Electronic devices may be provided with displays. A display may have a light guide plate. Backlight for the display may be launched into the light guide plate from an array of light-emitting diodes. The light-emitting diodes may be mounted on a metal core printed circuit board having a dielectric layer and a metal layer. The metal core printed circuit board may have an elongated shape that extends along the surface of a metal structure. A weld may be formed along a seam between the metal layer of the metal core printed circuit board and the metal structure. The metal structure may be an electronic device housing, a display chassis member, a heat spreader, a heat pipe, or other structures in an electronic device.