Abstract: An optical semiconductor element mounting package that has good adhesion between the resin molding and the lead electrodes and has excellent reliability is provided, as well as an optical semiconductor device using the package is also provided. The optical semiconductor element mounting package having a recessed part that serves as an optical semiconductor element mounting region, wherein the package is formed by integrating: a resin molding composed of a thermosetting light-reflecting resin composition, which forms at least the side faces of the recessed part; and at least a pair of positive and negative lead electrodes disposed opposite each other so as to form part of the bottom face of the recessed part, and there is no gap at a joint face between the resin molding and the lead electrodes.

Abstract: This invention provides a heat curable resin composition for light reflection, which, after curing, can realize high reflectance in a range of visible light to near ultraviolet light, has excellent heat deterioration resistance and tablet moldability, and is less likely to cause burrs during transfer molding, and a process for producing the resin composition, and an optical semiconductor element mounting substrate and an optical semiconductor device using the resin composition. The heat curable resin composition for light reflection comprises a heat curable component and a white pigment and is characterized in that the length of burrs caused upon transfer molding under conditions of molding temperature 100° C. to 200° C., molding pressure not more than 20 MPa, and molding time 60 to 120 sec is not more than 5 mm and the light reflectance after heat curing at a wavelength of 350 nm to 800 nm is not less than 80%.

Abstract: An optical semiconductor element mounting package that has good adhesion between the resin molding and the lead electrodes and has excellent reliability is provided, as well as an optical semiconductor device using the package is also provided. The optical semiconductor element mounting package having a recessed part that serves as an optical semiconductor element mounting region, wherein the package is formed by integrating: a resin molding composed of a thermosetting light-reflecting resin composition, which forms at least the side faces of the recessed part; and at least a pair of positive and negative lead electrodes disposed opposite each other so as to form part of the bottom face of the recessed part, and there is no gap at a joint face between the resin molding and the lead electrodes.