Imagine a solar panel on a rooftop. For 25 years, it must withstand gusting winds, heavy snow, and the daily cycle of heating and cooling. From the ground, it looks perfectly still. But at a microscopic level, it’s in constant motion—flexing, expanding, and contracting.
This relentless mechanical stress is a silent killer for solar modules. While we see a sturdy sheet of glass and silicon, the real battle for longevity is fought in connections smaller than a human hair: the solder joints that hold the solar cells together. As module technology has advanced, a hidden vulnerability has come to light that can lead to premature power loss and panel failure.
Here, we’ll explore that vulnerability—and more importantly, how data from advanced testing can help us engineer it out of future designs.
The Trade-Off of Modern Solar Design
For years, the solar industry has been moving toward multi-busbar (MBB) cell designs. Instead of a few flat, wide ribbons conducting electricity, MBB modules use many thin, round wires—often 12 or more. The benefits are clear:
- Higher Efficiency: Thinner wires cast smaller shadows on the cell, allowing more sunlight to be converted into electricity.
- Lower Material Costs: Using less of the expensive silver paste required for soldering reduces manufacturing costs.
But this innovation introduced a critical trade-off. In MBB modules, the copper ribbons used for cell interconnection are significantly thinner and more delicate than in older designs. While this is great for performance and cost, it also makes the solder joints more susceptible to fatigue and cracking under thermo-mechanical stress.
Think of it like bending a thick metal bar versus a thin paperclip. The paperclip bends easily and, with enough repetition, will break much faster. The same principle applies to the thousands of tiny solder joints inside an MBB solar panel. Each time the panel flexes, it puts stress on these joints. Over time, that stress can cause tiny cracks to form and grow, eventually severing the electrical connection.
Finding the Breaking Point: Why Dynamic Testing Matters
How do we find these microscopic weak points before they cause problems in the field? The answer is to simulate a lifetime of stress in a laboratory.
Traditionally, modules were subjected to a static load test—applying a heavy, constant pressure to simulate a worst-case snow load. But this doesn’t reflect reality. Wind doesn’t just push on a panel; it causes it to vibrate and oscillate.
That’s where Dynamic Mechanical Load (DML) testing comes in. Instead of one heavy push, a DML test applies thousands of push-and-pull cycles to the module, mimicking the flexing caused by years of wind and snow. This repetitive stress is far more effective at revealing the fatigue failures that occur in the real world.
After the DML test, we use a technique called Electroluminescence (EL) imaging to see the damage. An EL image essentially works like an X-ray for a solar panel, lighting up the active cell areas. Any cracks, breaks, or failed connections appear as dark spots or lines, showing us exactly where the interconnections have failed.
Image: A post-DML test electroluminescence (EL) image of a solar panel. The image clearly shows dark spots and lines indicating microcracks and failed solder joints, particularly concentrated in certain areas.
As the image above shows, analysis after DML testing often reveals that failures aren’t random. These failures are concentrated at the solder joints connecting the ribbon to the cell, confirming that this is the module’s weakest link under mechanical stress.
A Closer Look: What Really Happens to Solder Joints
When we zoom in on a single solder joint, we can see why it fails. The primary failure mechanism is a crack that starts at the edge of the joint—where stress is most concentrated—and slowly propagates through the solder.
Image: A close-up diagram or illustration showing a cross-section of a multi-busbar solder joint. It should highlight the copper ribbon, the solder, and the solar cell, with an arrow pointing to a potential crack initiation point at the edge of the joint.
What determines whether this crack forms? It’s not just the materials themselves, but the quality of the bond between them. The microstructure of the solder—how it cools and crystallizes—plays a crucial role in its durability, and this microstructure is directly influenced by the manufacturing process.
From Data to Durability: Tuning the Process for Resilience
Here, testing transforms from a simple pass/fail exercise into a powerful tool for improvement. The EL images don’t just tell us that a module failed; they show us how and where it failed. This data becomes a precise roadmap for creating a more robust product.
The key lies in the stringer, the machine that solders the ribbons to the cells. The soldering process parameters—such as the temperature profile, soldering time, and any pre-heating of the ribbon—are critical. By carefully analyzing the failure patterns from DML testing, engineers can fine-tune these settings.
“We often see that teams focus heavily on material selection but overlook the immense impact of the stringer’s soldering parameters,“ notes Patrick Thoma, PV Process Specialist at J.v.G. Technology. „A DML test followed by EL analysis gives us a precise roadmap. It shows us exactly where the interconnects are failing, allowing us to adjust the solar process parameters—not just the materials—to build a fundamentally more resilient module.”
For example, if cracks consistently appear at a certain point, it might indicate that the cooling rate is too fast, creating a brittle joint. By adjusting the temperature profile on the stringer, a stronger, more ductile joint can be formed—one that can withstand thousands of flex cycles without failing.
Building a Better Module From the Ground Up
For anyone developing next-generation solar technology, understanding the relationship between dynamic stress, solder joint fatigue, and process parameters is crucial. It shifts the focus from simply selecting materials to optimizing how they are assembled.
This knowledge is vital across all stages of development, from initial testing of new materials like ribbons and solder fluxes to the final validation of prototype PV modules. By integrating DML testing early in the design cycle, manufacturers can identify and eliminate these hidden weaknesses, ensuring their products deliver the long-term reliability and performance customers expect.
Frequently Asked Questions (FAQ)
What is a multi-busbar (MBB) module?
An MBB module uses many thin, round wires (busbars) to collect and conduct electricity from the solar cells, unlike older designs that used a few flat, wide ribbons. This design improves efficiency by reducing shading and electrical resistance.
What’s the difference between static and dynamic load testing?
Static load testing applies a single, constant pressure to a module to simulate a heavy snow load. Dynamic Mechanical Load (DML) testing applies thousands of smaller, repetitive push-pull cycles to simulate the flexing and vibration caused by wind over many years. DML is more effective at revealing fatigue-related failures in components like solder joints.
Can’t you just use stronger ribbons to prevent this?
While ribbon material properties are important, the failure often occurs in the solder joint itself, not the ribbon. This makes the module’s strength highly dependent on the quality of the soldering process. A perfectly optimized soldering process with a standard ribbon will often outperform a poorly executed process with a „stronger“ but incompatible ribbon.
How long does a typical DML test take?
A standard DML test protocol, such as the one specified by the IEC, involves applying 1,000 cycles at a specific pressure (e.g., 1,000 Pa). This can take several hours to complete, followed by EL imaging and analysis to assess the impact.
Your Next Step Toward More Reliable Solar Modules
Creating a solar module that can truly withstand the test of time begins with understanding its hidden vulnerabilities. By leveraging advanced diagnostic tools like DML and EL analysis, we can move beyond simply hoping a design is durable and start engineering it for resilience from the cell up.
If you’re exploring new materials, developing novel module designs, or looking to optimize your production processes, understanding the interplay between mechanical stress and interconnection quality is the essential first step.