Researchers from China's Shandong University have proposed a new methodology for the quantitative prediction of excess kerf loss caused by lateral vibration of diamond wire wafer cutting in solar wafer production.
Kerf is the crystalline material that is wasted when a solar ingot is sliced into wafers.
“The kerf loss model outlined in our research can predict the uniformity of silicon wafer thickness, which could help control the thickness uniformity of thinner silicon wafers in the future and offer a new perspective for the study of fracture strength of photovoltaic silicon wafers,” the research's corresponding author, Yufei Gao, told pv magazine.
The researchers warned that the newest diamond wire sawing used in the PV industry can operate at a faster pace than in the past, which increases vibrations during manufacturing. Furthermore, they also explained that the diameter of the saw wire is getting thinner while the wire span is getting larger, which increases lateral vibration. Both effects combined can, in turn, result in greater kerf loss and lower wafer thickness.
The lateral vibration of the saw wire will indirectly weaken the fracture strength of the silicon wafer, resulting in an increase in the fracture rate during the sawing process and subsequent cleaning and transportation,” they further explained. “Therefore, it is very significant to study the excess kerf loss caused by lateral vibration of saw wire during the sawing process for improving the process quality of wafers and sawing of ultra-thin wafer in the future.”
The scientists initially investigated the lateral vibration dynamics of the diamond saw wire during the sawing process under continuous excitation at the saw wire's both ends. Then they studied the frictional damping effect inside the sawing kerf.
Moreover, they used the finite difference method, which is commonly used to solve differential equations, to solve their model numerically. Furthermore, they measured the acceleration signals of the vibration and the displacement signals to assess the lateral vibration characteristics of the saw wire.
Finally, the academics developed their modeling through the Hertz contact theory, which describes the deformation and stress distribution when two non-conforming, elastic, and smooth bodies make contact. They also applied the principle of conservation of momentum, which states that the total momentum of a closed system remains constant over time.
To validate the predictive model, the research team conducted three groups of sawing experiments with a SH300 machine tool provided by Shenghai Numerical Control Co., Ltd, China. The diameter of the saw wire used was 350 μm and the wire speed ranged from 1,000 m/min to 1,400 m/min.
The experiments showed that kerf loss caused by the lateral vibration is mainly due to the vibration source signal of the main sawing stage, with the vibration source signal being stable. In addition, the researchers found that kerf loss decreases with a higher wire tension and a lower wire span, while it slightly increases with higher wire speed.
The analysis also showed that, with constant sawing parameters, smaller wafers suffer from greater excess kerf loss.
“The greater the excess kerf loss, the greater the unevenness in the thickness of the silicon wafer,” the scientists stressed. “For photovoltaic monocrystalline silicon wafers, significant thickness deviation is unacceptable.”
They also emphasized that lateral vibration mainly affects the edges of the wafers, which increases the risk of cracks.
Their findings are available in the paper “Prediction of excess kerf loss in diamond wire sawing based on vibration source signal measurement and processing,” published in Measurement.
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