You’ve done everything right. Your lamination process seems flawless, your materials are top-tier, and visual inspection shows a perfectly encapsulated solar module. But when you look closer—especially around the cell interconnect ribbons—you see them: tiny, persistent bubbles.
These small voids, often localized around solder joints and busbars, are more than just cosmetic imperfections. They are silent performance killers—indicators of a hidden struggle between your materials and process parameters. While they may seem minor, they can lead to reduced power output, hotspots, and long-term reliability issues.
So, what’s really going on? The answer often lies in outgassing from solder flux residue, a process that begins long before the module even enters the laminator.
Making the Invisible Visible: How to Diagnose Interconnect Bubbles
Standard visual inspection often misses these subtle defects, especially in their early stages. To truly understand the problem, you need to look at the module’s electrical and thermal behavior. Here, advanced diagnostics become your most powerful tool.
Electroluminescence (EL) Testing
An EL test is like an X-ray for a solar module. By passing a current through the cells, it reveals inactive or damaged areas as dark spots. When bubbles form around solder joints, they prevent the encapsulant from properly adhering to the cell, creating a non-conductive area that shows up clearly in an EL image.
The dark zones in the image above aren’t just bubbles; they represent lost power-generating real estate. Each dark spot is a micro-failure that, when multiplied across thousands of cells, can significantly impact the module’s overall efficiency.
Thermal Imaging
Where there’s poor electrical contact, there’s often heat. Bubbles around solder joints increase local series resistance, forcing electrical current to navigate around the void. This detour generates excess heat, creating a „hotspot.“
Thermal imaging cameras can detect these hotspots, which are invisible to the naked eye. Over time, these elevated temperatures can accelerate the degradation of the encapsulant and the cell itself, posing a serious risk to the module’s long-term durability and safety.
The Root Cause: Trapped Gasses from Solder Flux
The evidence points to bubbles, but the true culprit is often the solder flux used during the stringing process. Flux is essential for creating clean, strong, and reliable solder connections between cells. However, some of its chemical components can remain on the busbar after soldering.
When the module is heated inside the laminator, this residue vaporizes and releases gasses—a process known as outgassing. If this happens after the encapsulant (like EVA or POE) has started to melt and cure, the gas gets trapped, forming the very bubbles you see in EL and thermal tests.
The challenge is that different fluxes and encapsulants have different chemical compositions and react at different temperatures. A process that works perfectly for one set of materials may create widespread defects with another. This is why a one-size-fits-all lamination recipe is rarely effective.
The Solution: A Symphony of Vacuum and Heat
Eliminating these bubbles isn’t about finding a „better“ flux or encapsulant. It’s about synchronizing your lamination process with the specific properties of your materials. The two most critical levers you can pull are the vacuum cycle and the temperature profile.
1. Master Your Vacuum Ramp Rate
The primary purpose of the vacuum in a laminator is to remove air and moisture. It’s also your best tool for removing flux gasses before they get trapped. A common mistake is applying a deep vacuum too quickly.
A more effective approach is a two-stage vacuum process:
- Initial Low Vacuum: A slower, gentler initial pump-down allows outgassing to occur while the encapsulant is still porous enough for the gas to escape. This „pre-vacuum“ phase carefully removes volatile compounds without disturbing the module layup.
- Final High Vacuum: Once the outgassing phase is complete, the full vacuum can be applied to remove any remaining air and ensure a void-free bond.
Getting this timing right is critical. It requires understanding the outgassing temperature of your specific flux and aligning it with your vacuum steps. This alignment is typically discovered through structured lamination trials that test various ramp rates and hold times.
2. Fine-Tune Your Temperature Profile
The speed at which you heat the module is just as important. If the temperature rises too quickly, the encapsulant may cross-link (cure) before all the flux gasses have escaped.
By carefully controlling the heating ramp, you can ensure the outgassing is completed before the encapsulant reaches its gel point. This might mean:
- Adding a temperature „soak“ or hold period at a point just below the encapsulant’s melting temperature.
- Slowing the overall heating rate to give gasses more time to evacuate.
Optimizing these parameters is especially important when prototyping new module designs or introducing new materials into your production line. What worked for a PERC module with EVA might not work for a TOPCon module with a specific POE.
From Defect Detection to Flawless Production
Bubbles around solder joints are a classic example of a complex problem with an elegant solution. They reveal the deep connection between materials science and process engineering. By moving beyond simple visual checks and using advanced diagnostics to understand the root cause, you can transform your lamination process from a source of hidden defects into a predictable, reliable, and high-yield manufacturing step.
Ultimately, the goal is to create a process recipe where the materials, timing, and environmental conditions work in perfect harmony.
Frequently Asked Questions (FAQ)
1. What exactly is solder flux and why is it necessary?
Solder flux is a chemical cleaning agent used before and during the soldering of electronic components. Its main purpose is to remove oxides from the surfaces of the metal ribbons and silicon cells, ensuring a strong, uniform electrical connection. While essential for quality soldering, its residue can cause problems if not managed during lamination.
2. Can’t I just see these bubbles with my eyes after lamination?
Sometimes, but often the most damaging bubbles are small or located under the busbar, making them nearly invisible to the naked eye. Electroluminescence (EL) and thermal imaging are far more reliable because they detect the effect of the bubble (inactive areas and hotspots), not just its physical appearance.
3. Is this problem more common with certain types of encapsulants, like EVA or POE?
The problem is less about one encapsulant being „better“ than another and more about compatibility. POE and EVA have different melting points, curing speeds, and viscosity profiles. A lamination process optimized for EVA may trap gasses when used with POE, and vice versa. Each material combination requires its own validated process recipe.
4. Will a faster lamination cycle always make outgassing worse?
Not necessarily, but it increases the risk. A faster cycle reduces the time available for gasses to escape. If you need to increase throughput, you must optimize your vacuum and temperature profiles even more precisely to ensure outgassing is complete within the shorter window. Simply speeding up an existing recipe is a common cause of bubble formation.
5. How do I know what the right vacuum and temperature profile is for my materials?
You find the ideal profile through systematic experimentation. This involves running controlled tests where you adjust one variable at a time (e.g., vacuum ramp rate, temperature hold time) and measure the outcome using EL and other quality checks. This data-driven approach is the only way to build a robust process recipe tailored to your specific bill of materials, often done on a full-scale R&D production line to ensure the results are transferable to mass production.