Thermo-Mechanical Reliability of Flip-Chip Devices on Metal-Backed Flex Circuits

Flip chip packaging technology has become one of the mainstream choices for future chip level package solutions to meet the ever increasing demand of miniaturization and high I/O requirements [Lau, 1996]. Significant difference in the thermal expansion coefficients of silicon chip and organic-laminate substrate may subject the devices to several thermo-mechanical failure modes and mechanisms [Lau, 1996]. Solder joint fatigue is one of the dominant failure mechanisms in flip chip packages. There is a need for predictive tools and techniques in product design for optimization and trade-off studies. Accelerated testing is a time consuming and resource-intensive process. Modeling and simulation techniques are an attractive alternative for calculation of stresses, strains and life prediction. Previous studies have shown the effect of material and geometric parameters on the reliability of rigid organic laminate printed circuit boards [Yeh et al., 1996, Popelar 1998].

While many of the previous analyses reported in the literature were confined to either 2-D analysis or failed to address the material behavior of the solders accurately, like the plasticity for example [Lu, 1999]. Even if some succeeded in addressing the above characteristics, literature involving flip chip bump metallurgy (particularly the lead-free materials) as a parameter is very rare and still much work needs to be done to model the material behavior of lead-free solders. Further, work on flip chip on metal backed flex substrate substrates has also not been reported so far.

This paper presents the results of an investigation into the effect of material and geometric parameters on the thermo-mechanical reliability of an underfilled flip chip on metal-backed flex device. Several solder joint configurations have been analyzed with 3-D non-linear finite element models. The material properties and the geometric parameters investigated include bump metallurgy, underfill types (capillary-flow, reflow encapsulant), underfill glass transition temperature (Tg), solder alloy composition (SnAgCu, SnPbAg), solder joint height and bump size. ANSYS finite element software has been used for the simulation.

The assembly was modeled based on symmetry of geometry and boundary conditions. Diagonal symmetry model and quarter symmetry models have been developed. Hexahedral mapped meshing has been used for most cases. The material behavior of the solder was represented by the Anand’s viscoplastic constitutive model available in the ANSYS. Thermal cycles were simulated to study the accumulated plastic work per cycle in the solder. Inelastic strain energy density has been used as a damage proxy to analyze the effect of different parameters on the reliability of the solder joint.

The results of the study showed that the 95.5Sn3.5Ag0.5Cu solder joints exhibit lower inelastic strain energy density in temperature cycling compared to 62Sn36Pb2Ag solders for all the investigated underfills. Capillary flow underfills exhibited greater reliability than reflow underfills. Increase in stand-off height and bump size led to increase in reliability.