fix: QFN32 pad sizing to match IPC-7351B/KiCad standards

src/fn/qfn.ts

@@ -1,18 +1,49 @@
 import type { AnySoupElement } from "circuit-json"
-import { base_quad_def, quad, quad_def, quadTransform } from "./quad"
 import type { z } from "zod"
+import { base_quad_def, quad, quadTransform, quad_def } from "./quad"
 
 export const qfn_def = base_quad_def.extend({}).transform(quadTransform)
 
 export const qfn = (
-  parameters: z.input<typeof qfn_def>,
+  raw_params: z.input<typeof qfn_def>,
 ): { circuitJson: AnySoupElement[]; parameters: any } => {
-  parameters.legsoutside = false
-  if (!parameters.pl) {
-    parameters.pl = 0.875
+  raw_params.legsoutside = false
+  const pitchValue =
+    typeof raw_params.p === "string"
+      ? Number.parseFloat(raw_params.p)
+      : raw_params.p
+  if (raw_params.pw === undefined && raw_params.pl === undefined) {
+    if (pitchValue !== undefined && pitchValue > 0) {
+      // IPC-compliant defaults: pw = 0.5 * pitch, pl based on standard QFN sizing
+      raw_params.pw = pitchValue * 0.5
+      raw_params.pl = 0.875
+    } else {
+      raw_params.pl = 0.875
+      raw_params.pw = 0.25
+    }
+  } else {
+    if (raw_params.pl === undefined) {
+      raw_params.pl = 0.875
+    }
+    if (raw_params.pw === undefined) {
+      raw_params.pw =
+        pitchValue !== undefined && pitchValue > 0 ? pitchValue * 0.5 : 0.25
+    }
   }
-  if (!parameters.pw) {
-    parameters.pw = 0.25
+
+  // When body dimensions (w/h) are explicitly specified and padoffset is not,
+  // compute padoffset so pad centers match IPC-7351B / KiCad positioning.
+  // QFN pads extend slightly beyond the package body edge; the pad center
+  // sits approximately 0.0625mm inside the body edge.
+  if (raw_params.padoffset === undefined && raw_params.w !== undefined) {
+    const pl =
+      typeof raw_params.pl === "string"
+        ? Number.parseFloat(raw_params.pl)
+        : raw_params.pl
+    if (pl !== undefined && pl > 0) {
+      raw_params.padoffset = 0.0625 - pl / 2
+    }
   }
-  return quad(parameters)
+
+  return quad(raw_params)
 }

src/fn/quad.ts

@@ -27,6 +27,7 @@ export const base_quad_def = base_def.extend({
   pl: length.optional(),
   thermalpad: z.union([z.literal(true), dim2d]).optional(),
   legsoutside: z.boolean().default(false),
+  padoffset: z.number().optional(),
 })
 
 export const quadTransform = <T extends z.infer<typeof base_quad_def>>(
@@ -81,8 +82,9 @@ export const getQuadCoords = (params: {
   p: number // pitch between pins
   pl: number // length of the pin
   legsoutside?: boolean
+  padoffset?: number
 }) => {
-  const { pin_count, pn, w, h, p, pl, legsoutside } = params
+  const { pin_count, pn, w, h, p, pl, legsoutside, padoffset } = params
   const sidePinCount = pin_count / 4
   const side = SIDES_CCW[Math.floor((pn - 1) / sidePinCount)]
   const pos = (pn - 1) % sidePinCount
@@ -95,15 +97,17 @@ export const getQuadCoords = (params: {
   /** pad center distance from edge (negative is inside, positive is outside) */
   const pcdfe = legsoutside ? pl / 2 : -pl / 2
 
+  const offset = padoffset ?? 0.1
+
   switch (side) {
     case "left":
-      return { x: -w / 2 - pcdfe + 0.1, y: ibh / 2 - pos * p, o: "vert" }
+      return { x: -w / 2 - pcdfe + offset, y: ibh / 2 - pos * p, o: "vert" }
     case "bottom":
-      return { x: -ibw / 2 + pos * p, y: -h / 2 - pcdfe + 0.1, o: "horz" }
+      return { x: -ibw / 2 + pos * p, y: -h / 2 - pcdfe + offset, o: "horz" }
     case "right":
-      return { x: w / 2 + pcdfe - 0.1, y: -ibh / 2 + pos * p, o: "vert" }
+      return { x: w / 2 + pcdfe - offset, y: -ibh / 2 + pos * p, o: "vert" }
     case "top":
-      return { x: ibw / 2 - pos * p, y: h / 2 + pcdfe - 0.1, o: "horz" }
+      return { x: ibw / 2 - pos * p, y: h / 2 + pcdfe - offset, o: "horz" }
     default:
       throw new Error("Invalid pin number")
   }
@@ -130,6 +134,7 @@ export const quad = (
       p: parameters.p ?? 0.5,
       pl: parameters.pl,
       legsoutside: parameters.legsoutside,
+      padoffset: parameters.padoffset,
     })
 
     let pw = parameters.pw