You’ve invested in a state-of-the-art, high-speed shingling line. The goal is clear: produce next-generation TOPCon modules with higher power density and faster throughput. The line is running and cells are flying, but the results from your quality control team are disheartening. Power output is inconsistent, and worse, some of the first prototypes are showing signs of microcracking after just a few thermal cycles.
What’s going on? It’s not the cells, and it’s not the glass. The culprit is often hiding in plain sight: the tiny, millimeter-thick layer of electrically conductive adhesive (ECA) that bonds your shingled cells together.
In the race for speed, the intricate science of applying and curing this adhesive is often overlooked. But for sensitive TOPCon cells, this single process step can determine your module’s performance and long-term reliability.
What’s So Different About Shingling TOPCon Cells?
For years, soldering ribbons onto cells was the industry standard. But shingled module design, where cells are overlapped like roof shingles to eliminate gaps and maximize the active area, changed the manufacturing landscape. This design requires a more flexible and delicate bonding method.
Enter Electrically Conductive Adhesives (ECAs). These are specialized epoxies filled with conductive particles (usually silver) that create a strong, flexible electrical connection between the overlapped cells.
For TOPCon (Tunnel Oxide Passivated Contact) cells, ECAs are not just an option; they’re a necessity. Here’s why:
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Heat Sensitivity: TOPCon cells feature ultra-thin passivation layers that are highly sensitive to thermal stress. The high temperatures of traditional soldering (>200°C) can damage these layers, degrading cell efficiency from the start. ECAs cure at much lower temperatures, typically below 160°C, preserving the cell’s integrity.
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Mechanical Flexibility: The shingled structure needs to withstand mechanical stress during production and thermal expansion and contraction in the field. A flexible ECA bond absorbs this stress far better than a rigid solder joint, reducing the risk of microcracks.
But as manufacturers push shingling line speeds past 3,000 or even 4,000 cells per hour, the process window for applying and curing that ECA becomes incredibly small. This creates the triple challenge of speed, precision, and heat.
The Triple Challenge: Speed, Precision, and Heat
Successfully bonding shingled TOPCon cells at high speed isn’t just about using any ECA. It’s about orchestrating a perfect interplay between the adhesive’s properties and the machine’s process parameters. Getting it wrong introduces defects that are nearly impossible to fix later.
The Need for Speed: Rapid Curing Profiles
On a high-speed line, the curing station—the oven that hardens the ECA—is a major bottleneck. To keep up with the stringer, the adhesive must cure completely in under 10 seconds. This requires a precisely engineered thermal profile: the rate of temperature increase (ramp-up), the peak temperature, how long it’s held (dwell time), and the cool-down rate.
If you cure too aggressively, you risk „outgassing,“ where solvents in the ECA evaporate too quickly, creating tiny bubbles or voids in the bond line. Cure too slowly, and your production line grinds to a halt. Finding that perfect, sub-10-second profile is a critical process engineering task.
The Goldilocks Problem: ECA Viscosity and Bond Line Control
Viscosity is a measure of how thick or „runny“ an adhesive is. For ECA jetting systems, which dispense tiny, precise dots of adhesive, viscosity is everything. This is what we call the „Goldilocks problem“—it has to be just right.
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If Viscosity is Too Low: The ECA spreads out too much when the next cell is placed on top. This „squeeze-out“ can create unintended electrical connections (shunts) along the cell’s edge, draining power.
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If Viscosity is Too High: The ECA doesn’t flow enough to properly „wet“ the cell surfaces. This creates voids, or empty pockets, in the bond line, which increases electrical resistance and creates potential failure points.
The goal is a consistent, void-free Bond Line Thickness (BLT) that ensures maximum electrical contact without any squeeze-out. Achieving this balance is fundamental to any advanced solar module prototyping project, directly impacting both power and reliability.
The Silent Killer: Voids and Delamination
When the curing profile and ECA viscosity aren’t perfectly matched, you get the two most common and destructive defects: voids and delamination.
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Voids are air pockets trapped within the bond. They act like roadblocks for electrons, increasing the module’s series resistance and reducing its power output. Over time, thermal stress can cause these voids to grow, leading to hot spots.
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Delamination is the separation of the cell from the adhesive. It’s an immediate sign of a weak bond, often caused by improper curing or poor surface wetting. This completely breaks the electrical path and can propagate microcracks throughout the cell under mechanical stress.
