Model slotted (oblong) THT holes as stadiums, not circles

The drill's y dimension was discarded (padstack.drill.diameter.x only),
so a milled slot became a round hole of its x size - contact rings,
drill mouths and lead cones painted circles larger than the oblong pad
itself, and the cone was skipped outright (pad_min <= drill).

Slots now keep their true stadium shape, rotated with the pad (KiCad
CCW, y down): Electrode/ViaLink carry the end-cap offset vector,
drill_nm becomes the slot WIDTH, and a shared slot_distance() reduces
to the plain radius for round holes. The barrel wall ring, mouth
coverage, cone taper and the solver's attachment search all follow the
slot; barrel_resistance uses the stadium perimeter and bore area. The
stitching-coat fallback becomes a capsule along the slot (inscribed
disc when the axis is unknown) instead of a largest-dimension disc.
Slot fields round-trip through the JSON dumps.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -112,7 +112,11 @@ SWIG API. Requires KiCad **10.0.1+**.
   worth far more than the foil, so this is conservative. The pad face
   gets the average-thickness solder coat (exact pad shape) and the
   protruding-lead cone (see barrel contacts below; on oblong pads the
-  cone tapers within the inscribed circle). Whether a hole is a via or
+  cone tapers to the pad's short dimension). **Slotted (oval) holes**
+  keep their true stadium shape: the barrel wall, drill mouth, contact
+  ring and lead cone all follow the slot (rotated with the pad), and
+  the barrel conducts over the slot's real perimeter/bore area — not a
+  circle of the slot's long dimension. Whether a hole is a via or
   a THT pad, the owning footprint's side, and its **Do not populate**
   flag are all read from KiCad. **DNP pads** get an **open hole** and
   a plating-only barrel, no joint. At f > 0 the thickness scaling is
@@ -159,7 +163,8 @@ SWIG API. Requires KiCad **10.0.1+**.
   physically enters through the lead/wire soldered into the hole, so
   the spreading resistance across the pad and surrounding pour is part
   of the result (both contact models; verified against
-  R = ρ/(π·t)·acosh(d/2a) for two circular contacts on a sheet). A
+  R = ρ/(π·t)·acosh(d/2a) for two circular contacts on a sheet).
+  Slotted holes inject along the stadium-shaped slot wall. A
   soldered **THT joint** additionally assumes the **hole is filled with
   solder** (core in parallel with the plating) and the **pad face on
   the solder side carries an average-thickness solder coat**

fill_resistance/board_io.py

@@ -6,6 +6,7 @@ KiCad to extract without the dialog (all layers of the net, defaults).
 """
 from __future__ import annotations
 
+import math
 from dataclasses import dataclass, field
 from pathlib import Path
 
@@ -139,11 +140,33 @@ def _convert_poly(poly_with_holes) -> Polygon:
                    holes=[ring(h) for h in poly_with_holes.holes])
 
 
-def _pad_drill_nm(pad_or_via) -> int:
+def _drill_info(pad_or_via) -> tuple[int, int, int]:
+    """(width_nm, slot_dx_nm, slot_dy_nm) of a padstack drill. Round
+    holes: (diameter, 0, 0). Slotted (oblong) holes: width is the
+    NARROW dimension, (slot_dx, slot_dy) the board-frame offset from
+    the drill center to each end-cap center of the slot. The slot
+    follows the pad rotation (KiCad rotates CCW with y down:
+    x' = x cos + y sin, y' = y cos - x sin)."""
     try:
-        return int(pad_or_via.padstack.drill.diameter.x)
+        d = pad_or_via.padstack.drill.diameter
+        dx, dy = int(d.x), int(d.y)
     except Exception:
-        return 0
+        return 0, 0, 0
+    if dx <= 0 or dy <= 0 or dx == dy:
+        return max(dx, 0), 0, 0
+    half = (max(dx, dy) - min(dx, dy)) / 2.0
+    try:
+        th = math.radians(pad_or_via.padstack.angle.degrees)
+    except Exception:
+        th = 0.0
+    ux, uy = (1.0, 0.0) if dx > dy else (0.0, 1.0)
+    return (min(dx, dy),
+            int(round(half * (ux * math.cos(th) + uy * math.sin(th)))),
+            int(round(half * (uy * math.cos(th) - ux * math.sin(th)))))
+
+
+def _pad_drill_nm(pad_or_via) -> int:
+    return _drill_info(pad_or_via)[0]
 
