Vacuum reflow soldering is used to reduce voiding and oxidation in solder joints, particularly for thermal assemblies, power devices, BGA packages, and bottom-terminated components. Compared with conventional reflow soldering, the process adds controlled vacuum parameters to the thermal profile. This makes it possible to remove more gas from molten solder and achieve a lower, more stable void ratio.
1. Causes of Solder Voids
During reflow soldering, solder paste melts and then solidifies as the temperature decreases. Gases generated by flux and other materials may not escape before solidification. They are then trapped inside the solder joint as voids.
Research indicates that flux and other solder-paste constituents are the most significant factors affecting void formation. The selection of solder alloy and metal-powder particle size also has an important influence. Additional factors include the component package type, PCB land design, surface finish, stencil-aperture design, and reflow-profile settings.
Voiding is especially important for three types of assemblies:
- Thermal assemblies: Voids increase thermal resistance and reduce heat-transfer efficiency.
- Power assemblies: Voids reduce the effective soldering area and can affect power transfer and current-carrying performance.
- BGA and BTC assemblies: Voids reduce solder-joint strength and may affect long-term mechanical reliability.
2. Effects of Solder Voids
2.1 Impact on Product Reliability
Voids reduce solder-joint mechanical strength and increase thermal resistance. They can therefore degrade both the thermal and electrical performance of the joint, reducing component reliability.
For thermal-management and power applications, the effect is particularly serious. A void interrupts the effective heat-transfer path between the component and the PCB. As the thermal resistance increases, component temperature rises. Higher operating temperature can accelerate aging and increase the probability of failure. The referenced research indicates that approximately 60% of electronic-product failures are related to temperature rise and that component failure rates increase exponentially with temperature. It also states that a 10 °C temperature increase can double the failure rate.
2.2 Industry Requirements
IPC-A-610, IPC-7095, and IPC-7093 describe solder-joint voiding for BGA and BTC packages. For these package types, the referenced requirement specifies a void ratio of no more than 30%. Other applications may not have a universally defined limit, so the manufacturer and customer must establish the acceptance criteria together.
For the ground lands of high-power components, customers for high-reliability products often specify a void ratio below 10%, and in some cases an even lower value. Large-area thermal lands are difficult to control with conventional process adjustments alone, so equipment capability and process stability become critical.
3. Methods for Reducing Voids
3.1 Process-Based Methods
Several process methods can reduce voiding. These include using low-voiding solder paste, optimizing PCB land geometry, applying an array-style stencil-aperture pattern, soldering in a nitrogen atmosphere, and using preforms.
Although these methods can improve results, their effect is not always sufficient or stable. Large-area ground lands are particularly difficult to control, and conventional process methods may not consistently reduce the void ratio below 10%.
3.2 Equipment-Based Methods
Vacuum soldering can achieve a stable void ratio below 5% and is therefore an effective method for controlling voids. Vacuum vapor-phase soldering, however, is not well suited to high-volume production because of its process principle and equipment structure. Vacuum reflow soldering has become increasingly mature and provides a more practical approach for production lines.
According to the referenced analysis, when the vacuum level is below 20 mbar, gas bubbles can be reduced effectively, making a void ratio below 5% achievable. The result depends on the complete process combination, including the solder paste, land design, stencil pattern, thermal profile, and vacuum parameters.
4. Vacuum Reflow Soldering
Vacuum and controlled-atmosphere eutectic soldering equipment performs high-quality soldering in a vacuum environment. During the process, a reducing or protective atmosphere such as N2, formic acid, N2H2, or H2 may be introduced to protect component terminations and solder from oxidation. The combination of atmosphere control and vacuum processing helps reduce voiding.
4.1 Applications and Process Characteristics
The equipment uses infrared radiation heating. This provides relatively uniform heating, low-temperature-safe soldering, minimal temperature variation, reduced risk of localized overheating, and stable process parameters. The process does not require extensive process experimentation for every application, and its operating cost can be low when the system is properly configured.
The equipment has a relatively high thermal capacity, while the temperature difference across the PCB surface is small. These characteristics make it suitable for mixed-product, low-volume, and high-reliability manufacturing. The technology has been used in aerospace, defense electronics, automotive electronics, medical electronics, and other demanding applications.
