Route to inner plane layers with a via, not pad to pad

On a four-layer board with a GND plane on an inner power layer, freeroute
ignored the plane (it only used zones on routing layers) and chained every
GND pad to the next with tracks across the board. Now a through-hole pad
inside a plane is joined to it already, and for a net with a plane every
other group connects straight to the plane: a route may end on any cell
inside it where a via fits (checked lazily, like other vias), and ending
there places that via. Signal nets are unaffected.

Tested on a four-layer DSN: three SMD GND pads each get a short stub and
their own via, the THT GND pad needs nothing, and no track crosses the
board. Ahead of the TerraHawk ESC, a four-layer board.

src/freeroute/route/grid_router.py

@@ -212,6 +212,10 @@ class GridRouter:
         self._pad_own: dict[Node, set[int]] = {}
         self._keepout: set[Node] = set()
         self._via_keepout: set[Node] = set()
+        # inner power (plane) layers: not routed on, reached by vias
+        self._power_layers = {
+            i for i, layer in enumerate(board.layer_structure.arr) if not layer.is_signal
+        }
         self._zone_cells: dict[int, list[Node]] = {}
         # per-connection occupancy (owner = connection index)
         self._cell_owner: dict[Node, int] = {}
@@ -393,6 +397,9 @@ class GridRouter:
         self, conn: _Connection, allow_ripup: bool, conns: list[_Connection]
     ) -> list[Node] | None:
         goal_cell = self._group_target(conn.b_group)
+        if not conn.b_group[0] and any(z.layer in self._power_layers for z in conn.b_group[1]):
+            # the plane is everywhere: aim at where the route starts instead
+            goal_cell = self._group_target(conn.a_group)
         starts = self._group_ends(conn.a_group, conn.net_no, conns)
         goals = self._group_ends(conn.b_group, conn.net_no, conns)
         if not starts or not goals:
@@ -434,9 +441,18 @@ class GridRouter:
                 continue
             current = entry[1]
             if current in goals:
-                path = _reconstruct(came_from, current)
-                conn.a_end, conn.b_end = starts.get(path[0]), goals[current]
-                return path
+                end = goals[current]
+                cell = (current[0], current[1])
+                if isinstance(end, ConductionArea):  # a plane end needs its via to fit
+                    ok = via_ok.get(cell)
+                    if ok is None:
+                        ok = via_ok[cell] = self._via_placeable(cell, net_no, conns)
+                else:
+                    ok = True
+                if ok:
+                    path = _reconstruct(came_from, current)
+                    conn.a_end, conn.b_end = starts.get(path[0]), end
+                    return path
             cx, cy, cl = current
             base = g_score[current]
             for dx, dy in self._neighbours:
@@ -499,10 +515,12 @@ class GridRouter:
         return True
 
     def _zone_nodes(self, zone) -> list[Node]:
-        """Cells of a zone where a via of this net fits inside its copper."""
+        """Cells of a zone where a via of this net fits inside its copper; for a
+        plane on a power layer, on every routing layer (a via reaches it)."""
         key = id(zone)
         if key not in self._zone_cells:
             h = self.via_half
+            layers = [zone.layer] if zone.layer in self.layers else list(self.layers)
             nodes = []
             for tile in zone.tiles:
                 for cell in self._cells_in_box(tile.bounding_box(), 0):
@@ -511,7 +529,7 @@ class GridRouter:
                         zone.polygon.contains(IntPoint(c.x + dx, c.y + dy))
                         for dx in (-h, h) for dy in (-h, h)
                     ):
-                        nodes.append((*cell, zone.layer))
+                        nodes.extend((*cell, layer) for layer in layers)
             self._zone_cells[key] = sorted(set(nodes))
         return self._zone_cells[key]
 
@@ -549,8 +567,10 @@ class GridRouter:
             for node in sorted(self._end_nodes(pin, net_no, conns)):
                 ends.setdefault(node, pin)
         for zone in group[1]:
+            plane = zone.layer in self._power_layers
             for node in self._zone_nodes(zone):
-                ends.setdefault(node, None)
+                # a plane end is marked with the plane: the route ends in a via
+                ends.setdefault(node, zone if plane else None)
         return ends
 
     def _group_target(self, group) -> Cell:
@@ -704,6 +724,11 @@ class GridRouter:
 
