New package for CMOS sensors

Cost is the main drawback of existing packages for C-MOS sensors (mainly CLCC family). Alternative packages are thus developed world-wide. And in particular, S.T.Microelectronics has studied a low cost alternative packages based on QFN structure, still with a cavity. Intensive work was done to optimize the over-molding operation forming the cavity onto a metallic lead-frame (metallic lead-frame is a low cost substrate allowing very good mechanical definition of the final package). Material selection (thermo-set resin and glue for glass sealing) was done through standard reliability tests for cavity packages (Moisture Sensitivity Level 3 followed by temperature cycling, humidity storage and high temperature storage). As this package concept is new (without leads protruding the molded cavity), the effect of variation of package dimensions, as well as board lay-out design, are simulated on package life time (during temperature cycling, thermal mismatch between board and package leads to thermal fatigue of solder joints). These simulations are correlated with an experimental temperature cycling test with daisy-chain packages.