 
 def _pad_default_contact(pad: Pad) -> str:
@@ -250,7 +273,7 @@ def _to_electrode(board: Board, item, stackup: StackupInfo | None = None,
     if box is None:
         raise SelectionError(f"Could not get the bounding box of {label}.")
     rect = _box2_to_rect(box, "pad")
-    drill = _pad_drill_nm(pad)
+    drill, slot_dx, slot_dy = _drill_info(pad)
     return Electrode(rect=rect, contact=contact,
                      polygons=_pad_polygons(board, pad, contact), label=label,
                      # through-hole pad: current enters at the soldered
@@ -258,6 +281,7 @@ def _to_electrode(board: Board, item, stackup: StackupInfo | None = None,
                      # with a solder cone around the protruding lead
                      drill_nm=drill, pad_nm=_padstack_pad_nm(pad),
                      pad_min_nm=_padstack_pad_min_nm(pad),
+                     slot_dx_nm=slot_dx, slot_dy_nm=slot_dy,
                      center=(pad.position.x, pad.position.y),
                      solder=drill > 0,
                      protrusion_side=(_tht_protrusion_side(pad, pad_map or {})
@@ -547,12 +571,14 @@ def gather_barrels(board: Board, net_name: str,
                     populated = not fp.attributes.do_not_populate
                 except Exception:
                     pass
+            drill, slot_dx, slot_dy = _drill_info(pad)
             barrels.append(ViaLink(
                 x=pad.position.x, y=pad.position.y,
-                drill_nm=_pad_drill_nm(pad), z_top_nm=-1,
+                drill_nm=drill, z_top_nm=-1,
                 z_bot_nm=stackup.z_bot_nm + 1, kind="pad",
                 pad_nm=_padstack_pad_nm(pad),
                 pad_min_nm=_padstack_pad_min_nm(pad),
+                slot_dx_nm=slot_dx, slot_dy_nm=slot_dy,
                 solder_filled=populated,
                 protrusion_side=(_tht_protrusion_side(pad, pad_map,
                                                       quiet=True)

fill_resistance/geometry.py

@@ -113,11 +113,16 @@ class Electrode:
     contact: str = "all"
     polygons: list[Polygon] | None = None
     label: str = "rect"
-    drill_nm: int = 0                         # >0: barrel contact
+    drill_nm: int = 0                         # >0: barrel contact (slotted
+                                              # holes: the slot WIDTH)
     pad_nm: int = 0                           # pad diameter (search bound;
                                               # largest dimension if oblong)
     pad_min_nm: int = 0                       # smallest pad dimension (cone
                                               # taper bound); 0 = pad_nm
+    slot_dx_nm: int = 0                       # slotted (oblong) hole: offset
+    slot_dy_nm: int = 0                       # from `center` to each end-cap
+                                              # center of the slot, board
+                                              # frame; (0, 0) = round drill
     center: tuple[int, int] | None = None     # drill center; None = rect center
     barrel_z: tuple[int, int] | None = None   # (z_top, z_bot); None = full stack
     solder: bool = False                      # soldered THT joint (see above)
@@ -134,7 +139,7 @@ class ViaLink:
     layers whose z lies within [z_top_nm, z_bot_nm]."""
     x: int
     y: int
-    drill_nm: int
+    drill_nm: int                             # slotted holes: the slot WIDTH
     z_top_nm: int
     z_bot_nm: int
     kind: str = "via"                         # "via" | "pad"
@@ -144,6 +149,10 @@ class ViaLink:
     pad_min_nm: int = 0                       # smallest pad dimension (bounds
                                               # the lead-cone taper on oblong
                                               # pads); 0 = same as pad_nm
+    slot_dx_nm: int = 0                       # slotted (oblong) hole: offset
+    slot_dy_nm: int = 0                       # from (x, y) to each end-cap
+                                              # center of the slot, board
+                                              # frame; (0, 0) = round drill
     solder_filled: bool = False               # populated THT pad: the hole
                                               # holds lead + solder (in parallel
                                               # with the plating); False for
@@ -162,19 +171,22 @@ class ViaLink:
                           lead_nm: float = 0,
                           lead_rho_ohm_m: float | None = None) -> float:
         """Barrel segment resistance over length_nm: thin-wall annulus of
-        plating around the drill. With solder_rho_ohm_m the hole holds a
+        plating around the drill (slotted holes: thin wall around the
+        stadium-shaped slot). With solder_rho_ohm_m the hole holds a
         soldered THT joint: the component lead (a cylinder of lead_nm
         diameter, resistivity lead_rho_ohm_m) and the solder filling the
-        remaining annulus conduct in parallel with the plating."""
-        ga = math.pi * (self.drill_nm * 1e-9) * (plating_nm * 1e-9) \
-            / rho_ohm_m                       # conductance-area [m^2/ohm-m]
+        remaining bore conduct in parallel with the plating."""
+        ext = 2.0 * math.hypot(self.slot_dx_nm, self.slot_dy_nm) * 1e-9
+        wall = math.pi * (self.drill_nm * 1e-9) + 2.0 * ext
+        ga = wall * (plating_nm * 1e-9) / rho_ohm_m
+                                              # conductance-area [m^2/ohm-m]
         if solder_rho_ohm_m is not None:
             r_core = max(self.drill_nm / 2.0 - plating_nm, 0.0) * 1e-9
             r_lead = min(lead_nm * 1e-9 / 2.0, r_core)
             if lead_rho_ohm_m is not None and r_lead > 0:
                 ga += math.pi * r_lead * r_lead / lead_rho_ohm_m
-            ga += math.pi * (r_core * r_core - r_lead * r_lead) \
-                / solder_rho_ohm_m
+            ga += (math.pi * r_core * r_core + 2.0 * r_core * ext
+                   - math.pi * r_lead * r_lead) / solder_rho_ohm_m
         return (length_nm * 1e-9) / ga
 
