CST – Computer Simulation Technology

Transient Simulation of a System-in-Package (SiP)

Courtesy and Permission of AET Japan, Inc.

This article summarizes the simulation of a System-in-Package (SiP) model using the CST MICROWAVE STUDIO® (CST MWS) Transient Solver to determine the S-Parameters, field distribution and system response when excited with 10 and 20 GHz pulses with additional noise signals. An analysis of the SiP with a board mounting and its effect on the resonant frequency is also performed.

Figure 1: CST MWS System-in-Package model with material definitions

The model, shown in figure 1 and imported from the CDS Cad design System, consists of copper (lossy metal), polyimide and silicon with bond wires and through vias. The model is initially simulated without the board mounting which will be introduced later in this article.

Figure 2: Definition of the discrete ports in the SiP model...

Figure 2 shows the discrete port assignment for the power supply pin (1) and the signal pins (2, 3, 4, 5).

Figure 3: S-Parameters of the unmounted System-in-Package simulation

Figure 3 shows the S-Parameters of the board without the mounting board. It can be seen that there is a resonance at 10.29 GHz and that the noise coupling factor, S3,1 , is about -8 dB.

Figure 4: Surface currents in the SiP at 10 GHz for Port 1 excitation - some materials have been hidden for clarity

As a result of the Time-Domain method, the field at any frequency can be extracted by definition of so-called field monitors. Figure 4 shows an animated field plot of the surface currents at 10 GHz as a function of phase. This allows the behavior of the SiP to be investigated at any user-defined frequencies. The electric field as well as the radiating field can also be visualized.

Figure 5: Board Mounted SiP Model

The next step in the simulation process of the SiP is the addition of the mounting board and the investigation of its effects on the S-Parameters. Figure 5 shows the model with the mounting board which consists of 3 copper layers and an FR4 substrate. Vias connect the SiP package to the lower copper grounding layer.

Figure 6: Comparison of S-parameters for mounted and unmounted SiP

The change in the S-Parameters between the mounted and unmounted SiP models is shown in Figure 6. The resonance, S1,1, has shifted from 10.29 GHz to a lower frequency of 7.7 GHz and becomes sharper and more singular in nature.

Figure 7: Electric field at a distance of 3 m for the mounted and unmounted SiP configurations ( dBuV/m )

The electric field was extracted for both simulations via so-called far field monitors which provide field values at user-specified reference distances from the structure. This means that in addition to Signal Integrity analyses, the EMC characteristics of the system-in-package can also be investigated. Figure 7 shows the increase in the field when the SiP is mounted on the board. This is due to the resonant and almost antenna-like behavior of the board in the farfield.

This article has briefly shown how the Time-Domain solver in CST MWS can be applied to the simulation of a System in Package (SiP) and provide broadband S-Parameters and field results at any user-defined frequencies. The E-Field monitors can provide frequency based data in just a single simulation.

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