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name: matlab-model-via description: "Via modeling: pads, antipads, ground return vias, GRV placement, and signal integrity for high-speed layer transitions. TRIGGER: user asks to model a via, design a via transition, place ground return vias, analyze via performance, or check signal integrity through layer transitions. Invoke BEFORE writing code — only viaSingleEnded exists (no viaDifferential), and the location format is non-obvious. SKIP: general signal integrity without vias (use matlab-analyze-em), transmission line design (use matlab-design-pcb-transmission-line), PDN analysis (use matlab-analyze-pcb-pdn), material/stackup setup only (use matlab-manage-pcb-material)." license: https://www.mathworks.com/content/dam/mathworks/license/pmrl/license.md metadata: author: MathWorks version: "1.0"

Modeling Vias

When to Use

• Modeling signal vias through multi-layer PCB stackups (viaSingleEnded)
• Configuring pad and antipad geometry per layer
• Placing ground return vias for signal integrity
• Analyzing via S-parameters and identifying barrel resonances
• Evaluating GRV placement with criticalwavelength and gapratedistance

When NOT to Use

• Designing transmission lines or impedance control — use matlab-design-pcb-transmission-line
• Building custom PCB structures from shapes — use matlab-assemble-pcb-layout
• Setting up substrate or conductor materials — use matlab-manage-pcb-material
• Running full-wave EM analysis on non-via structures — use matlab-analyze-em
• PDN analysis on imported boards — use matlab-analyze-pcb-pdn

Typical Workflow

1. Before: matlab-manage-pcb-material — set up substrate and conductor for the stackup
2. This skill: Model the via, place GRVs, check signal integrity
3. After: matlab-integrate-pcb-circuit — cascade via model with trace/connector models → matlab-optimize-pcb-design — optimize GRV placement or pad geometry

Quick Reference

| Task | Code | |------|------| | Create via | via = viaSingleEnded | | Set signal layers | via.SignalLayer = [1 5] | | Set ground layers | via.GroundLayer = [3 7 9] | | Set substrate | via.Substrate = dielectric(...) | | Place signal via | via.SignalViaLocations = [x y startLayer stopLayer] | | Place ground return vias | via.GroundReturnViaLocations = [x y start stop; ...] | | Define ports | via.SignalTable = {viaNum layer traceWidth direction; ...} | | View pads | padsTable(via) | | View antipads | antipadsTable(via) | | Customize pads | pads = getpads(via); pads{k}.Radius = r; via.SignalViaPad = pads | | Customize antipads | ap = getantipads(via); ap{k}.Radius = r; via.SignalViaAntipad = ap | | Visualize | show(via) | | S-parameters | sp = sparameters(via, freq) | | Resonance risk check | cw = criticalwavelength(via, freq) | | Max GRV distance | gapratedistance(via, freq) |

viaSingleEnded Object

The viaSingleEnded object models signal vias through multi-layer PCB stackups with configurable pads, antipads, and ground return vias.

Basic Setup

via = viaSingleEnded;
via.SignalLayer = [1 5];
via.GroundLayer = [3 7 9];
via.Substrate = dielectric("Name","FR4","EpsilonR",4.8,...
    "LossTangent",0.026,"Thickness",1.27e-4,"Frequency",1e8);
via.Conductor = metal("Copper");
show(via);

Key Properties

| Property | Format | Description | |----------|--------|-------------| | SignalLayer | Vector | Layer indices where signal traces connect (odd numbers) | | GroundLayer | Vector | Layer indices for ground/return planes | | Substrate | dielectric | Dielectric between layers | | Conductor | metal | Via barrel and pad conductor | | SignalViaLocations | N×4 matrix | [x, y, startLayer, stopLayer] per via | | SignalViaDiameter | Scalar (m) | Via barrel diameter | | SignalViaFinishedDiameter | Scalar (m) | Finished hole diameter (after plating) | | SignalViaPad | Shape or cell | Pad shape(s) on signal via | | SignalViaAntipad | Shape or cell | Antipad (clearance) shape(s) | | RemoveUnusedPads | Logical | Remove pads on non-signal layers (default: true) | | GroundReturnViaLocations | M×4 matrix | [x, y, startLayer, stopLayer] per GRV | | GroundReturnViaDiameter | Scalar or vector | GRV barrel diameter | | GroundReturnViaFinishedDiameter | Scalar or vector | GRV finished hole diameter | | SignalTable | Cell array | Port definitions: {viaNum, layer, traceWidth, direction} |

Signal Via Configuration

Placing a Signal Via

via = viaSingleEnded;
via.SignalLayer = [1 5];
via.GroundLayer = [3 7 9];

X = 0; Y = 0;
startLayer = 1;
stopLayer = 7;
via.SignalViaLocations = [X Y startLayer stopLayer];

The via barrel spans from startLayer to stopLayer. Signal connections occur on layers listed in SignalLayer that fall within this range.

