What is a non-conformity?
In quality management, a nonconformity (NC) is a failure to meet a requirement, which can be typically classified by level of severity: Critical, Major, and Minor. A critical nonconformity is likely to result in hazardous or unsafe conditions for individuals or equipment; Major nonconformity is likely to endanger the ability of a product to fulfill its function; while Minor nonconformity does not have immediate consequences on the ability of the products to fulfill their function but represents a risk.
Requirements include product specification, applicable standards and product qualifications, manufacturing process parameters and quality control points all along the line, and pass/fail criteria for visual inspection, maximum power measurement, EL images… These requirements are agreed between the buyer and the vendor before the order manufacturing starts, and it is important that the buyer clearly specify his needs as early as possible in the order process.
STS provides contracting technical advisory services, performing risk assessment on quality deviation proposed by manufacturers to help the buyers make educated decisions.
Nonconformities distribution in a PV modules manufacturing line – No step is risk-free!
Stringing Process
Cells v-shape cracks, desoldering, and ribbons/wires deviations are major nonconformities generally happening at stringing process. Wire position deviation can be detected via visual inspection before lamination; wire desoldering can also be detected before lamination through measuring soldering strength on sampled cells, at least once per manufacturing shift. El images after lamination or final quality check can also catch this defect.
Wire position deviation and wire desoldering expose risk of hot spots leading to accelerated performance degradation and/or fire hazard.
Potential root causes for wire desoldering can be related to soldering equipment adjustment and maintenance, including soldering temperature outside of range, soldering heating process not uniform, and inaccurate cell or wire positioning.
Cell v-shape cracks mostly happen on half-cut cells, located at the interface between the edge of the cell and the wires, potentially due to local microdefect on the cell edge created during laser cutting or high mechanical pressure on the cell during stringing or lamination induced by technology changes (use of new wires design and materials or space reduction between cells…). This defect, if of small size (length 2 to 5mm), carries a risk of remaining undetected during quality control at factory. Crack length may increase during transportation, installation and/or operation, leading to hot spots.
To mitigate the risks of cell cracks and wire desoldering, we recommend performing product design and development audit or factory audit, ensuring that the manufacturer has validated all gates during new product introduction or capacity expansion. At STS, we support the buyer by preparing a technical exhibit based on STS Standard specific to multi-busbars and providing 24/7 production supervision onsite (EL test witnessing…).
Lamination Process
Air bubbles usually happen at the process of lamination, potentially due to flux-encapsulant reaction on the cell surface, laminator parameters (temperature, vacuum, time) not well adjusted, or not enough encapsulant inside the busbar holes on glass-glass modules, which can be detected via visual inspection after lamination. This nonconformity may result in delamination and hot spots leading to accelerated performance degradation and/or fire hazard.
To mitigate this risk, we recommend product design and development audit and production supervision to perform a thorough visual inspection after lamination as well as during final quality check.
Framing Process
Lack of silicon sealant is one of the major nonconformities happening at framing process, which is potentially caused by uneven silicone sealant application along the frame groove (especially at the corners) or module transferring to the next process step while the sealant curing is incomplete. This defect may lead to front glass breakage.
To mitigate this risk, we recommend a production supervision, ensuring that during the framing process the silicone sealant is evenly applied in the frame profiles groove, including at the corners, where there is a higher risk of missing sealant; pre-shipment visual inspection can help identify locations where there is no sealant overflow between the glass and the frame, or measurement with feeler gauge can also work to quantify the clearance between the glass and the frame on the front side; remember to verify during final quality check that there is silicone overflow on the rear side.
Overall, to mitigate the quality risk of PV module manufacturing, we recommend performing risk assessment on inspection requirements defined by the manufacturer vs STS Standard, requesting a production supervision at each step of the manufacturing, ensuring that Final Quality Control is efficiently performed, and staying informed about the changes in PV modules design and manufacturing process (new technology mostly means new risks).