 
@@ -261,6 +273,19 @@ def contact_solder_buildups(problem: Problem) -> list[str]:
     return sorted(set(touched))
 
 
+def slot_distance(xg, yg, dx_nm: int, dy_nm: int):
+    """Distance from points (xg, yg) (numpy-broadcastable, coordinates
+    RELATIVE to the hole center) to a slotted hole's axis - the segment
+    (-dx, -dy)..(+dx, +dy) between the end-cap centers. The slot wall
+    sits at distance width/2. Round drills (dx = dy = 0) reduce to the
+    plain radius, so callers need no special case."""
+    if dx_nm == 0 and dy_nm == 0:
+        return np.hypot(xg, yg)
+    l2 = float(dx_nm) * dx_nm + float(dy_nm) * dy_nm
+    t = np.clip((xg * dx_nm + yg * dy_nm) / l2, -1.0, 1.0)
+    return np.hypot(xg - t * dx_nm, yg - t * dy_nm)
+
+
 def _disc_polygon(x_nm: float, y_nm: float, r_nm: float,
                   n: int = 32) -> Polygon:
     th = np.linspace(0.0, 2.0 * math.pi, n, endpoint=False)
@@ -269,6 +294,19 @@ def _disc_polygon(x_nm: float, y_nm: float, r_nm: float,
         axis=1)).astype(np.int64))
 
