Reworking thermoplastic adhesives

Thermoplastic materials have curing temperatures that are lower and time durations that are shorter than thermosetting materials. Some compositions will cure at room temperature, making them suitable for temperature-sensitive components or when thermal expansion mismatch, residual stresses are of concern. Another advantage of thermoplastic adhesives is that because they readily soften at elevated temperatures, they can be easily removed to allow for the rework of components. 

Polyurethane adhesives are used primarily in the assembly of printed-wiring boards to attach large components such as capacitors, magnetics, heat sinks, and connectors. Their key benefits include flexibility, absorption of stresses, vibration damping, and thermal transfer. Being thermoplastic, polyurethanes are also easier to rework than epoxies. 

Thermoplastic hot melt adhesives have also found use in flat-panel displays (FPDs) because of their rapid bonding and ease of reworking. 

In general, hot-melt adhesives are in solid form at temperatures below 79°C, and as the temperature is increased beyond this point, the solid adhesive material rapidly melts to a relatively low-viscosity fluid that can be applied easily. Upon cooling, the adhesive solidifles rapidly. Since these adhesives are thermoplastics, the melting/resolidiflcation process is repeat-able with subsequent heating/cooling cycles. Consequently, these adhesives allow reworking of the bonded parts, when necessary. Typical application temperatures of hot-melt adhesives are 149-188°C. 

Both thermoplastic and thermoset resins can be used for ICA formulations. The main thermoplastic resin used for ICA formulations is polyimide resin. An attractive advantage of thermoplastic ICAs is that they are reworkable (e.g., can easily be repaired). A major drawback of thermoplastic ICAs, however, is the degradation of adhesion at high temperature. Another drawback of polyimide-based ICAs is that they generally contain solvents. During heating, voids are formed when the solvent evaporates. Most of commercial ICAs are based on thermosetting resins. Epoxy resins are most commonly used in thermoset ICA formulations because they possess superior balanced properties. Silicones, cyanate esters, and cyanoacrylates are also employed in ICA formulations [48-52]. 

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