Round-Robin Verification and Final Development of the IEC 62788-1-5 Encapsulation Size Change Test; NREL (National Renewable Energy Laboratory)
Polymeric encapsulation materials may a change size when processed at typical module lamination temperatures. The relief of residual strain, trapped during the manufacture of encapsulation sheet, can affect module performance and reliability. For example, displaced cells and interconnects threaten: cell fracture; broken interconnects (open circuits and ground faults); delamination at interfaces; and void formation. A standardized test for the characterization of change in linear dimensions of encapsulation sheet has been developed and verified. The IEC 62788-1-5 standard quantifies the maximum change in linear dimensions that may occur to allow for process control of size change. Developments incorporated into the Committee Draft (CD) of the standard as well as the assessment of the repeatability and reproducibility of the test method are described here. No pass/fail criteria are given in the standard, rather a repeatable protocol to quantify the change in dimension is provided to aid those working with encapsulation. The round-robin experiment described here identified that the repeatability and reproducibility of measurements is on the order of 1%. Recent refinements to the test procedure to improve repeatability and reproducibility include: the use of a convection oven to improve the thermal equilibration time constant and its uniformity; well-defined measurement locations reduce the effects of sampling size -and location- relative to the specimen edges; a standardized sand substrate may be readily obtained to reduce friction that would otherwise complicate the results; specimen sampling is defined, so that material is examined at known sites across the width and length of rolls; and encapsulation should be examined at the manufacturer’s recommended processing temperature, except when a cross-linking reaction may limit the size change. EVA, for example, should be examined 100 °C, between its melt transition (occurring up to 80 °C) and the onset of cross-linking (often at 100 °C).