Elastic modulus variation due to moisture absorption and permanent changes upon redrying in an epoxy based underfill

Abstract

In this study, the influence of moisture on the elastic modulus of a no-flow underfill is characterized over a wide range of accelerated testing environments, and the permanent changes are evaluated upon fully redrying. Moisture saturation is reached in all environments prior to testing, thus the inherent wet modulus of the underfill is identified for each respective level of moisture preconditioning. The change in the elastic modulus as a function of moisture concentration is measured, as well as the reversible and irreversible effects from moisture uptake in the underfill upon redrying. Thermal aging from the temperature component of the accelerated testing environment is found to have no effect on the elastic modulus, while the moisture uptake is found to notably decrease the modulus at higher humidity levels. Recovery experiments demonstrate that most of the loss in the elastic modulus from moisture exposure is recoverable, although more irreversible damage did occur at higher humidity levels. Hydrolysis contributed to the irreversible loss in the modulus, while the reversible component is attributed to plasticization of the underfill from moisture uptake.

Predictive Model for Adhesion Loss of Molding Compounds from Exposure to Humid Environments

“…As the aggregate absorbed moisture content increased, the interfacial adhesion continued to decrease. Since the moisture did not significantly change the elastic modulus of the underfill adhesive for the moisture conditions evaluated in this study [12], plasticization of the underfill from moisture uptake contributed little to the change in the interfacial fracture toughness. As a result, the reduction in the toughness is primarily attributed to the weakening of the underfill / copper interface due to the direct presence of moisture at the interface.…”

“…Using both the critical load of fracture and underfill elastic modulus value [12] for each respective level of moisture preconditioning, the interfacial fracture toughness of the underfill / copper interface was determined using Eq. 1.…”

“…It is important to note that all specimens in test groups 3, 4, and 5 had reached fully saturated moisture conditions at the conclusion of the 168 hour exposure time. Consequently, a gradient of moisture concentration did not exist in specimens during testing, allowing the identification of the intrinsic wet modulus [12] and interfacial fracture toughness for a particular level of moisture content inside the specimens. Test groups 6, 7, and 8 identify the reversible and irreversible effect of moisture preconditioning on adhesion.…”

“…Using the value for the density of water at room temperature (0.998 mg / mm 3 ), an average nanopore diameter of 5.5 Å, and the saturation concentration determined from the experimental portion of this study in conjunction with Eqs. (12) and (29), the number of active nanopores participating, N, and value of r debond can be determined by the intrinsic response of the material system to each level of moisture preconditioning. The results are shown in Table 4.…”

“…It has been shown that moisture presents several reliability issues once absorbed within a device. These include but are not limited to a reduction in adhesion [1][2][3][4][5][6], mechanical property change [2,[7][8][9][10][11][12], hygro-swelling [13][14], and steam driven delamination during solder reflow [15][16][17][18].…”