The Fine Line: Why Low-Temperature Soldering for TOPCon & HJT Cells is a High-Stakes Balancing Act
Imagine you’ve sourced the latest generation of high-efficiency solar cells—TOPCon or HJT—promising unprecedented power output. Your team is excited. But after manufacturing, the modules are underperforming. The efficiency gains you paid for have vanished, and worse, you’re seeing early field failures. The culprit? Not the cells, but a microscopic failure in a process that’s more of a delicate dance than a simple connection: soldering.
The shift to thermally sensitive cells like Heterojunction (HJT) and Tunnel Oxide Passivated Contact (TOPCon) has been a massive leap for the solar industry. But their advanced, delicate layers are easily damaged by the high heat of traditional soldering, a reality that has pushed manufacturers to adopt low-temperature soldering (LTS) processes.
While this solves one problem, it introduces another, more subtle one: mechanical fragility. This is where many promising module designs falter, and it’s a challenge that can only be solved with precise, data-driven process validation.
The New Frontier: Why TOPCon and HJT Cells Changed the Rules
To understand the problem, we first need to appreciate what makes these cells so special—and so sensitive.
Both HJT and TOPCon cells rely on incredibly thin, delicate passivation layers and transparent conductive oxide (TCO) coatings. Think of these like highly specialized, performance-enhancing films. They are essential for achieving high efficiency, but they are also extremely vulnerable to thermal stress.
Traditional soldering at temperatures above 200°C can irreversibly damage these layers. The result? A significant drop in the cell’s open-circuit voltage (Voc), a primary indicator of its performance potential. You could start with a world-class cell and end up with a mediocre one simply due to manufacturing heat damage. This is why the industry has shifted to low-temperature soldering pastes, often using bismuth (Bi)-based alloys that melt at much lower temperatures.
This new soldering method is essential for connecting the multiple busbars (MBB) that collect current from the cell. But solving the heat problem has created an entirely new mechanical challenge.
The Hidden Weakness of Low-Temperature Soldering
Here’s the „aha moment“ that many teams discover too late: the bismuth-based alloys used in low-temperature soldering are inherently more brittle than their traditional tin-lead or tin-silver counterparts.
This brittleness creates a major risk. While the solder joint might seem fine coming off the production line, it can be a ticking time bomb. Under the thermo-mechanical stress of manufacturing (like lamination) or real-world conditions (like daily temperature swings and wind or snow load), these brittle joints are prone to cracking.
These are not cracks you can see with the naked eye. They are microcracks that create tiny breaks in the electrical connection. Each microcrack increases the module’s series resistance (Rs)—think of it as adding electrical friction that wastes energy. As these cracks form and grow, the module’s power output steadily declines.
Advanced Electroluminescence (EL) imaging is critical for revealing this hidden damage. A module can look perfect on the outside while being riddled with power-sapping microcracks on the inside.
Finding the „Goldilocks Zone“: The Soldering Process Window
This reveals the core challenge: how do you create a solder joint strong enough to last 25 years without applying enough heat to damage the cell?
The answer lies in defining and maintaining a precise soldering process window. This is a delicate balance between three key parameters:
- Temperature: Hot enough to create a strong metallurgical bond, but cool enough to protect the cell’s Voc.
- Time: Long enough for the solder to flow and bond correctly, but short enough to minimize thermal exposure.
- Pressure: Enough force to ensure good contact, but not so much that it physically stresses the delicate cell.
Get this balance wrong, and you face one of two failures:
- Too Aggressive (Too Hot/Long): You get a strong bond, but you sacrifice cell efficiency. The Voc drops, and you lose the very performance you were aiming for.
- Too Gentle (Too Cool/Short): You preserve the cell’s efficiency, but you create a weak, brittle solder joint that is destined to fail under stress.
„Many teams focus exclusively on avoiding cell damage, so they keep their soldering process as cool as possible,“ notes Patrick Thoma, PV Process Specialist at PVTestLab. „But they forget that a weak solder joint is a latent failure waiting to happen. The goal isn’t just to protect Voc; it’s to guarantee a reliable interconnection for decades. True process optimization requires finding the perfect equilibrium where both goals are met, and that can only be found through structured testing.“
How Do You Prove It Works? The Role of Process Validation
You can’t find this „Goldilocks Zone“ through guesswork. It requires a systematic approach to process validation, using real industrial equipment to simulate a lifetime of stress.
That’s where controlled testing protocols become essential. To verify the long-term reliability of solder joints, modules must be subjected to accelerated stress tests that mimic years of outdoor exposure in just a few weeks. Key validation methods include:
- Thermal Cycling (TC): The module is exposed to repeated temperature swings, often from -40°C to +85°C, for hundreds of cycles. This simulates the stress of daily and seasonal temperature changes, which cause the module materials to expand and contract, putting strain on the solder joints.
- Mechanical Load Testing (MLT): The module is subjected to static and dynamic pressures to simulate wind and snow loads. This tests the physical resilience of the interconnections.
These protocols, often defined by standards like IEC 61215, are crucial for any serious solar module prototyping. By combining these stress tests with careful EL imaging and performance measurements before and after, you can prove definitively whether your soldering process has found the right balance. It’s the only way to move from hoping your modules will last to knowing they will. This kind of deep-dive analysis often involves extensive material compatibility testing to ensure the solder, ribbon, and cell coatings all work in harmony.
Frequently Asked Questions (FAQ)
What exactly is a multi-busbar (MBB) cell?
A multi-busbar cell uses many very thin, typically round wires (busbars) to collect electrical current, instead of a few flat ribbons. This reduces resistive losses and the amount of shading on the cell, increasing efficiency. However, it also means there are many more solder points that need to be perfect.
What is Voc and why is it so important?
Voc stands for Open-Circuit Voltage. It’s the maximum voltage a solar cell can produce when no current is flowing. It is a key indicator of the cell’s electronic quality. A drop in Voc after processing, like soldering, is a clear sign that the cell’s sensitive layers have been damaged.
Why is low temperature so critical for HJT and TOPCon cells specifically?
These cells have unique, nanometer-thin layers (amorphous silicon in HJT, tunnel oxide in TOPCon) that are responsible for their high performance. These layers are extremely sensitive to heat. High temperatures can cause them to break down or delaminate, permanently degrading the cell’s efficiency.
Can’t you just see microcracks by looking at the module?
No, almost never. Microcracks are often invisible to the naked eye and can even be hidden under busbars. The only reliable way to detect them is with specialized equipment like an Electroluminescence (EL) tester, which makes the cracks show up as dark, inactive areas on an image of the cell.
From Theory to Factory Floor: Your Next Steps
The promise of high-efficiency cells like TOPCon and HJT is real, but so are the manufacturing challenges. Achieving both peak performance and long-term reliability isn’t about choosing one over the other; it’s about finding the scientifically-proven process window where they coexist.
This requires moving beyond datasheets and assumptions and into the realm of applied research. By understanding the delicate balance between thermal protection and mechanical strength, and by using systematic validation protocols to verify your process, you can ensure that the potential you see in the lab is the performance your customers get in the field.
Building a truly reliable solar module is a journey of precision, one that begins with understanding these fundamental challenges and committing to a data-driven solution.