Structure and method are provided for plastic encapsulated semiconductor devices having a buffer layer of low dielectric constant and/or low loss tangent material separating the die surface from the plastic encapsulation. Semiconductor wafers with substantially completed SC die are coated with the buffer layer. The buffer layer is patterned to expose the die bonding pads but leave the buffer layer over some or all of the other die metallization. The die are then separated, mounted on a lead-frame or other support, wire bonded or otherwise coupled to external leads, and encapsulated. The plastic encapsulation surrounds the die and the buffer layer, providing a solid structure. The buffer layer reduces the parasitic capacitance, cross-talk and loss between metallization regions on the die. An optional sealing layer may also be provided at the wafer stage between the buffer layer and the plastic encapsulation to mitigate any buffer layer porosity.