Electrical Conductivity of SAC solder
It is generally 11-13% the conductivity of copper. 

SAC305 isn’t standardized, there can be some variation in the alloy and it really comes down to the manufacturer.  The surest way to find the conductivity is to just look in the data sheet for that vendor’s SAC305 alloy.  

It is generally given as resistivity rather than conductivity, but it’s the same thing really.   Just take the resisitivity ( in ohm meters - careful, some data sheets will use ohm centimeters or micro ohm meters or centimeters) and take the inverse, and that will give you the conductivity in siemens per meter.  

Most SAC305 I’ve seen has between 1.3 to 1.5 x 10^-7 (0.00000015) ohm meters of resistivity.  Which means the conductivity is about 6.66 million S/m. 

This is roughly the same conductivity as carbon steel, which is 6.99 million S/m.  Copper is 59.6 million S/m, so SAC305 is between 11-13% of this.