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

L_{MIN}=T_{R}*C_{Q}/2*
_{R}, BW=0.35/T_{R}

Example:
T_{R}=10_{ps} E_{R}=4-L_{MIN }=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=μ_{0}/2π
in(b/a)[H/M]

- Characteristic impedance

– Z_{0}=
=1/2π
_{0}/ԑ In (b/a)

- Phase constant

– β=w
=w
_{0}ԑ

- Propagation velocity

– V_{P}=ῳ/β=1
_{0}^{Z}=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

- F
_{resonance}=1.18*e^{9}/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 3
^{rd}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 Z
_{diff}=2*(z_{o}-∆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.