# Analysis and Experimental Test of Electrical Characteristics on Bonding Wire
## 3 Frequency Domain Simulation  
### Influence of Flat Length Ratio on S Parameter of Bonding Wire  
S-loop wire is the most common bonding wire. Due to its flat length, S-loop wire can withstand the impact from the epoxy compound in plastic molding process and prevent the gold bonding wire from having a horizontal sweep and vertical sag problem. However, too long a length of flat length wire will bring serious signal crosstalk and coupling problems. According to the Kulicke and Soffa package model library in Cadence APD, WL_LH3_SPXX series of models were selected for modeling and simulation. The value after SP is the ratio of the flat length with span, that is, the value of L1/L (as shown in Figure 5). According to the standard, there are six sizes in total: 00, 12, 20, 40, 60 and 70.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **Material** & **Dielectric Constant** \\ \hline S-loop wire & Gold & 0.99996 \\ Microstrip & Copper & 0.99991 \\ Chip & Silicon & 11.9 \\ Substrate & Rogers RO4350B & 3.66 \\ Ground & Copper & 0.99991 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Model material parameters.  
Figure 4: HFSS simulation model.  
Figure 3: Single bonding wire simulation model.  
According to the geometric relationship shown in Figure 5, the parameters of the model were selected (L = 4500 \(\upmu\)m, H1 = 250 \(\upmu\)m, H2 = 254 \(\upmu\)m, d = 25 \(\upmu\)m). The ratios of the flat length to the span L are 0, 12%, 20%, 40%, 60% and 70%, respectively through changing the length of L1, while sweep frequency ranges from 1 MHz to 20 GHz. The results of S11 and S21 with the ratio and frequency of the bonding wire are shown in Figure 6a,b respectively.  
In order to get a clear graph and accurate data, the S11 log coordinate was set from 1 to 20 GHz. The return loss describes the signal reflection performance of the transmission line. The smaller return loss means less signal loss and better signal integrity performance. From Figure 6, it can be seen that the loss increases with the frequency. In addition, the SP12 bonding wire has a higher cutoff frequency, which demonstrates that it can work in a higher frequency band. Compared with SP70 bonding wire, the working frequency increases by 16% (0.64 GHz). Figure 7 shows S21 (insertion loss) parameters. The abscissa is set from 0.1 to 20 GHz. It can be found that the insertion loss also increases with frequency, which suggests the ratio of the signal power delivered to the terminal prior to insertion versus the signal power delivered to the terminal after insertion is constantly decreasing. The value of the minimum point of S21 and its corresponding frequency under the condition of different flat length are selected in Table 2. The minimum point indicates the signal transmission is the worst when the ratio of the passing and input signal reaches the minimum. It is easy to get from Table 2 that the minimum values of S21 in SP12 are larger than that in other cases. At 17.38 GHz, the modulus of S21 reaches 8.71, which increases by 13.9% compared with SP70. According to the definition of S21, the ratio of passing signal with incoming signal is 36.69% after calculation.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Ratio of Flat Length to Span** & **SP00** & **SP12** & **SP20** & **SP40** & **SP60** & **SP70** \\ \hline Min S21 (Insertion Loss)/dB & \(-9.10\) & \(-8.71\) & \(-9.34\) & \(-9.58\) & \(-9.82\) & \(-9.92\) \\ Corresponding frequency/GHz & 17.63 & 17.38 & 17.58 & 17.58 & 17.48 & 17.48 \\ Passing signal/incoming signal/\% & 35.07 & 36.69 & 34.12 & 33.19 & 32.28 & 31.92 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of S21 extremum under different flat length.  
Figure 5: Three S-loop wires with different flat lengths.  
Figure 6: (a) S11 of bonding wire with different flat length; (b) S21 of bonding wire with different flat length.