jitx-substrate-modeler

name

jitx-substrate-modeler

description

This skill should be used when the user asks to "create a substrate", "define a stackup", "add via definitions", "set up routing structures", "configure impedance control", "define differential pairs", "set fabrication rules", "ring a shape with fence vias", "fence a pour outline", "fence an antipad", or "model a PCB layer structure". Ask the user which fabrication house they are targeting — if they confirm JLCPCB, predefined substrates from jitxlib.jlcpcb (JLC04161H_1080, JLC04161H_7628, JLC06161H_7628) are available with 4/6-layer FR-4, 50/90/100 ohm routing structures, vias, and fab rules. Otherwise, create a custom substrate. Covers Stackup, Symmetric, Conductor, Dielectric, Via (laser, mechanical, backdrilled, blind, buried, stacked), RoutingStructure, DifferentialRoutingStructure, NeckDown, via fencing along traces, fenced pour outlines (Tag + fence_via rule paired with a Pour + optional same-shape KeepOut — covers antipads, RF cavities, BGA breakouts), geometry, reference planes, and FabricationConstraints.

JITX Substrate Modeler

Generate complete JITX Python substrate definitions — stackups, materials, vias, routing structures, and fabrication constraints — all in a single file.

Predefined Substrates (JLCPCB Only)

If the user has confirmed they are targeting JLCPCB as their fabrication house, predefined substrates from jitxlib.jlcpcb are available. These are production-validated with correct materials, vias, fab rules, and impedance-matched routing structures:

Class	Layers	Prepreg	Routing Structures	Import
JLC04161H_1080	4	1080	RS_50, DRS_90, DRS_100	from jitxlib.jlcpcb import JLC04161H_1080
JLC04161H_7628	4	7628	RS_50, DRS_90, DRS_100	from jitxlib.jlcpcb import JLC04161H_7628
JLC06161H_7628	6	7628	RS_50, DRS_100	from jitxlib.jlcpcb import JLC06161H_7628

Each includes: Symmetric stackup, JLCPCBRules (FabricationConstraints), 11 JLCPCB via definitions (StdVia, StdViaPreferred, MultiLayerVia1-3 + Preferred variants, StdViaTentedFilled for via-in-pad), and routing structures for 50/90/100 ohm impedance targets.

Use directly — no substrate file needed:

from jitxlib.jlcpcb import JLC04161H_1080
substrate = JLC04161H_1080()

# Access routing structures for SI constraints:
# substrate.RS_50, substrate.DRS_90, substrate.DRS_100

When to use predefined: User has explicitly confirmed JLCPCB as fab house + 4 or 6 layer FR-4 + standard impedance targets (50/90/100 ohm). This covers USB, Ethernet, I2C, SPI, I2S, and most common protocols.

When to create custom (use the rest of this skill): User has not confirmed JLCPCB, non-FR-4 materials (Rogers, Megtron), unusual layer count, non-standard impedance, or additional routing structures needed. This is the default path — always create a custom substrate unless the user opts in to a predefined one.

Environment

Environment setup is handled by the base jitx skill. Ensure it has been invoked first.

Package Architecture

# Core imports — use these exactly
from jitx.stackup import Stackup, Symmetric, Conductor, Dielectric
from jitx.substrate import Substrate, FabricationConstraints
from jitx.via import Via, ViaType, ViaDiameter, Backdrill, BackdrillSet
from jitx.si import RoutingStructure, DifferentialRoutingStructure, symmetric_routing_layers
from jitx.layerindex import Side, LayerSet
from jitx.units import ohm
from jitx.constraints import ViaFencePattern
from jitx.feature import KeepOut, Soldermask
from jitxlib.physics import phase_velocity
from jitx.container import inline

These DO NOT EXIST — never import: jitx.material, jitx.layer, jitx.routing, jitx.impedance, jitx.pcb, jitx.dielectric, jitx.conductor, jitxlib.stackup, jitxlib.substrate

Anti-string-hacking — read before adding per-layer / per-via tables

Substrate-shaped data (layer-to-via maps, layer-pair tables, per-layer trace widths) belongs on the substrate, queried by the design — not duplicated as design-level constants. The design should write self.substrate.via[(a, b)], not maintain its own _SIGNAL_LAYER_TO_VIA dict. See jitx/references/architectural-patterns.md § "Substrate-shaped tables live on the substrate" before adding per-layer constant tables. Also: instantiate generic substrates (stackup = Generic_Stackup()), don't inline-subclass them (@inline class stackup(Generic_Stackup): pass) — § "Instantiate, don't inline-subclass".

