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
- Single-ended via
- Differential via
- Signal via
- Ground via
Anatomy of via
Via pads, anti-pads, barrel, NCP …..
- Conductive tube filling the drilled hole
- 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
- Substrate thickness =100mil
- Substrate material =FR-4
- 6 metal layers
- Via barrel radius = 5 mil
- No via pads
- 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
- 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
- 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 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.