Detect critical THT solder joints immediately

Why Design Rules Are Not Enough:

The Crucial Role of Simulation-Based DfM in THT Soldering in high power and high frequency applications

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Introduction to THT and pcb design rules

In the world of electronics manufacturing, ensuring that a printed circuit board (PCB) is designed for manufacturability is essential for producing reliable and high-quality products. While design rules (also known as design guidelines) provide a set of best practices for PCB layout, they often fall short when it comes to addressing the complex thermal behaviors of through-hole technology (THT) solder joints during the soldering process. This gap can lead to insufficient solderability, poor hole fill, and ultimately, unreliable electronic assemblies.

This article explores why traditional design rules are not the same as Design for Manufacturability (DfM), highlighting the limitations of design rules in handling the nonlinear thermal complexities of THT solder joints. We will showcase a real-world example to illustrate these challenges and explain how simulation-based DfM analysis can bridge the gap, ensuring optimal solderability and manufacturability.

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Understanding Design Rules and Their Limitations

What is The Essense of Design Rules?

Design rules are a set of guidelines that outline the best practices for designing PCBs [1, 2]. They primarily focus on the geometrical constraints to ensure that a solder joint can be properly accessed and processed by manufacturing tools, such as soldering equipment. These rules cover aspects like:

• Minimum trace widths and spacing
• Pad sizes and shapes
• Thermal pad configurations
• Via placements and sizes
• Component placement clearances

Design rules are intended to provide a one-size-fits-all solution, offering general recommendations that, if followed, should result in a manufacturable PCB design.

Why Design Rules Are Not the Same as DfM

While design rules offer valuable guidance, they are not synonymous with Design for Manufacturability (DfM). DfM is a comprehensive approach that considers all aspects of the manufacturing process, ensuring that the product design is optimized for efficient, high-quality production. In the context of THT soldering, DfM goes beyond geometric constraints to address:

• Thermal behaviors of solder joints
• Material properties and interactions
• Process parameter variations
• Nonlinear interdependencies between design elements

Design rules often fail to capture these complexities, particularly the nonlinear thermal effects that significantly impact solderability in THT solder joints.

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The Complexity of Thermal Behavior in THT Soldering

Nonlinear Thermal Interdependencies

The thermal behavior of a solder joint during the soldering process is influenced by various geometric parameters of the copper layout. These parameters include:

• Copper plane sizes and thicknesses
• Thermal vias and their configurations
• Trace widths connected to the plated through-holes (PTHs)
• PCB thickness

The relationships between these parameters and the thermal performance of the solder joint are highly nonlinear and interdependent. Small changes in one parameter can have significant and unpredictable effects on the thermal demand and solderability of a joint.

Limitations of Design Rules in Addressing Thermal Complexity

Design rules attempt to simplify this complexity by providing generic recommendations, such as:

• Adding thermal pads or heat traps to THT solder joints
• Specifying minimum or maximum hole sizes
• Recommending certain gap widths between pins and holes

However, these generic hints are often insufficient or unnecessary, depending on the specific PCB design situation. For instance, in applications requiring high current capacity, functional requirements may dictate that PTHs be connected to thicker traces or planes, increasing thermal mass and complicating solderability.

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Real-World Showcase: The Limits of Design Rules

To illustrate the shortcomings of relying solely on design rules, let’s examine a real-world example from the GCD Printlayout GmbH. The purpose of this case study was to identify thermal challenges in the soldering process early and address them effectively during manufacturing. Recognizing that the PCB layout presented thermal issues due to specific functional requirements and in accordance with customer requirements and specifications, our objective was to tailor the solder contact time individually for each solder joint. This customization aimed to minimize exposure to thermal stress during soldering, thereby mitigating the risk of thermal damage to materials, which could reduce the product’s lifespan and deteriorate the sensitive high-frequency (HF) functionality of the assembly.

Case Study: Conflicting Outcomes Despite Design Rule Compliance

Scenario:

• High power electronics PCB layout for high frequency (HF) testing application
  • 8 copper layers
  • PCB thickness: 1.6 mm
  • Outer layers thickness: 70 µm
  • Inner layer thickness: 35 µm
• Two THT components and their solder joints on the same PCB layout are evaluated.
• In Figure 1 the left solder joint does not comply with standard design rules due to minimal clearance around the joint.
• In Figure 1 the right solder joint fully complies with design rules, providing ample clearance for soldering tools.

Figure 1: Comparison of two THT solder joints—Left: Non-compliant but highly solderable; Right: Compliant but poorly solderable.