A "generic" substrate must be reusable across designs. Design-specific tags (AntipadFenceTag named after a particular escape design), design-specific trace widths (DESKEW_TRACE_WIDTH), or design-specific fence definitions do not belong in generic_*.py — push them into the consuming design. Comments and docstrings are part of this surface too: a generic substrate must not claim it's tuned for one downstream tool's extraction/export flow (jitx-ansys / HFSS, odb++) or state a fact the code doesn't back — that couples the reusable artifact to one consumer and asserts facts not in evidence. A neutral, evidenced mention of a tool isn't the problem; an unbacked tool-specific suitability claim is.

For a same-model self-critique pass on the substrate after writing (catches what these rules don't), invoke jitx-skills:jitx-code-review. Optional for single-task use.

File Structure

Everything goes in one Python file per substrate:

# 1. Material definitions (Dielectric, Conductor subclasses)
# 2. Stackup class (Stackup or Symmetric)
# 3. FabricationConstraints class
# 4. Substrate class containing:
#    - stackup instance
#    - constraints instance
#    - Via nested classes (all types needed)
#    - RoutingStructure instances
#    - DifferentialRoutingStructure instances

Materials

Set properties as class attributes, instantiate with thickness.

Soldermask is a Dielectric — define it like any other dielectric material:

class SoldermaskLayer(Dielectric):
    """Soldermask — typically Er ≈ 3.8"""
    dielectric_coefficient = 3.8
    loss_tangent = 0.02

class FR4_Prepreg(Dielectric):
    dielectric_coefficient = 4.4   # Dk (dielectric constant / relative permittivity)
    loss_tangent = 0.0168          # Df (dissipation factor)

class FR4_Core(Dielectric):
    dielectric_coefficient = 4.6   # Dk
    loss_tangent = 0.0168          # Df

class Copper1oz(Conductor):
    thickness = 0.035  # mm (can also be set at instantiation)

class CopperHalfOz(Conductor):
    thickness = 0.0175  # mm (can also be set at instantiation)

class SoldermaskLayer(Dielectric):
    thickness = 0.020

Terminology: dielectric_coefficient is the JITX attribute name for Dk (dielectric constant, also called relative permittivity or Er). loss_tangent is the JITX attribute name for Df (dissipation factor). Datasheets typically specify Dk and Df at a given frequency (e.g., 1 GHz or 10 GHz).

Common Dielectric Materials

Reference table of common PCB dielectric materials. Values are typical at 10 GHz unless noted. Always confirm with the manufacturer's datasheet for your specific construction.

Material	Manufacturer	Family	Dk	Df	Notes
Standard FR-4
FR408HR	Isola	High-Tg epoxy	3.68	0.0092	Workhorse high-Tg FR-4; widely available
I-Speed	Isola	Low-loss epoxy	3.64	0.0060	Step down in loss vs standard FR-4
N4000-13 EP	AGC/Nelco	High-speed epoxy	3.60	0.0090	High-speed digital backplanes
N7000-2HT	AGC/Nelco	High-speed laminate	3.50	0.0090	Dk/Df available at 2.5 and 10 GHz
Low-Loss
I-Tera MT40	Isola	Very low-loss epoxy	3.45	0.0031	High-speed digital/RF
Megtron 6	Panasonic	Low-loss multilayer	3.34	0.0037	Common in high-speed digital (at 13 GHz)
RO4350B	Rogers	Hydrocarbon/ceramic	3.48	0.0037	Popular RF laminate; FR-4 processable
RO4003C	Rogers	Hydrocarbon/ceramic	3.38	0.0027	Standard RF laminate
25N	Arlon	Ceramic-filled woven glass	3.38	0.0025	Low loss with standard FR-4 processes
Ultra-Low-Loss
Astra MT77	Isola	Ultra-low-loss	3.00	0.0017	RF/microwave and very-high-speed
Tachyon 100G	Isola	Ultra-low-loss	~3.05	~0.0017	Values vary by construction
Megtron 7	Panasonic	Ultra-low-loss	varies	varies	Capture exact row for glass style/resin
PTFE / RF
RT/duroid 5880	Rogers	Glass microfiber PTFE	2.20	0.0009	Ultra-low loss; microwave/RF
RT/duroid 5870	Rogers	Glass microfiber PTFE	2.33	0.0012	Low Dk/loss; antennas/stripline
RO3003	Rogers	Ceramic-filled PTFE	3.00	0.0010	Low loss PTFE; common RF choice
RO3035	Rogers	Ceramic-filled PTFE	3.50	0.0015	PTFE with Dk ~3.5
TLY-5A	Taconic/AGC	Low-loss PTFE	2.17–2.40	~0.0009	Selectable Dk range
TLX-0	Taconic/AGC	Fiberglass PTFE	2.45	0.0012	Lowest Dk in TLX series
High-Dk (miniaturization)
RO3006	Rogers	Ceramic-filled PTFE	6.15	0.0020	Higher Dk for size reduction
RO3010	Rogers	Ceramic-filled PTFE	10.20	0.0022	High Dk for compact RF
CER-10	Taconic/AGC	Organic-ceramic	10.0	0.0035	High Dk; check tolerances per lot

