Analysis and Design of Broadband Matching Interconnection of Millimeter-Wave Microstrip Bonding Wire

Gold wire bonding interconnection technology is widely used in microwave integrated circuits (MICs) and monolithic integrated microwave circuits (MMICs). It can be used to connect solid-state devices to passive circuits, and can also be used to connect chips. However, the use of this interconnection method in the high-end of millimeter waves is subject to certain restrictions. This is because the distribution current of the microstrip line is affected at the connection between the gold wire and the microstrip line, showing the characteristics of inductance. In order to solve this problem, people have proposed flip chips and some other electromagnetic coupling technologies. However, the gold wire bonding interconnection structure is still widely used, mainly because it has the advantages of simple process, low cost, and small thermal expansion coefficient, which is of great significance in applications with high requirements, especially in space systems. Practice has proved that as long as a suitable matching network is designed, the gold wire bonding interconnection structure can achieve good matching performance.

The accurate bonding parameter model can be obtained by adopting the method of electromagnetic field numerical calculation. The commonly used methods include the time-domain finite difference method based on full-wave analysis and the quasi-static analysis method. The fifth-order low-pass filter formed by bonding wires is used to connect two glass chips. Although it can achieve a very wide-band matching connection, it has more stringent requirements on the chips. In this paper, for the broadband matching interconnection of millimeter waves, the method of adding microstrip tuning branches is adopted to analyze and design the broadband matching interconnection of microstrip bonding wires suitable for V and E bands, and the influence of the thickness, number, spacing of bonding wires and microstrip tuning branch lines on the reflection and transmission characteristics of millimeter waves is compared and analyzed.

2 Bonding wire interconnection model and its equivalent circuit

The bonding wire interconnection model with microstrip adjustment branches studied in this paper is shown in Figure 1. Two microstrips with fixed width and gap are connected by gold wires on a Rogers 5880 dielectric substrate with a thickness of 0.127mm. In order to adjust the reactance effect of the bonding wire, a microstrip branch adjustment structure is added. The equivalent circuit of the entire interconnection structure is shown in Figure 2:

(a) Single gold wire

(b) Double gold wire

Figure 1. Gold wire interconnect model with microstrip stubs

Figure 2 Equivalent circuit of the gold bonding wire interconnection model

3. Simulation and analysis of bonding wire interconnection model

3.1 The influence of the number, thickness, spacing and branch lines of gold wires on the reflection characteristics

HFSS was used to calculate the reflection characteristic curves of different gold wire thicknesses, numbers, and spacings. The results are shown in Figures 3 to 5. Figure 3 shows the reflection coefficient frequency response curves of three gold wire diameters in a single gold wire structure. It can be seen that the thicker the gold wire, the smaller the reflection coefficient. Therefore, we should choose a thicker gold wire within the optional range. Figure 4 shows the influence of the spacing between the two gold wires on the reflection coefficient. It can be seen that the larger the spacing, the smaller the reflection parameter. This is because when the horizontal distance between the two wires is close to the line width of the microstrip, the equivalent series inductance can be reduced. Figure 5 shows the change of S11 with the number of bonding gold wires. It can be seen from the figure that the transmission characteristics of the double gold wire are obviously better than those of the single gold wire, but it is not the case that the more gold wires, the better. The transmission characteristics of three gold wires, four gold wires, and five gold wires become worse as the number of gold wires increases.

We introduce microstrip tuning branch lines to tune the inductance characteristics brought by the gold wire. Figures 6 and 7 show the influence curves of microstrip length and width on reflection characteristics. Adjusting the length and width of the microstrip can produce better reflection characteristics in the required frequency band.

Figure 3 Effect of gold wire diameter on S11 (single gold wire)

Figure 4 Effect of gold wire spacing on S11 (double gold wire)

Figure 5 Effect of the number of gold wires on S11

Figure 6 Effect of tuned microstrip branch line length on S11

Figure 7 Effect of tuned microstrip branch line width on S11

3.2 Optimization results and their main structural parameters

For the double gold wire structure shown in Figure 2(b), after investigating the influence of each main parameter on the transmission characteristics, we obtained a better output S parameter characteristic curve after optimization as shown in Figure 8. It can be seen that the insertion loss simulation results of the microstrip bonded gold wire interconnection structure in the frequency range of 50GHz-80GHz are all less than 0.65dB. At this time, the main structural parameters are shown in Table 1:

Table 1 Main structural parameters of double gold wire

Figure 8 Optimized S parameter characteristic curve

4 Conclusion

This paper analyzes and designs a millimeter-wave gold wire interconnect structure with microstrip tuning branch lines. At the same time, we also examine the thickness, number, spacing of the gold wires and the influence of the tuning microstrip branch lines on the reflection parameters. By optimizing the structural parameters, the insertion loss simulation results of the gold wire interconnect structure in the frequency range of 50GHz-80GHz are all less than 0.65dB, which is suitable for broadband matching interconnects in the V and E bands.