You’ve designed the perfect bifacial module. The materials are selected, the strings are laid out with robotic precision, and the bill of materials is optimized for performance. But after the lamination cycle, you see it—the subtle, costly misalignment of cells. Just a few millimeters of shift is all it takes to compromise module efficiency, aesthetics, and bankability.
What went wrong? It wasn’t the stringer or the layup station. The culprit is often an invisible force unleashed during the first few minutes in the laminator: the vacuum.
While many manufacturers focus on the final vacuum level and temperature, our research at PVTestLab shows that the rate of vacuum application is one of the most critical—and overlooked—variables in modern module lamination. This guide explores the physics of vacuum-induced cell shift and reveals a proven process to eliminate it.
The Anatomy of a Modern G2G Module
To understand the problem, we first need to examine the structure we’re working with. A typical glass-to-glass (G2G) bifacial module consists of a sandwich of materials, each with a specific purpose.
![Diagram showing a cross-section of a G2G module layup before lamination]()
From top to bottom, the layup includes:
- Top Glass: The first layer of protection and light transmission.
- Encapsulant (POE or EVA): A polymer sheet that bonds the layers and protects the cells.
- Solar Cells: The interconnected strings that form the heart of the module.
- Encapsulant (POE or EVA): The second sheet to fully encase the cells.
- Back Glass: Provides structural rigidity and allows for bifacial light capture.
This tightly packed, non-porous structure is fantastic for durability, but it creates a sealed chamber. When you place this layup into a laminator, you aren’t just laminating a module; you’re managing a delicate pneumatic system with a lot of trapped air. How you remove that air determines everything.
The Hidden Problem: Why a Single-Stage Vacuum Causes Chaos
The conventional approach in many production lines is to apply a single, aggressive vacuum to the laminator chamber, pulling the pressure down as quickly as possible to save cycle time. This is what we call a „pressure shock.“
Imagine trying to empty a full water bottle by turning it straight upside down—the water gushes out chaotically. A single-stage vacuum does the same to the air trapped inside the module layup. As this large volume of air rushes out from between the two sheets of glass, it creates powerful currents that can physically push and drag the delicate cell strings out of alignment.
![Animated GIF or sequence diagram illustrating the „pressure shock“ of a single-stage vacuum vs. the gentle air escape of a multi-stage vacuum]()
This is especially problematic in the initial phase when the encapsulant is still a cool, solid sheet. It has no „grip“ on the cells, allowing them to slide freely on its surface.
„We’ve seen it time and again in our lab,“ notes Patrick Thoma, PV Process Specialist at PVTestLab. „Manufacturers chase faster cycle times with aggressive vacuum ramps, but they inadvertently create forces strong enough to undo the precision of their automated stringers. The result is lower yield and compromised module aesthetics.“
The PVTestLab Solution: Mastering Control with a Multi-Stage Vacuum Profile
The key to preventing cell shift isn’t to remove air slower—it’s to remove it smarter. Through extensive solar module prototyping and testing on our full-scale R&D line, we’ve perfected a multi-stage vacuum profile that synchronizes pressure and temperature to keep cells perfectly locked in place.
![Graph showing a multi-stage vacuum profile (Pressure vs. Time) overlaid with a temperature ramp (Temperature vs. Time)]()
This process works by turning the encapsulant from a slippery surface into an anchor before applying the full force of the vacuum. Here’s how it breaks down.
Stage 1: The Gentle Purge (Initial Drawdown)
The cycle begins not with a rapid pressure drop, but with a slow, controlled vacuum application.
- Goal: To remove the bulk of the trapped air without creating disruptive air currents.
- Process: The pressure is gradually lowered to an intermediate level, typically around 200 mbar. During this time, the heating plates begin to warm the module.
- The Physics: By evacuating the chamber slowly, the air has time to escape gently around the cells rather than rushing past them. The encapsulant is still solid, but the forces are too low to cause any movement.
Stage 2: The Tacking Point (Heating & Softening)
Once the majority of the air is gone, the focus shifts to temperature.
- Goal: To heat the encapsulant to its softening point, where it becomes tacky.
- Process: The module’s temperature is raised to a specific point, often between 80-90°C for modern POE materials. The vacuum is held steady at the intermediate level.
- The Physics: As the polymer softens, it develops adhesive properties. It begins to grip the solar cells and the glass, effectively „tacking“ the entire string matrix into its precise position.
Stage 3: The Final Lock (Full Vacuum Application)
Only after the cells are secured by the tacky encapsulant do we apply the full vacuum.
- Goal: To remove the final traces of air and outgassing byproducts to ensure a void-free lamination.
- Process: The vacuum is now rapidly pulled down to its final target, often below 5 mbar.
- The Physics: Because the cells are already held firmly in place by the tacky encapsulant, this aggressive final drawdown can remove the remaining air without any risk of cell shift. The module is now locked in its ideal configuration for the final curing phase.
Perfecting this balance of pressure and temperature for different materials is the core focus of the lamination process trials we conduct for our clients.
The Result: Precision You Can See
By replacing a single, brute-force vacuum with an intelligent, multi-stage profile, manufacturers can eliminate cell shift as a source of yield loss. The result is a perfectly aligned module with superior aesthetic quality, optimized cell-to-module (CTM) performance, and reduced internal stresses.
This level of precision isn’t just a „nice-to-have“; it’s a critical factor in producing high-performance, bankable solar modules that meet the stringent quality demands of today’s market.
![High-resolution Electroluminescence (EL) image showing a perfectly aligned module laminated with the optimized process]()
Frequently Asked Questions (FAQ)
Does this multi-stage approach apply to backsheet modules too?
Yes, the principle is beneficial for all module types, but it’s especially critical for G2G constructions. This is because the two non-porous glass sheets create a much more effective seal, trapping air more intensely than a breathable backsheet would.
Will a multi-stage vacuum increase my cycle time?
Not necessarily. While the initial vacuum ramp is slower, the total cycle time can often be optimized to remain comparable to a single-stage process. The significant gains in yield, quality, and rework reduction almost always result in a net positive return. Fine-tuning this balance is a key part of process optimization.
What encapsulants are most sensitive to this?
While the principle applies to both EVA and POE, POE encapsulants can sometimes be more sensitive due to their different melt flow characteristics (rheology). Every material is unique, which is why testing your specific bill of materials is so important.
How do I determine the right „tacking point“ temperature?
The ideal temperature depends on the specific encapsulant you are using. It’s typically just above the material’s softening point, which can be found on its technical datasheet. This parameter must be validated under real industrial conditions, however, as factors like heating ramp rate and module thickness also play a role.
From Theory to Practice
Understanding the physics of vacuum drawdown is the first step toward mastering lamination. The invisible forces at play inside your laminator are powerful, but they are controllable. By shifting focus from sheer speed to intelligent control, you can transform a common source of defects into a repeatable, high-precision process.
Applying these principles to your specific materials, equipment, and module designs is the critical next step. At PVTestLab, we provide the industrial-scale R&D environment and German engineering expertise to help innovators bridge the gap between theory and reliable, high-yield production.