Copper Foil Types

Copper surface roughness affects insertion loss at high frequencies. Choose foil type based on your frequency range.

Rz values below are for the matte/bonding side (the side laminated to the dielectric core), which is the surface that dominates conductor loss. The drum/resist side is typically 2–5× smoother; use its Ra value when modelling the top surface of a trace.

Copper Type	Rz — Matte/Bonding Side	Rz — Drum/Resist Side	Use Case
Standard HTE (STD)	5–10 μm	3–5 μm	<1 GHz, general FR-4 inner layers
Reverse Treated Foil (RTF)	5–10 μm	3–5 μm	<5 GHz; adhesion treatment moves to drum side
Low Profile (LP / LoPro)	2–4 μm	1–2 μm	1–10 GHz signal layers
Very Low Profile (VLP)	2.5–5 μm	1–2 μm	5–25 Gbps; Megtron 6, Isola IS415/FR408HR
Hyper VLP (HVLP / SVLP)	1–3 μm	0.5–1 μm	25–56 Gbps; high-speed SerDes
Ultra Low Profile (ULP)	0.5–1.5 μm	0.3–0.5 μm	>56 Gbps, mmWave (>24 GHz)
Rolled Annealed (RA)	0.3–0.8 μm	0.3–0.8 μm	RF/microwave, flex circuits; both sides smooth

Rule of thumb: For signals above 5 GHz, use LP or smoother. Above 10 GHz, use VLP. For 25 Gbps+, use HVLP. For mmWave (>24 GHz) or >56 Gbps, use ULP or RA.

Cannonball-Huray parameters (for HFSS/EM simulation using the average HCPES+SCPES model):

• Nodule radius: a = 0.0573 × Rz (µm)
• Surface ratio: Sr = 5.117 (constant, independent of foil type)
• Use matte-side roughness for the bottom surface of a trace; drum-side for the top. The matte side is the rougher of the two — typical Ra range 0.18–0.51 µm for standard foils; the drum side is much smoother — Ra ≈ Rz × 0.0573 (matches the Cannonball-Huray nodule-radius formula, ≈ 0.18 µm at Rz = 3.05 µm).

Copper Type	Representative Rz (µm)	Nodule radius a (µm)
STD HTE	8.0	0.458
RTF	6.0	0.344
LP / LoPro	3.0	0.172
VLP	3.5	0.201
HVLP	2.0	0.115
ULP	1.0	0.057
RA	0.5	0.029

Stackup

Symmetric (preferred for most boards)

Define top half only — bottom auto-mirrors. Last layer MUST be dielectric (symmetry plane):

class My4LayerStackup(Symmetric):
    soldermask = SoldermaskLayer(thickness=0.015)
    top = Copper1oz()
    prepreg = FR4_Prepreg(thickness=0.076)
    inner = CopperHalfOz()
    core = FR4_Core(thickness=1.265)  # center — MUST be dielectric

Explicit Stackup (non-symmetric boards)

Top-to-bottom order. Named attributes or list. Give copper layers informative names describing their function (signal, ground, power) — these appear in the JITX UI and help users navigate the design:

class My8LayerStackup(Stackup):
    top_mask = SoldermaskLayer(thickness=0.02)
    L8 = ThinCopper(name="L8-Patch")
    sub7 = Prepreg326(thickness=0.068)
    L7 = ThinCopper(name="L7-GND3")
    sub6 = Prepreg322(thickness=0.104)
    L6 = ThinCopper(name="L6-Signal")
    # ... all layers top to bottom ...
    L2 = ThinCopper(name="L2-GND1")
    sub1 = Prepreg325(thickness=0.068)
    L1 = ThickCopper(name="L1-Signal")
    bottom_mask = SoldermaskLayer(thickness=0.02)

Inline Stackup (in Substrate class)

class MySubstrate(Substrate):
    @inline
    class stackup(Symmetric):
        soldermask = SoldermaskLayer(thickness=0.015)
        top = Copper1oz()
        prepreg = FR4_Prepreg(thickness=0.076)
        inner = CopperHalfOz()
        core = FR4_Core(thickness=1.265)

Via Types

Define as nested classes inside Substrate. All properties are ClassVar.