 
+def _capsule_polygon(x_nm: float, y_nm: float, dx_nm: float, dy_nm: float,
+                     r_nm: float, n: int = 16) -> Polygon:
+    """Stadium: two half-circle caps of radius r_nm centered at
+    (x +- dx, y +- dy), joined by straight flanks."""
+    a0 = math.atan2(dy_nm, dx_nm)
+    th = np.linspace(-0.5 * math.pi, 0.5 * math.pi, n) + a0
+    cap1 = np.stack([x_nm + dx_nm + r_nm * np.cos(th),
+                     y_nm + dy_nm + r_nm * np.sin(th)], axis=1)
+    cap2 = np.stack([x_nm - dx_nm + r_nm * np.cos(th + math.pi),
+                     y_nm - dy_nm + r_nm * np.sin(th + math.pi)], axis=1)
+    return Polygon(outline=np.round(np.vstack([cap1, cap2])).astype(np.int64))
+
+
 def tht_joint_buildups(problem: Problem,
                        shapes: dict | None = None) -> list[str]:
     """Solder coat of the net's populated STITCHING through-hole pads
@@ -292,7 +330,16 @@ def tht_joint_buildups(problem: Problem,
         if polys is None:
             if v.pad_nm <= v.drill_nm:
                 continue
-            polys = [_disc_polygon(v.x, v.y, v.pad_nm / 2.0)]
+            # oblong pads: never coat past the pad - a capsule along the
+            # slot axis, or the inscribed disc when the axis is unknown
+            w = v.pad_min_nm or v.pad_nm
+            hl = math.hypot(v.slot_dx_nm, v.slot_dy_nm)
+            if hl > 0.0 and v.pad_nm > w:
+                s = (v.pad_nm - w) / 2.0 / hl
+                polys = [_capsule_polygon(v.x, v.y, v.slot_dx_nm * s,
+                                          v.slot_dy_nm * s, w / 2.0)]
+            else:
+                polys = [_disc_polygon(v.x, v.y, w / 2.0)]
         problem.buildups.append(
             SurfaceBuildup(layer_name=v.protrusion_side,
                            polygons=list(polys)))
@@ -451,6 +498,8 @@ def _electrode_to_json(e: Electrode) -> dict:
         "drill_nm": e.drill_nm,
         "pad_nm": e.pad_nm,
         "pad_min_nm": e.pad_min_nm,
+        "slot_dx_nm": e.slot_dx_nm,
+        "slot_dy_nm": e.slot_dy_nm,
         "center": (None if e.center is None else list(e.center)),
         "barrel_z": (None if e.barrel_z is None else list(e.barrel_z)),
         "solder": e.solder,
@@ -468,6 +517,8 @@ def _electrode_from_json(d: dict) -> Electrode:
         drill_nm=int(d.get("drill_nm", 0)),
         pad_nm=int(d.get("pad_nm", 0)),
         pad_min_nm=int(d.get("pad_min_nm", 0)),
+        slot_dx_nm=int(d.get("slot_dx_nm", 0)),
+        slot_dy_nm=int(d.get("slot_dy_nm", 0)),
         center=(None if d.get("center") is None
                 else (int(d["center"][0]), int(d["center"][1]))),
         barrel_z=(None if d.get("barrel_z") is None
@@ -564,6 +615,8 @@ def problem_from_json(d: dict) -> Problem:
                     kind=vd.get("kind", "via"),
                     pad_nm=int(vd.get("pad_nm", 0)),
                     pad_min_nm=int(vd.get("pad_min_nm", 0)),
+                    slot_dx_nm=int(vd.get("slot_dx_nm", 0)),
+                    slot_dy_nm=int(vd.get("slot_dy_nm", 0)),
                     # older dumps: every THT pad counted as solder-filled
                     solder_filled=bool(vd.get(
                         "solder_filled", vd.get("kind", "via") == "pad")),

fill_resistance/raster.py

@@ -23,7 +23,7 @@ from scipy import ndimage
 
 from . import config
 from .errors import ElectrodeError, GridSizeError
-from .geometry import Electrode, Problem, Rect
+from .geometry import Electrode, Problem, Rect, slot_distance
 
 # 4-connectivity: matches the in-plane 5-point stencil of the solver
 _STRUCT4 = ndimage.generate_binary_structure(2, 1)
@@ -270,29 +270,31 @@ def _paint_lead_fillets(stack: RasterStack, problem: Problem) -> None:
             y = (e.rect.y0 + e.rect.y1) / 2.0
         seen.add((int(x), int(y)))
         if e.solder and e.protrusion_side:
-            # oblong pads: taper to the inscribed circle (conservative)
+            # oblong pads: taper from the (slot) wall to the inscribed
+            # dimension (conservative)
             jobs.append((x, y, e.drill_nm, e.pad_min_nm or e.pad_nm,
-                         e.protrusion_side))
+                         e.protrusion_side, e.slot_dx_nm, e.slot_dy_nm))
     for v in problem.vias:
         if v.kind == "pad" and v.solder_filled and v.protrusion_side \
                 and (v.x, v.y) not in seen:
             jobs.append((v.x, v.y, v.drill_nm, v.pad_min_nm or v.pad_nm,
-                         v.protrusion_side))
+                         v.protrusion_side, v.slot_dx_nm, v.slot_dy_nm))
 
