WCA January 2014

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62 Wire & Cable ASIA – January/February 2014
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❍ Figure 5: PlasmaPREPLATE tinning line for PV ribbon
production
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❍ Figure 6: HMI with a touch-screen, user-friendly interface on
PlasmaPREPLATE tinning line
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.
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.
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
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.