

Sunken joints in PCBs can be attributed to a variety of factors within the soldering process and PCB assembly. Understanding these underlying causes is essential for identifying and mitigating the risk of sunken joints. Here are some key factors contributing to the formation of sunken joints:

  1. Inadequate Solder Volume: One of the primary factors leading to sunken joints is an insufficient volume of solder applied during the soldering process. When there is not enough solder, it may not fully cover the component leads and PCB pads, resulting in depressions and gaps in the joint.
  2. Soldering Temperature and Duration: The temperature and duration of the soldering process play a critical role. If the soldering temperature is too low or the duration is too short, the solder may not flow properly, leaving behind incomplete joints with depressions. Conversely, excessive heat or prolonged exposure can lead to excessive solder flow and subsequent sinking.
  3. Solder Alloy Composition: The type and composition of the solder alloy used can impact joint formation. Solder alloys with specific compositions have different flow characteristics. Using an alloy ill-suited for a particular application may lead to sunken joints.
  4. PCB Design and Pad Size: PCB design elements, such as pad size and layout, influence solder joint formation. Smaller or improperly designed pads can hinder the proper flow and distribution of solder, resulting in sunken joints.
  5. Component and PCB Material Compatibility: Compatibility between the materials of the PCB and the components being soldered is crucial. Incompatibility can lead to uneven wetting and the formation of sunken joints.
  6. The Role of Flux in Sunken Joint Formation: Flux is used during the soldering process to remove oxides and contaminants, promoting good wetting and bonding. However, excessive or inadequate flux application can contribute to sunken joints. In some cases, flux residues can trap gas and prevent solder from properly wetting the joint.

By addressing these contributing factors, PCB manufacturers and assemblers can take proactive steps to prevent the formation of sunken joints during the soldering process. In the next section, we will explore the implications of sunken joints on PCB reliability and performance.

Effects of Sunken Joints

Sunken joints in Printed Circuit Boards (PCBs) can have significant ramifications for the overall reliability and performance of electronic devices. Understanding the effects of these sunken joints is essential for recognizing their potential impact. Here are some key consequences associated with sunken joints:

1. Poor Electrical Conductivity: Sunken joints often result in incomplete electrical connections between components and the PCB. This poor electrical conductivity can lead to intermittent or complete electrical failures, affecting the functionality of the device. Inconsistent electrical pathways can cause signal degradation, data loss, or malfunctioning of critical components.

2. Thermal Issues: Sunken joints can impede the efficient dissipation of heat within the PCB. Properly soldered joints help transfer heat away from sensitive electronic components, preventing overheating and potential damage. Sunken joints disrupt this heat transfer process, increasing the risk of thermal stress on components and the PCB itself.

3. Mechanical Stability: Reliable solder joints are essential for maintaining the mechanical stability of components on the PCB. Sunken joints compromise the structural integrity of these connections, making them susceptible to mechanical stress, vibrations, and shocks. Over time, this can result in component displacement, misalignment, or even detachment.

4. Risk of Component Detachment: Sunken joints create weaker bonds between components and the PCB, increasing the likelihood of component detachment. This detachment can occur due to mechanical stress during operation or environmental factors, such as temperature fluctuations or physical shocks. Detached components can render the entire PCB non-functional.

Real-World Examples and Case Studies:

By examining various instances across various industries, we can gain valuable insights into how sunken joints have the potential to escalate into severe problems, often resulting in dire consequences. Here, we explore the significance of these case studies and their role in shedding light on the profound impact of sunken joints:

  1. Product Failures: Sunken joints have been implicated in numerous product failures, ranging from consumer electronics to industrial machinery. For instance, consider a smartphone manufacturer who unknowingly incorporates PCBs with sunken joints in their devices. Over time, these sunken joints can compromise the device's functionality, leading to issues such as intermittent power loss or malfunctioning components. 
  2. Costly Recalls: In some cases, the consequences of sunken joints are so severe that they necessitate large-scale product recalls. An example is the automotive industry, where PCBs are integral to various vehicle systems, including engine control units and safety mechanisms. A sunken joint affecting critical components in a vehicle's control system can lead to erratic behaviour, putting drivers and passengers at risk. 
  3. Compromised Safety in Critical Applications: Sunken joints are particularly concerning in applications where safety is paramount. In the aerospace or medical industries, for instance, PCBs with sunken joints can compromise the performance of essential systems. In an aircraft, a sunken joint affecting communication or navigation equipment can jeopardize passenger safety. 

