WCA January 2014

❍ ❍ Figure 6 : HMI with a touch-screen, user-friendly interface on PlasmaPREPLATE tinning line Tin-plating of copper wire is traditionally performed by running the wire through a bath of molten tin/solder followed by wiping and cooling of the coated wire vertically in the cooling tower. Critical parameter: camber Low camber is important for ensuring straight laying of interconnect ribbon during stringing. Production of solar panels has become fully automated with increasing stringing speeds. High-output fully-automated stringers can suffer from unnecessary down-time due to excessive camber of processed interconnect ribbon. Ribbon with excessive camber can even cause weak solder joint or an increase in scrap rate on the stringer. Commonly pursued target camber today is <5mm/ metre. There has been a trend of ever-tighter camber requirements which require detailed assessments of PV ribbon production process as well as payoff on the stringer during panel manufacturing. To minimise camber, PV ribbon manufacturers have to look into the following areas of improvement: • Accuracy of layer winding on the spooler, which requires precision mechanics and accurate process control • Consistent ribbon quality, especially low tolerance of coating thickness • Select appropriate size of spool Manufacturers are well aware of the limitations to the minimum possible camber on the edge of the spool, where the ribbon changes direction during laying. Minimum possible camber on spool depends on the size of ribbon and barrel diameter of the spool. However, panel manufacturers or stringer suppliers themselves can examine possible improvements of the payoff system on the stringer in order to improve ribbon laying before soldering. Increasing the size of spool can also help in reducing the camber that is created on the edge of the spool. PV ribbon production: PlasmaPREPLATE tinning vs. traditional tinning

❍ ❍ Figure 5 : PlasmaPREPLATE tinning line for PV ribbon production Some panel manufacturers have adopted an alternative panel design with three or even four smaller ribbons per cell (instead of two), which further reduces the stress on the cells after stringing. Manufacturers are therefore forced to continuously improve their rolling, annealing, tinning and material handling techniques to meet ever more demanding product specifications. Critical parameter: yield strength The thermal expansion coefficient of copper is different to the thermal expansion coefficient of silicon. Interconnect ribbon is soldered onto the silicon cell at temperatures around 200°C. Cooling down after stringing results in warpage. This could lead to silicon crystal breakage. Interconnect ribbons with low yield strength reduce the stress on silicon cells after stringing and with it the scrap rate. The use of ever-thinner solar cells drives demand for ribbons with ever-lower yield strength (Rp0.2%). Only a few years ago solar 300-micron thick cells were commonly in use. They are able to sustain the stress from ribbons with yield strength of <120MPa. Today, 160 micron-180 micron thick cells became a common practice with it the ribbons of yield strength <70MPa-<80MPa. The average cell thickness is likely to continue its downward path putting further pressure on ribbon manufacturers to reduce yield strength below 65MPa. To reduce yield strength of PV ribbon the manufacturers should look into the following areas of improvement: • Select appropriate input copper material • Choose the right annealing and rolling techniques • Ensure precision handling of soft ribbon through the transport system on the tinning line • Ensure good payoff and precision winding on the takeup in the tinning line The panel manufacturers, who want to reduce the stress on the cell after stringing, should examine their payoff system on the stringer to avoid hardening of the ribbon and creation of camber during paying off.

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Wire & Cable ASIA – January/February 2014