F2-cPACTM-a manufacturing process for a low cost CSP

This paper presents a manufacturing line for a chip-size package. This CSP, called F²-cPAC^TM, is based on a flex circuit interposer. The entire package is 645 μm thick, including a 550 μm-thick chip and a 85 μm-thick tape carrier. For interconnection and redistribution of the chip I/Os, a three layer tape with gold or gold/nickel surface finish on the copper layer is used. Mechanical stability between the tape and the chip is achieved using a low stress adhesive. 63Sn/37Pb solder balls on the bottom side enable CSP attach to the board. For assembly, a new manufacturing line is established. The IC bumping process is done at wafer level. A very cost effective pretreatment method is to coat the Al IC pads with electroless Ni/Au. Besides the cost advantage of this process, a very high layer uniformity and a near hermetic sealing of the Al-pad is achieved. Subsequently, a novel process is used to apply a 10 μm-thin layer of AuSn20 solder to the Ni pads on the wafer. Interconnection of the bare die to the flex interposer is performed with a reel-to-reel chip-on-flex laser bonder. This bonder has the option to use a single fibre or multifibre bondhead. With the multifibre bondhead, the high speed bonder can be implemented in a fully-automatic production line. The advantage of this bonder system is the capability of bonding through the flex tape. Solder balls for CSP attach to the board are applied using a fluxless system for solder ball placement and controlled laser reflow. The package is fully surface mount compatible, allowing a wide process window