## 1 Introduction
### Literature Review
On the other hand, the backplane board (BPB) approach uses a common interface board, where all the other internal PCBs are connected to a motherboard, as shown in Figure 1.  
In terms of mechanical support, similar to the PC/104, four long stacking rods connect all of the individual PCB boards once electrical connection has been established with the connectors on the BPB. Cylindrical spacers are inserted into the rods between each consecutive PCB stack to constrain their position, as shown in Figure 2. Recently, several universities and some commercial vendors have increasingly adopted this interface approach, as it provides better flexibility in electrical connections. For instance, the University of Wurzburg in Germany developed a UWE-4 satellite in collaboration with other institutions that implements a backplane board electrical interface [17]. The backplane board type of interface is also the currently preferred approach used on most of the satellite missions at the Kyushu Institute of Technology (Kyutech) due to its high modularity [16]. So far, 19 CubeSats have been built based on the backplane style and 16 satellites have already been launched. However, from a fast delivery and quick assembly point of view, both electrical interface approaches with a stacking-type mechanical interface still pose some fundamental problems, mostly due to the mounting method of internal subsystems using long stacking rods. The method of inserting these long screws through the PCBs and the structure generally requires some level of effort for maintaining the alignment in this conventional type mechanical interface. The placement of the spacers between the PCBs particularly is often a challenging task during assembly, due to the required simultaneous task of inserting the stacking rods and placing the spacers, aligned with the holes in the PCBs, not to mention the substantial number of structural parts utilized, which consumes time during assembly. This potentially introduce workmanship errors to the assembly process,  
Figure 1: BIRDS satellite BPB interface [16]. Figure 2: Stacking of PCBs with long rods. damaging the connector pins, etc. In addition, the additional frames used to connect these stacking rods to the structural frames, as shown in Figure 3, increase the total part count of the structure. Both the above electrical interface approaches characterize the most conventional CubeSat interfaces. Several design solutions have been proposed to address the issues of flexibility and modularity. This study investigates similar design concepts available and tries to analyze their feasibility for the assumed applications. A commercial space company developed a card-slot-type structural design in which individual subsystems are inserted into defined slots using special spacers holding the internal subsystems. An example can be seen on the company's website [14]. This design concept can solve the above issue of interdependency between subsystems during integration. However, the retractable PCB holder/spacer used to prevent the direct contact of the subsystems with the main structural frames increases the total structural part count and possibly the complexity during assembly. Another study conducted by Istanbul Technical University for the PSAT-II mission [18] considered a modular 3U structure design with evenly spaced slit features on the interior faces of the four main frames to mount the internal subsystems. The PCBs were inserted into these tacks. The design in the study aimed to increase flexibility in rearranging internal subsystems, without the need to change the design of the structure, and to demonstrate it on a standardized bus in orbit. However, despite its concept of modularity, the study did not clearly show the electrical interface methods, nor the mechanical interface used to mount the internal PCB into the slots. insertion of PCBs directly into the given slits could potentially damage the PCB as well as the sensitive components, especially in the launch environment.