COVERSTORY
solar cells and modules. We need only to consider that as wafers have become thinner, larger and more delicate, and module layup is automated, the mechanical alignment systems that have until now been perfectly adequate for PV manufacture will necessarily give way to the sort of vision-assisted alignment techniques that have served the demanding semiconductor industry so well for years.
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Three lane processing exiting a dryer
that the landing area for the second printing pass, which is formed by the base pattern, is extremely limited.
High Cpk values are also fundamentally important for Selective Emitter Cells, another high-efficiency solution that, like PoP, requires a second print pattern to be aligned to within a few microns of a previously doped pattern. The added challenge here is that as the dopant, the first pattern to be deposited, is almost invisible to the naked eye, normal vision alignment techniques cannot be used to align the subsequent silver collection grid pattern to it. Most manufacturers therefore use multiple small diameter fiducials, positioned at the outer extremes of the cell, to which the deposition processes must be precisely aligned.
Dye Sensitised Solar Cell Technology, another relatively new approach to solar cell manufacture that is based on thin film technology, also calls for extreme precision in aligning successive printed layers within the cell, and here too, high Cpk is key.
A further way to increase efficiency is to move the relatively wide bus bars from the front of the cell to the rear, connecting them to the collection grid by means of metal wrap-through holes, solar’s version of plated through holes, which again requires high Cpk print processes.
From the above it is clear that repeatable accuracy, and therefore Cpk values of at least 2, are soon to become extremely important for manufacturers of
Another critical issue is machine stability, especially considering the enormous increases in print throughputs. Printing systems are typically equipped with large mechanical parts such as work tables, print heads and handling systems that may go through extensive linear or rotational excursions, thousands of times a day. As we go forward with new developments like those described above, repeated alignment to within just a few microns will become the norm. As this happens, it is essential that such masses and their movements are minimised as they may cause the machine to vibrate during printing, compromising print accuracy and quality, or equally damaging, they can over time progressively knock print alignment accuracies out of true.
A history of repeatable accuracy Achieving repeatable accuracy – and the high Cpk values that reflect this – requires a wealth of experience in process control and design. And this is where companies like DEK Solar come in. DEK has built repeatable accuracy into its systems ever since it opened for business 4 decades ago. As part of his printer demonstrations, for example, DEK’s founder would cover part of the print substrate with a strip of scotch tape, then he would print, remove the tape, and reprint. And then he would challenge his potential customer to spot the join. That was 40 years ago. If it was hard to find the joint then, it’s close to impossible today without
Eclipse inline wafer inspection module
www.solar-pv-management.com Issue VI 2010
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