Hygrothermal Aging Effects on Buried Molecular Structures at Epoxy Interfaces

Abstract

Interfacial properties such as adhesion are determined by interfacial molecular structures. Adhesive interfaces in microelectronic packages that include organic polymers such as epoxy are susceptible to delamination during accelerated stress testing. Infrared-visible sum frequency generation vibrational spectroscopy (SFG) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were used to study molecular structures at buried epoxy interfaces during hygrothermal aging to relate molecular structural changes at buried interfaces to decreases in macroscopic adhesion strength. SFG peaks associated with strongly hydrogen bonded water were detected at hydrophilic epoxy interfaces. Ordered interfacial water was also correlated to large decreases in interfacial adhesion strength that occurred as a result of hygrothermal aging, which suggests that water diffused to the interface and replaced original hydrogen bond networks. No water peaks were observed at hydrophobic epoxy interfaces, which was correlated with a much smaller decrease in adhesion strength from the same aging process. ATR-FTIR water signals observed in the epoxy bulk were mainly contributed by relatively weakly hydrogen bonded water molecules, which suggests that the bulk and interfacial water structure was different. Changes in interfacial methyl structures were observed regardless of the interfacial hydrophobicity which could be due to water acting as a plasticizer that restructured both the bulk and interfacial molecular structure. This research demonstrates that SFG studies of molecular structural changes at buried epoxy interfaces during hygrothermal aging can contribute to the understanding of moisture-induced failure mechanisms in electronic packages that contain organic adhesives.

Sum Frequency Generation Vibrational Spectroscopy for Characterization of Buried Polymer Interfaces

“…Using SFG-VS, Myers et al first correlated the interfacial molecular structures of cured epoxy resins on various substrates with the macroscopic adhesion strength during hygrothermal aging. 117 The results showed that water molecules tended to diffuse into the more hydrophilic interfaces during hygrothermal aging process and ordered at the interface (Figure 11d), replacing the original interfacial hydrogen bonding networks and reducing the adhesion strength. On the contrary, at the more hydrophobic interface such as PS, no ordered water was detected (Figure 11d), corresponding to a much smaller decrease in the adhesion strength.…”

“…Adapted from Myers et al. 117 Right: a schematic shows that water diffuses to and traps at the SiO 2 /epoxy interface.…”

“…The strong water signal at 3000-3500 cm -1 indicates that water can diffuse into and order at the interface during hydrothermal aging. Adapted from Myers et al 117 Right: a schematic shows that water diffuses to and traps at the SiO 2 /epoxy interface. correlated with the mechanical adhesion strength.…”

“…On the contrary, at the more hydrophobic interface such as PS, no ordered water was detected (Figure 11d), corresponding to a much smaller decrease in the adhesion strength. 117 The addition of silane adhesion promoters was found to be effective in preventing the water diffusion into the interface and improving the adhesion strength between polymer and epoxy. 118 The molecular structures between epoxy and copper were also studied by SFG-VS using a sandwiched window geometry.…”

“…One example is the interfaces between polymer and adhesives, which have been reviewed previously. [102][103][104][105][106][107][108][109][110][111][112][113][114][115][116][117][118][119][120] Another example is the rubbing and sliding between two polymers, which will be discussed in the following section.…”