Removable Polyurethane Encapsulants

Two castable thermoplastic polyurethane formulations have been developed for use as removable electronics packaging materials. The first formulation would be employed in situations where some elastomeric properties are desired. Mechanical properties for this formulation are: tensile strength 1675 psi, elongation 480%, and modulus 2.2 X 10⁴ psi. The second formulation more closely simulates an epoxy resin and the mechanical properties are: tensile strength 6040 psi, elongation 6.5%, and modulus 2.8 X 10⁵ psi. Both casting resins can be filled (e.g., with Al₂O₃) which reduces the coefficient of thermal expansion (~ 80 X 10^-6 to ~ 40 X 10^-6/°C) and susceptibility to creep. The two formulations are soluble in polar organic solvents such as dimethylformamide and tetrahydrofuran. Both formulations were synthesized in bulk and based on reaction rate studies, they exhibit a diffusion controlled process in the latter stages of reaction.