Development of plastic chip scale package for ATM switching systems

A plastic chip-scale package (CSP) smaller than conventional packages has been developed and applied to ATM switching systems. The package uses a low-cost glass-epoxy substrate. As glass epoxy is widely used in the printed circuit boards (PCBs) of ATM switching systems, its use in the CSPs mounted on a PCB reduces the difference in thermal expansion between PCB and CSPs, thus lengthening CSP life. The power plane, ground plane, and thermal vias of the CSP are designed to offset the increased thermal resistance due to the CSP's smaller size. Simulation showed a thermal resistance for this CSP with copper layers and thermal vias of as much as 22% less than that of a CSP with no copper layers or thermal vias. A six-layer test board mounting four CSPs was used to simulate a sub-board for ATM switching systems. The thermal resistance of the two CSPs located downstream was about 10% higher than that of the two located upstream. Two CSPs were developed for two types of application-specific ICs: a bus interface controller (BIC) and a peripheral interface circuit (PIC). Both CSPs were 18 mm square with a 0.8 mm outer solder-ball pitch and 256 outer balls. These CSPs occupied areas about 87% and 68% smaller than those of a conventional pin grid array (PGA) and quad flat package (QFP) respectively. The junction temperature of both CSPs satisfied the thermal conditions. These high-performance CSPs are thus attractive for use in ATM switching systems.