What is Via in High-Speed PCB Design?

In this tutorial we are going to learn about What is Via in High-Speed PCB Design?

What is via?

Vertical interconnect Access (VIA)

  • An electrical connection between layers to pass a signal from one layer to the other.
  • Single layer designs do not require vias and these are only for multi-layer PCBs or packages to route signals

Why Use Via?

  • Via generate discontinuity from the signal transition and significantly affect signal and power integrity in high speed designs  
  • Parasitic capacitance o via can increase signal rise time , making the signal speed slower
  • Designers should maintain a good impedance transition

Types of VIA

PTH, Blind, buried, and μ-vias

-By physical implementation….

  • PTH (plated through hole) via
  • Blind via
  • Buried via
  • μ-via (laser via), via-in-pad

-By signally

  • Single-ended via
  •   Differential via
  •   Signal via
  •   Ground via

Anatomy of via

Via pads, anti-pads, barrel, NCP …..

    Via barrel

  • Conductive tube filling the drilled hole

Via pads

  • Connects every end of the barrel to the component, plane, or trace.

Via anti-pads (or clearance)

  • Clearance hole between Metal layer & barrel  to which it is not connected

Non-functional via pads

  • Internal or external pads that are not connected to any traces or components

Return current path for Vias

  • The return current must find a path to return to the source
  •   At high frequencies, return currents will favor the path (s) of least impedance
  • Due to the skin effect, the current flows along the metal surface, not penetrating through
  • The closer the ground via to the signal via, the smaller the inductance of via is

Electrical model for vias

  • There should be not  simple via model for multi-gigabit signals or higher frequencies
  • Lumped or distributed
  • Is the via inductive, capacitive or something else?
  • For example, can we say that the impedance of three cases below are the same?
  • The answer is “it depends”

The return path affects the impedance characteristic of via

Via Impedance By Various Return Path

Simple single ended via w/o pads case

Test structure

  • Substrate thickness =100mil
  • Substrate material =FR-4
  • 6 metal layers
  • Via barrel radius = 5 mil
  • No via pads

                 Tested for:

  • Various sizes of via anti-pads
  • Various number of ground vias

 TDR (t) or impedance, Z(f)

  •  TDR (Time Domain Reflectometry)
  • Plots impedance vs. time or distance
  • The lower resolution depends on the rise time of step signal or signal bandwidth

   LMIN=TR*CQ/2* R, BW=0.35/TR

          Example: TR=10ps ER=4-LMIN =0.75mm or 29.53mil

  • Therefore , it is challenging to use TDR for the via itself since the feature size of via is typically very small impedance z
  • Plots impedance vs. frequency
  • Shows frequency dependent impedance characteristic

        As a coaxial transmission line

  • In coaxial transmission lines, the electric and magnetic fields are transverse to the direction of propagation, which makes TEM wave propagation
  • Capacitance per unit length

– C=2πl/in(b/a) in(b/a) [F/M]

  • Inductance per unit length

 –  L=μ0/2π in(b/a)[H/M]

  • Characteristic impedance

    –  Z0= =1/2π 0/ԑ In (b/a)

  • Phase constant

   –   β=w =w 0ԑ

  • Propagation velocity

–       VP=ῳ/β=1 0Z=C 1

Transmission Line Routing With Via

        Micro strip input +Via +micro strip output

  • The impedance of the full wave EM result(blue) is not the same as the cascaded impedance (red) of three sections, “micro strip+ via+ micro strip “
  • This is due to the dangling tail edge of the micro strip transmission line close to the via , which adds inductance to the impedance

       How Much of Inductance

  • Estimating Micro strip to via transition
  • The inductance value can be simply extracted by matching the impedance between full wave and cascaded results with additional inductors
  • The estimated inductance for the transition is 0.22nh in this case
  • This inductance can be compensated by decreasing the via anti-pads or increasing via pads

Improving impedance match

  • With Smaller Via Anti-Pads
  • Smaller radius should be  the via anti –pad could improve the impedance match performance by providing a little more capacitance
  •  However , the impedance variation (profile) is complex behavior ; so many other possible approaches may be available –for instance , a larger via pad

Via Stub

Stub Resonance

  • A dangling via stub acts as acts as a stub resonator, similar to a series LC resonator
  • At a quarter wavelength , the impedance turns into short impedance ;therefore , the insertion loss at that frequency
  • Depending on the Q value , the loss at other frequencies can be significant as shown in the DB (s21)plot

         Stub Resonance vs. Stub Length

  • The resonance frequency varies with the length of the Stub
  • By making the stub length shorter (movie the strip layer down), the stub resonance frequency can be pushed up to a higher frequency

    Estimating Via Stub Resonance Frequency

First order approximation formula

  • Fresonance=1.18*e9/4*stub length * R[Stub length in inch unit]
  • Example :93mil stub – 14.79GHz 

    Back Stub

  • The via stub resonance can be removed or pushed up to a higher frequency by back-drilling the via.
  • The stub resonance at 15GHz with 3rd layer strip line case is completely removed by the back-drilling.

    Differential Via

Differential signaling Via

  • Two single-ended vias used for differential signaling.
  • The minimum via patch size is determined by the manufacturing specification.
  • The coupling (overlap of E,H field lines) changes the differential impedance Zdiff =2*(zo-∆z)
  • The larger the coupling (light coupling)is , the lower the differential impedance is

Differential via crosstalk

  • Tight coupling vs. loose coupling
  • If have Tight coupling  then there will be  less area , but with a little higher loss
  • Tight coupling is also better for crosstalk performance
  • Tight coupling is minimum  sensitive to common signal noise

Main Point of Via:

  • It should be prefer vias as coaxial transmission lines to maintain good impedance match.
  • Via stubs act as a series LC resonator , adding significant loss to channels, that can be minimized by back-drilling the via stubs
  • Differential vias are used for differential signaling with a tight coupling more favored.
  • Tools for modeling vias:
  • ADS: design environment for high speed PCB analysis
  • Translate/convolution & s-parameter circuit simulators
  • FEM: finite element method.
  • Momentum: 3D planner EM simulator.
  • Via designer : new utility in ADS 2017 (coming this summer)
  • EMPro:3D modelling and EM simulation environment. 
  • FEM: Finite element method EM simulator.
  • FDTD: Finite difference time domain EM simulator.

About EEE

We have designing Experience for the last 40 years.

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