While vacuum reflow soldering significantly improves solder joint quality, its unique process characteristics also introduce a series of potential risks that require careful evaluation and management during application.
1.The risk of device package failure.
For non-hermetic components with internal cavities (such as some QFNs), in a high-temperature vacuum environment, the air inside the cavity expands due to heat, creating a significant pressure difference with the external vacuum. Simultaneously, when the ambient temperature exceeds the material's glass transition temperature (Tg), its mechanical strength decreases sharply. Under the combined effect of thermal stress and the internal/external pressure difference, the device package is at serious risk of cracking.
2.The issue of exceeding reflow time limits.
Vacuum reflow soldering typically involves longer times above the liquidus line (generally ≥80s), which may exceed the upper limit of specifications for some heating-time-sensitive components, leading to overheating and damage.
3.Changes in solder joint morphology present new challenges.
1) For BTC-type devices, the vacuum environment significantly reduces the stand-off height of the solder joints, making the solder more prone to spreading outwards, increasing the risk of bridging. Therefore, reducing the size of the stencil openings should be considered.
2) For fine-pitch BGAs (e.g., pitch ≤ 0.4mm), vacuum processing can easily cause bridging, and its use is generally not recommended. If it must be used, the stencil opening must be reduced while meeting the area ratio requirements.
3) For large-area grounding pads, reducing the number of openings may actually lead to a decrease in solder paste coverage; the stencil opening needs to be appropriately enlarged to ensure the soldering area.
4.The equipment itself carries specific risks.
1) Its three-section chain conveyor system has gaps in the vacuum section. For smaller PCBs (e.g., <100mm), there is a risk of board jamming and vibration during board passage, which may cause component displacement or drop. Fixtures must be used for support.
2) Moving parts in the vacuum zone are exposed to high temperatures for extended periods, requiring extremely high maintenance and upkeep of the chain, sensors, and sealing rings; otherwise, malfunctions or vacuum level loss may easily occur.
5.Proper operating procedures are crucial.
The board insertion interval must be strictly maintained. If the operator manually pushes the boards improperly, resulting in insufficient board spacing, "board collision" or board jamming accidents can easily occur in the vacuum zone, causing production interruptions and product scrap.
