The solar industry is in a constant race for higher efficiency. Next-generation cell technologies like heterojunction (HJT) and perovskites promise incredible performance gains, but they come with a crucial manufacturing constraint: they cannot handle the heat of traditional soldering processes.
Enter low-temperature solders (LTS), particularly those based on bismuth. These alloys allow manufacturers to create vital electrical connections without damaging heat-sensitive cells. But this innovation introduces a subtle, long-term challenge that can silently degrade module performance over its 25-year lifespan: thermomechanical fatigue.
While these modules may look perfect coming off the production line, the real test happens in the field, where daily temperature swings put a hidden strain on these softer solder joints. Understanding this strain isn’t just an academic exercise—it is fundamental to guaranteeing the long-term bankability of high-efficiency modules.
The Two Faces of Bismuth: Strong When Cold, Soft When Warm
To understand the challenge, we need to look at the unique properties of bismuth-based solders. Unlike traditional tin-lead or SAC (tin-silver-copper) alloys that melt around 220°C, bismuth alloys have a much lower melting point, typically between 138-170°C. This is their key advantage, enabling the production of HJT and other advanced modules.
But this low melting point creates a fascinating paradox. At room temperature, bismuth makes the solder alloy relatively brittle and strong. But as the module heats up under the sun—reaching operating temperatures of 40°C, 60°C, or even 85°C—the solder’s behavior changes dramatically. It doesn’t melt, but it becomes significantly softer and more susceptible to a phenomenon called „creep.“
Creep is the slow, permanent deformation of a material under constant stress. Think of a heavy bookcase causing a wooden shelf to sag over many years. That’s creep. For a solder joint in a solar module, the stress is constant, and warm operating temperatures are enough to activate this slow, plastic-like flow.
A concept called homologous temperature provides a critical insight here. It’s simply the ratio of a material’s operating temperature to its melting temperature. For bismuth solders, a routine 60°C day in the field puts them at a high homologous temperature, deep into the creep-active zone. In contrast, that same 60°C represents a much lower fraction of a traditional SAC solder’s melting point, making creep far less of a concern.
The Daily Push-and-Pull: How Thermal Cycles Drive Fatigue
Creep alone isn’t the full story. The real damage comes from the daily push-and-pull of thermal cycling, which causes thermomechanical fatigue.
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Materials Expand and Contract: Every material in a solar module expands when heated and contracts when cooled. Importantly, they do so at different rates—a property known as the Coefficient of Thermal Expansion (CTE).
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A Built-in Mismatch: The copper ribbon, the silicon cell, and the bismuth solder all have different CTEs. As the module heats up during the day and cools down at night, these materials work against each other. The copper ribbon wants to expand more than the silicon cell, creating shear stress directly in the solder joint that connects them.
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The Cycle Repeats: This isn’t a one-time event; it happens every single day for years. This repeated cycle of stress and strain is the definition of fatigue. Like bending a paperclip back and forth, each cycle contributes a tiny amount of damage that accumulates over time.
For soft, creep-prone bismuth solders, this daily stress cycle is particularly damaging. While the material’s deformation relieves some immediate stress, it also leads to irreversible changes in its internal structure.
From Micro-Cracks to Macro-Failure: Visualizing the Damage
What does this accumulated damage actually look like? It begins at the microscopic level.
Initially, a bismuth solder joint has a fine, uniform grain structure. Under repeated thermal stress, however, a process called microstructure coarsening occurs. The small, interlocking grains begin to grow and merge, weakening the overall structure of the solder.
This coarsened, weaker material becomes the perfect breeding ground for micro-cracks. These tiny fractures often start in areas of high stress concentration, like the corners of the joint, and slowly propagate through the solder with each thermal cycle.
Over hundreds or thousands of cycles, these micro-cracks can connect, leading to a partial or complete failure of the solder joint. This doesn’t mean the module instantly stops working. Instead, it causes a gradual, measurable increase in the module’s series resistance (Rs). As the electrical connection degrades, it becomes harder for current to flow, and more energy is lost as heat—a direct, irreversible loss of power output.
Designing for Durability: How Geometry and Process Matter
Fortunately, this degradation is not inevitable. Understanding the failure mechanism allows us to design more resilient solder joints. The key lies in managing stress.
Our research at PVTestLab, involving accelerated thermal cycling tests (e.g., from -40°C to +85°C for over 600 cycles), consistently shows that joint geometry is critical.
- Fillet Volume: A larger, more robust solder fillet helps distribute stress over a wider area, reducing peak stress at critical points.
- Standoff Height: The gap between the ribbon and the cell, when optimized, can also help accommodate the strain from CTE mismatch.
Careful engineering during the initial solar module prototyping phase can dramatically improve long-term fatigue life. This requires a deep understanding of the interplay between materials and process parameters. Through rigorous lamination and material testing, manufacturers can select the right combination of ribbons and solders and then validate their long-term performance.
Data from these tests clearly demonstrates this principle. Designs with optimized geometry show a much slower rate of Rs increase compared to less robust designs, highlighting how smart engineering can directly translate to better durability and less power degradation in the field. Achieving this requires a commitment to data-driven process optimization, where every parameter is fine-tuned for long-term reliability.
Frequently Asked Questions (FAQ)
What are low-temperature solders (LTS)?
LTS are soldering alloys designed to melt at temperatures significantly lower than traditional solders (e.g., 138-170°C vs. 220°C). They are essential for manufacturing solar cells like HJT or perovskites that would be damaged by higher temperatures.
Why can’t we just use traditional solder on HJT cells?
The high temperatures required for traditional SAC solders (around 220°C or more) can damage the sensitive amorphous silicon layers in HJT cells, permanently degrading their efficiency. LTS allows for a strong electrical connection without causing thermal damage.
Is creep the same as melting?
No. Melting is a phase change from solid to liquid. Creep is a slow, solid-state deformation that occurs far below the melting point, driven by sustained stress and elevated temperature.
How many thermal cycles are needed to see this effect?
Significant degradation, such as measurable increases in series resistance and visible micro-cracking, is often observed after several hundred cycles in accelerated testing (e.g., 400-600 cycles from -40°C to +85°C), simulating many years of service in the field.
Can this problem be detected with standard EL or flash tests right after production?
Not usually. The initial degradation is microscopic and does not typically show up as a defect in standard electroluminescence (EL) or flasher tests. The problem develops over time with thermal cycling, which is why accelerated reliability testing is so crucial for predicting long-term performance.
Your Next Step in Ensuring Long-Term Reliability
The shift to low-temperature soldering is a necessary and positive evolution for the solar industry, unlocking new levels of module efficiency. But it also demands a more sophisticated approach to reliability and quality assurance.
Understanding the unique mechanics of bismuth-based solder—its susceptibility to creep and its response to thermomechanical fatigue—is the first step. The next is to translate that theory into practice by quantifying how your specific combination of materials, joint designs, and process parameters will perform over a 25-year lifetime.
Proactive testing and validation are no longer optional; they are essential for anyone committed to producing high-performance modules that stand the test of time.