Effect of geometry and temperature cycle on the reliability of WLCSP solder joints

The wafer level-chip-scale package (WLCSP) is designed to have external dimensions equal to that of the silicon device. This new package type is an extension of flip chip packaging technology to standard surface mount technology. The package has been targeted for low pin count (less than 30) and has high volume applications such as cellular phones, hand-held PDAs, etc. The WL-CSP is typically used without underfill and so solder joint reliability is a prime concern. Thus it is imperative to have a good understanding of the various design parameters of the package that affect the reliability of the solder joint. This paper presents the effect of geometrical parameters such as die size, die thickness, solder joint diameter and height on the reliability of solder joints. The effects of different dwell times, temperature range and ramp rates on the reliability of the solder joints is also studied by applying different temperature cycles to the package. A 16 I/O ADI WLCSP called MicroCSP is used as the primary test vehicle for the thermal cycling tests performed with different ramp/hold profiles. The energy-based model developed by Robert Darveaux is used to assess the reliability of solder joints.