Thermal expansion coefficient history

When the flip-chip technique is used to connect two chips of different materials having different thermal expansion coefficients, the connection will be subjected to mechanical stress to a degree dependent on the the thermal history of the array. In JP-A-55150279 (Fujitsu Ltd, Japan, 22.11.80) the connection between an HgCdTe detector chip and a silicon read-out chip comprises bent buffers which absorb such mechanical stress and which prevent the fragile detector chip being damaged during the connection. 

Of more subtle influence is the biaxial strain in thin film samples, which is introduced upon different thermal expansion coefficients for the film and the substrate material. Furthermore, the biaxial strain can depend on the growth history. On this matter the amount of available information is not exhaustive. 

Stresses in solvent based coatings arise from the differential shrinkage between the thin film coatings and the corresponding substrates. These stresses are due to volume changes associated with solvent evaporation, chemical reaction (i.e. cyclization in polyimide formation) and differences in thermal expansion coefficients of the coating and substrate (4>5). The level of residual stress depends on the material properties such as modulus, residual solvent content and crosslinking (5) and its thermal-mechanical history. 

Most of the other properties of vitreous silica are only slightly affected by the hydroxyl concentration. While it is possible to measure the effect of hydroxyl content on the density, refractive index, thermal expansion coefficient, dielectric constant, and other properties of vitreous silica, the effects are very small as compared to those for the flow and optical transmission behavior. The density of commercial vitreous silica ranges from 2.197 to 2.203 gcm while the refractive index is 1.457 0.003 for most products. Since the effects of hydroxyl on the properties of vitreous silica are of the same magnitude as the effects of changes in fictive temperature, it is very difficult to separate the effect of hydroxyl from those of thermal history. 

The TMA technique provides a means by which the behavior of a sample may be characterized in response to specific experimental circumstances and allows a detailed fingerprint to be recorded as small sample history differences give rise to different dimension-temperature behaviors. Properties that are of immediate relevance to end users, including linear thermal expansion coefficient, thermal shrinkage and the shrinkage force, may be obtained by direct measurement from TMA characterization. 

Aromatic polyimides have found wide application in the microelectronics industry as alpha particle protection, passivation, and intermetallic dielectric layers, owing to their excellent thermal stability, mechanical properties and dielectric properties (7-5). Many microelectronic devices, such as VLSI semiconductor chips and advanced multi-chip modules (5), are composed of multilayer structures. In multilayered structures, one of the serious concerns related to reliability is residual stress caused by thermal and loading histories generated through processing and use, since polyimides have different properties (i.e., mechanical properties, thermal expansion coefficient, and phase transition temperature) from the metal conductors and substrates (ceramic, silicon, and plastic) com-... 

Table 7 gives the composition of gold alloys available for commercial use. The average coefficient of thermal expansion for the first six alloys Hsted is (14-15) X 10 j° C from room temperature to ca 1000°C two opaque porcelains used with them have thermal coefficient expansion of 6.45 and 7.88 X 10 from room temperature to 820°C (91). The HV values of these alloys are 109—193, and the tensile strengths are 464—509 MPa (67-74 X 10 psi). For the last four alloys in Table 7, the HV values are 102—216, and the tensile strengths are 358—662 MPa (52-96 x 10 psi), depending upon thermal history. 

Thermal Expansion. Most manufacturers literature (87,119,136—138) quotes a linear expansion coefficient within the 0—300°C range of 5.4 x 10"7 to 5.6 x 10 7 /°C. The effect of thermal history on low temperature expansion of Homosil (Heraeus Schott Quarzschmelze GmbH) and Osram s vitreous silicas is shown in Figure 4. The 1000, 1300, and 1720°C curves are for samples that were held at these temperatures until equilibrium density was achieved and then quenched in water. The effect of temperature on linear expansion of vitreous silica is compared with that of typical soda—lime and borosilicate glasses in Figure 5. The low thermal expansion of vitreous silica is the main reason that it has a high thermal shock resistance compared to other glasses. 

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