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50% Wafer Breakage Reduction By Ductile Polishgrinding
In today’s tight competition market waste reduction by wafer breakage reduction is a severe topic.
According to ductile polishgrinding specialist G&N, Erlangen in Germany, in a usual multi-crystalline production wafer factory each 10 million wafers lot (ca. 35 MW) has a wafer waste caused by broken wafers between 4% - 8%. Each wafer is a sales value of ca. 3 USD which equals losses of 1.2 - 2.4 mio. USD per year. “More than enough to seriously think and focus on this often underestimated and undervalued issue” says G&N´s CEO Gerhard Koennemann.
“With G&N´s unique ductile polishgrinding process breakage rate can be reduced by 50%, i. e. to 2% - 8%. Our best case was almost 0%.”, he goes on.
“These huge financial savings help that the machine basically pays itself already within 3 - 6 months.”
The TWIN-Spindle® of the German company KW-Abrichttechnik enables G&N´s machines to pre-grind and to polishgrind in a ductile mode with only 0.1 seconds delay in one clamping only. As a result the bricks are in perfect geometric shape and simultaneously have a mirror-like surface to reduce breakage as much as technically available today.
G&N´s ductile polishgrinding technology meanwhile is worldwide known and acknowledged and used in more than 100 machines. Chamfers are ground simultaneously and “for free” in the same machine during the same run.
Where does wafer breakage come from?
Wafer breakage is one of the biggest money wasting issues during the photovoltaic cell production process. If breakage rate of mono-crystalline material is set to 1, then the rate of multi-crystalline material is at a factor of ca. 2 and the one of UMG-Si is ca. 3.
It is necessary to think about, where breakage comes from. It is mostly caused by micro cracks - especially at the corners (this is why you champher the corners) and the edges. So a solution is simply to reduce the deep damage on the brick surface (which will be the wafer edges later) caused by the squaring cutting process with diamond band, wire or simple slurry cutting.
Slurry wire or diamond blade cutting process gives damage on the brick surfaces and edges (champher) in a value typical of 20 to 60 microns. The final wafer should be under 180 microns thickness. Already by the squaring process, it should be prepared for much lower damage, but it would need much more time to reach lower damage on the cut surfaces.
Using multi-crystalline material it is necessary to rework the squared surface , mostly by a polish-grinding process. Doing this simultaneously also the brick geometric errors are corrected and the surface gets well prepared for gluing. Nearly 50% of multi-crystalline wafer manufacturers polish their bricks completely on all 4 sides, including the champhers, but not in a correct way, yet.
Coming up with new materials like UMG–Si the breakage rate jumps up. G&N developed a complete new polishing and grinding process, so called simultaneous ductile polish-grinding for 4 sides of a brick and the 4 champhers.
This ductile mode allows also on standard multi-crystalline blocks to reduce the breakage close to "0".
Tests showed that also UMG-Si material had a significantly lower breakage rate when using the ductile polish grinding process which is installed in the G&N polish grinder type MPS 3-134.
Through the new combination of the ductile polish grinding mode with the patented TWIN-Spindle sytem the machine´s productivity rose up about 30%. Very important is the simultaneous process of face and chamfer ductile polish grinding. The bricks come out ready for ir-inspection, gluing and then, of course, wafering.
When it comes to wafers, the advantages are two-fold: Either wafer thickness stays the same and breakage rate is significantly reduced. Or, breakage rate stays the same and wafer thickness can be reduced significantly for extra sales.
The machine is prepared for full automatic robot handling. The MPS 3-134 has the smallest footprint and energy costs of all comparable machines worldwide.
Calculating the ROI (return on investment), this G&N polish grinder usually “pays itself” within 4 to 6 months by extra sales of more useable wafers which otherwise would have gone to the litter box.
