Microelectronic systems that are intended for use in high shock and vibration environments are encapsulated to achieve stable and reliable operation. The physical design of the electronic assembly, the material properties of the encapsulant, and the magnitude and frequency of the inertial loading must all be factored into the system design. Overall robustness to shock and vibration are improved by minimizing the physical size and mass of the system, which increases its stiffness and reduces the magnitude of the inertial forces that must be supported. This work describes the development of an encapsulation process and facilities that are optimized for building high-reliability microelectronic systems that range between one and five cubic centimeters in volume. Finite Element Analysis (FEA) is used to ensure that sensitive components are not overstressed by the encapsulant as a result of residual curing stresses and inertial loading effects. Computational Fluid Dynamics (CFD) software is used to model the filling process, with the objective of identifying locations prone to void formation. The CFD models are validated via cross sectioning mechanical replicas of the system and by encapsulating enclosures fitted with viewing windows to allow sequential photographs of the progression of the fill frontier. During fabrication, the encapsulant is dispensed under vacuum while being observed with a stereoscopic microscope. An essential component of the process is characterization of the encapsulant materials. Coefficient of thermal expansion and cure shrinkage of the encapsulant are determined by casting a sample onto a thin metal strip and extracting stress parameters with equations of a bimetallic strip. The surface energies of the encapsulant on materials in the system are measured by a modified sessile drop technique in which the material is dispensed on a coupon, cured and then measured with a profilometer. These tests are performed on each lot of material when received and periodically afterwards to monitor the condition of inventory. This paper provides a detailed description of the design process and facilities using examples from representative products.
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