This article was originally published in pv magazine - December 2024 Edition.

George Touloupas, senior director for technology and quality at Clean Energy Associates (CEA), reviews the quality assurance (QA) activity and methodologies used to ensure PV performance and emphasizes the importance of data-driven approaches and continuous monitoring.

Quality assurance in solar module production involves a series of technical consulting and inspection activities designed to ensure that modules meet the required standards before they are installed.

The main QA activities are CEA’s golden standard, factory audits, inline production monitoring, pre-shipment inspection, container-loading monitoring, and batch testing.

The golden standard involves all technical, quality, and inspection requirements being reviewed against best industry practices and agreed upon with suppliers, before contracts are signed. This includes standards, reliability tests, characterization, suppliers’ quality assurance plans, inspection processes and criteria, sample testing, and acceptance conditions.

This is a desktop activity and sets the framework for the other, on-site QA activities.

Before production begins, a team of engineers will audit a factory location using a checklist with at least 1,000 points. Every finding is recorded and classified according to its risk potential.

Engineers continuously monitor all stations of a factory location during the production of an order to carry out inline production monitoring, typically daily and using a 280-plus-point checklist. Every finding is recorded and classified according to its risk potential.

Before shipping, engineers perform visual, electroluminescence (EL), and I-V (current-voltage) inspections of a sample lot of modules, checking against a list of vetted quality criteria. Container-loading monitoring ensures conformance with packaging and loading specifications and confirms that the products being loaded into a shipping container come exclusively from inspected and approved lots.

Representative samples from a production batch are then tested for potential issues such as potential induced degradation (PID), light-induced degradation, light and elevated-temperature induced degradation, and ultraviolet-induced degradation.

Collection and analysis

*Findings during inline-production monitoring*

As a third party, performing quality assurance on behalf of PV module buyers, CEA collects data from pre-production factory audits, inline production monitoring, pre-shipment inspections, container loading inspections, and batch testing. The data is then analyzed using a risk-based methodology, assessing defect risks based on severity, detectability, and occurrence.

Findings are classified by severity as critical, major, and minor. The findings result in a numerical risk score and a corresponding letter grade. The grade ranges are decided based on the global distribution of factory audit risk scores.

Since 2016, CEA has performed factory audits in more than 300 factories. The audit data from 2020 to 2023 revealed that six out of 10 factories received a high-risk quality rating or worse (see chart below), with C or D ratings typically indicating multiple major risk findings and, possibly, one or more critical findings. Major findings may reduce a product’s functionality or safety and critical findings may result in severe safety and non-compliance risks.

*Electroluminescence imaging prior to shipment revealed cold soldering, grid breaks, and cell microcracks as the most common issues in PV modules.*

Common findings during inline production monitoring include contamination after manual rework, misalignment of module encapsulant, and cold soldering. Detecting problems before lamination is critical, as some of those issues may evade detection later. Additionally, affected modules can be reworked instead of being scrapped at the end of the line.

The main pre-shipment inspection points include visual inspection, EL imaging and I-V testing, safety testing (including high potential and ground bond tests), and certification and non-conformance inspection. High potential testing involves the application of very high voltage and ground-bond testing of high current. Cold soldering, grid breaks (from defective cell metallization), and cell microcracks are the most common issues found in the EL inspection. Both cold soldering and micro cracks have the long-term potential to evolve and cause serious underperformance and even module failure.

*Frequent defects during Container Loading Monitoring (CLM)*

Ensuring a product is packaged, marked properly, and verified to have passed inspections is critical, as there is very little room for error correction once a product is shipped. The most common issues found during container-loading monitoring are related to packaging, labeling, and data verification.

Known solar module degradation modes persist and new ones, such as ultraviolet- induced degradation, are appearing as cell and module technology evolves, so ensuring batch testing is crucial. Regarding PID, a well-known problem, newer solar technology such as tunnel oxide passivated contact (TOPCon) products may take some time to improve and stabilize. For example, one supplier recorded 6.4% initial PID performance loss. After ultraviolet stabilization, the loss fell to 1.2%. Ultraviolet stabilization is typically not performed where an initial loss is less than 5%.

EL case study

Although most suppliers have good ethics, in certain rare cases, we have discovered that suppliers have manipulated the EL images for the purpose of altering a sample inspection outcome.

In one such case, an inspector found several defects at the EL imaging station during inline production monitoring and noted down the serial numbers of the affected modules. When checking those no defects. That raised concerns that the images may have been altered, indicating suspicious behavior.

During one EL inspection, CEA noted soldering issues with one module (left). In a later pre-shipment check on the same module serial number (right), the faults had mysteriously disappeared, suggesting that the supplier had manipulated the images.

The supplier repeatedly delayed providing the EL images to CEA when requested, offering various excuses. Pre-shipment inspection took three days to get started, rather than the usual few hours. CEA detected that the EL data had been tampered with to cover the fact that the supplier had replaced defective samples to avoid rejection of the lot. The risk when this happens is that defective modules with hidden performance issues end up shipped and installed, leading to reduced performance, higher failure rate, safety hazards, and, ultimately, costly replacement or warranty claims.

Soldering case study

*The arrow points to a joint that was left unsoldered. Even a small percentage of bad soldering can lead to module failures, and sometimes severe fire incidents.*

The rapid introduction of automated production can backfire without the right safeguards. Module current is evacuated through soldered terminals and even a small percentage of bad soldering can lead to module failures, and sometimes severe fire incidents.

There has been an increase in fires in PV modules linked to poor junction box soldering and CEA has found that current soldering inspection methods at some factories were not effective in detecting these defects. As a result, we have required suppliers to strengthen control by closely monitoring resin, flux, and solder ratios and implementing 100% physical inspection.

As more factories adopt automated soldering and inspection for junction boxes, there has been a trend to reduce or eliminate physical inspection. This has led to inconsistent soldering quality because automated checks alone are not enough to catch all defects. Some manufacturers have been found to be trying to bypass physical inspections on 100% of their junction boxes, ensuring such issues go undetected. The risk here is that defective soldering can lead to thermal events and, in the worst cases, start a fire.

Ensuring the quality and reliability of PV modules is crucial for the success of solar energy projects. There is constant variability in materials, processes, and practices and new problems keep appearing even if the changes in technology are small. By employing rigorous QA activity and leveraging data and analysis, manufacturers and third parties can enhance the safety and performance of PV modules. Continuous monitoring and proactive identification of potential issues are key to maintaining high standards and delivering reliable products to market.

George Touloupas is the Senior Director for Technology and Quality at Clean Energy Associates. Since 2010, Touloupas has been active in technical consulting for PV manufacturing, project development, and solar and energy storage projects.

Using Data to Enhance Safety and Performance

Collection and analysis

EL case study

Soldering case study