Lead-free package interconnections for ceramic grid arrays

As the electronics industry moves to eliminate lead from its components and solder assembly processes, new challenges arise in developing reliable interconnect processes. For the surface mount attachment of multilayer ceramic packages, this challenge becomes increasingly difficult in second-level assembly because of the large coefficient of thermal expansion (CTE) mismatch between the ceramic chip carrier and epoxy glass printed circuit board (PCB). The Ceramic Column Grid Array (CCGA) technology has shown itself capable of withstanding this mismatch, for tin-lead assembly, with high reliability while extending the 32 mm practical body size of CBGA ceramic packages to 52.5 mm. Lead-free interconnect structures have been developed for second-level assembly of ceramic grid array packages. For smaller packages where a ball structure provides sufficient thermal fatigue life, a standard lead-free Tin-Silver-Copper (SnAgCu or SAC) ball may be used. SAC CBGA interconnections have been shown to provide better reliability than their predecessor tin-lead dual alloy CBGA interconnections, when tested under accelerated thermal cycling conditions. The results from recent evaluations of CBGA packages will be discussed. For larger packages that require enhanced thermal fatigue life of the interconnection, a new lead-free column structure is being introduced. The Copper Column Grid Array (CuCGA) replaces the high-lead solder column with a copper column, which achieves electrical properties and mechanical fatigue characteristics that are comparable to the existing tin-lead CCGA packages. Modeling and measurement of the electrical performance for various column lengths and diameters aided in the selection of a column geometry to meet or exceed electrical performance of existing tin-lead columns. The influence of the metallurgical and physical properties of the column on the fatigue life of the system was also considered. The thermal fatigue failure mode differs from that typically seen in tin-lead CCGA packages.