Reliability analysis of encapsulated components in 3D-circuit board integration

Abstract

The PCB embedding technology has continuously been developed and is increasingly used in the miniaturization efforts of today's electronic systems. Passive components and active IC components are desired to be integrated into printed circuit boards. The benefits are the obvious miniaturization effect due to 3D-packaging and the additionally environmental protection through the encapsulation. The reliability potential of these encapsulated components is overall not yet well understood. The paper presents the author's recent work towards understanding the thermo-mechanical reliability of such inhomogeneous systems. Detailed results of the experimental investigations in conjunction with finite-element-analysis will be presented. The experimental results are based on temperature cycling tests with over 5000 cycles applied to embedded ceramic passive components CR0805 and their surface-mount counterpart variant. The analysis results have shown that the damaging behavior in the solder joint in the encapsulated condition differs significantly compared to standard SMT mounted joints. The macro crack propagation under creep deformation is decelerated in case of embedded joints. However the observed grain refinement, indicates significant plastic deformation in the joint. Furthermore crack initiation and crack propagation is taking place in the polymer-matrix of the cavity surrounding the component. To understand the experimentally found differences between the encapsulated and standard SMT variant a finite-element model has been built to digitally represent the assessed structures. The finite-element models for the embedded and SMT variant have been adjusted and verified against deformation measurements using a digital image correlation setup. The simulation results show the field distributions of stress and strain and clarify the polymer cracking occurrences. To understand the considerable difference in damage accumulation during the temperature cycling test, the inelastic behavior of the solder joint has been evaluated. The FEA has shown that the deviatoric stress situations are comparable but the isotropic stress is different. In case of encapsulation the solder joint experiences hydrostatic pressure, which has been found to be responsible for the decelerated crack initiation seen in experimental testing. Consequently the currently established lifetime-prediction-laws based on the cyclic inelastic work or energy cannot be directly transferred to the embedded-component case. It will be shown that the existing lifetime correlations can however be corrected based on the isotropic stress situation. The experimental temperature cycling results underline the high reliability potential of embedded solder joints. Furthermore it is the authors believe that the found methodology can be transferred to more general encapsulation cases like potting or underfilling as well.