The development of the LED industry has created additional demand for low-voiding soldering. UV LED and Mini-LED products use very small package structures and can contain tens of thousands of solder joints on a single PCB. Flatness, heat dissipation, and long-term reliability therefore present substantial process challenges.
Manufacturers have evaluated vacuum reflow soldering for products such as P0.9 and P0.6 displays. In these applications, the process has been used to target void ratios below 2%, improving the reliability of the delivered assemblies.
As electronic products become smaller, reducing voids and oxidation in solder materials becomes increasingly important. Conventional reflow soldering may have difficulty consistently minimizing oxidation and controlling voiding, especially beneath large lands. These limitations are a major reason vacuum reflow soldering is considered for high-quality soldering processes.
4.2 Vacuum Process Parameters
In addition to the conventional reflow parameters—temperature, conveyor speed, and airflow—vacuum reflow soldering introduces four additional parameters:
- Vacuum level
- Vacuum-pull time
- Vacuum-hold time
- Atmospheric-pressure filling time
The equipment can also use staged evacuation. The chamber pressure is reduced step by step rather than dropping abruptly. This helps prevent excessive vacuum shock from disturbing molten solder joints. It also limits the rate of pressure change between bubbles inside the molten solder and the vacuum chamber.
If the pressure difference changes too quickly or becomes too large, the molten solder may spatter or exhibit a "solder explosion" effect. This can produce solder beads around the component. A staged vacuum profile is therefore important when the assembly or solder-paste system is sensitive to rapid pressure changes.
4.3 Equipment Configuration
A vacuum reflow oven is generally based on a conventional reflow oven with an additional vacuum chamber positioned near the end of the high-temperature reflow zone. Equipment designs differ. One configuration uses three modular sections, while another uses an integrated structure.
In a representative three-section configuration, the equipment consists of:
- Preheat and reflow module: typically divided into six to eight heating zones.
- Vacuum module: divided into two zones.
- Cooling module: divided into two to five cooling zones.
4.4 Main Advantages
- High solder-joint reliability: The soldering process is completed in a controlled atmosphere, which can reduce bubbles, impurities, other internal voids, and oxidation of the solder joints.
- Uniform heating: More uniform heating reduces the shadow effect and improves temperature consistency, including at solder joints located beneath or beside concealed component areas.
- Lower operator-skill requirements: The operating procedure is relatively simple, and operators can become familiar with it after a short training period compared with more complicated soldering methods.
- Lower operating cost: Energy consumption can be reduced, and the equipment may not require compressed or protective gases in configurations that use an integrated vapor-recovery system.
An integrated recovery system can reduce loss of the vapor-phase medium. A dedicated conveyor-maintenance system can also reduce routine maintenance costs.
5. Temperature Profiles and Soldering Comparisons
5.1 Measuring the Thermal Profile
The PCB must remain in the vacuum section for approximately 10–30 seconds during soldering. Consequently, thermal-profile measurement differs from measurement in a conventional reflow oven.
The equipment software normally provides a dedicated temperature-measurement mode. When this mode is enabled and the profiling board reaches the vacuum section, the conveyor stops. The upper cover of the vacuum chamber does not descend, preventing it from pressing against the profiler or thermocouple wires. The vacuum pump also remains off.
After the profiling board has remained in the vacuum section for the cumulative time specified by the vacuum parameters, the conveyor resumes operation. This procedure simulates the production reflow profile while protecting the temperature-measurement equipment.
5.2 Longer Time Above Liquidus
After entering the vacuum section, the PCB must remain there for the vacuum soldering operation, typically for approximately 30 seconds, before it can continue to the cooling section. The total reflow time is therefore longer than that of conventional reflow soldering.
The time above liquidus (TAL) may reach approximately 100 seconds. Components with a relatively short TAL specification may therefore be exposed to a process condition outside their recommended range. Such components must be identified and addressed during process design.