Via Diameter

via.SignalViaDiameter = 0.25e-3;          % Drill diameter
via.SignalViaFinishedDiameter = 0.20e-3;  % After copper plating

Unit Conversion Helper

The object provides a built-in mils-to-meters conversion:

via.SignalViaDiameter = 10 * via.mils2meters;   % 10 mils

Port Definition (SignalTable)

The SignalTable property defines which layers become ports for S-parameter extraction.

Format

Each row: {signalViaNumber, layerIndex, traceWidth, direction}

• signalViaNumber: Which signal via (1-based index into SignalViaLocations)
• layerIndex: The layer where the port is placed
• traceWidth: Width of the connecting trace (meters)
• direction: Angle in degrees (0 = +x direction, 90 = +y, etc.)

Example: Two-Port Via

via.SignalTable = {1, 1, 3e-4, 45;   % Port 1: via 1, layer 1, 0.3mm trace, 45 deg
                   1, 5, 3e-4, 0};    % Port 2: via 1, layer 5, 0.3mm trace, 0 deg

Using Table Syntax for Clarity

vPorts = table('Size', [2 4], ...
    'VariableTypes', ["cell","cell","cell","cell"], ...
    'VariableNames', ["Signal Via Num","Layer","Trace Width","Direction"]);
vPorts(1,:) = {{1} {1} {3e-4} {45}};
vPorts(2,:) = {{1} {5} {3e-4} {0}};
via.SignalTable = vPorts.Variables;

Ground Return Vias

Ground return vias provide a low-impedance return path near the signal via, critical for signal integrity.

Placement

via.GroundReturnViaLocations = [
    1.0015  2.001  1  9;
    1.0015  1.999  1  9;
    0.999   1.999  1  9;
    1.000   2.0015 1  9;
    0.999   2.001  1  9;
    1.0001  1.9987 1  9];

Each row: [x, y, startLayer, stopLayer]. Ground return vias typically span from the topmost to the bottommost ground layer.

GRV Diameter

via.GroundReturnViaDiameter = 0.25e-3;
via.GroundReturnViaFinishedDiameter = 0.20e-3;

These can be scalars (same for all GRVs) or vectors (one per GRV).

Pad and Antipad Customization

Viewing Pad/Antipad Tables

padsTable(via)       % Rows = signal vias, Columns = conductive layers; cells are pad shape objects (e.g. antenna.Circle) or []. Scalar SignalViaPad is expanded across all positions.
antipadsTable(via)            % Signal via antipads: rows = signal vias, columns = connected layers
antipadsTable(via, "ground")  % Ground return via antipads: rows = GRVs, columns = power planes

Default Pad Shape

By default, SignalViaPad is a circle:

via.SignalViaPad.Radius = 3e-4;    % Set pad radius

Per-Layer Pad Customization

Use getpads to get a cell array of pad shapes (one per layer), modify individual entries, then reassign:

pad_temp = getpads(via);
pad_temp{3}.Radius = 4.5e-4;   % Customize pad on the 3rd layer
via.SignalViaPad = pad_temp;

Antipad Customization

via.SignalViaAntipad.Radius = 5e-4;   % Clearance radius on ground layers

% Per-layer antipad customization (cell array: SignalViaLocations × GroundLayer)
antipad_temp = getantipads(via);
antipad_temp{1,2}.Radius = 6e-4;   % Customize antipad on 2nd ground layer
via.SignalViaAntipad = antipad_temp;

RemoveUnusedPads

When RemoveUnusedPads = true (default), pads are only placed on layers in SignalLayer. Set to false to place pads on all layers the via passes through:

via.RemoveUnusedPads = false;
padsTable(via)   % Now shows pads on all layers

Via Arrays

For modeling multiple signal vias (e.g., BGA breakout or bus routing):

