You’ve made the leap to Heterojunction (HJT) cells, drawn in by their impressive efficiency and performance. But as you move from the spec sheet to the production line, you face a critical challenge—one that can quietly undermine your entire module’s reliability. The very technology that makes HJT cells so powerful also makes them incredibly sensitive to heat.
Standard soldering processes, which run at temperatures around 245°C, are simply too hot. They risk damaging the delicate amorphous silicon layers that give HJT its edge. The solution seems straightforward: switch to a low-temperature solder (LTS) paste.
Problem solved, right? Not quite. In solving one problem, you may have unknowingly created another: one of mechanical strength. This is the central dilemma of HJT interconnection: how do you achieve a rock-solid mechanical bond without exceeding the cell’s thermal budget?
Why HJT Cells Demand a New Approach to Soldering
HJT cells are a marvel of solar engineering, but their unique structure is their Achilles‘ heel during manufacturing. The thin layers of amorphous silicon are highly susceptible to thermal degradation at temperatures above 200°C. Exceed this limit, and you risk compromising the cell’s efficiency and long-term stability—the very reasons you chose HJT in the first place.
Image: A close-up shot of a Heterojunction (HJT) solar cell showing the delicate busbars.
This thermal sensitivity makes traditional soldering methods unsuitable for HJT. The industry has pivoted to Low-Temperature Soldering (LTS), typically using tin-bismuth-silver (SnBiAg) alloys with a melting point around 140°C. This allows for a much gentler soldering process, keeping the cell well within its safe thermal operating window.
However, this lower temperature introduces a significant trade-off that many engineers discover the hard way: mechanical strength.
The Hidden Trade-Off: Brittleness vs. Bond Strength
Think of it like baking. If you lower the oven temperature to avoid burning a delicate cake, you risk it coming out crumbly and unable to hold its structure. Low-temperature solder alloys behave similarly; they are inherently more brittle than their high-temperature counterparts.
This brittleness directly impacts the adhesion between the cell’s busbar and the interconnecting ribbon. We measure this adhesion using a „pull strength“ test, which quantifies the force required to peel the ribbon off the cell. It’s a critical indicator of a reliable solder joint. While standard soldering aims for a pull strength over 2.0 Newtons per millimeter (N/mm), achieving this industry benchmark with low-temperature solder is a serious engineering challenge.
Initial trials often yield disappointing results. Our own research shows that a poorly optimized LTS process, even one that seems successful at first, can result in a catastrophic drop in mechanical integrity.
Finding the „Goldilocks Zone“: A Data-Driven Process Window
The key to overcoming the brittleness of LTS isn’t a new material but a mastery of the process—specifically, the thermal profile. The peak temperature during soldering is the single most important variable determining the final bond strength.
Through extensive material testing and controlled experiments, we’ve mapped the relationship between peak temperature and pull strength for HJT cells. The results are eye-opening.
- At a peak temperature of 180°C, the pull strength struggles to reach 1.4 N/mm—a concerning 30% reduction compared to the standard. The joint is simply too weak.
- Increasing the peak temperature to 190°C brings a significant improvement, with pull strength climbing to around 1.7 N/mm. We’re getting closer.
- At a peak temperature of 200°C—right at the edge of the HJT cell’s thermal limit—the pull strength finally exceeds 2.0 N/mm, matching the robustness of a standard solder joint.
Image: A graph comparing the pull strength of standard solder vs. low-temperature solder at different peak temperatures (180°C, 190°C, 200°C).
This data reveals a critical „Goldilocks Zone“ for HJT soldering: a narrow process window between 190°C and 200°C. It’s in this precise thermal range that you can achieve the required mechanical strength without damaging the cell. Operating below this window compromises reliability; operating above it risks cell degradation.
„Many teams focus only on staying below the 200°C limit,“ notes Patrick Thoma, a PV Process Specialist at J.v.G. Technology. „But the real engineering challenge is getting as close to that limit as possible without overshooting it. That’s where precise process optimization separates a lab concept from a production-ready module.“
The Silent Killer: Micro-Cracks and Long-Term Reliability
Why does a 30% drop in pull strength matter so much? Because a module in the field doesn’t experience a single, static day. It endures thousands of temperature cycles, from cold nights to hot days, causing materials to expand and contract.
A weak solder joint cannot withstand this repeated mechanical stress. Over time, microscopic cracks begin to form. These micro-cracks are invisible to the naked eye but are devastating to module performance. They increase series resistance, create hot spots, and can eventually lead to a complete interruption of the electrical circuit.
The most frightening part? A module with weak LTS joints can pass an initial quality check with a flawless Electroluminescence (EL) image. The damage only reveals itself after accelerated aging tests, like the industry-standard TC200 thermal cycling test.
Image: An Electroluminescence (EL) image showing micro-cracks in a poorly soldered LTS connection after thermal cycling.
This image shows the stark reality. Solder joints created at sub-optimal temperatures (below 190°C) are far more likely to fail during thermal cycling, jeopardizing the 25-year warranty of your module. This underscores the importance of not just building a module, but building it to last—a core principle in all solar module prototyping.
Frequently Asked Questions (FAQ)
What exactly is Low-Temperature Soldering (LTS)?
LTS is an interconnection technique that uses solder alloys, typically containing bismuth, which melt at much lower temperatures (around 140°C) than traditional tin-lead or tin-silver-copper solders (which melt above 220°C). It’s essential for processing thermally sensitive components like HJT solar cells.
Why can’t I just use standard solder on HJT cells?
The high temperatures (around 245°C) required for standard soldering would permanently damage the sensitive amorphous silicon layers in HJT cells. This degradation would reduce the cell’s efficiency and long-term stability, negating its primary advantages.
What is pull strength and why is it so important?
Pull strength is a measurement of the adhesive force between the solar cell’s busbar and the soldered ribbon. A high pull strength indicates a strong, durable mechanical bond that can withstand the stresses of thermal cycling and physical handling over the module’s lifetime. It’s a key predictor of long-term reliability.
Is a slightly lower pull strength acceptable for LTS joints?
While it may be tempting to accept a lower pull strength to maintain a larger safety margin from the 200°C thermal limit, our data shows this is a risky compromise. Lower pull strength correlates directly with a higher probability of micro-crack formation during thermal cycling, leading to premature field failures. The goal should be to optimize the process to achieve maximum strength within the safe thermal window.
Your Path to a Robust HJT Process
Integrating HJT cells into your production line is more than just a material swap; it’s a fundamental process engineering challenge. The transition to low-temperature soldering requires a new level of precision and a deep understanding of the interplay between thermal profiles and mechanical integrity.
The key takeaways are clear:
- Acknowledge the Trade-Off: Understand that LTS solves the heat problem but introduces a mechanical challenge due to its inherent brittleness.
- Define Your Process Window: Don’t guess. Use data-driven methods to precisely characterize your thermal profile and aim for the 190-200°C „Goldilocks Zone.“
- Validate for the Long Term: Initial quality checks are not enough. Use thermal cycling (TC200) and subsequent EL testing to ensure your solder joints are built to last.
Successfully navigating this challenge opens the door to producing high-efficiency, highly reliable solar modules. If you’re at the beginning of this journey, the next step is to move from theory to practice by validating these concepts with your specific materials and equipment. A controlled, scientific approach today is the best way to guarantee performance for the next 25 years.