MANUFACTURING I THERMAL
a prerequisite for capable processes. Machine capability only takes the capability of the machine into consideration [2]. The capability of a process/machine can be numerically described through the use of generally accepted capability coefficients.
Due to the fact that process capability depends upon the capability of the utilized production machines, higher numeric requirements are specified for machine capability. Sauer [2] indicates that in general, a capability value of Cmk > 1.67 is required for machines. Wohlrabe[4] describes the fundamental procedure used for capability analysis. Generally speaking, atleast 50 individual measurements are required.
Figure 3: Calculation of temperature difference on the solar cell
calibration and the ageing characteristics of the thermocouples, as well as test setup and execution, must also be taken into consideration.
On the one hand, this article discusses the measurement of temperature profiles, and it specifically addresses the question of which acquired temperature data are stable enough for the calculation of machine capability coefficients. In addition to theoretical observations, results are also presented which have been obtained during close collaboration between Bosch Solar Energy AG and Rehm Thermal Systems GmbH.
Machine capability
Process capability is defined by DIN ISO 21747:2006 [1] as a statistical estimate of the consequence of a characteristic of a process which is verifiably kept under control, wherein the estimated value describes the capability of the process to affect the characteristic such that it fulfills its respective requirement. Sauer [2] goes into the subject of the quality capability of processes in great detail. Process capability and machine capability describe the potential capability of a process (a machine) to produce a certain characteristic in a constant fashion within specified tolerance limits. Capable machines are
Figure 4: Calculation of standard deviation in dual lane systems
Problems become plainly apparent right from the start when attempting to ascertain capability coefficients for the firing process used for crystalline solar cells. Which measurable characteristics can be defined? The quality of the manufactured solar cells depends upon a great variety of influencing factors (e.g. wafer quality, POCl3 process, solar paste combination etc.), and the impact of the machine (firing system) accounts for only a small portion of overall influence. Consequently, cell parameters are unsuitable as a measurable characteristic of machine capability.
Temperature, conveyor speed and, if applicable, temperature differences are all possible candidates as measurable characteristics.
Conveyor speed is generally not used for the ascertainment of machine capability data. To some extent this is due to the knowledge that minimal variation in conveyor speed has nearly no significant effect on cell parameters. At the relatively high speeds typical of the firing process (5.5 to 6.0 meters per minute), the amount of heat absorbed by the individual wafers is nearly identical when the usual conveyor speed tolerances are taken into account (< 0.5%, ~ ± 3.5 cm/min.). A comparison of the temperature profiles for various conveyor speeds differing by ± 25 cm/min. (550 to 600 cm/min.) is shown in figure 2. The heat equivalent absorbed by the solar wafer in the peak zone deviates by no more than ± 4.5% of the mean value, although
Mean value Standard deviation Machine capability Cm Table 1: Statistical Values Associated with Figure 3 Issue IV 2014 I
www.solar-international.net 23
Temperature Difference (L-R) 5.5 2.0
1.72
Absolute Temperature (R) 771.8 1.7
1.18
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