Mechanical strength of glass ceramic substrates in land grid array packages

Alumina substrates have been used for many years in land grid array (LGA) packages. The mechanical strength of alumina is well understood in this application. New, higher performance LGA packages use copper conductor based glass ceramic substrates, which typically have lower strength. In this paper, finite element modeling (FEM) is used to evaluate the stress in a glass ceramic substrate as it is subjected to assembly conditions. The stress was modeled for a range of substrate thicknesses, under nominal and worst case assembly loads. The strength of glass ceramic was evaluated by breaking specimens in a novel square ring on square ring (SoS) fixture. FEM is used to relate the force required to break the specimen to the stress at fracture. Weibull statistics were computed from the results of these experiments. Finally, the Weibull strength model was combined with the FEM estimates of stress from assembly loads to determine the proper substrate thickness. A glass ceramic substrate with increased thickness compared to alumina is required to meet the mechanical strength requirements for module assembly.