Processing and reliability of flip chip with lead-free solders on halogen-free microvia substrates

Over the past several years, flip chip technology has been heavily focused on developing and refining the next generation of flip chip assembly processes and reliability; yet, little attention has been paid to the environmentally conscious aspects of manufacturing and high process throughput. With legislation pending on the use of lead in Europe and Japan and to limit environmental impact, flip chip technology must address new ways to meet safety and environmental requirements for the materials and processes used during assembly. The focus of this research is to characterize and implement environmentally conscious low cost flip chip material systems and processing, using two lead-free-solder interconnect systems and microvia halogen-free substrates, thus minimizing the environmental impact. The objective is to ensure that environmentally friendly materials are selected, along with acceptable process technology for all materials as well as for the flip chip assembly. An assembly process for environmentally conscious low cost flip chip assembly to microvia laminate substrates will be presented, based on a fully integrated high speed flip chip assembly line. The process includes the flux application, chip placement, reflow process, and underfill processing. Flux and underfill material compatibility will be discussed, and data will be presented analyzing the quality of the solder joint formation, and underfill adhesion to halogen-free solder masks. 204-mum pitch peripherally bump, daisy chain test chips with edge lengths of 5 nun and 10 mm respectively are used. Comprehensive reliability results are presented, comparing the two lead-free to tin/lead eutectic interconnect systems. The chips are assembled on microvia substrates with electroless nickel/immersion gold surface finish, comparing conventional to halogen-free FR-4 and solder masks. A fast flow snap cure underfill, qualified for use with eutectic tin/lead joints on conventional FR-4, is used for both board types. Reliability results from air-to-air thermal shock testing are presented, comparing lead-free to eutectic interconnect systems mounted on conventional and halogen-free microvia substrates. Process and failure mode analysis are presented, based on x-ray inspection, C-SAM analysis, and assembly cross sections.