Stress in adhesive

Stresses in adhesive-attached die become critical as the size of the die increases and as dissimilar adherends are used. Silicon die, smaller than 100-mil square, attached with high-modulus adhesives to leadframes or to other substrates have been quite reliable even when large differences in the CTEs of the adherends exist. However, for large devices (greater than 300-mil square) low-modulus adhesives are necessary to absorb or dissipate stresses. ... 

Besides delamination, actual cracking of silicon die is a failure mode that can occur from excessive adhesive stresses, voids, and moisture absorption. Residual stresses in adhesive-attached single-chip devices become critical as the size of the device increases and dissimilar die and leadframe materials are used. Several types of fractures that can occur within the die or within the adhesive are shown in Fig. 6.8. A hairline crack in an 1C chip that resulted from adhesive stress is shown in Fig. 6.9. 

Internal Stresses in Adhesive-Bonded Joints and Ways of Decreasing Them... 

The internal stresses in adhesive-bonded joints arise for two reasons. In the course of setting of the adhesive, its volmne decreases due to volatilization of solvents, polymerization or physical structurization. As a result of the adhesion interaction of the adhesive and the substrate, the film can contract only in thickness, which is why stresses that appear in it are parallel to the siuface. The film extends while contraction stresses appear in the substrates. Rapid growth of stresses, which tend to reduce the length of the film, begins from the moment the polymer loses yield. 

Determination of Internal Stresses In Adhesive-Bonded Joints... 

Shrinkage internal stresses in adhesive-bonded joints... 

The determination of edge stresses in adhesive-bonded joints is based on their property of counteracting the bonding forces between the adhesive and the substrate— in other words, of decreasing the adhesion strength. 

All the practically acceptable methods of decreasing the internal stresses in adhesive-bonded joints can be divided into three groups decrease of the adhesive shrinkage in the course of cure increase of the relaxation rate of the internal stresses in the adhesive and decrease of the difference between the coefficients of linear thermal expansion of the adhesive and the substrate. 

Controlling internal stresses in adhesive-bonded joints by taking account of the separation in time of the formation of linear and cross-linked polymers... 

These contradictions can to a considerable degree be resolved by a successfully chosen or properly developed theory of limiting stressed states, with the help of which it should be possible to find an explanation of the discrepancies between design, laboratory, and operating values of breaking stresses in adhesive joints. 

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