Semiconductor chips, such as photosensor arrays in a full-width scanner, are mounted on printed wiring boards. The printed wiring boards are in turn mounted on a second layer of printed wiring board material. The two layers of printed wiring board material are attached so that the seams between adjacent printed wiring boards in each layer alternate in a brick-like fashion. This structure enables arrays of semiconductor chips to be constructed in relatively long lengths, with minimal risk of damage caused by thermal stresses.