Flexible PCBs (flex/rigid-flex) make it possible to create a variety of products that require small form factors and light weight, such as wearable, mobile, military, and medical devices. As flexible PCB fabrication technology has matured in response to demands for smaller, lighter products, new design challenges have emerged. This paper discusses some of the key challenges to address and also introduces a new PCB design approach that enhances productivity through in-design inter-layer checks required to ensure correct-by-construction design.