Polyorganosiloxanes as electronic device encapsulants
Heat curable polysiloxane inorganic - organic hybrid materials, prepared from the hydrolysis and condensation of chloro - or alkoxysilanes, have been investigated as possible electric device encapsulants. Preparation - structure - property relationships have been determined for a simple difunctional system prepared by the direct hydrolysis of chlorosilanes. The system cyclisation was found to be highly dependent upon the system concentration, and most sensitive at low dilution. Due to the dilution associated with the addition of an aqueous base, even a solventless preparation was found to result in a large cyclic content, a result independent of the organic nature of the precursor employed. The copolymerisation of inorganic monofunctional end - blocking units with difunctional species was found to be beneficial in reducing the level of cyclisation within the system. However, high levels of end blocker with unreactive organic functionalities are to be avoided as the resulting high levels of oligomeric species are deleterious to the material's thermal and dielectric properties. The introduction of thermally reactive organics at much higher levels are possible without property degradation although high levels of reactive groups such as vinyl (-CHCH2) or allyl (CH2CH=CH2) are deleterious to the hermetic properties of the material. Introduction of tri - and tetrafunctional inorganic units into the difunctional systems, to prepare inorganically cross - linked materials, was easily achievable by the cohydrolysis of the precursors. The inorganic cross - linking afforded control over the system viscosity which was found to be particularly sensitive to the distribution of the di - and trifunctional species throughout the network, which in turn was a function of both the preparation pH and temperature. Useable materials were obtained for T group levels of less than 20%. Levels up to 50% were possible for more homogeneous T group distributions. Interpenetrating networks employing short and long chain components were successfully prepared. Their mechanical properties were assessed and correlated to their composition and structure. The role of the inorganic cross - links was found to be a larger determinant of the mechanical properties than the inorganic network. Extreme values of Young's moduli of 288kPa and 16.6 MPa were obtained for low and high vinyl containing materials respectively. Their dielectric properties were comparable to conventional encapsulation materials, with E' and tanS being in the range 3.36±O.06 to 3.9±O.1 and O.OOl±O.OO5 to 0.0370±O.OOO2 respectively. A number of IPN materials exposed to environmental testing (85°C I 85% RH), all afforded protection over the entire l000hrs test period, with no failure resulting from sample limitations.