Stress during cooling and

Because nanocomposites are made from different phases with different thermal expansion coefficients and elastic moduli, they inevitably develop residual thermal stress during cooling after sintering. Assuming the dispersion phase is spherical particulate in the matrix material, residual stresses can be developed due to differences in the thermal expansion and elastic constants between the matrix and the particles [23] ... 

The adhesive s thermal conductivity is important in minimizing transient stresses during cooling. This is why thinner bond thickness and adhesives or sealants with higher levels of thermal conductivity generally have better cryogenic properties. 

These arise inevitably from thermal contractions during cooling and may be supplemented by other stresses developed as a result of rigidity in the parts to be joined. 

Effect of Lead-Free Conversion. Lead-free conversion exacerbates these potential silicon-level failures. Since the assembly is subjected to a higher temperature range than with tin-lead assemblies, there is a higher propensity for early failures due to residual stresses during cool down. In addition, there are potential concerns over the effect on low dielectric constant (k) materials in the silicon during second level temperature cycling. This is discussed in more detail in Sec. 58.3.13.9.The effect of mold compound and underfill material selection on first-level interconnects is also discussed in more detail in Secs. 58.3.13.5 and 58.3.13.6 respectively. 

The factors that determine the strength of an adhesive at very low temperatures are (1) the difference in coefficient of thermal expansion between adhesive and adherend, (2) the elastic modulus, and (3) the thermal conductivity of the adhesive. The difference in thermal expansion is very important, especially since the elastic modulus of the adhesive generally decreases with falling temperature. It is necessary that the adhesive retain some resiliency if the thermal expansion coefficients of adhesive and adherend cannot be closely matched. The adhesive s coefficient of thermal conductivity is important in minimizing transient stresses during cooling. This is why thinner bonds have better cryogenic properties than thicker ones. 

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