OCCT 3D model export: fix copper fusing issues on StickHub GND and #20515

Fixes https://gitlab.com/kicad/code/kicad/-/issues/20515

pcbnew/exporters/step/exporter_step.cpp

@@ -171,6 +171,9 @@ bool EXPORTER_STEP::buildFootprint3DShapes( FOOTPRINT* aFootprint, VECTOR2D aOri
         SHAPE_POLY_SET holePoly;
         holeShape->TransformToPolygon( holePoly, maxError, ERROR_INSIDE );
 
+        // This helps with fusing
+        holePoly.Deflate( m_platingThickness / 2, CORNER_STRATEGY::ROUND_ALL_CORNERS, maxError );
+
         for( PCB_LAYER_ID pcblayer : pad->GetLayerSet().Seq() )
         {
             if( pad->IsOnLayer( pcblayer ) )
@@ -421,6 +424,9 @@ bool EXPORTER_STEP::buildTrack3DShape( PCB_TRACK* aTrack, VECTOR2D aOrigin )
         SHAPE_POLY_SET                 holePoly;
         holeShape->TransformToPolygon( holePoly, maxError, ERROR_INSIDE );
 
+        // This helps with fusing
+        holePoly.Deflate( m_platingThickness / 2, CORNER_STRATEGY::ROUND_ALL_CORNERS, maxError );
+
         LSET layers( via->GetLayerSet() & m_layersToExport );
 
         PCB_LAYER_ID top_layer, bot_layer;

pcbnew/exporters/step/step_pcb_model.cpp

@@ -785,6 +785,7 @@ STEP_PCB_MODEL::~STEP_PCB_MODEL()
 
 bool STEP_PCB_MODEL::AddPadShape( const PAD* aPad, const VECTOR2D& aOrigin, bool aVia )
 {
+    const double              c_padExtraThickness = 0.005;
     bool                      success = true;
     std::vector<TopoDS_Shape> padShapes;
 
@@ -806,9 +807,9 @@ bool STEP_PCB_MODEL::AddPadShape( const PAD* aPad, const VECTOR2D& aOrigin, bool
         {
             // Pad surface as a separate face for FEM simulations.
             if( pcb_layer == F_Cu )
-                thickness += 0.005;
+                thickness += c_padExtraThickness;
             else if( pcb_layer == B_Cu )
-                thickness -= 0.005;
+                thickness -= c_padExtraThickness;
         }
 
         TopoDS_Shape testShape;
@@ -860,6 +861,14 @@ bool STEP_PCB_MODEL::AddPadShape( const PAD* aPad, const VECTOR2D& aOrigin, bool
         double b_pos, b_thickness;
         getLayerZPlacement( F_Cu, f_pos, f_thickness );
         getLayerZPlacement( B_Cu, b_pos, b_thickness );
+
+        if( !aVia )
+        {
+            // Pad surface is slightly thicker
+            f_thickness += c_padExtraThickness;
+            b_thickness -= c_padExtraThickness;
+        }
+
         double top = std::max( f_pos, f_pos + f_thickness );
         double bottom = std::min( b_pos, b_pos + b_thickness );