Development of chip scale packages (CSP) for center pad devices

Memory chip scale package (MCSP) has been developed for center pad memory devices. The MCSP is classified into two designs, tapeless lead on chip (LOC) and flexible substrate, respectively. The MCSP with a tapeless LOC design consists of a polyimide-deposited chip which is bonded to a lead frame and then encapsulated with a molding compound. The MCSP made with a flexible substrate consists of a polyimide-deposited chip which is bonded to the flexible substrate with a thermoplastic polyimide and subsequently encapsulated with a liquid epoxy compound. Wire bonding technology was used for both of MCSPs. A 52 ball package are composed of solder balls (1.0 mm in diameter on a 1.27 mm pitch) attached to either the lead frame or the flexible substrate. The optimum material properties, the wafer fabrication and packaging process parameters, and the experimental and simulated reliability and performance results of the MCSPs are presented.