Large area focal planes for the next generation of astronomical instruments require very flat imaging surfaces (< 10 μm peak-valley) over significant sizes (20 - 100 cm), accurate alignment of detector height, stable operation at low temperature, and fully-buttable packaging with large I/O requirements to connect multiple amplifiers per detector. These requirements are often mutually exclusive and therefore difficult to obtain in a single focal plane. In this paper we discuss the hybridization or flip chip bonding technique and associated focal plane mounting methods to achieve these goals. Specifically, we describe a technique to hybridize CCD detectors onto high thermal conductivity ceramic with vias that lead to the I/O signals underneath the detectors. Packaging methods to mount such devices with a total flatness non-uniformity of less than 10 microns are presented. The requirements of achieving sub-5 microns flatness are also discussed.