Overcoming Voiding Problems in High-Reliability Electronic Assemblies Using Vacuum Reflow Processes

Abstract This document elaborates on the formation mechanism of solder voids during the reflow soldering process of Surface Mount Technology (SMT) and their severe impact on product reliability. It focuses on analyzing vacuum reflow soldering as an innovative process, including its working principle and outstanding effectiveness in eliminating solder voids and enhancing the density and integrity of solder joints. This paper aims to provide theoretical basis and process solutions for engineers in the field of high-reliability electronic manufacturing.

2. Introduction: Causes and Impacts of the Void Problem Solder voids are tiny air bubbles trapped inside the solder during the SMT soldering process. Their formation mainly stems from the following factors: Flux Volatiles: Vaporization of the flux in the solder paste at high temperatures. Oxidation of PCB and Component Pads: Gas generated by the reaction between oxides and flux. Plating Pores: Air trapped in the pores of the plating on PCB pads or component leads. Improper Temperature Profile: Excessively fast heating rate preventing gas from escaping in time. The hazards of voids are evident: Reduced Thermal Reliability: The presence of voids reduces the effective cross-sectional area for heat conduction, significantly increasing the thermal resistance of solder joints, which leads to a rise in chip junction temperature and affects performance and service life. Insufficient Mechanical Strength: Voids become stress concentration points. Under thermal cycling or mechanical vibration, cracks tend to initiate and propagate from voids, resulting in solder joint failure. Unstable Electrical Connection: In severe cases, it may affect the continuity and stability of electrical connections. For fields such as automotive electronics, aerospace, medical equipment, and high-power devices (e.g., IGBT, SiC modules), controlling the void rate is a necessary condition to ensure long-term product reliability, and the void rate is usually required to be below 3% - 1%.

3. Limitations of Traditional Reflow Soldering Traditional convection reflow ovens reduce oxidation by injecting nitrogen (N₂) under normal pressure. Although this can improve the void situation to a certain extent, it has physical limitations: Normal Pressure Environment: The atmospheric pressure inside the furnace "pressurizes" the bubbles in the molten solder, making it difficult for them to expand and escape effectively. The buoyancy of the bubbles is insufficient to overcome the surface tension of the viscous solder liquid. Narrow Process Window: The method of reducing voids by optimizing the temperature profile (such as extending reflow time and adjusting the soaking zone) has limited effect and is prone to causing other problems, such as excessive oxidation of components, warping of PCB boards, and damage to heat-sensitive components.4. Working Principle and Process Breakthrough of Vacuum Reflow Soldering The vacuum reflow soldering technology introduces a vacuum process at the critical stage of the standard reflow profile - around the peak temperature of the reflow zone, which fundamentally changes the dynamic environment of bubbles.

5. Summary of Advantages of Vacuum Reflow Soldering Extremely Low Void Rate: It can stably achieve a void rate of < 1%, easily meeting the most stringent industry standards. Excellent Solder Joint Quality: The internal structure of solder joints is denser, and the mechanical strength and fatigue resistance are significantly improved. Higher Reliability: Excellent thermal management performance ensures the stability of power devices under long-term harsh working conditions. Reduced Flux Residues: The vacuum environment helps volatilize and extract residues, reducing the difficulty of subsequent cleaning and the risk of corrosion. Process Compatibility: It is perfectly suitable for the soldering of QFN/BGA/LGA, PoP, SiP (System-in-Package), 2.5D/3D IC, and various power modules.

6. Application Case: Soldering of a Certain Automaker's Power Controller Module Challenge: A new energy vehicle motor drive controller produced by the customer requires the solder void rate of its main SiC power module to be ≤ 2%. After using high-end nitrogen reflow soldering, the average void rate was 5 - 8%, resulting in low yield. Solution: Adopting our company's VRS series vacuum reflow soldering oven. Process Parameters: The vacuum was activated when the peak temperature reached 245°C, and the vacuum degree inside the chamber was reduced to 5 mBar within 15 seconds and maintained for 15 seconds. Result: X-Ray inspection showed that the average void rate of solder joints was stably reduced to 0.8%, with excellent distribution uniformity. The product passed the rigorous thermal cycling test, the customer's yield was significantly improved, and mass production was successfully realized.

7. Conclusion and Outlook Vacuum reflow soldering is not a simple replacement for traditional reflow soldering, but a strategic process upgrade for future high-end electronic manufacturing. It directly solves the core void problem in high-reliability soldering and provides a reliable interconnection guarantee for the next generation of electronic devices (such as smaller-sized chips and higher-power modules). Choosing vacuum reflow soldering means choosing higher product quality, longer service life, and stronger market competitiveness.