High-modulus adhesives

The width of the lap shear specimen is generally 1 in. The recommended length of overlap, for metal substrates of 0.064-in thickness, is 0.5 + 0.05 in however, it is recommended that the overlap length be chosen so that the yield point of the substrate is not exceeded. In lap shear specimens, an optimum adhesive thickness exists. For maximum bond strengths, the optimum thickness varies with adhesives of different moduli (from about 2 mils for high-modulus adhesives to about 6 mils for low-modulus adhesives).5... 

High-modulus adhesives are likely to have high bending stresses, high deflections, and low radii of curvature. A comparison of typical and ultra-low-stress conductive adhesives was reported using 250-mil square silicon die (20 mils thick) bonded to 10-mil-thick copper leadframes." A conventional silver-filled adhesive had a deflection of 3 mm and an ROC of 1,070 mm while a low-stress adhesive showed a deflection of 0.01 mm and an ROC >125,000 mm." ... 

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. ... 

Property Adhesive A (high modulus) Adhesive B (high modulus) Adhesive C (intermediate modulus) Adhesive D (low modulus) ... 

Where the adherends have closely matched CTEs as for silicon die (CTE = approx. 3 ppm/°C) attached to Kovar leadframes (CTE = approx. 5 ppm/°C), a high-modulus adhesive could be used. As Kovar leadframes were replaced by copper leadframes (CTE = 16-17 ppm/°C), low-modulus, stress-free adhesives were required to compensate for the large mismatches in CTEs. Large CTE mismatches also occur when silicon devices are directly attached to epoxy printed-circuit boards as for COBs or CSPs. 

However, natural-based materials like starches and soybean oils are used in direct competition to animal or human food consumption. Technically, they are not at all suitable for high modulus adhesive and sealant joints. They usually are sensitive to hydrolysis and oxidization. Exposure to atmospherie eonditions or chemical attack can drastically limit their service life expectancy. They are sensitive to elevated temperatures and need to be protected against microbiological decay with more or less toxic preservatives. In light of those features and based on today s knowledge some important conclusions can be drawn ... 

If we reverse the situation and bond two very flexible pieces of rubber together with a rigid, high modulus adhesive, a different picture emerges. The tensile strain... 

Mixed modulus joints have been proposed in the past to improve the stress distribution and increase the joint strength of high modulus adhesives. The stiff, brittle adhesive should be in the middle of the overlap, while a low modulus adhesive is applied at the edges prone to stress concentrations. Sancaktar and Kumar (2000) used rubber particles to toughen the part of the adhesive located at the ends of the overlap and increase the joint strength. The concept was studied with the FE method and proved experimentally. Fires et al. (2003) also proved with an FE analysis and experimentally with two different adhesives that the mixed adhesive method gives an improvement in joint performance. Temiz (2006) used an FE analysis to study the influence of two adhesives in double-lap joints under bending and found that the technique... 

The higher the shear modulus of the adhesive, the higher and more concentrated at the edge are the stresses. It might therefore be expected that the damping of a high modulus adhesive will be low in the joint form. But remember that the energy stored per unit volume is... 

One major cause of failure in this field is large mismatch in the coefficient of thermal expansion of the substrates concerned, especially when bonded with high modulus adhesives, and so adhesives have been formulated from more elastomeric resins, which can effectively dissipate the stresses induced in these situations, and can be demonstrated by tbe accelerated test cycles described above. 

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