flexPAC-development and qualification of a low cost CSP

Common technologies to mount ICs on substrates such as SMT or COB technology are challenged by the increasing IC I/O density, by the drive to larger dies, and by the users´ desire to manufacture small, lightweight, highly reliable and cost effective devices. A new package must fit without large changes in current mass manufacturing processes, e.g. SMT compatibility is required. Various chip-size packages are in development worldwide using different approaches. This paper presents an approach in which standard low cost materials are used, the number of process steps is reduced and new bumping and bonding technologies are introduced in order to lower the overall system costs. The CSP described, the flexPAC, is based on the interconnection of the bare IC to a single metal layer flexible interposer using the fiber push connection (FPC) principle. The meniscus soldering of electroless Ni metallization is applied to wafers, using a new approach to low cost bumping for this assembly. Mechanical stability is incorporated in the CSP by using a low stress, low viscosity adhesive dispensed between chip and substrate prior to assembly. In a final step, the CSP is bumped on the backside using a highly flexible solder ball placement method. The package obtained is fully surface mount compatible, allowing for a wide process window. The package reliability is an important issue. The results of thermal cycling of the flexPAC^TM, repeated reflow and humidity tests are introduced and discussed in this paper