Imagine this: your new multi-busbar (MBB) production line is running smoothly. The modules look flawless rolling off the line—clean, uniform, and perfectly assembled. But when you check the final power output, the numbers are all over the place. Some modules hit their target, while others fall mysteriously short.

What’s going on? The answer is likely hidden from the naked eye, visible only through a special kind of „X-ray vision“ for solar cells: Electroluminescence, or EL imaging. These tiny, almost invisible imperfections in the soldering process are silently stealing your power, one solder joint at a time.

Why Your Stringer is a High-Stakes Game for MBB Cells

The switch from traditional 5-busbar (5BB) cells to multi-busbar technology was a huge leap forward. Using many thin, round wires instead of a few flat ribbons reduced silver paste consumption and lowered electrical resistance, boosting module efficiency.

But this innovation came with a trade-off. Where you once had a handful of robust solder joints, you now have hundreds of delicate ones on every cell. This makes the stringer process—the machine that solders the interconnecting ribbons to the cells—more critical than ever. The quality and consistency of these solder joints are now a primary driver of both initial power output and long-term module reliability, and even the slightest deviation can have a big impact.

Decoding the Dark Spots: A Visual Guide to Solder Defects

EL imaging is the single most powerful tool for diagnosing these issues. By passing a small electrical current through a solar cell, we make it light up. Healthy, efficient areas glow brightly, while problematic areas with high resistance or damage appear dark. These dark spots are a data-rich map pointing directly to specific failures in your production process.

Symptom #1: Cold Solder Joints

A „cold“ solder joint happens when the soldering iron isn’t hot enough or doesn’t dwell long enough to create a strong, uniform metallurgical bond. The solder fails to „wet“ the surfaces properly, resulting in a weak electrical connection.

What you’ll see in an EL image: You’ll notice small, distinct, and often circular dark spots precisely at the solder points along the busbar. In some cases, the entire ribbon may appear dimmer than its neighbors.

The cause: This high-resistance point creates a bottleneck for electrons. Since they can’t flow through easily, that part of the cell doesn’t light up properly. These spots not only reduce power but can also become hotspots over time, accelerating degradation.

Consider this a direct signal from your stringer. It’s telling you that the soldering temperature might be too low, the conveyor speed too high, or the flux isn’t activating correctly to clean the surface.

Symptom #2: Busbar Misalignment

With MBB technology, the round wires must be placed perfectly centered on the cell’s tiny solder pads. If the alignment is off by even a fraction of a millimeter, the wire may only make partial contact or, worse, create stress points on the delicate cell.

How this appears in an EL image: Instead of a small dot, misalignment typically creates a larger, linear dark area running along one side of the busbar. It’s a clear sign that one edge of the electrical contact is failing.

Why this happens: The off-center wire creates an uneven connection that increases resistance. More dangerously, it introduces mechanical stress. As the module heats and cools in the field, this stress can easily lead to microcracks propagating from the misaligned joint, causing severe power loss down the road.

The root of the problem is a mechanical issue tied to your stringer’s setup. It could be caused by improper ribbon guidance, incorrect cell positioning, or vibration in the transport system.

Symptom #3: The Sneaky Problem of Flux Residue

Flux is essential for a clean soldering process, but it’s designed to burn off completely. If the temperature isn’t quite right, a sticky, insulating residue can be left behind.

What it looks like in EL: This can be one of the trickiest defects to diagnose. Flux residue often appears as hazy, mottled, or irregularly shaped dark patches around the solder joints, which can sometimes be mistaken for other cell-level issues.

The underlying problem: The residue acts as an insulator, physically blocking the flow of current. Over time, it can also attract moisture and dust, potentially leading to corrosion and further degradation. This defect points to an imbalance in your soldering parameters—the temperature may be too low to fully burn off the flux, or you may be using a flux type that isn’t optimized for your process speed.

From Diagnosis to Action: Turning EL Insights into Process Improvements

Seeing these patterns is the first step. Fixing them requires a systematic approach. It’s rarely about one magic bullet; it’s about understanding the complex interplay between materials, machine settings, and process speed.

„An EL image isn’t just a pass/fail picture; it’s a process data map. A dark spot isn’t a defect—it’s a signal that your stringer temperature might be 5°C too low or your ribbon alignment is off by 0.1mm. This is where real process optimization begins.“
— Patrick Thoma, PV Process Specialist

The key is to troubleshoot these issues during solar module prototyping, preventing them from showing up in mass production. By creating a controlled environment, you can test one variable at a time—adjusting temperature, speed, or alignment—and see the immediate impact on the EL image.

Don’t forget that other materials can play a role, too. The type of encapsulant used, for instance, can affect the thermal dynamics during soldering and curing. This is why running integrated lamination trials alongside stringer tests provides a more complete picture of the final module’s quality.

Frequently Asked Questions (FAQ)

Can you see these solder defects with the naked eye?

Almost never. Cold joints, minor misalignment, and flux residue are microscopic. This is precisely why EL testing is an indispensable quality control tool in modern module manufacturing.

Are these small dark spots really that bad for module power?

Individually, the power loss from one cold solder joint is tiny. But a typical MBB cell has hundreds of solder points. If even 5% of them are faulty, the cumulative power loss becomes significant. What’s more, these high-resistance points can become „hotspots“ in the field, degrading the module much faster than its neighbors.

How is EL different from a flasher test (IV curve)?

Think of it this way: a flasher test tells you that you lost power. It gives you the final number. An EL test tells you where and why you lost it. The flasher diagnoses the symptom (low power), while EL diagnoses the root cause (e.g., cold solder joints in the third cell string).

Does this only apply to MBB cells?

While the principles of good soldering apply to all cell technologies, the sensitivity and frequency of these specific defects are much higher with MBB. The smaller solder pads and thinner wires leave a much smaller margin for process error, making precise stringer control absolutely essential.

Your Next Step in Mastering Module Quality

The next time you look at an EL image, don’t just see it as a quality check. See it as a conversation with your production line. Every dark spot, every shadow, and every inconsistency is a piece of data telling you how to build a better, more reliable, and more powerful solar module.

Understanding these signals is the first step. The next is creating a testing environment where you can safely connect cause and effect to build a truly robust and optimized production process.