Heterojunction (HJT) solar cells are a marvel of photovoltaic engineering, promising higher efficiencies and excellent performance. But their sensitivity to heat poses a unique manufacturing challenge, requiring low-temperature soldering to connect the cells. This necessity has driven the widespread adoption of Bismuth-Tin (BiSn) solders and specialized „no-clean“ fluxes.
The promise is alluring: a soldering process that skips the cleaning step, saving time and money. But what if „no-clean“ doesn’t mean „no-risk“? What if the invisible residue left behind could undermine the reliability of your modules over their 25-year lifespan?
This isn’t a hypothetical problem. It’s a critical issue that can lead to corrosion, power loss, and premature field failures. Let’s explore why this happens and how you can ensure the long-term integrity of your HJT modules.
Why Flux is Both Friend and Foe
Think of solder flux as a chemical assistant for soldering. Its main job is to clean the metal surfaces of cell ribbons and contacts, removing oxides so the molten solder can form a strong, reliable electrical connection.
After soldering, the flux leaves behind a residue. With „no-clean“ fluxes, this residue is designed to be chemically inert and non-corrosive, allowing it to be safely left on the cell and encapsulated within the module.
The challenge lies in HJT’s low-temperature process window, which limits the types of flux that can be used. These fluxes often rely on chemical „activators“ that are more aggressive at lower temperatures. If the flux chemistry isn’t perfectly matched to the process and materials, or if the residue isn’t fully encapsulated, it can become a long-term liability.
Expert Insight from Patrick Thoma, PV Process Specialist: „The industry’s focus on ’no-clean‘ for efficiency is understandable, but we’ve learned that ’no-clean‘ must also mean ’no-reaction‘ over a 25-year lifespan. This is a chemical validation challenge, not just a process step. The residue must be proven to be completely passive under decades of heat, humidity, and voltage stress.“
When „No-Clean“ Becomes a Problem: The Long-Term Risks
Under real-world conditions of heat and humidity, a seemingly harmless flux residue can become chemically active and create two major problems:
1. Corrosion of Cell Interconnectors
Many low-temperature fluxes contain hygroscopic activators, meaning they attract water molecules. If even a tiny amount of moisture penetrates the module over its lifetime, it can react with these activators. This reaction creates an acidic environment that slowly corrodes the delicate silver-coated copper ribbons connecting the cells, leading to increased resistance and eventual open circuits.
![Image of a corroded solder joint on a solar cell]()
2. Leakage Currents and PID
Flux residue can also contain mobile ions. In the field, a module operates under high voltage, and these ions can migrate under the influence of the electric field. This migration creates tiny, unintended electrical pathways known as leakage currents, which not only reduce the module’s power output but also make it significantly more susceptible to Potential-Induced Degradation (PID), a major cause of long-term power loss.
These issues are incredibly difficult to detect in a new module. They are slow-burning problems that manifest only after years in the field, long after the modules have been shipped and installed—and when the cost of failure is highest.
Putting Flux to the Test: How to Validate Reliability Before Mass Production
So, how can you be sure your chosen „no-clean“ flux won’t cause problems down the line? The answer lies in rigorous, accelerated aging tests that simulate decades of harsh environmental conditions.
This isn’t about checking a supplier’s datasheet. It’s about validating the entire system working together—the flux, the cells, the ribbons, and the encapsulant. This is where you need to build and validate new solar module concepts under controlled, industrial-scale conditions.
Two tests are absolutely critical for validating flux residue reliability:
Damp Heat (DH) Testing
In a Damp Heat test, finished modules are placed in a climatic chamber and exposed to 85°C and 85% relative humidity for 1,000 hours or more. This harsh environment is designed to accelerate any moisture-driven corrosion. After the test, modules are carefully inspected for any signs of corrosion at the solder joints and tested for power loss. A well-chosen, properly applied flux will show no degradation.
![Image of a solar module inside a climatic chamber for damp heat testing]()
Potential-Induced Degradation (PID) Testing
PID testing simulates the high-voltage stress that modules face in a large solar array. The module is exposed to high voltage (typically +/- 1000 V) at elevated temperature and humidity. This test is extremely effective at revealing ionic contamination from flux residue. If leakage currents increase or power drops significantly, it’s a clear red flag that the flux residue is not as inert as promised.
Beyond the Chemistry: Process Control is Everything
Even the best flux chemistry can fail if the manufacturing process isn’t precisely controlled. The key is to ensure the residue is minimal and perfectly sealed from the environment.
Key process control points include:
- Flux Application: The amount of flux applied must be exact. Too little, and you risk poor solder joints. Too much, and you leave behind excessive residue that is harder to effectively encapsulate.
- Post-Soldering Inspection: Techniques like high-resolution Electroluminescence (EL) testing can help spot anomalies in solder joints that might indicate process deviations.
- Encapsulation and Lamination: A flawless lamination process is the final line of defense. Conducting structured experiments on encapsulants ensures your chosen material (like EVA or POE) creates a perfect, void-free barrier, hermetically sealing the flux residue away from future moisture.
By combining rigorous material validation with tight process control, manufacturers can confidently use „no-clean“ fluxes to build efficient and reliable HJT modules that stand the test of time.
Frequently Asked Questions (FAQ)
What exactly is HJT technology?
HJT (Heterojunction Technology) combines crystalline silicon with amorphous thin-film silicon layers. This unique structure allows HJT cells to capture more energy and perform better in hot climates, leading to higher overall efficiency compared to traditional cell technologies like PERC.
Why can’t standard solder be used for HJT cells?
Standard solar cell soldering uses temperatures above 200°C. The delicate amorphous silicon layers in HJT cells can be damaged by such high heat, degrading their performance. Therefore, HJT requires low-temperature solders, typically Bismuth-Tin (BiSn) alloys, that melt around 140°C.
Is all „no-clean“ flux residue dangerous?
No. High-quality, well-formulated „no-clean“ fluxes are designed to leave behind a residue that is truly inert, non-corrosive, and non-conductive. The danger comes from using a flux that is not properly validated for long-term reliability or from poor process control that leaves behind active, unencapsulated residue.
How long do accelerated aging tests like Damp Heat take?
Standard certification tests like IEC 61215 require 1,000 hours of Damp Heat testing, which takes about 42 days. However, for validating new materials like a flux, it’s common to run extended tests (2,000 hours or more) to gain higher confidence in long-term reliability.
Can’t you just clean the flux residue off after soldering?
While possible, adding a cleaning step complicates the manufacturing line, increases costs, and introduces wet chemical processes that have their own risks (like ensuring the cleaning agent is completely removed). The goal of „no-clean“ flux is to eliminate this step entirely, which is why upfront validation is so crucial.
Next Steps for Deeper Exploration
Understanding the nuances of material interactions is key to building reliable solar modules. For those developing new module technologies or evaluating materials, the next step is applied research and prototyping. Exploring how a full-scale R&D production line can help validate your concepts is crucial for accelerating innovation with confidence.