Observation:

• Left Solder Joint (Non-Compliant): Clearance between SMT pads and THT solder joint too small. Despite violating design rules, simulation-based solderability analysis predicts excellent solderability for this joint.
• Right Solder Joint (Compliant): Sufficient clearance. Despite adhering to design rules, the solderability analysis indicates extremely high solder contact times, suggesting poor solderability.

Important note: In this specific case the heavy copper connection of the connector is a functional requirement for the HF and high power application and explicitly intended. Though, the use of the solderability analysis based on modelling approaches provides a deep insight into the thermal behavior of the solder joints. On this basis the soldering parameters can be recommended.

Figure 2 shows the x-ray image of experimentally soldered solder joints by DEEPTRONICS GmbH to show the validity of the solderability calculation as method to identify critical solder joints and the process limitations.

Figure 2: Validation soldering experiment showing the significant effect of the solderability as the thermal aspect of DfM

This example demonstrates that design rule compliance does not guarantee manufacturability or optimal solderability. The left joint, though non-compliant, has favorable thermal characteristics that promote good solderability. In contrast, the right joint, while compliant, has thermal properties that hinder effective soldering.

Why Design Rule Checks (DRC) Are Insufficient

The False Sense of Security

Design rule checks (DRC) provide a checklist approach to PCB design, ensuring that all guidelines are met. However, passing a DRC can give designers a false sense of security, believing that their design is fully optimized for manufacturing. This can lead to serious thermal risks and soldering issues that are not detected until production, resulting in:

• Poor hole fill and solder joint reliability
• Increased rework and scrap rates
• Delayed production schedules
• Higher manufacturing costs

Inability to Address Thermal Demand

Design rules are limited in their ability to account for the thermal demand of each individual solder joint. Since every THT solder joint has its unique design configuration and thermal behavior during soldering, a one-size-fits-all approach is inadequate. The thermal performance required to achieve proper soldering cannot be captured by generic rules alone.

Embracing Simulation-Based DfM Analysis

What Is Simulation-Based DfM?

Simulation-based Design for Manufacturability (DfM) involves using advanced simulation tools to model and analyze the thermal and physical behaviors of solder joints during the soldering process. This approach allows engineers to:

• Predict solderability issues before production
• Optimize PCB designs for thermal performance
• Balance functional requirements with manufacturability
• Reduce trial-and-error in the design process

Benefits of Simulation-Based DfM

• Customized Analysis: Accounts for the specific thermal properties of each solder joint.
• Improved Solderability: Ensures that solder joints can be reliably formed within the process window without causing thermal damage.
• Risk Mitigation: Identifies potential soldering problems early in the design phase.
• Cost Savings: Reduces rework, scrap, and production delays.

DEEPTRONICS Simulation-based dfm as the new state of the art of Dfm

At DEEPTRONICS GmbH, we specialize in simulation-based DfM analysis for THT soldering processes. Our expertise bridges the gap between design rules and true manufacturability, ensuring that your PCB designs are optimized for reliable, efficient production.

Why Choose DEEPTRONICS?

• Advanced Simulation Tools: We utilize cutting-edge simulation software to analyze thermal behaviors and solderability.
• Expertise in THT Soldering: Our team has extensive experience in addressing the complexities of THT solder joints.
• Customized Solutions: We provide tailored recommendations based on your specific PCB design and manufacturing processes.
• Proven Track Record: Our methods have helped numerous clients improve solderability and reduce manufacturing costs.

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Design for Manufacturing (DfM) and design rules are frequrently understood synonymously. Yet, DfM is actually the intersection between design rule compliance, and the solderabiliy which describes the thermal feasiblity of a THT solder joint.

Don’t let inadequate design rules compromise your PCB manufacturability and product reliability. Take the next step toward optimizing your designs with simulation-based DfM analysis.

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Conclusion

Design rules are a valuable starting point in PCB design but fall short in ensuring true manufacturability, especially for complex THT solder joints. The nonlinear thermal behaviors and interdependencies inherent in these joints require a more sophisticated approach.

Simulation-based DfM analysis offers the solution by providing detailed insights into the solderability of each joint, allowing for optimization that design rules alone cannot achieve. By embracing this approach, manufacturers can avoid costly soldering problems, improve product quality, and gain a competitive edge in the market.

Final Thoughts

Ensuring the manufacturability of PCBs with THT components requires more than just adherence to design rules. By leveraging simulation-based DfM analysis, you can proactively address thermal challenges, optimize your designs, and achieve superior manufacturing outcomes.

At DEEPTRONICS, we are uniquely equipped to provide this specialized service, helping you navigate the complexities of THT soldering and deliver high-quality electronic products.

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References

1. PCB Layout Guidelines
2. Copper Layers in PCB Design