Invar thickness ratio in order to more closely match the CTE of the substrate with that of the chip carriers.
While it might appear desirable to form the substrate from the same material as the chip carrier in order to better match the CTEs, most chip carriers are formed of aluminum oxide which is a brittle ceramic having marginal thermal conductivity. The brittle nature of the material make it highly susceptable to fracture during manufacture and handling.
Generally, it is the desire of the art to increase the density of electronic circuitry and to further minimize the overall size and weight of electronic systems. Problems of heat dissipation, differential thermal expansion and vibrational stress are limiting factors which have resulted in thick substrates formed of very dense materials. It would(March 22, 2007) -- Marshall Faulk is expected to announce his retirement from the NFL at a press conference March 26 at the NFL's annual meeting in Phoenix.

Faulk issued the following statement, "It has been an honor and pleasure to play in the National Football League. I'd like to thank all my teammates and coaches with whom I've been associated over the past 13 seasons as well as the fans who supported me throughout the years. I gave the game my passion, and it gave me a wonderful career and memories that I will always be thankful for."

One of, if not the most exciting player to watch in the last 10 years IMO....