Thermal expansion of morphologically textured short-fiber composites
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INTRODUCTION
SHORT-FIBER composites are increasingly being used in numerous applications. They are most often manufactured in the form of short glass or ceramic fibers embedded in a polymer, metal, or ceramic matrix. The processing route has a strong influence on the orientation distribution of the fibers in the composite, which in turn strongly influences its mechanical and physical properties: stiffness, conductivity, thermal expansion, strength, etc. Despite this realization, parts fabricated from short-fiber composites are currently designed using inefficient trial-and-error approaches. These costly approaches are necessary because there exists a gap in the understanding of the relations between processing, microstructure, and overall properties of short-fiber composites. A promising application of short-fiber composites is in electronics packaging. To take advantage of existing surface mount technology, printed circuit board materials must have a low thermal expansion coefficient (CTE). This is required to minimize the thermal expansion mismatch between the printed circuit board material and ceramic chip carriers mounted on the board. Thermal expansion mismatch induces thermal stresses that can lead to cracking and failure of solder connections under thermal loading. As silicon chip technology improves, chip sizes will increase, thus further increasing the importance of this requirement. More precisely though, it is desired to match the in-plane CTE with the low CTE of the ceramic chip carriers (;5.0 3 1026/7C). The out-of-plane CTE of the printed circuit board material is also an important design criterion, because the basis for two-sided and multilayer printed circuit board MARTIN L. DUNN is with the Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309-0427. HASSEL LEDBETTER is with the Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Boulder, CO 80303. ZHUANG LI is with the Materials Science Division, Argonne National Laboratory, Argonne, IL 60439. Manuscript submitted June 3, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
interconnections are plated through holes in the printed circuit boards. To establish electric connection between two sides of a board, holes are drilled through the printed circuit board and then plated with copper. Mismatch between the out-of-plane CTE of the printed circuit board material and that of copper (;17.0 3 1026/7C) can lead to cracking in the plated holes, resulting in electrical open circuits that can cripple a system. An ideal printed circuit board material, thus, will not exhibit isotropic thermal expansion, but will exhibit anisotropic thermal expansion with the in-plane component matching that of the ceramic chip carriers and an out-of-plane component matching that of copper. Estimation of the effective CTE of composite materials is generally approached in one of two ways. If the corresponding effective elastic moduli of the composite are known, either by theory or by experiment, the CTE can
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