High-Density Interconnection or HDI substrates are multi-layer, high-density circuits with features including the fine line and well-defined space patterns. Increasing adoption of HDI substrates enhances the overall functionality of PCBs and limit the operational area.
One of the key factors that differentiate HDI boards from other types of boards is their unique design, which includes multiple layers of copper filled micro-vias. These multi-layers of micro-vias enables vertical interconnections. Further, HDI substrates offer advantages including higher integration level and better placement of components on both sides. Additionally, HDI boards consist of a higher number of I/O in smaller geometries. Other features of HDI substrates include faster transmission of signals and a significant reduction in signal loss and crossing delays.
The recent technology adopted for the preparation of HDI boards deals with the miniaturization of components and the adoption of high-end devices. However, challenges such as crosstalk can severally affect the performance of HDI boards. Thus, it becomes critical to avoid crosstalk in HDI boards.
Crosstalk Generation in HDI PCB
An unintentional electromagnetic coupling among the traces and components is defined as crosstalk in electronic circuitry. Moreover, electromagnetic field disturbance can take place in PCBs due to outside interference. The crosstalk causes undesirable effects which effect clock, periodic signals, system critical nets like data lines, control signals and I/Os. Additionally, the affected clock and periodic signals create serious functionality concerns to the working PCB and assembly components. Crosstalk leads to capacitive and inductive coupling. Capacitive coupling in HDI substrates occurs when one of the traces lies on another trace.
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Methods to avoid Crosstalk
Crosstalk in HDI substrates is reduced by the shorter coupled lengths and by the lower dielectric constant by as much as 50 percent. Other factors that can limit crosstalk in HDI substrate include,
- Use of lower Dk materials.
- The lower dielectric constant of the HDI material system may allow a board to shrink up to 28 percent.
- The thinner the distance to the reference plane, the lower the near-end crosstalk will be.
HDI miniaturization provides shorter interconnect lengths, and if the lower dielectric constant material is used, then crosstalk in HDI substrates is reduced. Eric Bogatin, Signal Integrity Evangelist at Teledyne Lecroy, provides the following example: ”A typical line-width in HDI technology is 3 mils (75 microns). The figure below shows the characteristic impedance of 3-mil-wide traces for various dielectric thicknesses.
The dielectric thickness will be less for a lower dielectric constant. This means a lower dielectric constant material system will either result in less crosstalk for the same spacing, or the traces can be moved closer together and have the same amount of crosstalk.”
Eric Bogation continues, ”In the two cases studied, the line-width was 3 mil, and the dielectric thickness was adjusted so that for the two different dielectric constants, the line impedance was the same. From these curves, it can be seen that if the routing pitch is crosstalk constrained, just the lower dielectric constant of the HDI material system may allow a board to shrink up to 28 percent.
For coupled lengths less than the saturation length, the magnitude of the near-end voltage noise will scale with length. The saturation length will depend on the rise time. For a rise time of 1 nanosecond, the saturation length with an effective dielectric constant of 2.5 is about 7.6 inches, which would include many of the traces in a small card application. The relative coupled near-end noise would be given by:
Crosstalk in HDI substrates is reduced by the shorter coupled lengths and by the lower dielectric constant by as much as 50 percent. Shorter trace lengths will radiate less, and traces with thinner dielectric will radiate less. The example below shows that the shorter the coupled length, the less the mutual inductance (Lm), and the thinner the traces, the less the mutual capacitance (Cm).
Moreover, the thinner the distance to the reference plane, the lower the near-end crosstalk will be, or the same crosstalk for a longer coupled length. With length reductions of 2x and dielectric thickness reductions of 2x over conventional boards, the radiated field from HDI signal loops might be reduced by as much as 4x, which is 12 dB.”
Eric Bogatin further states, ”If the entire board is HDI, rather than just a few outer layers, controlling the return path can be a bigger challenge than in through-hole boards.”
Key Takeaways by Eric Bogatin:
“You have to pay attention to the same issues in HDI substrates:
- Providing a continuous return path.
- Engineering controlled impedance interconnects.
- Routing in a linear, daisy chain path with minimal stub lengths.
- Managing reflection noise with terminations.
- Controlling via to via crosstalk by return path control.
- Using low inductance capacitors connected to the IC pins.
In conjunction with a through-hole core, HDI interconnects can be incredibly valuable.”
Signal Integrity Challenges and Conclusion
We are well familiar with signal integrity and its role in determining the performance of a high-speed PCB design. Crosstalk is one of the critical parameters that affect signal integrity. Crosstalk can directly lead to distortion in the receiver signal. Thus, it should be a prime concern among designers to minimize the effect of crosstalk in HDI substrates.
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