-    for x, y, drill_nm, pad_nm, side in jobs:
+    for x, y, drill_nm, pad_nm, side, sdx, sdy in jobs:
         li = index.get(side)
         if li is None or pad_nm <= drill_nm:
             continue
         ra, rb = drill_nm / 2.0, pad_nm / 2.0
-        j0 = max(0, math.floor((x - rb - stack.x0_nm) / h))
-        j1 = min(nx, math.floor((x + rb - stack.x0_nm) / h) + 1)
-        i0 = max(0, math.floor((y - rb - stack.y0_nm) / h))
-        i1 = min(ny, math.floor((y + rb - stack.y0_nm) / h) + 1)
+        ex, ey = rb + abs(sdx), rb + abs(sdy)
+        j0 = max(0, math.floor((x - ex - stack.x0_nm) / h))
+        j1 = min(nx, math.floor((x + ex - stack.x0_nm) / h) + 1)
+        i0 = max(0, math.floor((y - ey - stack.y0_nm) / h))
+        i1 = min(ny, math.floor((y + ey - stack.y0_nm) / h) + 1)
         if i0 >= i1 or j0 >= j1:
             continue
         xs = stack.x0_nm + (np.arange(j0, j1) + 0.5) * h - x
         ys = stack.y0_nm + (np.arange(i0, i1) + 0.5) * h - y
-        r = np.sqrt(ys[:, None] ** 2 + xs[None, :] ** 2)
+        r = slot_distance(xs[None, :], ys[:, None], sdx, sdy)
         t_sn = H * np.clip((rb - r) / (rb - ra), 0.0, 1.0)
         t_eq = t_sn * (problem.rho_ohm_m / problem.solder_rho_ohm_m)
         factor = 1.0 + t_eq / problem.layers[li].thickness_nm
@@ -353,18 +355,20 @@ def _apply_via_mouths(stack: RasterStack, problem: Problem) -> None:
         if via.kind == "pad" and via.solder_filled:
             continue
         r = via.drill_nm / 2.0
-        j0 = max(0, math.floor((via.x - r - stack.x0_nm) / h))
-        j1 = min(nx, math.floor((via.x + r - stack.x0_nm) / h) + 1)
-        i0 = max(0, math.floor((via.y - r - stack.y0_nm) / h))
-        i1 = min(ny, math.floor((via.y + r - stack.y0_nm) / h) + 1)
+        ex, ey = r + abs(via.slot_dx_nm), r + abs(via.slot_dy_nm)
+        j0 = max(0, math.floor((via.x - ex - stack.x0_nm) / h))
+        j1 = min(nx, math.floor((via.x + ex - stack.x0_nm) / h) + 1)
+        i0 = max(0, math.floor((via.y - ey - stack.y0_nm) / h))
+        i1 = min(ny, math.floor((via.y + ey - stack.y0_nm) / h) + 1)
         if i0 >= i1 or j0 >= j1:
             continue
         xs = stack.x0_nm + (np.arange(j0, j1)[:, None] + sub[None, :]) * h \
             - via.x
         ys = stack.y0_nm + (np.arange(i0, i1)[:, None] + sub[None, :]) * h \
             - via.y
-        cov = ((ys[:, None, :, None] ** 2 + xs[None, :, None, :] ** 2)
-               <= r * r).mean(axis=(2, 3))
+        cov = (slot_distance(xs[None, :, None, :], ys[:, None, :, None],
+                             via.slot_dx_nm, via.slot_dy_nm)
+               <= r).mean(axis=(2, 3))
         if not (cov > 0).any():
             continue                            # mouth far smaller than h
         if stack.thick_scale is None:
@@ -477,7 +481,8 @@ def _electrode_cells2d(stack: RasterStack, e: Electrode) -> np.ndarray:
 def _barrel_ring2d(stack: RasterStack, e: Electrode,
                    mask2d: np.ndarray) -> np.ndarray:
     """Contact cells of a barrel electrode on one layer: the copper ring
-    at the drill wall (cell centers within one cell of radius drill/2),
+    at the drill wall (cell centers within one cell of radius drill/2;
+    slotted holes: within one cell of the stadium-shaped slot wall),
     where the lead/wire soldered into the hole actually meets the layer.
     If rasterization or an antipad leaves no copper there, fall back to
     the nearest copper ring within the pad footprint (+1 cell of slop) -
@@ -491,16 +496,17 @@ def _barrel_ring2d(stack: RasterStack, e: Electrode,
         y = (e.rect.y0 + e.rect.y1) / 2.0
     r = e.drill_nm / 2.0
     rw = max(e.pad_nm, e.drill_nm + 300_000) / 2.0 + h
+    ex, ey = rw + abs(e.slot_dx_nm), rw + abs(e.slot_dy_nm)
     out = np.zeros((ny, nx), dtype=bool)
-    j0 = max(0, math.floor((x - rw - stack.x0_nm) / h))
-    j1 = min(nx, math.floor((x + rw - stack.x0_nm) / h) + 1)
-    i0 = max(0, math.floor((y - rw - stack.y0_nm) / h))
-    i1 = min(ny, math.floor((y + rw - stack.y0_nm) / h) + 1)
+    j0 = max(0, math.floor((x - ex - stack.x0_nm) / h))
+    j1 = min(nx, math.floor((x + ex - stack.x0_nm) / h) + 1)
+    i0 = max(0, math.floor((y - ey - stack.y0_nm) / h))
+    i1 = min(ny, math.floor((y + ey - stack.y0_nm) / h) + 1)
     if i0 >= i1 or j0 >= j1:
         return out
     xs = stack.x0_nm + (np.arange(j0, j1) + 0.5) * h - x
     ys = stack.y0_nm + (np.arange(i0, i1) + 0.5) * h - y
-    d = np.sqrt(ys[:, None] ** 2 + xs[None, :] ** 2)
+    d = slot_distance(xs[None, :], ys[:, None], e.slot_dx_nm, e.slot_dy_nm)
     m = mask2d[i0:i1, j0:j1]
     ring = m & (np.abs(d - r) <= h)
     if not ring.any():

