A Physics-of-Failure Investigation of Flip Chip Reliability Based on Lead-Free Solder Fatigue Modeling

Abstract

Solder fatigue failure is one of the primary wear out failure mechanisms limiting reliability in microelectronics flip chip packaging. Solder fatigue failure occurs when a crack initiates and subsequently propagates through the entirety of the solder joint causing an electrical open. Crack initiation and propagation within a solder joint is caused by a cyclic application of stress, typically induced by exposure to temperature cycle excursions. Understanding the relationship between thermal cycling during product use and the accelerated thermal cycles used for testing is critical for predicting device reliability. MIL-PRF-38535 is the guiding aerospace and high reliability General Specification for Integrated Circuits (Microcircuits) Manufacturing that defines manufacturing, qualification and certification requirements for products to be listed on the Defense Logistic Agency’s (DLA) Qualified Manufacturer Listing (QML). Revision M of this standard, targeted for release in 2022, for the first time includes the use of lead-free solder alloys and organic substrates in the manufacturing of QML flip chip products. Hence there is a strong need to understand the physics-of-failure (PoF) concerning solder fatigue of lead-free flip chip assemblies. This paper demonstrates how finite element modeling can be used to predict solder fatigue for flip chip package assemblies. The authors have leveraged fatigue lives measured for different flip chip daisy chain package configurations, as well as fatigue lives defined and available in published papers. The authors then use the resulting lead-free solder fatigue model to perform parametric studies looking at the impact of different die sizes, underfill material properties, and package substrate materials. Comparisons are also made between eutectic Sn/Pb and lead-free fatigue life predictions. In addition, the authors demonstrate how solder fatigue predictions can be de-rated to use conditions in order to make reliability assessments for flip chip package assemblies. This ultimately leads to a better understanding of the impact of solder alloy and material selection on the mission life of aerospace and high reliability products that fall under the changes introduced in MIL-PRF-38535 Revision M.