5.3 Three-Section Conveyor System
The conveyor in a vacuum reflow oven is divided into three sections: the reflow section, the vacuum section, and the cooling section. Under normal conditions, the conveyor speed of all three sections is set to the same value.
When the vacuum function is enabled, the cooling-section speed may be configured independently. The speeds of the front and rear conveyor sections can therefore differ. This changes the effective thermal-profile parameters and can also affect the cooling slope and the temperature of the PCB as it exits the oven.
6. Risk Points in Vacuum Reflow Applications
6.1 Component-Package Failure
Most components can tolerate the vacuum reflow process. However, non-hermetic components with internal cavities may be at risk. For example, air inside the cavity of a QFN package expands when heated. When this internal expansion is combined with the external vacuum environment, the pressure difference across the package can become greater than during conventional reflow soldering.
When the temperature exceeds the glass-transition temperature, or Tg, the material coefficient of thermal expansion can increase significantly while mechanical strength decreases sharply. The combined effect of material thermal stress and internal-to-external pressure may cause package cracking.
6.2 Excessive Reflow Time
Vacuum reflow soldering generally has a longer reflow time than conventional reflow soldering, commonly at least 80 seconds and sometimes at least 100 seconds. Components with a short TAL specification may exceed their allowable process range, creating a risk of component damage.
6.3 Solder-Joint Risks
- BTC packages: The stand-off height of the solder joints may decrease noticeably. Solder can spread outward, increasing the risk of bridging. Where necessary, selected stencil apertures should be reduced appropriately.
- BGA packages: When the BGA pitch is 0.4 mm or less, vacuum processing may increase the risk of solder bridging. Vacuum processing is therefore not recommended for fine-pitch BGAs under the stated condition. If it must be used, the stencil apertures should be reduced appropriately while still satisfying the required aperture-area ratio.
- Large-area ground lands: As voiding is substantially reduced, the apparent solder coverage may also decrease. The stencil aperture area for the ground land may need to be increased appropriately.
6.4 Equipment Risks
The principal equipment risks come from the three-section conveyor and the vacuum chamber. A gap exists between the conveyor in the vacuum section and the conveyors in the adjacent sections. The gap is approximately 20–30 mm, while the conveyor return radius is approximately 15 mm. As a result, the PCB may experience an unsupported contact region of approximately 50–60 mm while crossing the gap.
For PCBs smaller than 100 mm, this condition can increase the probability of board jamming and vibration. Possible defects include component displacement, falling components on the underside of the PCB, and short circuits caused by BGA solder balls. A fixture may be required to reduce these risks during oven transfer.
The vacuum section also contains multiple moving parts that operate at high temperature, at least 250 °C over extended periods. Maintenance and inspection must therefore be performed carefully. Conveyor chains, sensors, sealing rings, and other components must remain in good condition. Otherwise, vacuum-control accuracy may deteriorate, and board jams or transfer failures may occur.
6.5 Operator and Board-Spacing Risks
During production, a PCB remains in the vacuum section for a defined period while boards continue to move through the upstream preheat section. The spacing between boards entering the oven must therefore be controlled strictly.
The equipment may use the SMEMA interface to coordinate board-entry signals. However, manually pushing boards into the oven can create a risk if the spacing between two boards is less than the minimum interval defined by the equipment. The boards may collide or jam in the vacuum section, resulting in avoidable damage.
Conclusion
Vacuum reflow soldering addresses one of the most persistent challenges in high-reliability electronic assembly: voids trapped in solder joints. By combining controlled heating, protective atmosphere management, and staged pressure reduction, the process can significantly reduce voiding and oxidation compared with conventional reflow methods.
Its benefits are particularly relevant to thermal lands, power assemblies, BGA and BTC packages, Mini-LED products, and other applications where thermal resistance, solder-joint strength, or reliability is critical. However, vacuum reflow is not a universal replacement for conventional reflow. Longer TAL, package pressure stress, solder bridging, conveyor gaps, maintenance requirements, and board-spacing control must all be evaluated during process qualification.
The most reliable implementation is based on a complete process review that includes component limitations, stencil design, PCB support, conveyor settings, vacuum parameters, thermal-profile measurement, and inspection criteria.
