In today’s high-speed digital systems, the demand for advanced data processing and computing power has led to continuous improvements in chip technology. As chip process sizes shrink and operating frequencies rise, modern processors now operate at GHz levels. This results in shorter signal transition times and higher harmonic components, making the system a high-frequency, high-bandwidth environment. Both the PCB and the package (Pkg) can exhibit resonant frequencies within this range. If the Power Delivery System (PDS) is not properly designed, structural resonance may occur, leading to poor power quality and potential system failures.
With increasing component density, low-voltage, low-swing designs are commonly used to reduce power consumption. However, these low-voltage signals are more susceptible to noise. Common noise sources include coupling, crosstalk, and electromagnetic interference (EMI), with simultaneous switching noise (SSN) being the most significant. The PDS system includes both the circuit and the electromagnetic field formed by the power source and ground plane. Understanding how these elements interact is crucial for managing noise effectively.
When analyzing ground bounce noise (GBN), it's common to focus only on the PCB and measure its S-parameter |S21| as an indicator of GBN magnitude. However, GBN originates from the IC, passes through the Pkg power system, and then travels via solder balls and vias to the PCB. Therefore, a comprehensive analysis must consider both the Pkg and PCB together.
To study this interaction, we designed a PDS structure that represents the Pkg power system mounted on the PCB. Using a network analyzer (HP8510C) and a probe station (Microtech), we measured the S-parameters of this structure from 50 MHz to 5 GHz. Two 450 µm-pitch GS probes were connected to the power and ground rings of the Pkg signal layer. The measurement setup is shown in the diagram below.
The results of the Pkg+PCB structure showed significant differences in GBN behavior compared to standalone Pkg or PCB measurements. A single Pkg exhibited capacitive behavior up to 1.3 GHz, followed by a resonant mode above 1.5 GHz. A single PCB had resonant modes at 0.73 GHz, 0.92 GHz, and 1.17 GHz, resulting in worse GBN performance. When combined, the Pkg and PCB introduced additional resonance points before 1.5 GHz, indicating that PCB noise was coupled into the Pkg through solder balls and vias, worsening the overall noise situation.
Decoupling capacitors are traditionally used to suppress power plane noise. However, their placement, size, and number are often based on empirical rules. To evaluate the ideal location, we tested capacitors on the Pkg, PCB, or both. Results showed that adding capacitors reduced impedance and GBN, especially at lower frequencies. However, at higher frequencies, the effectiveness of the capacitors diminished due to their ESL limitations.
The ESR and ESL of decoupling capacitors also significantly impact their performance. Higher ESR tends to flatten the pole but reduces the resonance frequency, while higher ESL increases impedance after resonance, reducing noise suppression. Increasing the number of capacitors improves noise suppression, especially in the low-frequency range. Capacitor value selection should be based on the target noise frequency band, and mixing different capacitance values can broaden the noise suppression range but may introduce more resonance points.
Additionally, the thickness of the Pkg and PCB power layers affects the S-parameters. Thinner Pkg layers reduce noise coupling from the PCB, while PCB thickness has minimal impact at higher frequencies. The distance between the capacitor and the noise source also matters; placing capacitors closer reduces parasitic inductance, improving noise suppression.
In conclusion, effective noise suppression in high-speed digital systems requires a holistic approach. Placing decoupling capacitors on the Pkg, optimizing their ESR and ESL, increasing their number, selecting appropriate capacitance values, and controlling power plane thickness all contribute to better power integrity. Understanding how each factor influences the system ensures stable and reliable performance.
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