Advances in chip technology have escalated I/O densities and complex interconnects, demanding careful consideration of signal integrity in high-speed PCB designs to prevent data loss and system errors. This study on Flip-chip packages explores the impact of data rate, vias, and material losses on system performance.
To contribute to this research effort, we took a flip-chip package design from a PhD thesis paper and simulated it using our electromagnetic virtual prototyping software, HFWorks. To study the intricacies of signal integrity in high-speed ICs, we investigated three design issues which are mainly high density of interconnects, vias, and material properties.
Today’s high-level functionality electronics stipulate the use of HDI PCBs that run on frequency of 10 GHz and higher. Boards have become more compact and tighter wiring density gives rise to the importance of crosstalk analysis.
Coupled Microstrip Lines
The coupled microstrip lines are simulated; the following results are obtained:
As illustrated in the two figures above, the near-end (NEXT)and far-end crosstalk (FEXT) are not major concerns at lower frequencies, however, they are considerably important factors at higher frequencies. By providing adequate spacing between traces, crosstalk noise is reduced. Therefore, it is crucial to consider a proper trace spacing in high-speed ICs to minimize signal distortion.
A via is one of the major discontinuities in high-speed ICs and can give rise to a reflection noise. The reflection noise increases time delay and produces overshoot, undershoot, and ringing.
A Package with Multiple Vias
We can notice from the above figure on the left that vias and anti-pads act as a low-pass filter for high-speed signals. Increasing the via diameter, i.e., decreasing the via-aspect ratio, has a little effect at lower frequencies and significant effect at higher frequencies. The via is one of the major impedance mismatches which can result in big reflections at high frequencies, as illustrated in the above figure on the right.
The electrical properties of dielectric materials used in packages and PCB can affect the electrical performance of digital interconnects. Using the right material helps reduce signal attenuation and maintain signal integrity of high-speed PCB.
Animation of Electric Field of a Package with Multiple Vias (10 GHz)
Insertion Loss for Different Substrate Materials
At frequencies up to 3GHz, there is almost no difference in the insertion loss between silicon, RO4350B and glass. However, as the frequency increases, the loss in silicon becomes significantly higher. In comparison between the dielectric materials, TGV (Through Glass Via) shows the best performance in terms of the insertion loss; hence, TGVs represent a promising technology for next generation 3D IC packaging and can take over the traditional TSV technology.
As per the eye diagram of the two receivers, the TSV attenuates the signal more.
