You’ve done everything right. You’ve sourced the latest high-efficiency PERC cells, selected a high-quality encapsulant, and designed a state-of-the-art solar module. But when the final flash test results come in, the power output is consistently lower than your calculations predicted.
Where did the power go?
This frustrating gap between theoretical potential and real-world performance is a common challenge. Often, the culprit isn’t the materials themselves, but a subtle, overlooked variable in the manufacturing process: lamination pressure. It’s the silent factor that can create an invisible traffic jam for electrons, quietly stealing your module’s power.
Let’s explore how this happens and what a recent study reveals about finding the „Goldilocks zone“ for pressure.
The Most Important Connection You Can’t See
Inside every solar module, thousands of tiny, critical connections are made. Solder ribbons are meticulously bonded to the silver busbars on the solar cells, forming the electrical highway that carries generated power.
A close-up shot of a solar cell’s busbars and solder ribbons, highlighting the delicate connection points.
This bond is everything. If it’s weak, incomplete, or stressed, it creates resistance. And in the world of photovoltaics, resistance is the enemy of efficiency. This is particularly true for modern high-efficiency cells like PERC (Passivated Emitter and Rear Cell), which are thinner and more sensitive to mechanical stress.
What is Series Resistance (Rs) and Why Does It Matter?
Think of series resistance (Rs) as the total electrical friction electrons encounter as they travel out of the solar cell. It’s the sum of all the little roadblocks in the current’s path—from the silicon wafer itself to the solder joints and ribbons.
A diagram illustrating the concept of series resistance (Rs) in a solar cell, showing resistance points along the current path.
When Rs is low, electrons flow freely, like cars on an open freeway. When Rs is high, it’s like a rush-hour traffic jam. The energy that should be powering a home is instead lost as waste heat within the module.
The formula for this power loss is simple but unforgiving: P_loss = I² * Rs. Since the power loss is proportional to the square of the current (I), even a small increase in series resistance (Rs) can lead to a significant drop in the module’s final power output (Pmax).
A PVTestLab Study: Putting Lamination Pressure to the Test
We know that lamination—the process of using heat and pressure to bond a module’s layers together—is crucial for durability. But how does the pressure variable specifically affect the delicate solder joints on modern PERC cells?
To find out, our team conducted a focused study.
The Setup:
- Cells: Standard high-efficiency PERC M10 cells with 12 busbars.
- Encapsulant: A commonly used EVA (ethylene vinyl acetate).
- The Variable: We created identical mini-modules and laminated them at three different pressure levels: Low (0.2 bar), Medium (0.5 bar), and High (0.8 bar), while keeping all other parameters (temperature, time) constant. This type of controlled experiment is central to the work we do in material testing and lamination trials.
The Results: A Clear „Goldilocks Zone“ Emerges
After lamination, we analyzed each module’s performance using a Class AAA flasher to generate precise I-V curves. The results were striking.
An IV curve graph comparing the results from low, medium, and high lamination pressure tests, with the „knee“ of the curve clearly showing the impact of Rs.
- Low Pressure (0.2 bar): This group showed a significant increase in series resistance. The gentle pressure was insufficient to create a robust, uniform bond between the solder ribbon and the cell’s busbars. The I-V curve showed a noticeably shallower „knee,“ a classic sign of high Rs and lost power.
- High Pressure (0.8 bar): While better than low pressure, this setting also resulted in a measurable increase in Rs compared to the optimal group. The excessive force likely induced mechanical stress on the solder joint and the cell itself, potentially creating micro-cracks that impede current flow.
- Medium Pressure (0.5 bar): This was the clear winner. Modules laminated at this pressure exhibited the lowest series resistance and, consequently, the highest power output. The pressure was strong enough to form a solid connection but gentle enough to avoid damaging the delicate cell structure.
As our PV Process Specialist Patrick Thoma notes, „Many teams are laser-focused on lamination temperature profiles and cycle times, which are of course critical. But pressure is the silent variable that can make or break a solder joint’s integrity. Our data confirms that assuming ‚more is better‘ or ‚gentler is safer‘ is a risky simplification.“
For visual confirmation, we used Electroluminescence (EL) imaging, which reveals defects invisible to the naked eye. The module laminated with low pressure clearly showed darker, inactive areas around the solder joints—providing direct visual evidence of the poor electrical contact causing the high Rs.
An Electroluminescence (EL) image of a mini-module laminated with low pressure, revealing dark spots or uneven areas indicative of poor soldering.
What This Means For Your Solar Project
The key takeaway is that lamination pressure is not a „set it and forget it“ parameter. It’s a critical control point that needs to be dialed in for your specific combination of cells, ribbons, and encapsulants.
- Don’t Assume, Validate: The lamination recipe that worked for older cell technologies may not be optimal for today’s thinner, more complex PERC or TOPCon cells.
- Data is Your Best Friend: Relying on datasheets alone is not enough. The only way to know for sure is to test. This is why iterative process optimization is essential for staying competitive.
- Test Before You Invest: Before scaling to full production, validating these parameters during the solar module prototyping phase can save you from costly, large-scale yield loss down the line.
That hidden power thief in your module might just be a simple pressure setting. By understanding its impact and testing to find the optimal „Goldilocks zone,“ you can ensure your modules deliver the performance they were designed for.
FAQ: Your Lamination Pressure Questions Answered
What exactly is lamination pressure?
Lamination pressure is the force applied by the laminator’s diaphragm onto the solar module stack during the heating cycle. It ensures that the encapsulant flows properly around the cells and ribbons, removes air bubbles, and creates a strong, void-free bond between all layers: glass, encapsulant, cells, and backsheet.
Why are PERC cells more sensitive to lamination pressure?
Modern PERC cells are thinner than their predecessors to save on silicon costs. They also have a delicate, multi-layer passivation coating on their rear side that is crucial for their high efficiency. Excessive or non-uniform pressure can induce micro-cracks in the thin wafer or damage this passivation layer, both of which can increase series resistance or create other recombination losses.
How can I tell if my lamination pressure is incorrect?
The most reliable methods are quantitative. A high-precision I-V curve measurement from a flasher will reveal an increase in series resistance (Rs) and a decrease in Fill Factor (FF). Additionally, Electroluminescence (EL) testing before and after lamination can visually expose new micro-cracks or areas of poor soldering that resulted from the process.
Can’t I just use the highest pressure to ensure a good bond?
As our study showed, this is not a safe bet. While very high pressure might prevent the issues seen with low pressure (like poor contact), it introduces a new set of risks, primarily mechanical stress. This can lead to cell cracking, damaged busbars, and long-term reliability problems that may not be immediately apparent but can cause premature module failure in the field.