The invention relates to a plastic-welding device for connecting a first, thin-walled plastic part (6) to a second plastic part in a planar manner, comprising a hand-held mounting aid (1) and a welding head (2) that can be held in a retainer of a hand soldering iron; wherein the welding head (2) has a main body (3) and at least one welding tip (4) arranged on the main body (3); wherein the mounting aid (1) has at least one spacer (5), which can be placed onto the thin-walled plastic part (6) by means of an underside of the at least one spacer and on the surface of which the main body (3) of the welding head (2) can be supported; and wherein the at least one spacer (5) is designed in such a way that, when the main body (3) is supported on the at least one spacer (5), a depth of penetration of the at least one welding tip (4) into the plastic parts to be connected is limited to a predetermined maximum value.