According to this answer (correct me if I'm wrong), copyleft software licenses don't do much when applied to hardware. Unlike software which is protected by copyright in any form, HDL/CAD hardware is transformed to "circuit" / "netlist" / "layout" when compiled, and these cannot be copyrighted. As a result, one can make a hardware device based on modified open-source blocks, and they are not required to release their modifications if they produce and distribute a physical product (unless the hardware blocks are also protected by a patent):
Legally, you can use copyright to protect your drawings, but you can't protect the actual circuit, nor the PCB layout, and therefore you can't insist on share-alike clauses to be followed. What we are left with is a "social contract", i.e. a statement that a design is shared for general use but please be nice and share your modifications.
But there's TAPR license which seems to require exactly that:
* You may distribute products you make to third parties, if you either include the documentation on which the product is based, or make it available without charge for at least three years to anyone who requests it.
Another example is Arduino (licensed under CC BY-SA) which states in their FAQ:
Deriving the design of a commercial product from the Eagle files for an Arduino board requires you to release the modified files under the same Creative Commons Attribution Share-Alike license. You may manufacture and sell the resulting product.
Did I understand these clauses correctly and are they really enforceable?