Board level reliability of CSP

In recent years, portable devices have become smaller, lighter, and offering greater functionality. For mounted packages, the requirements for higher densities have been becoming extremely high. In order to meet this demand, various companies are pushing the development of CSP (Chip Size Package or Chip Scale Package), which has almost no limit to LSI chip size. Sharp has developed a CSP, which can be manufactured using existing equipment, and commenced mass production in August 1996. The CSP, a face-up structure on single-sided wiring polyimide substrate, was developed using wire bonding and transfer molding technology, which are proven packaging technologies. Further, the external connection terminals are an area array structure using solder balls, with a pitch of 0.8-1.0 mm. The area array package, which has a structure using solder balls or "bumps" as external connection terminals similar to Sharp's CSP, is different from current flat type plastic packages (SOP (Small Outline Package), QFP (Quad Flat Package), etc.) in that the mechanism of stress on the solder connectors being reduced due to the lead frame does not operate. For that reason, after mounting on the motherboard, heat or mechanical stress can be expected to concentrate in the solder connections, and there is some concern that this reduces the post-mounting reliability. The factors that are believed to affect CSP reliability after mounting are the CSP's outline size, structure, and material, the motherboard's material, design, and manufacturing methods, etc. Sharp conducted a temperature cycle test similar to those currently performed to evaluate resistance to heat stress. For resistance to mechanical stress, Sharp also conducted bending and drop impact tests on a mounted motherboard under various conditions, taking into consideration the severe operating conditions of devices such as celluler telephones. This clarified the CSP structure which affects the reliability of the CSP after mounting and the effect on the motherboard. In addition, during each test, by measuring the amount of strain occurring mounted sections on a motherboard and its occurrence speed (strain speed), the possibility of quantitative evaluation of mounting reliability for various kinds of mechanical stress was investigated.