A new multiscale model of thermal contact resistance (TCR) between real rough surfaces is presented, which builds on Archard’s multiscale description of surface roughness. The objective of this work is to construct the new model and use it to evaluate the effects of scale dependent surface features and properties on TCR. The model includes many details affecting TCR and is also fairly easy to implement. Multiscale fractal based models often oversimplify the contact mechanics by assuming that the surfaces are self-affine, the contact area is simply a geometrical truncation of the surfaces, and the pressure is a constant value independent of geometry and material properties. Concern has grown over the effectiveness of frequently used statistical rough surface contact models due to the inadequacies in capturing the true multiscale nature of surfaces (i.e., surfaces have multiple scales of surface features). The model developed in this paper incorporates several variables, including scale dependent yield strength and scale dependent spreading resistance to develop a new model that can be used to evaluate TCR. The results suggest that scale dependent mechanical properties are more influential than scale dependent thermal properties. When compared to an existing TCR model, this very inclusive model shows the same qualitative trend. Results also show the significance of capturing multiscale roughness when addressing the thermal contact resistance problem.
In this study, the influence of moisture on the elastic modulus of a no-flow underfill is characterized over a wide range of accelerated testing environments, and the permanent changes are evaluated upon fully redrying. Moisture saturation is reached in all environments prior to testing, thus the inherent wet modulus of the underfill is identified for each respective level of moisture preconditioning. The change in the elastic modulus as a function of moisture concentration is measured, as well as the reversible and irreversible effects from moisture uptake in the underfill upon redrying. Thermal aging from the temperature component of the accelerated testing environment is found to have no effect on the elastic modulus, while the moisture uptake is found to notably decrease the modulus at higher humidity levels. Recovery experiments demonstrate that most of the loss in the elastic modulus from moisture exposure is recoverable, although more irreversible damage did occur at higher humidity levels. Hydrolysis contributed to the irreversible loss in the modulus, while the reversible component is attributed to plasticization of the underfill from moisture uptake.
Moisture poses a significant threat to the reliability of microelectronic assemblies and can be attributed as being the principal cause of many premature package failures. Of particular concern is characterizing the role of moisture with respect to the acceleration of the onset of package delamination. In this paper the effect of moisture on the interfacial fracture toughness of two no-flow underfill materials with a commercially available solder mask coated FR-4 board is experimentally determined. Bilayer specimens with prefabricated interface cracks are used in a four-point bend test to quantify the interfacial fracture toughness. Two groups of test specimens of varying underfill thickness were constructed. The first group was fully dried while the other was moisture preconditioned at 85°C/85%RH for 725 hours. The results of this study show that the interfacial toughness is significantly affected by the presence of moisture.
In a previous study, we found that moisture preconditioning strongly influenced the interfacial fracture toughness of the underfill/solder mask interface, decreasing the interfacial adhesion by approximately one-half for both classifications of underfill/solder mask interfaces after 725 h of exposure at 85°C/85%RH. To better understand the rate and mechanisms for moisture transport through the interfacial fracture test specimens, a diffusion analysis was implemented based on traditional, analytical solutions of Fick’s second law of diffusion. Test specimens were constructed to experimentally determine the diffusion coefficient for each underfill. Since both underfill encapsulants proved to exhibit non-Fickian behavior at 85°C/85%RH, the application of the analytical Fickian solution for the test specimens was limited to the associated JEDEC criteria of 168 hours for 85°C/85%RH. A finite element analysis was performed to illustrate the moisture concentration in the interfacial fracture test specimens for initial times of exposure to the humid environment. The results of this study demonstrate that the presence of amine functional groups considerably retard moisture penetration through underfill encapsulants.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.