Abstract
Moisture diffusion and vapor pressure development analyses are the key to understand the moisture-induced failure mechanisms in electronic packages. In this chapter, theories and applications of moisture diffusion modeling and vapor pressure analysis are reviewed. The unique characteristics of moisture diffusion in multi-material system are described. The commonly used normalization methods to remove interfacial discontinuity are presented, and the details of thermal-moisture analogy and implementations using commercially available finite element software are discussed. The applications of normalization methods to moisture diffusion in a PBGA package are illustrated. Furthermore, moisture diffusion in a reflow process, in which ambient temperature and humidity loading conditions vary with time, is examined. Caution must be made to apply normalization methods to solve desorption problems when saturated moisture concentration is a function of temperature. A direct concentration approach (DCA) is introduced. In the DCA, the moisture concentration is used directly as a basic field variable, which is discontinuous at interfaces. Constraint equations are applied at interfaces to satisfy the interface continuity requirement. The detailed numerical treatment and implementation procedures using the DCA method are presented. “Over-saturation” phenomenon is observed. Over-saturation refers to a situation in which a material continues to absorb more moisture due to the increase of its saturated moisture concentration despite that desorption takes places during soldering reflow. Finally in this chapter, a whole-field vapor pressure model is introduced. This model is based on a multi-scale micromechanics analysis and considers the phase change of moisture. The model links the macroscopic moisture concentration to a moisture state at a microscopic level. Examples are given to show the differences in moisture and whole-field vapor pressure distributions in a package over time at reflow.
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Appendix: Table of the Saturated Water Vapor Density and Vapor Pressure at Different Temperatures
Appendix: Table of the Saturated Water Vapor Density and Vapor Pressure at Different Temperatures
T (°C) | 60 | 70 | 80 | 90 | 100 | 110 | 120 |
ρ g (kg/m3) | 0.13 | 0.2 | 0.29 | 0.42 | 0.6 | 0.83 | 1.12 |
p g (MPa) | 0.02 | 0.03 | 0.05 | 0.07 | 0.1 | 0.15 | 0.2 |
T (°C) | 130 | 140 | 150 | 160 | 170 | 180 | 190 |
ρ g (kg/m3) | 1.5 | 1.97 | 2.55 | 3.26 | 4.12 | 5.16 | 6.4 |
p g (MPa) | 0.28 | 0.38 | 0.5 | 0.65 | 0.84 | 1.1 | 1.37 |
T (°C) | 200 | 210 | 220 | 230 | 240 | 250 | 260 |
ρ g (kg/m3) | 7.86 | 9.59 | 11.62 | 14 | 16.76 | 19.99 | 23.73 |
p g (MPa) | 1.72 | 2.14 | 2.65 | 3.25 | 3.97 | 4.83 | 5.84 |
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Fan, X.J., Tee, T., Shi, X., Xie, B. (2010). Modeling of Moisture Diffusion and Whole-Field Vapor Pressure in Plastic Packages of IC Devices. In: Fan, X., Suhir, E. (eds) Moisture Sensitivity of Plastic Packages of IC Devices. Micro- and Opto-Electronic Materials, Structures, and Systems. Springer, Boston, MA. https://doi.org/10.1007/978-1-4419-5719-1_4
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