Assembly structure for injection molded substrate and for mounting component

A substrate (1) includes conductive portions (7) formed by press working and a resin portion (11) integrally injection-molded with the conductive portions (7). The conductive portions (7) are formed from, for example, a copper alloy. The resin portion 11 is formed from, for example, PPS. A surface-mount component (3), which is an electronic surface-mount component, is mounted on the substrate (1). The surface-mount component (3) has electrodes (5) at its opposite sides, and the electrodes (5) and the respective conductive portions (7) are electrically connected by means of a solder (9). The substrate (1) has a hole (13), which functions as a stress relaxation mechanism, formed in the resin portion (11) (a portion extending therethrough) between connection portions (15) under the surface-mount component (3). The substrate (1) also has resin-exposed portions (13), which function as a stress relaxation mechanism, formed on opposite sides of the surface-mount component (3).