These real-world examples serve as stark reminders of the far-reaching consequences of sunken joints in PCBs. They highlight the urgent need for stringent quality control measures and robust soldering processes in PCB manufacturing and assembly. 

Detection and Inspection of Sunken Joints

Ensuring the integrity of printed circuit boards (PCBs) involves rigorous detection and inspection procedures to identify sunken joints promptly. Detecting these anomalies early in the manufacturing process is crucial for maintaining product quality and reliability. We will explore various methods and techniques employed in the detection and inspection of sunken joints:

Visual Inspection Techniques: Visual inspection remains one of the primary methods for identifying sunken joints in PCBs. Experienced technicians and automated optical inspection (AOI) systems are used to scrutinize solder joints for any irregularities. Key aspects of visual inspection include:

  • Solder Joint Profile: Technicians examine the profile of solder joints to identify any depressions or irregular shapes that may indicate a sunken joint.
  • Surface Level: A straightforward visual check ensures that the solder joints align uniformly with the surface of the PCB. Any deviations are red flags for further investigation.
  • Consistency: Inspectors assess the consistency of solder joints across the entire PCB, looking for variations that may suggest soldering issues.

X-ray Inspection for Hidden Defects: While visual inspection can identify surface-level sunken joints, it may not reveal defects hidden beneath components or within complex PCB assemblies. X-ray inspection, also known as X-ray imaging or radiographic testing, plays a pivotal role in identifying concealed sunken joints. Key aspects of X-ray inspection include:

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  • Internal Assessment: X-ray imaging allows inspectors to peer inside components and PCB layers, providing a comprehensive view of the solder joints' internal structure.
  • Detecting Hidden Sunken Joints: X-ray scans can reveal sunken joints that are not visible through conventional visual inspection.
  • Quality Verification: X-ray inspection ensures that solder joints meet the required standards and are free from defects, enhancing overall product quality and reliability.

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The Importance of Quality Control and Testing: Effective quality control and testing procedures are integral to detecting sunken joints and ensuring the reliability of PCB assemblies. These practices involve:

  • Quality Control Checks: Implementing stringent quality control processes at various stages of PCB manufacturing, including solder paste application, component placement, and reflow soldering.
  • Functional Testing: Conducting functional tests on finished PCBs to verify that all components are properly soldered and functioning as intended.
  • Environmental Testing: Subjecting PCB assemblies to environmental tests, such as temperature cycling and vibration testing, to evaluate their performance under real-world conditions.
  • In-Circuit Testing (ICT): Employing ICT to assess the electrical connectivity and functionality of PCBs, identifying any issues with solder joints that may affect performance.

Detection and inspection techniques are critical in the prevention of sunken joints and the maintenance of PCB reliability. By combining visual inspections, X-ray imaging, and comprehensive quality control measures, manufacturers can identify and rectify soldering issues, ensuring that PCB assemblies meet the highest standards of quality and performance.

Preventing and Repairing Sunken Joints

Strategies for Preventing Sunken Joints during PCB Assembly

Maintaining the integrity of printed circuit boards (PCBs) and preventing sunken joints requires a proactive approach to PCB assembly. Manufacturers employ several strategies to minimize the occurrence of sunken joints:

  1. Proper Solder Paste Application: The foundation for reliable solder joints lies in the correct application of solder paste. Key considerations include:
    • Stencil Design: Designing stencils with appropriate thickness and aperture sizes ensures consistent and accurate solder paste deposition.
    • Solder Paste Quality: Using high-quality solder paste with optimal flux properties enhances soldering reliability.
  2. Reflow Soldering Profile Optimization: The reflow soldering process plays a pivotal role in achieving robust solder joints. Optimization involves:
    • Temperature Profiles: Creating precise temperature profiles during reflow soldering to ensure proper wetting and solder joint formation.
    • Ramp-up and Cool-down Rates: Controlling the rates at which the temperature rises and falls to prevent thermal shock and defects.
  3. Component and Pad Design Considerations: PCB design influences solder joint quality. Key design considerations include:
    • Pad Size and Shape: Properly sized pads with appropriate shapes accommodate solder volume and component placement.
    • Component Orientation: Ensuring correct component orientation and alignment prevents soldering issues.
  4. Material Selection and Compatibility: The choice of materials for PCBs and components affects solder joint reliability:
    • RoHS-Compliant Materials: Using materials that comply with Restriction of Hazardous Substances (RoHS) directives promotes soldering consistency.
    • Component Compatibility: Ensuring that components and PCBs are compatible in terms of thermal expansion and solderability.
  5. Best Practices for Ensuring Robust Solder Joints: Incorporating industry best practices, such as:
    • Solder Alloy Selection: Choosing solder alloys that match the specific requirements of the application, considering mechanical and thermal properties.
    • Flux Management: Using fluxes that are appropriate for the soldering process, ensuring flux residues are minimal and non-corrosive.
    • Soldering Equipment Calibration: Regularly calibrating and maintaining soldering equipment to ensure consistent and precise soldering.

