An Examination of Glass-fiber and Epoxy Interface Degradation in Printed Circuit Boards

Estimated reading time: 9 minutes

Figure 3:

Figure 3 illustrates a typical alkoxysilane coupling agent and its hydrolysis reaction. Usually, the silane is functional at both ends. R is an active chemical group such as amino (NH2), mercapto (SH), or isocyanato (NCO). This functionality reacts with functional groups in an industrial resin or biomolecule such as DNA fragments. The other end consists of a halo (most often chloro) or alkoxy (most often methoxy or ethoxy) silane. This functionality is converted to active groups on hydrolysis called silanols. The silanols can further react with themselves, generating oligomeric variations. All silanol variations can react with active surfaces that contain hydroxyl (OH) groups.

The three main classes of silanes are chloro-, methoxy- and ethoxy-silanes. Chlorosilanes are the most reactive but evolve corrosive hydrogen chloride on hydrolysis. Methoxy silanes are of intermediate reactivity ,and ethoxy silanes are least reactive and evolve non-toxic ethanol ^[8]. The difference in reactivity between methoxy and ethoxy silanes is low. At typical hydrolysis pH (acidic ~5, basic ~9), both versions hydrolyze in under 15 minutes at 2% silane concentrations.

Figure 4: Schematic for a conventional interdiffusion and IPN ^[10].

The bifunctional silane molecules act as a link between the glass fiber and resin by forming a chemical bond with the glass surface through a siloxane bridge, and the organofunctional group bonds to the polymeric resin ^[9]. This allows silanes to function as surface-treating or coupling agents. The formation of an interpenetrating network (IPN) through interdiffusion is the next step in the process. Figure 4 shows a schematic for interdiffusion and creation of an IPN in a silane-treated glass fiber ^[10]. Interdiffusion and intermixing take place in the coupling agent-polymer resin interface region due to penetration of resin into the chemisorbed silane layer and migration of the physisorbed silane molecules into the resin phase. The migration and intermixing of silane and other sizing ingredients with polymer resin create an interphase of substantial thickness.

Theoretically, a single layer of silane may be sufficient to bond with the glass surface; however, to ensure uniform coverage, more than one layer of silane is usually applied. This results in a tight siloxane polymeric network close to the inorganic surface that diffuses into subsequent overlays. The siloxane remains in high concentration at the glass-epoxy interface and may be dissolved into the matrix during the curing of the matrix resin (Figure 5).

Figure 5: Schematic for a hydrolyzed diffused interface after aging in water in a silane-treated glass fiber ^[11].

Coupling to the organic matrix is a complex phenomenon. The reactivity of a thermoset polymer is matched to its reactivity with the silane. For example, an epoxysilane or aminosilane will bond to an epoxy resin, an aminosilane will bond to a phenolic resin, and a methacrylate silane will bond through styrene crosslinking to an unsaturated polyester resin. The large differences in composition and chemical characteristics of the individual components—such as antistats on the glass, lubricants, surfactants, and film formers—further complicate the formation of this interphase with different formulations.

While the silane chemistry and its interactions with the glass surface and the polymer have been extensively studied, relatively little information is available about the influence of these sizing components on the formation of the interphase. The interphase is the region where stress transfer occurs between the two composite constituents, but its material properties and effective thickness are unknown. Investigation of the mechanical properties of the interphase presents a challenge to probing into materials in extremely low dimensions.

The silicon atoms in the silanes are bonded to the silicon atoms in the glass through oxygen atoms, and the silicon atoms in the glass are bonded to each other. If the water produced in the forward reaction is continuously removed by evaporation, for instance, the bonding of silane to glass will continue until either there is no more silane or are no more attachment sites on the glass surface. Conversely, if water is added to the silane bonded to glass, the reverse reaction can debond the silane from the glass. The rate of hydrolysis is influenced by the pH.

For pathway formation in the conductive anodic filament (CAF), the degradation reaction is the second hydrolysis of silane bonds, which is typically observed after long-term exposure to elevated temperature and humidity conditions. Hydrothermal degradation of silane bonds in glass-epoxy composites ^{[10, 12, & 14]} enables the path formation due to hydrolysis, which is the rate-limiting step in PCB failure due to CFF. The hydrolysis degradation reaction of the interfacial bonds formed with silane coupling agents is shown in Equation 4 where Mis the substrate.

Equation 4: 

The rate of degradation is shown in Equation 5 where k_f is an elementary rate constant given by Equation 6.

Equation 5: 

Equation 6: 

In Equation 6, k is the Boltzmann constant, h is the Planck constant, R is the gas constant, T is the absolute temperature, f_W is the activity coefficient of the attacking chemical, f_B is the activity coefficient of the interfacial bond, C_W is the concentration of the attacking chemical, C_B is the concentration of the interfacial bond, ΔG_0* is the change of Gibbs free energy of the activated state, P is the pressure, and ΔV* is the activated volume per mole ^[8].

Conclusions

The two-step process of conductive filament formation includes the resin-glass fiber bond degradation and pathway formation followed by an electrochemical reaction between the conductors. One method by which the pathway forms is due to breakage of the organosilane bonds at the glass and resin interface. This breakage occurs by hydrolysis (adsorption of water at the fiber glass-epoxy resin interface) or by repeated thermal cycling, which induces stresses at the interface due to CTE mismatches. This article reviews the organosilane-resin bonding process and summarizes processing and environmental phenomenon that can result in breakage of the bonds. The article also motivates the requirement for systematic investigation of mechanical properties at the glass-resin interphase, and to track the influences of physiochemical stresses such as moisture and pH that can affect the strength of this vital interface.

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