Through-Hole (Standard)

class THVia(Via):
    type = ViaType.MechanicalDrill
    start_layer = 0        # Side.Top also works
    stop_layer = -1        # Side.Bottom also works
    diameter = 0.45        # pad diameter (mm)
    hole_diameter = 0.3    # drill hole (mm)

Through-Hole (Tented + Filled, Via-in-Pad)

class THViaFilled(Via):
    type = ViaType.MechanicalDrill
    start_layer = Side.Top
    stop_layer = Side.Bottom
    diameter = 0.45
    hole_diameter = 0.3
    tented = True
    filled = True
    via_in_pad = True

Laser Microvia (Single Span)

class MicroVia_L1_L2(Via):
    type = ViaType.LaserDrill
    start_layer = 0
    stop_layer = 1
    diameter = 0.356
    hole_diameter = 0.178
    filled = True
    via_in_pad = True

Stacked Microvia (Multi-Span Laser)

class StackedVia_L1_L3(Via):
    type = ViaType.LaserDrill
    start_layer = 0
    stop_layer = 2
    diameter = 0.356
    hole_diameter = 0.178
    filled = True
    via_in_pad = True

Buried Via (Internal Only)

class BuriedVia_L3_L12(Via):
    type = ViaType.MechanicalDrill
    start_layer = 2
    stop_layer = 11
    diameter = 0.356
    hole_diameter = 0.178
    filled = True

Backdrilled Via

Backdrill depth is set via stop_layer — set it to the target signal layer, then use BackdrillSet to remove the stub. The backdrill side is opposite to the signal entry:

bd = Backdrill(
    diameter=0.5, startpad_diameter=0.7,
    solder_mask_opening=0.8, copper_clearance=0.6,
)

class BackdrilledVia_L3(Via):
    """Signal enters from top, connects at L3 — backdrill from bottom removes stub"""
    type = ViaType.MechanicalDrill
    start_layer = Side.Top
    stop_layer = 3             # target signal layer controls backdrill depth
    diameter = 0.6
    hole_diameter = 0.3
    filled = True
    via_in_pad = True
    backdrill = BackdrillSet(bottom=bd)  # backdrill from opposite side

Dual backdrill (both sides) — incredibly uncommon, almost never needed:

    backdrill = BackdrillSet(
        top=Backdrill(diameter=0.5, startpad_diameter=0.7,
                      solder_mask_opening=0.8, copper_clearance=0.6),
        bottom=Backdrill(diameter=0.5, startpad_diameter=0.7,
                         solder_mask_opening=0.8, copper_clearance=0.6),
    )

Per-Layer Pad Diameter (NFP Removal)

class AdvancedVia(Via):
    type = ViaType.MechanicalDrill
    start_layer = 0
    stop_layer = -1
    diameter = 0.6
    hole_diameter = 0.3
    diameters = {
        0: 0.5,
        1: ViaDiameter(0.5, nfp=0.2),  # non-functional pad on layer 1
    }

Via SI Models

from jitx.si import PinModel

class ModeledVia(Via):
    # ... standard attributes ...
    models = {
        (0, -1): PinModel(5e-12, 0.05),  # top-to-bottom: 5ps delay, 0.05dB loss
        (0, 1): PinModel(2e-12, 0.02),
    }

Routing Structures

Single-Ended (RoutingStructure)

RS_50 = RoutingStructure(
    impedance=50 * ohm,
    layers=symmetric_routing_layers({
        0: RoutingStructure.Layer(
            trace_width=0.12,       # mm
            clearance=0.2,          # mm
            velocity=phase_velocity((4.4 + 1) / 2),  # mm/s — microstrip effective Dk
            insertion_loss=0.018,   # dB/mm
        )
    }),
)

Velocity Calculation

from jitxlib.physics import phase_velocity

vel_microstrip = phase_velocity((Dk + 1) / 2)   # microstrip effective Dk
vel_stripline = phase_velocity(Dk)               # stripline uses full Dk
vel_mixed = phase_velocity((Dk_pp + Dk_core) / 2)  # mixed dielectric

velocity must be in mm/s, NOT m/s. phase_velocity() returns mm/s. Passing a raw m/s value will be 1000x too small, producing wrong timing constraints.