fill_resistance/solver.py

@@ -47,7 +47,7 @@ from scipy.sparse import linalg as sla
 
 from . import config, skin
 from .errors import ConnectivityError, ElectrodeError, SolverError
-from .geometry import Problem
+from .geometry import Problem, slot_distance
 from .raster import RasterStack, electrodes_touch
 
 
@@ -156,12 +156,14 @@ def _barrel_links(stack: RasterStack, problem: Problem
         span = [li for li, layer in enumerate(problem.layers)
                 if via.spans(layer.z_nm)]
         r_nm = max(via.pad_nm, via.drill_nm + 300_000) / 2.0 + h
-        win = int(r_nm // h) + 1
-        i0, i1 = max(0, i - win), min(ny, i + win + 1)
-        j0, j1 = max(0, j - win), min(nx, j + win + 1)
+        win_j = int((r_nm + abs(via.slot_dx_nm)) // h) + 1
+        win_i = int((r_nm + abs(via.slot_dy_nm)) // h) + 1
+        i0, i1 = max(0, i - win_i), min(ny, i + win_i + 1)
+        j0, j1 = max(0, j - win_j), min(nx, j + win_j + 1)
         xs = stack.x0_nm + (np.arange(j0, j1) + 0.5) * h - via.x
         ys = stack.y0_nm + (np.arange(i0, i1) + 0.5) * h - via.y
-        d2 = ys[:, None] ** 2 + xs[None, :] ** 2
+        d2 = slot_distance(xs[None, :], ys[:, None],
+                           via.slot_dx_nm, via.slot_dy_nm) ** 2
         d2 = np.where(d2 <= r_nm * r_nm, d2, np.inf)
         present = []                            # (layer, i, j) per layer
         for li in span:

tests/test_barrel_contacts.py

@@ -318,6 +318,162 @@ def test_vialink_solder_json():
     assert problem_from_json(d).vias[0].solder_filled is False
 
