# Accelerated Testing and Reliability of FDM-Based Structural Electronics
## 1 Introduction  
Electronics development is currently focused not only on increasing efficiency and performance but on inventing new forms and functionalities. Today, electronics do not have to be bulky, rigid plates but often become flexible [1], soft [2; 3], injectable [3], or even invisible to human eyes [4]. Another unconventional form of electronics is structural electronics (SE), in which electronic circuits and components are embedded in device structures such as frames or housing [5]. Thanks to placing components in three dimensions, this approach can reduce device size by increasing design complexity [6], as shown in Figure 1. Miniaturization can reduce manufacturing cost, and this effect can be amplified by adapting SE through a reduction the number of parts and manufacturing steps of a given device. Overall, some structural electronics technologies in specific applications can save up to 70% of mass and 50% of cost [7]. Structural electronics can be based on 3D additive manufacturing, which allows customer-tailored and agile production. As a result, industries such as aerospace, automotive, and production lines are interested in using SE. Structural electronics, thanks to the elimination of tool cost, is a promising technology for electronics prototyping and small scale production. However, for this technology to become widespread, especially in the aforementioned sectors, it is necessary to determine the reliability of devices constructed with it. Reliability has always been an important aspect of electronics. One of the many advantages of the first transistors was their much longer lifetime than that of tubes [8]. It is still relevant now because electronic systems consist of hundreds or thousands of components and the failure of any one of them can damage the entire device and generate maintenance costs [9]. It has to be remembered that for electronics devices, even 95% of life cycle costs can be generated after the design phase [10], which shows the importance of reliability-oriented designing and the selection of suitable components. Currently, electronic modules are not only widely available consumer electronics (like smartphones, TVs, computers, etc. ), butan essential part of most modern devices. For example, in the automotive industry, since the 2000s, electronics have accounted for ca. 25% of manufacturing costs [11]. Furthermore, electronic sensors are often parts of safety systems, which has led to the zero-defect concept [12]. These requirements have led to a situation in which reliability is not an auxiliary property of a given device but one of the main requirements or even objectives of the design stage of a given electronic system. To accurately estimate the reliability of a given system, besides data on its components (such as mean time between failures), it is crucial to know the mechanisms of failure of the entire device. The estimation of failure mechanisms that occur in structural electronics is a complex issue. These devices combine two functions (mechanical and electronic), so it is necessary to consider both groups. The typical failure, well known from engineering practice, in electrical systems is a capacitor defect. It is true that these components are responsible for 30% of electronics failures, but the proportion of damages caused by printed circuit boards (PCBs) is similar, and together with solder-based connections, they account for 39% of failures in electronics [12]. In structural electronics, standard components such as resistors, capacitors, microchips etc. are also used. The most significant difference between PCBs and SE is observed for paths and joints (which may be uniform in SE systems), so this work will focus on them. One common PCB failure is delamination of the copper foil from polymer substrate. It may be caused by moisture entering the PCB and expanding during the soldering process or operation of electronics at high temperatures.