You’ve done everything right: your new Multi-Busbar (MBB) solar modules are coming off the line, the materials are top-tier, and initial flash tests look good. Yet, when you analyze long-term performance data, the power output is consistently a fraction of a percent lower than simulations predicted.
It’s not a catastrophic failure, but it is a frustrating and costly mystery. In modern solar manufacturing, this scenario is all too common. The culprit is often a microscopic flaw invisible to the naked eye: solder bridging—a tiny, unwanted electrical shortcut that silently drains power from an otherwise perfect module.
Let’s break down what solder bridging is, why it poses a particular challenge for MBB technology, and how you can diagnose and prevent it on your production line.
What Are MBB Cells, and Why Do They Have This Problem?
For years, solar cells typically had four, five, or six wide, flat busbars to collect electrical current. Multi-Busbar (MBB) technology was a game-changer. By using 10, 12, or even 16 very thin, round wires instead, MBB cells offer two significant advantages:
-
Reduced Shading: The thin wires block less sunlight from reaching the cell, boosting light absorption.
-
Shorter Electron Travel Path: The dense grid of wires means electrons don’t have to travel as far to be collected, significantly cutting resistive losses.
The result is a more efficient and powerful solar cell. But this elegant design introduces a new manufacturing challenge: the spacing between these busbars is incredibly tight. This proximity dramatically increases the risk of solder accidentally connecting two adjacent busbars and creating a „bridge.“
This bridge forms a parallel resistance path—essentially a micro-short circuit. While a single tiny bridge won’t cause a module to fail, hundreds of them can create a steady power drain, leading to underperformance and even localized hotspots that pose a long-term reliability risk.
Research confirms that issues like solder bridging, solder balls, and ribbon misalignment are primary concerns in the soldering process, directly impacting module efficiency and quality.
You Can’t Fix What You Can’t See: The Diagnostic Challenge
Solder bridges are often microscopic, which makes visual inspection impractical and unreliable on a production scale. So, how do you find an invisible problem?
The answer lies in making it visible through advanced imaging and electrical analysis.
Electroluminescence (EL) Testing: The Solar Cell „X-Ray“
The most powerful tool for detecting solder bridging is Electroluminescence (EL) testing. Think of it as an X-ray for solar modules. By applying a current to the module in a dark environment, the silicon cells light up. A healthy cell glows uniformly.
But areas with defects like micro-cracks or short circuits don’t light up properly. A solder bridge appears as a distinct dark spot or a dark line along the busbar, showing exactly where current is being diverted and providing undeniable visual confirmation of a short circuit.
Dark I-V Measurement: Quantifying the Leak
While EL shows you where the problem is, a Dark I-V curve helps you understand how severe it is. This test measures the „leakage current“ when the module is not producing power. A higher leakage current—specifically, a lower shunt resistance (Rsh)—often points to the presence of shorts and confirms the findings from the EL image.
Getting to the Root Cause: A Process Qualification Methodology
Finding a solder bridge is one thing; preventing it is another. The solution lies in precise control over your soldering process. The primary culprits behind solder bridging are almost always related to the solder paste and its application.
„Solder bridging is a classic example of where a small process variation can have a measurable impact on performance. It’s not about the quality of the cells or the ribbons; it’s about the precision of their assembly. The volume, viscosity, and placement of your solder paste are the three pillars of a stable MBB soldering process.“
— Patrick Thoma, PV Process Specialist
Here’s a methodology for systematically troubleshooting and optimizing your process:
- Control Solder Paste Volume
Too much solder is the most common cause of bridging. The paste must be just enough to form a strong bond without overflowing, which requires precise calibration of your dispensing or printing equipment. A Design of Experiments (DOE) can help you find the optimal volume that ensures robust interconnection with the lowest risk of bridging.
- Manage Solder Paste Viscosity and Composition
The properties of the solder itself are critical. If the paste is too thin (low viscosity), it can slump and flow during the heating process, easily connecting adjacent busbars. Work closely with your supplier and conduct thorough material validation to ensure the solder paste’s rheology is perfectly suited to your process parameters and the tight tolerances of MBB cells.
- Ensure Placement Accuracy
The stringer machine must place the ribbons with extreme precision. Even a slight misalignment can position a busbar too close to a neighboring solder deposit, inviting a bridge to form. This makes regular calibration and maintenance of your stringer’s vision and placement systems non-negotiable.
Controlling these elements in a single environment is key. Structured solar module lamination trials allow you to test how different solder pastes, application volumes, and stringer settings behave under real thermal conditions. This systematic approach—a core part of effective PV module prototyping—helps you build a robust and repeatable process that eliminates guesswork.
Frequently Asked Questions (FAQ)
Q: What’s the difference between a solder bridge and an electrical short?
A: A solder bridge is a specific physical cause of an electrical short. The „short“ is the resulting electrical condition where current takes an unintended, low-resistance path. Solder bridging is one of many potential causes of shorts in a solar module.
Q: Can you repair a module with solder bridging?
A: Unfortunately, once the module is laminated, it is practically impossible to repair solder bridges. The focus must be on prevention during the cell stringing and layup process.
Q: How much power loss can solder bridging actually cause?
A: The impact varies. A single, small bridge might have a negligible effect. However, widespread or severe bridging across multiple cells can lead to power losses of 1-3% or even more. It also increases the risk of hotspot formation over the module’s lifetime, which can lead to more severe degradation.
Q: Are newer cell technologies like TOPCon or HJT also affected?
A: Yes, absolutely. Solder bridging is a function of the interconnection method (MBB), not the underlying cell architecture. Any module that uses MBB for current collection is susceptible to this issue if the soldering process is not perfectly controlled.
From Mystery to Mastery
The hidden power drain from solder bridging doesn’t have to be a permanent mystery. By combining advanced diagnostics like EL testing with a systematic approach to process control, you can turn this manufacturing challenge into a competitive advantage.
Perfecting the balance between solder volume, viscosity, and placement is the key to unlocking the full potential of MBB technology. This ensures that the power generated by your high-efficiency cells actually makes it out of the module, delivering the value and reliability your customers expect.
For manufacturers looking to accelerate process development, testing and validating materials in a dedicated R&D environment can provide the data-driven confidence needed to scale production successfully.