Thermal mismatch adhesive shear stresses

Adhesive shear stresses in thermally mismatched double-lap bonded joint (adapted from Hart-Smith, 1973). 

The solutions covered in this introductory chapter all fall into a class of mechanics solutions known as mechanics of materials solutions because they involve assumptions that are typical of those made in the undergraduate level mechanics of materials courses. These closed form solutions are easy to apply, and can provide fundamental insights into the stress fields present within many idealized bonded joints. The shear lag concept is of fundamental importance to any bonded configuration where load is transferred from one adherend to another, primarily through shear. stresses within the adhesive layer. The beam on elastic foundation solution provides the basis for explaining the nature of bonded beams or plates subjected to lateral loads or applied moments. The material on residual stres.ses and curvature are important in understanding the significant stresses that can result from mismatches in properties such as the coefficients of thermal expansion. 

For flip-chip solder joiiung, plastic strain of solder bumps is a critical parameter that governs the joint reliability. Increasing bump height can reduce the bump strain and thus increase the joint reliability, as shown in Fig. 15(a). However, a systematical study on the effect of bump height showed that the failure mechanism of ACA flip-chip joints is totally different (Ref 37). In ACA joints, the bump and pad are usually made of metals that are much stiffer than adhesives. In other words, thermal mismatch stresses can hardly deform the bump and pad, and the shear strain is localized in the adhesive between the mating bump and pad (Fig. 15b). In this case. 

Typically, the adhesive and/or the matrix in FRP retrofitting applications transfers three different stress categories. These are shear, peel and thermal residual stresses. The latter occur in FRP composite joints either upon fabrication due to mismatch in the hygrothermal and elastic properties of the fibres, matrices/adhesives and adherends or due to the difference between curing and operating temperatures of the FRP material. These three stress categories can be referred to as the good, the bad and the unavoidable, respectively. 

Lap shear test conditions are not really convenient in electronics involving small dice and two substrates with different thermal expansion coefficients (CTE). The stresses generated during the cure cycle by this CTE mismatch account for possible crack formation and delamination, which in turn may degrade the adhesive strength. The die shear strength method consists in the measurement of the force required to shear the die from the chip carrier. Although any die size can be used, a standard technique is to bond 1.27 X 1.27 mm non-functional silicon dice to silver-plated lead frames. 

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