  We analyze the performance of a microwave chip mount that uses wirebonds to
connect the chip and mount grounds. A simple impedance ladder model predicts
that transmission crosstalk between two feedlines falls off exponentially with
distance at low frequencies, but rises to near unity above a resonance
frequency set by the chip to ground capacitance. Using SPICE simulations and
experimental measurements of a scale model, the basic predictions of the ladder
model were verified. In particular, by decreasing the capacitance between the
chip and box grounds, the resonance frequency increased and transmission
decreased. This model then influenced the design of a new mount that improved
the isolation to -65 dB at 6 GHz, even though the chip dimensions were
increased to 1 cm by 1 cm, 3 times as large as our previous devices. We
measured a coplanar resonator in this mount as preparation for larger qubit
chips, and were able to identify cavity, slotline, and resonator modes.
