Meaning
A localized manufacturing defect describes an microscopic void or bubble that forms within the polymer seal layer of a pouch cell during the thermal welding process. This void, known as a seal channel micro cavity, creates a weak point that can develop into a continuous leak path over time. Process engineers monitor the formation of these voids to optimize sealing bar profiles and temperature settings on the assembly line.
The defect is defined as any void that reduces the effective seal width below the specified safety margin.
Formation Mechanism
Moisture or volatile solvent droplets trapped on the polymer surface before welding vaporize when the hot sealing bars are applied, creating trapped gas bubbles. Alternatively, uneven pressure distribution across the sealing bars can leave localized low-pressure zones where the polymer does not fully flow and fuse. These micro cavities are often located near the metal tab feedthroughs, where the transition in thickness makes it difficult to apply uniform pressure.
If the weld cools too quickly, the polymer shrinks rapidly, which can also initiate internal voids.
Leak Path Development
While a single micro cavity may not immediately span the entire width of the seal, it reduces the distance that moisture must travel to enter the cell. Under the action of internal gas pressure and external mechanical vibrations, these individual cavities can merge to form a continuous open channel. This channel allows volatile electrolyte solvents to escape and atmospheric moisture to enter the cell casing.
This gradual moisture ingress slowly degrades the active materials, resulting in cell swelling and accelerated capacity fade during field use.
Detection and Control
Non-destructive testing methods, such as high-resolution ultrasonic imaging and thermographic scanning, are used to detect these internal cavities on the production line. Standard visual inspection cannot identify these buried defects, making automated instrumentation necessary for high-volume manufacturing. Process engineers adjust the sealing jaw geometry and implement pre-drying steps to eliminate surface moisture before the thermal weld is made.
These actions help to maintain the long-term reliability of the battery pouch seals.
