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As the appearance of the printed circuit board (PCB) in the 1960s and the development of polymer materials, more and more attention have been paid on the investigation of applying the polymer material as a base material to process circuit board. In 1983, US scientific community proposed the concept and processing method of molded circuit board (MCB), which is based on the combination of injection molding process and circuit printing process. With the birth of MID, it represents a new concept and process of mechanical and electrical integration. Specifically, MID means to make the wires with electrical function and install the electronic components on the surface of injection molded polymer base structure, then integrate the electrical interconnection and bearing component functions of common circuit board and the support and protection functions of polymer molded base structure as a whole, eventually form a 3D structural circuit board—molded interconnect devices (MID) (Glendennin, et al., 2011; Islam, et al., 2009). Compared with the traditional electromechanical system, MID is a new integration technology of electromechanical system and has some unique advantages, such as to realize the mechanical support function using the base structure of 2D or 3D device, simultaneously to complete the electrical connection and information transmission on the surface of the structure profile, therefore, to save the space occupied by devices, improve the integrated level of system, simplify the assembly process as well as lower the product cost. Fig. 1 gives the steering wheel of a luxury car branded by Volkswagen (VW), using MID technology, the cables and buttons in traditional steering wheel have been processed on the framework of the injection molded steering wheel, which saves space occupied by system, enhances the integration level, shortens the process time and lowers the cost (Islam, et al., 2009; Hueske, et al., 2001; Heineger, et al., 2004).
Figure 1. Two-shot injection molding process
The study of MID technology mainly concentrates in Europe, especially in Germany. Institute of manufacturing automation and production system in University of Erlangen Nuremberg is the biggest MID technology research institution all over the world, which has made remarkable achievements through decades of exploration and research. They Cooperated with LPKF company to develop laser direct structuring technology and corresponding equipments, then successfully brought the MID products and equipments to the market, besides that, they also conducted a thorough investigation on the welding technique and process equipment of electronic components in 3D-MID device (Kilian, et al., 2008; Worgull, 2009; Khuntontong, et al., 2009). HSG-IMAT institute in Germany has developed many kinds of MID devices based on laser direct structuring technology, two-shot injection molding technology and hot embossing technology during the past 10 years, based on which, they conducted an in-depth research on the conceptual and mold design of MID products, as well as assembly, modeling, reliability, quality test and application in microsystem of the chips and electronic components (Schomburg, et al., 2011; Khuntontong, 2008). Aminul Islam research group in Technical University of Denmark has been focusing on the two-shot injection molding technology applied in MID for many years. They applied two different kinds of polymer material, which can be electroless copper plated and cannot be electroless copper plated, designed two-shot mold according to the device and circuit wiring requirements, processed base structures of the devices by two-shot injection molding method, then conducted electroless copper plating on the surface of bare polymer devices which can be electroless copper plated, finally, obtained MID. Using this technology, a variety of micro MID have been processed successfully, for example, micro switch, miniature antenna, hearing aid, etc (Becker, et al., 2005). Erwin Quarder Werkzeugbau Company in Ace Peckham & City of Germany developed the embedded “flexible” thin-film-cladding molding technology by improving injection molding machine and mold (Schomburg, et al., 2011).