High Density Interconnect, or HDI, has been the answer to double demand of higher functionality in smaller devices. But HDI raises its own problems, specifically with signal integrity, of which we will explore two in our article for today: crosstalk and impedance discontinuities.

Crosstalk

In modern HDI PCB layouts, crosstalk management is a primary design focus. Designers emphasize that HDI PCB routing requires strict adherence to spacing rules. Crosstalk is a problem that arises when electromagnetic fields generated by one trace interferes with another trace, reducing signal clarity, resulting in noise, and sometimes in timing errors. Th fine lines and their narrow spacing that define HDI PCB structures are also precisely what increases these problems. In modern HDI PCB layouts, crosstalk management is a primary design focus.

A high frequency, in a 5G module for example, worsens crosstalks, by which even slight interference can distort the data transfer process and increase big error rates. Remember to avoid these situations that inevitably give rise to crosstalk: long parallel routing of high-speed signals, inadequate spacing in fine-pitched layouts, and lack of proper isolation between differential pairs. 

Impedance Discontinuities 

Engineers working with HDI PCBs know these discontinuities must be carefully minimized. Controlled impedance is a condition for signals to travel predictably through a transmission line. In HDI PCB layouts, discontinuities occur when abrupt changes of geometric or material properties cause signal reflection.

-        Sudden changes in the diameters of microvias and stacked vias can cause localized impedance mismatches

-        Minor deviations of trace width and spacing cause amplified errors at higher frequencies

-        Inconsistencies with dielectric thickness, and shifts even at the micron-level can affect impedance stability

-        Changes in the dielectric constant or loss factor can also skew calculations

Unaddressed, these discontinuities manifest as reflections, insertion loss, and distorted eye diagrams.

Stack-Up Strategies 

A well-planned HDI PCB stack-up helps reduce both signal loss and warpage. A good stack-up strives for a balance between electrical performance and manufacturability:

-        Maintain consistent dielectric thickness in order to stabilize impedance

-        Avoid asymmetric copper weights that can cause warpage and distort impedance

-        Build up layers with resin-coated copper films in order to create fine-pitch interconnects

-        Place these ground and power planes with strategy, shielding more sensitive signals and so as to reduce electromagnetic interference.

Symmetrical stack-ups are often recommended for advanced packaging for the better mechanical reliability they provide that can reduce skew. 

Trace Routing Guidelines 

These routing rules are particularly important in dense HDI PCB designs.

-        The consecrated industry-wide practice is to always do trace spacing of three times the line width for standard signals, and five times for critical signals. These are also known as the 3W rule and the 5W rule

-        Tight coupling within differential pairs can guarantee noise immunity, but it is no less important to maintain adequate spacing from neighboring signals

-        On adjacent layers, orient signals perpendicular to each other in what is known as orthogonal routing. This is for the purpose of reducing broadside coupling

-        Grounded guard traces between high-speed signals can also reduce coupling in dense routing areas

Via Design 

Optimizing vias in HDI PCBs reduces inductance and enhances overall reliability. Robust via design practices are crucial when manufacturing HDI PCBs for telecom and aerospace applications. HDI PCBs are heavy on microvias and via-in-pad designs, which require careful optimization, as vias are themselves a source of impedance discontinuities:

-        Remove unused stubs by back-drilling in order to reduce reflections in through-hole vias

-        Use resin-filled vias to improve reliability and ensure planar surfaces for component mounting

-        Reinforce your stacked microvias when they are dense, again to improve reliability

-        Use Via-in-Pad Plated Over, or VIPPO, to reduce inductance and shorten signal paths, so as to improve performance under higher frequencies

Materials 

Selecting advanced materials is essential for stable HDI PCB performance. This choice of materials determines how well an HDI PCB performs under stress. We ourselves find standard FR-4 insufficient for the high speeds of HDI PCB due to its higher dielectric loss, over which we recommend these materials:

-        Low-Dk and low-Df laminates can provide stable impedance and also reduce signal loss

-        Polyimide systems offer high thermal stability, very valuable for aerospace and defense applications

-        Adhesiveless laminates can also minimize variability in dielectric thickness and thus improve consistency

-        Advanced resin systems are also great for applications where moisture absorption is going to be a problem

Simulations

Simulation tools make it possible to anticipate HDI PCB issues before production. Profit from the many simulation tools unavailable only so many years before, by which you can anticipate problems before moving onto the next step

-        Signal Integrity, or SI simulations can help you predict crosstalk and impedance mismatches

-        Time Domain Reflectometry, or TDR, can provide exact impedance profiles and detect discontinuities

-        Eye diagram analysis can reveal the cumulative effects of crosstalk, jitter, reflections, and other actions

Take advantage of these simulation tools before fabrication and then combine their results with testing done on samples to prepare your product for every combination of circumstances imaginable.

Designing for Manufacturability

Consistent communication with fabricators ensures HDI PCBs meet both cost and performance targets.

• Remember to confirm with your manufacturer your requirements for minimum trace and minimum space
• Keep your microvia aspect ratios within limits to avoid plating voids
• Dense layouts will inevitably reduce yields if tolerances were not realistic

Looking forward, HDI PCBs will remain a core enabler of miniaturization and high-speed functionality. HDI PCBs are the unique solution for the smaller and smaller devices that our times demand, but their signals are prey to the double threat of crosstalk Impedance Discontinuities. But these problems can be controlled with good design, and you can always refer to the guidelines above to keep your project safe.