Grid-Based Via Array

obj = viaSingleEnded;
obj.SignalLayer = [1 3];
obj.GroundLayer = [1 3];   % Mixed signal/ground layers
obj.Conductor = metal('Name','Copper','Thickness',3*obj.mils2meters,'Conductivity',10e9);
obj.Substrate = dielectric('EpsilonR',3.7,'LossTangent',0.03,'Thickness',12*obj.mils2meters);
obj.SignalViaDiameter = 10 * obj.mils2meters;
obj.SignalViaPad.Radius = 1.00001 * obj.SignalViaDiameter/2;
obj.SignalViaAntipad.Radius = 15 * obj.mils2meters;

% Create grid of all via positions
[X,Y] = meshgrid(1:8, 1:8);
allXYs = [X(:) Y(:)];

% Signal via subset
[X,Y] = meshgrid([1 3 5 8], [1 2 4 6 8]);
SVXYs = [X(:) Y(:)];

% Ground via locations = everything else
GRVXYs = setdiff(allXYs, SVXYs, 'rows');

Assigning Multiple Signal Vias

startLayer = 1; stopLayer = 3;
obj.SignalViaLocations = [SVXYs, repmat([startLayer stopLayer], size(SVXYs,1), 1)];
obj.GroundReturnViaLocations = [GRVXYs, repmat([startLayer stopLayer], size(GRVXYs,1), 1)];

Open Signal Vias (No Ports)

To model signal vias without assigning ports (open-circuited), leave SignalTable empty or assign ports only to specific vias:

% Only port via #1 on layers 1 and 3
obj.SignalTable = {1, 1, 5*obj.mils2meters, 0;
                   1, 3, 5*obj.mils2meters, 0};

All other signal vias remain as open (unported) stubs — useful for studying coupling in dense via fields.

Multi-Layer Stackup

Layer Numbering Convention

Layers alternate metal and dielectric, numbered sequentially:

Layer 1:  Metal (signal or ground)
Layer 2:  Dielectric
Layer 3:  Metal (signal or ground)
Layer 4:  Dielectric
Layer 5:  Metal (signal or ground)
...

Only odd-numbered layers are metal. SignalLayer and GroundLayer use these odd indices.

Example: 10-Layer Board

via = viaSingleEnded;
via.SignalLayer = [1 7];         % Signal on layers 1 and 7
via.GroundLayer = [3 5 9];      % Ground on layers 3, 5, and 9
via.Substrate = dielectric("Name","FR4","EpsilonR",4.4,...
    "LossTangent",0.02,"Thickness",0.1e-3);

The substrate Thickness is the thickness of each dielectric layer (uniform). For non-uniform stackups, use a multi-element dielectric.

Analysis

S-Parameters

freq = linspace(1e9, 20e9, 51);
sp = sparameters(via, freq, 'Behavioral', true);
rfplot(sp);

% Verify insertion loss meets threshold across band
S21_dB = 20*log10(abs(squeeze(sp.Parameters(2,1,:))));
idx = find(S21_dB <= -1, 1);
if isempty(idx), fprintf('PASS: IL < 1 dB across band\n');
else, fprintf('FAIL: IL exceeds 1 dB at %.1f GHz\n', sp.Frequencies(idx)/1e9); end

TDR (Time Domain Reflectometry)

Use the tdr function from the Signal Integrity Toolbox. Thumb rules for parameters: RiseTime = 1/fmax, SampleTime = 1/(100*fmax), EndTime = 0.1e-9, where fmax is the maximum frequency of the S-parameter sweep:

fmax = 20e9;
tdrObj = tdr(sp, RiseTime=1/fmax, SampleTime=1/(100*fmax), EndTime=0.1e-9);
plot(tdrObj)

SI Analysis Functions

Two functions help evaluate whether ground return vias are close enough to the signal via at a target frequency:

| Function | Syntax | Returns | |----------|--------|---------| | criticalwavelength | cw = criticalwavelength(via, freq) | Number of wavelengths between the signal via and its nearest ground return via at freq. Values approaching 0.25 or 0.5 indicate resonance risk. | | gapratedistance | gapratedistance(via, freq) | Maximum allowable center-to-center distance between signal via and nearest ground return via for a default critical-wavelength threshold of 0.25. |