Thermomechanical Analysis of Conformally Coated QFNs for High-Reliability Applications

Abstract

The use of Quad Flat No-leads (QFNs) in high reliability applications is receiving increased interest. To address concerns of harsh working environments, conformal coatings are used to protect the components and associated printed circuit board (PCB). However, using conformal coatings can pose a reliability risk for the components on the PCB. This paper details an investigation into the effect of conformal coatings on the reliability of QFN second-level solder interconnections. Five QFN package types and two types of conformal coatings were investigated. In the experiments, a large population of QFN assemblies coated with conformal coatings were subjected to a thermal cycling test condition. These components, designed with daisy chain connections, were monitored during the thermal cycling tests and the exact time for failure to occur for each component was recorded and analyzed. Detailed finite element models for the tested QFN assemblies were developed, the inelastic strain energy density was used as the damage indicator to evaluate the impact of conformal coating on the solder joint reliability. Modelling results reveals that conformal coating plays a complex role in QFN solder joint reliability, the presence of conformal coating results in a reduction in the shear stress in the solder joint and plays a positive role in solder reliability, but it also induces more stress in the solder joint in the out-of-plane direction and plays a negative role. These two mechanisms are competing and the relative magnitude of them will determine the overall effect of conformal coating on solder joint reliability.

Prediction of Solder Joint Reliability with Applied Acrylic Conformal Coating

“…These include ball grid array (BGA) and quad flat no-lead (QFN) packages, since when exposed to temperature fluctuations the conformal coating material may induce different expansion and contraction rates compared to other materials, which is dependent on the materials' coefficients of thermal expansion (CTE). [10][11][12][13] This consecutive expansion and contraction of the solder joints can compromise the reliability, which results in thermomechanical fatigue (TMF). TMF is one of the main reasons for the failure of solder joints, and it accumulates during the cycles.…”