Reliability and applications of adhesives for microsystem packaging

Doctoral thesis, 2005

The electronics industry is one of the most dynamic, fascinating and important industries. Polymeric adhesives are widely used in electronics packaging and play a critical role. The objective of this study is to gain a better understanding of the reliability performance and degradation mechanisms of adhesives for interconnection and encapsulation applications in microsystems. Effects of mechanical loading on the electrical conductivity of isotropic conductive adhesive (ICA) joints were investigated both experimentally and numerically. At either loading or unloading stage of the lap shear tensile test, the resistance varied monotonically with the joint strain, and the conductivity recovered almost completely after the elastic load was removed. In the low cycle fatigue test, the joint conductivity degraded gradually with the number of cycles. Finite element modeling (FEM) revealed that three mechanisms; stress concentration around fillers, filler separation, and filler sliding, may explain the observed resistance increase of ICA joints under mechanical straining. The thermomechanical fatigue cracking of the leadless surface mount ICA joints was studied with microscopic observation and FEM simulation. Vertical and horizontal interfacial cracks were observed to initiate at the top and inner ends of the adhesive/component interface, respectively. Bulk cracking occurred around the knee of the joint. The FEM stress analysis revealed that the interfacial cracking is due to the singularity of peel stress, while the bulk cracking results from the concentration of equivalent stress. Also the numerical studies indicated that higher standoff height and lower fillet height would be helpful to release thermal stresses. A global/local FEM procedure was proposed to realize the full-scaled modeling of anisotropic conductive adhesive flip-chip assembly. It consists of two independent models with different mesh sizes. The coarse global model studies the overall behavior of the entire package and identifies the critical region. The refined local model incorporates the global displacement field as boundary conditions and performs the detailed analyses in the region of interest. It was found that due to the global thermal mismatch between the chip and substrate, the joint stress pattern becomes asymmetric as the package cools down to room temperature. The shear strain from the global mismatch is localized in the region between the bump and pad. Additionally, the particle-electrode contact behaviors as well as some failure mechanisms were discussed. The reliability of a tunable light modulator was evaluated with temperature cycling tests. It was found that the delamination of encapsulation adhesive would initiate around the site suffering the maximum peel stress. The brittle cracking of the glass prism wound also be a problem if large enough flaws have been introduced in handling or processing. For design optimization, the effects of several geometry and material parameters were evaluated based on FEM calculations and some preliminary guidelines were proposed. The concept and realization of integration of highly thermal-conductive nanotubes with conventional microchannel coolers were proposed for enhancement in cooling capability. Using lithography process, chemical vapor deposition and adhesive bonding, the nanotube fins were grown on the silicon substrate and then sealed in the macrochannel where the coolant flows. Though it has fairly low cooling capability at the present stage, this new cooler has shown promise in the experimental characterization. Additionally, further modifications have been proposed and some possible reliability concerns were sited.