A low cost approach to CSP based on meniscus bumping, laser bonding through flex and laser solder ball placement

The drive toward larger die size with high I/O counts together with manufacturer demand for small, lightweight, reliable and low cost devices is a challenge for packaging technologies. Chip size/scale packages (CSPs) were designed to meet these demands without the drawbacks of direct chip attach (DCA). The techniques presented until now allow complete package testing, thereby solving the known good die issue and can easily cope with changing die size and I/O design. In future, the customer will expect to receive a tested package with standardized I/O layout. However, manufacturing costs are still too high. 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 used in order to lower overall system costs. The CSP described, the flexPAC, is based on interconnection of the bare IC to a single metal layer flexible interposer using the fiber push connection (FPC) principle. The solder for this assembly is applied to a wafer with electroless Ni metallization using a new low cost bumping approach: meniscus soldering. Mechanical stability is incorporated by using a low stress, low viscosity adhesive dispensed between chip and substrate prior to assembly. Finally, the CSP is bumped on the backside using a highly flexible high speed solder ball placement method. The package is fully surface mount compatible, allowing for a wide process window. One important issue is package reliability. The results of thermal cycling of the flexPAC, and repeated reflow and humidity tests are discussed here.