# WRONG — velocity in m/s (1000x too small, timing constraints will be wrong)
velocity = 1.5e8  # m/s — DO NOT USE

# CORRECT — always use phase_velocity() which returns mm/s
velocity = phase_velocity(4.2)  # returns ~1.46e11 mm/s

symmetric_routing_layers()

Define top half only — mirrors to bottom using -layer - 1 index:

layers = symmetric_routing_layers({
    0: RoutingStructure.Layer(...),   # → also creates layer -1
    2: RoutingStructure.Layer(...),   # → also creates layer -3
})

Layer with NeckDown

RoutingStructure.Layer(
    trace_width=0.15, clearance=0.1,
    velocity=vel, insertion_loss=0.05,
    neck_down=RoutingStructure.NeckDown(
        trace_width=0.09, clearance=0.075,
    ),
)

Layer with Via Fence

RoutingStructure.Layer(
    trace_width=0.203, clearance=0.076,
    velocity=phase_velocity(1.99), insertion_loss=0.05,
).fence(
    MicroVia_L1_L2,   # via class
    ViaFencePattern(
        pitch=0.4,     # via-to-via spacing along route
        offset=0.43,   # trace center to via center
        num_rows=1,
    ),
    reference_layer=1, # ground reference for fence net
)

Offset formula: offset = trace_width/2 + gap + via_pad_radius

Layer with Geometry and Reference

RoutingStructure.Layer(
    trace_width=0.12, clearance=0.08,
    velocity=phase_velocity(3.26), insertion_loss=0.08,
)
.geometry(Soldermask, 0.25, side=Side.Top)        # soldermask opening
.geometry(KeepOut, 1.2, layers=LayerSet(1), pour=True)  # keepout on layer 1
.reference(2, 1.0)                                 # reference plane on layer 2
.fence(FenceViaClass, ViaFencePattern(pitch=0.5, offset=0.35, num_rows=1),
       reference_layer=2)

Differential Routing Structure

DRS_100 = DifferentialRoutingStructure(
    name="100 Ohm Differential",
    impedance=100 * ohm,
    layers=symmetric_routing_layers({
        0: DifferentialRoutingStructure.Layer(
            trace_width=0.09,
            pair_spacing=0.137,    # edge-to-edge between P and N
            clearance=0.2,
            velocity=vel,
            insertion_loss=0.018,
        )
    }),
    uncoupled_region=RoutingStructure(
        name="50 Ohm SingleEnded, Uncoupled",
        impedance=50 * ohm,
        layers=symmetric_routing_layers({
            0: RoutingStructure.Layer(
                trace_width=0.09, clearance=0.2,
                velocity=vel, insertion_loss=0.018,
            )
        }),
    ),
)

Differential with NeckDown (for BGA escape or constrained areas):

DRS_100_ND = DifferentialRoutingStructure(
    name="100 Ohm Differential w/ NeckDown",
    impedance=100 * ohm,
    layers=symmetric_routing_layers({
        0: DifferentialRoutingStructure.Layer(
            trace_width=0.09,
            pair_spacing=0.137,
            clearance=0.2,
            velocity=vel,
            insertion_loss=0.018,
            neck_down=DifferentialRoutingStructure.NeckDown(
                trace_width=0.075,
                pair_spacing=0.1,
                clearance=0.15,
            ),
        )
    }),
    uncoupled_region=RoutingStructure(
        name="100 Ohm Differential w/ NeckDown, Uncoupled",
        impedance=50 * ohm,  # half of differential impedance
        layers=symmetric_routing_layers({
            0: RoutingStructure.Layer(
                trace_width=0.09, clearance=0.2,
                velocity=vel, insertion_loss=0.018,
                neck_down=RoutingStructure.NeckDown(
                    trace_width=0.075, clearance=0.15,
                ),
            )
        }),
    ),
)

Multi-layer differential (different trace widths per layer):

DRS_82 = DifferentialRoutingStructure(
    impedance=82 * ohm,
    layers=symmetric_routing_layers({
        0: DifferentialRoutingStructure.Layer(
            trace_width=0.154, pair_spacing=0.2,
            clearance=0.23, velocity=VEL, insertion_loss=0.018,
        ),
        2: DifferentialRoutingStructure.Layer(
            trace_width=0.137, pair_spacing=0.15,
            clearance=0.21, velocity=VEL, insertion_loss=0.018,
        ),
    }),
    uncoupled_region=RoutingStructure(
        impedance=41 * ohm,
        layers=symmetric_routing_layers({
            0: RoutingStructure.Layer(trace_width=0.154, clearance=0.15,
                                      velocity=VEL, insertion_loss=0.018),
            2: RoutingStructure.Layer(trace_width=0.137, clearance=0.15,
                                      velocity=VEL, insertion_loss=0.018),
        }),
    ),
)