These defects are often invisible to the naked eye but show up glaringly in electroluminescence (EL) testing, appearing as dark spots or entire dead cell strings.
Finding the Right Formula: How to Match Your ECA to Your Process
The solution isn’t to slow down your line. It’s to adopt a data-driven approach to select and optimize your ECA process. This means treating the adhesive, the dispenser, and the curing oven as one integrated system.
Step 1: Characterize Your Adhesive
Don’t just rely on the manufacturer’s datasheet. Different ECA formulations behave differently under the pressures of high-speed jetting and rapid curing. At PVTestLab, we conduct structured experiments to analyze key properties:
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Viscosity vs. Temperature: How does the ECA’s thickness change as it’s heated in the dispenser?
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Rheology: How does it behave under the high shear forces of jetting? Does it hold its dot shape or slump?
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Curing Kinetics: How quickly does it reach full strength at different temperatures?
Step 2: Engineer Your Thermal Profile
There is no „one-size-fits-all“ curing profile. The ideal profile depends on your chosen ECA, your cell type, and your line speed. This requires controlled experimentation—what we call lamination process trials for the interconnection stage. By testing various combinations of ramp-up speeds and dwell times, you can identify the optimal thermal recipe that achieves a full cure in the shortest time without creating internal stress or voids.
For example, our tests have shown that a two-stage heating profile—a quick initial ramp to start the reaction followed by a slightly lower dwell temperature—can often reduce voids compared to a single, aggressive peak temperature.
Step 3: Integrate and Validate
Once you have a promising ECA and curing profile, the final step is to validate the entire process. This involves producing a series of mini-modules or full-size prototypes and subjecting them to accelerated reliability testing, including:
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Thermal Cycling: (e.g., 200 cycles from -40°C to +85°C) to check for delamination and crack formation.
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Damp Heat Testing: (e.g., 1000 hours at 85°C and 85% humidity) to test the bond’s long-term stability.
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High-Resolution EL Imaging: To inspect for new or growing defects after each round of testing.
This validation provides concrete data, confirming your chosen process is not just fast but also robust enough to produce reliable modules that will last for 25+ years in the field.
Frequently Asked Questions (FAQ)
What is an ECA?
An ECA, or Electrically Conductive Adhesive, is a specialized adhesive, usually an epoxy, filled with conductive particles like silver. It is used in solar modules to create both a physical bond and an electrical connection between components, especially in heat-sensitive or flexible designs like shingled cells.
Why can’t we just use traditional solder for TOPCon shingled cells?
Traditional soldering requires high temperatures (often >200°C), which can damage the delicate, passivated layers of high-efficiency TOPCon cells, reducing their performance. Solder also creates a rigid joint, which is more prone to causing microcracks in the thin, overlapping cells under mechanical stress.
How do I know if my ECA process has voids?
While severe voids can be detected with high-resolution EL imaging, definitive confirmation often requires laboratory analysis. This can involve cross-sectioning the bond line and inspecting it under a microscope or using advanced non-destructive techniques like scanning acoustic microscopy.
Can I use the same ECA for PERC and TOPCon cells?
Not always. While some ECAs may work for both, the process parameters will be very different. TOPCon cells require a much lower peak curing temperature, so you need an ECA specifically formulated for low-temperature, rapid curing to avoid thermal degradation.
What’s more important: ECA viscosity or the curing profile?
Neither. They are two sides of the same coin. The best ECA formulation in the world will fail with a poor curing profile, and a perfect curing profile cannot fix the problems caused by an incorrect viscosity. They must be optimized together as a single, integrated system.
From Lab Theory to Production Reality
The push for higher throughput in shingled TOPCon manufacturing is relentless, but speed cannot come at the expense of quality and reliability. The interconnection process itself is a sophisticated engineering challenge that requires a holistic approach.
Success depends on matching your material (the ECA) to your machinery (the jetting and curing system) through rigorous, data-driven testing. By moving beyond datasheets and validating your process under real-world conditions, you can turn your production line’s biggest potential weakness into a source of competitive advantage.
If you are navigating these complex process variables, consulting with a PV process specialist can provide the clarity and data needed to bridge the gap between material selection and high-yield, reliable production.