     def _commit(self, conn: _Connection, path: list[Node]) -> None:
         wire_segs, via_cells = _split_path(path)
+        via_cells = list(via_cells)
+        if isinstance(conn.a_end, ConductionArea):
+            via_cells.append((path[0][0], path[0][1]))
+        if isinstance(conn.b_end, ConductionArea):
+            via_cells.append((path[-1][0], path[-1][1]))
         conn.trace_cells = [(cell, layer) for layer, cells in wire_segs for cell in cells]
         conn.via_cells = list(via_cells)
         for cell, layer in conn.trace_cells:
@@ -724,8 +749,8 @@ class GridRouter:
                     conn.via_halo.append(c)
         ends = ((conn.a_end, wire_segs[0], 0), (conn.b_end, wire_segs[-1], -1)) if wire_segs else ()
         for pin, (layer, cells), at in ends:
-            if pin is None:
-                continue  # ended on a zone: no stub
+            if not isinstance(pin, Pin):
+                continue  # ended on a zone or a plane: no stub
             cell = cells[at]
             for c in self._stub_cells(pin.location, cell):
                 node = (*c, layer)
@@ -735,9 +760,9 @@ class GridRouter:
         conn.wires = []
         for idx, (layer, cells) in enumerate(wire_segs):
             points = [self._cell_center(c) for c in cells]
-            if idx == 0 and conn.a_end is not None:
+            if idx == 0 and isinstance(conn.a_end, Pin):
                 points.insert(0, conn.a_end.location)
-            if idx == len(wire_segs) - 1 and conn.b_end is not None:
+            if idx == len(wire_segs) - 1 and isinstance(conn.b_end, Pin):
                 points.append(conn.b_end.location)
             points = _simplify(points)
             if len(points) >= 2:
@@ -835,12 +860,26 @@ class GridRouter:
         for zone in self.board.get_conduction_areas():
             if zone.layer in self.layers and zone.net_nos and self._zone_is_solid(zone):
                 zones_by_net.setdefault(zone.net_nos[0], []).append(zone)
+        # planes: zones of a net on an inner power layer, solid by construction
+        planes_by_net: dict[int, list] = {}
+        for zone in self.board.get_conduction_areas():
+            if zone.net_nos and zone.layer in self._power_layers:
+                planes_by_net.setdefault(zone.net_nos[0], []).append(zone)
+        plane_ids = frozenset(id(z) for zs in planes_by_net.values() for z in zs)
         conns: list[_Connection] = []
-        nets = set(pins_by_net) | set(zones_by_net)
+        nets = set(pins_by_net) | set(zones_by_net) | set(planes_by_net)
         # deterministic: fewer-pin nets first, then by net number
         for net_no in sorted(nets, key=lambda n: (len(pins_by_net.get(n, [])), n)):
-            groups = _connected_groups(pins_by_net.get(net_no, []), zones_by_net.get(net_no, []))
-            for ga, gb in zip(groups, groups[1:], strict=False):
+            zones = zones_by_net.get(net_no, []) + planes_by_net.get(net_no, [])
+            groups = _connected_groups(pins_by_net.get(net_no, []), zones, plane_ids)
+            hubs = [g for g in groups if any(id(z) in plane_ids for z in g[1])]
+            if hubs:
+                # a plane net: every other group goes straight to the plane
+                # (usually a stub and a via), not pad to pad across the board
+                pairs = [(g, hubs[0]) for g in groups if g is not hubs[0]]
+            else:
+                pairs = list(zip(groups, groups[1:], strict=False))
+            for ga, gb in pairs:
                 conn = _Connection(len(conns), net_no, _first(ga[0]), _first(gb[0]))
                 conn.a_group, conn.b_group = ga, gb
                 conns.append(conn)
@@ -900,8 +939,15 @@ def _ends_of(conn) -> tuple[str, str]:
     return _group_label(conn.a_group), _group_label(conn.b_group)
 
 
-def _pin_touches_zone(pin: Pin, zone) -> bool:
-    """A pad on the zone's layer whose centre or a corner lies in the zone."""
+def _pin_touches_zone(pin: Pin, zone, plane: bool = False) -> bool:
+    """A pad on the zone's layer whose centre or a corner lies in the zone.
+
+    For a plane on an inner power layer: a through-hole pad (on more than one
+    copper layer) inside it passes through the plane and is joined to it; an
+    SMD pad is not, and needs a via.
+    """
+    if plane:
+        return len(pin.layers) > 1 and zone.polygon is not None and zone.polygon.contains(pin.location)
     if zone.layer not in pin.layers or zone.polygon is None:
         return False
     if zone.polygon.contains(pin.location):
@@ -913,7 +959,9 @@ def _pin_touches_zone(pin: Pin, zone) -> bool:
     return any(zone.polygon.contains(c) for c in corners)
 
