As solar module technology pushes beyond the limits of PERC, the industry’s focus is shifting. For developers working with high-efficiency cells like Heterojunction (HJT) and TOPCon, the biggest cell-to-module (CTM) gains are no longer found in the cell itself but in how it’s connected.
Traditional high-temperature soldering, the industry workhorse for decades, is now a liability. The intense thermal stress it creates can damage the sensitive, advanced layers of HJT and TOPCon cells, leading to microcracks, efficiency loss, and long-term reliability issues.
This isn’t a theoretical problem—it’s a critical manufacturing challenge that directly impacts your product’s performance, bankability, and profitability. Choosing the right interconnection strategy is no longer just a process step; it’s a core design decision.
This guide takes a data-driven look at the three leading interconnection technologies: low-temperature soldering, electrically conductive adhesives (ECA), and zero-busbar (ZBB) designs. We’ll move beyond datasheets to explore the practical realities of implementation, drawing on insights from our applied research at PVTestLab.
The New Reality: Why Advanced Cells Demand a New Interconnection Playbook
The market has chosen its path forward. In 2024, TOPCon technology is set to dominate n-type production, capturing between 38% and 70% of the market. HJT, while holding a smaller share, remains the premium choice for peak efficiency (around 26.5%) and superior performance in hot climates, thanks to its excellent temperature coefficient.
But both technologies share a common vulnerability: their complex, multi-layered structures are highly sensitive to heat. For HJT, it’s the amorphous silicon layers; for TOPCon, the ultra-thin tunneling oxide layer. Both can be compromised by the 230-260°C temperatures of conventional soldering.
This thermal stress introduces a range of risks:
- Immediate Yield Loss: Microcracks formed during soldering can lead to cell breakage or rejection during EL testing.
- Reduced CTM Efficiency: Thermal damage can degrade the cell’s electrical performance before it ever leaves the factory.
- Long-Term Reliability Concerns: Hidden defects can propagate over time, leading to premature module degradation and field failures.
Unlocking the full potential of these cells requires an interconnection process designed for their unique characteristics.
Evaluating the Top 3 Interconnection Technologies
The industry has developed three primary pathways to address the thermal stress problem. Each comes with its own set of trade-offs in cost, complexity, and performance. Here’s how they compare based on our hands-on process validation work.
Low-Temperature Solders (LTS): The Evolutionary Step
Low-temperature soldering is the most direct evolution from traditional processes. It involves replacing standard tin-lead or tin-silver-copper alloys with bismuth-based alloys (like SnBi or SnBiAg) that melt at temperatures below 180°C.
Our process trials show this seemingly simple change has a profound impact. By reducing peak process temperatures, LTS immediately lowers the thermal stress on the cell, preserving its passivation layers and minimizing the risk of microcracks. It’s an attractive option for manufacturers looking to adapt existing production lines, as it can often use the same stringer equipment with modified thermal profiles.
However, bismuth alloys can be more brittle than traditional solders. This introduces a new set of process challenges related to mechanical stress during handling and long-term climate cycling.
PVTestLab Process Insight: Success with LTS depends on mastering the thermal profile. It’s not just about a lower peak temperature; the ramp-up and cool-down rates are critical to preventing latent defects. Through our Material Testing and Lamination Trials, we help clients design and validate precise thermal recipes that maximize bond strength while protecting against bismuth brittleness—a common failure point that often only becomes visible after rigorous testing like TC600.
Electrically Conductive Adhesives (ECAs): The Flexibility Play
ECAs represent a fundamental shift away from soldering. These are polymer-based adhesives filled with conductive particles (typically silver) that are dispensed onto the cell and cured at low temperatures, often around 150°C. From a thermal perspective, this process is almost entirely stress-free.
The adoption of this technology is accelerating; industry projections show that over 15% of modules will utilize ECAs by 2032. The primary advantage is mechanical flexibility. The cured adhesive forms a pliable bond that can better absorb thermomechanical stress throughout the module’s life. This makes ECAs exceptionally well-suited for advanced concepts like ultra-thin wafers and flexible modules.
Here, the challenge shifts from thermal control to material science and process precision. The long-term stability of the polymer, the consistency of the dispensing, and the thoroughness of the curing process are all critical for a 25-year product warranty.
PVTestLab Process Insight: ECA performance is defined by the dispensing and curing steps. Using our full-scale Prototyping and Module Development line, we fine-tune dispense patterns, dot sizes, and curing profiles to ensure a void-free, highly reliable interconnection. By optimizing these parameters, we’ve helped partners reduce material waste by up to 15% while validating the bond’s integrity through accelerated lifetime testing in our climate chambers.