Techniques for Repairing Sunken Joints

Despite preventive measures, sunken joints may occasionally occur. In such cases, repair techniques can salvage PCB assemblies and maintain their functionality:

  • Re-soldering: Re-soldering involves adding additional solder to the sunken joint, increasing its volume to the required level. Careful re-soldering can restore electrical connectivity and mechanical stability.
  • Reflow Soldering: Applying controlled heat during reflow soldering can help re-melt and reflow the solder to the desired shape and volume, eliminating the sunken appearance.
  • Component Replacement: In cases where sunken joints persist, replacing the affected component with a new one may be necessary. Proper desoldering and rework techniques are crucial to ensure a reliable replacement.

Industry Standards and Guidelines

The electronics manufacturing industry relies on a set of well-established standards and guidelines to ensure the quality and reliability of printed circuit board (PCB) assemblies. One of the primary standards that addresses the acceptability of electronic assemblies is IPC-A-610, which is published by the Association Connecting Electronics Industries (IPC). This widely recognized standard provides comprehensive criteria for the acceptance of electronic assemblies, including solder joint quality.

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Compliance and Quality Assurance Measures

Adherence to industry standards and guidelines, such as IPC-A-610, is essential for electronics manufacturers to maintain consistent quality and prevent defects like sunken joints. 

How Adherence to Standards Can Prevent Sunken Joints

Adhering to industry standards and guidelines, particularly IPC-A-610 plays a pivotal role in preventing sunken joints and ensuring the overall quality of PCB assemblies. Here's how compliance with these standards contributes to the prevention of sunken joints:

  1. Standardized Acceptance Criteria: IPC-A-610 provides clear and standardized criteria for evaluating the acceptability of solder joints. By following these criteria, manufacturers can identify and rectify sunken joints during inspections, preventing them from reaching the final product.
  2. Process Optimization: Industry standards often include best practices for processes like solder paste application and reflow soldering. Adhering to these best practices helps optimize the soldering process, reducing the likelihood of sunken joints.
  3. Material Selection: Standards may recommend materials that comply with regulations such as Restriction of Hazardous Substances (RoHS). Using RoHS-compliant materials ensures compatibility and reduces the risk of soldering issues.
  4. Quality Control: Industry standards emphasize the importance of quality control measures, including personnel training and equipment calibration. These measures are crucial for maintaining solder joint reliability.
  5. Continuous Improvement: Adherence to standards encourages a culture of continuous improvement. Manufacturers regularly review and update their processes to align with the latest industry guidelines, reducing the risk of sunken joints over time.

Industry standards and guidelines, such as IPC-A-610, provide a structured framework for electronics manufacturers to follow. Compliance with these standards not only enhances the overall quality of PCB assemblies but also plays a vital role in preventing sunken joints and other soldering defects. 


This article has explored "Sunken Joints," deviations from the norm in these connections, highlighting their significance in achieving reliable PCB assembly. Sunken joints can result from various factors: inadequate solder volume, temperature issues, solder alloy, PCB design, and flux management. They jeopardize reliability, leading to electrical problems, thermal issues, instability, and component detachment. 

Early detection through visual inspection and X-ray imaging, coupled with rigorous quality control and testing, is vital to ensure PCB reliability. When sunken joints do occur, repair techniques like re-soldering and reflow soldering can salvage PCBs.

Industry standards like IPC-A-610 are critical, providing a roadmap for consistent quality. Compliance with these standards not only improves PCB quality but also helps prevent sunken joints and other defects. Understanding sunken joints is vital in the ever-changing world of electronics. By addressing this issue, manufacturers can ensure their PCB assemblies perform reliably, keeping our devices running smoothly.

Additionally, if you are seeking PCB manufacturers or require PCB components, we recommend visiting PCB Directory, the largest directory of PCB Manufacturers and Fabricators on the Internet. PCB Directory provides comprehensive listings of manufacturers worldwide and valuable resources covering various aspects of PCB design, manufacturing, and testing processes.

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