Abstract: A method for sealing an electronic component mounting substrate including: preparing an electronic component mounting substrate including a substrate and electronic components mounted thereon and having a space between an electronic component and the substrate; placing a thermosetting sheet on the electronic component mounting substrate; and heat-molding the placed thermosetting sheet, to allow a melt of the thermosetting sheet to fill the space between the electronic component and the substrate and be cured. When a distance between the center of the substrate and an optional point P on a frame line that surrounds all the electronic components and minimizes the surrounded area is denoted by Lp, a distance Lq between the center of the substrate and a point Q at which a straight line passing through the point P and the center of the substrate intersects with an outer periphery of the thermosetting sheet is 0.9Lp or more.

Abstract: [Problem] To provide a multi-layer sheet for mold underfill encapsulation, which exhibits good infiltrability between electrodes. [Solution] In order to solve the aforementioned problem, the present invention provides a multi-layer sheet for mold underfill encapsulation, which is characterized by having provided as an outermost layer thereof an (A) layer that comprises a resin composition having a local maximum loss tangent (tan ?) value of 3 or more at a measurement temperature of 125° C. for a measurement time of 0-100 seconds.