3D printed electronics introduces new opportunities in product design. On the other side, it also brings challenges regarding many aspects in the design and manufacturing process of prototypes/products. In this paper, based on our preliminary investigations, we summarize the opportunities of using 3D printed electronics in product design as: 1) it offers designers more freedom in their designs; 2) it promotes miniaturization of design; 3) it accelerates the design and manufacturing process; 4) it is able to improve the efficiency of producing customized/personalized mechatronic/electronic products and 5) it improves the sustainability in the product design and manufacturing process. Motivated by those opportunities, we conducted four case studies regarding four key aspects of 3D printed electronics: the conductive materials, geometric modelling, multiphysics simulation and manufacturing tools. Based on the findings in those case studies, we identified the challenges in 3D printed electronics and highlight the future works which may provide a better support to the needs of product designers.