QFN (Quad Flat No–lead) SAC Solder Joints under Thermal Cycling: Identification of Two Failure Mechanisms

Abstract

One of the main barriers for using QFN (Quad Flat No-lead) packages in the semiconductor industry is the board-level solder joint reliability. Despite the small form factor and better heat dissipation that offer the QFN devices, their reduced standoff height remains a concern for solder joint thermal fatigue failure. The primary failure mechanism often involved in thermomechanical fatigue of lead-free solder joints is tin grain recrystallization and strain-enhanced Ag <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn precipitate coarsening. The combination of these microstructural processes leads to intergranular cracks which propagate in the high strain region depending on the solder geometry (generally close to the package or PCB). However, another failure mechanism was observed when studying the behavior of QFN solder joints during thermal cycling. An approach that combines in-situ electrical monitoring, microstructural investigation and failure analysis is presented in this study to identify phenomena leading to QFN solder joints cracking. Electron Back Scattered Diffraction (EBSD) analysis was conducted to assess the microstructural changes at different thermal cycling levels. Lifetime data are then correlated to the microstructural evolution analysis to understand QFN solder joint damaging during thermal cycling. Results show that QFN solders are likely to fail by two failure mechanisms. A first mechanism characterized by a mixture of the known intergranular crack and a brittle interfacial crack that does not need tin grain recrystallization to propagate. The other mechanism illustrates only the presence of the interfacial cracks. The different behavior of QFN joints compared to other solder geometries can be due to several factors such as the presence of an unusual as-reflowed microstructure. The assembly design and its materials properties (aspect ratio, green epoxy molding compound with low CTE, copper lead-frame) may also affect the solder joint failure mechanism for QFN packages.