  The dimpling in the CuO_2 planes of overdoped Y_{1-y}Ca_yBa_2Cu_3
O_{6.96-6.98} (y=0.02-0.2) has been measured by x-ray absorption-fine-
structure spectroscopy (Y-K EXAFS). A step-like decrease around 12% Ca
indicates a percolation threshold for distorted sites of 5 cells, and thus
phase segregation. We conclude the charge carriers added by substitution of
Y{3+} by Ca{2+} to be trapped at the Ca sites and their nn environment.
