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A wafer-utilization factor is shown for computing the number of chips per wafer for any chip size, kerf, pad cage, and wafer diameter.
English (United States)
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Chips Per Wafer Count
A wafer-utilization factor is shown for computing the number of chips per
wafer for any chip size, kerf, pad cage, and wafer diameter.
Rather than using an iterative computer program to determine the number of
chips per wafer, a closed form equation is used to evaluate wafer utilization. A
trade-off analysis can be made quickly and accurately between the chip sizes
being evaluated and the number of chips per wafer.
Referring to Fig. 1, for a given wafer diameter (Dw), the useful diameter Dwe
= Dw - dead zone (approx. 2 to 3 mm). Fig. 2 shows a chip with a side (S) and
side plus kerf (S + K). Fig. 3 is a graph showing an approximate chip count and
actual chip count per wafer. L X "Take the closest integer smaller than this
Square chips cannot fully utilize wafers. Due to this fact, some silicon is lost.
The amount of unusable silicon increases as the chip edge increases.
In the limit of chip edge ---> 0, i.e., (S+K) ---> 0, the utilization factor ---> 1. In
the limit of chip edge, (S+K) > Dwe/!L, and the utilization factor = 0.
For rectangular chips (Fig. 4), if edge E and aE, where a>1, E = (S+K), where
the aspect ratio a is the length-to-width ratio of the chip.
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