Castable Thermoplastic Urethane Elastomers I. Formulation and Applications as Removable Encapsulants

Abstract

Castable thermoplastic polyurethane formulations with im proved upper and lower temperature capability have been developed for use as removable electronics packaging matenals. Two formulations have been developed employing a two step prepolymer processing technique. One consists of 58/42 ratio of polyether to polyester diols reacted with bitolylene diisocyanate to form a prepolymer; this step is followed by reaction with 1,4 butanediol. The second contained a 64/36 ratio of poly ether to polyester diol reacted in the same manner as above. The first formulation exhibited a tensile strength of 13.1 MPa, an elongation of 500%, and a modulus of 145 MPa; the second gave a tensile strength of 7.58 MPa, elongation 275%, and modulus of 124 MPa. These property differences resulted from differences in microphase structure between hard and soft segments in these linear polyurethanes. Both formulations were essentially free of creep over an 80 hour period at 70°C. These casting resins can be processed with a filler (e.g., Al₂O₃) which reduces the coeffi cient of thermal expansion from ∼ 160 × 10^-6 to ∼ 100 × 10 ^-6 /°C. The formulations are soluble in polar organic solvents such as dimethyl formamide and tetrahydrofuran. Functional electronic components have been successfully potted and depotted using these solvent-removable formulations.