  Solid-state qubits with transition frequencies in the microwave regime, such
as superconducting qubits, are at the forefront of quantum information
processing. However, high-fidelity, simultaneous control of superconducting
qubits at even a moderate scale remains a challenge, partly due to the
complexities of packaging these devices. Here, we present an approach to
microwave package design focusing on material choices, signal line engineering,
and spurious mode suppression. We describe design guidelines validated using
simulations and measurements used to develop a 24-port microwave package.
Analyzing the qubit environment reveals no spurious modes up to 11GHz. The
material and geometric design choices enable the package to support qubits with
lifetimes exceeding 350 {\mu}s. The microwave package design guidelines
presented here address many issues relevant for near-term quantum processors.