Fabrication Constraints

All values in mm.

class MyFabRules(FabricationConstraints):
    min_copper_width = 0.09           # minimum trace width
    min_copper_copper_space = 0.09    # minimum copper spacing
    min_copper_hole_space = 0.254     # copper-to-hole spacing
    min_copper_edge_space = 0.3       # copper-to-board-edge
    min_annular_ring = 0.13           # via annular ring
    min_drill_diameter = 0.3          # minimum drill hole
    min_hole_to_hole = 0.5            # hole-to-hole spacing
    min_pitch_leaded = 0.217          # leaded package pitch
    min_pitch_bga = 0.377             # BGA pitch
    max_board_width = 500
    max_board_height = 400
    min_silkscreen_width = 0.153
    min_silk_solder_mask_space = 0.15
    min_silkscreen_text_height = 1.0
    solder_mask_registration = 0.05
    min_soldermask_opening = 0.0
    min_soldermask_bridge = 0.08
    min_th_pad_expand_outer = 0.2
    min_pth_pin_solder_clearance = 0.0

Custom attributes are allowed for fab-house-specific rules (not engine-enforced).

Design Constraints (Tags)

This section defines the rules (design_constraint(...)) a tag triggers. Choosing which layout objects to tag and why — fanout/escape tags on package escapes, direct-connect on high-current pads, tagging a code-based Route — is covered in the jitx-physical-layout subskill.

For net-to-net clearances and via stitching rules:

from jitx.constraints import Tag, design_constraint

class RFSignalTag(Tag): pass
class GNDTag(Tag): pass

# Trace width for tagged nets (unary constraint — single tag)
self.rule1 = design_constraint(RFSignalTag(), priority=1).trace_width(0.102)

# Net-to-net clearance (binary constraint — two tags)
self.rule2 = design_constraint(RFSignalTag(), RFSignalTag()).clearance(1.05)
self.rule3 = design_constraint(RFSignalTag(), GNDTag()).clearance(0.15)

Board-wide defaults belong on the Design class, not the substrate. The four canonical defaults — trace width, copper clearance, thermal relief, wider power/ground — go in self.rules on the top-level Design via UnaryDesignConstraint(IsTrace) / BinaryDesignConstraint(IsCopper, IsCopper) / UnaryDesignConstraint(IsPad) / UnaryDesignConstraint(PowerTag() | GroundTag(), priority=1). See jitx/references/project-builder-flow.md "Default design rules" for the full pattern. The substrate's FabricationConstraints are the fab-minimum floor; the Design rules are the production-friendly defaults that sit above the floor.

design_constraint(...) and UnaryDesignConstraint(...) / BinaryDesignConstraint(...) are equivalent — the lowercase form is a factory that returns the right subtype based on arity. Use either.

Tag inheritance & proliferation

Tags form a hierarchy through class inheritance, and a rule on a base tag applies to every subclass tag. This is a first-class JITX feature, not a trick — a tag can subclass another tag, not just Tag:

class FenceTag(Tag): pass
class AntipadFenceTag(FenceTag): pass        # subclass of FenceTag
class DeskewAntipadFenceTag(AntipadFenceTag): pass   # subclass of AntipadFenceTag

# Applies to ALL fence tags — antipad, deskew, and any future FenceTag subclass:
self.fence_clearance = design_constraint(FenceTag(), GNDTag()).clearance(0.15)

# Applies only to the deskew variant; give it higher priority to override the base
# rule where they overlap (higher priority wins when multiple rules match):
self.deskew_fence = design_constraint(DeskewAntipadFenceTag(), priority=10).fence_via(...)

A net/pour/object tagged DeskewAntipadFenceTag() matches rules written against DeskewAntipadFenceTag, AntipadFenceTag, and FenceTag. Where two matching rules conflict, the higher priority= wins — that's how a specific subtag rule overrides the general base-tag rule. (Tags also combine with & / | / ~ and Tag.any(...) when a hierarchy isn't the right shape.)

Flat tag proliferation is a smell. A row of near-identical sibling tags that all inherit straight from Tag and differ only by name — each wired to its own rule that mostly restates the others — usually wants one of:

• a base tag carrying the shared rule, with subtags only where behavior actually differs (the neckdown case: one clearance rule for all neckdown via NeckDownTag, plus a higher-priority rule for the one neckdown level that's special), or
• a single combined rule (design_constraint(TagA() | TagB())…) when the tags aren't really distinct concepts.

Reach for many flat tags only when the rule sets are genuinely distinct. Mapping a spreadsheet of per-combination rules into a flat tag-per-row table is the usual way this goes wrong — the hierarchy expresses the same intent with far fewer rules.

