# Space-Borne System-in-Package Based on High Reliability Microwave Interconnections
## 3 A Novel High Reliability Interconnection Structure  
A highly reliable packaging method for space-borne microwave systems is proposed. The metal airtight packaging shell is used, and a new type of horizontal and vertical transition structure is adopted. This structure not only guarantees high reliability, but also achieves good microwave matching in the Ka and above bands, and also avoids the inner core shielding of the microwave dielectric substrate in the vertical direction. The proposed novel microwave horizontal transmission structure is based on the air coaxial structure with a gradual opening. Specifically, the air coaxial structure with a gradual opening is arranged at the end of the inner core, protruding towards the planar  
Figure 8: The photo and test results of the gold-ribbon wrap bonding with improvement. (**a**) Photo of the wrap bonding with improvement; (**b**) Reflection loss of the wrap bonding with improvement. Figure 7: Improving design for gold-ribbon wrap bonding. Figure 9: Diagrammatic sketch of assembly blocks. microstrip circuit. The coaxial structure takes the inner core of the seal as the axis, and air as the coaxial dielectric. The air cylinder is cut along the vertex at one end to the center of the circle at the other end to form a gradually opening hollow cylinder. The cutting surface is from the edge of the circular cavity to the diameter of the other end, so as to form a gradual opening on the circular cavity. It makes the complete circular cavity gradually change into the missing semicircular cavity in the upper half, and the missing upper half of the semicircular cavity is an opening structure, which provides an operating space for bonding. One end of the inner core of the seal is exposed to the upper semicircle surface. After bonding, the microstrip circuit is connected to realize the reliable transmission between the inner core and the planar circuit, as shown in Figure 10.  
The design uses a gradient approach to transition the coaxial TEM mode to the quasi-TEM mode of the microstrip line (shown in Figure 11) without a height drop, so that the matching transition from coaxial structure to the microstrip circuit can be achieved in a wide bandwidth [19]. The method ensures that the inner core of the coaxial seal protruding to the interior does not block the microstrip substrate and can be softly connected together through gold or copper ribbon. The design achieves highly reliable interconnection with excellent electromagnetic transmission performance and an efficient assembly process at the same time. Figure 11: Transition from coaxial TEM mode to microstrip quasi TEM mode. Figure 10: Structure diagram of the novel horizontal interconnection. The structure is mainly divided into four parts: coaxial seal, gradually opening air coaxial structure, gold ribbon crossing gap, and planar microstrip line, as shown in Figure 12. \(D\) is the dielectric diameter of the coaxial seal, \(d\) is the diameter of the inner core of the coaxial seal, \(dd\) is the diameter of the air cylinder, \(L_{1}\) is the length of the coaxial dielectric, \(L_{2}\) is the air coaxial length, \(del\) is the gap width between the dielectric substrate and the package wall, the thickness of the microwave dielectric substrate is \(h\), the width of the microstrip line is \(w\), the arch height of the gold ribbon is \(h_{b}\), and the width of the gold ribbon is \(W_{b}\). The dielectric constant of the coaxial seal is \(\varepsilon_{r1}\), the dielectric constant of the microwave dielectric substrate is \(\varepsilon_{r2}\), and the dielectric constant and magnetic permeability of air are approximate to those of vacuum, \(\varepsilon_{0}\) and \(\mu_{0}\), respectively. The equivalent transmission line theory [19; 20] is used to analyze the microwave signal transmission. Impedance matching is required at the connection of each part to ensure good microwave transmission performance. Therefore, the equivalent characteristic impedance of the four parts can be calculated.