A method for making dimensionally stable polyester films comprising the steps of: (a) providing a treatment assembly (10, 30, 40) comprising a plurality of layer packets (12a, 12b, 12c, 12d; 36a, 36b, 36c; 40a, 40b), each layer packet comprising a polyester film (14, 44b, 46b), the polyester film comprising one or more semicrystalline polyesters having a T g and a T m, having first major surface and second major surface, and a release layer (20, 44a, 46a) having first major surface and second major surface, the first major surface of the release layer being attached to the second major surface of the polyester film, wherein the layer packets are arranged in a stack (34, 38) such that the second major surface of the release layer of an overlying layer packet is separably attached to the first major surface of the polyester film of an underlying layer packet; (b) bi-axially orienting the polyester films (14, 44b, 46b) by heating the treatment assembly (10, 30, 40) such that the polyester films reach a temperature of at least T Orient wherein T Orient is above the T g of the polyester film and biaxially stretching the treatment assembly while the layer packets (12a, 12b, 12c, 12d; 36a, 36b, 36c; 40a, 40b) are at a temperature of at least T Orient to yield an oriented assembly; characterized in that it then comprises the steps of: (c) heat setting the polyester films (14, 44b, 46b) by heating the oriented treatment assembly (10, 30, 40) to a temperature T Set wherein T Set is above T Orient and below T m while maintaining the orienting configuration and dimensions so as to raise the crystallinity of the polyester component to yield a heat set assembly; and then (d) heat relaxing the heat set treatment assembly (10, 30, 40) by heating to a temperature T Relax wherein T Relax is below T Set and above T Orient while being substantially unrestrained in the x and y axial dimensions to yield a heat relaxed assembly. (a) providing a treatment assembly (10, 30, 40) comprising a plurality of layer packets (12a, 12b, 12c, 12d; 36a, 36b, 36c; 40a, 40b), each layer packet comprising a polyester film (14, 44b, 46b), the polyester film comprising one or more semicrystalline polyesters having a T g and a T m, having first major surface and second major surface, and a release layer (20, 44a, 46a) having first major surface and second major surface, the first major surface of the release layer being attached to the second major surface of the polyester film, wherein the layer packets are arranged in a stack (34, 38) such that the second major surface of the release layer of an overlying layer packet is separably attached to the first major surface of the polyester film of an underlying layer packet; (b) bi-axially orienting the polyester films (14, 44b, 46b) by heating the treatment assembly (10, 30, 40) such that the polyester films reach a temperature of at least T Orient wherein T Orient is above the T g of the polyester film and biaxially stretching the treatment assembly while the layer packets (12a, 12b, 12c, 12d; 36a, 36b, 36c; 40a, 40b) are at a temperature of at least T Orient to yield an oriented assembly; characterized in that it then comprises the steps of: (c) heat setting the polyester films (14, 44b, 46b) by heating the oriented treatment assembly (10, 30, 40) to a temperature T Set wherein T Set is above T Orient and below T m while maintaining the orienting configuration and dimensions so as to raise the crystallinity of the polyester component to yield a heat set assembly; and then (d) heat relaxing the heat set treatment assembly (10, 30, 40) by heating to a temperature T Relax wherein T Relax is below T Set and above T Orient while being substantially unrestrained in the x and y axial dimensions to yield a heat relaxed assembly. (c) heat setting the polyester films (14, 44b, 46b) by heating the oriented treatment assembly (10, 30, 40) to a temperature T Set wherein T Set is above T Orient and below T m while maintaining the orienting configuration and dimensions so as to raise the crystallinity of the polyester component to yield a heat set assembly; and then (d) heat relaxing the heat set treatment assembly (10, 30, 40) by heating to a temperature T Relax wherein T Relax is below T Set and above T Orient while being substantially unrestrained in the x and y axial dimensions to yield a heat relaxed assembly.