Adhesives degradation

Abstract—The adhesion of pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide to fluorine-contaminated silicon dioxide (F-SiO,) with y-aminopropyllriethoxysilane (APS) adhesion promoter has been studied as a function of the peel ambient humidity. The peel strength was not affected by the change in peel ambient relative humidity (RH) from 11-17% to 35-60% when APS was used at the interface. Without APS, the adhesion degraded significantly with this change in RH. It was found that although the dip application of APS caused the removal of about 80% of the initial atomic percentage of fluorine on the surface, it could not be totally removed even after several days in water at elevated temperature. 

Time and economics generally allow only short-term tests to verify the selection of the adhesive system relative to the environment. It is tempting to try to accelerate service life in the laboratory by increasing temperature or humidity, for example, and then to extrapolate the results to actual conditions. However, often too many interdependent variables and modes of potential adhesive degradation are in operation, and a reliable estimate of life using simple extrapolation techniques cannot be achieved. 

The above analysis applies to degradation processes that relate to the bulk adhesive. Interfacial degradation processes such as corrosion can be similarly determined. Thermal and oxidative stability, as well as corrosion and water resistance, depends on the adherend surface as well as on the adhesive itself. Epoxy-based adhesives degrade less rapidly at elevated temperatures when in contact with glass or aluminum than when in contact with copper, nickel, magnesium, or zinc. The divalent metals have a more basic oxide surface than the higher-valence metal oxides and hence serve to promote dehydrogenation reactions, which lead to anion formation and chain scission.7... 

As in the case of 73-series adhesives, degradation of 87 and 88-series polyurethanes was bimodal (Fig. 9). These adhesives were somewhat more stable than the 73-series (compare Fig. 6 with Fig. 9). Volatilization began at about 250°C. Assuming that the hard segments degrade first, this implies that the polyurea hard segments of 87- and 88-series polymers have better thermal stability than the polyurethane hard segments of the 73-series adhesives. 

Cured adhesives must be resistant to the effects of solvents and chemicals used in subsequent cleaning and processing steps. Dissolving or swelling of adhesives degrades their mechanical, electrical, and physical properties. Adhesives resistance to solvents is evaluated based on the function or application. 

However, it should be remembered that the vast majority of adhesives degrade rapidly as the temperature approaches and passes 200°C. Consequently, whenever a service temperature of 120°C or more is likely, the expected service life of the adhesive should be discussed thoroughly with the manufacturer. 

Laminate and Cu/Laminate Adhesion Degradation. When a PCB is exposed to elevated temperatures for long periods of time, the adhesion between the Cu and the laminate and the flexural strength of the laminate itself will gradually degrade. Discoloration is usually an early symptom. 

The heat stability of the adhesive is a very important property. Running temperatures that are greater than the adhesive degradation temperature result in charring and decreased overall properties. The addition of stabilizers to the adhesive formulation contributes to the stability of the material by hindering the acceleration of degradation due to the presence of oxygen. 

Long-term stabihty in hostile environments such as heat, humidity, and thermal cycling (e.g., resistance to adhesion degradation)... 

Adhesion degradation under these conditions results from various phenomena. The diffusion of tin (Sn) from the solder into the interfacial regions and subsequent formation of intermetalhc compounds is the primary failure mechanism. This was discussed earher for Ag, Ag-Pd, and Cu formulations. Formation of these compounds creates interfacial stresses as a result of thermal expansion mismatches of the various phases. The result is mechanical... 

D. H. R. Sarma et al., An Accelerated Lot Acceptance Test for Adhesion Degradation of Soldered Copper Thick Films in Temperature Cycling, Proc. Inti. Symp. Microelec., Minneapolis, pp. 554-561,1987. 

Starch-based adhesives degrade in humid service environments. 

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