For the semi custom option with Sifive,. Are we talking 100k unit runs? 10k? At what QTY does the price match COTS MCUs of a similar capability?
You'd have to talk to
[email protected] to get exact costs. I don't think you'd ever do it because it was *cheaper*, unless you're talking pretty large quantities and the savings come from avoiding distribution chain markups.
If you go for full-reticle production then the marginal cost per chip should be lower right from the first wafer of chips (typically something like 1000 to 5000, depending on chip die size and wafer size). You're looking under $1 a chip for something like the FE310 -- maybe even $0.10. At some point the ~$0.20 cost of putting the chips into packages starts to dominate...
The question is how many chips *you'd* feel you'd need to amortize the NRE costs over to make it worth your while.
According to this three year old article, getting a mask set made for 180nm costs around $100k. That would have been what I've said if you'd just asked me, but it may be less by now. Older processes such as 350nm may be less.
https://anysilicon.com/semiconductor-wafer-mask-costs/SiFive's FE310-G000 and FE310-G002 are made in 180nm and run at up to 320 MHz (they *all* run that fast -- some do 400). If you don't need 320 MHz then you could use a cheaper and older process: 250nm, 350nm, 500nm (originally and maybe still called 0.35um and 0.5um) I think 0.5um is the largest TSMC still does, but others may still do larger. I don't know how much cheaper the mask set costs get with the older process nodes.
So let's say you're looking at FAB NRE costs of $100k, so you spread that over however many chips you make. If you're making under 100k chips then probably this costs more than the actual cost of the chips. But maybe getting exactly the right chip for your purpose is worth paying $10 or $20 each -- only you can know that.
Note that you don't have to make them all at one time. Once the FAB has the masks they keep them in storage and you can order I think just one wafer worth of chips at a time whenever you want.
Then you need to add SiFive's NRE costs. We're trying to automate that as much as possible to bring the price down, especially for minor variations on things we've made before, but aren't there yet. Call that another low six figures, or a dollar or two per chip if you're making 100k. We want to drive this part of the costs down by an order of magnitude over the next few years but aren't there yet.
If you want a million chips or ten million ships then the per chip NRE costs start to disappear anyway.
If you want only a small number of chips then using Multi-Project-Wafer services is useful. This lets you share the mask making costs with 5 or 10 or 20 other customers, but the flip side is that you get only about 40 to 100 chips per wafer processed (for 200mm wafers and 300mm wafers respectively). This brings your total cost from the FAB down to maybe around $20k or $30k for that minimum order of 40 or 100 chips, so that's maybe $30 to $50 per chip. Again, if you have the FAB run multiple wafers using those same masks then the additional chips.
The current FE310-G000 and FE310-G002 chips on the HIFive1 and LoFive are made using repeat runs of the same Multi-Project masks. We're selling the chips for $5 each and Crowd Supply take a significant cut. I'd expect we're covering our FAB costs on those no problem.
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