Error-Proofing and Visual SOP in Lithium Cell Assembly

Cell assembly error-proofing starts from a premise most industries do not have: the steps are effectively irreversible. Once the tabs are welded, the top cap sealed and the electrolyte filled, there is no rework path — only scrap or downgrade, and disassembling a charged cell carries its own safety risk. That means the effective move is to push the judgement point ahead of welding and filling: confirm tab position, insulation film presence, can orientation and fill port state at the station, rather than sorting the problem out at formation or in the module line. Two hard constraints come with the territory — metal particles and dust are tied directly to internal short risk, and live cells with a flammable electrolyte limit what equipment can be used and where it can sit.

Why the window is narrower than in the pack line

The main line runs roughly: electrode coating and slitting, winding or stacking, can insertion, tab welding, top-cap welding, filling, sealing, formation, grading. Problems found at module or pack level are mostly assembly issues and can be opened up and corrected. Problems found at cell level are already sealed inside the can. So the question of when a defect is discovered carries far more weight here than in most industries — and that in turn settles where the money should go: the stations before welding and filling. The shared characteristics of thin, high-precision and irreversible work are discussed for semiconductor processes.

• Winding and stacking. Electrode alignment, separator wrinkling or displacement, overhang beyond tolerance. Some of this is only visible under X-ray — it is internal structure.
• Tab welding. Weld spot offset, missing welds, cold joints from ultrasonic or laser welding. Position and presence can be judged by vision; strength cannot.
• Top-cap welding. Blowholes, pinholes and weld offset on the cap seam. Appearance defects can be screened visually; hermeticity needs helium leak testing.
• Filling and sealing. A missing or reversed seal at the fill port, foreign material present before sealing.
• Labels and film. Insulation film or blue film left off, QR or batch marking legible or not.

Placing equipment under clean and Ex constraints

The common mistake is to specify the way you would on a general assembly line: an industrial PC with a cooling fan bolted next to the machine, cables run casually across the conveyor, light power turned up high to guarantee brightness. None of that causes trouble on an ordinary line, and all of it meets resistance in a clean zone. A steadier approach puts the compute unit in a cabinet outside the clean zone, routes only the necessary cabling to cameras and lights with sheathing matched to the clean class, and prefers low-heat lighting that solves contrast through geometry rather than wattage. Camera and shutter choice affects how sharp a moving target is, which is covered under global versus rolling shutter.

How far vision can actually reach

Using that table as a requirements list is far more useful than a blanket target of higher detection rate. Write “what will be judged and what will not” as acceptance items during selection and most of the later argument disappears. On the trade-off between thresholds, misses and false alarms, reducing false alarms goes into more detail.

What belongs to a different inspection method

Internal structure and electrical performance are outside what vision can cover. Electrode alignment and separator condition belong to X-ray. Cap seam hermeticity belongs to helium leak or pressure testing. Capacity and internal resistance belong to the grading equipment. Written into a vision scope, these give no measurable acceptance criteria. What vision should carry is the layer above: whether the action was performed, whether the part went in at the right place, whether the marking is legible. It complements X-ray and leak testing. For splitting the work across the line, see vision versus sensor division; to keep a process record for every cell, how visual SOP connects to MES covers three common channels.