A method of bonding a flexible printed circuit board (FPCB). The method includes disposing a bonding material on a connection unit of a panel, disposing a connection unit of the FPCB on the bonding material; disposing a thermally conductive sheet on the connection unit of the FPCB; and heating and pressing the connection unit of the FPCB onto the connection unit of the panel. The thermally conductive sheet includes through holes that are formed in the thermally conductive sheet or grooves that are formed in a bottom surface of the thermally conductive sheet. In the operation of heating and pressing the connection unit of the FPCB, extended portions of the FPCB are accepted into the through holes or the grooves in the thermally conductive sheet.