The solar industry is in a constant race for higher efficiency, lower costs, and longer-lasting modules. The introduction of N-type TOPCon (Tunnel Oxide Passivated Contact) cells was a massive leap forward in efficiency, promising more power from every panel.

But a leap in one area often creates a new challenge in another.

For TOPCon cells, that challenge lies in one of the most critical stages of manufacturing: soldering. These high-efficiency cells are more sensitive to high temperatures. The industry’s trusted, go-to solder alloy, SAC305 (containing 3% silver), requires a soldering temperature that pushes these sensitive cells to their thermal limits, risking micro-cracks and long-term performance loss.

This raises a critical question across the industry: Can we switch to more cost-effective, lower-temperature, low-silver (Low-Ag) solder alloys without sacrificing the 25-year reliability that customers expect?

The answer is yes—but it’s not as simple as swapping one spool of solder for another. It requires a deeper understanding of how these new materials behave under stress.

The High-Stakes Balancing Act: Cost, Performance, and Durability

For years, SAC305 has been the workhorse for cell interconnection. It’s known for its excellent mechanical properties and predictable long-term behavior. Its one major drawback? The „Ag“ in its name—silver—is expensive, and its price is volatile.

Low-Ag alloys, which significantly reduce or eliminate silver, present a compelling alternative. They offer two key advantages:

1. Lower Cost: Reducing silver content directly cuts material costs, a significant factor in large-scale module production.
2. Lower Melting Point: Many Low-Ag formulations have a lower melting temperature, reducing the thermal stress on sensitive TOPCon cells during the crucial stringing process.

But these new materials also introduce new questions. The mechanical properties of Low-Ag solders are different. Do they fatigue faster? How do they hold up after decades in the field, enduring blistering heat, freezing cold, and daily temperature swings? Answering these questions requires moving beyond datasheets and into real-world testing.

Putting Solder to the Test: What 600 Thermal Cycles Reveal

To understand the real-world lifetime of these alloys, we conducted a study at PVTestLab. We built TOPCon modules using a standard SAC305 solder and compared them against two common Low-Ag alternatives (Sn96.5Ag0.5Cu3.0 and Sn99.3Cu0.7).

We then put them through an accelerated aging process called thermal cycling. The modules were cycled 600 times between -40°C and +85°C, simulating the harsh conditions they would face over many years in the field.

The initial results were promising. Straight off the production line, the modules built with Low-Ag solders showed comparable, and in some cases slightly better, initial power output (Pmax). But the real story emerged as the cycles stacked up.


While all modules experienced some expected power degradation, the reason for that degradation was fundamentally different. In older technologies, power loss after thermal cycling was often dominated by cell cracks. Our analysis revealed a new primary culprit for TOPCon modules using Low-Ag alloys: a significant increase in series resistance (Rs).

Think of series resistance as the internal friction electricity encounters as it flows through the module. As this resistance increases, more energy is lost as heat, and less power makes it out of the panel. The solder joints connecting the cells were fatiguing and weakening, acting like a slowly clogging artery in the module’s electrical system.

 images comparing a module before and after TC600, highlighting increased resistance in cell interconnects.)

This „aha moment“ is critical. It means that to ensure the long-term reliability of Low-Ag solders, we can’t just look for cracks. We must focus on the integrity of the solder joint itself and the manufacturing process that creates it.

The Narrow Path to a 25-Year Lifespan

So, does this increased series resistance mean Low-Ag solders are a dead end for long-term reliability? Not at all.

Our research, using the Coffin-Manson lifetime prediction model, showed that these alloys can achieve the desired 25-year lifespan. However, they operate within a much narrower process window. Every parameter in the manufacturing line—from the soldering temperature profile to the speed of the tabber-stringer—must be perfectly dialed in.

That leaves far less room for error compared to the more forgiving SAC305.


„What this data tells us is that you can’t treat Low-Ag solder as a simple drop-in replacement,“ explains Patrick Thoma, PV Process Specialist at PVTestLab. „Success hinges on precise process control. You need an environment where you can test, measure, and validate how small changes in your parameters affect long-term joint integrity.“

This is where a methodical approach to solar module prototyping becomes essential. By building and testing small batches under real industrial conditions, manufacturers can define the exact parameters needed to ensure their lamination process and soldering stages are optimized for these new materials. It’s a crucial step in the overall material validation journey, de-risking the transition before committing to mass production.

Key Takeaways for Module Manufacturers

1. Low-Ag Solder is Viable: These cost-effective alloys are a strong candidate for TOPCon cells, but they demand more process expertise.
2. The Failure Mode Has Shifted: Don’t just look for physical cracks. Monitor series resistance (Rs) as the key indicator of solder joint fatigue and long-term degradation.
3. Process is Everything: The path to 25-year reliability with Low-Ag alloys is narrow. Invest time in defining and controlling your process parameters.
4. Test Before You Scale: Use prototyping and pilot lines to validate your process and ensure your chosen alloy will meet reliability targets before scaling to full production.

Your Questions on Low-Ag Solders, Answered

What exactly is a Low-Ag solder alloy?

A Low-Ag (low-silver) solder is a tin-based alloy where the silver content is significantly reduced (typically to 1% or less) or eliminated entirely, often replaced with other elements like copper (Cu) or bismuth (Bi). This makes them less expensive than traditional SAC (Tin-Silver-Copper) alloys like SAC305.

Why is SAC305 not ideal for TOPCon cells?

While an excellent solder, SAC305 has a melting point around 217-220°C. Reaching the ideal processing temperature for strong solder joints can expose sensitive N-type TOPCon cells to thermal stress, potentially causing invisible micro-cracks that compromise long-term efficiency and reliability.

What is thermal cycling and why is it important?

Thermal cycling is an accelerated reliability test where solar modules are repeatedly exposed to extreme temperature changes (e.g., -40°C to +85°C). This process simulates the stress of daily and seasonal temperature swings over a 25+ year period, helping to identify potential failure points like solder joint fatigue or cell cracking in a matter of weeks or months.

Does „low-silver“ mean lower quality?

Not necessarily. It simply means the material has different mechanical and thermal properties. Our research shows that when the manufacturing process is properly optimized for these properties, Low-Ag solders can deliver performance and reliability on par with their high-silver counterparts. This makes them a high-quality, cost-effective alternative.

From Lab Research to Your Production Line

The transition to new, advanced cell technologies like TOPCon is an incredible opportunity for the solar industry. But seizing this opportunity requires embracing the manufacturing challenges that come with it.

Low-silver solders represent a clear path toward reducing costs without compromising the initial performance of TOPCon modules. However, ensuring that performance lasts for 25 years requires a shift in focus—from the material itself to the manufacturing process that brings it to life. By understanding the unique degradation pathways of these alloys and investing in precise process optimization, manufacturers can confidently build the next generation of efficient, affordable, and reliable solar modules.