Wiring board unit and design method therefor

The purpose of the present invention is to provide a wiring board unit in which internal stress is reduced to reduce susceptibility to the formation of cracks from a location where stress is concentrated. The present invention comprises a first wiring board and a second wiring board bonded to the first wiring board. Further, a semiconductor element is resin-sealed on the reverse surface of the second wiring board from the surface that is bonded to the first wiring board. Furthermore, tensile strength of an insulating resin material used in the second wiring board and the Cu pattern width on the aforementioned reverse surface from the first wiring board are configured such that the value of the numerical formula 1 below is less than 0.5. (1) ... 1/(1+Exp(-A)), wherein A = -15.45 - 0.1654 * tensile strength + 11.31 * log (Cu pattern width).