For piezoelectric semi-infinite strips with mixed edge-data, the Betti-Rayleigh reciprocal theorem and the general solution of plane piezoelectricity are applied in a novel way to obtain the appropriate edge conditions accurate to all order. A decay analysis technique is used to establish necessary conditions which the prescribed data on the edge of the strips must satisfy in order that it should generate a decaying state within the strips. For the case of transverse bending of the strips, these decaying state conditions are obtained explicitly for the first time when the mixed conditions are imposed on the edge.