Conditions toolbox — builtin tags, layers, expressions

Rule conditions are not limited to tags you define:

• Builtin tags — IsCopper, IsTrace, IsPour, IsVia, IsPad, IsBoardEdge, IsThroughHole, IsNeckdown, IsHole (import from jitx.constraints or top-level jitx). The engine matches them by object kind; they are conditions only — assign() on a builtin raises TypeError. The four canonical Design defaults use these (see jitx/references/project-builder-flow.md "Default design rules").
• OnLayer(index) — layer-scoped condition (import from jitx.constraints; not re-exported top-level). OnLayer.external() matches the top and bottom copper layers; OnLayer.internal() is its inverse.
• AnyObject — matches everything; useful as the second condition of a binary rule.
• Expressions — conditions combine with & / | / ~, and n-ary Tag.any(*tags) / Tag.all(*tags).

from jitx.constraints import design_constraint, AnyObject, OnLayer

# Wider high-speed traces on external layers only:
self.hs_outer = design_constraint(HighSpeedTag() & OnLayer.external()).trace_width(0.15)

# Keep everything 0.3 mm away from tagged power copper:
self.pwr_keepaway = design_constraint(PowerTag(), AnyObject).clearance(0.3)

Which objects can carry a tag (Net, TopologyNet, Copper, Pour, Route, Component, Circuit, Landpattern, Pad, Via, ControlPoint), container inheritance (tagging a landpattern tags its pads), and tagging self to tag all instances of a class are covered in jitx-physical-layout "Layout-intent tags".

Constraint effects — the full surface

A rule's effects are chainable methods; one rule can set several. The arity boundary: unary rules (one condition) chain any effect below except clearance; binary rules (two conditions) support only .clearance(). Everything a design_constraint(...) can do (all dimensions in mm):

Effect	Signature	Notes
Trace width	.trace_width(width)	example above
Clearance	.clearance(clearance)	binary rules only — design_constraint(cond1, cond2)
Via fencing	.fence_via(via_cls, ViaFencePattern(...))	along traces/pour outlines — see "Fenced Pour Outlines" below
Via stitching	.stitch_via(via_cls, grid)	grid = SquareViaStitchGrid(pitch=, inset=) or TriangularViaStitchGrid(pitch=, inset=); inset = boundary-to-outermost-via-center distance
Thermal relief	.thermal_relief(gap_distance, spoke_width, num_spokes)	pad-to-pour connections
Serpentine params	.serpentine_params(min_radius=, min_pitch=)	bend radius / segment pitch of length-matching serpentines
Coupled-pair params	.coupled_pair_params(deskew_bump_radius=, skew_tolerance=, min_bump_spacing=, max_bump_length=, long_lookahead=)	deskew-bump geometry for diff pairs; skew_tolerance is in mm (distance, not time — the time-domain skew budget lives in jitx-interconnect-constraints)
Pour feature size	.pour_feature_size(min_width)	clips pour regions not coverable by a circle of min_width diameter fully inside the pour (sliver removal; thermal-relief spokes excluded)
Routing structure	.routing_structure(rs, ...)	see below

from jitx.constraints import design_constraint, SquareViaStitchGrid, IsPour

# Stitch tagged ground pours on a 2 mm square grid:
self.gnd_stitch = design_constraint(GNDPourTag()).stitch_via(
    GndVia, SquareViaStitchGrid(pitch=2.0, inset=0.5)
)

# Board-wide pour sliver removal:
self.no_slivers = design_constraint(IsPour).pour_feature_size(min_width=0.3)

Routing structures as a rule effect

.routing_structure(...) assigns an impedance-controlled structure (defined on this substrate) to every trace matching the condition — including plain Nets and code-based Routes that have no >> topology. Reference planes resolve one of three ways:

# (a) one net references every reference layer the structure declares:
self.hs = design_constraint(HighSpeedTag()).routing_structure(self.RS_50, ref_net=gnd)

# (b) per-layer mapping:
self.hs = design_constraint(HighSpeedTag()).routing_structure(
    self.DRS_100, ref_layer_nets={1: gnd, 4: gnd}
)

# (c) neither argument — requires an active jitx.si.ReferencePlanes context,
#     else ValueError at rule-construction time.

The keyword names are ref_net / ref_layer_nets (not reference_*); passing both raises ValueError. For ordered point-to-point paths where the structure travels with timing/loss constraints, the topology-based Constrain(...).structure(...) flow is usually the better fit — the choice is covered in jitx-interconnect-constraints "Tag-based routing structures".

