Tensile hygrothermal stress

The absorption of moisture in underfill adhesives induces a tensile hygrothermal stress on the solder-ball connections causing electrical opens in the connections and cracking in the adhesive. These tensile stresses offset the compressive stresses that underfill adhesives provide in improving the reliability of flip-chip and ball-grid-array devices. 

Figure 12.4 depicts the comparison of the measured moisture uptake history for the laminate with NOVA-3D predictions. In general, the model prediction agrees reasonably well with test data over the 112 day period used for this comparison, especially during the absorption cycles. Figure 12.5 shows the predicted evolution of the in-plane stress at the exposed laminate surface with hygrothermal cycling. The increase in in-plane tensile... 

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