Materials effects on reliability of FC-PBGA packages for Cu/low-k chips

Reliability of the flip-chip plastic ball grid array (FC-PBGA) packages is highly dependent on the properties of the constituent components and the interface formed between them. The relative mechanical compliances and thermal mismatch between the silicon chip, the underfill material and the organic laminate substrate are particularly important to the design and performance the package. Strong coupling between the chip and the substrate can cause chip cracking, delamination of interlayer dielectrics (ILD), delamination of underfill and problems associated with BGA interconnection when the package is assembled to a printed circuit board (PCB). The problem became more severe as we migrate to the 90nm and 65nm silicon technology nodes where low-k ILD is widely used. Combined experimental and modeling methods were used to investigate the thermo-mechanical behavior and failure mechanisms controlling FC-PBGA package reliability. Materials effect of new generation of underfill materials was first studied for minimizing the chip-substrate thermo-mechanical coupling. Fully assembled FC-PBGA packages with various underfill materials were evaluated following a carefully designed analysis and screening flow. Thermo-mechanical response of the package was measured and analyzed using high resolution moire interferometry and numerical modeling techniques. Four-point bending test was also used to characterize interfacial fracture energy for the critical interface between die passivation and underfill material. The experiments and modeling were correlated with the JEDEC standard component-level reliability testing results. The combined experimental and numerical analysis confirmed our selection of the substrate, underfill and other package materials and demonstrated that significantly improved reliability of the flip-chip PBGA packages can be achieved by controlling thermo-mechanical coupling of the silicon die and the package, and by controlling various important interfaces within the package