Fenced Pour Outlines (Antipads, RF Cavities, BGA Breakouts)

Trick for placing fence vias along an arbitrary closed shape — antipad rings around signal-via pairs, RF cavity perimeters, BGA breakout boundaries, deskew arc cutouts. Three pieces compose:

1. Substrate-side rule — a Tag and a design_constraint(...).fence_via(...) declaring that any pour carrying the Tag gets fence vias of the given class placed along it.
2. Design-side Pour — created on the fence net (typically GND) with the Tag assigned. The Pour exists to give the constraint engine a shape to ring with vias; its copper may or may not be wanted.
3. Optional matching KeepOut — same shape, voids the pour's copper. Add it when the pour is purely a fence-via trigger (you want the vias, not the copper). Omit it when the pour's copper is real (e.g. a stitching region that doubles as a return-path pour).

Substrate-side declaration

from jitx.constraints import Tag, design_constraint, ViaFencePattern

class FenceOutlineTag(Tag):
    """Pours with this tag get fence vias along their outline."""

class MySubstrate(Substrate):
    # ... other vias ...
    class uGndStitch(Via):
        """Example fence via — adjust to your fab's microvia capability."""
        type = ViaType.LaserDrill
        start_layer = 0
        stop_layer = 6
        diameter = 0.25
        hole_diameter = 0.1
        filled = True

    _FENCE_PATTERN = ViaFencePattern(
        pitch=0.35,   # via-to-via spacing along each row
        offset=0.15,  # row-to-row spacing; also the default boundary-to-first-row offset
        num_rows=1,
    )

    outline_fence_rule = design_constraint(
        FenceOutlineTag(), priority=20
    ).fence_via(uGndStitch, _FENCE_PATTERN)

ViaFencePattern.input_shape_only (pour-only, defaults to True) controls which pour shape gets fenced — the pre-isolation input outline (default) or the post-isolation computed copper. Leaving it default is correct for nearly every fenced-outline case; set False only if downstream clearance rules will reshape the pour and the fence vias should track the reshaped boundary.

Design-side usage

The tagged pour must sit on a conductor layer the fence via reaches — fence vias inherit the pour's net, so the pour has to be on a layer they can land on. Typically the pour goes on the reference/termination layer being fenced (here, the via's stop_layer = 6, so the pour goes on layer=6).

from jitx import Pour
from jitx.feature import KeepOut
from jitx.layerindex import LayerSet

# `shape` is the outline to fence — e.g. a capsule around a signal-via pair,
# an RF cavity perimeter, a BGA breakout boundary, or a deskew arc cutout.
fence_pour = Pour(shape, layer=6)
FenceOutlineTag().assign(fence_pour)
self.GND += fence_pour

# Add this ONLY when the pour copper itself is unwanted (cavity / antipad opening).
# Omit when the pour doubles as a real GND region.
self.fence_outline_keepout = KeepOut(shape, layers=LayerSet(6), pour=True, via=True)

Do not set isolate= on the fence Pour — it's legacy. Pour clearance is governed by FabricationConstraints + Tag-based design_constraint(...).clearance(...).

Via Mixin Pattern

Reuse via definitions across substrates:

class MyVias:
    class StdVia(Via): ...
    class StdViaFilled(Via): ...

class SubstrateA(Substrate, MyVias):
    stackup = StackupA()
    constraints = RulesA()

class SubstrateB(Substrate, MyVias):
    stackup = StackupB()
    constraints = RulesB()

Layer Index Convention

• 0 / Side.Top = top conductor
• 1 = second conductor from top
• -1 / Side.Bottom = bottom conductor
• -2 = second from bottom
• symmetric_routing_layers maps layer i to -i - 1

Workflow

1. Gather specs — stackup cross-section, dielectric properties (Dk, Df), copper weights, fab house rules, impedance targets
2. Define materials — Dielectric and Conductor subclasses with Dk/Df/roughness
3. Build stackup — Symmetric for symmetric boards, Stackup for asymmetric
4. Set fab constraints — FabricationConstraints with all manufacturing rules
5. Define vias — all via types needed (through, micro, stacked, blind, buried, backdrilled)
6. Add routing structures — RoutingStructure and DifferentialRoutingStructure for each impedance target
7. Add design rules — Tags and design_constraint() for clearances if needed
8. Verify — pyright type check, then jitx build with a test design (sequence builds — don't parallelize against the same project; see jitx/SKILL.md "Build Safety")

API Reference

For complete class definitions, all parameters, method signatures, and additional examples, see JITX Documentation.

Formatting

ruff format path/to/substrate.py