The invention relates to a substrate (10) provided with a stack of thin films comprising alternate "n" functional layers (40, 80) with infrared reflection and/or solar radiation reflection properties, and "(n+1)" coatings (20, 60, 200) where n is an integer = 2, said coatings consisting of a plurality of dielectric layers (24, 26; 64, 66; 104) such that each functional layer (40) is arranged between the two coatings (20, 60, 100), at least two functional layers (40, 80) being respectively arranged on a wetting layer (30, 70) which is in turn respectively arranged directly on a subjacent coating (20, 60). The inventive substrate is characterised in that two subjacent coatings (20, 60) each comprise at least one dielectric layer (24, 64) and at least one non-crystallised smoothing layer (26) consisting of a material different from the material of the dielectric layer inside each coating, said smoothing layer (26) being in contact with the subjacent wetting layer (30).The invention is also characterised in that since the two subjacent coatings (20, 60) have different thicknesses, the thickness of the smoothing layer (26, 66) of the subjacent coating (20, 60) having a total thickness which is lower than that of the other subjacent coating is lower than or equal to the thickness of the smoothing layer (66, 26) of said other subjacent coating (60, 20).