The thread is closed but the logic still checks out. Hexagons, octagons, etc. are more efficient to fill a circle than squares.
If a transistor has three leads shouldn't it be a triangle?
6 triangles make a hexagon right?
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3\$\begingroup\$ Have you ever tried to cut a bunch of hexagons out of a sheet? \$\endgroup\$– DKNguyenCommented Feb 17, 2021 at 4:27
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\$\begingroup\$ A round sheet? No. why do we have to cut the wafer? Just run the whole thing and sell the runtime/ \$\endgroup\$– kpieCommented Feb 17, 2021 at 4:27
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\$\begingroup\$ Any shaped sheet. Think about how many cuts do you need to make. Now think about how short each edge of a tiny hexagon has to be and how tiny the saw blade needs to be to squeeze in there. \$\endgroup\$– DKNguyenCommented Feb 17, 2021 at 4:28
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4\$\begingroup\$ You do realize many hundreds or thousands of chips are made from one wafer and at some point someone has to cut it up into individual chips, right? \$\endgroup\$– DKNguyenCommented Feb 17, 2021 at 4:32
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1\$\begingroup\$ The answers in that reddit thread seem correct to me. Cutting hexagons would be harder and the reticle is actually exposing using a square/rectangular mask anyway, so hexagons would actually be less efficient (more dead space between dies). \$\endgroup\$– user1850479Commented Feb 17, 2021 at 4:43
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1 Answer
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- The edges of a wafer are not useful for manufacturing so there is already some waste there.
- Areas around the edge of a wafer (and some internal spots) that you think might be wasted are actually used for test circuits. These test transistors are evaluated by the manufacturing fab to validate that the wafer was processed correctly before giving them to the customer. If the test transistors anywhere around the edge are out of tolerance then this means something in the process shifted and yield of the die may suffer.
- It is easier to cut straight across the wafer with a saw. One doesn’t saw along the edge of one die right up to the next one like with a jigsaw. Therefore only a four sided polygon is possible. If you wanted a hexagon die, then your cuts through the wafer would not only double because you now have diagonal cuts but you would end up sawing right through the neighboring die.
- Many die are very small, so they pack in around the edges of the wafer quite efficiently already. Sure, some die are very large, but then again that is why you pay a premium for an eight core Pentium. (The cost to process a wafer is nearly constant, but die size determines how much it costs per die to manufacture.)
- Some die are constrained in their dimension and size due to application. For example, a display row/column driver die would be very narrow and very long, on the order of a 1x10 ratio, or greater. They cannot be hexagonal.
Those are the reasons that came to my mind.
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1\$\begingroup\$ I will add that a hexagonal die might actually be easier for bond wires from the pad ring around the die to the lead frame when packaging. The bond wires can get a little dense right around the corners. \$\endgroup\$– TimBCommented Feb 17, 2021 at 4:58
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\$\begingroup\$ 1) SMU 2) The fabricator is the end user 3) The wafer is not being cut 4) The edge chip-lets are implemented for alpha partial mitigation. \$\endgroup\$– kpieCommented Feb 17, 2021 at 5:33
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2\$\begingroup\$ if the wafer is not going to be diced, then you'd think that a circle would be the best shape. \$\endgroup\$– KartmanCommented Feb 17, 2021 at 9:06
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3\$\begingroup\$ @Kartman What would you do with an 8" silicon wafer? I don't see a situation where this really makes sense. The silicon defect density is going to make significant parts of the wafer unusable...best to keep the good bits and throw out the bad. \$\endgroup\$ Commented Feb 17, 2021 at 12:59
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1\$\begingroup\$ You actually can buy laser wafer dicing tools that let you cut arbitrary shapes. I imagine they are much slower than traditional mechanical systems. They arent commonly used, probably because there is no benefit to doing so. \$\endgroup\$– MattCommented Feb 17, 2021 at 17:51