Exposed Pad Soldering and PCB Heat Management Guide
With modern electronic devices getting smaller, effective thermal management is more critical than ever. High-density PCB designs often face serious heat dissipation challenges, especially when using QFN (Quad Flat No-lead) packages. Unlike traditional leaded packages, QFNs rely on their exposed pad for both electrical grounding and heat dissipation. Without proper design and soldering techniques, poor heat dissipation can cause overheating, reducing circuit stability and lifespan.
In high-power chips, thermal performance is directly associated with circuit stability and lifespan. A poorly designed heat dissipation path may cause localized overheating, leading to device failure. Optimizing the design of exposed pads and soldering processes can significantly improve heat dissipation efficiency.
What Is an Exposed Pad?
The exposed pad, also known as the thermal landing, is part of the chip’s lead frame. It serves a dual purpose:
- Thermal conduction: It provides a direct path for heat transfer from the die to the PCB copper layers.
- Electrical connection: It helps with grounding and reduces electrical noise.
The pad forms a heat conduction path of low thermal resistance by directly connecting the silicon die inside the chip to the external PCB copper foil.
This structure offers:
Low parasitic inductance, making it suitable for high-frequency applications.
Excellent thermal performance, lowering the overall thermal resistance of the package.
The exposed pad design also places higher demands on the soldering process in terms of controlling gas emissions and solder volume during reflow soldering.
Key Challenges in Exposed Pad Soldering
Solder Voids & Poor Heat Transfer
If gas is trapped during soldering, voids form within the solder joint, reducing thermal and electrical conductivity.
Component Floating
Excessive solder paste can lift the chip off the PCB, misaligning surrounding pads and causing weak joints.
Incorrect Via Design
Poorly designed vias can cause solder leakage, leading to poor bonding and heat dissipation inefficiencies.
How to Optimize Exposed Pad Design for Better Heat Dissipation?
Optimize Copper Pad Size
Ensure that the PCB pad is slightly larger than the chip’s exposed pad to maximize heat transfer.
Thermal Via Design
- Use 13-mil diameter vias within the solder pad, following the manufacturer’s datasheet.
- Use 25-mil diameter vias outside the solder area to provide additional heat dissipation.
Segmented Solder Paste Application
Instead of covering the entire pad, divide it into smaller sections (e.g., 9-segment pattern) covering about 60% of the area. This helps gases emission and prevents excessive solder buildup.
Advanced Copper-Core PCB: A Breakthrough in Thermal Management
Traditional PCBs rely on standard copper or aluminum substrates for heat dissipation. However, these materials have limitations due to the thermal resistance of their insulating layers. LCSC introduces a cutting-edge Direct Heatsink Copper-Cored PCB, designed to enhance thermal performance for high-power applications.
How Does the Direct Heatsink Copper-Cored PCB Work?
- Principle: By separating the thermal conductive tabs from the conductive pads and using them only for heat conduction, the thermally separated copper substrate allows heat to be quickly exported from the heat source and improves the overall thermal performance, thus effectively avoiding the risk of short circuits.
- Heat conduction efficiency: With a thermal conductivity of 380 W/m·K, this new technology vastly outperforms traditional PCBs (which typically range from 1–18 W/m·K).
| Comparison | Single-Sided Copper/Aluminum Substrate | Thermoelectric Separation Copper Substrate |
| Heat Conduction Mechanism | Heat must pass through the insulating layer (thermal conductivity of only 1–18 W/m·K) before reaching the copper or aluminum base. The insulating layer limits overall heat dissipation efficiency. | Heat is directly transferred to the copper base through a thermal conduction boss, bypassing the insulating layer entirely, while FR4 material provides structural support. This design significantly enhances heat dissipation efficiency. |
| Summary | Due to the thermal limitations of the insulating layer, heat dissipation efficiency is low, making it difficult to meet the cooling demands of high-power chips. | Direct heat conduction maximizes the thermal advantages of the copper substrate, significantly improving thermal management for high-power chips. |
Case Study: Thermal Optimization for MP4560 DC-DC Converter
In a MP4560 buck converter design, the initial PCB used FR-4 material, which struggled to handle the high-frequency and high-current conditions. The poor heat dissipation affected long-term reliability.
The magic bullet to this problem, the design team used a Direct Heatsink Copper-Cored PCBs and added a special heat sink underneath the exposed pad on the bottom of the MP4560DN-LF-Z. The module’s thermal performance is effectively improved, resulting in a significant reduction in the PCB surface temperature. In addition, the heat from the module can be quickly transferred to the copper substrate and diffused, further improving the cooling efficiency.
For high-power applications, using optimized PCB materials can significantly enhance system efficiency and reliability.
After testing, the thermal efficiency of this design is significantly improved compared to the original solution. This thermal management method is not only applicable to the MP4560, but can also be extended to other high power DC-DC converter designs. The use of a copper substrate and a rational thermal structure not only significantly improves circuit performance, but also extends product life.
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