In many cases, customers move from a 25um Au wire to a 20um Au wire to save money. In the case of Au, it’s simple arithmetic, 20um is 20% cheaper than 25um. However, in the conversion to Cu wire, the savings are already so dramatic to go from Au wire to Cu wire, and the wire sweep is so critical during EMC qualifications, that the cost/effort vs. benefit of going to a 20um Cu wire vs a 25um Cu wire isn’t justified.
Therefore, unless the I/O count is very high, most customers converting from Au-wire to Cu-wire simply convert from 20um Au wire to 25um Cu wire in low-pin-count devices.
On the other hand, some customer don’t see any big disadvantages of using 20um vs 25um Cu wire so they see no need to move to 25um copper wire first. Wiresweep is less of an issue due to the greater stiffness of the wire. Wirebonding capability, especially with smaller bondpads, 20um copper wire seems even better.
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