Zero-Busbar (ZBB) Interconnection: The Efficiency Maximizer
Often called the „hottest topic“ in module manufacturing, ZBB eliminates traditional flat busbars entirely. Instead, it uses an array of up to 18 fine, round wires or specially coated ribbons laminated directly onto the cell surface, often with an ECA or a specialized thermal adhesive layer.
This design delivers three main performance benefits:
- Reduced Shading: Round wires cast significantly smaller shadows than flat ribbons, increasing the active cell area and boosting power output.
- Lower Material Cost: ZBB designs drastically reduce the amount of expensive silver paste required on the cell.
- Enhanced Reliability: The dense mesh of wires provides redundant electrical paths, making the module far more resilient to power loss from microcracks.
While ZBB offers the highest potential for CTM gains, it also requires the most significant process adaptation. It’s not just a stringer modification; it’s a fully integrated system that impacts cell metallization, layup, and lamination.
PVTestLab Process Insight: ZBB is an ecosystem, not a single component. The key to success is ensuring seamless integration between the wire application, layup, and lamination stages. Our applied research focuses on the lamination recipe itself. The pressure and temperature profile must be perfectly calibrated to fully encapsulate the wires without creating new stress points or damaging the cell. This is a core focus of our Process Optimization and Training programs, where we help teams bridge the gap between theory and real-world industrial production.
The Decision Matrix: Comparing Your Options
To simplify your evaluation, here is a comparison of these technologies across key factors.
Low-Temperature Solder (LTS)
- CTM Efficiency Gain: Baseline improvement due to lower thermal stress on cells.
- Relative Cost per Wp: Lowest. Similar material cost to traditional solder.
- Reliability: Good, but requires process control to manage bismuth brittleness.
- CapEx Requirement: Low. Often compatible with existing stringer equipment.
- Best Suited For: Cost-effective upgrades for TOPCon lines, bifacial modules.
Electrically Conductive Adhesive (ECA)
- CTM Efficiency Gain: Moderate gain from reduced stress and compatibility with thin wafers.
- Relative Cost per Wp: Medium. Higher material cost but potential for savings on thin wafers.
- Reliability: Very Good. Excellent thermomechanical stress resistance.
- CapEx Requirement: Medium. Requires investment in precision dispensing and curing systems.
- Best Suited For: Premium HJT modules, ultra-thin wafers, flexible module designs.
Zero-Busbar (ZBB)
- CTM Efficiency Gain: Highest gain (1-3% relative) due to reduced shading and resistive loss.
- Relative Cost per Wp: Medium-High. Reduces silver paste cost but has higher CapEx.
- Reliability: Excellent. High redundancy protects against microcrack power loss.
- CapEx Requirement: High. Requires dedicated stringer/layup and lamination systems.
- Best Suited For: Flagship products demanding maximum power output and reliability.
Frequently Asked Questions
Question: Is ECA technology proven enough for a 25-year warranty?
Answer: Absolutely. While newer than soldering, ECAs have been used in demanding electronics applications for decades. The key is rigorous validation. Our reliability testing protocols, including extended damp heat (DH2000) and thermal cycling (TC600), are designed to verify that the specific ECA formulation and process you choose will maintain its mechanical and electrical integrity over the module’s full lifetime.
Question: What is the biggest challenge when transitioning from soldering to ZBB?
Answer: The primary challenge is shifting from a linear process (stringing) to an integrated one (wire layup and lamination). The interplay between the lamination materials (encapsulant, backsheet) and the ZBB wires is complex. Getting this right requires a holistic approach to process design, which is where a pilot-line environment like PVTestLab becomes invaluable for de-risking the transition before making major CapEx investments.
Question: Can I upgrade my existing PERC line for TOPCon using low-temp soldering?
Answer: Yes, this is one of the most common pathways and a key reason for TOPCon’s rapid adoption. Because LTS can often use existing stringer hardware, it represents the lowest-capital route to producing high-efficiency TOPCon modules. However, it’s crucial to conduct thorough process validation to ensure your new thermal profiles are optimized for both the TOPCon cell and the LTS alloy.
Find Your Optimal Process Before You Scale
The choice between low-temp solders, ECAs, and ZBB isn’t about finding the single „best“ technology—it’s about finding the optimal strategy for your product goals, manufacturing capabilities, and market position.
Making the right decision requires more than datasheets. It requires real-world data from an industrial production environment.
At PVTestLab, we provide that environment. You can test new materials, prototype next-generation module designs, and fine-tune your process parameters on a complete, full-scale production line, all guided by experienced German process engineers. Don’t leave your CTM efficiency gains to chance.
Ready to validate your interconnection strategy? Contact our engineering team to discuss your project and schedule a trial on our R&D line.