Today cells are screened for hotspots based on Shunt Resistance as measured in the Flash Tester
But occurrence of hotspots depends on size and distribution of shunts – this method is inadequate.
Shown is a graphical representation of the distributions of defective vs. nondefective cells as a function of inverse shunt resistance. The designation of a cutoff shunt resistance, as shown by the dotted green line, creates a population of false-positives (diagonal lines) which represent lost revenue for the manufacturer and a population of false-negatives (cross-hatched region)which represent increased warranty liability for the manufacturer. It is notable that the lost revenue represented by the false-positive region has a high margin since the costs have already been incurred for production of the cells.
An industry trend toward third party financing is increasing warranty liability and causing manufacturers to be more conservative in setting the cutoff shunt resistance. In parallel, ongoing downward price pressure in the market makes it infeasible to address defect root cause. As such, in-line screening is becoming a standard part of the manufacturer’s toolset.
Diagram showing only the lowest binning cells being screened and sorted after flash testing. This has the primary advantage that it requires fewer test stations to be installed and maintained – depending on the threshold set and the reliability of the process, a single system can serve several manufacturing lines. However a population of low shunt resistance cells passed by the Flash Tester (false-negatives) will still get built into modules.
Diagram showing all cells are inspected either prior to or as part of the end-of-line sort. This has the advantage of minimizing warranty liability by screening every cell in the manufacturing line -- minimizing both false-positive and false-negative populations. This method also enables statistical process control by identifying the types and frequency of defects in the line, allowing the recapture of lost entitlement by identifying manufacturing excursions at their onset. Ultimately the best strategy for screening depends on the frequency and distribution of defects across the cell population.
The are three opportunities for hotspot defect screening in the module production line--
- Incoming cell screening (unnecessary if they've been previously inspected)
- String inspection to look for solder induced defects. If found, there is an opportunity to rework the string.
- Module quality control to look for soldering, lamination, or mechanically induced hotspot defects. Modules can be reworked (if appropriate) or scrapped to minimize warranty liability.