 
+# --- slotted (oblong) holes --------------------------------------------------
+# The lead/barrel of a slotted hole is a stadium, not a circle: modeling
+# it as a circle of the slot's LONG dimension painted contact rings,
+# mouths and cones bigger than the oblong pad itself.
+
+def _slot_dist_mm(stack, ii, jj, x_mm, y_mm, dx_nm):
+    """Distance of cells (ii, jj) to a slot axis (+-dx_nm along x)."""
+    xs = stack.x0_nm + (jj + 0.5) * stack.h_nm - x_mm * NM
+    ys = stack.y0_nm + (ii + 0.5) * stack.h_nm - y_mm * NM
+    t = np.clip(xs / dx_nm, -1.0, 1.0)
+    return np.hypot(xs - t * dx_nm, ys), xs, ys
+
+
+def test_slot_ring_hugs_slot_wall():
+    """The contact ring of a slotted THT pad follows the stadium-shaped
+    slot wall: it reaches around the end caps but never pokes past the
+    oblong pad's short side (the old circular model of the slot's long
+    dimension put cells at radius 1.5 mm straight above/below)."""
+    p = make_problem([(PLATE20, [])],
+                     rect1_mm=(0, 0, 1, 20), rect2_mm=(19, 0, 20, 20))
+    e = _barrel(10, 10, drill_mm=1.0, pad_mm=3.6)   # slot 3.0 x 1.0 mm
+    e.pad_min_nm = int(1.6 * NM)                    # pad 3.6 x 1.6 mm
+    e.slot_dx_nm = 1 * NM
+    p.electrodes1 = [e]
+    stack = raster.rasterize_stack(p, 0.1 * NM)
+    e1, _ = raster.electrode_masks(stack, p)
+    ii, jj = np.nonzero(e1[0])
+    d, xs, ys = _slot_dist_mm(stack, ii, jj, 10, 10, 1 * NM)
+    assert len(ii) >= 16
+    assert (np.abs(d - 0.5 * NM) <= stack.h_nm + 1).all()
+    assert xs.max() > 1.2 * NM and xs.min() < -1.2 * NM   # rings the caps
+    assert np.abs(ys).max() < 0.8 * NM      # stays inside the 1.6 mm side
+
+
+def test_slot_mouth_is_stadium():
+    """A DNP slotted pad cuts a stadium-shaped hole: open along the whole
+    slot, copper kept just past the slot width and the end caps."""
+    p = make_problem([(PLATE20, [])],
+                     rect1_mm=(0, 0, 1, 20), rect2_mm=(19, 0, 20, 20))
+    v = _pad_link(populated=False)
+    v.slot_dx_nm = 1 * NM                   # slot 3.0 x 1.0 mm along x
+    p.vias = [v]
+    stack = raster.rasterize_stack(p, 0.1 * NM)
+    m = stack.masks[0]
+    assert not m[stack.cell_of(10 * NM, 10 * NM)]
+    assert not m[stack.cell_of(int(10.9 * NM), 10 * NM)]  # slot end: open
+    assert not m[stack.cell_of(int(9.1 * NM), 10 * NM)]
+    assert m[stack.cell_of(10 * NM, int(10.8 * NM))]  # past the width: copper
+    assert m[stack.cell_of(10 * NM, int(9.2 * NM))]
+    assert m[stack.cell_of(int(11.8 * NM), 10 * NM)]  # past the cap: copper
+
+
+def test_slot_cone_follows_slot():
+    """The lead cone of a slotted oblong pad tapers from the slot WALL
+    to the pad's short dimension. The old circular-drill model (diameter
+    = the slot's long dimension) skipped the cone entirely
+    (pad_min <= drill) and, for the mouth, ate the pad's short side."""
+    p = make_problem([(PLATE20, [])],
+                     rect1_mm=(0, 0, 1, 20), rect2_mm=(19, 0, 20, 20))
+    e = _barrel(10, 10, drill_mm=1.0, pad_mm=3.6, solder=True)
+    e.pad_min_nm = int(1.6 * NM)
+    e.slot_dx_nm = 1 * NM
+    e.protrusion_side = "F.Cu"
+    p.electrodes1 = [e]
+    stack = raster.rasterize_stack(p, 0.1 * NM)
+    assert stack.thick_scale is not None
+    ny, nx = stack.shape2d
+    jj, ii = np.meshgrid(np.arange(nx), np.arange(ny))
+    r, _, _ = _slot_dist_mm(stack, ii, jj, 10, 10, 1 * NM)
+    ra, rb, H = 0.5 * NM, 0.8 * NM, p.tht_protrusion_nm
+    t_eq = H * np.clip((rb - r) / (rb - ra), 0, 1) \
+        * (p.rho_ohm_m / p.solder_rho_ohm_m)
+    expect = np.where(stack.masks[0],
+                      1.0 + t_eq / p.layers[0].thickness_nm, 1.0)
