Design of underfill materials for lead free flip chip applications

The reliability of flip chips (FC) is enhanced several orders of magnitude by the introduction of an underfill material between the chip and the substrate. The design of such underfill materials is not trivial due to the varied failure mechanisms that occur in the underfilled configuration. Reliability of underfilled flip chips not only depends on the thermo-mechanical properties of the underfill but also the processing conditions. Hence, the design of an underfill material must involve the development of the optimal processing conditions for the underfill material. Legislation against the use of lead in electronics has led to extensive research on alternative alloy systems and such a change will lead to a number of modifications at the various levels of packaging. Underfill materials are no exception to such a change. The differences between the designs of the underfill material for a lead solder based and a lead-free solder based package are examined in this paper. Among the different lead free solder options, the tin-3.9%silver-0.8%copper (SAC) alloy was compared against the standard eutectic tin-lead solder. Higher CTE and lower modulus resulted in shorter lifetimes of the package for either solder alloy. The SAC alloy does not plastically deform as much as the eutectic tin-lead solder and its creep deformation is less at lower stresses and more at higher stresses than the tin-lead solder. Overall, when subjected to the -55°C to 125°C cycle, the SAC exhibited superior performance than the tin-lead solder. This meant that a flip chip package with SAC alloy can last as long as a tin-lead solder based package while using a higher CTE underfill than the tin-lead solder package. However, it would be incorrect to conclude that SAC will always have smaller deformation than the tin-lead solder. Firstly, the processing conditions, especially in a no-flow underfilling scenario are different. This requires the underfill not only to provide fluxing action at higher reflow temperatures but also to withstand the higher residual stresses during subsequent cooldown. Secondly, the creep behavior of the SAC alloy is poorer at higher stress levels and applications operating at such stress levels would place more demands on the underfill.