Bond wires
The standard bondwire is 1mil in diameter and 2mm in length. It has very roughly 1nH/mm. [T. Lee, CMOS RF] A D-PAK MOSFET (AOD452) has 6.5mOhms Rds on with 2x12mils wire bond; the wire resistance is ~0.6mOhm. [Alpha & Omega, AN911]. The package resistance is about 20% of total MOSFET resistance. For DPAK, the package resistance is 0.5mOhm with Powerbond II [Infineon, PowerBond].
Flex circuits
For Minco, 16 layers max, 1.5mil min trace/space, 2mil min hole diameter, 6mil thick per layer.
Solder Conductivity
The 63% tin 37% lead solder alloy electric resistivity is 144e-9 Ohm-m, thermal conductivity 40.9 W/m-K, and thermal expansion coefficient 21.4e-6/C. As a comparison, the copper resistivity is 17e-9 Ohm-m. So the solder has more than eight times the resistance. Plugging a via hole with solder would do very little to reduce its resistance.
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