Lamination is the most important process in making of solar modules. A good laminate ensures long life and durability of PV modules whereas an improper one can lead to early or premature module failures. Understanding the complete process not only helps in producing a better product but also reduces losses like cell breakage, air bubbles and delamination, which mostly occurs due to incorrect processing parameters.
Majority of module laminators follow the following 3-step process for proper melting and curing of the encapsulant (EVA) and achieving a good quality laminate:
1. Heating of the module lay-up to required temperatures to perform the EVA cross-linking step.
2. Applying a vacuum to remove the air and other volatiles to prevent bubbles.
3. Application of pressure to ensure a good surface contact and adhesion between the different layers of the PV module.
The so-called flatbed laminator consists of a processing chamber that is divided by a silicon membrane (silicon diaphragm) in an upper and lower chamber. Both chambers can be individually evacuated and the module lay-up is normally heated in the lower chamber by a heating plate (Electric or hybrid).
To dig deeper, the three step process can be further divided into following:
1. Lay up: An arrangement of glass-> EVA -> cells -> EVA -> Backsheet, in that order, is prepared for lamination.
2. Pin lift: Due to the relative large temperature difference of about 100°C between the heating plate and the PV module lay-up upon insertion, glass warping (curving) of the 3-4mm thick glass is observed. To avoid this glass warping and achieve homogenous heating profile, the flat-bed laminator is equipped with pins that are used to lift the PV module lay-up about 5mm from the heating plate, which results in a more gentle and homogeneous heating of the lay-up.
3. Vacuum - Upper chamber: The lower processing chamber of the laminator is evacuated to remove the air and avoid bubble formation. The time of applying a vacuum as well as the rate of evacuation can be varied to optimise the process and hence the end-result. Reducing the pressure too early or at a high rate will result in significant outgassing of the additives in the EVA like adhesion promoters and/or stabilisers, and hence in a decreased quality of the PV modules, whereas applying the vacuum too late will lead to air inclusion and hence unwanted
bubble formation.
4. Pin Release: After obtaining a homogeneous temperature in the glass plate and crossing the so called EVA softening point at about 60°C-80°C, the PV module is directly pressed on the heating plate by releasing the pins and the actual EVA cross-linking process is initiated.
5. Vent - Upper Chamber: By venting the upper chamber, improved contact & heat transfer takes place between the module lay-up and the heating plate. Here again, care has to be taken when applying the pressure, to what extend and at what rate. Applying the pressure too early increases the chance of unwanted cell breakage whereas applying the pressure too late will most often result in a shift of the cell strings or so-called cell-swimming phenomenon.
6. Controlled Cooling: A subsequent controlled cooling step leads to a controlled stopping of the radical induced chemical reactions and completes the lamination cycle making the stable PV module ready for the post-processing steps and testing.
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