Manufacturing method of mounting structure, and sheet therefor

A manufacturing method of a mounting structure, the method comprising:

• a step of preparing a mounting member including a first circuit member and a plurality of second circuit members placed on the first circuit member via bumps, the mounting member having a space between the first circuit member and the second circuit member;
• a step of preparing a sheet having a space maintaining layer;
• a disposing step of disposing the sheet on the mounting member such that the space maintaining layer faces the second circuit members;
• and
• a sealing step of pressing the sheet against the first circuit member and heating the sheet, to seal the second circuit members so as to maintain the space, and to cure the sheet, wherein
• the bumps are solder bumps,
• the space maintaining layer after curing has a glass transition temperature of higher than 125° C., and a coefficient of thermal expansion at 125° C. or lower of 20 ppm/K or less, and
• the space maintaining layer has a loss tangent tan δ1t of 0.1 or more and 1 or less, at a temperature t when the second circuit members are sealed.