+    assert np.allclose(stack.thick_scale[0], expect, rtol=1e-12)
+    assert stack.thick_scale[0].max() > 3.0
+
+
+def test_slot_barrel_resistance():
+    """Slotted barrel: plating wall = stadium perimeter, solder core =
+    stadium bore area (both reduce to the circle for dx = dy = 0)."""
+    v = ViaLink(x=0, y=0, drill_nm=1_000_000, z_top_nm=-1, z_bot_nm=1,
+                slot_dx_nm=800_000, slot_dy_nm=600_000)   # ext = 2 mm
+    rho, sn = 1.68e-8, 1.32e-7
+    ga = (math.pi * 1e-3 + 2 * 2e-3) * 18e-6 / rho
+    r_plain = v.barrel_resistance(1_600_000, rho, 18_000)
+    assert r_plain == pytest.approx(1.6e-3 / ga, rel=1e-12)
+    rc = 0.5e-3 - 18e-6
+    ga += (math.pi * rc * rc + 2 * rc * 2e-3) / sn
+    r_fill = v.barrel_resistance(1_600_000, rho, 18_000, solder_rho_ohm_m=sn)
+    assert r_fill == pytest.approx(1.6e-3 / ga, rel=1e-12)
+
+
+def test_slot_coat_fallback_within_pad():
+    """Without an exact pad shape the stitching coat falls back to a
+    capsule along the slot (width = pad_min), not the old pad_nm disc
+    that stuck out past an oblong pad's short side."""
+    p = make_problem([(PLATE20, [])],
+                     rect1_mm=(0, 0, 1, 20), rect2_mm=(19, 0, 20, 20))
+    v = _pad_link()                          # pad_nm = 2.4 mm
+    v.pad_min_nm = 1_600_000
+    v.slot_dx_nm = 1 * NM
+    p.vias = [v]
+    assert tht_joint_buildups(p) == ["F.Cu"]
+    pts = p.buildups[0].polygons[0].outline.astype(float)
+    xs, ys = pts[:, 0] - 10 * NM, pts[:, 1] - 10 * NM
+    t = np.clip(xs / (0.4 * NM), -1.0, 1.0)  # caps at +-(2.4-1.6)/2 mm
+    d = np.hypot(xs - t * 0.4 * NM, ys)
+    assert np.allclose(d, 0.8 * NM, atol=2)
+    assert np.abs(xs).max() <= 1.2 * NM + 2  # never past pad_nm / 2
+    assert np.abs(ys).max() <= 0.8 * NM + 2  # never past pad_min / 2
+
+
+def test_slot_json_roundtrip():
+    p = make_problem([(PLATE20, [])],
+                     rect1_mm=(0, 0, 1, 20), rect2_mm=(19, 0, 20, 20))
+    e = _barrel(10, 10, drill_mm=1.0, pad_mm=3.6)
+    e.slot_dx_nm, e.slot_dy_nm = 700_000, -700_000
+    p.electrodes1 = [e]
+    v = _pad_link()
+    v.slot_dx_nm = 1 * NM
+    p.vias = [v]
+    q = problem_from_json(problem_to_json(p))
+    assert (q.electrodes1[0].slot_dx_nm, q.electrodes1[0].slot_dy_nm) \
+        == (700_000, -700_000)
+    assert (q.vias[0].slot_dx_nm, q.vias[0].slot_dy_nm) == (1 * NM, 0)
+    # legacy dumps: round drills
+    d = problem_to_json(p)
+    for vd in d["vias"]:
+        del vd["slot_dx_nm"], vd["slot_dy_nm"]
+    assert problem_from_json(d).vias[0].slot_dx_nm == 0
+
+
+def test_drill_info_slot_rotation():
+    """_drill_info: slot axis from the drill x/y sizes, rotated with the
+    pad (KiCad angles are CCW with y down: 90 deg sends +x to -y)."""
+    from types import SimpleNamespace as NS
+
+    from fill_resistance.board_io import _drill_info
+
+    def pad(dx_mm, dy_mm, angle_deg):
+        return NS(padstack=NS(
+            drill=NS(diameter=NS(x=int(dx_mm * NM), y=int(dy_mm * NM))),
+            angle=NS(degrees=angle_deg)))
+
+    assert _drill_info(pad(1.0, 1.0, 0.0)) == (1 * NM, 0, 0)    # round
+    assert _drill_info(pad(3.0, 1.0, 0.0)) == (1 * NM, 1 * NM, 0)
+    assert _drill_info(pad(1.0, 3.0, 0.0)) == (1 * NM, 0, 1 * NM)
+    w, dx, dy = _drill_info(pad(3.0, 1.0, 90.0))
+    assert (w, dx, dy) == (1 * NM, 0, -1 * NM)
+    w, dx, dy = _drill_info(pad(3.0, 1.0, 45.0))
+    assert w == 1 * NM
+    assert dx == pytest.approx(1 * NM / math.sqrt(2), abs=2)
+    assert dy == pytest.approx(-1 * NM / math.sqrt(2), abs=2)
+
+
 def test_barrel_electrode_json_roundtrip(tmp_path):
     p = make_problem([(PLATE20, [])],
                      rect1_mm=(0, 0, 1, 20), rect2_mm=(19, 0, 20, 20))