A solar panel can look perfect from the outside—clean glass, solid frame—yet its performance can be slowly degrading. This silent loss of power is one of the most significant long-term challenges in the solar industry, and its cause is often microscopic: tiny cracks growing in the solder joints that connect solar cells.
This isn’t a random defect; it’s a predictable process called thermo-mechanical fatigue. It’s a failure mode that can be understood, mapped, and engineered against. Solder joint cracking is a leading long-term failure mode in PV modules. But with the right approach, it can be transformed from a hidden threat into a known variable you can manage.
The Hidden Stress: What is Thermo-Mechanical Fatigue?
Imagine a solar panel on a rooftop. During the day, it bakes in the sun, reaching high temperatures. At night, it cools down, sometimes to below freezing. This daily temperature swing happens thousands of times over a module’s 25-year lifespan.
Every material in that panel expands when heated and contracts when cooled. The problem is that they don’t all do it at the same rate—a property known as the Coefficient of Thermal Expansion (CTE).
Inside a solar module, three key materials are soldered together:
- Silicon Solar Cell: The heart of the module.
- Copper Interconnect Ribbon: The thin metal strip that carries electricity.
- Solder: The metallic glue holding the cell and ribbon together.
The copper ribbon wants to expand and contract much more than the silicon cell, leaving the solder joint caught in the middle of this constant push-and-pull. This daily tug-of-war creates mechanical stress on the solder joint.
Think of it like bending a paperclip back and forth. The first few bends do nothing, but if you keep going, the metal weakens and eventually snaps. The daily thermal cycle does the same thing to the solder joint, creating microscopic cracks that grow over time. This process is the essence of thermo-mechanical fatigue.
Simulating a Lifetime of Stress: The Role of Thermal Cycling
We can’t wait 25 years to see if a new module design or material choice will stand the test of time. Instead, we use accelerated testing to simulate decades of wear and tear in just a few weeks.
The industry standard for this is the Thermal Cycling Test (TC200), defined by the IEC 61215 certification. In this test, a solar module endures 200 cycles of extreme temperature swings in a climate chamber, typically from -40°C to +85°C.
To pass, a module must show less than 5% power degradation after the test. While this provides a simple pass/fail result, it doesn’t reveal why a module failed, how close it came to failing, or where its weaknesses lie.
Making the Invisible Visible: Mapping Cracks with Sequential EL Testing
To truly understand how solder joints fail, you need to watch them fail in slow motion. This is where a more advanced diagnostic approach comes in: sequential Electroluminescence (EL) testing.
An EL test is like an X-ray for a solar module. By passing a current through the module in a dark room, we can see areas that are no longer electrically active. A crack in a solder joint severs the electrical connection, causing parts of the cell to go dark in the EL image.
The standard approach is to perform an EL test before and after the full 200 thermal cycles. But at PVTestLab, we’ve found that performing the EL test sequentially—for example, after every 50 cycles—provides far more valuable data. This method doesn’t just give you a „before and after“ snapshot; it creates a time-lapse video of the failure in progress.
This allows us to pinpoint exactly when a crack initiates, how fast it propagates, and where it originates. Crack propagation often starts at the outer edge of the solder ribbon and moves inward, slowly isolating more of the cell’s surface.
This detailed mapping transforms the conversation from „Did it fail?“ to „Why, when, and how did it fail?“
Not All Failures Are Created Equal: Key Factors in Solder Joint Cracking
By analyzing data from sequential EL testing, we can directly correlate the failure rate to specific material choices and manufacturing processes. Two factors stand out as critical.
The Solder Itself: A Matter of Material Choice
The solder alloy itself has a dramatic impact on fatigue resistance. For years, tin-lead (SnPb) solders were common, but environmental regulations have pushed the industry toward lead-free alloys, like tin-silver-copper (SnAgCu) formulations.
These different alloys have distinct mechanical properties. Some are more brittle, while others are more ductile, and their performance under repeated stress varies significantly. Through controlled testing, we can compare different solder types head-to-head under identical thermal cycling conditions, revealing which ones provide the longest operational lifespan. This kind of targeted material testing and lamination trials is essential for validating new material suppliers or formulations.
The Starting Point: How Lamination Sets the Stage for Failure
Perhaps the most overlooked factor is the initial manufacturing process. The lamination process—where the module’s layers are fused together with heat and pressure—can lock in residual stress before the module ever leaves the factory.
If the lamination parameters (like temperature, pressure, or cooling rate) are not perfectly optimized for the specific combination of materials, the solder joints can be „pre-stressed.“ These joints start their life already weakened, making them far more susceptible to failing early during thermal cycling.
This is a crucial insight: long-term reliability isn’t just about the quality of the raw materials; it’s about the precision of the assembly process. This is why prototyping new solar module concepts in a controlled, industrial-scale environment is so vital for preventing these latent defects.
FAQ: Understanding Solder Joint Cracking
What exactly is a solder joint in a solar module?
A solder joint is a small, metallic connection that electrically and mechanically bonds the copper interconnect ribbons to the front and back of the silicon solar cells. It creates the series circuit that lets electricity flow from cell to cell.
Can I see these cracks with my naked eye?
No. These cracks are typically microscopic and occur underneath the interconnect ribbon, making them invisible from the surface. They can only be detected with specialized equipment like an Electroluminescence (EL) tester.
Does this affect all types of solar panels?
Thermo-mechanical fatigue is a fundamental challenge for all standard crystalline silicon solar modules that use soldered interconnects. However, the severity and speed of failure are heavily influenced by the specific materials (solder, ribbons) and manufacturing processes used.
Why is a small power loss from one crack a big deal?
A single crack can make a small part of one cell inactive. Over time, this crack can grow, deactivating more of the cell. If a crack severs a ribbon completely, it can shut down an entire string of cells. What’s more, inactive cell parts can sometimes lead to „hotspots,“ where energy builds up as heat, potentially damaging the module’s encapsulant and backsheet over time.
From Insight to Action: Building More Reliable Modules
Solder joint cracking is not a matter of „if,“ but „when.“ Caused by the fundamental physics of thermal expansion, it’s a primary hurdle to achieving a 30+ year module lifespan.
However, by understanding the root causes—CTE mismatch, material selection, and process-induced stress—we can engineer more resilient products. Accelerated testing, particularly with sequential EL analysis, provides the foresight needed to identify weaknesses long before they become field failures.
With this data-driven approach, developers and manufacturers can make informed decisions that directly improve the long-term bankability and reliability of their solar modules. If you’re developing a new module design or evaluating new materials and want to understand their long-term performance, you can consult with our process engineers to design a testing protocol that provides the answers you need.