Evaluation of lead(Pb)-free ceramic ball grid array (CBGA): Wettability, microstructure and reliability

Flip-chip carriers have become the preferred solution for high-performance ASIC and microprocessor devices. Typically these are packaged in organic or ceramic Ball Grid Array (BGA) packages. IBM has developed both Ceramic Ball Grid Array (CBGA) and Ceramic Column Grid Array (CCGA) packages which cover a wide range of package I/O capabilities required for high-performance devices, typically between 300 to more than 1600 I/O. Recently, there has been a lot of interest in Pb-free solders to replace typical Pb-based solders such as eutectic Sn-Pb used for balls in plastic BGAs (PBGA) for their assembly. The common feature of all proposed Pb-free solder alloys to date is that they are all Sn-based solders with the wt.% of Sn in the alloy typically greater than 90%. Antimony-based lead-free solders have been considered, but recently there has been a concern over the use of antimony (Sb) as well. The leading antimony-free (Sb-free) solder that has emerged from various Pb-free solder evaluations by industry and academic consortia at present is: 95.5Sn/3.8Ag/0.7Cu (SAC). The primary issues with changing from high-Pb based solders that are used in BGA assembly to electronic cards are: (1) wettability of the Pb-free solders to both Ni/Au I/O pads typically used in ceramic BGAs and card lands (Cu or Cu tinned with solder paste) with water soluble or no-clean fluxes; (2) the rate of intermetallic growth and its structure, with much higher Sn containing Pb-free solders; and (3) thermal fatigue reliability of the CBGA joints to organic cards. In this paper, initial assessment of a few leading Pb-free solders regarding wettability, microstructure and thermal fatigue life for CBGA applications is presented.