While it has been recognized that science, technology, engineering, and mathematics (STEM) education requires an interdisciplinary approach, integrating multiple subjects in a meaningful way remains challenging for teachers. This study aimed to design a STEM curriculum, emphasizing explicit and continuous scaffolding of students' reflection on scientific and engineering knowledge. The primary goal was to foster knowledge integration in their engineering designs and enhance their attitudes toward STEM. The study involved fifty tenth-grade students who were guided to discuss and reflect on relevant scientific and engineering knowledge and to apply mathematics for data collection and analysis during the design of their technology products. The research instruments included an assessment of the progression of knowledge integration in students' engineering designs through student journals and pre- and post-test surveys on attitudes toward science, technology, engineering, and the learning environment. The results reveal that the introduction and explicit scaffolding students' reflection on scientific and engineering knowledge led to a gradual improvement in knowledge integration within their engineering designs. Students also significantly enhanced their attitudes toward STEM and the learning environment compared to the general school curriculum. This study contributes to interdisciplinary learning that promotes the integration of scientific and engineering knowledge in students' engineering design processes, and to interdisciplinary assessment that evaluates students' knowledge integration across learning progressions and outcomes.