 
-def _connected_groups(pins: list[Pin], zones: list) -> list[tuple[list[Pin], list]]:
+def _connected_groups(
+    pins: list[Pin], zones: list, planes: frozenset = frozenset()
+) -> list[tuple[list[Pin], list]]:
     """Pins and zones of one net that copper already joins: a pad in a zone
     is connected to it, and two zones sharing a pad are one. Only the groups
     need routing between them. Pin groups come first, in pin order."""
@@ -927,7 +975,7 @@ def _connected_groups(pins: list[Pin], zones: list) -> list[tuple[list[Pin], lis
 
     for j, zone in enumerate(zones):
         for i, pin in enumerate(pins):
-            if _pin_touches_zone(pin, zone):
+            if _pin_touches_zone(pin, zone, id(zone) in planes):
                 parent[find(i)] = find(len(pins) + j)
     groups: dict[int, tuple[list[Pin], list]] = {}
     order: list[int] = []

tests/route/test_real_board_geometry.py

@@ -164,3 +164,50 @@ def test_cli_names_the_connections_it_could_not_route(tmp_path, capsys):
     assert main([str(dsn), "--engine", "exact", "-do", str(tmp_path / "out.ses")]) == 0
     err = capsys.readouterr().err
     assert "1 connection(s) not routed" in err and "N: A-1 to B-1" in err
+
+
+# --- inner plane layers (a 4-layer board with a GND plane on In1.Cu) --------
+
+def _four_layer_dsn():
+    return """(pcb four (parser) (resolution um 10) (unit um)
+  (structure
+    (layer F.Cu (type signal)) (layer In1.Cu (type power))
+    (layer In2.Cu (type power)) (layer B.Cu (type signal))
+    (boundary (path pcb 0  0 0  30000 0  30000 -30000  0 -30000  0 0))
+    (plane GND (polygon In1.Cu 0  500 -500  29500 -500  29500 -29500  500 -29500  500 -500))
+    (via "Via[0-3]_600:300_um") (rule (width 250) (clearance 150)))
+  (placement
+    (component S (place G1 5000 -5000 front 0) (place G2 25000 -5000 front 0)
+                 (place G3 15000 -25000 front 0) (place A 5000 -15000 front 0) (place B 25000 -15000 front 0))
+    (component T (place GT 15000 -15000 front 0)))
+  (library
+    (image S (pin smd 1 0 0))
+    (image T (pin tht 1 0 0))
+    (padstack smd (shape (rect F.Cu -300 -300 300 300)) (attach off))
+    (padstack tht (shape (circle F.Cu 1000)) (shape (circle B.Cu 1000)) (attach off))
+    (padstack "Via[0-3]_600:300_um" (shape (circle F.Cu 600)) (shape (circle B.Cu 600)) (attach off)))
+  (network (net GND (pins G1-1 G2-1 G3-1 GT-1)) (net SIG (pins A-1 B-1))))"""
+
+
+def test_smd_pads_reach_an_inner_plane_by_their_own_via():
+    dsn = parse_dsn(_four_layer_dsn())
+    result, scale, names = route_dsn_board_exact(dsn)
+    gnd = next(i + 1 for i, n in enumerate(dsn.nets) if n.name == "GND")
+    sig = next(i + 1 for i, n in enumerate(dsn.nets) if n.name == "SIG")
+    assert len(result.result.vias.get(gnd, [])) == 3  # one per SMD pad, none for the THT pad
+    for _layer, pts in result.result.wires.get(gnd, []):  # stubs, not tracks across the board
+        length = sum(abs(q.x - p.x) + abs(q.y - p.y) for p, q in zip(pts, pts[1:], strict=False))
+        assert length < 3000 * scale  # under 3 mm: a stub to its via
+    assert sig in result.result.routed_net_numbers  # signals still route on the outer layers
+    assert all(names[layer] in ("F.Cu", "B.Cu") for segs in result.result.wires.values() for layer, _ in segs)
+
+
+def test_a_through_hole_pad_inside_a_plane_needs_no_route():
+    from freeroute.route.grid_router import _connected_groups
+
+    board = build_board(parse_dsn(_four_layer_dsn()))
+    plane = [z for z in board.get_conduction_areas() if z.name == "GND"]
+    gnd_pins = [p for p in board.get_pins() if p.component_ref in ("G1", "G2", "G3", "GT")]
+    groups = _connected_groups(gnd_pins, plane, frozenset(id(z) for z in plane))
+    hub = next(g for g in groups if g[1])
+    assert [p.component_ref for p in hub[0]] == ["GT"]