# Space-Borne System-in-Package Based on High Reliability Microwave Interconnections
## 2 Analysis of Traditional Interconnections in Space-Borne Microwave Systems  
The microwave transceiver is the core part of all kinds of space-borne microwave equipment and also the most complex circuit part. It can realize microwave signal reception, amplification, filtering, frequency conversion, and specific gain or temperature regulation functions. There are many functional components and circuits involved in the radio-frequency (RF) chain. Generally, microwave RF dies, which have excellent performance and are easy to integrate, are used. Once the airtight packaging fails to meet the reliability, tiny air molecules will diffuse into the package. The space-borne microwave SIP will be damaged in the ground test. The coaxial seal can not only realize the transmission of high frequency microwave signals, but also realize the airtightness due to the dielectric filling between the internal and external conductive layers. Therefore, the coaxial seal is usually used to realize the transmission of microwave signals between the cavities in the microwave package and the external environment [11; 12; 13].  
On the other hand, if the inner core of the microwave interconnected structure with the coaxial seal is soldered directly to the substrate, the transition structure will be invalidated due to thermal deformation because of a mismatch in thermal expansion coefficients between different materials and geometric position restrictions. As shown in Figure 1, the inner core of coaxial seal and the substrate are directly soldered as a whole and the thermal expansion coefficients between them are significantly different. However, there is no means to release deformation stress in the directly soldered interconnection structure. The temperature change of the package causes a large stress in the solder joint. After multiple temperature cycles, cracks and ultimate failure of the microwave interconnection can occur. The test temperature is \(-\)35-70 \({}^{\circ}\)C, and the rate of the temperature change is less than \(10\)\({}^{\circ}\)C/min. The limit temperature is maintained for at least 15 min, and a total of 200 cycles are conducted. Eventually, the cracks shown in Figure 2 will appear. Normally, direct current (DC) characteristics will not be affected, but its microwave and RF signals will be blocked.  
Figure 1: Directly soldered structure. (**a**) Photo of the directly soldered structure; (**b**) Model of the directly soldered structure.  
Three-dimensional electromagnetic simulation and mechanical analysis of the transition structure were carried out according to the actual design. The microwave transmission characteristics and the solder joint stresses were simulated. The basic structure can be abstracted as two uniform rods fixed together, one of which is fixed at only one end. The stress can be estimated according to the thermal expansion coefficient, elastic modulus, cross-section area, and the temperature change of each material. Because the actual structure is relatively complex, a simulation method was adopted. By establishing the same 3D model as the real structure, and giving mechanical parameters such as thermal expansion coefficient, tensile strength, and Young's modulus to each part of the structure as Table 1, the mechanical simulation analysis was carried out after the finite element grid was divided and the connection relationship was set. Taking the seal applied in an aerospace microwave package as an example, the dielectric diameter as D was 1.6 mm, and the inner core diameter as d was 0.3 mm. The materials used for packaging and interconnection structures are as follows:  
Various traditional interconnection structures are analyzed in detail, below.